1. Introduction
Fuel cells offer a viable alternative to the non-renewable energy sectors, such as petroleum and natural gas, by harnessing catalysts to accelerate the oxygen reduction reaction (ORR) crucial for their operation. Platinum (Pt) catalysts are favored for their high ORR efficiency and minimal overpotential [1]. Nevertheless, Pt’s high cost poses significant economic challenges for widespread commercial adoption [2]. The escalating global demand for energy, driven by a growing population, higher living standards, and the ubiquity of portable devices, has led to an increased reliance on fossil fuels, which account for more than 85% of the energy used worldwide for stationary and automotive purposes [3]. This reliance on fossil fuels, which are depleting while also damaging the environment through contributions to global warming [4], highlights the urgent need for a shift toward alternative and renewable energy sources [5]. Efforts to utilize geothermal, hydro, wind, and solar power for energy production are underway, paralleled by research into batteries, fuel cells, and electrochemical supercapacitors for efficient energy storage [6,7,8]. Fuel cells stand out among alternative energy storage technologies, offering higher energy density compared to both flow batteries and lithium-ion batteries [9]. Although lithium-ion batteries boast portability, they are hindered by short lifespans, environmental pollution, and reduced effectiveness in extreme temperatures [10].
The interest in alternative energy systems is on the rise, with Proton Exchange Membrane Fuel Cells (PEMFCs) emerging as the leading technology in the field. PEMFCs are distinguished among fuel cell technologies for their superior energy conversion efficiency and power density, which ranges between 40% and 65% [11]. These fuel cells are also valued for their rapid startup and warmup times, ability to operate at relatively low temperatures (60 to 80 °C) [12], and their lightweight, compact design [13]. In PEMFCs, electricity is generated through an electrochemical reaction as protons move from the anode to the cathode. This setup includes a bipolar plate (flow field plate), a gas diffusion layer (GDL), and a catalyst layer (CL) at both the anode and cathode sides. At the anode, the hydrogen oxidation reaction (HOR) occurs, where hydrogen molecules are absorbed onto the catalyst layer, resulting in the detachment of electrons and the release of protons (H+). These electrons then make their way to the cathode via an external circuit, whereas the protons move through the membrane to reach the cathode. At this point, the oxygen reduction reaction (ORR) occurs, facilitating the production of electricity. The reactions within PEMFCs can be summarized as follows:
| Anode (HOR): | 2H2 → 2H+ + 4e− | E° = 0.00 V vs. SHE |
| Cathode (ORR): | O2 + 4H+ + 4e− → 2H2O | E° = 1.23 V vs. SHE |
| Overall reaction: | 2H2 + O2 → 2H2O | SHE = Standard Hydrogen Electrode |
The primary challenge faced by Proton Exchange Membrane Fuel Cells (PEMFCs) is the need for high loadings of noble metal catalysts, such as platinum (Pt) and its alloys, at the cathode to accelerate the inherently slow ORR. These catalysts are essential for both the hydrogen oxidation reaction (HOR) and the ORR, with Pt and its alloys currently recognized as the most effective electrocatalysts due to their high catalytic activity, electronic conductivity, low overpotential, and outstanding stability [14,15,16]. However, the high cost of Pt significantly hinders the mass commercialization of PEMFCs [17,18]. This has led to efforts to develop non-precious metal catalysts (NPMCs) aimed at replacing Pt-based catalysts for the ORR, with recent research making considerable progress in improving the ORR performance of NPMCs, particularly those based on iron and cobalt [19]. A carbon-based bifunctional electrocatalyst offers a more efficient approach to constructing superior electrocatalysts. This is likely achieved by combining specific heteroatom doping and engineered carbon defects, which simultaneously have positive effects. To create porous B and N co-doped nanocarbon (also known as B, N-carbon) materials that are composed of interconnected cuboidal hollow nanocages with fine graphitization and ample carbon defects, a convenient strategy for facile construction has been developed. The resultant nanocarbon material, which combines carbon defects and B and N co-dopants, is an extremely reactive and long-lasting electrocatalyst for the ORR and the oxygen evolution reaction (OER) [20]. These advances have positioned NPMCs as viable alternatives to Pt and its alloys.
