Thickness effects of anode catalyst layer on reversal tolerant performance in proton exchange membrane fuel cell

Real-world driving conditions will likely cause hydrogen starvation at the anode chambers of stacks to trigger voltage reversal events, posing a tremendous challenge to the durability of proton exchange membrane fuel cells (PEMFCs). The reversal-tolerant anode (RTA), a material-based solution, that inclusion of oxygen evolution reaction (OER) catalyst into the anode is usually employed to cope with the voltage reversal issue. In this work, we investigate the impact of anode catalyst layer thickness on the voltage reversal performance of the membrane-electrode assemblies (MEAs) with conventional anodes (Pt/C catalyst) and RTAs doped with IrO₂ catalyst, a representative OER catalyst. We find that regardless of how thick the anode catalyst layer is, the conventional MEAs exhibit almost similar voltage reversal behaviors and times, only about 1 min to reach the shutdown voltage (?2.5 V). As for the RTA MEAs, a surprising thickness effect that the thinner RTA with the same IrO₂ loading shows superior voltage reversal tolerance. Notably, an ultra-thin RTA (~2 μm) exhibits the reversal tolerance time of 310 min, which is five times higher reversal tolerance time than most of the reported RTAs. We conclude that this thickness effect mainly results from the ionomer distribution on the OER catalyst. Besides, we observe that the RTA with a higher ionomer content shows the better reversal tolerance performance. Our work highlights the importance of the OER Triple-Phase-Boundary (TPB) and the need for improved electrode designs for robust RTAs.     

Effect of iridium oxide as an additive on catalysts with different Pt contents in cell reversal conditions of polymer electrolyte membrane fuel cells

Cell reversal is observed when a current load is applied to the polymer electrolyte membrane fuel cell under fuel starvation conditions. Cell reversal causes severe corrosion (or oxidation) of the carbon support in the anode, which leads to a decrease in overall fuel cell performance. To suppress the corrosion reaction of carbon under cell reversal conditions and to increase the durability of fuel cells, studies on anode additives are being conducted. However, studies on the effect of additives on catalysts with different platinum contents have not been conducted. In this study, 20 wt%, 40 wt%, 60 wt% commercial Pt/C catalyst was applied to the anode, and 50 cycles of cell reversal were performed. Furthermore, the performance change with and without IrO₂ as an additive was observed and its effect was assessed. Changes in the morphologies of the electrodes before and after cell reversal tests were also observed using a transmission electron microscope and a scanning electron microscope. The higher the platinum content of the catalyst, the more resistant to cell reversal. In addition, the addition of IrO₂ to the anode effectively prevents performance degradation due to cell reversal.     

Ir-Pt/C composite with high metal loading as a high-performance anti-reversal anode catalyst for proton exchange membrane fuel cells

Voltage reversal induced by hydrogen starvation can severely corrode the anode catalyst support and deteriorate the performance of proton exchange membrane fuel cells. A material-based strategy is the inclusion of an oxygen evolution reaction catalyst (e.g., IrO₂) in the anode to promote water electrolysis over harmful carbon corrosion. In this work, an Ir-Pt/C composite catalyst with high metal loading is prepared. The membrane-electrode-assembly (MEA) with 80 wt% Ir-Pt(1:2)/C shows a first reversal time (FRT) of up to 20 hours, which is about ten times that of MEA with 50 wt% Ir-Pt(1:2)/C does. Furthermore, the MEA with 80 wt% Ir-Pt(1:2)/C exhibits a minimum cell voltage loss of 6 mV@1 A/cm² when the FRT is terminated in 2 hours, in which the MEA with 50 wt% Ir-Pt(1:2)/C exhibits a voltage loss of 105 mV@1 A/cm². Further physicochemical and electrochemical characterizations demonstrate that the destruction of anode catalyst layer caused by the voltage reversal process is alleviated by the use of the composite catalyst with high metal loading. Hence, our results reveal that the combination of OER catalyst on the Pt/C with high metal loading is a promising approach to alleviate the degradation of anode catalyst layer during the voltage reversal process for PEMFCs.     

