Investigation of tungsten carbide supported Pd or Pt as anode catalysts for PEM fuel cells

In this contribution, we present results of electrochemical characterization of prepared tungsten carbide supported palladium and platinum and Vulcan XC-72 supported palladium. These catalysts were employed as anode catalysts in PEMFC and results are compared to commercial platinum catalyst. Platinum seems to be irreplaceable as a proton exchange membrane fuel cell (PEMFC) catalyst for both the anode and the cathode, yet the high price and limited natural resources are holding back the commercialization of the PEMFCs. Tungsten carbide is recognized as promising catalyst support having the best conductivity among interstitial carbides. Higher natural resources and significantly lower price make palladium good candidate for replacement of the platinum catalyst. The presented results show that all prepared catalysts are very active for the hydrogen oxidation reaction. Linear sweep voltammetry curves of Pd/C and Pd/WC show existence of peaks at 0.07 V vs. RHE, which is assigned to absorbed hydrogen. H₂|Pd/WC|Nafion117|Pt/C|O₂ fuel cell has almost the same efficiency and similar power output as commercial platinum catalyst.     

The poisoning level of Pt/C catalysts used in PEM fuel cells by the hydrogen feed gas impurities: The bonding strength

Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed despite it nearly always contains carbon monoxide or ammonia. In this paper, the microcalorimetry technique was employed to study and compare the poisoning effect of pollutants such as CO and NH₃ on three commercial carbon-supported platinum catalysts with high Pt loading, aimed to be used in PEMFCs applications. Microcalorimetric measurements were performed at 80 °C and the results were compared with those obtained from hydrogen adsorption in similar conditions. All the catalysts exhibited significantly higher differential heats of CO adsorption in comparison with NH₃ and hydrogen adsorption, indicating that carbon monoxide will be primarily adsorbed in case of co-adsorption, while ammonia and hydrogen will compete in the adsorption process on the same type of active sites. The irreversibly (chemically) amount of adsorbed molecules on Pt/C surfaces decreases in the order: CO >> NH₃ > H₂.     

Electrostatic self-assembly of Pt nanoparticles on hexagonal tungsten oxide as an active CO-tolerant hydrogen oxidation electrocatalyst

Developing CO-tolerant electrocatalysts is of great importance for the practical use of proton exchange membrane fuel cells (PEMFCs) fed with reforming hydrogen. Transitional metal oxides are a class of promising component to (i) alleviate the CO adsorption on Pt and (ii) provide as a stabilized support for Pt nanoparticles. Herein, we developed an electrostatic assembly strategy to deposit Pt nanoparticles uniformly on the hexagonal tungsten oxides (hex-WO₃) modified by polyethleneimine (PEI). It is the first time employing hex-WO₃ with biomimetic proton channels and mixed ionic electronic conductivity as supports in PEMFCs. Also, this work is the first report using PEI as a linker to assemble Pt nanoparticles and metal oxide supports. The Pt/PEI-hex-WO₃ composites possess excellent dispersion of Pt nanoparticles with average size less than 3 nm even at Pt loadings as high as 40 wt%. The Pt/PEI-hex-WO₃ catalysts exhibit superior catalytic activity and electrochemical stability for the hydrogen electro-oxidation (HOR) in the presence of CO, and good PEMFC performance compared to the conventional carbon-supported Pt catalysts, attributed to the bifunctional mechanism and a strong metal-support interaction (SMSI).     

Kinetics and electrocatalytic activity of IrCo/C catalysts for oxygen reduction reaction in PEMFC

The purpose of this study is to develop a novel binary Iridium-Cobalt/C catalyst as a suitable substitute for platinum/C applied in proton exchange membrane fuel cells (PEMFCs). The carbon-supported IrCo catalysts were successfully synthesized using IrCl₃ and C₄H₆CoO₄ as the Ir and Co precursors respectively, in ethylene glycol (EG) refluxing at 120 °C. The nanostructured catalysts were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscope (TEM). Homogeneous catalyst particles supported on carbon showed a size of proximately 2 nm. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted for the characterization of the catalyst performances. With a cathodic loading of 0.4 mg_Ir cm⁻², 20%Ir-30%Co/C achieved a maximum power density of 501.6 mW cm⁻² at 0.418 V, with a 50 cm² H₂/O₂ single cell. Although such a performance is about 26% lower than commercial Pt/C catalyst, it is still helpful in terms of Pt replacement and cost reduction.     

