Graphene-Based Nanostructured Cathodes for Polymer Electrolyte Membrane Fuel Cells with Increased Resource

Abstract

Pt on carbon black (Pt/C) has been widely used as a catalyst for both ORR and hydrogen oxidation reaction (HOR), but its stability is compromised due to carbon corrosion and catalyst poisoning, leading to low Pt utilization. To address this issue, this study suggests replacing carbon black with graphene in the catalyst layer. The importance of this work lies in the detailed examination of novel electrocatalysts with high electrocatalytic activity for large-scale power generation. In this paper, we discuss the use of regulatory techniques like structure tuning and composition optimization to construct nanocatalysts impregnated with noble and non-noble metals on graphene supports. Finally, it highlights the limitations and advantages of these nanocatalysts along with some future perspectives. Our objective is that this summary will help in the research and rational design of graphene-based nanostructures for efficient ORR electrocatalysis. The results of this study showed that the performances of graphene-based catalysts show high electrochemical active surface areas for Pt-Fe/GNPs and Pt-Ni/GNPs catalysts (132 and 136 m² g⁻¹, respectively) at 100 operating cycles. Also, high current densities and power densities were observed for Pt₃-Ni/G and Pt-Co/G catalysts used at the cathode. The values for current density were 1.590 and 1.779 A cm⁻², respectively, while the corresponding values for power density were 0.57 and 0.785 W cm⁻².

1. Introduction

Electrochemical conversion is another way to obtain electrical energy, as it involves the direct, non-polluting, and silent transformation of continuous chemical energy in a wide variety of substances into electrical energy. Fuel cells are different from primary cells, which are commonly called batteries or accumulators (secondary cells), in that reactants are constantly transported to the electrodes and the reaction products are eliminated permanently [1,2,3]. The key components of this entire chain are the electrochemical energy conversion devices—electrolysis and fuel cells. The most promising technology for these electrochemical systems is polymer electrolyte membrane fuel cells (PEMFCs). The ability of these cells to provide clean and sustainable energy for transport, stationary, and portable applications is attributed to their high power density, fast start-up time, and low-temperature operation. PEMFCs are projected to be one of the most challenging focus technologies in the world, as it is expected to provide a zero-emission energy conversion solution of vital importance in achieving complete decarbonization not only in the automotive energy sector, but in almost all other energy sectors. The reason for this is its high power density, quick start-up time, and ability to operate at relatively low temperatures [4,5,6,7,8,9]. However, limited performance, durability, and high production costs have hindered the widespread adoption of this technology. One of the major bottlenecks of this technology is identified by performance experts as the cathode oxygen reduction reaction (ORR). Proton-coupled electron transfer is involved in multiple steps during the complex process of cathodic oxygen reduction reaction (ORR). Due to its slow kinetics, it limits the performance of fuel cells, unlike the fast hydrogen anodic oxidation. The essential element in the design of PEM fuel cells is the configuration of the catalytic layer. Currently, Pt or Pt-alloy nanoparticles are the best electrocatalysts for ORR, but they have drawbacks such as high cost and limited availability. To solve these problems, several microporous carbon black (CB) supports are used to enhance the mass transport of gaseous species, provide electrical wiring for nanoparticles, and regulate water. The catalyst nanoparticles’ properties are greatly influenced by the support materials, which include their size, shape, and dispersion. Although carbon black is commonly used, it has limitations that can degrade catalyst activity and performance. The commercialization of Pt-based catalysts has been impeded by the low stability and limited availability of Pt [10,11,12,13].

In a standard design, platinum nanoparticles are evenly distributed on the surface of a suitable support to increase the active surface area and catalytic activity, enhance stability, and lower electrode cost. PEMFCs often use carbon blacks, which are readily available and inexpensive, as a support for electrocatalysts. However, they contain deep micropores that are poorly connected and reduce the flow of reagents, affecting the support material’s electrical conductivity. Additionally, Pt nanoparticles may be confined to micropores without access to ionomers and reagents, leading to a drastic reduction in the catalyst’s electrochemically active surface area. The durability of the catalytic layer is directly affected by the presence of impurities. Carbon corrosion accelerates the removal or aggregation of Pt nanoparticles and raises the resistance to oxygen transport in the catalytic layer [14,15,16].

One of the key challenges in using catalyst nanocomposites is to ensure their durability. There are currently two known degradation mechanisms for catalyst composites. The first is the electrochemically induced dissolution of Pt, which is directly related to the dynamics of the formation and reduction of Pt-oxide, resulting in Ostwald ripening and the formation of metallic Pt bands in the membrane. Pt nanoparticles are agglomerated and detached by the electrochemical and chemical corrosion of the carbon support, which is the second mechanism. Separate analysis of both mechanisms is difficult because they are interconnected. The reduction in the electrochemically active surface area (ECSA) of Pt is caused by either of these results, leading to a loss of catalyst performance over time [17,18,19,20].

Over the past two decades, significant research and development efforts have been focused on improving the catalytic performance of Pt catalysts using two major strategies: regulating the chemical composition and optimizing the morphological structure. Adjusting the electronic structure, atomic arrangement, and component coordination is the initial approach to improving the catalytic adsorption interface and achieving better intrinsic activity. External morphologies such as wires, nanosheets, flowers, core–shell structures, nano-frames, and assemblies are used in the second strategy to efficiently utilize active sites, facilitate electronic transmission, and enhance mass exchange [21,22].

The development of carbon-based 2D materials, which is a significant scientific breakthrough, could potentially provide a potential solution to our problem, which is good news. The increasing advancements in nanotechnologies and nanomaterials have led to the validation of using 2D graphene as a support for Pt-based catalysts in the past decade. It is important to note that graphene-based materials offer several key benefits, including high specific surface area, chemical stability, high electrochemical durability, high electronic conductivity, and strong metal support interaction that stabilizes Pt nanoparticles and prevents their growth and leaching [23,24,25]. The structure of the catalysts also has the potential to significantly enhance mass transfer. Layered graphene-based materials can avoid restacking and agglomeration in aqueous solutions due to van der Waals interactions [24]. These remarkable attributes of graphene have the potential to improve current performance and play a significant role in enhancing the efficiency of PEMFCs. In addition, different graphene derivatives, such as rGO (reduced graphene oxide) and graphene nanoribbons, have distinct chemical and physical properties compared to carbon-based materials (CBs) [24,25,26]. Graphene derivatives could be suitable for supporting ORR catalysts due to their thermodynamic stability and other properties. If used effectively, these features can significantly improve long-term durability by enhancing carbon corrosion resistance, which is crucial in achieving the ambitious aim of 30,000 h of system lifespans for HDV fuel cell applications (Figure 1) [27,28].