Quantifying potential losses of non-Pt MEAs for gas-phase HBr PEM electrolyzer to produce hydrogen

Gas-phase HBr can be converted to hydrogen and bromine in a proton exchange membrane (PEM) electrolyzer. However, due to high cost and the poisoning of bromine and bromide ions on Pt electrodes, non-Pt MEAs (membrane electrode assembly) need to be developed and evaluated. In this paper, RuO₂, carbon (Vulcan XC 72R) and TiO₂–Nb (10% wt.) are prepared as anodes, and IrO₂/C and MoS₂ are prepared as cathodes for incorporation into MEAs. The individual electrodes in these MEAs are then evaluated by de-convoluting the individual voltage losses in-situ from the total electrolyzer voltage. On the anode, Pt, Vulcan XC 72R, TiO₂–Nb (10% wt.) and RuO₂ are all found to have comparable activity toward bromine evolution. On the cathode, Pt was more active toward the hydrogen evolution reaction (HER) compared to IrO₂/C, and both were far superior to MoS₂.     

C16MI.OTf ionic liquid on Pt/C and PtMo/C anodes improves the PEMFC performance

The effect of 1-hexadecyl-3-methylimidazolium trifluoromethanesulfonate (C₁₆MI.OTf) ionic liquid (IL) on the catalytic activity of Pt/C or PtMo/C anodes is studied in a proton exchange membrane fuel cell (PEMFC). PtMo nanoparticles (NPs) are synthesized with two different Pt:Mo proportions (13 or 31 at.% Mo) by a borohydride method on the carbon support. The composition, crystalline structure, morphology of the PtMo/C are evaluated by energy-dispersive X-ray spectroscopy, X-ray diffraction and transmission electron microscopy, respectively. The stability tests of the electrocatalysts are carried out in acid medium using cyclic voltammetry measurements. Pt/C or PtMo/C electrocatalysts containing C₁₆MI.OTf are assessed in the anode in a H₂/air PEMFC by polarization curve and ac electrochemical impedance spectroscopy. The synthesized PtMo nanoparticles show spherical shape and average particle size of 3.5 nm. The PEMFC performance of PtMo (13 at.% Mo) at anode is very similar than of Pt/C anode. The presence of 15 wt% C₁₆MI.OTf in the Pt/C or PtMo/C (13 at.% Mo) anodes let to an increase of the maximum power values, 71 and 107 W g_Pt^?1 cm^?2, respectively. The catalytic surfaces of nanoparticles are modified due to C₁₆MI.OTf presence which improved the PEMFC performance. This result agrees with the EIS analysis, where the resistances of charge transfer and mass transfer decrease in the C₁₆MI.OTf presence. However, this effect is more pronounced for PtMo/C (13 at.% Mo) catalyst, demonstrate that PtMo/C anodes with a small amount of Mo and C₁₆MI.OTf ionic liquid improve significantly the PEMFC performance.     

Performance of carbon-supported PtPd as catalyst for hydrogen oxidation in the anodes of proton exchange membrane fuel cells

In this work, the replacement of platinum by palladium in carbon-supported catalysts as anodes for hydrogen oxidation reaction (HOR), in proton exchange membrane fuel cells (PEMFCs), has been studied. Anodes with carbon-supported Pt, Pd, and equiatomic Pt:Pd, with various Nafion^® contents, were prepared and tested in H₂|O₂ (air) PEMFCs fed with pure or CO-contaminated hydrogen. An electrochemical study of the prepared anodes has been carried out in situ, in membrane electrode assemblies, by cyclic voltammetry and CO electrooxidation voltammetry. The analyses of the corresponding voltammograms indicate that the anode composition influences the cell performance. Single cell experiments have shown that platinum could be replaced, at least partially, saving cost with still good performance, by palladium in the hydrogen diffusion anodes of PEMFCs. The performance of the PtPd catalyst fed with CO-contaminated H₂ used in this work is comparable to Pt, thus justifying further work varying the CO concentration in the H₂ fuel to assert its CO tolerance and to study the effect of the Pt:Pd atomic ratio.     

