Electrochemical cells using proton-conducting ceramic electrolyte membranes have recently been revived, which is attributed to the numerous studies that have demonstrated that proton-conducting ceramic fuel cells and electrolysis cells can be readily manufactured, and their performances have been significantly enhanced in the last decade.[1–13] Protonic ceramic electrochemical cells (PCECs) are also emerging for synthesizing chemicals more efficiently and sustainably.[14–25] PCECs are versatile and capable of synthesizing broad and diversified chemicals from earth-abundant feedstocks (e.g., H2O and N2) and CO2, and upgrading natural gas (methane) with reduced carbon footprint and enhanced energy efficiency. PCECs offer various fascinating advantages that distinguish them from other chemical production technologies. These advantages include the intermediate operating temperature (300–600 °C) that thermodynamically and kinetically favors the electrode chemistry while potentially allowing the co-utilization of renewable power and waste heat.[26] Therefore, PCECs can be readily integrated with renewable or nuclear power plants to convert electrical energy. PCECs could also be integrated with fossil assets to capture and utilize CO2 and waste heat. Moreover, the intermediate operating temperature will relax the system design and control,[27] consequently reducing both capital and operational costs. The modular configuration of PCECs enables them to be deployed as distributed systems, which further facilitates their integration with renewable power plants and enables the use of various feedstocks, creating substantial market potential for renewable energy sectors and chemical industries.
As schematically illustrated in Figure 1 and summarized in Table 1, PCECs have been adapted to various applications, including synthesizing ammonia that can be utilized as fertilizers to improve crop yield, converting CO2 to sustainable CO, CH4, and syngas, and upgrading CH4 to high-value aromatics and light olefins. The reactor configurations for these three applications are displayed in Table 1. Additionally, Table 1 presents the reactions that occur at both positive electrodes and negative electrodes. Table 1 also summarizes the representative electrode materials for these reactions and the state-of-the-art performances. As shown in Figure 1, the PCEC can function as a fuel cell in its reversed mode to interconvert chemical energy and electrical energy; thus, the PCEC can be employed for the chemical energy storage system. Numerous lab-scale PCECs have been demonstrated to realize these applications.[5,14,17,23,28] These previous studies have centered on proving the concept of producing chemicals in PCECs. However, the reaction thermodynamics, energy efficiency, potential reaction mechanisms, critical barriers, and potential strategies to overcome the challenges are not discussed in detail. For example, the energy efficiency of synthesizing ammonia in PCECs has not been determined and compared with the conventional Haber–Bosch (HB) process. The electrochemical reactions in PCECs have not been clearly and systematically elaborated. Additionally, as listed in Table 1, despite synthesizing chemicals in PCECs displays multiple compelling benefits, challenges concurrently exhibit, some of which have been noted while some issues have not been recognized. Therefore, comprehensively summarizing these challenges and disclosing the corresponding strategies can unlock additional opportunities for synthesizing chemicals in PCECs.
Table 1 Summary of PCECs for synthesizing chemicals
| Ammonia Synthesis | CO2 reduction | Natural gas conversion | |
| Configuration | [IMAGE OMITTED. SEE PDF.] | [IMAGE OMITTED. SEE PDF.] | [IMAGE OMITTED. SEE PDF.] |
| Reactions at the positive electrode and representative positive electrode materials |
Steam methane reforming and water gas shift reaction: BaZr0.8Y0.2O3+δ+Ni[1] H2 oxidation reaction: BaZr0.8Y0.2O3+δ+Ni or BaCe0.7Zr0.1Y0.1Yb0.1O3+δ+Ni[1,2,29] Oxygen evolution reaction: BaCo0.4Fe0.4Zr0.1Y0.1O3+δ[22] PrBa0.5Sr0.5Co1.5Fe0.5O5+δ[7] Ba1−xGd0.8La0.2+xCo2O6−δ[30] |
H2 oxidation reaction: BaZr0.8Y0.2O3+δ+Ni or BaCe0.7Zr0.1Y0.1Yb0.1O3+δ+Ni[1,2,29] Oxygen evolution reaction: BaCo0.4Fe0.4Zr0.1Y0.1O3+δ[22] PrBa0.5Sr0.5Co1.5Fe0.5O5+δ[7] Ba1−xGd0.8La0.2+xCo2O6−δ[30] |
Methane dehydroaromatization: Mo/H-MCM-22[23] Alkane dehydrogenation: PrBa0.5Sr0.5Co1.5Fe0.5O5+δ[7] |
| Reactions at the negative electrode and representative negative electrode materials |
Nitrogen reduction reaction: VN-Fe[14] Hydrogen evolution reaction (HER, side reaction) |
CO2 reduction reaction: PBMBZY/IrO or PBMBZY/IrIr[4] HER (side reaction) |
HER BaZr0.8Y0.2O3+δ+Ni or BaCe0.7Zr0.1Y0.1Yb0.1O3+δ+Ni[1,2,29] |
| Advantages | Intermediate operating temperatures (400–500 °C) thermodynamically and kinetically favor NH3 synthesis |
High CO2 conversion High chemical production rate Low overpotentials |
Enhanced methane conversion Co-generation of hydrogen Lower operating temperature |
| Disadvantages |
HER is thermodynamically more favorable than NRR High temperature favors NH3 decomposition |
High temperature favors thermochemical CO2 reduction High temperature favors HER |
Potentially exacerbated coking Complicated catalyst integration |
| State of the art | >5 × 10−9 mol cm−2 s−1[31] | >95% selectivity to CO/CH4 at 400 °C[28] |
CH4 conversion > 11% >40 h of stable operation at 710 °C[23] |
| Critical Barriers |
Low NH3 Faradaic selectivity; Severe HER |
Severe HER; Poor stability; Low Faradaic selectivity toward C2 and C2+ products |
Low methane conversion; Coking |
Herein, to provide insights into designing PCEC reactors, probing reaction mechanisms, identifying associated challenges, and proposing corresponding strategies, the most up-to-date PCECs are comprehensively analyzed. Depending on the chemicals that can be produced in PCECs, the PCECs are divided into three main categories, including nitrogen reduction in PCECs for ammonia synthesis, CO2 reduction reaction for producing carbon-containing chemicals, and PCECs for upgrading natural gas (methane and other alkanes) to aromatics and light olefins. This review aims to present the PCEC design and configurations that have been developed and experimentally validated for these three applications. The corresponding reactions thermodynamics and mechanisms will be discussed. The recent progress and achievements of synthesizing chemicals in PCECs are summarized. We will subsequently identify and discuss the factors that impact the conversion, product yield, energy efficiency, and durability, guiding the future research and development of PCECs. Additionally, the challenges associated with chemical synthesis in PCECs will be outlined. Finally, the review offers strategies to overcome these challenges concerning materials design and synthesis, PCEC reactor design, and operating conditions.
Nitrogen Reduction Reaction for Ammonia SynthesisThere is significant commercial and economic incentive to produce ammonia more efficiently and sustainably, as ammonia is widely used as fertilizer, refrigerant gas, the building block for other chemicals, and fuel for power generation.[32–34] The ammonia production industry based on the conventional Haber–Bosch (HB) process is responsible for >1.0% global greenhouse gas (GHG) emissions,[32] striking the development of alternative ammonia synthesis technologies and the optimization of conventional HB ammonia synthesis processes. The main challenges of the HB process arise from 1) harsh operating conditions, especially high pressure (e.g., 300 bar), which is required to activate the N2 triple bond and thermodynamically favor the ammonia production,[35,36] and 2) hydrogen is produced primarily from natural gas via steam reforming,[37–39] which is of high emissions and energy-intensive. PCECs with different configurations have been employed to address these challenges by integrating electrochemical ammonia synthesis with other thermochemical and electrochemical processes, aiming to increase ammonia yield while simultaneously enhancing overall energy efficiency and reducing GHG emissions.
Configurations of PCECs for Ammonia SynthesisFour major PCEC reactor configurations have been designed and validated for ammonia synthesis, which are schematically illustrated in Figure 2. Some of these reactors integrate electrochemical/thermochemical processes and separation processes with ammonia synthesis, aiming to circumvent the thermodynamic limitations of ammonia synthesis that are typically encountered in a classical HB reactor. This integration also allows using renewable power for ammonia synthesis, which enhances the sustainability of ammonia production and reduces the carbon footprint of ammonia manufacturing industries. Ammonia produced via this sustainable approach can also serve as energy storage media or hydrogen carrier, which is attributed to its high energy density, high compressibility, carbon-neutral nature, and lower transportation costs than hydrogen.[40] When additional electricity is needed to supplement the intermittent renewable power sources, PCECs can function as fuel cells to directly convert ammonia to electrical energy without any changes to the configuration of PCECs.[5,17]
Figure 2 shows that PCECs for synthesizing ammonia can be classified according to the configurations and feedstocks. Reactor 1 and Reactor 2 have been developed to electrochemically reduce nitrogen to ammonia to enhance the ammonia production rate under mild conditions, such as ambient pressure. Reactor 3 intensifies steam methane reforming with ammonia synthesis, eliminating the external reformer and separating the H2 stream from the CO2 stream, which consequently simplifies CO2 capture and reduces carbon footprint. Reactor 4 can convert H2O and N2 to ammonia via nitrogen reduction reaction and water electrolysis that provides alternative hydrogen.
The detailed processes, unique features, innovations, and challenges of these four reactors are outlined as follows:
In Reactor 1, H2 is fed to the positive electrode while N2 is delivered to the negative electrode. Under an electrical potential, protons transport to the negative electrode where N2 is reduced to ammonia. The chemisorption of both nitrogen and hydrogen on catalysts, and the binding energy of nitrogen with the catalyst, are essential to high ammonia yield. A high hydrogen coverage can poison the catalysts, while a lower hydrogen coverage is insufficient for nitrogen reduction.[41,42] Therefore, the polarization current density applied on PCECs, which determines the proton flux, can affect hydrogen coverage on the negative electrode and ammonia production rate. Additionally, as the negative electrode can have proton conduction, the hydrogen is primarily absorbed on the surface of proton conductors, which creates additional active sites on the metallic phase for the dissociative adsorption of nitrogen.[43] Reactor 1 can therefore accelerate the kinetics of nitrogen reduction ammonia production. Reactor 1 can directly convert the electrical energy to chemical energy, which could circumvent the thermodynamic limits encountered in conventional HB reactors, eliminating the requirements of high-pressure operation.[44,45] Therefore, the objective of employing Reactor 1 is to enhance ammonia yield under mild conditions (e.g., ambient pressure). However, Reactor 1 cannot address the issue associated with hydrogen production, which requires integrating Reactor 1 with hydrogen production units, including steam reforming of fossil fuels or renewable hydrogen production technologies (e.g., electrolysis, biomass, photoelectrochemical catalysis, and solar thermochemical H2).
Reactor 2 presents PCECs with a single-chamber configuration that is similar to the conventional HB reactor counterpart, which can convert H2 and N2 to ammonia in a single chamber. The Reactor 2 configuration is also similar to the single-chamber solid oxide fuel cells (SOFCs),[46,47] where a mixture of fuel and oxygen is supplied to the same chamber. The anode selectively oxidizes the fuel while the cathode selectively reduces O2, enabling the generation of power. The concept of designing electrodes with selective activity to enable power generation can assist in designing positive and negative electrodes of PCECs with a Reactor 2 configuration. A mixture of H2 and N2 is used as the feedstock in Reactor 2. The positive electrode can selectively oxidize H2 while the negative electrode can selectively reduce N2. Compared to the conventional HB process, the main difference is that an ammonia synthesis catalyst is applied as the negative electrode of PCECs. Applying a polarization current can control the hydrogen coverage on the catalyst and the binding energy of nitrogen, which consequently leads to a non-Faradaic effect and significantly improve the ammonia yield.[48] In terms of the hydrogen source, Reactor 2 displays the same characteristic as Reactor 1. Moreover, the ammonia synthesis in Reactor 2 is constrained by thermodynamics, which exhibits the same issue as the HB process. Therefore, the ammonia yield in Reactor 2 under ambient pressure is poor. It has been demonstrated Reactor 2 can operate at a slightly high operating pressure (e.g., 30 bar), allowing to significantly enhance ammonia production by applying a current.[48] The central benefit of implementing Reactor 2 is to reduce the operating pressure of the HB process while achieving a higher ammonia yield.
