Optimization of RuxSey electrocatalyst loading for oxygen reduction in a PEMFC

The synthesis, characterization and optimization of Ru_xSe_y catalyst loading as a cathode electrode for a single polymer electrolyte membrane fuel cell, PEMFC were investigated. Ru_xSe_y catalyst was synthesized via a decarbonylation of Ru₃(CO)₁₂ and elemental selenium in 1,6-hexanediol under refluxing conditions for 2 h. The powder electrocatalyst was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and examined for the oxygen reduction reaction (ORR) in 0.5M H₂SO₄ by rotating disk electrode (RDE) and in membrane-electrode assemblies, MEAs for a single PEMFC. Results indicate the formation of agglomerates of crystalline particles with nanometric size embedded in an amorphous phase. The catalyst exhibited high current density and lower overpotential for the ORR compared to that of Ru_x cluster catalyst. Dispersed Ru_xSe_y catalyst loading on Vulcan carbon was optimized as a cathode electrode by performance testing in a single H₂–O₂ fuel cell.     

Synthesis and characterization of Pd0.5NixSe(0.5−x) electrocatalysts for oxygen reduction reaction in acid media

Pd_0.5Ni_xSe_(0.5−x) electrocatalysts with different chemical composition, X = 0.25, 0.35, 0.45, were synthesized by a NaBH₄ reduction of PdCl₂, NiCl₂ and SeO in a THF solution and evaluated for the oxygen reduction reaction (ORR) in acid media by electrochemical techniques of rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE). The electrocatalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and CO adsorption/stripping. The relation of Se to Ni in the samples has a profound effect on the nature of the cathode catalytic activity toward the oxygen reduction process, enhanced as the Se content in the electrocatalysts was reduced. Electrochemical results show that the ORR takes place by a multi-electron charge transfer process (n = 4e⁻) with the formation of less than 1% of hydrogen peroxide. The enhanced activity was attributed to the high active surface area deduced from CO adsorption/stripping analysis. The research activities are reported within the focus on the activity-stability of the bimetallic chalcogenides electrocatalysts.     

Electrocatalytic reduction of dioxygen on PdCu for polymer electrolyte membrane fuel cells

The present research is aimed to study the oxygen reduction reaction (ORR) on a PdCu electrocatalyst synthesized through reduction of PdCl₂ and CuCl with NaBH₄ in a THF solution. Characterization of PdCu electrocatalyst was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. Characterization results showed that the synthesis method produced spherical agglomerated nanocrystalline PdCu particles of about 10 nm size. The electrochemical activity was evaluated using cyclic voltammetry (CV), rotating disc electrode (RDE) and electrochemical impedance spectroscopy (EIS) in a 0.5 M H₂SO₄ electrolyte at 25 °C. The onset potential for ORR on PdCu is shifted by ca. 30 mV to more positive values and enhanced catalytic current densities were observed, compared to that of pure Pd catalyst. The synthesized PdCu electrocatalyst dispersed on a carbon black support was tested as cathode electrode in a membrane-electrode assembly (MEA) achieving a power density of 150 mW cm⁻² at 0.38 V and 80 °C.     

Highly efficient supported PtFe cathode electrocatalysts prepared by homogeneous deposition for proton exchange membrane fuel cell

A simple and efficient approach has been developed for the synthesis of carbon-supported binary PtFe (50:50) electrocatalyst with high metal loading that combines homogeneous deposition (HD) of PtFe hydroxide complex species through generation of OH⁻ ions realized by in situ hydrolysis of urea and subsequent uniform reduction of the complex by ethylene glycol (EG) in a polyol process, providing control over the size and dispersion of PtFe nanoparticles (NPs). Compared to PtFe catalysts prepared with other common synthesis methods using NaBH₄ and EG and commercial PtFe catalyst, the supported PtFe catalyst prepared by the HD-EG method reveals more uniform homogenous dispersion of PtFe NPs with much smaller particle size, thus demonstrating excellent electrocatalytic ability and fuel cell performance. The structural properties and catalytic activities of Pt–Fe catalysts prepared in various synthesis methods were evaluated on the basis of the analysis of HR-TEM, HR-SEM, XRD, electrochemical surface area and fuel cell polarization performance.     