The use of graphene derivatives as supports for PEMFC catalysts has been widely attempted in recent years, but there have been no significant breakthroughs, due to a number of challenges. Specifically, compared to carbon black-supported catalysts, graphene derivatives have a more hydrophobic nature due to their preparation methods, and they also tend to restack more, making it difficult to make sure that Pt-based NPs are loaded with more metal and are distributed uniformly. In order to achieve higher current density performance and a higher roughness factor of the catalyst layer, both of these properties are essential [29,30,31].

This paper aims to examine recent advances in graphene-based materials integrated into membrane electrode assemblies (MEAs) designed for proton exchange membrane fuel cells (PEMFCs) for large-scale power generation.

2. Durability of Graphene-Based Cathode: Catalyst Degradation in Operating MEAs

One of the green energy conversion devices is the hydrogen fuel cell—a device functioning by converting chemical energy (of mainly hydrogen and oxygen/air) into electric power through a redox reaction. It is considered one of the most important methods of providing clean and emission-free energy. Among fuel cells (FCs) [15], proton exchange membrane fuel cells (PEMFCs) have attracted increased attention due to operation using simply hydrogen and air, low-temperature operation, high power density, and relatively low degradation [15,16]. The cathodic oxygen reduction reaction (ORR) is slower than fast anodic oxidation because it involves complex, multi-step proton-coupled electron transfer processes. This has an effect on the performance and cost of fuel cells [26,32,33]. The cathode receives electrons from an external circuit, and protons (H⁺) diffuse through the electrolyte membrane. Oxygen is brought into the cathode, and then transformed into O²⁻ by electrons from the anode. Following that, O²⁻ reacts with the protons diffusing through the membrane, completing the circuit and producing water. During ORR electrocatalysis, oxygen molecules diffuse to the electrode surface from the solution and adsorb onto the catalyst reaction sites. Depending on the mass concentration and the reaction conditions in the electrolyte, the oxygen molecules are transformed into oxygenated intermediates adsorbed on the active surface site of the catalysts, where an electrocatalytic reduction takes place (O₂ + 4H⁺ + 4e⁻ → 2H₂O) (Figure 2) [26,34,35]. The slow kinetics of the ORR is the primary problem for PEMFCs, and the high production cost of the precious metal required for fuel cell reactions is the primary reason for this [36,37]. Additionally, there are issues with the inadequate performance and durability of the electrode catalysts. Pt nanoparticles on black smoke (Pt/C) are commonly used as a catalyst for both ORR and HOR, but they suffer from poor stability due to carbon corrosion, catalyst poisoning, and low Pt utilization. In order to tackle these obstacles, there is a proposal to replace carbon black in the catalyst layer with graphene [18,36]. Graphene, a 2D carbon material, has demonstrated excellent performance as an ORR catalyst because of its outstanding thermal and electrical properties, as well as its high specific surface area [38,39]. The use of Pt nanoparticles supported on graphene for PEMFCs, ORR, and HOR has been extensively studied in the literature.

Das et al. [18] reported the replacement of carbon black with graphene oxide (GO), graphene nanoplatelets (GNPs), and graphene-based hybrids for PEMFC reactions. The electrochemical properties of the Pt-M/GNP catalysts were studied using the half-cell test method. According to observations, the Pt-Fe/GNPs electrode had an ECSA of 136 m² g⁻¹ Pt, while the Pt-Ni/GNPs and Pt-Cu/GNPs electrodes had ECSAs of 132 and 122 m² g⁻¹ Pt, in that order. The durability of catalysts was evaluated in terms of HOR and ORR activities using two different accelerated stress tests (ASTs). The electrochemical surface area (ECSA) decreased after ASTs, which could be interpreted as a reduction in the area associated with hydrogen adsorption and desorption. After the potential holding test (1.2 V for 24 h), the ECSA decreased for Pt-Ni/PNG by 9%, Pt-Fe/GNP by 30%, and Pt-Cu/GNP by 51%. Additionally, after the potential cyclic test (1000 cycles), the ECSA decreased for Pt-Ni/GNP by 7%, Pt-Fe/GNP by 19%, and 47% for Pt-Cu/GNP. Based on these findings, it appears that the Pt-Ni/GNP catalyst has greater durability than the other two catalysts. Improved ADT durability of the Pt-Ni/GNPs catalyst may be due to the strong metal–support interaction that reduces Pt migration, but further investigation is needed to understand in detail the mechanism for the improved ECSA. Rotating disk electrode (RDE) measurements were used to examine the catalytic activities of the Pt-M/GNPs catalysts for ORR, resulting in a half-wave potential of the Pt-Ni/GNP catalyst of 0.79 V, which is higher than that of the PtFe/GNP catalyst (0.74 V) and Pt-Cu/GNP catalyst (0.74 V). Moreover, the Pt-Ni/GNP catalyst shows a higher limiting current density compared to the other two catalysts under the same operating conditions, and it was concluded that the Pt-Ni/GNP catalysts show a higher ORR activity compared to other catalysts. In addition to high ORR activity, the durability of catalysts in the harsh conditions of the fuel cell is essential. In this regard, the durability of Pt-M/GNP catalysts was investigated, and ADT and ORR curves were produced. The activity of all catalysts decreased, but the Pt-Cu/GNP catalyst showed a slower rate of decline compared to the others. After testing the MEA with the Pt-Ni/GNP catalyst, better performance was observed in the ohmic bias region. It was also found that Pt-Ni/GNi/GNP had a power density of 0.378 Wcm⁻² at 0.6 V, Pt-Fe/GNP had a power density of 0.293 W cm⁻² at 0.6 V, and the Pt-Cu/GNP catalyst showed a power density of 0.256 W cm⁻² at 0.6 V. Furthermore, MEA’s polarization experiments were conducted, with the use of 67.7% Tanaka as the cathode catalyst and Pt-M/GNP catalyst as the anode. The activation polarization regime showed no change in performance, but ohmic polarization and concentration regions showed a significant decrease in performance. This drop strongly depends on the structure of the electrode and the activity of the reactants.

