The effect of low concentrations of CO on H2 adsorption and activation on Pt/C. Part 1: In the absence of humidity

The presence of CO in the H₂-rich gas used as fuel for hydrogen fuel cells has a detrimental effect on PEMFC performance and durability at conventional operating conditions. This paper reports on an investigation of the effect of CO on H₂ activation on a fuel cell Pt/C catalyst close to typical PEMFC operating conditions using H₂-D₂ exchange as a probe reaction and to measure hydrogen surface coverage. While normally limited by equilibrium in the absence of impurities on Pt at typical fuel cell operating temperatures, the presence of ppm concentrations of CO increased the apparent activation energy (E_a) of H₂-D₂ exchange reaction (representing H₂ activation) from approximately 4.5-5.3 kcal mole⁻¹ (Bernasek and Somorjai (1975) [24], Montano et al. (2006) [25]) (in the absence of CO) to 19.3-19.7 kcal mole⁻¹ (in the presence of 10-70 ppm CO), similar to those reported by Montano et al. (2006) [25]. Calculations based on measurements indicate a CO surface coverage of approximately 0.55 ML at 80 °C in H₂ with 70 ppm CO, which coincide very well with surface science results reported by Longwitz et al. (2004) [5]. In addition, surface coverages of hydrogen in the presence of CO suggest a limiting effect on hydrogen spillover by CO. Regeneration of Pt/C at 80 °C in H₂ after CO exposure showed only a partial recovery of Pt sites. However, enough CO-free Pt sites existed to easily achieve equilibrium conversion for H₂-D₂ exchange. This paper establishes the baseline and methodology for a series of future studies where the additional effects of Nafion and humidity will be investigated.     

Synthesis, characterization and electrocatalytical behavior of Nb-TiO2/Pt nanocatalyst for oxygen reduction reaction

In order to point out the effect of the support to the catalyst for oxygen reduction reaction nano-crystalline Nb-doped TiO₂ was synthesized through a modified sol-gel route procedure. The specific surface area of the support, S_BET, and pore size distribution, were calculated from the adsorption isotherms using the gravimetric McBain method. The support was characterized by X-ray diffraction (XRD) technique.The borohydride reduction method was used to prepare Nb-TiO₂ supported Pt (20 wt.%) catalyst. The synthesized catalyst was analyzed by TEM technique.Finally, the catalytic activity of this new catalyst for oxygen reduction reaction was investigated in acid solution, in the absence and the presence of methanol, and its activity was compared towards the results on C/Pt catalysts.Kinetic analysis reveals that the oxygen reduction reaction on Nb-TiO₂/Pt catalyst follows four-electron process leading to water, as in the case of C/Pt electrode, but the Tafel plots normalized to the electrochemically active surface area show very remarkable enhancement in activity of Nb-TiO₂/Pt expressed through the value of the current density at the constant potential.Moreover, Nb-TiO₂/Pt catalyst exhibits higher methanol tolerance during the oxygen reduction reaction than the C/Pt catalyst.The enhancement in the activity of Nb-TiO₂/Pt is consequence of both: the interactions of Pt nanoparticles with the support and the energy shift of the surface d-states with respect to the Fermi level what changes the surface reactivity.     

The effect of low concentrations of CO on H2 adsorption and activation on Pt/C: Part 2—In the presence of H2O vapor

CO affects H₂ activation on supported Pt in the catalyst layers of a PEMFC and significantly degrades overall fuel cell performance. This paper establishes a more fundamental understanding of the effect of humidity on CO poisoning of Pt/C at typical fuel cell conditions (80 °C, 2 atm). In this work, direct measurements of hydrogen surface concentration on Pt/C were performed utilizing an H₂-D₂ switch with Ar purge (HDSAP). The presence of water vapor decreased the rate of CO adsorption on Pt, but had very little effect on the resulting CO surface coverage on Pt_S (θ_CO) at steady-state. The steady-state θ_COs at 80 °C for Pt exposed to H₂ (PH₂=1 atm) and a mixture of H₂/H₂O (1 atm H₂, 10%RH) were 0.70 and 0.66 ML, respectively. Furthermore, total strongly bound surface hydrogen measured after exposure to H₂/H₂O was, surprisingly, the sum of the exchangeable surface hydrogen contributed by each component, even in the presence of CO. In the absence of any evidence for strong chemisorption of H₂O on the carbon support with/without Pt, this additive nature and seemingly lack of interaction from the co-adsorption of H₂ and H₂O on Pt may be explained by the repulsion of strongly adsorbed H₂O to the stepped-terrace interface at high coverages of surface hydrogen.     