Polydopamine (PDA) has emerged as a novel, bio-inspired material attracting significant attention for its unique properties and applications in energy, the environment, and catalysis [21]. PDA, a synthetic polymer, demonstrates a strong affinity for solid substrates through chemical bonds and physical interactions, courtesy of its functional groups like amines, imines, and catechol [22,23]. Its excellent biocompatibility and surprising properties in terms of optics, electricity, and magnetism [21], along with similarities to mussel proteins, have spurred interest in its application across various domains, including coatings, environmental and catalytic applications, and energy storage and conversion [24,25,26]. The synthesis of N-doped carbon materials typically involves direct reactions with nitrogen precursors or the carbonization of nitrogen-containing polymers, with PDA polymerization offering a straightforward method for creating carbonaceous nanostructures [27,28,29].
Cellulose, the most abundant natural polymer, presents an opportunity for the synthesis of N-doped carbonaceous materials, including those containing metals [30]. Nanocellulose (NC), derived from cellulose, is noted for its environmental friendliness and excellent mechanical properties attributed to its nano-scale structure. This has led to its widespread application in engineering and functional materials [31]. NC, with dimensions in the nanometer range, is obtained from a variety of sources including plants, algae, and bacteria, and is characterized by its high surface area, mechanical strength, and biodegradability, making it suitable for high-performance energy devices [32]. The increasing demand for renewable energy solutions has catalyzed research into NC-based conductive materials. Despite the advancements, the stability of NPMCs remains a challenge, preventing them from being considered direct replacements for Pt/C catalysts in PEMFCs [19].
In this manuscript, we discuss the synthesis and application of a cobalt and nitrogen-doped carbonaceous catalyst. The catalyst is synthesized by pyrolyzing a mixture of cobalt complex, NC, PDA, and urea at 500 °C under a nitrogen atmosphere, aiming to enhance the oxygen reduction reaction (ORR) efficiency in acidic fuel cells. We hypothesize that incorporating cobalt and nitrogen into the carbonaceous material matrix significantly boosts the electrocatalytic activity for the ORR in acidic environments, a critical aspect for fuel cell technology. The innovative aspect of our work lies in the synthesis strategy, which involves a unique blend of a well-dispersed cobalt complex, nanocellulose, and polydopamine, followed by pyrolysis with urea under an inert atmosphere. Characterization techniques such as SEM, XRD, and EDS have confirmed the presence of uniformly dispersed metal nanoparticles and an even elemental distribution. CV assessments demonstrated robust ORR activity, evidenced by a significant peak at 0.30 V against a reversible hydrogen electrode (RHE), particularly under acidic conditions and with uniformly layered electrodes. RDE experiments have further validated the four-electron reduction pathway of oxygen to water, highlighting the catalyst’s potential in improving fuel cell performance.
2. Results and Discussions
2.1. Synthesis
The PPh4+ salt of the cobalt complex and nanocellulose were mixed together in the presence of dopamine (hydrochloride salt), which was allowed to polymerize under an oxidative environment at a slightly elevated pH. The reaction mixture turned black, and a composite material precipitated, which is supposed to contain the cobalt complex along with nanocellulose coated with polydopamine. PDA plays a dual role: it is involved in the synthesis of the nanocellulose and cobalt complex nanocomposite, and it contains nitrogen atoms. Upon pyrolysis, these nitrogen atoms can dope the carbonaceous material, altering its structure and properties. After collecting the nanocomposite material post-polydopamine reaction, we added urea so that during pyrolysis, the carbonaceous material could be further doped with nitrogen atoms, which in turn can synergistically enhance ORR (oxygen reduction reaction) activities. NC contains several hydroxyl groups (-OH), which allow well dispersion and possible binding of the cobalt complex on its surface to enable uniform distribution of the catalytic site. NC also acts as a source of carbon when the catalyst is synthesized under pyrolysis. This unique synthesis route aims to achieve a homogeneous distribution of cobalt nanoparticles and nitrogen doping within the carbonaceous material matrix, hypothesized to synergistically improve the ORR performance by facilitating a more efficient four-electron reduction pathway of oxygen to water.