Han et al. [38] conducted a study to synthesize graphene (N, P-G) with a metal-free electrode substrate using a one-step synthesis technique. They used activated carbon (AC) and varied the amounts of nitrogen and phosphorus as well as the spacer content. The electrocatalytic performance of the synthesized material was tested for oxygen reduction reaction (ORR) in a 0.5 M H₂SO₄ solution saturated with O₂. The results showed that the sample with 10% AC@N, P-G exhibited the highest absolute current density limit value at 0.608 mA cm⁻², indicating improved ORR activity compared to other samples. After testing for a certain duration without changing conditions, it was observed that the performance of 10% AC@N, P-G deteriorated by less than 30%, while the catalytic activity of (0–5%) AC@N, P-G decreased by nearly 40% due to particle dissolution and aggregation over time. Further experiments were conducted using a composite catalyst with a mass ratio of 2:1, 10% Pt/C@(10%AC@N, P-G), under different operating temperature conditions. It was found that the maximum output power density improved as the cell temperature increased from 40 to 60 °C. However, an increase to 70 °C led to a decrease in performance by more than 50% due to an imbalance in heat and moisture in the membrane, causing water deficiency. Compared to the traditional 10%Pt/C, the mixed electrode materials with 10%Pt/C@(10%AC@N, P-G) exhibited excellent stability and higher output power at equal values for cell temperature. The maximum power density output at 40 °C, 60 °C, and 70 °C was 0.292 W cm⁻², 0.551 W cm⁻², and 0.495 W cm⁻², respectively, with a performance reduction of less than 10.34% from 60 °C to 70 °C. The improved performance of the fuel cell can be attributed to the adsorption function and catalytic reaction of the doped graphene in the 10% AC@N, P-G substrate, as well as the enhanced transport performance and temperature distribution of the electrode surface, leading to reduced electrochemical heat accumulation.

Velayuthan et al. [39] successfully synthesized a stable electroactive Ni-anchored bimetallic catalyst with a low Pt nanoparticle loading by using graphene as a support material for the integration of the catalyst (Pt3-Ni/G). This catalyst exhibited a maximum electrochemical surface area (ECSA) of 108.56 m²/g Pt, mass activity of 2.2 A mg Pt, and specific activity of 3.47 mA cm⁻², indicating very good ORR (oxygen reduction reaction) activity. They constructed a scalable proton exchange membrane fuel cell (PEMFC) using 0.2 mg cm⁻² Pt, Pt3-Ni/G as the cathode with an active area of 25 cm² and SS-314L-stainless steel for the serpentine flow field. The operating temperature of the single-cell PEMFC was maintained at 50°C, with a relative humidity of 50%, and 99.99% pure O₂ and 99.999% H₂ were supplied to the cathode and anode, respectively. Prior to testing, the membrane electrode assembly (MEA) was conditioned using constant-voltage pulses of 0.7 V and 0.4 V to reach a constant current range. The PEMFC testing resulted in a maximum power output of 71.25 W mg⁻¹ Pt at a current density of 1.59 A cm⁻². Furthermore, the PEMFC-based Pt3-Ni/C//Pt/C system provided a consistent power output of 68.75 W mg⁻¹ Pt even after 4 h of continuous cycling.

Chen et al. [40] incorporated a high-temperature PEMFC with high-quality single-layer graphene (SLG), which improved the performance and durability. Their suggestion was to utilize monolayer graphene to manage phosphoric acid leaching and hydrogen crosslinking. The membrane electrode assembly (MEA) that was loaded with SLG at different positions after accelerated stress tests (ASTs) exhibited lower electrode resistances, higher electrochemical active surface areas, and higher peak power densities compared to pure polybenzimidazole membranes highly doped with phosphoric acid. MEAs with the SLG-loaded cathode and anode achieved a peak power density of 480 mW cm⁻² after AST, while those based on pure membranes reached 249 mW cm⁻². To investigate the impact on hydrogen crosslinking and control phosphoric acid leaching, the researchers tested samples with SLG on an ultra-thin membrane (7.5 µm). After 100 h of galvanostatic discharge, the hydrogen crosslinking of single-layer graphene-loaded samples on the anode did not exceed 1.75 × 10⁻⁴ mol s⁻¹, a value much lower than that of MEAs made using ultra-thin membranes with no graphene (8.16 × 10⁻⁴ mol s⁻¹).

Martini et al. [41] reported that they developed and applied suitable electrocatalysts using Co and Mo oxides with or without MoSe₂, combined or not with graphene nanoribbons (GNRs), which can support small amounts of Pt and are highly efficient for OER (oxygen evolution reaction) and HER (hydrogen evolution reaction). Based on the findings, CoMoSe/GNR was determined to be the most effective and durable electrocatalyst for OER. The observed improvements in the OER electrocatalytic properties were attributed to the synergistic effects of binary metal ions, including Co²⁺, Co³⁺, Mo⁴⁺, and Mo⁶⁺ (transition metals intercalated into nanostructures, with Co²⁺ and Mo⁴⁺ acting as active sites), along with interactions between the metals and ionic support (charge transfer, geometric effects) and between the different oxides used. The results showed that the CoMoSe/GNR/CP electrode exhibited the best OER electrocatalytic responses with η = 330 mV at 10 mA cm⁻² iR-free, overpotential (η) = 240 mV at 10 A g⁻¹ iR-free, and a current density of 10.4 mA cm⁻² at ηECSA@330 mV. Compared to HO⁻, the CoMoSe/GNR electrocatalyst for OH oxidation mainly produced O₂ (OER). The observed enhancements in the OER electrocatalytic properties for CoMoSe/GNRs were attributed to the synergistic effects of binary metal ions, including Co²⁺ (active site), Co³⁺, Mo⁴⁺ (active site), and Mo⁶⁺ (transition metals intercalated in the nanostructures), along with the interaction between different oxides and metal/ion-support interactions (charge transfer; geometrical effects). The long-term stability test for OER led to just a 20 mV increase in η at 10 mA cm⁻² iR-free for the CoMoSe/GNR/CP electrode, confirming the electrode’s stability.