Enhanced performance of Ca-doped Pt/γ-Al2O3 catalyst for cyclohexane dehydrogenation

The promotion effects of Ca on catalytic performance of Ca-doped Pt/Al₂O₃ catalyst were investigated by varying Ca content and impregnation orders. The Ca-doped Pt/Al₂O₃ catalyst with atomic rate Ca/Pt = 5 exhibited the highest catalytic activity and stability. Detailed analyses show that low Ca content (Ca/Pt ＜ 5) could not efficiently promote hydrogen spillover from Pt-Al interface to the support and reduce the strong acid sites on the support. However, high Ca content (Ca/Pt > 5) decreases the initial activity due to the coverage of Ca on the support and increases the amount of Pt strongly interacting with Ca, which inhibits Pt reduction. Furthermore, the Ca promotion effect is more pronounced when Ca is added prior to Pt due to surface modification of Ca on Al₂O₃ support. This modified catalyst possesses more dispersed Pt, lower acidity and larger amount of spilled-over hydrogen.     

Electro-catalytic activity of enhanced CO tolerant cerium-promoted Pt/C catalyst for PEM fuel cell anode

Cerium-promoted Pt/C catalysts were prepared by one-pot synthesis process and applied as an anode material for CO tolerance in PEM fuel cell. Its physical properties were characterized by XRD and TEM techniques, which indicated that Pt nano-particles are highly dispersed on the carbon supports. The investigation focused on examining the CO tolerance in sulfur acid solution of Pt–CeO₂/C compared to Pt/C (JM). The hydrogen oxidation activity was strongly depended on the content of the cerium in the Pt catalyst which was detected by CV, LSV, CO-stripping and EIS techniques. Effect of the anode catalyst poisoning on hydrogen oxidation in the presence of CO was studied in single cells. Pt–CeO₂/C catalyst at the appropriate content of 20% Ce presented a very higher CO tolerant activity. A tentative mechanism is proposed for a possible role of a bi-functional synergistic effect between Pt and CeO₂ for the enhanced electro-oxidation of CO. CeO₂-promoted Pt/C catalyst may be one of the attractive candidates as CO tolerance anode material in PEMFC.     

Pt/ZrO2 catalyst for a single-stage water-gas shift reaction: Ti addition effect

Ti modified Pt/ZrO₂ catalysts were prepared to improve the catalytic activity of Pt/ZrO₂ catalyst for a single-stage WGS reaction and the Ti addition effect on ZrO₂ was discussed based on its characterization and WGS reaction test. Ti impregnation into ZrO₂ increased the surface area of the support and the Pt dispersion. The reducibility of the catalyst was enhanced in the controlled Ti impregnation (∼20 wt.%) over Pt/ZrO₂ by the Pt-catalysed reduction of supports, particularly, at the interface between ZrO₂ and TiO₂. The significant CO₂ gas band in the DRIFTS results of Pt/Ti[20]/ZrO₂ indicated that the Ti addition made the formate decomposition rate faster than the Pt/ZrO₂ catalyst, linked with the enhanced Pt dispersion and reducibility of the catalyst. Consequently, Ti impregnation over the ZrO₂ support led to a remarkably enhanced CO conversion and the reaction rate of Pt/Ti[20]/ZrO₂ increased by a factor of about 3 from the bare Pt/ZrO₂ catalyst.     