CO tolerance and durability study of PtMe(Me = Ir or Pd) electrocatalysts for H2-PEMFC application

In the present work, carbon supported PtMe (Me = Ir or Pd) electrocatalysts, with different atomic ratios (Pt/Me (20 wt%) = 3:1, 1:1, 1:3), are thoroughly investigated towards CO tolerance and durability, as anode and cathode for H₂-PEMFCs (hydrogen fed proton exchange membrane fuel cells) application. The electrocatalysts are prepared via a pulse-microwave assisted polyol synthesis method and their durability and electrocatalytic activity in presence and absence of CO are evaluated using the techniques of electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and rotating disk electrode (RDE). For the investigation of CO tolerance a protocol is set that could be used by other research groups, since various procedures are reported in literature. It is found that Pd/C shows higher CO tolerance than Pt/C, while the PtPd₃/C exhibits the highest CO tolerance ability, even after being exposed for 9 h at 400 ppm CO. Despite the fact that Pt₃Ir/C shows higher CO tolerance ability than Pt/C, it cannot resist at such high CO concentrations for more than 6 h. Finally, it is found that PtIr/C and PtPd/C exhibit very good durability even after 5000 accelerated durability test (ADT) cycles, while Pt₃Pd/C and PtPd/C present the highest mass activities (339.4 and 410 mA/mg_Pt respectively at 0.9 V), which are 4 and 5 times higher than the one observed over commercial Pt/C (82.75 mA/mg_Pt).     

Performance of novel bimetallic carbonyl clusters as PEM fuel cell anodes,a comparative study

This work describes the performance of novel bimetallic catalysts, prepared from ruthenium, rhodium and iridium carbonyl clusters by a thermolysis procedure in o-dichlorobenzene. The electrochemical characterization by the rotating disk electrode technique in 0.5 mol L⁻¹ H₂SO₄ showed that the Ru_xIr_y(CO)_n, Ru_xRh_y(CO)_n and Rh_xIr_y(CO)_n clusters are able to perform both the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR), even in the presence of fuel cell contaminants such as CO and methanol, respectively. These promising results led us to evaluate the new materials as electrodes in a single fuel cell, using a Fuel Cell Test System designed and built in our group. The performance results of the three bimetallic clusters as anodes in a hydrogen PEM fuel cell are presented in this work. In the tests different H₂ and O₂ gas flows were fed into the cell to determine the most adequate ratio for maximum power. In the absence of CO, the results showed that although the three bimetallic materials had a similar performance to that of platinum with low flows of both reactants, Ru_xIr_y(CO)_n showed the best electrocatalytic parameters. When the hydrogen fuel feed was contaminated with 100 ppm and 0.5% CO, the commercial platinum activity decreased considerably or was completely lost. However, while the current density of the novel materials also decreased in the presence of CO, it was significantly above that of platinum nanoparticles, the Rh_xIr_y(CO)_n and Ru_xIr_y(CO)_n catalysts showing the best performance in the presence of 100 ppm CO and 0.5% CO respectively. These results are promising in the context of PEM fuel cells using reforming hydrogen.     

In-situ investigation on the CO tolerance of carbon supported Pd–Pt electrocatalysts with low Pt content by electrochemical impedance spectroscopy

Carbon supported Pd–Pt electrocatalysts (Pd–Pt/C) with low Pt content were investigated in proton exchange membrane fuel cells (PEMFCs) with pure H₂ and CO/H₂ as the feeding fuels, respectively. The Pd–Pt/C catalysts showed high activity for hydrogen oxidation reaction (HOR) and improved CO tolerance. Electrochemical impedance spectroscopy (EIS) was employed to probe the in-situ information of the improved CO tolerance. The dependence of Nyquist plots and Bode plots on current density and feeding gas was investigated in low polarization region. The results of EIS analysis indicated that the improved CO tolerance of Pd–Pt/C catalysts can be attributed to the lower coverage of CO on the Pd–Pt bimetal than that on the pure Pt.     