In a study by Ji et al. [42], a combination of materials including reduced graphene oxide, nitrogen-doped electrochemical exfoliate, and carbon black-bearing platinum (Pt/NrEGO₂-CB₃) was used to enhance the durability and performance of elevated-temperature PEMFCs while using a lower amount of Pt. The study found that Pt/NrEGO₂-CB₃, with its strong interaction between Pt and nitrogen (N), prevented the accumulation of Pt particles, which were measured at 5.46 ± 1.46 nm, a smaller size compared to 6.78 ± 1.34 nm in Pt/C. The effective surface area (ECSA) for Pt/NrEGO₂-CB₃ decreased by 13.65% after an accelerated stress test (AST), decreasing from the initial value of 97.99% in Pt/C. Additionally, this study observed that NrEGO flakes in MEA_ac acted as a barrier, reducing the redistribution of phosphoric acid and improving the formation of triple-phase boundaries (TPBs), leading to the stable operation of MEA_ac with a degradation rate of less than 0.02 mV h⁻¹ within 100 h. After reaching a balanced state, Pt/NrEGO₂-CB₃ demonstrated the highest power density of 0.411 W cm⁻², three times higher than the maximum conventional power of Pt/C (0.134 W cm⁻²) in high-temperature PEMFCs. Furthermore, during AST, the mass transfer resistance of the Pt/NrEGO₂-CB₃ electrode was found to be lower than that of Pt/C (0.560 Ω cm² and 0.728 Ω cm², respectively). Overall, over a period of 70 h, this study showed that MEAAC (Pt/NrEGO₂-CB₃ at both the anode and the cathode), MEAC (Pt/C at the anode and Pt/NrEGO₂-CB₃ at the cathode), and MEAAC (PT/NREGO₂-CB₃ at the anode and PT/C at the cathode) demonstrated stability and minimal decrease at 0.3 Acm⁻², while MEA0 (Pt/C at both the anode and cathode) gradually decreased from 0.411 V to 0.288 V following AST. MEA_c showed stability at a higher current density but experienced a faster drop in Open-Circuit Voltage (OCV) after the third cycle. MEA_ac has a higher OCV compared to other MEAs. Additionally, MEA_o (0.890 V) and MEA_a (0.768 V) initially had higher OCVs, which could be attributed to the weak oxygen reduction reaction (ORR) activity of Pt/C, resulting in a reduced potential between the anode and cathode.

Ji et al. [43] reported synthesizing reduced electrochemically exfoliated N-doped graphite support materials and hybrid graphene oxide/carbon black (NrEGO-CB) as well as studying the effect of nitrogen doping to achieve improved durability and performance for the hydrogen fuel cell. The performance of the MEA was tested in a single cell at temperature values of 40 °C, 50 °C, and 60 °C with a relative humidity of 100% and with 100 mL min⁻¹ of H₂ and O₂.

Two distinct ASTs were performed to evaluate the corrosion of the support carbon and the electrocatalyst degradation. The results of the in situ AST show that through the NrEGO-CB, the materials present improved ORR activity by controlling the size of Pt nanoparticles and the corrosion resistance of carbon with elevated electrochemical stability in the fuel cell (with a maximum power density value of 1.18 W cm⁻² at a maximum current density of 2.22 A cm⁻²). Thus, the authors found potential in improving fuel cell durability and performance through its use as an advanced electrocatalyst support.

Gomez et al. [44] introduced a new catalytic ink for making electrodes in open-cathode fuel cells. They integrated a small amount of graphene-nanoplatelet (GN)-doped epoxy into the catalytic ink. This was carried out because the epoxy has excellent mechanical properties and has good chemical stability in acidic and basic environments, and it can reduce the risk of water flooding in the electrodes due to its hydrophobic nature. Additionally, inexpensive graphene nanoplatelets (GNs) were added to the epoxy to enhance the electrical conductivity of the final composite and overcome its disadvantage of being an electrical insulator. The authors created four different electrodes using electrospray deposition and then analyzed them using different catalytic inks. They used Nafion and graphene-nanoplatelet-doped epoxy as cohesive agents. They studied the behavior of the electrodes in a single proton electrolyte membrane of open-cathode fuel cells. They found that adding graphene-doped epoxy as a binder improved fuel cell performance and increased the mechanical stability of the electrode. This helped to avoid reducing the fuel cell’s durability and losing the catalyst during the handling of the electrode in the fuel cell assembly process.

The proton conductivity of a graphene oxide (GO) membrane (GOM) is much lower than that of a high-performance polymer electrolyte, such as Nafion^®. When the GOM comes into contact with hydrogen gas in fuel cells, the loss of surface functional groups due to hydrogen reduction is accompanied by increased electronic conductivity. To improve the proton conductivity of GOM, further research is needed.

Chowdury et al. [15] used (3-mercaptopropyl) trimethoxysilane (MPTS) to react with graphene oxide (GO) to form MPTS-modified GO (M-GOM). Excess MPTS was then incorporated as a binding agent for the composite electrolyte (MGC), which further increased proton conductivity. The synthesized membrane was tested in a single-station fuel cell at a temperature of 40 °C and 100% relative humidity (RH) without backpressure and hot-press membrane electrode assembly (MEA). Both MGC and Pd/MGC-based MEAs were found to enhance fuel cell efficiency without hot pressing, as they provided uniform pressure on the MEA, as compared to regular hot-pressed Nafion^®-based MEAs.

Recent advancements in graphene oxide (GO)-based nanocomposites for proton exchange membranes (PEMs) have focused on improving proton conductivity, self-moistening, gas permeability, and stability. These advancements have improved proton conduction by providing a large surface area of oxygen-based functional groups for proton hopping and enhanced water retention abilities, particularly under dry conditions. Incorporating GO with Nafion has shown improved electrical and mechanical properties, with potential microstructural changes for proton exchange membrane fuel cells (PEMFCs).

Marinoiu et al. [45] created Pt-rGO-based membrane electrode assemblies (MEAs) and studied their durability without fuel, comparing them to commercial Pt/C-based MEAs. Their analysis of the electrocatalytic activity showed that the Pt-rGO MEAs exhibited higher stability during accelerated degradation tests without fuel compared to commercial Pt/C MEAs. When comparing the performance of the proton exchange membrane fuel cells (PEMFCs) containing commercial Pt/C and Pt/rGO, it was found that the commercial catalyst displayed better responsiveness than the prepared Pt/rGO MEA. The commercial catalyst showed higher power density values: 0.62 W/cm² at 1.15 A cm⁻² (BoL—beginning of life), 0.58 W cm⁻² at 1.09 A cm⁻² (intermediate stage), and 0.49 W cm⁻² at 1.05 A cm⁻² (EoL—end of life). The maximum power densities for Pt/rGO were slightly lower: 0.52 W cm⁻² at 1.18 A cm⁻² (BoL), 0.49 W cm⁻² at 1.16 A cm⁻² (intermediate stage), and 0.46 W cm⁻² at 1.13 A cm⁻² (EoL). Although the maximum power density of Pt/rGO-based MEAs was slightly lower than that of Pt/C-based MEAs, the degradation in Pt/rGO preparation was significantly lower at about 10% compared to approximately 19% for Pt/C. This improved performance is likely due to the unique structure of the rGO support, which led to a considerable decrease in mass transport losses in the third region.