Effect of polyoxometalate amount deposited on Pt/C electrocatalysts for CO tolerant electrooxidation of H2 in polymer electrolyte fuel cells

Polyoxometalate-deposited Pt/C electrocatalysts are prepared by impregnation with various amounts of polyoxometalate (POM) anions (from 2 to 16.7 wt.% PMo₁₂O₄₀^3–) on the Pt/C catalyst. The prepared electrocatalysts show a high CO electrooxidation performance over a half-cell system for CO stripping voltammetry, and CO tolerant electrooxidation of H₂ is further demonstrated over a proton exchange membrane fuel cell by using CO-containing H₂ gas feeds (0, 10, 50, and 100 ppm CO in H₂). In the CO stripping voltammograms, the onset and peak potentials for the CO oxidation appear to decrease as the POM deposition is increased, indicating that the electrooxidation of CO undergoes more efficiently on the catalyst surface with the deposited POMs on the Pt/C catalysts. In the single fuel cell tests with the CO-containing H₂ gases, the higher current density is also generated with the larger amounts of deposited POMs on the Pt/C catalysts. Importantly, the charge transfer resistance R_p appears to decrease monotonically with the POM amounts, which was measured by electrochemical impedance spectroscopy. Physico-chemical characterizations with electrocatalytic analyses show that the deposited POMs hardly affect the active phase of Pt catalyst itself but can help the electrooxidation of H₂ by efficiently oxidizing CO to prevent the Pt catalyst from poisoning. Consequently, this POM-deposited Pt/C catalyst can serve as a promising CO tolerant anode catalyst for the polymer electrolyte fuel cells that are operated with hydrocarbons-reformed H₂ fuel gases.     

Identification and quantification of performance losses for PEM fuel cells as determined by selective chemisorption and ESA measurements

Membrane electrode assemblies (MEAs) were fabricated using a high frequency spraying technique. Electrocatalyst powders were directly sprayed onto an electrolyte membrane by ultrasonic spraying. The weight ratios of Nafion to Pt/C were studied, and the ratio about 50–62% yields the maximum current density at the ohmic and gas diffusion region of the polarization curve. Cross sections of the MEA were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); images from SEM indicate that the supported catalysts are arranged in a layered manner and that pores in the catalyst coated layer (CCL) are formed. TEM images show the ionomer layer exists between the reaction zone and the electrolyte membrane. The concentration of Pt surface sites in the MEA was measured by selective H₂ chemisorption methods at dry conditions and by cyclic voltammetry (CV) for the electrochemical reaction. The chemisorption measurements indicate that ∼52% of the active Pt surface are lost due to ionomer coverage and CV shows an additional 28% of the Pt surface are lost due to blockage by condensed water and the isolation of Pt sites from ionomer and electrical conduction. In total 80% of the Pt surface sites present on the 20 wt% Pt/C starting material are lost during preparation and at operating conditions. Thus, both mechanisms of activity loss are separately identified and quantified.     

Water-gas shift reaction over magnesia-modified Pt/CeO2 catalysts

Fuel cells have risen as a clean technology for power generation and much effort has been done for converting renewable feedstock in hydrogen. The water-gas shift reaction (WGS) can be applied aiming at reducing the CO concentration in the reformate. As Pt/CeO₂ catalysts have been pointed out as an alternative to the industrial WGS catalysts, the modification of such systems with magnesium was investigated in this work. It was shown that the addition of MgO to Pt/CeO₂ increased the activity and stability of the catalyst irrespective of the preparation method used, either impregnation or co-precipitation. Based on TPR and IR spectroscopy experiments, it was seen that the presence of magnesium improved ceria reduction favoring the creation of OH groups, which are considered the active sites for the WGS reaction. The evolution of the surface species formed under reaction conditions (CO, H₂O, H₂) observed by DRIFTS evidenced that the formation of formate species and the generation of CO₂ is closely attached to each other; under a reaction stream containing hydrogen the presence of formate species showed to be more relevant while the CO₂ formation was hindered. It is suggested that the addition of MgO favors the formate decomposition and lower the carbonate concentration on the catalyst surface during WGS reaction.     

Enhanced hydrogen generation from methanol aqueous solutions over Pt/MoO3/TiO2 under ultraviolet light

The photocatalytic activity in hydrogen production from methanol reforming can be significantly enhanced by Pt/MoO₃/TiO₂ photocatalysts. Compared with Pt/P25, the photocatalytic activity of optimized Pt/MoO₃/TiO₂ shows an evolution rate of 169 μmol/h/g of hydrogen, which is almost two times higher than that of Pt/P25. XRD and Raman spectra show that MoO₃ are formed on the surface of TiO₂. It is found that with the bulk MoO₃ just formed, the catalyst shows the highest activity due to a large amount of heterojunctions and the high crystallinity of MoO₃. The HRTEM image showed a close contact between MoO₃ and TiO₂. It is proposed that the Z-scheme type of heterojunction between MoO₃ and TiO₂ is responsible for the improved photocatalytic activity. The heterojunction structure of MoO₃/TiO₂ does not only promote the charge separation, but also separates the reaction sites, where the oxidation (mainly on MoO₃) and reduction (on TiO₂) reactions occurred.     

