Dynamic Behavior of a PEM Fuel Cell During Electrochemical CO Oxidation on a PtRu Anode

The dynamic behaviour of a single PEM fuel cell (PEMFC) with a PtRu/C anode catalyst using CO containing H₂ as anode feed was investigated at ambient temperature. The autonomous oscillations of the cell potential were observed during the galvanostatic operation with hydrogen anode feed containing CO up to 1000 ppm. The oscillations were ascribed to the coupling of the adsorption of CO (the poisoning step) and the subsequent electrochemical oxidation of CO (the regeneration step) on the anode catalyst. The oscillations were dependent on the CO concentration of the feed gas and the applied current density. Furthermore, it was found that with CO containing feed gas, the time average power output was remarkably higher under potential oscillatory conditions in the galvanostatic mode than during potentiostatic operation. Accompanying these self-sustained potential oscillations, oscillation patterns of the anode outlet CO concentration were also detected at low current density (<100 mA/cm²). The online measurements of the anode outlet CO concentrations revealed that CO in the anode CO/H₂ feed was partially electrochemically removed during galvanostatic operation. More than 90% CO conversion was obtained at the current densities above 125 mA/cm² with low feed flow rates (100–200 mL/min).     

The inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant PEMFC anodes

The effect of the inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported anode electrocatalysts on CO tolerance in proton exchange membrane fuel cells (PEMFC) has been investigated by cyclic voltammetry and fuel cell tests. CO stripping voltammetry on binary Pt_xM/C (M: Mo, Nb, Ta) reveals partial oxidation of the CO adlayer at low potential, with PtMo (4:1)/C exhibiting the lowest value. At 80 °C, the operating temperature of the fuel cell, CO oxidation was observed at potentials close to 0 V versus the reversible hydrogen electrode (RHE). No significant difference for CO electro-oxidation at the lower potential limit, compared to PtRu/C, was observed for PtRuM_y/C (M: Mo, Nb). Fuel cell tests demonstrated that while all the prepared catalysts exhibited enhanced performance compared to Pt/C, only the addition of a relatively small amount of Mo to PtRu results in an electrocatalyst with a higher activity, in the presence of carbon monoxide, to PtRu/C, the current catalyst of choice for PEM fuel cell applications.     

Comparative study of carbon-supported Pt/Mo-oxide and PtRu for use as CO-tolerant anode catalysts

Carbon-supported Pt/Mo-oxide catalysts were prepared, and the reformate tolerances of Pt/MoOx/C and conventional PtRu/C anodes were examined to clarify the features and differences between these catalysts. Fuel cell performance was evaluated under various reformate compositions and operating conditions, and the CO concentrations at the anode outlet were analyzed simultaneously using on-line gas chromatography. Pt/MoOx showed better CO tolerance than PtRu with CO(80 ppm)/H₂ mixtures, especially at higher fuel utilization conditions, which is mainly due to the higher catalytic activity of Pt/MoOx for the water-gas shift (WGS) reaction and electro-oxidation of CO. In contrast, the CO₂ tolerance of Pt/MoOx was much worse than that of PtRu with a CO₂(20%)/H₂ mixture. The results of voltammetry indicated that the coverage of adsorbates generated by CO₂ reduction on Pt/MoOx was higher than that on PtRu, and therefore, the electro-oxidation of H₂ is partly inhibited on Pt/MoOx in the presence of 20% CO₂. With CO(80 ppm)/CO₂(20%)/H₂, the voltage losses of Pt/MoOx and PtRu are almost equal to the sum of the losses with each contaminant component. Although the adsorbate coverage on Pt/MoOx increases in the presence of 20% CO₂, CO molecules in the gas phase could still adsorb on Pt through an adsorbate ‘hole’ to promote WGS or electro-oxidation reactions, which leads to a reduction in the CO concentration under CO/CO₂/H₂ feeding conditions.     