Reactor 3 intensifies steam methane reforming with ammonia synthesis. Renewable power is utilized to pump the hydrogen from the positive electrode to the negative electrode, which shifts the methane reforming equilibrium toward a higher methane conversion.[18] Under a specific current density, methane could achieve complete conversion at an intermediate temperature (e.g., 400–550 °C). The outlet gas stream will be CO2-rich gas that can be significantly captured. This process intensification gives rise to multiple benefits. First, it eliminates the external reformer and simplifies the system. Additionally, this integrated reactor couples the endothermic reforming reactions with exothermic processes (e.g., nitrogen reduction, heat generated via Ohmic loss), creating a thermoneutral regime and dramatically enhancing the overall energy efficiencies. Moreover, this reactor separates the CO2 gas stream from the ammonia gas stream. The CO2 capture will reduce the emissions during ammonia production. Additionally, as CH4 is fed to the positive electrode along with CO2 produced, a highly active and CO2-tolerant electrode material is required to achieve both high performance and durable operation. Therefore, rationally designed positive electrode for steam methane reforming is also crucial to achieving high cell performance and long-term durability
Reactor 4 represents the ultimate alternative that could transform the energy-intensive and high-carbon emission HB process toward sustainable ammonia production. Ammonia can be directly produced from H2O and N2, which are cheap and abundant feedstocks, by using PCECs powered by renewable power sources. Oxygen evolution reaction occurs at the positive electrode, which has been well-established for renewable hydrogen production.[4,5,7,30] Protons will then transport across the electrolyte membrane and react with nitrogen at the negative electrode, producing ammonia.
These four PCEC configurations primarily differ in the feedstocks fed to the positive electrode and corresponding reactions. All these reactors, which are powered by renewable electricity, can enhance the ammonia production rate by modulating hydrogen coverage on the catalyst and promoting the dissociative adsorption of nitrogen. Moreover, Reactor 1, Reactor 3, and Reactor 4 potentially bypass the thermodynamic constraints of ammonia synthesis in conventional HB reactors, allowing them to achieve a higher ammonia yield under mild conditions. Both Reactor 3 and Reactor 4 concurrently increase ammonia yield and reduce carbon emissions.
The proton-conducting electrolyte membrane has minor electronic leakage under an oxidizing atmosphere, which might lead to low Faradaic efficiency and reduced ammonia production rate. Prior work on PCECs has identified that electronic leakage is attributed to the oxidation of electrolytes.[5,49] As both positive electrodes and negative electrodes of Reactors 1–3 are under a reducing atmosphere that has a much lower oxygen partial pressure, the electronic leakage can be significantly suppressed. The electronic charge carrier transference numbers of two typical electrolyte materials, BaZr0.8Y0.2O3-δ and BaCe0.7Zr0.1Y0.1Yb0.1O3-δ, under a reducing atmosphere is lower than 0.01%, indicating the electronic leakage can be negligible. Therefore, both BaZr0.8Y0.2O3-δ and BaCe0.7Zr0.1Y0.1Yb0.1O3-δ can be employed as the electrolyte for Reactors 1–3. Considering BaZr0.8Y0.2O3-δ exhibits improved CO2 and H2O tolerance than BaCe0.7Zr0.1Y0.1Yb0.1O3-δ, BaZr0.8Y0.2O3-δ-based PCECs can achieve better stability than that of BaCe0.7Zr0.1Y0.1Yb0.1O3-δ-based PCECs.
In Reactor 4, the positive electrode is exposed to an oxidizing atmosphere. The oxidization of electrolyte membrane can result in p-type electronic conduction. Under the humidified oxidizing atmosphere (e.g., 10% H2O balanced with air), BaZr0.8Y0.2O3-δ exhibits an electronic charge carrier transference number of >30% and BaCe0.7Zr0.1Y0.1Yb0.1O3-δ has an electronic transference number of <15%,[5] suggesting BaCe0.7Zr0.1Y0.1Yb0.1O3-δ is a better electrolyte membrane for Reactor 4. Additionally, reducing oxygen partial pressure, increasing steam partial pressure, and decreasing operating temperature favor electrolyte hydration and inhibit oxidization, which will reduce electronic leakage.
Thermodynamics of Ammonia Synthesis in PCECsFigure 3 shows the thermodynamics and reversible electrochemical potential required to electrochemically reduce nitrogen to ammonia. The Gibbs free energy change (ΔG) represents the minimum electrical energy consumption. The voltage denotes the minimum applied voltage (onset voltage) to drive the ammonia synthesis. Ammonia can be produced if the applied voltage is more negative (i.e., more cathodic) than that shown in Figure 3. The negative voltage indicates the reactions shown in Figure 3 cannot occur spontaneously. Electrical energy is consumed to break the associated chemicals for ammonia synthesis.
At an operating temperature of <180 °C, PCECs with H2 and N2 as the feedstocks possess a positive onset voltage, suggesting PCECs could potentially co-generate electricity and ammonia. However, due to the sluggish kinetics of nitrogen reduction, especially at such a low operating temperature, the co-generation of power and ammonia is not facile. At a typical PCEC operating temperature of 300–500 °C, converting H2 and N2 to ammonia consumes the least electrical energy. Converting CH4, H2O, and N2 to ammonia requires a slightly higher amount of electrical energy, which is attributed to the highly endothermic nature of the steam methane reforming. The total energy consumption of ammonia synthesis via the HB process is ≈500 kJ mol−1,[32] which is much higher than the electrical energy consumption of Reactors 1–3 that is projected based on thermodynamics. As extensive electrical energy is utilized for water electrolysis, the energy consumption of Reactor 4 is much higher than Reactors 1–3, which is similar to the HB process. Therefore, A higher voltage (>1.1 V) is applied to drive the ammonia production in Reactor 4. Despite Reactor 4 consuming much higher electrical energy than the other reactors, ammonia produced in Reactor 4 is carbon-free and sustainable. With the broad deployment of renewable power plants, the renewable electricity cost has been drastically reduced, which stimulates sustainable ammonia production by converting H2O and N2.
Current Progress in Utilizing PCECs for Ammonia Synthesis and Associated ChallengesPCECs with all these four configurations have been experimentally validated for ammonia synthesis. Table 2 presents a summary of notable PCECs for ammonia synthesis. An ammonia production rate of >5.0 × 10−9 mol cm−2 s−1 has been demonstrated in Reactor 1,[31] which is higher than that of Reactor 3 and Reactor 4. At a relatively high operating pressure (e.g., 50 atm), the PCEC with the Reactor 2 configuration achieves an ammonia production rate of ≈ 5 × 10−9 mol cm−2 s−1,[48] which is similar to the performance realized in Reactor 1. However, under ambient operating pressure, Reactor 2 does not achieve a higher ammonia production rate than that achieved in Reactor 1. Additionally, PCECs with the Reactor 4 configuration obtain lower ammonia production rates than other PCECs, which could be ascribed to the electronic leakage of electrolytes. The ammonia synthesis was conducted under a relatively low current density (e.g., <100 mA cm−2). Prior work on PCECs has recognized that the electronic leakage tends to be more severe at a current density of <100 mA cm−2 as the p-type electronic conduction of the electrolyte creates a H2 permeation flux from the negative electrode to the positive electrode, leading to high electronic leakage and low ammonia production rate.[5,7]
Table 2 A summary of notable PCECs with four configurations for ammonia synthesis
| N2 reduction electrode (Negative electrode) | Counter electrode (Positive Electrode) | Electrolyte | Atmosphere | NH3 production rate [mol cm−2 s−1] x 109 | NH3 Faradaic Efficiency [%] | Ref. | |
| Reactor1 | Ba0.5Sr0.5Co0.8Fe0.2O3−δ | NiOBaCe0.85Y0.15O3-δ | BaCe0.85Y0.15O3-δ | N2|H2 | 4.10 | N/A | [50] |
| Cu film | NiO-CeO- BaZr0.7Ce0.2Y0.1O2.9 | BaZr0.7Ce0.2Y0.1O2.9 | N2|H2 | 1.71 | <2.7 | [51] | |
| PtFe oxide- BaCe0.5Zr0.3Y0.16Zn0.04O3-δ | NiO- BaCe0.5Zr0.3Y0.16Zn0.04O3-δ | BaCe0.5Zr0.3Y0.16Zn0.04O3-δ | N2|H2 | 3.95 | N/A | [52] | |
| Pd | Pd | SrCe0.95Yb0.05O3 | N2|H2 | 4.50 | N/A | [53] | |
| AgPd | AgPd | Ba3Ca0.9Nd0.28Nb1.82O9-δ | N2|H2 | 2.16 | N/A | [54] | |
| AgPd | AgPd | BaCe0.8Gd0.1Sm0.1O3-δ | N2|H2 | 5.82 | N/A | [31] | |
| Reactor 2 | The BASF S6-10RED | Ag | CaIn0.1Zr0.9O3-δ | N2|H2 | 5.0 | >100% (Non-Faradaic effect) | [48] |
| Rh film | NiO- BaCe0.2Zr0.7Y0.1O3-δ | BaCe0.2Zr0.7Y0.1O3-δ | Wet N2|H2 | 2.89 | N/A | [55] | |
| Reactor 3 | VN-Fe | Ni- BaCe0.2Zr0.7Y0.1O3-δ | BaZr0.8Ce0.1Y0.1O3-δ | N2|CH4, H2O | 1.88 | 5.32 | [14] |
| Reactor 4 | Ru-Ag/MgO | Pd | SrCe0.95Yb0.05O3−δ | N2, He|H2O, He | 0.0003 | N/A | [56] |
| La0.6Sr0.4Co0.2Fe0.8O3-δ | La0.6Sr0.4Co0.2Fe0.8O3- δ | BaZr0.8Y0.2O3-δ | N2, He|H2O, Ar | 0.085 | 0.33 | [57] |
To better understand the current status and evaluate the economic viability of synthesizing ammonia in PCECs, determine the potential challenges, and identify the approaches to addressing them, the guiding performance metric for comprehensively evaluating PCECs should be established. This metric should synergize multiple performance indicators, including the current density, voltage, Faradaic selectivity, and ammonia production rate. Prior efforts have focused on achieving a high production rate or enhancing Faradaic selectivity toward ammonia, which cannot completely represent the practical value of producing ammonia in PCECs. Here, we perform an analysis of the electrical energy consumed for producing one mole of ammonia that can be the guiding performance metric, which provides some insights that may inform future PCEC development. This metric has been used for evaluating the electrical energy consumption of other electrochemical ammonia synthesis technologies.[58] Therefore, Figure 4 is presented to report the electrical energy consumption of synthesizing ammonia in PCECs, which comprehensively summarizes the performances of PCECs with the consideration of current density, selectivity toward ammonia, applied voltage, and ammonia production rate. The electrical energy consumption of ammonia synthesis is determined by using the following equation. [Image Omitted. See PDF]
Figure 4 presents our survey of the published work that simultaneously reports the applied voltage, current density, and ammonia production rate.[14,48,50–52,55,56,59] Some published papers did not report either the applied voltage or the current density, which cannot be used to thoroughly evaluate its performances,[16,31,54,60–66] especially the electrical energy consumption. At a current density of <10 mA cm−2, PCECs with a configuration of Reactor 3 and Reactor 2 have led to electrical energy consumption that is lower than the conventional HB process.[14,55,59] Once the current density is increased to 20 mA cm−2, its electrical energy consumption is ten times as high as that at a current density of 10 mA cm−2, which is due to the decreased ammonia Faradaic selectivity. Increasing the current density can increase the overpotential attributed to the negative electrode. The more cathodic potential at the negative electrode can facilitate the HER and consequently reduce the Faradaic selectivity toward ammonia. PCECs with a configuration of Reactor 1 are intensively studied for ammonia synthesis as this PCEC architecture simplifies the reactor design and operation.[52] Reactor 1 also delivers the highest current density. However, its electrical energy consumption is more than ten times higher than that of the HB process, mainly ascribed to the low Faradaic selectivity toward ammonia at a relatively high current density. It is moreover clear that the PCECs with a configuration of Reactor 4 consume a much higher amount of electrical energy than the other configuration as the applied voltage for water electrolysis alone is dramatically high, which is consistent with the thermodynamics shown in Figure 3. Additionally, the current density of Reactor 4 is much lower than other reactors. Despite Reactor 4 requiring much higher energy consumption, Reactor 4 does not need any fossil fuel as the feedstock and eliminates CO2 emissions. The electrical energy consumption in Reactor 4 could be dramatically reduced once the HER is suppressed and ammonia selectivity is improved. As shown in Figure 4, it is expected that PCECs with a next-generation Reactor 4 configuration and optimized electrode materials can produce ammonia at a higher current density and selectivity, leading to comparable or lower energy consumption than the HB process. The strategies to achieve Next-generation Reactor 4 will be discussed in great detail in later sections of this review.
