Scanning electrochemical microscopy: Investigation of hydrogen oxidation at polycrystalline noble metal electrodes

Scanning electrochemical microscopy (SECM) was used to study the kinetics of hydrogen oxidation on platinum, iridium, and rhodium substrates in acidic media. Hydrogen was generated at a platinum tip by reduction of H+ and oxidized at a Pt, Ir, or Rh substrate. The Tafel slope (117 mV, corresponding to a transfer coefficient, alpha, of 0.50) and the rate constant for hydrogen oxidation on Pt obtained by fitting approach curves (0.22 cm/s at the halfwave potential) agree with a previous SECM result. Kinetic data were also obtained on Ir (Tafel slope = 113 mV, kdegrees = 0.25 cm/s, alpha = 0.48), and Rh (Tafel slope = 138 mV, kdegrees = 0.010 cm/s, alpha = 0.58) substrates. These results demonstrate that hydrogen oxidation on Pt and Ir occurs at a similar rate, but is much slower on Rh.