Pt/C catalyst impregnated with tungsten-oxide – Hydrogen oxidation reaction vs. CO tolerance

Hydrogen Oxidation Reaction (HOR) is anode reaction in Proton exchange membrane fuel cells (PEMFCs) and it has very fast kinetics. However, the purity of fuel (H₂) is very important and can slow down HOR kinetics, affecting the overall PEMFC performance. The performance of commercial Pt/C catalyst impregnated with WO_x, as a catalyst for HOR, was investigated using a set of electrochemical methods (cyclic voltammetry, linear scan voltammetry and rotating disk electrode voltammetry). In order to deepen the understanding how WO_x species can contribute CO tolerance of Pt/C, a particular attention was paid to CO poisoning. In the absence of CO, HOR is under diffusion limitations and HOR kinetics is not affected by WO_x species. Appreciable HOR current on the electrodes pre-saturated with CO_ads at potentials above 0.3 V vs. RHE, which is not observed for pure Pt/C, was discussed in details. HOR liming diffusion currents for higher concentrations of W are reached at high anodic potentials. The obtained results were explained by donation of OH_ads by WO_x phase for CO_ads removal in the mid potential region and reduced reactivity of Pt surface sites in the vicinity of the Pt|WO_x interface. The obtained results can provide guidelines for development of novel CO tolerant PEMFC anode catalysts.     

Direct borohydride fuel cell using Ni-based composite anodes

In this study, nickel-based composite anode catalysts consisting of Ni with either Pd on carbon or Pt on carbon (the ratio of Ni:Pd or Ni:Pt being 25:1) were prepared for use in direct borohydride fuel cells (DBFCs). Cathode catalysts used were 1 mg cm⁻² Pt/C or Pd electrodeposited on activated carbon cloth. The oxidants were oxygen, oxygen in air, or acidified hydrogen peroxide. Alkaline solution of sodium borohydride was used as fuel in the cell. High power performance has been achieved by DBFC using non-precious metal, Ni-based composite anodes with relatively low anodic loading (e.g., 270 mW cm⁻² for NaBH₄/O₂ fuel cell at 60 °C, 665 mW cm⁻² for NaBH₄/H₂O₂ fuel cell at 60 °C). Effects of temperature, oxidant, and anode catalyst loading on the DBFC performance were investigated. The cell was operated for about 100 h and its performance stability was recorded.     

Theoretical insights into electronic structures and durability of single-atom Pd/TiN catalysts

Density functional theory (DFT) calculations have been performed to evaluate the metal-support interactions between palladium atom and titanium nitride surface (Pd–TiN). N vacancy sites on defective TiN surface can stabilize Pd single atom under strongly oxidizing conditions, and surface defect-mediated stabilization is accompanied by obvious charge redistributions between Pd and substrate. The adsorption of several gas species on stable Pd–TiN surfaces is also explored to understand catalytic reactions. It is found that O, H, OH, O₂, and CO favorably adsorb on Ti atop site, while CO₂ and H₂ prefer the hollow and Pd atop site, respectively. Moreover, co-adsorption with either H or OH weakens the CO-surface interactions, indicating the CO poisoning effect would be alleviated under both acidic and alkaline conditions. This study provides a perspective to understand the support effect of Pd–TiN, which sheds light on the design of high-performance Pd-based electrodes for proton exchange membrane fuel cells (PEMFCs).     

Effect of Pd-impregnation on performance,sulfur poisoning and tolerance of Ni/GDC anode of solid oxide fuel cells

Sulfur tolerance of Ni/Gd₂O₃–CeO₂ (Ni/GDC) anodes promoted by impregnated palladium nanoparticles is investigated using the electrochemical impedance spectroscopy (EIS) and galvanostatic polarization techniques in the H₂–H₂S fuels at 800 °C. The anodes are alternately polarized in pure H₂ and H₂S-containing H₂ fuels with H₂S concentration gradually increased from 5 to 700 ppm at 200 mA cm⁻². The degradation in performance for the hydrogen oxidation in H₂S-containing H₂ fuels especially at low H₂S concentration is substantially smaller on Pd-impregnated Ni/GDC cermet anodes, as compared to that on pure Ni/GDC anodes. The potential of Pd-impregnated Ni/GDC electrodes measured in pure H₂ decreases by 0.07 V after exposure to H₂S-containing H₂ fuels, substantially smaller than 0.13 V observed on pure Ni/GDC anodes under identical test conditions. The results show that Pd impregnation significantly enhances the sulfur tolerance of Ni/GDC cermet anodes particularly in the low H₂S concentration range (e.g., <100 ppm). The results indicate that the enhanced sulfur tolerance of Pd impregnated Ni/GDC anodes is most likely due to the promotion effect of impregnated Pd nanoparticles on the hydrogen dissociation and diffusion processes. The reduced moderation of the morphology and microstructure of the anodes in the presence of Pd nanoparticles may be the result of weaker interaction or adsorption of sulfur on Ni and GDC phases.     