Experimental validation of the “EasyTest Cell” operational principle for autonomous MEA characterization

The paper presents the experimental validation of the “EasyTest Cell” operational principle via comparative electrochemical tests on MEAs carried out in three types of electrochemical hydrogen energy conversion (EHEC) testing cells: conventional polymer electrolyte membrane fuel cells (PEMFC) and polymer electrolyte membrane water electrolyzers (PEMWE), properly equipped with all the required auxiliaries (products conditioning and supplying, reagents removal, etc.), and the simple, autonomous EasyTest Cell. Along with EasyTest Cell validation and demonstration of its advantages, the influence of argon pressure during sputtering on the electrode characteristics, including gas diffusion limitations was investigated. The electrodes under investigation were magnetron sputtered C/Ti/IrO_x (IrO_x loading in the range 0.12–0.4 mg cm⁻²), C/Ti/IrO_x/Pt/IrO_x (IrO_x 0.08/Pt 0.06/IrO_x 0.08 mg cm⁻²), sputtered at various argon pressure C/Ti/Pt (0.15 and 0.25 mg cm⁻²), and commercial ELAT electrode (V.21, Lot # MB030105-1, Pt loading 0.5 mg cm⁻², E-TEK). The results obtained proved the reliability, simplicity (running-periphery-free) and broadened experimental possibilities of EasyTest Cell over PEMFC and PEMWE single cell testing. Thus, significant cost reduction and resource saving in R&D laboratory can be achieved. Moreover, validation of EasyTest Cell contributes not only to testing facilitations, but potentially to standardization of MEA testing since it gives possibilities for precise control and more uniform distribution of the working parameters applied to the testing object, which are both compulsory for performance comparison and qualifying.     

Effects of Pt loading in the anode on the durability of a membrane–electrode assembly for polymer electrolyte membrane fuel cells during startup/shutdown cycling

A polymer electrolyte membrane fuel cell (PEMFC) stack of a fuel cell vehicle (FCV) is inevitably exposed to reverse current conditions, which are formed by the oxygen reduction reaction (ORR) induced at the anode with a hydrogen/air boundary during startup/shutdown processes. With an increase in the reverse current, the degradation rate of the cathode that experiences a highly corrosive condition (locally high potential) increases. In this work, the anode Pt loading is decreased from 0.4 to 0.1 mg cm⁻² to decrease the reverse current. The decrease in the anode Pt loading is found to decrease the hydrogen oxidation rates (HOR) during normal operation, but this loading decrease barely affected the cell performance. However, a decrease in the anode Pt loading can significantly decrease the reverse current, leading to a diminished cathode degradation rate during startup/shutdown cycling. It is revealed by slow decreases in the cell performance (i–V curves) and electrochemical active surface area (EAS), and a slow increase in the charge-transfer resistance (R_ct), which can be attributed to corrosion of the carbon support and dissolution/migration/agglomeration of the platinum catalyst.     

Highly efficient,cell reversal resistant PEMFC based on PtNi/C octahedral and OER composite catalyst

In order to obtain a fuel cell with both enhanced power generation performance and cell reversal resistance, the composite catalyst consisting of the self-made PtNi/C octahedral and the oxygen evolution reaction (OER) catalyst IrO₂ and RuO₂ is mixed and applied in the anode, and the only octahedral catalyst is employed as the cathode to prepare the membrane electrode assembly (MEA). The electrochemical activity of the composite catalyst decreases slightly, but its performance retention after the accelerated durability test (ADT) is higher. In the single cell test, the MEA fabricated using the composite catalyst maintains good single cell power generation performance. Compared with the control fabricated with Pt/C (JM), the cell voltage at 1 A cm⁻² and the maximum power density are increased by 23 mV and 119 mW cm⁻², respectively. Especially, its durability under continuous cell reversal condition is also improved significantly, and the holding time is prolonged by 1 h. This work realizes the transformation of the octahedral catalyst from the laboratory research to the actual application, and solves the difficulties in fuel cell application, and promotes its commercialization.     

