Relationship Between Equilibrium Hydrogen Pressure and Exchange Current for the Hydrogen Electrode-Reaction at Mmni(3.9-X)Mn(0.4)A1(X)Co(0.7) Alloy Electrodes

We present a theoretical relationship between equilibrium hydrogen pressure and exchange current for the hydrogen electrode reaction which considers the degree of hydrogen coverage at the electrode surface. Electrochemical measurements at MmNi3.9–xMn0.4AlxCo0.7 (0 x 0.8) electrodes were performed to prove the theoretical model. The equilibrium hydrogen pressures were analyzed from electrochemical pressure-composition isotherms, and the exchange currents were determined by linear polarization measurements. Fitting the experimental data to the theoretical model indicated that the rate constants for the chargetransfer reaction as well as the charge-transfer coefficient were influenced by the partial substitution of nickel by aluminum. Also, the exchange current passed through a maximum with decreasing equilibrium hydrogen pressure, i.e., with increasing aluminum content, indicating that it is possible to design new materials which combine high electrocatalytic activity with an appropriate equilibrium hydrogen pressure. Because of its high energy density and high power density, MmNi3.6Mn0.4Al0.3Co0.7 was found to be the most appropriate composition.