Jyoti et al. [46] discussed the potential applications of three-dimensional graphene–carbon nanotube hybrid materials (3D-GCNTs) for storage devices and energy conversion. They highlighted the progress in this emerging field. The maximum power density delivered by the Pt-GNS-CNT hybrid fuel cell was 1072 mW cm⁻². Studies have reported that graphene–CNT hybrid composites used in oxygen reduction reaction (ORR) electrocatalysts have good durability, high stability, and high activity in both alkaline and acidic solutions. The performance of the PEMFC electrochemical using porous graphene/CNT hybrid composites demonstrated an improvement in power density due to the smaller charge transfer resistance of the graphene/CNT electrode composites compared to Pt-based graphene electrodes. The electrochemical active surface area (ECSA) of Pt in Pt-graphene/CNT hybrid composites was recorded at 38.2 m² g⁻¹, a value exceeding that of Pt in graphene cathodes (34.5 m² g⁻¹). The improved PEMFC performance using graphene/CNT composites fabricated by the CVD technique was studied, and it was found that the Pt/graphene/CNT cathode displayed a maximum power density of 1071.8 mW/cm² due to low electron transfer resistance. The mass ratio of rGO and PMWCNTs (MWCNT-polyethyleneimine) of 1/1 showed excellent performance (1200 cycles) and better durability compared to the other composites with 1/3 and 3/1 ratios. The power density of Pt/rGO, Pt/PMWCNT, Pt/1rGO-3PMWCNT, Pt/1rGO-1PMWCNT, and Pt/3rGO-1PMWCNT was 0.26, 0.44, 0.56, 0.68, and 0.48 W cm⁻², respectively.

In a study by Barik et al. [47], graphene oxide (GO) functionalized with acidic groups was used as filler material in a Nafion electrolyte for next-generation fuel cell applications. They introduced a potential Nafion-modified one-step phosphorylated graphene oxide (sPGO) (sPGO/NF) to enhance Nafion’s power density, proton conductivity, and chemical durability. The sPGO/NF demonstrated a maximum proton conductivity of 0.306 S cm⁻¹ under real fuel cell operating conditions, which was 1.7 times and 1.6 times higher than those of rNF and GO/NF, respectively. The sPGO/NF also achieved a maximum power density of 0.652 W cm⁻² (at 80 °C and 100% RH), surpassing the power densities of rNF (0.51 W cm⁻²) and GO/NF (0.53 W cm⁻²) under similar conditions. The fuel cell’s performance trend aligned with the proton conductivity results. The authors attributed the low energy yield of the GO/NF membrane to the limited occurrence of hydrophilic oxygen moieties and low resistance compared to sPGO/NF. They also noted that rNF, which only has specific sulfonic functions, exhibited the observed energy yield.

Chen et al. [48] synthesized phosphonate graphene oxide (PGO) with a P/O ratio of 8.5% using a one-step electrochemical exfoliation process in a three-dimensionally (3D)-printed reactor using natural graphite flakes. The authors observed that synthesizing PGO through one-step electrochemical exfoliation resulted in better performance of the membrane electrode assembly (MEA) based on the polybenzimidazole (PBI) membrane in high-temperature proton exchange membrane fuel cells (HT-PEMFCs) compared to graphene oxide (GO) prepared using a two-step electrochemical exfoliation method. The results indicated that doping 1.5 wt% GO synthesized by electrochemical exfoliation with the two-stage process or the reactor method in PBI increased the peak power density by 17.4% and 35.4%, respectively, compared to the pure PBI membrane MEA at a temperature of 150 °C. Furthermore, doping PGO in PBI showed improved durability under accelerated stress testing (AST).

Developing durable, high-temperature PEMFCs with polymer electrolyte membranes that have highly stable phosphoric acid loading continues to be a challenge. A new class of highly conductive and durable composite membranes is being developed for high-temperature application in fuel cells under anhydrous conditions. 2,6-pyridine-functionalized polybenzimidazole (Py-PBI) is used as a substrate to host the phosphoric acid moiety.

Lotf et al. [49] studied the performance of fuel cells using phosphonate graphene oxide (PGO) membranes doped into the Py-PBI substrate at different levels before acid doping. They found that the performance of the fuel cells with PGO composite membranes was better than the performance of the original acid-doped 2,6-pyridine-functionalized polybenzimidazole (Py-PBI) membrane, despite similar initial acid doping levels. The peak power density values were 331 mW cm⁻² for Py-PBI/1.0% PGO and 359 mW cm⁻² for Py-PBI/1.5% PGO, while the power density value for Py-PBI doped with the original PA was 202 mW cm⁻². This improvement in performance was attributed to the superior stability of polyphosphoric acid (PA) in the structure of PGO-containing membranes.

Simari et al. [50] tested nanocomposite membranes based on graphene oxide (GO) and its organo-sulfonate derivative (SGO) dispersed in polysulfone sulfonate (PSU) in a single PEMFC operating under harsh conditions. They found that only SGO nanoplatelets significantly improved PEMFC performance, with sPSU-SGO providing a peak power density of 216.1 mW cm⁻², surpassing the performance of the reference Nafion 212 cell. The composite membrane also provided a peak current density 10% higher than that of Nafion 212, efficiently enlarging the mass transport region.

The dependence of fuel cell parameters on membrane composition can be seen in Table 1. The ion exchange capacity for the membranes mentioned fluctuated between 0.09 and 1.86 meq g⁻¹, with the highest value being observed in the Nafion coating with 4.5% graphene. The water absorption capacity showed values ranging from 18.18 8 meq g⁻¹ (Nafion+Pd/GOM) to 33.05 5 meq g⁻¹ (3-mercaptopropyl) trimethoxysilane MPTS-(G).

3. The PEMFC Cathode In Situ Stability Is Based on the Pt Degradative Catalyst in the Functioning of MEA

PEMFCs are electrochemical energy sources that can convert chemical energy directly into electrochemical energy, such as H₂. The reaction rate in PEMFCs is primarily based on the ORR, which is considerably slower than the HOR at the anode. The ORR necessitates a greater proportion of the Pt-based catalyst than HOR, unlike the rate-determination stage (RDS) [51]. The manufacturing cost of PEMFCs depends on the price of Pt-based catalysts. PEMFC stability is deteriorated most notably because of the degradation of carbon-backed Pt catalysts used at the cathode. The main goals of preparing Pt cathode catalysts involve minimizing the use of Pt as a precious metal and increasing the sustainability of the catalyst. Researchers have introduced various transition metals into Pt as the main crystal structure to create catalysts that are based on Pt-based alloys for ORRs over the years. Producing Pt electrocatalysts with high dispersion and efficiency is achieved by using Pt nanoparticles on a backing material. The electrocatalyst support is crucial in PEMFCs and can impact its physical properties (particle size, dispersion, and specific surface area) as well as its electrochemical properties (activity and stability). The properties of the support materials can influence the performance, stability, and catalytic activity of the catalysts toward the oxygen reduction reaction. Adequate support is crucial for the dispersion and binding of nanoparticles to the catalyst, increasing the active area of the catalyst, and developing activity and stability [52,53].