Ammonia Synthesis MechanismsFigure 5 presents two primary nitrogen reduction and ammonia production pathways that occur at PCEC negative electrodes. The relatively high operating temperature (300–600 °C) of PCECs drives the NN bond cleavage; thus, the dissociative nitrogen reduction mechanism is relatively favorable. Furthermore, the NRR electrode is usually polycrystalline metals or metallic nanoparticles (e.g., Ru) with stepped surfaces, which favors nitrogen reduction via the dissociative mechanism.[58] These two pathways presented in Figure 5 are based on the dissociative mechanism. However, these two pathways can lead to distinct ammonia selectivity. Pathway 1 (Figure 5A) shows the electrochemical ammonia synthesis via the dissociative mechanism. The proton could directly take electrons and reduce the dissociative adsorbed nitrogen on the catalyst to ammonia. The direct electrochemical ammonia synthesis does not require the stoichiometric amount of gaseous hydrogen production. Thus, this pathway can result in a much higher Faradaic selectivity. To favor Pathway 1, the NRR electrode should inhibit HER while exhibiting good electrocatalytic activity to reduce nitrogen. Pathway 2 displays the conventional HB reaction where nitrogen is reduced by the gaseous hydrogen produced via HER. Thermochemical catalytic ammonia synthesis dominates overall ammonia production. Therefore, the H2/N2 ratio is essential for the ammonia production rate as the overall yield will obey the thermodynamics of thermochemical ammonia synthesis. The PCECs will have to operate at a high current density to achieve a H2/N2 ratio that approaches 3/1 and thereby increasing ammonia yield. However, in these scenarios, the ammonia Faradaic selectivity is very poor as a significant amount of hydrogen cannot be utilized.
PCECs have been demonstrated for ammonia synthesis over the last decades. The ammonia production rates typically range from 10−10 to 10−8 mol cm−2 s−1, which are higher than that obtained in low-temperature (<100 °C) electrochemical devices.[62] However, the Faradaic selectivity toward ammonia is low, which is ascribed to the parasitic HER. There is a lack of rationally designed negative electrodes for ammonia synthesis. As summarized in Table 2, perovskite-based oxides or proton-conducting oxide-metal composite or metal, for instance, Ba0.5Sr0.5Co0.8Fe0.2O3−δ, Pd, AgPd, and Rh, are directly applied as the negative electrodes.[43,48,50,55,62,66] These materials also favor HER, inevitably leading to significant hydrogen production and poor selectivity to ammonia. Furthermore, the current PCEC operating temperatures (e.g., 500–600 °C) are slightly high, which favors the dissociative adsorption of nitrogen. However, the decomposition of ammonia is also favorable at 500–600 °C, further reducing the selectivity toward ammonia. Therefore, both the negative electrode and operating temperatures can influent the HER and ammonia selectivity.
Nitrogen Reduction Reaction PathwayIt has been noted that increasing the current density increases the ammonia production rate because increasing the current density can enhance the H2/N2 ratio and thereby could facilitate ammonia production via the HB process. On the other hand, increasing current density also affects the hydrogen and nitrogen coverage on the negative electrode, enhancing the electrochemical ammonia production rate. However, there is no well-established tool to differentiate the ammonia produced via electrochemical nitrogen reduction from the HB process (i.e., two pathways shown in Figure 5). Therefore, it is challenging to directly prove electrochemical ammonia synthesis in PCECs.
Discrepant Faradaic SelectivityAdditionally, discrepancies in the Faradaic selectivity toward ammonia still exist. An ammonia selectivity of >50% has been demonstrated in certain PCECs. In comparison, some studies present an ammonia selectivity of <1%,[14,48,50–52,55,56,59] suggesting it is necessary to develop more rigorous procedures to test the PCECs and measure the ammonia production rate, which will subsequently provide more reliable PCEC performances.
Strategies to Enhance Ammonia Selectivity and Yield, as well as Energy EfficiencyThe intermediate operating temperatures of PCECs offer the thermodynamic and kinetic sweet spot for high-efficiency ammonia synthesis. However, the catalysts that have been developed for the HB process cannot be directly applied to PCECs, which is due to the catalyst is not electronically conductive. The HB catalysts typically fall into two categories, including fused-iron and supported metallic catalysts. Fused-iron catalysts are derived from iron oxides, which are not highly conductive. The supported metallic catalysts are generally activated carbon or metal oxide-supported ruthenium or cobalt.[68] The activated carbon cannot be used for PCECs as the carbon can be easily oxidized during fabrication. Additionally, as the weight percentage of the metallic phase is normally <10% while the metal oxides are not electronically conductive, the metal-oxide-supported metallic catalyst cannot be applied on PCECs as the negative electrode. However, as PCECs can operate at temperatures similar to the HB process, the catalyst design and development strategies established for the HB process might be the starting points to develop negative electrode materials for PCECs, especially the most recent HB catalysts that are active under mild conditions.[69,70]
The proton conductivity of the electrolyte is essential for providing sufficient protons/hydrogen to the negative electrode for reducing nitrogen. There has been extensive work on enhancing its intrinsic conductivity and fabricating thin electrolytes to minimize Ohmic losses.[28,63] A current density of >500 mA cm−2 can be easily achieved in protonic ceramic electrolysis cells with a low Ohmic loss.[4,7,17,21,22,30] Therefore, future work should focus on deliberately designing and synthesizing negative electrodes to improve the catalytic activity of negative electrodes and suppress the HER. The negative electrodes for PCECs should be electronically conductive to expedite charge transfer. Moreover, it should exhibit excellent catalytic activity to activate nitrogen and produce ammonia. It should also simultaneously hinder HER. Prior works have explored various metallic catalysts to achieve a higher ammonia yield while understating the impacts of catalyst support or the second phase of a composite electrode is limited.[61,64] Tailoring both the metallic phase and the support could diminish the active sites for HER while enhancing the active sites for nitrogen reduction.
Herein, as shown in Figure 6, this review intends to provide a set of strategies from different perspectives to enhance ammonia selectivity and yield. These strategies include deliberately designing NRR electrocatalysts and negative electrodes of PCECs, optimizing PCEC architectures, and modulating the PCEC operation conditions.
Applying metal nitride as the NRR catalyst is expected to enhance ammonia selectivity and yield. The proton directly reacts with the lattice nitrogen to produce ammonia.[65–69] Gaseous nitrogen will then adsorb on the metal nitride to replenish the lattice nitrogen. Experimental evidence, for instance, isotope-based experiments, should be performed to prove that the gaseous nitrogen can replenish the nitrogen-vacancy in transitional metal nitride.[70] Furthermore, due to the low electronic conductivity of metal nitride, employing metal nitride might increase overpotentials and electrical energy consumption. To tackle this issue, metal nitride could be dispersed on a mixed electronic and protonic support to create electrochemically active sites. For example, transition metal-doped BaCexZr0.8-x-yTMyY0.2O3 (BCZTMY, TM = Mn, Ni, Fe) or SrCe0.7Zr0.2Eu0.1O3-δ, which simultaneously displays electronic and protonic conduction, can be used as the electrode support.[57,71] The support should also be carefully engineered to inhibit HER.
Employing NRR catalyst with appropriately strong nitrogen binding with the catalysts to ensure high selectivity to ammonia and inhibit HER. According to the Brønsted–Evans–Polanyi relation, the dissociative adsorption of nitrogen requires strong binding energy on the surface of negative electrodes. However, enhancing the nitrogen binding cannot continually increase ammonia selectivity. The strong binding energy can lead to high desorption energy due to the scaling relations.[72,73] Furthermore, strengthening the nitrogen binding energy could lead to a higher overpotential and consequent low energy efficiency. It is therefore important to rigorously design the electrode with an optimal nitrogen binding to maximize ammonia production and energy efficiency. It is suggested to perform molecular modeling to determine the nitrogen binding energy, assisting in designing new catalysts.
It has been recognized that the Ru sites with oxygen vacancies can serve as the active sites that stabilize *NNH, destabilize *H, and enhance N2 adsorption, which suppresses HER and favors NRR.[74] Therefore, oxygen vacancies on the surface of transition metal oxides could accelerate the electron charge transfer while it might not be active for HER; thus, increasing the oxygen vacancies of the oxide support can alter the NRR electrocatalytic activities.[74] The oxides that can be applied as the support possess a high surface oxygen vacancy concentration and can also inhibit HER, implying they cannot transport protons as the proton conductor can facilitate HER.[40] For example, the proton-conducting oxides might not function well as the catalysts support as its high proton conductivity could lead to favorable HER. Therefore, ZrO2, CeO2, and acceptor-doped ZrO2/CeO2 are expected to be effective catalyst support that can create active sites for nitrogen reduction to ammonia. Moreover, doping ZrO2 or CeO2 with an acceptor (e.g., Sm, Gd) can increase its oxygen vacancy concentration, which could improve its NRR activity.
PCEC Architecture Design Positive Electrode-Supported PCECsA positive electrode-supported PCEC is expected to be an optimal architecture as this design offers substantial opportunities for using broad NRR electrocatalysts as the negative electrodes. The current PCECs mainly use Ni-based cermet negative electrodes as the support. The researcher would try to use the Ni-based cermet electrode as the NRR electrode, which favors HER and consequently leads to poor Faradaic selectivity toward ammonia. Alternatively, positive electrode-supported PCECs can address the abovementioned challenges. A positive electrode scaffold with a thin electrolyte can be fabricated by conventional tape casting, dry pressing, or other coating technologies followed by sintering at a high temperature. The deliberately designed NRR electrocatalyst is directly coated on the electrolyte layer. This architecture allows the evaluation of various NRR electrocatalysts.
Electrolytes-Supported PCECsAlternatively, electrolyte-supported PCECs offer the same advantage as that of positive electrode-supported PCECs. As the electrolyte-support PCECs normally use an electrolyte membrane with a thickness of 300–500 µm, which gives rise to 10 times higher ohmic resistance. Thanks to the recent advancements in highly-conductive proton-conducting oxides.[9,21,29] an electrolyte with a thickness of 300–500 µm can achieve a current density of >500 mA cm−2. Thus, electrolyte-supported architecture will not impact the ammonia production rate. However, electrical energy consumption will be slightly higher than that of positive electrode-supported PCECs. The excess electrical energy consumption ascribed to ohmic loss is finally converted to heat, which can be utilized for water electrolysis and maintain the intermediate operating temperature. Therefore, the overall energy efficiency might not be drastically reduced. The electrolyte-supported PCECs also simplify the preparation of positive electrodes. The conventional positive electrode can be easily coated on one side of the electrolyte, eliminating the complicated infiltration process. Therefore, electrolyte-supported PCECs possess higher scalability than electrode-supported PCECs.
Operating Conditions Current DensityAs aforementioned, PCECs can facilely achieve a current density of >500 mA cm−2 for H2 production via steam electrolysis.[4,5,7] However, increasing the current density cannot continuously increase the ammonia production rate.[17] Therefore, it is essential to operate PCECs under an optimal current density to obtain high ammonia yield and energy efficiency. As shown in Figure 7, at a relatively low current density, the negative electrode is covered with sufficient chemisorbed nitrogen that will be readily reduced by hydrogen/proton. Therefore, in Zone 1, increasing the current density can significantly enhance the ammonia production rate while a reasonable Faradaic selectivity toward ammonia is achieved. With further increasing the applied current density to Zone 2, the surface hydrogen concentration tends to increase and occupies the sites on the metal, enhancing the possibility of hydrogen production. The ammonia production rate can slightly increase due to the increased surface hydrogen concentration. However, as HER tends to be more facile, the Faradaic selectivity toward ammonia decreases. The ammonia production rate will peak at an intermediate current density, under which condition a maximum ammonia production rate could be obtained while the Faradaic selectivity and energy efficiency are not maximized. Therefore, the operating conditions that give rise to the highest ammonia yield might not be optimal for synthesizing ammonia. Zone 3 in Figure 7 displays the ammonia production rate and Faradaic selectivity at a higher current density. The catalyst surface is predominantly occupied by hydrogen, leading to severe hydrogen evolution, reduced ammonia production rate, and poor Faradaic selectivity toward ammonia.
