On the current-potential curve of a very fast hydrogen evolution process up to high current densities

Abstract

Abstract The steady-state polarization curve of the hydrogen evolution at highly active Pt-electrodes was determined in 5 M and 0.5 M H 2 SO 4 up to high current densities. The polarization curve in 5 M H 2 SO 4 is characterized by four regions, which differ from one another by the slope (b) of the E -log( i ) curve. The first three regions at low and medium current densities with b 1 ⋍ 29 mV decade −1 and b 2 ⋍ 19 mV decade −1 and b 3 → 0 mV decade −1 are due to the reversible hydrogen evolution, if the rate determining step is the transport of hydrogen into the bulk of solution, the electron exchange is reversible and the overvoltage is determined by the H 2 -supersaturation at the electrode surface. The slope b 1 ⋍ 29 mV decade −1 is due to the transport of the evolved H 2 into the bulk of solution by diffusion; b 2 ⋍ 19 mV decade −1 is due to the transport by diffusion and by gas bubbles; b 3 → 0 mV decade −1 is due to the transport by diffusion and by gas bubbles provided that the H 2 -supersaturation at the electrode surface does not further change with growing overvoltage. At high current densities ( i ⪢ 350 mA cm −2 ) the slope of the polarization curve is b 4 ⋍ 120 mV decade −1 and the rate determining step is the charge transfer (Volmer) reaction.