Rotating disk electrode measurement

HOR performances of the as-prepared electrocatalysts were examined using the RDE technique. The catalyst thin film electrodes were first activated by CV in H₂-saturated 0.1 M NaOH between −0.1 and 0.2 V versus RHE at 20 mV s⁻¹ for several cycles until a steady polarization curve was obtained. The HOR polarization curves were then collected with a rotating speed of 400, 900, 1600 and 2500 r.p.m and a scanning rate of 0.5 mV s⁻¹ to minimize the capacitive charge contribution.

The HOR kinetic current (i_K) was calculated based on the Koutecky–Levich equation⁶⁸

where i is the measured current, and i_D is the diffusion limited current.

The HOR exchange current (i₀) was obtained subsequently by fitting i_K to the Butler–Volmer equation⁶⁹

where i₀ is the HOR exchange current, α_a is transfer coefficient for the HOR, F is the Faraday constant (96485 C mol⁻¹), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), T is the temperature, and η is the overpotential. α_a was between ~0.1 and ~0.4.

Activation energy (E_a) of the HOR was measured at different temperatures from 2 °C to 30 °C. The E_a can be calculated from the Arrhenius equation

where i₀ (mA cm⁻²) is the exchange current density at different temperatures, E_a is the activation energy (J mol⁻¹), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), and T is the temperature (K).