HER is parasitic and might be inevitable. Moreover, PCECs utilize intermittent renewable power to synthesize chemicals, suggesting the PCEC cannot always operate under the optimal condition to achieve the highest NH3 selectivity, which requires the PCECs have operational flexibility. Therefore, recycling the unreacted H2 and N2 to the negative electrode can be a feasible approach to enhancing NH3 yield and improving the utilization of protons for NH3 production. As shown in Figure 2, N2 is fed to the negative electrode; thus, the negative electrode outlet gas is a mixture of H2, N2, and NH3. NH3 can be captured via condensation and the unreacted H2 and N2 can be recycled back to PCECs. The H2 and N2 gas recycling can reduce the electric energy consumption of PCECs as a certain amount of H2 is recycled and PCECs can operate at a lower current density. Additionally, recycling unreacted H2 and N2 can adjust the H2-to-N2 ratio at the negative electrode, indicating the N2 feeding rate and the current density need to be dynamically adjusted to maximize NH3 production. Despite the H2 gas recycling has not been widely accepted, recycling H2 has been systematically analyzed for the HB reactor,[84,85] which concludes that the reactor can operate under a wide range of conditions by maintaining operational and ammonia production flexibilities.
CO2 Reduction Reaction in PCECs for Chemicals Production CO2-PCECs for Carbon-Containing Chemical ProductionConverting CO2 to value-added chemicals via CO2 reduction reaction (CO2RR) simultaneously overcomes three critical energy and environmental challenges, including CO2 mitigation, sustainable chemical synthesis, and long-term chemical energy storage. The wide deployment of CO2 reduction technologies reduces CO2 emissions while decreasing atmospheric CO2 concentration by producing sustainable chemical building blocks and curbing reliance on fossil fuels. Among various CO2RR technologies, electrochemical CO2 reduction reaction (ECO2RR) utilizes renewable electricity to power the electrochemical reactions, further reducing the carbon emissions ascribed to the power generation assets. Moreover, ECO2RR enables the interconversion between electrical energy and chemical energy, which can be employed for long-term chemical energy storage.
PCECs have been demonstrated for the co-conversion of CO2 and H2O into chemicals.[20,22,31,75–80] Prior attempts to integrate CO2 reduction and H2O electrolysis have been plagued by the disparate temperatures between electrolyzers and CO2 reduction chemistries. Conventional solid oxide electrolyzers based on oxygen-ion-conducting membranes deliver exceptional CO2 conversion and efficiency but require operating temperatures above 700 °C at which the reaction of CO2 reduction to hydrocarbons (e.g., CH4) becomes thermodynamically unfavorable. Therefore, the end product will be limited to syngas (the mixture of H2 and CO).[81–84] Similarly, aqueous proton-exchange membrane (PEM) CO2 electrolyzers and anion exchange membrane (AEM) CO2 electrolyzers can directly convert CO2 into high-value chemicals at temperatures below 100 °C, but their CO2 reduction is kinetically limited.[85–87] PCECs overcome these issues by providing high proton conductivity with doped ceramic membranes at versatile operating temperatures and conditions compatible with heterogeneous gas-phase CO2 hydrogenation to hydrocarbons and CO2 conversion to CO. The high-flux proton generated at an intermediate operating temperature (300–600 °C) from water electrolysis, the versatility to selectively synthesize CH4 or CO, cost-effective and platinum group metal (PGM)-free catalysts, and high energy conversion efficiency—all demonstrate that the PCEC is the “holy grail” for efficient CO2 conversion and value-added chemicals production.[5,26] The major advantage is that the highly active protons transported across the electrolyte membrane, which operates at an intermediate temperature (300–500 °C), encounter a favorable kinetic and thermodynamic regime for CO2 reduction.
In the last decade, the advancements in proton-conducting oxides have sparked the CO2 conversion and utilization in PCECs for synthesizing carbon-containing chemicals. Various PCECs have been developed for reducing CO2 via hydrogen sourced from different feedstocks, including sustainable hydrogen, alkenes, and water.[20,22,31,75–79] As shown in Figure 8, CO2 is fed to the negative electrode (CO2 reduction electrode) while the feedstocks for providing protons are delivered to the positive electrode. Like the PCECs designed for synthesizing ammonia, the reactions at the negative electrode will not vary with the reactants fed to the positive electrode, which simplifies the system design, enhances the feedstock flexibility, and relaxes the requirements for CO2 electroreduction catalysts. Furthermore, as shown in Figure 8, it is expected PCECs equipped with rationally designed CO2 electroreduction catalysts could be employed to produce carbon-containing chemicals, including carbon monoxide, methane, and other hydrocarbons (C2 and C2+). However, converting CO2 to other hydrocarbons in PCECs has not been experimentally validated. Herein, we therefore center on discussing CO2 reduction to CO and CH4.
As shown in Figure 8, the PCECs for CO2 conversion can be categorized into three configurations according to the reactants delivered to the positive electrode, which include CO2-PCEC 1, CO2-PCEC 2, and CO2-PCEC 3. CO2-PCEC 1 aims to electrochemically reduce CO2 using hydrogen pumped from the positive electrode. The proton/hydrogen activates CO2 adsorbed on the negative electrode. The applied current density controls the surface hydrogen coverage and consequently affects the CO2 conversion and product yield. Additionally, the operating temperatures of PCECs range from 300 to 600 °C, which impacts the selectivity toward a particular product. The CO2 reduction to CO is endothermic; thus, CO is more prone to be produced at a relatively high operating temperature. CO2 reduction to methane is exothermic, which is therefore more thermodynamically favorable at low operating temperatures. However, the chemicals produced in PCECs are not solely determined by the operating temperatures. By modulating the kinetics of CO2RR over the negative electrode, CO can be favorable at relatively low operating temperatures while CH4 can also be selectively produced at slightly high operating temperatures.
CO2-PCEC 2, as illustrated in Figure 8, displays the PCEC configuration for CO2 conversion with the highest sustainability, which enables the co-conversion of CO2 and H2O into value-added chemicals. Oxygen evolution reaction occurs at the positive electrode, while CO2 reduction occurs at the positive electrode. The chemicals produced via CO2RR will be ultimately oxidized to CO2 and H2O. Thus, synthesizing chemicals via co-conversion of CO2 and H2O can achieve a net-zero closed carbon cycle.
CO2-PCEC 3 integrates the dehydrogenation of alkanes with CO2 reduction. Dehydrogenation of alkanes upgrades cheap and abundant hydrocarbons to value-added light olefins. However, the conversion of alkanes is thermodynamically limited. CO2-PCEC 3 pumps hydrogen from the positive electrode, shifting the dehydrogenation equilibrium toward higher conversion and enhancing the yield of light olefins. Furthermore, hydrogen/proton transports to the negative electrode where CO2 is reduced to value-added chemicals. CO2 reduction to hydrocarbons is exothermic, which generates heat that could be directly used for the dehydrogenation of alkanes. Therefore, CO2-PCEC 3 intensifies multiple reactions, which has multiple benefits, including improved product yield at both electrodes, enhanced energy efficiency, and raised CO2 conversion.
The thermodynamics of CO2 reduction to CH4 and CO2 reduction to CO as a function of operating temperatures are summarized in Figure 9. However, the CO2 reduction and product yield under realistic conditions are also governed by factors that affect the CO2 reduction kinetics. For example, although the CO2 reduction to CH4 is thermodynamically favorable at low operating temperatures, the CO2 reduction electrode can kinetically favor the CO2-to-CO conversion, and CO2 can be quickly reduced to CO with high selectivity toward CO.[28] However, at a high operating temperature, it is challenging to kinetically enhance the CH4 production selectivity as a high operating temperature simultaneously favors the steam reforming of CH4, which subsequently converts CH4 back to CO. Therefore, the intermediate operating temperature of PCECs offers the feasibility and versatility of producing either CO or CH4. Additionally, the applied current density and potential also affect the CO2 reduction kinetics. CO2 reduction to CH4 requires more protons than CO production. Therefore, the proton supplied for CO2 reduction, which is proportional to the current density, can impact the Faradaic selectivity toward CH4 or CO.[5]
Synthesizing chemicals via CO2RR in CO2-PCEC 1 and CO2-PCEC 2 has been experimentally validated. Figures 10 and 11 display the most recent demonstrations of PCECs for producing CO and CH4. Table 3 summarizes additional details in reducing CO2 in PCECs. Duan and O'Hayre et al. conducted a preliminary study on reducing CO2 in CO2-PCEC 2.[5] A composite of BaCe0.7Zr0.1Y0.1Yb0.1O3 (BCZYYb7111) and Ni was employed as the negative electrode, which enables the conversion of CO2 to CO and CH4. Figure 10A–C shows BCZYYb7111-Ni electrode favors HER, although a CO2 conversion of >20% has been achieved. The production rates of CO and CH4 increase with increasing the operating temperature (Figure 10A,B). Additionally, a high current density enhances the H2 utilization/permeation and leads to a higher H2/CO2 ratio, which is beneficial for increasing the CH4 selectivity (Figure 10C). Furthermore, a mixture of CH4 and CO is produced, indicating BCZYYb7111-Ni cannot selectively reduce CO2 to either CH4 or CO. As shown in Figure 10D, synthesizing CH4 in large-area CO2-PCEC 2 via co-conversion of CO2 and H2O has been also validated using BaCe0.4Zr0.4Y0.1Yb0.1O3 (BCZYYb4411)+Ni as the negative electrode.[98] However, the selectivity toward CH4 is <80% and a significant portion of the applied current contributes to H2 production. Therefore, an optimized negative electrode is needed to inhibit HER and selectively produce one single product. Techno-economic analysis suggests synthesizing CH4 in CO2-PCEC 2 with BCZYYb4411+Ni negative electrode can lead to a levelized cost of fuel production (LCOFP) of $104 MWh−1 if the total Faradaic efficiency is increased to >90%, the electricity cost is reduced to $0.02 kWh−1, and the H2 is recycled for CO2 conversion.[98] It is expected the CH4 production cost can be further reduced if the CO2-to-CH4 selectivity is enhanced to >95%.