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.     

Characterizations of polybenzimidazole based electrochemical hydrogen pumps with various Pt loadings for H2/CO2 gas separation

Carbon capture and storage (CCS) technologies have been intensively researched and developed to cope with climate change, by reducing atmospheric CO₂ concentration. The electrochemical hydrogen pumps with phosphoric acid doped polybenzimidazole (PBI) membrane are evaluated as a process to concentrate CO₂ and produce pure H₂ from anode outlet gases (H₂/CO₂ mixture) of molten carbonate fuel cells (MCFC). The PBI-based hydrogen pump without humidification (160 °C) can provide higher hydrogen separation performances than the cells with perfluorosulfonic-acid membranes at a relative humidity of 43% (80 °C), suggesting that the pre-treatment steps can be decreased for PBI-based systems. With the H₂/CO₂ mixture feed, the current efficiency for the hydrogen separation is very high, but the cell voltage increase, compared to the pure hydrogen operation, mainly due to the larger polarization resistance at electrodes, as confirmed by electrochemical impedance spectroscopy (EIS). The performance evaluation with various Pt loadings indicates that the hydrogen oxidation reaction at anodes is rate determining, and therefore the Pt loading at cathodes can be decreased from 1.1 mg/cm² to 0.2 mg/cm² without significant performance decay. The EIS analysis also confirms that the polarization resistances are largely dependent on the Pt loading in anodes.     

A novel bifunctional electrocatalyst for unitized regenerative fuel cell

5 wt.% of platinum (Pt) nanoparticles are highly dispersed on the surface of IrO₂ by chemical reduction, and the catalyst is mixed with Pt black to be used as a novel bifunctional oxygen electrocatalyst for the unitized regenerative fuel cell (URFC). The novel cell has been evaluated in the hydrogen and oxygen fuel cell and water electrolysis modes, and compared to a similar cell with an oxygen electrode using conventional mixed Pt black and IrO₂ catalyst. With the novel oxygen electrode catalyst, the highest fuel cell power density is 1160 mW cm⁻² at 2600 mA cm⁻²; the overall performance is close to that with the commercial Pt supported on carbon catalyst and about 1.8 times higher than that with the conventional mixed Pt black and IrO₂ catalyst. Additionally, the cell performance for water electrolysis is also slightly improved, which is probably the result of lower interparticle catalyst resistance with 5 wt.% Pt on IrO₂ compared to no Pt on IrO₂.     

Characterization of nitrogen gas crossover through the membrane in proton-exchange membrane fuel cells

Gas crossover phenomenon through a membrane is inevitable in a proton-exchange membrane fuel cell (PEMFC). For nitrogen, the concentration at the cathode side is usually higher than that at the anode side, so N₂ permeates to the anode side. Nitrogen gas crossover (NGC) may cause fuel starvation, if N₂ gas accumulates in the hydrogen recirculation loop. Thus, it is important to determine the NGC under various PEMFC operating conditions. In this study, characterization of NGC under both open circuit voltage (OCV) and power generation conditions is investigated using a mass spectrometer. Under OCV conditions with the PEMFC membrane fully hydrated, N₂ concentration in the anode exit stream increases as cell temperature increases. Nitrogen permeability coefficients (NPC) are calculated based on the obtained N₂ concentration data. The results show that NPC exhibits an Arrhenius type relationship. Under OCV conditions, the maximum NPC is 5.14 × 10^?13 mol m^?1 s^?1 Pa^?1 with an N₂ activation energy of 19.83 kJ. Under power generation conditions, the NGC increases with increasing current density, which is the result of elevated membrane temperature and increased water content. When the anode stoichiometric ratio (SR_A) is lowered, the N₂ concentration increases under all tested current densities. A low hydrogen flow rate, along with a low SR_A at low current density, significantly increases N₂ concentration at the anode outlet.     