CO tolerance of PdPt/C and PdPtRu/C anodes for PEMFC

The performance of H₂/O₂ proton exchange membrane fuel cells (PEMFCs) fed with CO-contaminated hydrogen was investigated for anodes with PdPt/C and PdPtRu/C electrocatalysts. The physicochemical properties of the catalysts were characterized by energy dispersive X-ray (EDX) analyses, X-ray diffraction (XRD) and “in situ” X-ray absorption near edge structure (XANES). Experiments were conducted in electrochemical half and single cells by cyclic voltammetry (CV) and I-V polarization measurements, while DEMS was employed to verify the formation of CO₂ at the PEMFC anode outlet. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with the PdPt/C and PdPtRu/C anodes containing 0.4 mg metal cm⁻², with the cell presenting potential losses below 200 mV at 1 A cm⁻², with respect to the system fed with pure H₂. For the PdPt/C catalysts no CO₂ formation was seen at the PEMFC anode outlet, indicating that the CO tolerance is improved due to the existence of more free surface sites for H₂ electrooxidation, probably due to a lower Pd-CO interaction compared to pure Pd or Pt. For PdPtRu/C the CO tolerance may also have a contribution from the bifunctional mechanism, as shown by the presence of CO₂ in the PEMFC anode outlet.     

Influence of temperature and relative humidity on performance and CO tolerance of PEM fuel cells with Nafion-Teflon-Zr(HPO4)2 higher temperature composite membranes

The carbon monoxide (CO) poisoning effect on carbon supported catalysts (Pt-Ru/C and Pt/C) in polymer electrolyte membrane (PEM) fuel cells has been investigated at higher temperatures (T > 100 °C) under different relative humidity (RH) conditions. To reduce the IR losses in higher temperature/lower relative humidity, Nafion^®-Teflon^®-Zr(HPO₄)₂ composite membranes were applied as the cell electrolytes. Fuel cell polarization investigation as well as CO stripping voltammetry measurements was carried out at three cell temperatures (80, 105 and 120 °C), with various inlet anode relative humidity (35%, 58% and 100%). CO concentrations in hydrogen varied from 10 ppm to 2%. The fuel cell performance loss due to CO poisoning was significantly alleviated at higher temperature/lower RH due to the lower CO adsorption coverage on the catalytic sites, in spite that the anode catalyst utilization was lower at such conditions due to higher ionic resistance in the electrode. Increasing the anode inlet relative humidity at the higher temperature also alleviated the fuel cell performance losses, which could be attributed to the combination effects of suppressing CO adsorption, increasing anode catalyst utilization and favoring OH_ads group generation for easier CO oxidation.     

The influence of CO on the current density distribution of high temperature polymer electrolyte membrane fuel cells

In this work the poisoning effect of carbon monoxide (CO) on the performance of high temperature polymer electrolyte membrane (PEM) fuel cell is reported. The poisoning of the anode is assessed at 160 °C and 180 °C based on the transient behavior of the fuel cell potential and current density distribution. The current density distribution at similar cell potential and global current density is also critically compared for CO-free hydrogen feed and for CO-contaminated hydrogen feed. Furthermore, the current–cell potential (I–V) and power density curves and impedance spectra are obtained.The presence of CO causes a performance loss which is aggravated for higher CO concentrations and higher current densities and for lower temperatures. The transient behavior of the fuel cell potential and current density distribution show that the poisoning effect of carbon monoxide at the anode is very fast.The use of CO contaminated hydrogen at the anode yields an anisotropic distribution of carbon monoxide, which is accentuated for higher carbon monoxide concentrations and current densities.     