Table 3 A summary of notable PCECs for CO2RR
| Configuration: Positive electrode | Electrolyte | Negative (CO2) electrode | T [°C] | Current density [mA cm−2] | Voltage [V] | Positive electrode atmosphere | Negative electrode atmosphere | CO2 conversion [%] | CO selectivity [%] | CH4 selectivity | Total FE [%] | CO Faradaic yield [%] | H2 Faradaic yield [%] | CH4 Faradaic yield [%] | Stability | Ref. |
| Ni- BaCe0.5Zr0.3Y0.16Zn0.04O3-δ |BaCe0.5Zr0.3Y0.16Zn0.048O3-δ |Fe- BaCe0.5Zr0.3Y0.16Zn0.04O3-δ | 614 | 1500 | 2.60 | 3% H2O+H2 | CO2 | 65.0 | 92.5 | 7.5 | 35.9 | 29.5 | 4.0 | 2.4 | 1 h, 1500 mA cm−2 at 614 °C | [86] |
| SrEu2Fe1.8Co0.2O7-δ+BaZr0.2Y0.8O3-δ| BaZr0.2Y0.8O3-δ -|Ni- BaZr0.2Y0.8O3-δ - | 550 | 0 | 0.97 | 10% H2O+Air | 6% CO2+24% H2+70% Ar | 76.8 | 89.3 | 9.9 | N/A | 95 h, 800 mA cm−2 at 550 °C | [87] | |||
| 100 | 1.13 | 75.8 | 87.6 | 9.9 | 22.5 | 8.4 | 9.7 | 4.5 | ||||||
| 200 | 1.20 | 74.9 | 88.0 | 10.3 | 32.6 | 8.7 | 19.3 | 4.6 | ||||||
| 400 | 1.29 | 72.3 | 88.8 | 10.3 | 42.8 | 9.6 | 28.1 | 5.1 | ||||||
| (La0.75Sr0.25)0.95Mn0.5Cr0.5O3-δ |BaCe0.5Zr0.3Y0.16Zn0.04O3-δ |Ni- BaCe0.5Zr0.3Y0.16Zn0.04O3-δ | 600 | 100 | 2.00 | 5%H2O+Ar | 100%CO2 | N/A | 90 | 10.75 | 90 | 90 | 10 | 0.0 | 30 min, 2 V at 600 °C | [89] |
| La0.8Sr0.2MnO3-δ – BaCe0.2Zr0.7Y0.1O3-δ | BaCe0.2Zr0.7Y0.1O3-δ |Pt | 700 | 0 | 0.85 | 7.5%H2O+Ar | 9%CO2+Ar | 15 | 100 | 0 | N/A | N/A | N/A | 0.0 | 8 h at a current density ranging from 0 to 10.4 mA cm−2 at 600 °C | [90] |
| 5 | 1.25 | 17.8 | 100 | 0 | 28 | 5.0 | 23.0 | 0.0 | ||||||
| 10 | 1.62 | 19.3 | 100 | 0 | 39 | 7.5 | 31.5 | 0 | ||||||
| BaCo0.4Fe0.4Zr0.1Y0.1O3-δ | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ |Ni- BaCe0.7Zr0.1Y0.1Yb0.1O3-δ | 600 | 1231 | N/A | 5.5%H2O +Air | 5.8% CO2+ Ar | 23.7 | 95.5 | 4.5 | 97.0 | 22.0 | 74.0 | 1.0 | N/A | [5] |
| 550 | 1231 | N/A | 15.0 | 75.9 | 24.1 | 97.0 | 11.1 | 82.4 | 3.5 | |||||
| 500 | 1231 | N/A | 9.7 | 44.8 | 55.2 | 97.0 | 4.2 | 87.6 | 5.2 | |||||
| Ni- BaCe0.7Zr0.1Y0.1Yb0.1O3-δ | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ | PrBaMn2O5+δ+ BaZr0.7Y0.3O3−δ | 400 | 32 | 0.25 | 100%H2 | 5%CO2+Ar | 31.4 | 76.3 | 23.7 | 97.0 | 64.8 | 8.6 | 64.8 | N/A | [28] |
| 102 | 0.50 | 31.6 | 67.3 | 32.7 | 97.9 | 56.3 | 8.9 | 56.3 | ||||||
| 166 | 0.75 | 32.7 | 60.0 | 40.0 | 99.1 | 50.7 | 8.4 | 50.7 | ||||||
| 225 | 1.00 | 31.5 | 53.8 | 46.2 | 98.4 | 40.6 | 11.7 | 40.6 | ||||||
| Ni-BaCe0.7Zr0.1Y0.1Yb0.1O3-δ | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ | PrBaMn2O5+δ+ BaZr0.7Y0.3O3−δ|Ir-Ir | 400 | 32 | 0.25 | 30.4 | 9.1 | 90.9 | 99.1 | 8.2 | 0.1 | 90.9 | 100 h, 0.5 V at 400 °C | |||
| 102 | 0.50 | 30.5 | 3.7 | 96.3 | 98.8 | 2.5 | 0.1 | 96.3 | ||||||
| 166 | 0.75 | 28.5 | 3.7 | 96.3 | 98.5 | 2.2 | 0.0 | 96.3 | ||||||
| 225 | 1.00 | 23.7 | 1.0 | 99.0 | 99.0 | 0.0 | 0.0 | 99.0 | ||||||
| Ni-BaCe0.7Zr0.1Y0.1Yb0.1O3-δ | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ | PrBaMn2O5+δ+ BaZr0.7Y0.3O3−δ|IrO | 400 | 64 | 0.25 | 21.6 | 100.0 | 0.0 | 97.3 | 97.1 | 0.1 | 0.0 | 100 h, 0.5 V at 400 °C | |||
| 156 | 0.50 | 20.7 | 100.0 | 0.0 | 98.3 | 98.2 | 0.1 | 0.0 | ||||||
| 281 | 0.75 | 20.9 | 93.4 | 6.6 | 99.0 | 92.3 | 0.1 | 6.6 | ||||||
| 405 | 1.00 | 18.9 | 86.9 | 13.2 | 98.2 | 85.0 | 0.0 | 13.2 | ||||||
| Ni BaCe0.7Zr0.1Y0.1Yb0.1O3-δ BaCe0.4Zr0.4Y0.1Yb0.1O3-δ | Sr2Fe1.4Mo0.5O6-δ-Ni0.175 | 600 | 50 | 0.07 | 20%H2+N2 | 100%CO2 | N/A | 99.7 | 0.3 | N/A | 50 h, 50 mA cm−2 at 500 °C | [99] | |||
| 100 | 0.07 | 99.8 | 0.2 | |||||||||||
| 150 | 0.08 | 99.7 | 0.3 | |||||||||||
| 200 | 0.09 | 99.7 | 0.3 | |||||||||||
| 300 | 0.11 | 99.7 | 0.3 | |||||||||||
| 400 | 0.14 | 99.6 | 0.4 | |||||||||||
| 550 | 50 | 0.085 | 99.0 | 1.0 | ||||||||||
| 100 | 0.090 | 99.4 | 0.6 | |||||||||||
| 150 | 0.010 | 99.3 | 0.7 | |||||||||||
| 200 | 0.104 | 99.3 | 0.7 | |||||||||||
| 300 | 0.130 | 99.3 | 0.7 | |||||||||||
| 400 | 0.147 | 99.3 | 0.7 | |||||||||||
| 500 | 400 | 0.159 | 98.4 | 1.6 | ||||||||||
| 450 | 300 | 0.152 | 97.2 | 2.8 | ||||||||||
| 400 | 50 | 0.108 | 97.0 | 3.0 | ||||||||||
| 100 | 0.122 | 97.3 | 2.7 | |||||||||||
| 150 | 0.128 | 95.1 | 4.9 | |||||||||||
| 200 | 0.140 | 95.5 | 4.5 | |||||||||||
| 300 | 0.161 | 96.7 | 3.3 | |||||||||||
To overcome the challenges associated with product selectivity, Ding et al. have demonstrated novel negative electrodes to modulate the selectivity toward CO or CH4.[24,31] They observed that the interactions between the oxide support and metal clusters can tune the electronic structure of the clusters and consequently affect product selectivity. Both computational modeling and experimental testing reveal that a single Ir atom or a relatively small Ir cluster (IrO) on an Sm-doped CeO2 (SDC) surface displays strong interaction and ionic features, which favors CO production. On the other hand, a relatively large Ir cluster (Ir-Ir) on SDC tends to show metallic features that favor CH4 generation. Figures 11A and 10C show the CO2-PCEC 1 with the Ir-based negative electrode. Both suppressed HER and high selectivity toward either CO or CH4 are achieved. For example, as for the PBM-BZY-Ir-Ir catalyst, the CO2-to-CH4 Faradaic selectivity reaches up to 96.3% at 400 °C with an Ebias of 0.5 V. On the other hand, under the same operating conditions, PBM-BZY/IrO favors the CO2-to-CO production, with a CO2-to-CO Faradaic selectivity of 98.2% achieved, which suggests modulating the negative electrode can tune the selectivity while suppressing HER.
As CO2RR in PCECs is an emerging technology, as shown in Table 3, prior work has focused on performing proof-of-concept or enhancing the product yield, while fewer efforts were devoted to enhancing the selectivity of the desired product and inhibiting hydrogen evolution reaction.[20,22,31,75–79] Therefore, the selectivity of CO2RR in PCECs is summarized here to identify challenges and offer suggestions to address them.
The selectivity of CO2 reduction in PCECs is shown in Figure 12A,B. The selectivity toward methane tends to increase with reducing the operating temperatures while high operating temperatures favor CO production, which is consistent with the thermodynamics of CO2RR, indicating that CO2 reduction in PCERs is thermodynamically controlled. As CH4 is thermodynamically stable at low temperatures while CO is thermodynamically stable at high temperatures. Therefore, the products synthesized are primarily controlled by reaction thermodynamics rather than kinetics. At relatively low operating temperatures (e.g., <400 °C), CH4 is thermodynamically stable, and thus CH4 production is more favorable. A CO2 reduction electrode that reduces the activation energy of CH4 production can further kinetically facilitate the production of CH4. However, to enable selective CO2-to-CO production, the CO2 reduction electrode should be rigorously designed to significantly reduce the CO2-to-CO reaction activation energy and increase the CO production at a faster speed than that of CH4 production, which will give rise to a high selectivity toward CO. At an operating temperature of around 600 °C, the thermodynamic stability of CO and CH4 are similar. The selectivity cannot be thermodynamically controlled and both CO and CH4 can be easily produced. To enhance the selectivity toward either CO or CH4, the kinetics of corresponding reactions should be controlled to accelerate the speed of producing a target product. However, due to the high operating temperature, CH4 can be converted back to CO via steam reforming. Therefore, the negative electrode should also be designed to suppress the steam methane reforming.
Figure 12C presents the Faradaic selectivity toward hydrogen as a function of current density, implying the HER concurrently occurs with the CO2 reduction. Without using the CO2 reduction electrode that is rationally designed for PCECs, HER is the predominant reaction in PCECs that are equipped with conventional Ni-based CO2 reduction electrodes (Duan 2019 in Figure 12C).[5] Furthermore, the HER tends to be more significant at a high current density as the surface hydrogen or proton cannot be quickly utilized for CO2 reduction. The PCECs with Ir-based CO2 reduction electrode deliver a H2 Faradaic selectivity <10% (Ding 2021 in Figure 12C), which efficiently and selectivity produce either CH4 or CO at a current density <400 mA cm−2.[24,28] These achievements suggest both the operating conditions, including temperature and current density, and the properties of the CO2 reduction electrode control the selectivity toward a specific product.
Mechanisms of CO2 Reduction in PCECsA detailed understanding of the CO2 reduction mechanisms will open the “black box” of CO2RR in PCECs. Due to the lack of appropriate tools to conduct in situ operando experiments and probe the active intermediates involved in CO2 reduction, the CO2 reduction mechanisms in PCECs are not illuminated. Establishing this understanding, especially by leveraging the thermochemical knowledge,[88,89] will therefore assist in rationally designing CO2 reduction catalysts that could be applied to PCECs.
Figure 13 displays the proposed CO2 reduction pathways, including the associative CO2 reduction pathways and dissociative pathways. The associative pathways denote the pathways where the CO bond cleavage occurs after its hydrogenation while the dissociative pathways represent the CO bond will first break and then be hydrogenated. The CO2 reduction pathway could also be divided into two categories depending on the product, which is CO or CH4. Despite other hydrocarbons, such as ethylene and life olefins, could be produced in PCECs that operate at an intermediate temperature at which the CC bond coupling is thermodynamic favorable, this concept has not been experimentally validated. Thus, the CO2-to-light olefins mechanisms are not discussed in this review. As shown in Figure 13, the protons from the positive electrode could first pair with electrons to form hydrogen adsorbed on the negative electrode (*H). The negative electrode is typically a composite of oxide support and a metallic phase. The proton is therefore reduced at the oxide-metal interface and the hydrogen will then adsorb on the surface of the metallic phase. It will subsequently reduce the intermediate species adsorbed on the oxide. The potential CO2 reduction pathways are outlined as follows:
Path 1: CO2 is associatively adsorbed on the oxide surface and forms carbonate that will be then reduced to formate. The formate species will be further reduced to CO.
Path 2: Oxygen vacancies are available on the oxide support. One oxygen atom in a CO2 molecule occupies the oxygen vacancy. CO2 will then be dissociatively adsorbed on the oxide surface. The resultant CO will desorb and produce CO.
Path 3: CO2 molecules associatively adsorb on the negative electrode followed by the formation of bidentate carbonate. The bidentate carbonate will be subsequently hydrogenated to bicarbonate which can be further reduced to CO.
Path 4: This path displays the associative formate path for CH4 production. CO2 molecules associatively adsorb on the negative electrode and form formate species that can be subsequently reduced to CH4.
Path 5: CO2 molecules dissociatively adsorb on the negative electrode and produce *CO that will be consecutively reduced to CH4.
Path 6: CO2 molecules associatively adsorb on the negative electrode and produce formate that will be further reduced to CH4.
Challenges Associated with CO2RR in PCECsAccording to the current achievements of CO2 reduction that are summarized in Figures 10–12, and the PCEC performances have been demonstrated,[20,22,28,86–90] we have identified the challenges of converting CO2 in PCECs. As shown in Figure 14, these challenges include 1) high H2 selectivity; 2) poor Faradaic selectivity to either CO or CH4; 3) poor chemical stability; and 4) the lack of established CO2 reduction mechanisms. These challenges are detailed as follows:
As PCECs for CO2 conversion is a truly nascent technology, most previous studies were performed directly using the negative electrode of protonic ceramic fuel cell (PCFC), which is a composite of proton-conducting oxide and Ni,[20,5] as the CO2 reduction electrode. Ni-based cermet also favors HER. Therefore, the Faradaic selectivity toward carbon-containing molecules is poor. Additionally, oxide-supported Ni is not prone to selectively produce either CO or CH4. Thus, the product is commonly a mixture of H2, CO, and CH4. Furthermore, the negative electrode that favors HER leads to poor CO2 conversion.