Graphene oxide influence on selected properties of polymer fuel cells based on Nafion

Graphene oxide (GO) was used as an additive to the anode, to modify the electrochemical properties of polymer fuel cells (PEMFC) based on Nafion. GO was obtained by modified Hummers method and fully characterized by Raman, FTIR, X-ray, TEM, electrochemically (CV) and Surface Area and Porosity Analyzer. PEMFC with a GO-based anode containing about 30% less Pt, was constructed and compared with a cell with standard anodes. The electrodes were electrochemically tested at 25 and 60 °C. A maximum power density of 134 mW/cm² with a current density of 374 mA/cm² was achieved for PEMFC with GO-based anode at 60 °C. The electrochemical surface area (ECSA) of the PEMFC with GO-based anode was about two times higher than that of the reference device. The electrochemical characterization as well as the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analysis indicate that GO in the anode reduced Pt agglomeration, as a consequence of the increased surface area and decreased average pore width, compared with the reference electrode. Well-fitted equivalent circuits were proposed and discussed after an electrochemical impedance spectroscopy study of the constructed devices.     

Reduction of hydrogen peroxide production at anode of proton exchange membrane fuel cell under open-circuit conditions using ruthenium–carbon catalyst

This study examines the effect of hydrogen peroxide (H₂O₂) on the open-circuit voltage (OCV) of a proton exchange membrane fuel cell (PEMFC) and the reduction of H₂O₂ in the membrane using a ruthenium/carbon catalyst (Ru/C) at the anode. Each cathode and anode potential of the PEMFC in the presence of H₂O₂ is examined by constructing a half-cell using 1.0 M H₂SO₄ solution as an electrolyte and Ag/AgCl as the reference electrode. H₂O₂ is added to the H₂SO₄ solution and the half-cell potential is measured at each H₂O₂ concentration. The cathode potential is affected by the H₂O₂ concentration while the anode potential remains stable. A Ru catalyst is used to reduce the level of H₂O₂ formation through O₂ cross-over at the interface of a membrane and the anode. The Ru catalyst is known to produce less H₂O₂ through oxygen reduction at the anode of PEMFC than a Pt catalyst. A Ru/C layer is placed between the Nafion^® 112 membrane and anode catalyst layer and the cell voltage under open-circuit condition is measured. A single cell is constructed to compare the OCV of the Pt/C only anode with that of the Ru/C-layered anode. The level of hydrogen cross-over and the OCV are determined after operation at a current density of 1 A cm⁻² for 10 h and stabilization at open-circuit for 1 h to obtain an equilibrium state in the cell. Although there is an increase in the OCV of the cell with the Ru/C layer at the anode, excessive addition of Ru/C has an adverse effect on cell performance.     

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.     

Deposited RuO2–IrO2/Pt electrocatalyst for the regenerative fuel cell

A bifunctional RuO₂–IrO₂/Pt electrocatalyst for the unitized regenerative fuel cell (URFC) was synthesized by colloid deposition and characterized by analytical methods like TEM, XRD, etc. The result reveals that RuO₂–IrO₂ was well dispersed and deposited on the surface of Pt black. With deposited RuO₂–IrO₂/Pt as the catalyst of oxygen electrode, the performance of fuel cell/water electrolysis of unitized regenerative fuel cell (URFC) was studied in detail. URFC with deposited RuO₂–IrO₂/Pt shows better performance than that of URFC with mixed RuO₂–IrO₂/Pt catalyst. Cyclic performance of URFC with deposited RuO₂–IrO₂/Pt is very stable during 10 cyclic tests.     