Effect of the PtCu/C electrocatalysts initial composition on their activity in the de-alloyed state in the oxygen reduction reaction

The promising ORR electrocatalysts with the low platinum content have been obtained by a simple method in one pot. The morphology of Pt-based nanoparticles supported on carbon was controlled to enhance the catalytic performance in oxygen reduction reaction (ORR). A detailed study of the microstructural characteristics and electrochemical behavior of the de-alloyed electrocatalysts made it possible to obtain a correlation between the effect of PtCu/C materials initial composition and their ORR activity. It has been established that the initial content of the alloying component has a significant effect on the structural reorganization of bimetallic nanoparticles. Electrochemical study results showed that the ORR performance of de-alloyed PtCu/C catalyst with large Cu content in the initial structure was better than that of de-alloyed PtCu/C catalysts with low Cu content due to the morphology advantages. A successful reorganization of NPs structure increased the ORR activity of the de-alloyed catalysts by two times. A presented approach can be applied to design of different ORR electrocatalysts based on the PtM nanoparticles.     

Effects of stabilizers on the synthesis of Pt3Cox/C electrocatalysts for oxygen reduction

Pt₃Co_x/C electrocatalysts for use as cathodes in proton exchange membrane fuel cells are fabricated using various stabilizers to control the different reduction speeds between Pt and Co ions. Four different types of stabilizers—sodium acetate, oleylamine, tetraoctylammonium bromide (TOAB), and hexadecyltrimethylammonium bromide (CTAB)—differing in molecular structures and ionic states are tested. Primarily, Pt₃Co_x/C alloy nanoparticles are synthesized with 0.6 < x < 0.8 after heat treatment to remove the residual stabilizers. A significant improvement in the activity for oxygen reduction reaction is observed in the case of TOAB- and CTAB-mediated Pt₃Co_x/C catalysts. In particular, CTAB-mediated catalysts exhibit the best activity, which is about 2-times higher mass activity than commercial Pt/C catalyst. The higher mass activity is believed to result from not only the alloying effects with small atomic size Co but also better dispersion and smaller particle size after heat treatment at relatively low temperature.     

Effect of mesoporous carbon on oxygen reduction reaction activity as cathode catalyst support for proton exchange membrane fuel cell

In this paper, a new carbon support with a large number of mesoporous-structures is selected to prepare Pt/C catalysts. Transmission electron microscope (TEM) results show that the Pt/3# catalyst presents a sponge-like morphology, Pt particles are not only evenly distributed on the surface of carbon support, but also the smaller Pt particles are deposited in the mesoporous inside the support. The average diameter of Pt particles is only 2.8 nm. The membrane electrode assembly (MEA) based on Pt/3# catalyst also shows excellent performance. In conclusion, the 3# support is an idea carbon support for PEMFC, which helps to improve the oxygen reduction reaction (ORR) activity of the catalyst. Based on the “internal-Pt” structure of the support mesoporous, the efficient three-phase boundaries (TPBs) are construct to avoid the poisoning effect of ionomer on the nano-metal particles, reduce the activation impedance and oxygen mass transfer impedance, and improve the reaction efficiency.     

Oxygen reduction reaction increased tolerance and fuel cell performance of Pt and RuxSey onto oxide-carbon composites

Pt and Ru_xSe_y nanoparticles were selectively deposited onto oxide sites of oxide-carbon composite substrates using the photo-deposition process and compared to conventional carbon support materials. The oxide was essentially anatase phase. Cyclic voltammetry and rotating disk electrode measurements for the oxygen reduction reaction (ORR) in formic acid containing-electrolyte showed a tolerance improvement for ORR of Pt supported on composite substrates. This positive substrate effect on platinum, turned out not to be favorable for Ru_xSe_y catalyst centers. On the other hand, the methanol tolerance for ORR was increased for both nanostructured materials supported on the oxide-carbon composite. Single H₂/O₂ fuel cell results were in agreement with half-cell electrochemical measurements on Pt, showing an improvement of the power density when using the oxide-carbon as substrate for the cathode. The composites were evaluated as cathode catalysts of an HCOOH laminar-flow fuel cell (LFFC) in which commercial Pd/C was used as an anode catalyst. The cathodes with Ru_xSe_y and Pt supported on TiO₂/C improved the specific power density by 15% and 24%, respectively, with respect to carbon as support.     

Synthesis and characterization of PtNx/C as methanol-tolerant oxygen reduction electrocatalysts for a direct methanol fuel cell

Platinum nitride supported on carbon (PtN_x/C) is synthesized by the novel strategy of chelating the Pt precursor followed by pyrolysis and is characterized as a possible cathode electrocatalyst for direct methanol fuel cells (DMFCs). The prepared PtN_x/C is shown to possess high methanol tolerance and catalytic activity for the oxygen reduction reaction (ORR). The results indicate that the temperature of the heat treatment and the molar ratio of Pt to N in the precursor solution play important roles in the catalytic performance. A sample of PtN_x/C prepared at 700 °C with a Pt:N ratio of 1:2 shows a significant decrease in the potential loss associated with the mixed potential and the poisoning effect by adsorbed methanol, and this results in a high power density of 180 mW cm⁻². The performance is 30% higher than that of Pt/C under 4 M of methanol concentration.     