To reduce PEMFC costs, the main goal should be achieved by a catalyst support that possesses high conductivity, large active specific surface area, adequate pore structure, high conductivity, and good electrochemical and thermal stability.

Types of Support Materials That Contribute to the Stability of the Pt-Based PEMFC Cathode

The catalyst performance of carbon-based support materials is crucial because they facilitate interactions between the metal and support. They also enhance the stability, durability, and activity of the catalyst [54]. Carbon, with its various forms and textures, is commonly used in electrochemical applications due to its morphological characteristics. In order to improve the performance of Proton Exchange Membrane Fuel Cells (PEMFCs), the catalyst support needs to have a large area to distribute the catalyst evenly. The number of electrocatalysts varies depending on the cell’s geometry, operating state, and catalyst size. Carbon-based materials like graphene and carbon nanotubes (CNTs) are commonly employed as catalyst supports for PEMFCs. Additionally, combining carbon-based supports with non-carbonaceous supports such as silica and ceramics has shown promising results in PEMFCs. The performance of electrocatalysts is also influenced by the interaction between the substrate and the catalyst, which can alter the electronic structure of the catalyst’s surface [53].

Chen et al. [55] explained that among all the support materials researched for use, carbon is the most desirable. Carbon has distinct characteristics such as high stability in acidic and basic environments, even at high temperatures, as well as physical mass transfer, controllable pore structure, modifiable surface chemistry, and low cost. Another important characteristic for high-performance electrocatalysts is high electron conductivity. The authors addressed the current advancement of Pt/nanocarbon support electrocatalysts for PEMFCs, emphasizing the influence of pore structure, heteroatom doping, and carbon-based functional supports on electrochemical functionality.

Sun et al. [52] provided information about research focused on reducing or eliminating the amount of Pt in cathodes, leading to catalysts based on the low use of platinum group metals and alternative platinum-free catalysts. A major challenge in commercializing platinum-free catalysts is increasing their durability and improving their catalytic activity under MEA operating conditions. To enhance catalytic activity, porous structures with micropores, mesopores, and macropores are desired. The stability and output power density of platinum-free catalysts are significantly influenced by the porous structure and surface properties of the Pt/carbon support. The stability of the active sites of the carbon support is closely related to its surface properties and pore structure. Optimizing the surface properties and pore structure of the carbon support can be adapted through the configuration and distribution of the ionomer in the MEA fabrication to reduce the local diffusion of oxygen. To improve the corrosion resistance of the carbon support, increasing the degree of graphitization can effectively mitigate the three-phase limit, the risk of water flooding, and the demetallation of active sites. Developing MEA-specific in situ characterization techniques for optimizing the cathode layer can help establish the mechanism of influence of different parts of the cathode layer under the working conditions of PEMFCs.

Sui et al. [56] developed a new method for incorporating Pt nanoparticles (Pt-NPs) into a carbon matrix and growing in situ Pt nanowires (Pt-NW). This innovative approach can be applied to other alloy electrocatalysts or alloy nanowires. To prevent the aggregation of Pt nanowires, the authors created low-energy interfaces for the Pt nucleation of Pt nanoparticles, which then led to the growth of Pt nanowires. Excessive Pt nanoparticles were found to decrease the length and crystallinity of Pt nanowires, affecting the overall structure. The matrix carbon content was found to influence the matrix thickness and the Pt profile. The high performance of Pt-NW cathodes can be attributed to several factors: the uniform growth of Pt-NWs induced by Pt-NPs, which is from the high use of Pt due to the fact that Pt-NW grows directly on the wall of the pores and is completely exposed to oxidants, as well as the dominant facets of Pt-NW with the high-catalytic-activity ORR. A Pt-NP load of 0.005 mg cm⁻² and a carbon load of 0.02 mg cm⁻² were achieved by the Pt-NW cathode, which was the best.

Sahoo et al. [57] observed a significant improvement in the performance of a hybrid carbon structure composed of multi-dimensional (1D) carbon nanotubes (MWNTs), and two-dimensional (2D) graphene sheets were observed when used as a catalyst support material for proton exchange membrane fuel cells (PEMFCs). In their experiments, longitudinally cut carbon nanotubes with multiple walls were used to obtain partially exfoliated nanotubes (PENTs), which served as the support material for platinum deposition. The Pt/PENT catalysts were employed for the oxygen reduction reaction (ORR) in PEMFCs. A comparison was made between cathode materials, namely Pt/C and Pt/PENT, revealing an improved performance of 1000 mW cm⁻² for Pt/PENT compared to 418 mW cm⁻² for a commercial Pt/C cell. This enhancement was attributed to the specific structure of the support material, which provided higher conductivity and an improved effective surface area.

Ortiz-Herrera et al. [53] discuss the influence of support materials on the catalytic activity, stability, and performance of catalysts. They focus on the modification of carbon nanotubes (CNTs) as a support for Pt-based cathode catalysts in PEM-FCs. CNT-supported catalysts show improved electrochemical oxidation resistance, performance, and durability compared to carbon black-supported catalysts. The ORR’s catalytic activity, stability, and performance can be modified by the support material and its dispersion effect. Important properties of a good support material include good conductivity, high porosity, high surface area, and great corrosion resistance. Pure carbon nanotubes have low chemical reactivity, but modifications such as functionalization, doping, and hybrid supports (CNT metal oxide) can enhance their properties. CNTs are composed of a laminated graphene sheet closed at both ends and can be classified into single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) based on the number of graphene sheets. Functionalized Pt/CNT-based nanoparticles have a negative impact on their catalytic activity and stability, which is influenced by the preparation method and degree of functionalization. The functionalization of CNTs with acids (H₂SO₄ and HNO₃) is particularly useful, and the degree of functionalization can be adjusted by controlling the amount and type of acid used.