2.2. Characterizations
2.2.1. SEM and EDS
SEM characterization was conducted to assess the morphology of the carbonaceous material. The SEM images, presented in Figure 1a,b, depict the formation of the cobalt-doped material, characterized by aggregates of various shapes [33]. The SEM analysis also reveals that the carbon produced through pyrolysis exhibits flake-like structures with some porosity [34]. The surface features distinct white and gray areas, clearly distinguishable from each other. To verify the composition of the cobalt-doped carbonaceous material, EDS analysis was performed. Figure 1c displays the EDS results and the associated spectrum for the selected area, indicating the atomic percentages of elements. The EDS spectrum shows peaks for C, Co, and O, suggesting the formation of metal oxides. The higher atomic percentage of carbon (C) can be attributed to the use of carbon tape during the EDS analysis, where the powder sample was affixed to the sample holder with carbon tape. Furthermore, the presence of nitrogen in the sample is highly likely. It is assumed that the peak nearest to C corresponds to nitrogen.
2.2.2. XRD
X-ray Diffraction (XRD) characterization was conducted to investigate the phase, structure, and crystallinity of the pyrolyzed material. The XRD patterns reveal the presence of cobalt oxides. The XRD pattern, depicted in Figure 2, confirms the crystalline nature of the sample. The diffraction peaks corresponding to Co3O4 were observed at 2θ values of 27.33°, 31.66°, 45.43°, 56.45°, 66.23°, and 75.32°, which correspond to the (111), (220), (400), (422), (440), and (620) planes, respectively [35,36]. All of the diffraction peaks match with the COD database file number 1538531. The XRD spectrum suggests that Co3O4 has the preferential orientation along the (220) plane. To further confirm the phase and chemical composition, the Raman spectrum was collected (Supplementary Information, Figure S1) to support that Co3O4 is present in significant amounts, with trace amounts of CoO. Thus, even if CoO is not detected directly from XRD, both Co3O4 and CoO are produced with a spinel structure during pyrolysis, which are likely the active catalytic sites for the ORR. To determine the crystallite size of Co3O4 in the composite, the Scherrer equation was used. The equation is D = Kλ/β cosθ, where D represents the crystal size, K is the Scherrer constant, λ is the wavelength of the X-rays, β is the full width at half maximum (FWHM) of the peaks in radians, and θ is the Bragg angle. The average crystal size for Co3O4 was calculated to be 20.3 nm. The distinguishable (002) peak at 2θ = 26.5° signifies the graphitic carbon formed in the material [1,37]. The XRD findings indicate that cobalt oxide constitutes the primary phase in the oxidized form of the prepared carbonaceous material, with no evidence of additional oxide impurities.
2.2.3. XPS Study
X-ray Photoelectron Spectroscopy (XPS) analysis was performed on the pyrolyzed carbonaceous material to determine its surface elemental composition and the chemical states of carbon and cobalt (Co), which are crucial for its electrocatalytic activity. Initially, a survey scan of the material revealed the presence of major elements including C, N, O, and Co. During this survey, peaks were observed for both nitrogen and oxygen atoms. An oxygen peak was noted at approximately 532.6 eV, suggesting the presence of carbonyl oxygen in the sample [38]. A nitrogen peak was detected at 400.3 eV, indicating nitrogen doping in the carbonaceous material [39]. Nitrogen doping can enhance the material’s properties, potentially altering its electronic structure, improving conductivity, or enhancing its catalytic properties. These modifications can be tailored for specific applications, such as energy storage devices, catalysis, sensors, and electronic devices. The C1s XPS peaks shown in Figure 3b reveal the chemical composition and bonding present in the carbonaceous material. The C1s spectrum exhibits distinct peaks at binding energies of 284.8 eV, 286.0 eV, and 288.5 eV, corresponding to various carbon functional groups in the material [40]. The primary peak at 284.8 eV represents carbon atoms in sp3-hybridized structures prevalent in the sample. The peak at 286.0 eV is attributed to carbon atoms involved in C-O (carbon-oxygen) bonds, indicating the presence of oxygen-containing functional groups. Furthermore, the peak at 288.5 eV suggests the presence of carbon atoms in carbonyl (C=O) functionalities [41]. The relative intensities and positions of these C1s peaks offer insights into the surface chemistry, molecular structure, and composition of the carbonaceous material.