Development of durable copper catalyst for hydrogen production by high temperature methanol steam reforming

Highly durable catalyst for high temperature methanol steam reforming is required for a compact hydrogen processor. Deactivation of a coprecipitated Cu/ZnO/ZrO₂ catalyst modified with In₂O₃ is very gradual even in the high temperature methanol steam reforming mainly at 500 °C, but the initial activity is considerably low. Addition of Y₂O₃ to Cu/ZnO/ZrO₂/In₂O₃ increases its initial activity due to the higher Cu surface amount, while the activity comes gradually close to that for the catalyst without Y₂O₃ during the reaction. Coprecipitation of Cu/ZnO/ZrO₂/Y₂O₃/In₂O₃ on a zirconia support triply increases the overall activity by keeping the durability while the amount of the coprecipitated portion is a half of that without the support. On the composite catalyst, sintering of Cu particles is suppressed. The surface Cu amount is similar to that without the support, but the Cu surface activity is much higher probably because of the small Cu particle size.     

Nickel on a macro-mesoporous Al2O3@ZrO2 core/shell nanocomposite as a novel catalyst for CO methanation

A novel nickel catalyst supported on Al₂O₃@ZrO₂ core/shell nanocomposites was prepared by the impregnation method. The core/shell nanocomposites were synthesized by depositing zirconium species on boehmite nanofibres. This contribution aims to study the effects of the pore structure of supports and the zirconia dispersed on the surface of the alumina nanofibres on the CO methanation. The catalysts and supports were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR), nitrogen adsorption–desorption, and thermogravimetry and differential thermal analysis (TG-DTA). The catalytic performance of the catalysts for CO methanation was investigated at a temperature range from 300 °C to 500 °C. The results of the characterization indicate that the metastable tetragonal zirconia could be stably and evenly dispersed on the surface of alumina nanofibres. The interlaced nanorods of the Al₂O₃@ZrO₂ core/shell nanocomposites resulted in a macropore structure and the spaces between the zirconia nanoparticles dispersed on the alumina nanofibres formed most of the mesopores. Zirconia on the surface of the support promoted the dispersion and influenced the reduction states of the nickel species on the support, so it prevented the nickel species from sintering as well as from forming a spinel phase with alumina at high temperatures, and thus reduced the carbon deposition during the reaction. With the increase of the zirconia content in the catalyst, the catalytic performance for the CO methanation was enhanced. The Ni/Al₂O₃@ZrO₂-15 exhibited the highest CO conversion and methane selectivity at 400 °C, but they decreased dramatically above or below 400 °C due to the temperature sensitivity of the catalyst. Ni/Al₂O₃@ZrO₂-30 exhibited a high and constant rate of methane formation between 350 °C and 450 °C. The excellent catalytic performance of this catalyst is attributed to its reasonable pore structure and good dispersion of zirconia on the support. This catalyst has great potential to be further studied for the future industrial use.     

Influence of ink preparation with the untreated and the burned Pt/C catalysts for proton exchange membrane fuel cells

This research discovers the burning reaction of the Pt/C catalyst on ink preparation and the effect of the untreated and burned Pt/C catalysts for the proton exchange membrane fuel cells (PEMFCs). The platinum nanoparticles on the carbon support aggregate to form bigger cluster sizes due to the burning reaction of the untreated Pt/C catalyst reacting with the Nafion solution or isopropyl alcohol. After several times of “purposely” burning reaction, the specific surface area of the fully burned Pt/C reduces from 150.9 to 46.6 m² g⁻¹ which is 3 times smaller than the untreated Pt/C catalyst. The crystallite size of platinum catalyst changes from 8.4 to 46.2 nm via the calculation of Debye–Scherrer equation from X-ray diffraction (XRD) and the electrochemical surface area (ECSA) obviously decreases from 85.6 to 14.8 m² g⁻¹. The variation of the ratio of Pt/C to Nafion influences the consequent electrochemical performances. Three catalyst coated membranes (CCMs) coated with untreated, fully burned, and partial burned Pt/C catalysts are analyzed and compared in this study. The CCM coated with the untreated Pt/C catalyst shows the best polarization curve which presents the peak power density, 897 mW cm⁻². Moreover, it presents the slowest degradation rate (0.1 mA min⁻¹) at a constant voltage of 0.4 V for 4000 min, even though the result of Nyquist plots is slightly worse than others The work confirms that the burning reaction of Pt/C catalyst influences the electrochemical performance and structural balance of the catalyst layer.     