Design of a reference electrode for high-temperature PEM fuel cells

In this work, we present the design of an external reference electrode for high-temperature PEM fuel cells. The connection between the reference electrode with one of the fuel cell electrodes is realized by an ionic connector. Using the same material for the ionic connection as for the fuel cell membrane gives us the advantage to reach temperatures above 100 °C without destroying the reference electrode. This configuration allows for the separation of the anode and cathode overpotential in a working fuel cell system. In addition to the electrode overpotentials in normal hydrogen/air operation, the influence of CO and CO + H₂O in the anode feed on the fuel cell potentials was investigated. When CO poisons the anode catalyst, not only the anode potential increased, but also the cathode overpotential, due to fewer protons reaching the cathode. By the use of synthetic reformate containing hydrogen, carbon monoxide and water on the anode, fuel cell voltage oscillations were observed at high constant current densities. The reference electrode measurements showed that the fuel cell oscillations were only related to reactions on the anode side influencing the anode overpotential. The cathode potential, in contrast, was only negligibly affected by the oscillations under the applied conditions.     

Electrochemical promotion of CO conversion to CO2 in PEM fuel cell PROX reactor

The possibility of electrochemically promoting the water–gas-shift reaction and the CO oxidation reaction in a PEM fuel cell reactor supplied with a methanol reformate mixture was investigated in PEM fuel cells with Pt or Au state-of-the-art E-TEK anodes, in order to explore the use of PEMFC units as preferential oxidation of CO (PROX) reactors. The electropromotion of CO removal was investigated both with air or H₂ fed to the cathode side and also by O₂ bleeding to the anode during normal PEMFC operation. It was found that the catalytic activity of the anode for CO conversion to CO₂ can be modified significantly by varying the catalyst potential. The magnitude of the electrochemical promotion depends strongly on the anodic electrocatalyst (Pt or Au), on the CO concentration of the fuel mixture, on the operating temperature and on the presence of oxygen. The electropromotion effect and the Faradaic efficiency were found to be much higher in CO-rich anode environments.     

PEM fuel cell reaction kinetics in the temperature range of 23-120 °C

The performance of a Nafion 112 based proton exchange membrane (PEM) fuel cell was tested at a temperature range from 23 °C to 120 °C. The fuel cell polarization curves were divided into two different ranges based on current density, namely, <0.4 A/cm² and >0.4 A/cm², respectively. These two ranges were treated separately with respect to electrode kinetics and mass transfer. In the high current density range, a linear increase in membrane electrode assembly (MEA) power density with increasing temperature was observed, indicating the advantages of high temperature operation.Simulation based on electrode reaction kinetic theory, experimental polarization curves, and measured cathodic apparent exchange current densities all gave temperature dependent apparent exchange current densities. Both the calculated partial pressures of O₂ and H₂ gas in the feed streams and the measured electrochemical Pt surface areas (EPSAs) decrease with increasing temperature. They were also used to obtain the intrinsic exchange current densities. A monotonic increase of the intrinsic exchange current densities with increasing temperature in the range of 23-120 °C was observed, suggesting that increasing the temperature does promote intrinsic kinetics of fuel cell reactions.There are two sets of cathode apparent exchange current densities obtained, one set is for the low current density range, and the other is for the high current density range. The different values of cathode current densities in the two current density ranges can be attributed to the different states of the cathode Pt catalyst surface. In the low current density range, the cathode catalyst surface is a Pt/PtO, and in the high current density range, the catalyst surface becomes pure Pt.     

The effect of current density and temperature on the degradation of nickel cermet electrodes by carbon monoxide in solid oxide fuel cells

The oxidation of dry carbon monoxide (CO) in intermediate temperature solid oxide fuel cells (IT-SOFCs) has been studied using a three electrode assembly. Ni/CGO:CGO:LSCF/CGO three electrode pellet cells at 500, 550 and 600 °C were exposed to dry carbon monoxide for fixed periods of time, at open circuit and under load at 50 and 100 mA cm⁻², in an aggressive test designed to accelerate electrode degradation. It is shown that if the anode is kept under load during exposure to dry CO, degradation in anode performance can be minimised, and that under most conditions the anode showed significant irreversible degradation in performance after subsequent load cycling on dry H₂. Only at 500 °C and at 100 mA cm⁻² was the degradation in performance after operation on dry CO and subsequent load cycling on dry H₂ within the background degradation rates measured. Where anode performance was compromised, this appeared to be caused by a reduction in the exchange current density for hydrogen oxidation, and the relatively large degradation after load cycling on dry H₂ was primarily caused by an increase in the series resistance of the anode. It is suggested that this increase in series resistance is associated with the removal of carbon deposited in the non-electrochemically active region of the electrode during operation on dry CO, and that operation under load inhibits carbon deposition in the active region.     