Etesami et al. [54] discussed the need for environmentally friendly conversion devices using PEMFCs. However, they mentioned the scarcity and high cost of Pt for both the cathode and anode. The authors also talked about improving the catalyst support, the structural and geometric development, and narrowing the catalyst dimensions to enhance performance. They also mentioned depositing an atomic layer (ADL) in the synthesis of the catalysts to ensure high interaction between the Pt layer/carbon support and improved stability of the catalysts during operation. ALD was used to deposit platinum on GDL to create a membrane electrode assembly (MEA). The parameters obtained in this case were a high mass activity of 4.80 kW g⁻¹ Pt and a maximum power density of 0.864 W cm⁻². The loading amounts of the catalysts must be above 0.05 mg cm⁻² Pt per anode and 0.2 mg cm⁻² Pt per cathode at a current density below 1 A cm⁻² in PEMFCs to prevent voltage drops. The size value of Pt nanoparticles has a significant influence on PEMFCs. The Pt nanoparticle size values for the synthesized samples were 1.9 nm, 3.2 nm, 7.1 nm, and 12.7 nm. The minimum initial performance of Pt nanoparticles was observed at 12.7 nm, and their degradation was slowest. The use of Pt/C, AuPt, PtNiCo/NC, and PtZn nanoparticles as cathode materials is important for generating power densities higher than 1 W cm⁻² in PEMFC systems. The commercialization and price of PEMFC hydrogen are influenced by the following parameters: activity, durability, and cost of electrocatalysts.

Cao et al. [58] presented data that were provided on low Pt loading in MEAs for PEMFCs, covering both the ORR electrocatalysts and CL (catalyst loading) structure in MEAs at various levels. Nanocatalyst alloying, morphology design, and catalyst support development are the three parts of ORR electrocatalyst progress. To optimize the structure and composition of the catalyst, the support materials are particularly important for improving the catalytic performance. The combination of Pt-based nanoparticles loaded on a carbon support as a catalyst support is the most commonly adopted in MEA. The detachment and agglomeration of metal particles are a result of the vulnerability of carbon-based supports to oxidation and corrosion reactions in harsh conditions. To reduce this inconvenience, it is proposed to introduce transition metal oxides as a support material. Thus, the excellent durability of the TiNiN@Pt catalyst can be attributed to the good stability of the nitride and the synergistic effect between the Pt atoms and the stable TiN supports. Other ways to increase the stability of the catalyst include using highly graphitized nanocarbons as a support, such as carbon nanotubes, carbon nanocages, and graphene. Their oxidation and corrosion are less severe than those of traditional supports.

The degradation of catalysts in MEA operation involves the deterioration of Pt-based ORR catalysts, including the degradation of Pt-based nanoparticles and corrosion of the carbon-based support [59,60]. The tendency of metal nanoparticles (platinum) to dissolve can be estimated by analyzing the thermodynamic properties of the nanoparticles. The reduction in ORR activity, catalyst stability, and active sites of platinum is caused by the increase in particle size [61,62]. The thickening of Pt particles can occur in several situations: when the prepared Pt nanoparticles do not have the same dimensions, when the Pt nanoparticles are not perfectly bound to the support and sinter together to form larger particles, and when the platinum particles dissolve and unite to form larger particles due to the corrosion effect of the carbon support [63,64]. The carbon support is the most commonly used Pt-based support electrocatalyst due to its high electrical conductivity, large surface area, and abundant porous structure [65,66,67].

Figure 3 shows the performances of the graphene-based catalysts analyzed in this study, with reference to electrochemical surface area (ECSA), current density, and power density.

The performance of graphene-based catalysts is depicted in Table 2 and Figure 3. The electrochemically active surfaces of the Pt-Fe/GNP and Pt-Ni/GNP catalysts were observed to have high values, ranging from 132 and 136 m² g⁻¹, during 100 operating cycles. The Pt₃-Ni/G and Pt-Co/G catalysts employed at the cathode exhibited high current densities and power densities. The current density was recorded at 1.590 and 1.779 A cm⁻², and the power density was at 0.57 and 0.785 Wcm⁻² accordingly. The catalytic materials based on Au and RGO (PtAu/RGO and PtAu/rGO+CNT at the cathode and Pt/C at the anode) achieved the lowest current density performance and power density performances, respectively. The current density measured 0.134 and 0.247 A cm⁻², whereas the power density measurement measured 0.07 and 0.128 W cm⁻². The Au/u/RGO PtNW-Pd/d/RGO had the lowest values for ECSA, with the Au/u/RGO PtNW-Pd/d/RGO exhibiting a value of 12.9 and 19 m² g⁻¹, respectively.

4. Conclusions

The importance of electrocatalysts cannot be overstated when it comes to fuel cell performance. Pt-based materials are commonly employed as catalysts, but due to their high cost and limited availability, alternative catalysts have been investigated to make fuel cells commercially viable. When catalysts are combined with other supports, performance can be enhanced. Graphene, which has multiple advantages, is one of the support options that has gained significant attention and has been explored for Pt loading. Our objective is to present this summary to support the investigation and judicious design of graphene-based nanostructures for efficient ORR electrocatalysis. PEMFCs are facing major obstacles such as the slow kinetics of the oxygen reduction reaction (ORR) and the high cost of precious metals used in fuel cell reactions, particularly platinum. Also, there are problems with the catalysts for obtaining electrodes, such as insufficient performance and durability, and the degradation of the carbon-supported material. Pt catalysts in the cathode have a significant impact on the stability of PEMFCs. Pt/C is commonly utilized as a catalyst for both ORR and the hydrogen oxidation reaction (HOR). In order to solve this problem, this study recommends using graphene instead of carbon black in the catalyst layer. This review examines the efficacy of graphene-based catalysts in the PEMFC cathode and the stability of Pt-based PEMFC cathodes, as well as the degradation of the catalysts during MEA operation. The results obtained from analyzing graphene-based catalysts showed that Pt-Fe/GNP and Pt-Ni/GNP catalyst surfaces had a higher electrochemically active surface, with values ranging from 132 to 136 m² g⁻¹ during 100 operating cycles. In the cathode, the Pt3-Ni/G and Pt-Co/G catalysts are capable of exhibiting high current densities and power densities. The current density was recorded at 1.590 and 1.779 A cm⁻², while the power density was recorded at 0.57 and 0.785 W cm⁻². PtAu/RGO and PtAu/rGO+CNT at the cathode and Pt/C at the anode received the lowest current density and power density performances, respectively. The current density measured 0.134 and 0.247 Acm⁻², while the power density measured 0.07 and 0.128 W cm⁻². Au/u/RGO PtNW-Pd/d/RGO had the lowest ECSA values, with Au/u/RGO PtNW-Pd/d/RGO exhibiting a value of 12.9 and 19 m² g⁻¹, respectively.

In conclusion, for an effective commercialization of PEMFCs, a reduction in the Pt content is required, as well as modifications in nanoparticles to improve the durability of the fuel cells, and to prevent the agglomeration of metal nanoparticles, polymeric materials can be used as coating agents. The high performance parameters for the fuel cell can be achieved by using graphene’s excellent properties and the synergy of doped heteroatoms.