Nanosized Pt/IrO2 electrocatalyst prepared by modified polyol method for application as dual function oxygen electrode in unitized regenerative fuel cells

A new generation of highly efficient and non-polluting energy conversion and storage systems is vital to meeting the challenges of global warming and the finite reality of fossil fuels. In this work, nanosized Pt/IrO₂ electrocatalysts are synthesized and investigated for the oxygen evolution and reduction reactions in unitized regenerative fuel cells (URFCs). The catalysts are prepared by decorating Pt nanoparticles (2–10 nm) onto the surface of a nanophase IrO₂ (7 nm) support using an ultrasonic polyol method. The synthesis procedure allows deposition of metallic Pt nanoparticles on Ir-oxide without causing any occurrence of metallic Ir. The latter is significantly less active for oxygen evolution than the corresponding oxide. This process represents an important progress with respect to the state of the art in this field being the oxygen electrocatalyst generally obtained by mechanical mixing of Pt and IrO₂. The nanosized Pt/IrO₂ (50:50 wt.%) is sprayed onto a Nafion 115 membrane and used as dual function oxygen electrode, whereas 30 wt.% Pt/C is used as dual function hydrogen electrode in the URFC. Electrochemical activity of the membrane-electrode assembly (MEA) is investigated in a single cell at room temperature and atmospheric pressure both under electrolysis and fuel cell mode to assess the perspectives of the URFC to operate as energy storage device in conjunction with renewable power sources.     

A highly stable TiB2-supported Pt catalyst for polymer electrolyte membrane fuel cells

Pt nanoparticles supported on TiB₂ conductive ceramics (Pt/TiB₂) have been prepared through a liquid reduction method, where the TiB₂ surfaces are stabilized with perfluorosulfonic acid. The prepared Pt/TiB₂ catalyst is characterized with X-ray diffraction (XRD) and TEM techniques, and a rotating disk electrode (RDE) apparatus. The Pt nanoparticles are found to uniformly disperse on the surface of the TiB₂ particles with narrow size distribution. The electrochemical stability of Pt/TiB₂ is evaluated and found highly electrochemically stable compared to a commercial Pt/C catalyst. Meanwhile, the catalyst also shows comparable performance for oxygen reduction reaction (ORR) to the Pt/C. The mechanism of the remarkable stability and comparable activity for ORR on Pt/TiB₂ is also proposed and discussed.     

A comparative study of Pt/C cathodes in Sn0.9In0.1P2O7 and H3PO4 ionomers for high-temperature proton exchange membrane fuel cells

New Pt/C cathodes with many reaction sites for the oxygen reduction reaction as well as high tolerance to Pt corrosion have been designed for high-temperature proton exchange membrane fuel cells (PEMFCs), wherein a composite mixture of Sn_0.9In_0.1P₂O₇ (SIPO) and sulfonated polystyrene-b-poly(ethylene/butylene)-b-polystyrene (sSEBS) functioned as an ionomer. The microstructure of the Pt-SIPO-sSEBS/C cathode was characterized by homogeneous distribution of the ionomer over the catalyst layer and close contact between the ionomer and the Pt/C powder. As a result, the activation and concentration overpotentials of the Pt-SIPO-sSEBS/C cathode between 100 and 200 °C were lower than those of an H₃PO₄-impregnated Pt/C cathode, which suggests that the present ionomer can avoid poisoning of Pt by phosphate anions and the limitation of gas diffusion through the catalyst layer. Moreover, agglomeration of Pt in the Pt-SIPO-sSEBS/C cathode was not observed during a durability test at 150 °C for 6 days, although it was significant in the Pt-H₃PO₄/C cathode. Therefore, it is concluded that the Pt-SIPO-sSEBS/C electrode is a very promising cathode candidate for high-temperature PEMFCs.     

Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell

Thin film Pt/TiO₂ catalysts are evaluated in a polymer electrolyte electrochemical cell. Individual thin films of Pt and TiO₂, and bilayers of them, were deposited directly on Nafion membranes by thermal evaporation with varying deposition order and thickness (Pt loadings of 3–6 μg cm⁻²). Structural and chemical characterization was performed by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) polarization plots show that the presence of a thin TiO₂ layer between the platinum and the Nafion increases the performance compared to a Pt film deposited directly on Nafion. Based on the TEM analysis, we attribute this improvement to a better dispersion of Pt on TiO₂ compared to on Nafion and in addition, substantial proton conduction through the thin TiO₂ layer. It is also shown that deposition order and the film thickness affects the performance.     

The effect of low concentrations of tetrachloroethylene on H2 adsorption and activation on Pt in a fuel cell catalyst

The poisoning effect of tetrachloroethylene (TTCE) on the activity of a Pt fuel cell catalyst for the adsorption and activation of H₂ was investigated at 60 °C and 2 atm using hydrogen surface concentration measurements. The impurity was chosen as a model compound for chlorinated cleaning and degreasing agents that may be introduced into a fuel cell as a contaminant at a fueling station and/or during vehicle maintenance. In the presence of only H₂, introduction of up to 540 ppm TTCE in H₂ to Pt/C resulted in a reduction of available Pt surface atoms (measured by H₂ uptake) by ca. 30%, which was not enough to shift the H₂-D₂ exchange reaction away from being equilibrium limited. Exposure of TTCE to Pt/C in a mixed redox environment (hydrogen + oxygen), similar to that at the cathode of a fuel cell, resulted in a much more significant loss of Pt surface atom availability, suggesting a role in TTCE decomposition and/or Cl poisoning. Regeneration of catalyst activity of poisoned Pt/C showed the highest level of recovery when regenerated in only H₂, with much less recovery in H₂ + O₂ or O₂. The results from this study are in good agreement with those found in a fuel cell study by Martínez-Rodríguez et al. [2] and confirm that the majority of the poisoning from TTCE on fuel cell performance is most likely at the cathode, rather than the anode.     

Pd nanoparticles supported on WO3/C hybrid material as catalyst for oxygen reduction reaction

Pd nanoparticles supported on WO₃/C hybrid material have been developed as the catalyst for the oxygen reduction reaction (ORR) in direct methanol fuel cells. The resultant Pd–WO₃/C catalyst has an ORR activity comparable to the commercial Pt/C catalyst and a higher activity than the Pd/C catalyst prepared with the same method. Based on the physical and electrochemical characterizations, the improvement in the catalytic performance may be attributed to the small particle sizes and uniform dispersion of Pd on the WO₃/C, the strong interaction between Pd and WO₃ and the formation of hydrogen tungsten bronze which effectively promote the direct 4-electron pathway of the ORR at Pd.     

Cyclic voltammetry and X-ray photoelectron spectroscopy studies of electrochemical stability of clean and Pt-modified tungsten and molybdenum carbide (WC and Mo2C) electrocatalysts

The electrochemical stability of tungsten carbide (WC), Pt-modified WC, molybdenum carbide (Mo₂C), and Pt-modified Mo₂C has been examined using an in situ electrochemical half-cell in combination with X-ray photoelectron spectroscopy (XPS). The WC surface, created via the carburization of a tungsten foil, was electrochemically stable to ∼0.8 V with respect to the normal hydrogen electrode (NHE) when exposed to dilute sulfuric acid. At higher potentials, XPS confirmed the surface oxidation of WC to form W_xO_y species. The deposition of submonolayer coverage of Pt on the WC surface increased the region of stability of WC, extending the onset of catalyst oxidation to ∼1.0 V (NHE). These results suggest that both WC and Pt/WC have the potential to be used as anode electrocatalysts. In contrast, both Mo₂C and Pt-modified Mo₂C underwent oxidation at ∼0.4 V (NHE), indicating that molybdenum carbides are not stable enough for applications as anode electrocatalysts.