PEM fuel cell Pt anode inhibition by carbon monoxide: Non-uniform behaviour of the cell caused by the finite hydrogen excess

Inhibition of platinum surfaces by carbon monoxide, in particular in polymer membrane electrolyte fuel cells (PEMFC) has been observed for decades by electrochemists. Significant effects have been observed in the hydrogen stream fed to the anode of the fuel cell with concentrations ranging from 1 to 100 ppm depending on the operating conditions e.g. temperature, pressure and excess in reacting gases. As a matter of fact, the gas composition and the surface coverage by CO and H₂ vary in the cell, because of the hydrogen consumption at the anode: this is to result to non-uniform distributions of electrode poisoning, current density, and overvoltage, from the inlet to the outlet of the cell. A simple 1D-model has been developed for prediction of the profiles of the above variables in the fuel cells, with the support of experimental data obtained with a 25 cm² PEMFC: interpretation of polarization curves and impedance spectra yielded the kinetic laws of the two electrode reactions, with both neat hydrogen and CO-containing hydrogen at ppm levels. Simulations show that for low excess in hydrogen – as for practical use of fuel cells – the coverage fractions of the various species can greatly vary in the cell, resulting in non-uniform distributions of current density in the cell and enhanced electrode poisoning near the cell outlet. In contrast working with very high hydrogen excess, as can be done at bench scale, leads to uniform behaviour of the cell, and far less visibility of the anode poisoning by carbon monoxide.     

Conversion of methane to syngas in a solid oxide fuel cell with Ni-SDC anode and LSGM electrolyte

A solid oxide fuel cell constructed from Ni-SDC anode and LSGM electrolyte was applied to the partial oxidation of methane to syngas (CO+H₂) at 700-800 °C with the merits of co-generation of electricity and controllable O₂ supply. It was found that the co-generated syngas at H₂/CO ratio of 1.4-2.0 varied with applied current densities, CH₄ flow rates and operating temperatures. The cell voltage at 100 mA cm⁻² and 800 °C was 0.90 V, i.e. about 90 mW cm⁻² power density could be obtained. The cell operating at 50 mA cm⁻² for 24 h almost showed no degradation of the cell performance. The observed carbon deposition seemed mainly taking place by CH₄ cracking reaction.     

Electrochemical reforming of CH4−CO2 mixed gas using porous Gd-doped ceria electrolyte with Cu electrode

This paper reports on the composition and flow rate of outlet gas and current density during the reforming of CH₄ with CO₂ using three different electrochemical cells: cell A, with Ni−GDC (Gd-doped ceria: Ce_0.8Gd_0.2O_1.9) cathode/porous GDC electrolyte/Cu−GDC anode, cell B, with Cu−GDC cathode/ porous GDC electrolyte/Cu−GDC anode and cell C, with Ru−GDC cathode/ porous GDC electrolyte/ Cu−GDC anode. In the cathode, CO₂ reacts with supplied electrons to form CO fuel and O²⁻ ions (CO₂+2e⁻→CO+O²⁻). Too low affinity of Cu cathode to CO₂ in cell B reduced the reactivity of the CO₂ with electrons. The CO fuel, O²⁻ ions and CH₄ gas were transported to the anode through the porous GDC mixed conductor of O²⁻ ions and electrons. In the anode, CH₄ reacts with O²⁻ ions to produce CO and H₂ fuels (CH₄+O²⁻→2 H₂+CO+2e⁻). The reforming efficiency at 700−800 °C was lowest in cell B and highest in cell A. The Cu anode in cells A and C worked well to oxidize CH₄ with O²⁻ ions (2Cu+O²⁻→Cu₂O+2e⁻, Cu₂O+CH₄→2Cu+CO+2H₂). However, a blockage of the outlet gas occurred in all the cells at 700−800 °C. The gas flow is inhibited due to a reduction in pore size in the cermet cathode, as well as sintering and grain growth of Cu metal in the anode during the reforming.     