A comprehensive analysis of the cobalt complex through Co 2p XPS scans unveiled two distinct peaks, depicted in Figure 3c. The first peak, observed at 781.8 eV, is attributed to the Co 2p3/2 orbital, and the second peak, located at 796.9 eV, is associated with the Co 2p1/2 orbital. The 15.1 eV difference between the Co 2p3/2 and Co 2p1/2 levels signifies the presence of Co(II) in the sample, as indicated by references [42,43,44]. Additionally, satellite peaks at binding energies of 802.6 eV and 786.8 eV, with a 15.8 eV separation between the Co 2p3/2 and Co 2p1/2 levels, further corroborate the existence of Co(II), supported by references [45,46,47,48,49]. Given the XRD and Raman (Figure S1) results indicating both CoO and Co3O4, it is plausible that our sample contains both Co(II) and Co(III) states, and Co3O4 itself comprises both.
Distinguishing between Co(III) and Co(II) in XPS analysis is challenging due to their similar peak positions, leaving room for both possibilities. Prior XPS data for similar, non-pyrolyzed complexes showed peak positions akin to those reported here [50], albeit identified in the Co(III) oxidation state. The presence of deprotonated amide peaks complicates the assignment further, as deprotonated amides significantly donate electrons to the metal. Despite the sample being subjected to a pyrolysis temperature of 500 °C, which typically leads to the decomposition of such complexes into metal oxides, it is conceivable that some original complex structures were retained, especially when polydopamine was involved. Pyrolysis is expected to break down the complex, as indicated by previous XRD and Raman analyses, potentially reducing Co(III) to Co(II) through the production of reducing gases during the process. The pyrolysis of our sample, enriched with nitrogen doping from dopamine, the amidomacrocyclic ligand, and urea, fundamentally altered the original complex structure. This transformation decomposed the metal complex and integrated metal and nitrogen into the matrix, potentially enhancing the material’s oxygen reduction reaction (ORR) activities. This enhancement could be attributed to a synergistic interaction, with the metal or the nitrogen dopants serving as active sites for the ORR, illustrating the intricate relationship between material composition and electrocatalytic performance.
2.3. Cyclic Voltammetry (CV)
Electrochemical tests were conducted to evaluate the carbonaceous material’s capability to electrochemically reduce oxygen in the oxygen reduction reaction (ORR). The most common method for efficiently evaluating the performance of a catalyst is to measure its half-wave potential (E1/2). Assessing the ORR performance of a catalyst entails benchmarking against the ORR performance of a cutting-edge commercial Pt/C catalyst, typically featuring 20% Pt loading. The established average E1/2 value of 0.84 ± 0.03 V (vs. RHE) serves as the “Golden reference” for commercial Pt/C (with Pt 20 wt%), facilitating the assessment of other ORR catalysts in both acidic and alkaline electrolytes [51]. The electrocatalytic activity of the cobalt-doped carbonaceous material toward the ORR was initially examined using cyclic voltammetry (CV). CV measurements were taken at various scan rates (100 mV/s, 50 mV/s, 25 mV/s, and 10 mV/s) across different pH levels (1, 3, 5, and 7) of a 0.1 M H2SO4 electrolyte solution, within a potential range of 0.0 V to 1.1 V (vs. RHE), under both O2- and Argon (Ar)-saturated conditions, as illustrated in Figure 4a,b. The results from the pH studies indicated that the catalyst displayed a sharper reduction peak in the electrolyte with a pH of 3 (Figure 4a), noting a significant reduction in the current at 0.30 V (vs. RHE) and 0.35 V (vs. RHE) at scan rates of 100 mV/s and 10 mV/s, respectively, in the O2-saturated environment. Additionally, a CV test was performed under the same conditions but in an Ar-saturated environment to determine if the catalyst could reduce oxygen in its absence, where no peak was observed (Figure 4b).