Single cell study of electrodeposited cathodic electrodes based on Pt-WO3 for polymer electrolyte fuel cells

Cathodic electrodes based on electrodeposited Pt-WO₃ material for proton exchange membrane fuel cells (PEMFC) were studied in single cell configuration. Preparation of the electrodes was carried out by electrodeposition of Pt and WO₃ on commercial gas diffusion layer substrates (microporous carbon black layer on carbon cloth, ELAT E-TEK). The process of simultaneous electrodeposition of Pt and WO₃ is first analyzed from voltammetric curves. It is observed that the deposition of Pt is enhanced when WO₃ is present. Compositional analysis of the electrodes shows metallic platinum and WO₃ in variable proportions. The electrodeposited electrodes were characterized in single PEMFC. Membrane-electrode assemblies were prepared with Nafion^® 117 electrolyte membrane and a standard Pt/C anode. Pt-WO₃ electrodes showed enhanced stability and good response in single cell up to 1500 h. Performance degradation is attributed to a decrease in Pt electroactive area and increase of the internal resistance of the cell. These effects are possibly a consequence of the production of mobile tungsten species, like soluble WO₂ at high current demands and low cathode potentials.     

In situ Van der Pauw measurements of the Ni/YSZ anode during exposure to syngas with phosphine contaminant

Solid oxide fuel cells (SOFCs) represent an option to provide a bridging technology for energy conversion (coal syngas) as well as a long-term technology (hydrogen from biomass). Whether the fuel is coal syngas or hydrogen from biomass, the effect of impurities on the performance of the anode is a vital question. The anode resistivity during SOFC operation with phosphine-contaminated syngas was studied using the in situ Van der Pauw method. Commercial anode-supported solid oxide fuel cells (Ni/YSZ composite anodes, YSZ electrolytes) were exposed to a synthetic coal syngas mixture (H₂, H₂O, CO, and CO₂) at a constant current and their performance evaluated periodically with electrochemical methods (cyclic voltammetry, impedance spectroscopy, and polarization curves). In one test, after 170 h of phosphine exposure, a significant degradation of cell performance (loss of cell voltage, increase of series resistance and increase of polarization resistance) was evident. The rate of voltage loss was 1.4 mV h⁻¹. The resistivity measurements on Ni/YSZ anode by the in situ Van der Pauw method showed that there were no significant changes in anode resistivity both under clean syngas and syngas with 10 ppm PH₃. XRD analysis suggested that Ni₅P₂ and P₂O₅ are two compounds accumulated on the anode. XPS studies provided support for the presence of two phosphorus phases with different oxidation states on the external anode surface. Phosphorus, in a positive oxidation state, was observed in the anode active layer. Based on these observations, the effect of 10 ppm phosphine impurity (or its reaction products with coal syngas) is assigned to the loss of performance of the Ni/YSZ active layer next to the electrolyte, and not to any changes in the thick Ni/YSZ support layer.     

Application of Pt loaded graphite felt in SO2-depolarized electrolyzer

The three-dimensional anodes for SO₂ depolarized electrolysis (SDE) cells are prepared by loading Pt/C on high void content graphite felts, with the method of ultrasonic spray and vacuum suction. SEM results confirm the three-dimensional space distribution of Pt in graphite felts, which ensures sufficient contact between Pt/C and SO₂ in anolyte. Comparing with the two-dimensional anodic catalyst layer loaded on the proton exchange membrane in a conventional SDE cell, the application of the three-dimensional anode decreases cell impedance greatly and improves the SDE performance significantly. In this study, 0.63 mg/cm² Pt loading amount shows the best performance when Pt/C is double-side-sprayed on graphite felt, and the current density reaches 1.24 A/cm² at cell voltage of 1.2 V, as operating at 60 °C and anolyte flow rate of 360 mL/min. The effects of the Pt loading amount, operating temperature and anolyte flow rate on the SDE performance are investigated.