Oxygen-ion solid oxide electrochemical cells (O-SOECs) have been widely used for efficiently producing syngas via co-electrolysis of CO2 and H2O or producing pure CO via CO2 electrolysis.[102] However, the products are limited to either syngas or CO. As the operating temperature of PCECs is lower than that of O-SOECs, it is feasible to produce CH4.[5,24,28] However, this competitive advantage also poses challenges in selectively producing high-purity chemicals. The selection of negative electrode materials will determine the Faradaic selectivity. There is a lack of rationally designed negative electrode materials that can lead to the production of pure CO, syngas, or CH4. To selectively produce pure CO or CH4, the negative electrode should also significantly suppress the HER.
One of the innovations of CO2 reduction in PCECs is that PCECs can operate at a lower temperature than O-SOECs. However, as the CO2 molecules are thermodynamic stable, the CO2 reduction kinetics is slower than that at high operating temperatures, especially for CO production. Therefore, the CO2 conversion to CO in PCECs is lower than that in O-SOECs. The intermediate operating temperatures thermodynamically favor CO2 reduction to hydrocarbons. Therefore, it is suggested future research should be geared toward enhancing the electrocatalytic activity of negative electrodes at intermediate operating temperatures (e.g., 300–500 °C) to improve CO2 conversion to hydrocarbons.
One of the impediments to the development of commercially relevant PCECs is the long-term durability of PCECs, especially the negative electrode under an atmosphere with CO2. Reduced operating temperatures thermodynamically favor the CO2 reduction to hydrocarbons. However, lowering the operating temperature may compromise the chemical stability of negative electrodes that are more prone to react with CO2 and form BaCO3 at low temperatures. The BaCO3 phase is an insulator and catalytically inert, leading to the degradation of PCECs.[2,86]
Despite coking (or carbon deposition) not being observed in PCECs, it does not indicate coking is not possible as the current CO2 conversion in PCECs is poor. The future PCECs developed for CO2 reduction to CO could achieve a high CO/CO2 ratio. The intermediate operating temperature further favors the exothermic Boudouard reaction, posing potential risks of coking.[103]
The CO2 reduction mechanism in PCECs is the “black box”. It is widely accepted that CO2 is reduced by either the proton or the gaseous H2. However, there is no direct evidence of how electrocatalysis occurs and what intermediate species are involved in the reactions. The lack of this fundamental understanding hinders the rational design of negative electrodes for PCECs. Therefore, without understanding the CO2 reduction pathway over negative electrodes, it is impossible to deliberately modulate the negative electrode composition and structures to tune the CO2 reduction pathways.
Strategies to Address the Challenges of CO2 Conversion and Utilization in PCECsFigure 14 displays the challenges of synthesizing chemicals in PCECs via CO2 reduction and corresponding strategies. There is a variety of strategies that can tackle these problems, which include 1) designing CO2 reduction electrodes for PCECs to inhibit HER, enhance CO2RR activity, and tune the CO2 reduction pathways for enhancing the propensity to produce desired products; 2) enhancing chemical stability of CO2 reduction electrode by improving the CO2 tolerance of proton-conducting oxide support; 3) alternating the operating conditions to modulate selectivity while achieving high CO2 conversion and product yield; and 4) conducting in situ operando experiments to probe the CO2RR mechanisms in PCECs. These strategies can also be intertwined to better control the selectivity and further enhance CO2 conversion.
Inhibit Hydrogen Evolution ReactionHER is thermodynamically more favorable than CO2RR, suggesting HER can only be inhibited by modulating the kinetics of HER and CO2RR on the negative electrode. Two sets of strategies could be applied to suppress HER, which include reducing the HER activity of the negative electrode and improving its CO2RR kinetics. The negative electrode is typically a composite of transition metal and oxide. The transitional metal, including Ni, Co, Cu, and Fe, is key to the HER activity. Although there is limited study of the HER activity on transition metals in gaseous media, it has been recognized that the intrinsic properties of transition metals alternate the HER activity. For example, in acidic media, the HER activity of the most common transition metals shows the following trend: Ni>Co>Cu>Fe, which suggests tailoring the transition metal can inhibit the HER.[104] Additionally, an Ir-based negative electrode has been applied as the PCEC negative electrode for CO2 reduction, which significantly inhibits the HER without compromising the CO2RR activity, validating the metallic phase is essential for HER.[28] However, in acidic media, HER on Ir is more favorable than that on Ni.[104] Therefore, the volcano plot developed for HER in acidic media cannot be directly used to develop CO2 reduction electrodes for PCECs, suggesting the HER activity on transition metal in gaseous media should be better studied.[105–107]
Faradaic Selectivity toward CO or CH4Thermochemical CO2 reduction over heterogeneous catalysts has been well studied, establishing strategies to tune the selectivity toward CO and CH4. Despite the electrochemical CO2 reduction pathways in PCECs being different from thermochemical CO2 reduction, the approaches to creating and modulating the active intermediate species are similar. Therefore, the strategies established for tuning the selectivity of thermochemical CO2 reduction could be potentially applied for tailoring the CO2 reduction electrode of PCECs, especially the approaches to selecting a metallic phase.[108–110]
For instance, NiCo bimetallic heterogeneous catalysts have been widely used for thermochemical CO2 methanation because cobalt is the most active metal for methanation reaction and the NiCo alloy (e.g., Co/Ni = 1) can lead to synergistic effects which can significantly enhance the catalytic activity and CH4 yield.[111–115] To selectively produce CO, using Ni-free nanoparticles, such as FeCo alloy nanoparticles, could be a plausible approach. Fe tends to absorb CO2 and can selectively produce CO. However, the activation energy to produce CO is high and thus its overall catalytic activity is low. FeCo alloy with a Fe/Co ratio of >3 can drastically enhance its catalytic activity while it does not reduce the CO selectivity because 25 mol% Co can decrease the CO2-to-CO reaction activation energy.[116]
The oxide support is also essential for CO2 reduction as it typically functions as the site for forming active species, including carbonate, hydroxyl groups, formate, and other functional groups. However, the oxide support developed for thermochemical CO2 reduction, such as Al2O3, TiO2, and SiO2, might not be directly used for PCECs as they are not conductive. The negative electrodes of PCECs should have electrocatalytic interfaces that are the triple-phase boundaries, namely the junctions of metal, oxide support, and gas phases. The oxide support should be a mixed ionic and electronic conductor to create the electrocatalytic interfaces.
Improve Chemical Stability of Negative ElectrodeThe chemical stability CO2 reduction electrode is key to the durable operation of PCECs, which depends on the CO2 tolerance of the oxide phase (i.e., proton-conducting oxide). Therefore, the approaches developed to enhance the CO2 tolerance of proton-conducting oxides or related materials can be leveraged. Herein, we focus on reviewing the proton-conducting oxide with a generalized formula of ABO3 as it is the most widely used oxide in PCECs. Its chemical stability could be improved by doping the A-site or B-site with other cations that inhibit its chemical reaction with CO2. For example, doping the A-site with Ca and La can increase CO2 tolerance. Moreover, it has been recognized that increasing the amount of Zr at the B-site also leads to improved chemical stability.[117–121]
Establish the Experimental Tools to Probe the CO2RR Mechanisms in PCECsThough previous studies have attempted to electrochemically reduce CO2 in PCECs by integrating it with water electrolysis or dehydrogenation of hydrocarbons, it is not fully clear if the CO2 conversion pathway involves any electrochemical processes, as CO2 reduction in negative electrodes may proceed through either electrochemical pathway or thermochemical pathway, or both.
Therefore, complementing the electrochemical cell design, CO2 reduction electrode design, fabrication, and characterization, it is important to probe and understand the CO2RR mechanisms and establish the relationship between CO2 reduction mechanisms and electrode materials. This fundamental understanding will allow us to establish strategies to deliberately design the CO2 reduction electrode. The active functional groups involved in CO2 reduction are not systematically studied and correlated with the CO2 reduction electrode. Without a fundamental understanding of CO2 reduction pathways, it is unfeasible to deliberately design and fabricate highly active CO2 reduction electrodes. It is essential to establish the in situ operando spectroscopy technologies, such as in situ operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), to probe the intermediates and better understand the CO2RR mechanisms.
The in situ operando DRIFTS platform is designed for studying thermochemical heterogeneous catalysis,[80,122–124] which cannot directly apply a potential on the samples. The in situ operando DRIFTS chamber available on the market should be optimized or customized to allow performing in situ operando electrochemical DRIFTS. The establishment of this platform will advance the fundamental study of electrochemical CO2RR mechanisms in PCECs.
As the choice of proton-conducting oxide support and metallic phase in negative electrodes can significantly impact CO2RR mechanisms, the factors that have relations with the CO2RR mechanism include the support, metallic phase, and the intertwining effects between support and metallic phase.[125] It is expected that different supports, metals, and the combination of support and metal could result in distinct intermediate species, which consequently leads to different CO2RR pathways, reaction kinetics, and selectivity. Therefore, the in situ operando electrochemical DRIFTS is suggested to be utilized to thoroughly illuminate these impacts, which can provide extensive guidance on designing CO2 reduction electrode materials.
Adjust Operating Conditions to Modulate the Product Yield, Selectivity, and CO2 ConversionThe operating temperature has a huge impact on CO2 reduction kinetics and CO2 conversion. Therefore, the operating temperature should be rigorously selected considering both CO2 reduction kinetics over the negative electrode and CO2 conversion. In general, to produce CO with a high CO2 conversion and energy efficiency, the negative electrode should be designed to kinetically favor CO production while operating PCECs at an appropriately high temperature. To selectively produce CH4, the negative electrode should be engineered to reduce CO2 to CH4 and PCECs should operate at relatively low temperatures (e.g., 400 °C) to further enhance CH4 production. It is also essential to keep in mind that reducing the operating temperature can certainly enhance the overpotential of oxygen evolution reaction at positive electrodes. The production of CH4 could be enhanced by reducing operating temperature, but at the cost of reduced energy efficiency.
Furthermore, the applied current density is the second factor that affects the CO2 reduction in PCECs as the current density is proportional to the proton flux and hydrogen concentration in the negative electrode. Therefore, adjusting the current density will affect the CO2 reduction performances. It has been noted that electrochemical promotion does exist for ammonia synthesis (nitrogen reduction reaction) after integrating an ammonia synthesis catalyst with a proton-conducting ceramic membrane.[48] The electrochemically promoted ammonia production rate is 13 times higher than the thermochemical ammonia production rate while the proton consumed for ammonia synthesis is six times higher than that of electrochemically supplied, indicating a significant non-Faradaic impact exists which is attributed to the enhanced proton concentration on the catalyst surface. Therefore, we anticipate that similar electrochemical promotion also plays a role in CO2RR because the CO2RR is also mediated by the local proton/hydrogen concentration on the catalyst surface.[26,126] Referring to Figures 7 and 14, the Faradaic selectivity toward carbon-containing chemicals as a function of the current density should display a similar relationship, implying that increasing the applied current density cannot continuously enhance CO2 conversion, product yield, and Faradaic selectivity. Therefore, an appropriately high current density, as shown in Zone 1 of Figure 14 (Top middle panel), could concurrently achieve a high product yield and selectivity.
Finally, we would like to emphasize that PCECs provide versatility in terms of operating conditions and products. There is no straightforward principle of selecting the optimal operating conditions, which should be carefully determined after considering the chemical to be produced, selectivity, CO2 conversion, and energy efficiency, especially when those factors are considered in the techno-economic analysis.
Upgrading Natural Gas in PCECs Motivations of Employing PCECs for Distributed Natural Gas ConversionPCECs offer several intriguing advantages for upgrading natural gas to value-added chemicals and provide potential opportunities for distributed chemical production. First, we provide a brief overview of the industrial context that stimulates the research of employing PCECs for upgrading natural gas.
The depletion of conventional oil reserves has shifted much oil production to remote and offshore reservoirs, which imposes severe restrictions on the utilization of co-produced natural gas. Facilities for natural gas conversion rarely exist onsite, while transporting gas to centralized facilities is cost-prohibitive and/or infeasible. Consequently, flaring and venting are commonly used to dispose of co-produced natural gas. Nonetheless, both practices flagrantly waste precious resources with deleterious environmental impacts.