2.3.1. Electrocatalyst Layer Studies
The oxygen reduction reaction (ORR) studies utilized different numbers of deposited layers (1, 3, 5, 7) on glassy carbon electrodes to investigate variations in the peak potential position and/or current density. Cyclic voltammetry tests were extended to seven layers, revealing that electrodes with a greater number of uniform layers displayed a more pronounced reduction peak (Figure 5) in an oxygen-saturated solution. Notably, a distinct peak was recorded at 0.31 V (vs. RHE) for the electrode with seven layers in an O2-saturated electrolyte solution at a scan rate of 100 mV/s. Incrementally adding layers of the carbonaceous material on the electrode surface resulted in a rise in current density [50], suggesting enhanced ORR activity. In comparison, the electrolyte solution saturated with argon exhibited a significantly weaker reduction peak across all layers relative to the O2-saturated 0.1 M electrolyte solution. The peak’s potential progressively shifted to lower values and its intensity decreased with a lesser number of layers. Adding layers potentially increases both the active sites and the electrical conductivity of the electrode, as more carbonaceous material enhances the overall ORR efficiency, leading to a higher current and shifts toward more positive potentials. The sample with seven layers showcased a sharper reduction peak, a result of the additional layers increasing the number of active sites and improving the electrical conductivity, thereby expanding the surface area. This aligns with the observed trend, underscoring the importance of optimizing layer thickness for improved electrocatalytic performance.
2.3.2. Electrocatalyst pH and Stability Studies
The performance of the catalyst was evaluated using cyclic voltammetry (CV) tests under various pH conditions (1, 3, 5, 7) in both O2-saturated and Ar-saturated environments. In Figure 6a, the cobalt-doped carbonaceous catalyst exhibited a pronounced reduction peak at pH 3 in an oxygen (O2)-saturated environment at a scan rate of 100 mV/s, indicating its optimal performance in acidic conditions. At pH 1 and 5 in an O2-saturated environment, only modest reduction peaks were detected, suggesting a decrease in catalytic activity compared to pH 3. Remarkably, at pH 7, the catalyst showed no reduction peak, indicating a significant drop in activity in less acidic environments. This trend confirms that the catalyst’s efficiency is maximized in more acidic conditions. In contrast, the CV tests conducted in an Ar-saturated environment revealed no significant reduction peaks for any pH level, with only a very slight peak observable for pH 1, 3, and 5 (Figure 6b), and again, no peak was detected from pH 7 upwards. The absence of significant activity in the Ar-saturated tests across all pH levels emphasizes the catalyst’s specificity for the oxygen reduction reaction in acidic media, highlighting its potential application in environments where efficient ORR activity is critical. The electrochemical stability study aims to examine the stability of the carbonaceous material through cyclic voltammetry (CV) measurements. The stability of the catalyst is an important requirement for its operation. It has been recently discovered that ensuring the long-term stability of the Membrane/Electrode Assembly (MEA) is crucial for commercializing fuel cells. An investigation through XPS data has revealed that the amount of fluorine atoms in the fuel cell gradually decreases over time due to the degradation of Nafion [52]. In this study, we investigated the chemical stability of our material up to 500 cycles at pH 3 under an oxygen (O2)-saturated environment at a scan rate of 100 mV/s with a voltage window from 0.0 V to 1.1 V (vs. RHE). During the 500 cycles, drop-cast working electrode materials experienced a significant loss during the experiment, and it resulted in a consequential decrease in the reduction peak (Figure 6c). This degradation of active materials on the electrode surface may have resulted from irreversible reactions in the electrolyte. The effect of binders can be another factor for this kind of decrease in reduction peak. We used Nafion as a binder due to its high ionic conductivity and chemical stability. However, Nafion tends to swell in certain electrolytes or solvent environments, which can compromise the stability and electrocatalytic performance of the material over time. Although Nafion is chemically stable, its hydrophobic nature may impede the wetting of electrode surfaces, affecting electrochemical reactions and leading to reduced performance, particularly in aqueous environments. We have planned to experiment with different electrolytes and binders to improve the stability of the pyrolyzed cobalt-doped carbonaceous material.