Tuning lattice strain in biphenylene for enhanced electrocatalytic oxygen reduction reaction in proton exchange membrane fuel cells

As proton-exchange membrane fuel cell technology has grown and developed, there has been increasing demand for the design of novel catalyst architectures to achieve high power density and realize wide commercialization. Herein, based on the two-dimensional biphenylene, we compare the oxygen reduction reaction (ORR) activity on the active sites with different biaxial lattice strains using first-principles calculations. The ORR free energy diagrams of biphenylene monolayers with varying lattice strains suggest that the biaxial tensile strains are unfavorable for catalytic activity. In contrast, the biaxial compressive strains could improve the catalytic performance. The biphenylene systems with the strain of ?2% ～ ?6% (S_{-0.02～-0.06}) display overpotentials of 0.37–0.49 V. This performance is comparable to or better than the Pt (111) surface. The Bader charge transfer of adsorbed O species on various biaxial strain biphenylene catalysts could be a describer to examine the catalytic activity. The catalysts possessed the moderate transferred charge of O adsorbed species often promotes catalytic process and give the high catalysis efficiency. Overall, this work suggests that the lattice strain strategy can significantly enhance the catalytic activity of biphenylene materials and further provide guidance to design biphenylene-based catalysts in various chemical reactions.     

Nitrogen-modified carbon-based catalysts for oxygen reduction reaction in polymer electrolyte membrane fuel cells

Nitrogen-modified carbon-based catalysts for oxygen reduction were synthesized by modifying carbon black with nitrogen-containing organic precursors. The electrocatalytic properties of catalysts were studied as a function of surface pre-treatments, nitrogen and oxygen concentrations, and heat-treatment temperatures. On the optimum catalyst, the onset potential for oxygen reduction is approximately 0.76 V (NHE) and the amount of hydrogen peroxide produced at 0.5 V (NHE) is approximately 3% under our experimental conditions. The characterization studies indicated that pyridinic and graphitic (quaternary) nitrogens may act as active sites of catalysts for oxygen reduction reaction. In particular, pyridinic nitrogen, which possesses one lone pair of electrons in addition to the one electron donated to the conjugated π bond, facilitates the reductive oxygen adsorption.     

Co-Fe/MIL-101(Cr) hybrid catalysts: Preparation and their electrocatalysis in oxygen reduction reaction

The development of highly efficient electrocatalysts with low cost for oxygen reduction reaction (ORR) is urgently required for metal-air batteries and fuel cells. In this work, FeCo/MIL-101(Cr) with various molar ratios of Fe/Co as hybrid catalysts was prepared by a facile and mild impregnation method. MIL-101(Cr) increased the specific surface areas of the hybrid catalysts, thus improving the dispersion of Fe and Co species at their surfaces. The effects of Fe and Co species on ORR activity of the hybrid catalysts were investigated. It is found that the synergistic effects between the well-dispersed Fe and Co species contribute mainly to ORR activity. More specifically, Fe species exert a partial-charge-transfer-activation effect on Co ones, which reduces the charge transfer resistance and thus improves the catalytic activity. As a result, FeCo/MIL-101(Cr) showed the excellent ORR activity, in which Co₅₀Fe₅₀/MIL-101(Cr) exhibited the superior ORR activity to the other prepared hybrid catalysts.     

High-yield, size-controlled synthesis of silver nanoplates and their applications as methanol-tolerant electrocatalysts in oxygen reduction reaction

