Kinetics of hydrogen evolution reaction on Zr_0.5Ti_0.5V_0.6Cr_0.2Ni_1.2 alloy in KOH electrolyte

Abstract

A hydrogen-storage alloy of the composition Zr_0.5Ti_0.5V_0.6Cr_0.2Ni_1.2 has been investigated for corrosion resistance and hydrogen-evolution reaction (HER) in KOH electrolyte of varying concentrations. Activation of the electrode by absorption of hydrogen takes place after prolonged cathodic polarization in the potential range of HER. Prior to activation, the open-circuit potential is about -0.4 V vs Hg/HgO, OH-, at which the alloy electrode tends to undergo corrosion with oxygen-reduction reaction (ORR) as the conjugate reaction. The corrosion-current density measured from Tafel polarization of ORR is found to be independent of KOH concentration and has an average value of about 30 μ A cm^-2. Subsequent to activation, the open circuit potential of the electrode is shifted to about -0.93 V vs Hg/HgO, OH^-, which is equal to the reversible potential of HER. The exchange current density values measured from Tafel polarization of HER are marginally higher in relation to the values obtained before the electrode is activated. Alternating-current impedance spectra in the Nyquist form contain two overlapped semicircles. The high-frequency semicircle is attributed to the electrode geometry, while the low-frequency semicircle is due to the charge-transfer reaction and double-layer capacitance. The impedance data are analyzed by a non-linear least square curve fitting technique and impedance parameters are evaluated.