2.3.3. Rotating Disk Electrode (RDE)
RDE studies were conducted on the carbonaceous material to determine the number of electrons involved in the ORR. In acidic conditions, oxygen can be reduced to water in a four-electron process. However, sometimes, oxygen can be reduced to hydrogen peroxide through a two-electron exchange process. Even though the reduction of oxygen to hydrogen peroxide is also an ORR process, generating peroxide is undesirable since it has an oxidative character that degrades the catalyst, which decreases its ORR activity. RDE tests were performed by rotating a drop-cast glassy carbon electrode at different rotation speeds (ω = 400 to 2500 rpm) with a 10 mV/s scan rate in O2-saturated 0.1 M H2SO4 (Figure 7a).
To calculate the number of electrons involved in an electrochemical process, the convective movement between the analyte solution and the electrode surface is related using the Koutecky–Levich equation. The Levich current (Jlev) is calculated using the equation Jlev = 0.620nFCD2/3ω1/2υ−1/6, where n is the number of electrons transferred, F is the Faraday constant, C is the molar concentration of the analyte, D is the diffusion coefficient of O2, ω is the angular rotation rate of the electrode, and υ is the kinematic viscosity of the solution. The kinetic current (Jk) and the observed limiting current (Jlim) from the experimental RDE data are used to construct the Koutecky–Levich equation. The slope of Jlev is obtained by plotting the graph between Jlim−1 and ω−1/2.
1/Jlim = 1/Jlev + 1/Jk
where Jlim is the limiting current density, Jlev is the Levich current, and Jk is the kinetic current.From the slope of the K-L plot, the number of electrons involved in the oxygen reduction mechanism is determined using the equations mentioned above. According to the calculated results, the catalyst performs the ORR through a 3.58 electron process at pH 3, which matches the theoretical n = 4 value (Figure 7b).
2.4. Proposed Mechanism
The RDE studies provide clear evidence that the material catalyzes the oxygen reduction reaction (ORR) through a four-electron mechanism in acidic environments, a conclusion supported by electrochemical analyses at pH 3. This efficient four-electron pathway is shown in Figure 8. XPS analysis has confirmed the presence of both Co(II) and Co(III) species within the catalytic material, indicating that these Co centers could serve as the critical active sites for the ORR, with molecular O2 initially binding to them. Although Co (II) is likely better for O2 interaction, we also consider that Co (III) could interact with O2. Adding nitrogen to the material is expected to greatly improve the reactivity and effectiveness of these active sites for the ORR process. Interestingly, during the ORR, the Co center temporarily reaches a higher oxidation state when it first binds with an O2 molecule. The reaction ends with the reduction of O2 to water, which interestingly brings the Co center back to its original oxidation state, either II or III, thus completing the catalytic cycle in the challenging environment of acidic conditions. This reversible oxidation–reduction cycle highlights the durability and effectiveness of the catalytic process, pointing out the catalyst’s potential as an excellent catalyst for ORRs [53,54].
3. Materials and Methods
3.1. Materials
The chemicals used in this study were sourced from Aldrich Chemical Co., St. Louis, MO, USA, and Fisher Scientific Company, San Jose, CA, USA, and were utilized as received unless specified otherwise. Nanocellulose was procured from Blue Goose Biorefineries, Saskatoon, SK, Canada. Scanning Electron Microscopy (SEM) analysis was conducted using a JEOL SEM (JSM 7000F, Tokyo, Japan), while Energy-Dispersive X-ray Spectroscopy (EDS) analysis was performed with a JEOL SEM (JSM 2100F) equipped with an EDAX Genesis EDS system. X-ray Diffraction (XRD) measurements were carried out using a Rigaku MiniFlex instrument (Tokyo, Japan). The valency of elements in the carbonaceous material was determined using X-ray Photoelectron Spectroscopy (XPS) on a Thermo Scientific (Waltham, MA, USA) K-Alpha instrument with Al Kα radiation. Cyclic voltammetry (CV) experiments to assess electrocatalytic activity were conducted with a Pine Instruments potentiostat (Raleigh, NC, USA) in a 0.1 M H2SO4 solution. pH and layer deposition studies also utilized the Pine Instruments potentiostat in a 0.1 M H2SO4 solution. Pyrolysis was performed in an MTI Corporation (GSL-1100X, Richmond, CA, USA) high-temperature furnace. Oxygen and argon gases for the research were supplied by Airgas, and deionized water was used for all washing procedures.