Efficient and economical utilization of flared and vented natural gas motivates the development of distributed and modular technologies for producing liquid or solid chemicals to enable economical transportation and utilization of the resulting value-added products.[127,128] Figure 15 summarizes the pathways of synthesizing chemicals from methane (the primary component of natural gas). Currently, the syngas and Fischer–Tropsch synthesis route is the most mature technology for converting natural gas to commodity chemicals (Figure 15, top panel). However, this centralized technology is not suitable for onsite chemical production because it is a complicated process that is difficult to downsize and requires costly infrastructure to transport natural gas.[129] Additionally, Fischer–Tropsch synthesis consumes either extra CO or H2 to remove the oxygen from CO, leading to lower carbon utilization or consumption of valuable H2.[130] Alternatively, syngas can be converted to methanol followed by methanol-to-olefins conversion[131] or methanol-to-aromatics conversion.[132] However, this technology also relies on energy-intensive and centralized syngas production.
There is an urgent need for modular and distributed chemical production systems (Figure 15, bottom panel) that can directly, efficiently, and economically convert natural gas to high-value and transportable chemicals. It should be noted that the global demand for benzene has reached about 43 million tonnes in 2020.[133] Moreover, the U.S. imports 20% of all the benzene it uses because of a shortage of benzene supply.[133] These facts are driving the industry toward producing aromatic compounds directly from methane via nonoxidative methane dehydroaromatization (MDA), oxidative methane aromatization, or oxidative coupling of methane to ethylene followed by further oligomerization to aromatics.[134–138]
Direct, nonoxidative conversion of methane to aromatics has attracted widespread attention because it enables high-value liquid chemicals production, eliminates complex reactors and processes, and avoids methane overoxidation, as well as significantly reduces CO2 emissions.[138] However, the nonoxidative MDA over heterogeneous catalysts in conventional packed bed reactors is challenging because of its low methane conversion (typically ≈10% at 750 °C),[139] coking, and the high operating temperatures (>800 °C) for CH bond activation.[138–142] Although a high methane conversion of 48.1% and a stable operation exceeding 60 h have been demonstrated over single-atom iron sites embedded in a silica matric, its operating temperature is impractically high (>1000 °C), and the selectivity of the aromatic compounds is low (<50%).[138] In the last decades, a host of technologies have been proposed for addressing these challenges. These include injecting oxygen to mitigate coke and improve durability, and extracting hydrogen to circumvent the severe thermodynamic limitations and thereby enhancing methane conversion and product yield.[143–146] Unfortunately, it is infeasible to achieve simultaneous hydrogen extraction and oxygen injection in packed bed reactors to tackle all the above challenges.
As an alternative strategy, PCECs (Figure 16),[1,5,23,127] which can operate in power-driven (electrolytic reactor, Figure 16A) mode and hydrogen permeation mode (Figure 16B), enable continuous and high-flux hydrogen extraction, and minor oxygen injection. The hydrogen extraction aims to enhance methane conversion and aromatics yield while minor oxygen injection can improve its coking tolerance. Figure 16A displays the PCECs with a mixed proton and oxygen-ion conductor as the membrane, which extracts H2 and injects minor O2 under an external potential. Figure 16B shows the PCECs that employ a mixed proton, oxygen-ion, and electronic conductor as the membrane. The chemical potential drives H2 permeation and oxygen injection.
The oxygen-ion conduction of the membrane is key to durability. Additionally, the transference number of oxygen ions can affect the selectivity of products. A relatively low oxygen-ion transference number can mitigate coking while selectively producing aromatics. However, an excessively high oxygen-ion conduction can oxidize the hydrocarbons and reduce the selectivity of aromatics. Therefore, the transference number of oxygen ions should be controlled at lower than 10% to simultaneously achieve durable operation and high aromatics yield. It has been widely recognized that the operating temperature and atmosphere can alter the transference number of oxygen ions.[5,49,147] In general, increasing the operating temperature and oxygen partial pressure can increase the transference number of oxygen ions. Duan and O'Hayre et al. also noted BaCe0.7Zr0.1Y0.1Yb0.1O3-𛿠exhibits a higher transference number of oxygen ions than BaZr0.8Y0.2O3-ð›¿, indicating the electrolyte composition also affects the oxygen-ion injection.[147] As one of the benefits of employing PCECs for producing aromatics is the reduced operating temperature, increasing the temperature to achieve a relatively high transference number of oxygen ions is not favorable. However, the oxygen partial pressure of the cathode side or electrolyte composition can be adjusted to control the oxygen-ion injection.
Incorporating deliberately designed methane aromatization catalysts into PCECs and synergizing the mixed proton and oxygen-ion conduction address the two main issues of methane aromatization in conventional thermochemical catalytic processes: 1) low methane conversion due to thermodynamic limitations and 2) rapid catalyst deactivation due to coking. By continually removing hydrogen (blue arrows in Figure 16) from the methane aromatization electrode side, this hybrid process pushes the reaction equilibrium toward aromatics, thereby increasing methane conversion (Figure 16, yellow dot) at reduced temperatures (e.g., 600 °C).[142,148] Second, the electrolyte membrane exhibits minor oxygen-ion conductivity,[49,147] which enables the concomitant injection of oxygen (red arrows in Figure 16) to the methane aromatization electrode which can act to burn off coke or selectively react with H2 and hydrocarbons to produce H2O and CO2 which can then mitigate coking (blue dot in Figure 16).
A computational study[136] found that the addition of an appropriate amount of oxygen facilitates methane aromatization by the formation of ethylene via oxidative coupling of methane followed by the aromatization of ethylene. Cao et al.[137] have experimentally demonstrated oxidative methane aromatization in mixed oxygen-ion and electronic membrane reactors. Although enhanced stability and inhibited coke formation were achieved in the reactor, the aromatics yield, in particular the initial aromatics yield, was not significantly enhanced.[137] In conventional fixed bed rectors, there have been attempts to regenerate the methane aromatization catalyst by burning out the carbon using oxygen at 700 °C.[149–151] However, due to relatively high oxidation temperature, Mo2C, the active site on methane aromatization catalysts, tends to be reoxidized into mobile Mo-oxides (i.e., overoxidation). Therefore, the regeneration of catalysts at lower temperatures (500–600 °C) has been employed to reactivate the catalysts.[143,152] However, temperature cycling in a fixed-bed reactor is inefficient and complex.
Extracting a considerable amount of hydrogen to shift reaction equilibrium has been achieved in PCECs to enhance methane conversion and aromatics yield.[145,146] The potential of PCECs for improving methane aromatization has been validated.[23] If MDA can run at lower temperatures, coke formation will be suppressed since the formation of polyaromatic type carbon, the main reason for catalyst deactivation, is more prone to occur at high temperatures.[142,148] Liu et al. also recognized that reduced operating temperatures will inhibit coke formation.[153] However, further dehydrogenation favors undesired naphthalene and thus will lead to its oligomerization to polyaromatic hydrocarbons, which are the deleterious coking species.[139,145] Rival et al. noted that dehydrogenation via the Pd-alloy membrane reactor results in more severe graphitic coke.[145] Therefore, although enhanced methane conversion was achieved in hydrogen-permeation membrane reactors (e.g., Pd and Pd-alloy membrane reactor), the exacerbated coking makes it impractical for economically viable methane conversion.
In summary, either hydrogen extraction or oxygen injection exhibits both beneficial and detrimental impacts, which are summarized in Figure 17, implying the challenges of methane aromatization cannot be solely addressed via hydrogen extraction or oxygen injection. As depicted in Figure 17, it is expected that the integration of hydrogen extraction with oxygen injection could potentially address all issues of low conversion, low yield, and poor durability. Thanks to the unique defect chemistry and transport properties of the protonic ceramic electrolyte (e.g., BaCexZryY0.1Yb0.1O3-δ, x+y = 0.8), simultaneous hydrogen extraction and oxygen injection can be easily achieved in PCECs. Table 4 summarizes the amalgamated strategies employed by the PCECs to tackle the two primary challenges of methane aromatization. Therefore, we fully anticipate PCECs will be a significant step toward producing aromatics from distributed natural gas.
Table 4 Summary of combined strategies employed in PCECs to address the challenges of upgrading natural gas via methane aromatization
| Approaches and strategies | Challenge 1: Low methane conversion | Challenge 2: Severe coking and fast degradation |
| Hydrogen extraction via proton conduction | Hydrogen extraction drives the reaction toward a higher methane conversion | Hydrogen extraction lowers the operating temperatures to inhibit coking[142,148] |
| Oxygen injection via oxygen-ion conduction | Oxygen injection leads to oxidative methane aromatization and oxidative coupling of methane, which are thermodynamically favorable reactions and thus can enhance methane conversion[137] | Oxygen injection mitigates coking via direct burn-off coke, selectively oxidation of Mo2C followed by the Mo2O3-catalyzed coke mitigation, or the formation of intermediates (H2O, CO2) which can mitigate both graphitic and polyaromatic coke[143] |
|
Natural gas as the feedstock. Direct natural gas conversion |
C2-C4 alkanes in natural gas can make MDA thermodynamically more favorable[154] | Natural gas contains sulfur. The membrane has excellent sulfur tolerance.[1] Sulfur will not lead to exacerbated degradation |
Converting methane to aromatics in PCECs has centered on developing PCECs in either planar or tubular geometry.[23,78,155,156] The most recent demonstrations of employing PCECs and protonic ceramic membrane reactors are summarized in Table 5. Tubular PCECs are more favorable as they can achieve a longer gas stream residence time than the planar PCECs, consequently enhancing the hydrogen extraction rate, methane conversion, and aromatic yield. According to the membrane that is used for building PCECs, the two primary PCEC configurations shown in Figure 16 have been developed. Herein, we focus on reviewing the PCECs that fall into these two categories.[23,157,158]
Table 5 A summary of PCECs for upgrading natural gas
| PCEC Application | PCEC Configuration | T [°C] | P [atm] | Current density [mA cm−2] | Voltage [V] | Reactants Composition | Conversion [%] | Selectivity [%] | Stability | Ref. |
| PCEC for ethane deprotonation | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ-Ni (Ethane electrode) | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (Electrolyte)|PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (Hydrogen evolution reaction electrode) | 400 | 1 | 1000 | 0.41 | 10% C2H6/Ar | 2.8 (Ethane) | 100 (Ethylene) | 90 h, 400 °C, and 1000 mA cm−2 | [25] |
| PCEC for ethane deprotonation | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ-(PrBa)0.95(Fe0.9Mo0.1)2O5+δ -PtGa/ZSM-5 (Ethane electrode) | BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (Electrolyte)| BaCe0.7Zr0.1Y0.1Yb0.1O3-δ -Ni (Hydrogen evolution reaction electrode) | 550 | 1 | 0 | N/A | 10% C2H6/Ar | 21.7 (Ethane) | 81.4 (Ethylene) | 150 h, 550 °C, and 40 mA cm−2 | [157] |
| 10 | 25.9 (Ethane) | 79.3 (Ethylene) | ||||||||
| 20 | 30.6 (Ethane) | 77.6 (Ethylene) | ||||||||
| 30 | 36.1 (Ethane) | 72.1 (Ethylene) | ||||||||
| 40 | 39.9 (Ethane) | 66.6 (Ethylene) | ||||||||
| PCEC for methane upgrading | Cu-Mo/H-MCM-22(Methane electrode) | BaZr0.7Ce0.2Y0.1O3–δ (Electrolyte)|Ni-BaZr0.7Ce0.2Y0.1O3–δ (Hydrogen evolution reaction electrode) | 710 | 1 | 40 | N/A | 10% CH4/H2 | 11.6 (Methane) | 86.2 (Aromatics) | 40 h, 710 °C, and 40 mA cm−2 | [23] |
| Proton membrane reactor for methane upgrading | Fe-SiO2+SrCe0.8Zr0.2O3–δ (Methane side) | SrCe0.7Zr0.2Eu0.1O3–δ (Membrane) | 980 | 1 | N/A | 90% CH4/Ar | 5.9 (Methane) | 42.9 (Aromatics) | 50 h, 1030 °C, and 1 atm | [158] | |
| 1000 | 10.1 (Methane) | 46.4 (Aromatics) | ||||||||
| 1030 | 18.2 (Methane) | 47.4 (Aromatics) | ||||||||
| Proton membrane reactor for methane upgrading | Fe@SiO2±SrCe0.8Zr0.2O3–δ (Methane side) | SrCe0.7Zr0.2Eu0.1O3–δ (Membrane) | 950 | 1 | N/A | 90% CH4/Ar | 2.1 (Methane) | >90 (C2 products) | 50 h, 1030 °C, and 1 atm | [155] | |
| 1000 | 8.3 (Methane) | |||||||||
| 1030 | 15.5 (Methane) | |||||||||
| 1050 | 23.7 (Methane) | |||||||||
| Proton membrane reactor for methane upgrading | Mo/H-ZSM5 (Methane side) | SrCe0.95Yb0.05O3-α (Membrane) | 677 | 1 | N/A | 85% CH4/Ar+CO2 | 6.5–8.5 (Methane) | 72 (C6-C12 products) | 25 h, 720 °C, and 1 atm | [153] | |
Figures 18–20 present the most recent demonstrations of converting low-cost alkane molecules, including CH4 and C2H6, to aromatics and light olefins. As shown in Figure 18, J. M. Serra and C. Kjølseth et al. have focused on employing tubular PCECs with proton-conducting membranes to upgrade methane in PCECs.[23] The electrolyte membrane they employed exhibits mixed protonic and oxygen-ion conduction. Under an external power, as shown in Figure 16A, H2 extraction and minor oxygen injection are simultaneously achieved. By using PCECs for methane aromatization, the methane conversion and aromatic yield have been enhanced. Furthermore, the coking has been significantly inhibited, which enables durable operation. It should also be recognized the operating temperature of methane aromatization in PCECs is around 200 °C lower than methane aromatization in conventional packed bed reactors, validating the benefits of using PCECs for upgrading methane to aromatics.[138]
Ding et al. have developed a planar PCEC for upgrading C2H6 to C2H4 and other light olefins.[25,157] Figure 19A displays the reactor designed for C2H6 dehydrogenation and H2 separation, which enhances C2H6 conversion. Upon applying a current to extract the H2, C2H6 conversion is enhanced while the selectivity to C2H4 is reduced, which is attributed to the increased selectivity to C3+ and CH4. This work has validated that extracting H2 from the C2H6 stream can shift the reaction equilibrium towards a higher conversion. Additionally, the extent of H2 extraction affects product selectivity. Increasing the current density tends to increase the number of carbon atoms in hydrocarbon molecules. However, this work does not identify if the C3+ compounds are alkanes or olefins. As light olefins have a higher economic value than alkanes, it is expected that increasing the current density could lead to a higher yield of C3+ light olefins.