Methane-fueled thin film micro-solid oxide fuel cells with nanoporous palladium anodes

Methane-fueled thin film micro-solid oxide fuel cells (μSOFCs) based on palladium (Pd) anodes are discussed in this article. The μSOFCs are composed of porous platinum (Pt) cathodes, 8 mol.% yttria-stabilized zirconia (YSZ) ultrathin electrolytes, and Pd anodes - specifically, a Pt/YSZ/Pd heterostructure synthesized by physical vapor deposition. The Pt/YSZ/Pd μSOFCs exhibit a power density of 385 mW cm⁻² and an open circuit voltage of 0.77 V at 550 °C. A detailed study on synthesis, microstructure and functional properties of the nanoporous Pd films is presented. Possible anodic methane reactions, carbon deposition on Pd anodes, and carbon suppression approaches are discussed. The results are of potential relevance to advancing low temperature micro-fuel cell technology using hydrocarbon fuels.     

Enhancement of anodic oxidation of formic acid on palladium decorated Pt/C catalyst

A palladium decorated Pt/C catalyst, Pt@Pd/C, is prepared by a colloidal approach with a small amount of platinum as core. It is found that the catalyst shows excellent activity towards anodic oxidation of formic acid at room temperature and its activity is 60% higher than that of Pd/C. Decoration of palladium shell on the platinum core is supported by XPS results. Due to the use of platinum as core, active components are dispersed very well and the particle sizes are smaller than those of Pd/C. The cyclic voltammetry measurement clearly shows synthetic electro-oxidation effects of formic acid on Pt@Pd/C. It is speculated that the high performance of Pt@Pd/C may result from the unique core-shell structure and synergistic effect of Pt and Pd at the interface. The preparation method for Pt@Pd/C reported in this work will provide additional options for the design of catalysts for direct formic acid fuel cell (DFAFC).     

Deactivation of Pt/VC proton exchange membrane fuel cell cathodes by SO2, H2S and COS

Sulfur contaminants in air pose a threat to the successful operation of proton exchange membrane fuel cells (PEMFCs) via poisoning of the Pt-based cathodes. The deactivation behavior of commercial Pt on Vulcan carbon (Pt/VC) membrane electrode assemblies (MEAs) is determined when exposed to 1 ppm (dry) of SO₂, H₂S, or COS in air for 3, 12, and 24 h while held at a constant potential of 0.6 V. All the three sulfur compounds cause the same deactivation behavior in the fuel cell cathodes, and the polarization curves of the poisoned MEAs have the same decrease in performance. Sulfur coverages after multiple exposure times (3, 12, and 24 h) are determined by cyclic voltammetry (CV). As the exposure time to sulfur contaminants increases from 12 to 24 h, the sulfur coverage of the platinum saturates at 0.45. The sulfur is removed from the cathodes and their activity is partially restored both by cyclic voltammetry, as shown by others, and by successive polarization curves. Complete recovery of fuel cell performance is not achieved with either technique, suggesting that sulfur species permanently affect the surface of the 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.     

Toluene hydrogenation over Pd and Pt catalysts as a model hydrogen storage process using low grade hydrogen containing catalyst inhibitors

The effects of CO and H₂S as catalyst inhibitors on the rate of toluene hydrogenation were studied as a means of hydrogen storage using low-grade hydrogen. Pd/SiO₂ suffered serious negative effects from catalyst inhibitors; however, Pd/TiO₂–SiO₂ exhibited high CO and H₂S tolerance because the acidic support decreased the electron density of the Pd metal particles, which, in turn, decreased the interaction between the Pd surface and CO (or H₂S). The TiO₂–SiO₂-supported Pd catalyst exhibited activity greater than that of the TiO₂–SiO₂-supported Pt catalyst in the presence of CO; however, it exhibited lower activity in presence of H₂S. Catalyst characterization after sulfidation with H₂S revealed that Pd particles were fully sulfided, whereas Pt particles were sulfided only on their surface. We concluded that Pd catalysts supported on acidic oxides exhibit excellent activity toward toluene hydrogenation in the presence of CO and that Pt catalysts exhibit excellent activity in the presence of H₂S.     

