A.S. Al-Zakri

Effect of carbon dioxide on the performance of Ni/PTFE and Ag/PTFE electrodes in an alkaline fuel cell

The effect of carbon dioxide as impurity in hydrogen and oxygen on the performance of electrodes was studied in a half cell arranged at different concentrations and temperatures. The presence of CO2 in hydrogen was investigated on Ni/PTFE at different concentrations (0–4%) and three temperatures (28, 52, 72° C). Carbon dioxide was found to increase the overpotential due to ionic concentration polarization, but this effect was completely reversible. Impurity levels of CO2 up to 1% in oxygen had no effects on the Ag/PTFE electrode in the short term. Long term performance tests were carried out with CO2 impurity in oxygen at two different concentrations (0.03%, 1%) and at two different temperatures (25° C, 72° C). All experiments showed no degrading effect on the Ag/PTFE electrode with the exception of one at 25° C with 1% CO2. At this run a steady drop of current density was observed due to the formation of K2CO3 in the micropores which was verified by XRD. In all runs the concentration of KOH electrolyte was kept constant at 25%. The effect of adding K2CO3 to KOH was also investigated and no loss in electrode currents was observed for 48 h on both Ni/PTFE and Ag/PTFE electrodes.

Preparation of Raney–Ni gas diffusion electrode by filtration method for alkaline fuel cells

A novel filtration method for preparation of gas diffusion electrodes for fuel cells is proposed. This method, which is a modification of the conventional dry method, has the merits of both wet and dry techniques. The electrode performance is improved due to better structure, controlled hydrophobicity and less compaction. To compare the effectiveness of the method, Raney–Ni/PTFE anodes for use in a KOH fuel cell were made. Their electrochemical performance was compared with similar electrodes produced by the dry method by other research groups, under the same conditions. The filtration method electrodes performed better between temperatures of 25°C and 75°C. The electrode exhibited no significant degradation of activity in the first 180h at 100mAcm−2 anodic load.

Steady state performance of copper impregnated Ni/PTFE gas diffusion electrode in alkaline fuel cell

Abstract The steady-state polarization measurements on a Raney nickel gas diffusion electrode impregnated with copper oxide were carried out in a half-cell setup with 25% KOH electrolyte solution. Pure hydrogen gas was used at a pressure of 1.2 bars in the temperature range of 25–75 °C. The results were compared with almost the same electrode without copper. There was an improvement in the performance of the electrode impregnated with about 8 wt% Cu. This improvement is much more pronounced at higher temperatures and higher current densities. The spherical Raney catalyst grain model was used to determine the kinetic parameters such as exchange current density ( i 0 ) and charge transfer coefficient (α) for the electrode. The values found for the exchange current densities at various temperatures were 6.6 × 10 −6 -3.1 × 10 −4 mA cm − 2 and for the charge transfer coefficient was about 0.6. The exchange current density followed an exponential relation with temperature. The apparent activation energy for the electrode reaction at zero mV overvoltage was found to be lower (28 kJ mol −1 ) than that reported in the literature (32 kJ mol −1 ). The higher values for the exchange current densities and lower values for the activation energies are indication of better performance of the electrode used in this study.

Novel methods of stabilization of Raney-Nickel catalyst for fuel-cell electrodes

Abstract Two new methods of stabilizing Raney-Nickel (Raney-Ni) catalyst for making fuel-cell anodes were studied. In the first method, the catalyst was oxidized with aqueous H2O2 solution, while in the second, oxygen/air (O2/air) was used in a slurry reactor. Effects of different concentrations of H2O2 (5–25 wt.%) and different pressures (10–20 psig) of gas were investigated. The stabilized catalyst was characterized using BET surface area, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The catalyst was used in fuel-cell anodes and the electrochemical performance was determined in an alkaline half-cell. The results were compared with electrodes prepared using conventionally stabilized catalysts. The hydrogen peroxide-treated catalyst has higher BET surface area and produces electrodes with lower polarization. In addition to this, H2O2 treatment is convenient, fast and needs simple equipment which involves no instrumentation. Use of oxygen in a slurry reactor to stabilize the catalyst is also convenient but electrode performance is relatively poor.