The reactors that are shown in Figures 18A and 19A consume electrical energy to drive the H2 separation.[157,158] The reactor equipped with a mixed ionic and electronic conductor can extract H2 and inject O2, which is driven by the chemical potential. As shown in Figure 20, Wachsman et al. have demonstrated intriguing results using a mixed protonic and electronic conductor, SrCe0.7Zr0.2Eu0.1O3-□, as the electrolyte membrane and integrating the rationally designed methane aromatization catalyst with tubular reactors.[78,155,156] Without external applied current/voltage, the reactor functions as a hydrogen permeation membrane reactor, which extracts H2 from the CH4 side, shifting the reaction toward a higher CH4 conversion. This membrane reactor leads to higher CH4 conversion and product yield than the conventional fixed-bed reactor. Moreover, the sweeping gas to remove the H2 flux affects the performance. Using an inert gas, such as helium, results in the highest CH4 conversion and aromatics yield. However, as both sides of this reactor are under a reducing atmosphere, the H2 permeation exacerbates the coking as no oxygen injection is simultaneously achieved. Using oxygen as the sweep gas changes the membrane as a mixed proton, oxygen-ion, and electronic conductor, which concurrently extracts H2 and injects O2. As shown in Figure 20, despite the oxygen injection leading to the production of CO, both CH4 conversion and aromatics yield are higher than that achieved in conventional fixed-bed reactors. Furthermore, using oxygen as the sweep gas mitigates coking and achieves durable operation. However, as the H2 permeation is thermally activated, the operating temperature of reactors shown in Figure 16B is higher than that of PCECs displayed in Figure 16A.
These works present new scenarios of producing value-added chemicals (aromatics and light olefins) from natural gas, which enhances the onsite utilization of natural gas and reduces emissions, offering an economically viable and environmentally benign alternative to the conventional conversion of natural gas.
Challenges and Opportunities for Upgrading Methane in PCECsUpgrading natural gas in PCECs also faces some challenges. These challenges and corresponding strategies are outlined as follows:
There is a limited catalyst that has been deliberately designed for PCECs.[159] The current methane aromatization catalysts integrated with PCECs are developed for thermochemical reactors, which generally operate at a higher temperature than PCECs. Furthermore, the chemical compatibility of methane aromatization catalysts with proton-conducting ceramic has not been studied. It is therefore suggested to rationally design methane aromatization catalysts that are chemically and physically compatible with PCECs. Furthermore, if the methane aromatization catalyst simultaneously exhibits excellent conductivity and catalytic activity, the catalyst can be directly applied to PCECs as an electrode or catalytic layer, spatially expanding the active electrochemical reaction sites and simplifying PCEC manufacturing.
The current PCEC configuration is limited to the tubular geometry, which allows to directly fill the PCEC tube with methane aromatization catalysts. However, the tubular PCEC requires more complicated manufacturing processes. Therefore, developing PCECs and catalysts that could also be integrated with planar PCEC cells is essential for its future development, further enhancing its versatility for upgrading natural gas to high-value chemicals.
Prospects and OpportunitiesThe research and development of PCECs for synthesizing chemicals are under enlargement. Electrochemical approaches for chemical production are being proven to be economical, sustainable, and environmentally benign as they can utilize fossil fuels more efficiently or fully avoid the utilization of fossil fuels while using renewable power and abundant feedstocks such as nitrogen and water. Producing chemicals in PCECs on the other hand opens an attractive avenue for chemical energy storage as PCECs interconvert the electrical energy and chemical energy.
Interdependency between Chemical ReactionsThinking beyond synthesizing chemicals via a single reaction, intensifying two or more reactions in one PCEC to produce more chemicals can enhance the energy efficiency, reduce capital cost, and improve economic benefits. For example, N2 or CO2 reduction PCEC can be integrated with the CH4 dehydroaromatization reactor to utilize the H2 produced via CH4 conversion. N2 or CO2 reduction electrodes can be applied as the negative electrodes of the CH4 conversion reactor (Table 1); thus, H2 will directly reduce N2 to ammonia or convert CO2 to value-added chemicals. This process intensified reactor increases the hydrogen utilization and reduces the carbon footprint of producing ammonia, carbon monoxide, and hydrocarbons.
Due to the higher operating temperature (300–500 °C) of PCECs than low-temperature (<100 °C) electrochemical devices, HER is favorable. Both CO2 reduction and N2 reduction in PCECs suffer from high Faradaic selectivity toward H2. Additionally, the high operating temperature leads to thermally activated decomposition or conversion of the chemicals produced in PCECs. For example, at 500 °C, ammonia can be cracked to H2 and N2, which further reduces the ammonia production rate and Faradaic selectivity. Therefore, some common strategies can be established to suppress HER for both CO2 reduction and N2 reduction. For example, using Ni-free negative electrode to inhibit hydrogen production.
Impacts of Renewable Electricity Cost on the Economic Viability of Producing Chemicals in PCECsThe economic viability of producing chemicals in PCECs depends on both capital and operational costs. The reduction in electricity cost, especially the renewable electricity cost, is essential for facilitating the future implementation of PCECs for synthesizing chemicals in the real world. In the last decade, with the growing deployment of wind- and solar-based renewable power plants, the renewable electricity cost has been drastically reduced. It is expected that renewable electricity will become more widely available. However, the global power demand is also growing, and electricity cost is determined by the demand and supply. Therefore, to accurately evaluate the economic viability of producing chemicals in PCECs, it is suggested to conduct a comprehensive techno-economic analysis to quantitively determine the chemical production cost in PCECs and compare it with conventional manufacturing processes.
Electrodes and ScaleupFuture work should primarily focus on the development of advanced electrodes and catalysts for PCECs, which could significantly improve the feedstock conversion, reduce the electrical energy consumption, enhance the product yield, and modulate the selectivity toward a certain chemical. Nowadays, most efforts have been devoted to synthesizing chemicals in lab-scale PCECs that exhibit an active area of up to 5 cm2, which is far from practical applications. The scaleup and commercialization of PCECs for synthesizing chemicals also lag far behind other technologies. Therefore, future studies should also be centered on designing, fabricating, and characterizing PCERs at the bench scale or commercially relevant scale.
Fundamental Understanding of the Reactions at ElectrodesSubstantial increasing efforts have been devoted to validating the concepts of using PCECs for chemical production. Although compelling performances have been demonstrated for ammonia synthesis, CO2 reduction, and natural gas conversion, there is a lack of fundamental understanding of the electrochemical reactions and reaction pathways. Therefore, it is necessary to design and build the experimental tools to assist in probing mechanisms of nitrogen reduction reaction, CO2 reduction reaction, and methane aromatization in PCECs. A deeper understanding of the reaction mechanisms could be of great benefit to rationally designing electrodes and catalysts for PCECs.
Optimize Positive Electrodes to Improve the Feedstock FlexibilityThe configurations of PCECs employed for producing chemicals vary with feedstocks and products, which limits the flexibility and versatility of PCECs. For example, the positive electrodes of PCECs for CO2 reduction should be modified to use different reactants delivered to the positive electrode. With a positive electrode that is active for the oxidation of various feedstocks, PCECs will be more capable of converting broader reactants to high-value chemicals.
Operating ConditionsFinally, we have recognized that operating conditions, including temperature, atmosphere, current density or voltage, and pressure, can affect conversion, selectivity, and yield. All these factors are intertwined to impact the PCEC performances, suggesting a detailed computational system can be developed to control and optimize the processes, aiming to further enhance the economic viability of producing chemicals in PCECs.
ConclusionIn this review, we have focused on summarizing and analyzing the recent demonstrations of PCECs for synthesizing chemicals, which include ammonia synthesis, reducing CO2 to fuels, and upgrading natural gas to high-value chemicals. For each application, we have provided the background and fundamental understanding of materials, with a particular focus on the electrode where the essential reactions occur. We have also summarized the corresponding reactions and thermodynamics. Moreover, we have highlighted the promising demonstrations of PCECs. For each application, the challenges have been summarized. The strategies to address these challenges, enhance performance, and improve stability have been proposed. Overall, this review aims to provide comprehensive guidance for the researchers to design and optimize PCECs for synthesizing sustainable chemicals and fuels.
AcknowledgementsThis work was primarily supported by the faculty research funding from Kansas State University. The publication of this article was financed with support from the Kansas State University Open Access Publishing Fund. Additional support was provided by the Department of Energy, Office of Fossil Energy and Carbon Management under award no. DE-FE0032005. D.D. would like to acknowledge the support of the U.S. Department of Energy (USDOE), Office of Energy Efficiency and Renewable Energy (EERE), Advanced Manufacturing Office (AMO) R&D Projects Emerging Research Exploration and the Rapid Advancement in Process Intensification Deployment (RAPID) Institute seed project under DOE Idaho Operations Office under contract DE-AC07-05ID14517.
Conflict of InterestThe authors declare no conflict of interest.
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Abstract
Protonic ceramic electrochemical cells (PCECs) have been intensively studied as the technology that can be employed for power generation, energy storage, and sustainable chemical synthesis. Recently, there have been substantial advances in electrolyte and electrode materials for improving the performance of protonic ceramic fuel cells and protonic ceramic electrolyzers. However, the electrocatalytic materials development for synthesizing chemicals in PCECs has gained less attention, and there is a lack of systematic and fundamental understanding of the PCEC reactor design, reaction mechanisms, and electrode materials. This review comprehensively summarizes and critically evaluates the most up-to-date progress in employing PCECs to synthesize a wide range of chemicals, including ammonia, carbon monoxide, methane, light olefins, and aromatics. Factors that impact the conversion, selectivity, product yield, and energy efficiencies are discussed to provide new insights into designing electrochemical cells, developing electrode materials, and achieving economically viable chemical synthesis. The primary challenges associated with producing chemicals in PCECs are highlighted. Approaches to tackle these challenges are then offered, with a particular focus on deliberately designing electrode materials, aiming to achieve practically valuable product yield and energy efficiency. Finally, perspectives on the future development of PCECs for synthesizing sustainable chemicals are provided.
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Details
; Ding, Dong 2 ; Duan, Chuancheng 1
1 Department of Chemical Engineering, Kansas State University, Manhattan, KS, USA
2 Energy and Environmental Science and Technology, Idaho National Laboratory, Idaho Falls, ID, USA