3.2. Synthesis of Cobalt-Doped Carbonaceous Material
The cobalt complex of the amidomacrocyclic ligand was synthesized using a previously published synthetic method with slight modifications [55]. Initially, the lithium salt of the cobalt complex was synthesized. Tetraphenylphosphonium (PPh4+) salts of inorganic or metalloorganic anions are commonly utilized due to their ease of crystallization. Consequently, the countercation was switched from lithium (Li+) to tetraphenylphosphonium (PPh4+) by dissolving 300.0 mg of the initial cobalt complex in deionized water and incrementally adding a 3.0 M solution of tetraphenylphosphonium chloride (PPh4Cl). The resulting precipitate was collected through filtration, and the product was subsequently dried under vacuum for 5–6 h.
Next, 120.0 mg of nanocellulose was combined with the newly synthesized cobalt complex in deionized water, and the pH of the solution was adjusted to 8. To facilitate binding, 338.0 mg of dopamine hydrochloride (DA HCl) was introduced to the mixture. This mixture was then placed on a hot plate and stirred gently for 12 h in open air to allow for polydopamine formation via an oxidative polymerization process. Afterward, the contents were collected and dried under vacuum. Finally, 20.0 mg of urea (CH4N2O) was finely ground and added to the dried sample. The prepared mixture (referenced as Figure 9) underwent pyrolysis at 500 °C under a nitrogen atmosphere.
3.3. Electrochemical Studies
A potentiostat facilitated the cyclic voltammetry (CV) experiments. For these tests, a glassy carbon electrode from BASi Research Product (Lafayette, IN, USA) served as the working electrode. The reference electrode, Ag/AgCl, and the counter electrode, a platinum wire, were sourced from Pine Instruments and BASi Research Product, respectively. The setup included a 100 mL glass vial as the electrochemical cell, sealed with a three-holed stopper. Concentrated sulfuric acid was diluted to a 0.1 M H2SO4 solution, yielding a predicted pH of 1.0. CV measurements spanned potential ranges from 0.0 V to 1.1 V (vs. RHE) at various scan rates. These electrochemical studies were conducted at an ambient temperature (25 °C) using a freshly prepared 0.1 M H2SO4 electrolyte solution. An O2-saturated environment was established for the experiments by bubbling oxygen gas through the electrolyte solution for at least one hour prior to testing, a condition that was maintained during the experiments. To evaluate the activity of the synthesized carbonaceous material, the electrolyte solution was deoxygenated with argon (Ar) gas. Throughout this study, all potential values are reported relative to the reversible hydrogen electrode (RHE), as per the specified expression.
ERHE = EAg/AgCl + E˚Ag/AgCl + 0.059pH
ERHE is the potential versus RHE in this equation. The potential measured against the Ag/AgCl reference electrode is denoted by EAg/AgCl. The standard electrode potential of the Ag/AgCl reference electrode in 0.1 M H2SO4 is E˚Ag/AgCl.
Layer and pH studies were carried out by performing CV tests at different scan rates ranging from 0.0 V to 1.1 V (vs. RHE) in both O2- and Ar-saturated 0.1 M H2SO4 or buffered electrolytes.
3.4. Electrode Preparation
A homogeneous catalyst suspension was prepared by dissolving 5 mg of the catalyst in 5 mL of methanol, achieving a 1:1 weight/volume (w/v) ratio. To ensure uniformity, the mixture was sonicated for 10 min. Subsequently, after sonication, 40 µL of Nafion solution (5 wt.%) was added to the homogeneous suspension. The surface of the glassy carbon electrode was initially polished with a 0.05 µm alumina slurry and then thoroughly rinsed with deionized water. Following this, 20 µL of the catalyst suspension was drop-cast onto the polished glassy carbon electrode surface and allowed to dry under vacuum. For the electrocatalyst layer studies, the procedure was similar to that used for cyclic voltammetry (CV) testing; however, 10 µL of the catalyst suspension was drop-cast onto the glassy carbon surface for each layer, with each application followed by drying under vacuum.