Electrocatalyst materials for fuel cells based on the polyoxometalates [PMo(12 − n)VnO40] (n = 0-3)

We report on the use of the polyoxometalate acids of the series [PMo_(12 − n)V_nO₄₀]^(3 + n)− (n = 0-3) as electrocatalysts in both the anode and the cathode of polymer-electrolyte membrane (PEM) fuel cells. The heteropolyacids were incorporated as catalysts in a commercial gas diffusion electrode based on Vulcan XC-72 carbon which strongly adsorbed a low loading of the catalyst, ca. 0.1 mg/cm². The moderate activity observed was independent of the number of vanadium atoms in the polyoxometalate. In the anode the electrochemistry is dominated by the V^3+/4+ couple. With a platinum reference wire in contact with the anode, polarization curves are obtained withV_OC of 650 mV and current densities of 10 mA cm⁻² at 100 mV at 80 °C. These catalysts showed an order of magnitude more activity on the cathode after moderate heat treatment than on the anode,V_OC = 750 mV, current densities of 140 mA cm⁻² at 100 mV. The temperature dependence of the catalysts was also investigated and showed increasing current densities could be achieved on the anode up to 139 °C and the cathode to 100 °C showing the potential for these materials to work at elevated temperatures.     

Overpotential Behavior of Carbon Monoxide Fuel in a Molten Carbonate Fuel Cell

The overpotential of carbon monoxide (CO) fuel was analyzed with a 100‐cm² class molten carbonate fuel cell. The overpotential at the anode was measured using the steady state polarization, inert gas step addition, and reactant gas addition methods. Then, the overpotential was compared between normal hydrogen fuel (H₂:CO₂:H₂O = 0.69:0.17:0.14 atm, inlet composition) and CO fuels (CO:CO₂:H₂O = 0.5:0.5:0 atm and 0.43:0.43:0.14 atm, inlet compositions). The CO fuel without H₂O showed a much greater overpotential at 150 mA cm^–2 than the CO fuel with H₂O. This implies that the water‐gas‐shift reaction prevails at the anode and humidification of CO fuel is an efficient way to reduce anodic overpotential. The anodic overpotential with CO:CO₂:H₂O = 0.43:0.43:0.14 atm was about 73% of that of the H₂ fuel at 150 mA cm^–2. The anode showed gas‐phase mass‐transfer limitations with CO fuels.     

In situ analysis of oxygen partial pressure at the cathode catalyst layer/membrane interface during PEFC operation

To understand the concentration overpotential in the polymer electrolyte fuel cell (PEFC), we have performed an in situ analysis of the oxygen partial pressure (p[O₂]_CL/PEM) at the interface between the cathode catalyst layer (CL) and the polymer electrolyte membrane (PEM). Diffusion-limited oxygen reduction current was measured, with Pt probes inserted into the PEM, during cell operation by supplying H₂ to the anode and O₂ + N₂ to the cathode at 80 °C. It was found that the p[O₂]_CL/PEM decreased by ca. 20% when the current density was stepped from 0 to 2.0 A cm⁻² at p[O₂]_gas = 54 kPa and 100% RH at the cathode inlet, irrespective of the oxygen utilization UO₂ (from 10% to 50%). Such a change in p[O₂]_CL/PEM might result in a concentration overpotential of ca. 10 mV, based on the Tafel slope of 120 mV decade⁻¹ in the high current density region. It was also found that ohmic losses in the ionomer phase of the CL increased with decreasing humidity, from 100% to 80% RH, and became a dominant factor in the increased total overpotential, while the corresponding concentration overpotential was unchanged. The present results provide new insight into the transport of oxygen and water at the CL/PEM interface, especially at the high current densities required for the electric vehicle application.     

