Hydrogen evolution in acid solution at Pd electrodeposited onto Ti2AlC

Abstract

The hydrogen evolution reaction (HER) was studied in 0.5 M H2SO4 at 25 degrees C on Pd electrodeposited onto Ti2AlC substrate, as an excellent substrate due to its high conductivity and high stability in concentrated sulfuric acid. Pd was electrodeposited onto Ti2AlC by pulse technique from the solutions containing different concentrations of PdCl2 in 1 M NH4Cl. It was shown that in all investigated solutions Pd electrodeposition occurs by the diffusion-controlled 3D nucleation and growth. Two types of Pd electrodeposits were submitted to the study of the HER by the polarization measurements and the electrochemical impedance spectroscopy (EIS). The surface of Pd samples was characterized by scanning electron microscopy (SEM), by energy dispersive X-ray spectroscopy (EDS) and by atomic force microscopy (AFM). High catalytic activity for the HER was observed for all investigated samples. The Tafel slope of -64 mV dec(-1) was recorded for the Pd layer electrodeposited at 20 mV vs. saturated calomel electrode (SCE) for 5 s, mainly composed of isolated hemispherical 3D Pd centers and dendrites (sample (2)), while for Pd electrodeposit with the theoretical thickness of 1000 nm (sample (1)) Tafel slope of -143 mV dec(-1) was detected. All Tafel slopes recorded for the investigated samples could be explained by the Volmer-Tafel mechanism for the HER assuming Temkin adsorption isotherm, with either of the steps (Volmer or Tafel step) being the rate-determining one. By the analysis of the EIS results similar slopes for the E vs. log (R-F)(-1) dependences were observed for all investigated samples respectively, while the electrochemically active surface area (EASA), obtained by the integration of the charge for Pd-oxide reduction was found to be the highest for the sample (1). The exchange current densities corrected for the EASA were presented for all samples.