A novel method for the synthesis of as-prepared Ag nanoplates in high yield and the control of their dimensions has been developed. In this method, hexadecyltrimethyl ammonium ions (CTA⁺) are used as a trace additive in a seed solution for blocking the seed surface to govern the growth direction on nanoplate in the growth pathway, leading to a high-yield production of the Ag nanoplates with mixed morphologies, mainly triangular nanoplates and nanodisks. The spectra of the obtained nanoplate solution showed a high-intensity peak attributed to the in-plane dipole resonance and a low-intensity peak at 400 nm. By decreasing the amount of CTA⁺, the mean edge length of triangular nanoplates could be changed from ∼78.7 nm-∼124.8 nm. The in-plane dipole resonance peak corresponding to change in the mean edge length shifted from 630 nm to 785 nm, respectively. The mean edge length of triangular nanoplates could also be controlled from 70 nm to 148 nm by decreasing the CTA⁺-adsorbed seed amount. To investigate the practical feasibility of application of the proposed method, the prepared nanoplates were used as a methanol-tolerant electrocatalyst in an oxygen reduction reaction (ORR). An analysis conducted using a rotating ring-disk electrode showed that these nanoplates have high activity towards the ORR and that the electron transfer numbers (n) were 3.85, 3.83, 3.81, and 2.94 for 70 nm, 124 nm, 148 nm nanoplates, and macroscopic Ag electrode, respectively. If the present of methanol, the corresponding n values of 3.82, 3.81, 3.78, and 2.30 were detected. Despite working in the methanol-tolerant solution, the prepared Ag nanoplates still exhibited high electroactivity and their ORR proceeded via an approaching 4-electron pathway.     

Recent progress in heteroatom doped carbon based electrocatalysts for oxygen reduction reaction in anion exchange membrane fuel cells

Oxygen reduction reaction (ORR) is a key step in many electrochemical devices such as fuel cells and metal-air batteries. However, the reaction proceeds at a significant overpotential requiring Pt-based catalysts. The scarcity and economical challenges associated with Pt is one of the major limitations for the commercialization of the devices. In this context, the electrochemical research community is constantly exploring other low-cost and earth-abundant materials as ORR catalysts. Carbon nanomaterials are identified as promising electrocatalysts due to their superb electronic conductivity together with high specific surface area. However, the low reactivity of carbon is the major limiting factor in the fabrication of ORR catalysts. Recent studies have proved that chemical modification of the carbon network (substitution of foreign atoms, Ex: N, S, B, F, P) could alter the reactivity of carbon nanomaterials for ORR. Many doping strategies have been proposed including single atom doping, co-doping and multi-atom doping. The heteroatom doped carbons have delivered promising results towards ORR in alkaline media. This review presents a rational approach of doping methods and the electrochemical properties of heteroatom doped carbons, and we believe that this review could be a guiding material to design advanced non-noble catalysts for ORR in the coming future.     

Carbon supported Ru1−xFexSey electrocatalysts of pyrite structure for oxygen reduction reaction

Structure, activity, and stability of the Ru_1−xFe_xSe_y/C (x = 0.0–0.46, y = 0.4–1.9) catalysts, synthesized via hydrogen annealing of carbonyl precursors, are analyzed using X-ray diffraction, transmission electron microscopy, rotating (ring) disk electrode voltammetry, and cyclic voltammetry. Hydrogen annealing at 400 °C enables nucleation of the pyrite phase, which strengthens the catalyst stability for oxygen reduction reaction (ORR) in the acidic electrolyte. Evaluation on the stable Ru_1−xFe_xSe_y/C catalysts indicates that the ORR activity increases with increasing iron content and Se content. Substitution of base metal Fe enhances the activity and reduces the material cost, but it also increases the H₂O₂ yield, which is measured less than 3.0% between 0.7 and 0.9 V in contrast to 1.0% of the catalyst without iron. The high activity of Ru_0.54Fe_0.46Se_1.9/C decays more rapidly than that of RuSe_2.0/C in the stability test, probably because Fe and Se are preferentially leached in the potential cycling up to 1.2 V. Still the two catalysts with pyrite structure exhibit much higher durability than the cluster-type catalyst of RuSe_cluster/C (Ru:Se = 1:0.3) that was not annealed.     

RuxNb1−xO2 catalyst for the oxygen evolution reaction in proton exchange membrane water electrolysers

Bimetallic catalyst system of ruthenium oxide (RuO₂) and niobium oxide (Nb₂O₅) was prepared using the Adams method and the hydrolysis method. Physical and electrochemical characterizations of the catalysts were studied using X-ray diffraction (XRD), Scanning electron microscopy (SEM), cyclic voltammogram (CV) and polarization measurements. Nb₂O₅ addition to RuO₂ was found to increase the stability of RuO₂. In Adams method the sodium nitrate was found to be forming complex with Nb₂O₅ at high temperature reaction. This makes Adams method unsuitable for the synthesis of RuO₂–Nb₂O₅ bimetallic system. Hydrolysis method on other hand does not have this problem. But a proper mixture of two oxides was not obtained in hydrolysis method. A lower crystallite size for bimetallic system was obtained with Adams method compared to hydrolysis method. RuO₂ prepared by Adams method had higher activity compared to the hydrolysis counterpart in electrolyzer operation with nafion membrane. A cell voltage of 1.62 V was obtained with RuO₂ (A) at 1 A/cm². A higher stability for Ru_0.8Nb_0.2O₂(A) compared to RuO₂(A) was observed in continuous cyclic voltammogram and electrolyzer cell test.     