On the electrocatalysis of ethylene glycol oxidation

In the present paper, PtRu electrodes are used to study the electrooxidation of ethylene glycol (EG) in acid medium. The voltammetric results show that current-potential curves are shifted by about 0.2 V towards more negative potentials through the promoting effect of Ru on Pt. As in the case of pure platinum, FTIR spectra on PtRu surfaces prove that CO₂, glycolic acid and, possibly, oxalic acid are formed in parallel reactions. The catalytic activity, measured as the current density at constant potential, increases with the Ru content. But the complete oxidation of EG to CO₂ is favored by a high Pt content. In order to compare the catalytic activity of different PtRu compositions, currents measured at electrodeposited electrodes are normalized using the data for the oxidation of a CO monolayer.Results of a test using a laboratory PEM EG/O₂ cell with Pt:Ru (50:50) are presented.     

Preparation of supported PtRu/C electrocatalyst for direct methanol fuel cells

In this work, high-surface supported PtRu/C were prepared with Ru(NO)(NO₃)₃ and [Pt(H₂NCH₂CH₂NH₂)₂]Cl₂ as the precursors and hydrogen as a reducing agent. XRD and TEM analyses showed that the PtRu/C catalysts with different loadings possessed small and homogeneous metal particles. Even at high metal loading (40 wt.% Pt, 20 wt.% Ru) the mean metal particle size is less than 4 nm. Meanwhile, the calculated Pt crystalline lattice parameter and Pt (2 2 0) peak position indicated that the geometric structure of Pt was modified by Ru atoms. Among the prepared catalysts, the lattice parameter of 40-20 wt.% PtRu/C contract most. Cyclic voltammetry (CV), chronoamperometry (CA), CO stripping and single direct methanol fuel cell tests jointly suggested that the 40-20 wt.% PtRu/C catalyst has the highest electrochemical activity for methanol oxidation.     

Mo oxide modified catalysts for direct methanol, formaldehyde and formic acid fuel cells

Pt black and PtRu black fuel cell anodes have been modified with Mo oxide and evaluated in direct methanol, formaldehyde and formic acid fuel cells. Mo oxide deposition by reductive electrodeposition from sodium molybdate or by spraying of the fuel cell anode with aqueous sodium molybdate resulted in similar performance gains in formaldehyde cells. At current densities below ca. 20 mA cm⁻², cell voltages were 350–450 mV higher when the Pt catalyst was modified with Mo oxide, but these performance gains decreased sharply at higher current densities. For PtRu, voltage gains of up to 125 mV were observed. Modification of Pt and PtRu back catalysts with Mo oxide also significantly improved their activities in direct formic acid cells, but performances in direct methanol fuel cells were decreased.     

Components for PEM fuel cell systems using hydrogen and CO containing fuels

Proton exchange membrane fuel cells (PEMFC) show a significant performance drop in CO containing hydrogen as fuel gas in comparison to pure hydrogen. The lower performance is due to CO adsorption at the anode thus poisoning the hydrogen oxidation reaction. Two approaches to improve the cell performance are discussed. First, the use of improved electrocatalysts for the anode, such as PtRu alloys, can significantly enhance the CO tolerance. On the other hand, CO poisoning of the anode could be avoided by the use of non-electrochemical methods. For example, the addition of liquid hydrogen peroxide to the humidification water of the cell leads to the formation of active oxygen by decomposition of H₂O₂ and the oxidation of CO. In such a way a complete recovery of the CO free cell performance is achieved for H₂/100 ppm CO.