Author Contributions

Conceptualization, A.M.; methodology, A.M. and M.I.; software, A.M.; vali-dation, A.M., M.I. and A.O.; formal analysis, A.M., M.I., A.O. and E.S.B.; investigation, A.M., M.I., A.O. and E.S.B.; resources, A.M.; data curation, M.I.; writing—original draft preparation, A.M.; writing—review and editing, A.M., M.I. and A.O.; visualization, A.M.; supervision, A.M.; project administration, A.M.; funding acquisition, A.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within this article.

Conflicts of Interest

The authors declare no conflicts of interest regarding this work.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. ORR catalyst diagram.

Figure 2. A schematic representation of a typical membrane electrode assembly (MEA).

Figure 3. Performance of graphene-based electrocatalysts in PEMFCs. (a) Electrochemical durability (ESCA); (b) the MEA cathode catalyst’s current density; (c) the MEA cathode catalyst has a high power density.

Figure 3. Performance of graphene-based electrocatalysts in PEMFCs. (a) Electrochemical durability (ESCA); (b) the MEA cathode catalyst’s current density; (c) the MEA cathode catalyst has a high power density.

Table 1

The dependence of fuel cell parameters on membrane composition based on graphene and its derivatives.

Polymers	Composite Membranes	Ion Exchange Capacity (IEC)(meq g−1)	Water Uptake (%)	OCV(V)	Power Density (mW cm−2)/mA cm−2	Reference
Nafion	4.5 wt% (G)	1.86	32.86	-	490 at 120 °CmW cm−2	[16]
Nafion	4.0 wt% (G)	-	-	-	415 at 120 °CmW cm−2
Nafion	5 to 10 wt% (G)	0.96	28.75	-	150 at 120 °CmW cm−2
Nafion	GO membrane (GOM)	0.09	35.0	-	3 at 40 °CmW cm−2	[15]
Nafion	(3-mercaptopropyl) trimethoxysilane (MPTS)-GOM	1.15	36.05	-	26 at 4 °CmW cm−2
Nafion	1 wt% MPTS-GOM	1.20	21.0	-	23.25 at 40 °CmW cm−2
Nafion	5 wt% MPTS-GOM	1.20	23.54	-	27.25 at 40 °CmW cm−2
Nafion	10 wt% MPTS-GOM	1.23	24.02	-	33 at 40 °CmW cm−2
Nafion	50 wt% MPTS-GOM	1.27	25.88	-	56.45 at 40 °CmW cm−2
Nafion	50 wt% MPTS-GOM	1.42	24.23	-	-
Nafion	Pd/GOM (GOHM)	-	18.18	-	21 at 40 °CmW cm−2
Nafion	Pd/MPTS-GOM (GOHM)	1.11	18.20	-	43 at 40 °CmW cm−2
Nafion	Pd/50 wt% MPTS-GOM (GOHM)	1.30	27.77	-	73.80mW cm−2
Nafion/sGO/Fe3O4	3 wt% of sulfonated GO	1.36	35.63	-	783.7mW cm−2	[16]
Nafion	Pt/rGO	-	-	0.89	520 at 1.18mA cm−2	[45]
Nafion	Pt/graphene/CNT	-	-	-	1071.8mW cm−2	[46]
Nafion	Pt/PMWCNTPt/rGO-functionalized MWCNT-polyethyleneimine	-	-	-	440.0mW cm−2
Nafion	Pt/1rGO-1PMWCNT (mass ratios 1:1)	-	-	-	680.0mW cm−2
Nafion	PtNi/GNP	-	-	-	378 at 632mA cm−2	[18]
Nafion	PtFe/GNP	-	-	-	293 at 489mA cm−2
Nafion	PtCu/GNP	-	-	-	256 at 428mA cm−2
Nafion	Pt/NrEGO2-CB3	-	-	-	411.0mW cm−2	[42]
Nafion	Py-PBI/1.0% PGO	-	-	-	331.0mW cm−2	[49]
Nafion	Py-PBI/1.5% PGO	-	-	-	359.0mW cm−2
Nafion	sPSU-SGO	1.51		0.984	216.0mW cm−2	[50]

Table 2

Performances of graphene-based electrocatalysts for PEMFCs.

Catalyst MEA		Potential(V)	Current Density(A cm−2)	Power Density(W cm−2)	ECSA(m2 g−1)	Electrochemical Durability (h)	Reference
Anode	Cathode
67.7% Tanaka	Pt-Fe/GNPs	0.6	0.490	0.293	132	100	[18]
	Pt-Ni/GNPs		0.632	0.378	136
	Pt-Cu/GNPs		0.428	0.256	122
Pt-Fe/GNPs	67.7% Tanaka	0.6	0.643	0.385	120
Pt-Ni/GNPs			0.839	0.502	95
Pt-Cu/GNPs			0.700	0.419	60
Pt-Fe/GNPs	Pt-Fe/GNPs	0.6	0.414	0.247	110
Pt-Ni/GNPs	Pt-Ni/GNPs		0.615	0.368	123
Pt-Cu/GNPs	Pt-Cu/GNPs		0.322	0.193	65
Pt/GNPs	Pt/GNPs		0.503	0.301	-	-
Pt/C	Pt/rGO	0.5	0.032	0.020	-	-	[53]
Pt/C	PtAu/rGO		0.134	0.070	-	-
Pt/C	PtAu/rGO+CNT		0.247	0.128	-	-
Pt/C	PtAu/rGO +PtAu/CNT		0.691	0.365	-	-
Pt/C	Pt–CeO2-GO			0.50	-	-
Pt/C (20%)	Pt/G	0.9	1.226	0.46	98.15	4	[39]
Pt/C (20%)	Pt3-Ni/G		1.590	0.57	108.56
Pt/C (20%)	Pt/C		1.1975	0.36	84.92
Pt-Ru/C	Pd3-Co1/G	0.59	0.200	-	-	-	[38]
Pt-Ru/C	Pd3-Co1/G-C			-	120.154	50
Pt-Ru/C	Pd3-Co1/C			-	36.0
-	Pt/NG	0.95	0.300	-	34.7	120
Pt/C	Au@AuPd-rGo	-	-	0.362	-	-	[64]
-	Pt-Co/G		1.779	0.785	-		[17]
-	PtNW-Pd/rGO	-	-	0.226	19.0	200
-	Pt/rGO-O	-	0.5	-	51.0	-
-	Pt/rGO-FCB	-	-	1.344	62.0	5000
-	Au/rGO	-	1.4	0.65	12.9	9200

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