Characterization of composite materials of electroconductive polymer and cobalt as electrocatalysts for the oxygen reduction reaction

Platinum-free electrocatalysts based on electroconductive polymer, modified with cobalt, were prepared and characterized for the oxygen reduction reaction (ORR). The carbon-supported materials were: carbon/polyaniline/cobalt, carbon/polypyrrole/cobalt and carbon/poly(3-methylthiophene)/cobalt. Also the corresponding cobalt-free precursors were studied. EDAX studies show that in cobalt-modified catalysts, significant percentages of cobalt, between 5 and 7% in weight, are present. FTIR, TGA, and EDAX studies confirmed that the addition of cobalt modifies the chemical structure of C–Pani, C–Ppy, and C–P3MT materials. Cyclic voltammetry shows reduction peaks corresponding to the ORR for all materials and kinetic parameters were calculated based on lineal voltammetry using RDE at different rotating speeds. It was found that C–P3MT–Co has highest exchange current densities, followed by C–Ppy and C–Ppy–Co. All samples have Tafel slopes between −110 and −120 V/dec, indicating that the first electron transfer is the decisive step in the global ORR. Potentiostatic tests showed an adequate stability of cobalt-modified samples in acid medium at ORR potentials. Based on the potential range at which ORR occurs, the exchange current density and stability tests, it is concluded that the best material for potential application as fuel cell cathode catalyst is C–Ppy–Co.     

IrxCo1−x (x = 0.3–1.0) alloy electrocatalysts, catalytic activities, and methanol tolerance in oxygen reduction reaction

Novel methanol-tolerant catalysts for oxygen reduction reaction (ORR), Ir_xCo_1−x/C (x = 0.3–1.0), were synthesized by a conventional impregnation method. These carbon-supported catalysts showed particle sizes of 2.7–5.0 nm. The catalyst activity and the catalyzed ORR kinetics were characterized by cyclic voltammetry and rotating disk electrode methods. Among these Ir_xCo_1−x/C catalysts, the alloy with a formula of Ir_xCo_1−x/C with x value in the range of 0.7–0.8 exhibited the highest mass and specific activities. Compared to a Pt/C catalyst, these alloy catalysts have much stronger methanol tolerance in terms of ORR onset potential (or open-circuit potential). Based on the rotating disk electrode measurements, it was confirmed that these Ir_xCo_1−x/C alloy catalysts could catalyze a complete four-electron transfer reaction of oxygen to water. These results strongly suggest that the novel Ir–Co metal alloy catalysts synthesized in this work could be promising for DMFC cathodes.     

Kinetics of oxygen reduction reaction on Corich core-Ptrich shell/C electrocatalysts

The nanostructured Co_{rich core}-Pt_{rich shell}/C electrocatalysts were prepared by combining the thermal decomposition and the chemical reduction methods. The particle size of homemade Co_{rich core}-Pt_{rich shell}/C analyzed by TEM was significantly greater than that of Pt grain size calculated from the XRD data due to the existence of Co in core. The mass activity and specific activity of oxygen reduction reaction (ORR) at the overpotential (η) of 0.1 V were 6.69 A g⁻¹ and 1.51 × 10⁻⁵ A cm⁻² for Pt/C, and 10.22 A g⁻¹ and 2.73 × 10⁻⁵ A cm⁻² for Co_{rich core}-Pt_{rich shell}/C in 0.5 M HClO₄ aqueous solution at 25 °C. The Tafel slopes of ORR on Pt/C and Co_{rich core}-Pt_{rich shell}/C electrocatalysts were obtained as 64 and 67 mV dec⁻¹ at a lower η (50–100 mV), and 116 and 110 mV dec⁻¹ at a higher η (120–200 mV). The exchange current densities of ORR on Pt/C and Co_{rich core}-Pt_{rich shell}/C evaluated based on the higher Tafel slope regions were 6.76 × 10⁻⁵ and 9.21 × 10⁻⁵ A cm⁻², respectively. The experimental results indicated that the ORR on Co_{rich core}-Pt_{rich shell}/C electrocatalyst in 0.5 M HClO₄ aqueous solution was a four electron transfer mechanism and first order with respect to the dissolved oxygen.