Performance of porous nickel electrode for alkaline H2/O2 fuel cell

Abstract Polarization measurements on specially prepared porous nickel electrodes were carried out in a half set-up to determine the intrinsic parameters (exchange current density and charge transfer coefficient) using the spherical grain model developed earlier. The values found for exchange current densities were (6 × 10−6 to 2.8 × 10−4 A cm−2) higher than that reported in the literature which shows the superiority of this electrode. Exchange current density followed an exponential relation with temperature. Apparent activation energies were found to decrease with increasing overpotential and the activation energy corresponding to zero overpotential by extrapolation was found to be 31.4 kJ mol−1.

A Mathematical Model for the Performance of Raney Metal Gas Diffusion Electrodes

A mathematical model is developed to examine the effect of intragrain diffusion on the operation of Teflon-bonded Raney metal gas diffusion electrodes. The model is based on the concept of «electrolyte flooded spherical catalyst grains» while the gas is filling the space between grains. Analytical equations are derived to evaluate the performance of a single catalyst grain in the presence of diffusion limitations. By incorporating the current generation expression for each grain into the differential equation of potential distribution, complete electrode performance under the combined influence of diffusion, activation, and ohmic overpotentials is predicted

Deactivation studies on the porous Ni-gas diffusion electrode in H2/O2 alkaline fuel cell

Abstract Long term performance tests were carried out potentiostatically for the Cu-impregnated Ni-gas diffusion electrode at various temperatures. To measure the deactivation with time, the voltage between the working electrode and the reference electrode was kept constant and the fall of current density (i.e. the reaction rate) was monitored as a function of time. At almost every temperature there was a decay of activity. The higher the temperature the faster the observed deactivation rate was. The deactivation model assumed a first order reactant-gas-concentration-independent deactivation. The spherical flooded grain model was used for the determination of the deactivation rate constants ( k d s ) at different conditions. Deactivation is attributed to: (a) dissolution of promoter Cu and Ni, hence reducing the electronic contact; (b) formation of Ni(OH) 2 which passivates the electrode; (c) the change in the hydrophobicity of PTFE; and (d) dissolution of atmospheric CO 2 in the KOH, forming K 2 CO 3 .

Modified flooded spherical agglomerate model for gas-diffusion electrodes in alkaline fuel cells

Abstract The spherical-grain mathematical model is modified and tested against experimental data for single-layer, gas-diffusion electrodes of alkaline fuel cells. The model assumes that the electrode is made of spherical agglomerates of Raney metal and polytetrafluoroethylene (PTFE) that are flooded with electrolyte; the gas occupies the macropores of the electrode. In addition to previous analysis of the diffusion and reaction in the grains, the modified model includes the resistance of gas diffusion into the macropores and a thin electrolyte film surrounding the grain. The original model and the modified model are both compared with experimental polarization data for hydrogen oxidation on an Ni PTFE electrode in alkaline electrolyte. The newly developed model predicts accurately the experimental data in all regions.

Parametric study of the preparation of gas-diffusion electrodes for alkaline fuel cells by a filtration method

Abstract The effects of various parameters in preparing Raney–Ni/PTFE anodes for alkaline fuel-cells are investigated. A partial factorial experiment design (2 5−1 ) analysis reveals that the important parameters are PTFE content and milling time, and their interaction. Thus, one parameter at a time is varied to study its effect on electrode performance. Electrodes prepared with 8 wt.% PTFE and with milling for 60 s exhibit the best performance.

Effect of CO impurity in H2 on the performance of Ni/PTFE diffusion electrodes in alkaline fuel cells

Abstract The deactivation behavior of Ni/PTFE electrodes operating with hydrogen gas containing CO impurity was studied in half-cell experiments under potentiostatic conditions. The decay of electrode current densities with respect to time was measured. It was observed that, under the experimental conditions studied, the electrode was deactivated and the electrode current densities reached lower steady-state values upon being subjected to CO impurity in the hydrogen stream. The recovery, which was only partial under all conditions except at 72°C, upon removal of the contaminant CO impurity is also recorded. A total of 12 runs were carried out at three different temperatures. The effect of the concentration of CO impurity was investigated at each temperature. The effect of electrode overpotential was investigated only at 25°C. Also, the effect of electrolyte concentration was studied at 52°C.