4. Conclusions
A cobalt complex along with nanocellulose, polydopamine, and urea composite material was synthesized for use as a cathode catalyst in fuel cell applications. Characterizations of the metal-doped carbonaceous material were conducted using SEM, EDS, XRD, and XPS. SEM analysis revealed the morphology and nanostructure of the material, with some agglomeration observed. EDS confirmed the uniform distribution of elements within the carbonaceous material, while XRD identified the crystalline structures of Co3O4 and CoO. The electrocatalytic behavior of the cobalt-doped carbonaceous material toward the oxygen reduction reaction (ORR) in acidic media was investigated through cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. CV analysis demonstrated the ORR activity of the Co-doped carbonaceous material across different pH levels, with a pronounced peak at pH 3, showing a peak potential of 0.30 V (vs. RHE) in an oxygen-saturated 0.1 M H2SO4 electrolyte solution at a scan rate of 100 mV/s. This indicates the material’s capability for ORRs. Further pH and layering studies were conducted using the CV system. RDE measurements determined that the cobalt complex catalyzes the ORR via a four-electron pathway. These findings offer new insights that could be beneficial for various applications, including acidic fuel cells, electrocatalytic reduction processes, batteries, biosensors, and supercapacitors.
M.M.P., S.H., P.S., G.H., R.S.K., F.W., K.K., D.W. and A.G. contributed to the experimental design and data interpretation. A.G. conceived the study and experimental design. M.M.P., S.H., F.W., K.K., P.S. and A.G. contributed to the experimental design and wrote the manuscript. All authors have read and agreed to the published version of the manuscript.
All data are kept in the corresponding laboratories and are available upon request.
A.G. would also like to thank the UA-Little Rock Signature Experience for partially funding the research. The authors would like to thank The Center for Integrative Nanotechnology Science (CINS) at UA-Little Rock for all their help and support. The authors would also like to acknowledge the use of
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. SEM images of cobalt-doped carbonaceous ORR catalytic material at (a) 50k magnification and (b) 1k magnification. (c) EDS spectra showing element mapping image of elements of carbonaceous material.
Enlarge this image.Figure 2. XRD pattern of pyrolyzed cobalt-doped carbonaceous ORR catalytic material.
![View Image - Figure 3. XPS results of cobalt-doped carbonaceous material: (a) survey scan, (b) C 1s narrow scan, and (c) Co 2p narrow scan. [The red line means sum of peaks, the green line means background line and other color line represents peak fitting curve].](https://media.proquest.com/media/hms/THUM/6/PtF4a?_a=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%3D%3D&_s=DKBR%2B%2FUFKA8mDqVJAsMbulA2dK0%3D "View Image - Figure 3. XPS results of cobalt-doped carbonaceous material: (a) survey scan, (b) C 1s narrow scan, and (c) Co 2p narrow scan. [The red line means sum of peaks, the green line means background line and other color line represents peak fitting curve].")
Enlarge this image.Figure 3. XPS results of cobalt-doped carbonaceous material: (a) survey scan, (b) C 1s narrow scan, and (c) Co 2p narrow scan. [The red line means sum of peaks, the green line means background line and other color line represents peak fitting curve].
Enlarge this image.Figure 4. Cyclic voltammograms of pyrolyzed cobalt-doped carbonaceous material at pH 3 in (a) O2-saturated 0.1 M H2SO4 and (b) Ar-saturated 0.1 M H2SO4, with v = 100 mV/s.
Enlarge this image.Figure 5. Cyclic voltammogram comparison for different layers in 0.1 M H2SO4 electrolyte solution, with v = 100 mV/s.
Enlarge this image.Figure 6. Cyclic voltammograms of cobalt-doped carbonaceous catalyst with different pH values in (a) O2-saturated 0.1 M H2SO4 and (b) Ar-saturated 0.1 M H2SO4. (c) Electrochemical stability at pH 3 in 0.1 M H2SO4 with v = 100 mV/s.
Enlarge this image.Figure 7. (a) RDE plot of cobalt-doped carbonaceous catalyst in O2-saturated 0.1 M H2SO4 at different rotation speeds (400 to 2500 rpm), with v = 10 mV/s; (b) Koutecky–Levich plot (j−1 vs. ω−1/2).
Enlarge this image.Figure 9. Schematic illustration of the synthesis of the cobalt-doped carbonaceous material catalyst for enhanced ORR activity.
Supplementary Materials
The following supporting information can be downloaded at:
References
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