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.     

Significant role of surface activation on Pd enriched Pt nano catalysts in promoting the electrode kinetics of ethanol oxidation: Temperature effect,product analysis & theoretical computations

The present investigation involves the electrode kinetic studies on ethanol electro-oxidation in alkaline medium within the temperature range 20–80 °C on carbon supported platinum and platinum–palladium alloys of different compositions obtained through NaBH₄ reduction of the respective precursor salts. The experimental work was further substantiated by computational work based on DFT calculations. Different textural properties of the catalyst matrix were determined by the application of BET equation to the adsorption isotherms. Surface morphology, structure and composition of the catalyst matrices were revealed through XPS, XRD, TEM and EDAX analyses. Electro-analytical techniques were deployed to derive the kinetic parameters along with the activation energies for the oxidation reactions, studied over the mentioned range of the temperature. Further attempt was made to estimate the intermediates formed during the course of the reaction by the help of ion exchange chromatography. The incorporation of Pd into Pt matrix was found to decrease the charge transfer resistance and activation energy of the ethanol oxidation, enabling faster reaction kinetics and better conversion of the fuel into the end products, presumably by alleviating the problem of CO poisoning which remains as one of the critical issues with bare Pt catalyst. The investigation finally include the density functional theory computations on some Pt–Pd mixed cluster configurations along with pure Pt and Pd to realize the effect of local electronic structures and geometry on the relative catalytic activity of the alloyed and single metal particles. It was predicted that the highest activity of the alloyed catalyst Pt₃₀Pd₇₀/C is due to the optimal presence of Pt & Pd in the matrix, rendering the surface activation by OH^– and favorable charge distribution among the Pt and Pd sites leading to improved CO oxidation within the fuel cell potential range.     

A novel proton exchange membrane fuel cell anode for enhancing CO tolerance

A novel proton exchange membrane fuel cell (PEMFC) anode which can facilitate the CO oxidation by air bleeding and reduce the direct combustion of hydrogen with oxygen within the electrode is described. This novel anode consists of placing Pt or Au particles in the diffusion layer which is called Pt- or Au-refined diffusion layer. Thus, the chemical oxidation of CO occurs at Pt or Au particles before it reaches the electrochemical catalyst layer when trace amount of oxygen is injected into the anode. All membrane electrode assemblies (MEAs) composed of Pt- or Au-refined diffusion layer do perform better than the traditionary MEA when 100 ppm CO/H₂ and 2% air are fed and have the performance as excellent as the traditionary MEA with neat hydrogen. Furthermore, CO tolerance of the MEAs composed of Au-refined diffusion layer was also assessed without oxygen injection. When 100 ppm CO/H₂ is fed, MEAs composed of Au-refined diffusion layer have the slightly better performance than traditionary MEA do because Au particles in the diffusion layer have activity in the water gas shift (WGS) reaction at low temperature.     

The effect of H2S and CO mixtures on PEMFC performance

Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed which always contains carbon dioxide (CO) and hydrogen sulfide (H₂S). Trace amount of CO and H₂S can cause considerable cell performance losses. A comparison between the effect of CO and that of H₂S on PEMFC performance was made in this paper. Under the same conditions, the H₂S poisoning rate is much higher than CO because of different adsorption intensity. When the fuel stream contains the gas mixture (25 ppm CO and 25 ppm H₂S), the fuel cell performance deteriorates more quickly than 50 ppm CO but slowly than 50 ppm H₂S and can be only partially recovered by reintroducing neat H₂. The resulting effects of the mixtures can be divided into two parts roughly: during the inception phase, the cell voltage drops quickly and the actual values of anode overvoltage are bigger than the corresponding calculated values; then the deterioration rate of the cell performance decreases gradually.