Key Parameters for the Design of Composites Electrodes for Water Splitting in Alkaline Medium

Abstract

A study on the hydrogen evolution performances in alkaline medium (KOH 6M) of nickel-based composite electrodes has been carried out. Surface tension characteristics have been tuned by using various polymer binders of different hydrophobic/hydrophilic balances: polyvinyl alcohol (PVOH), polybenzimidazole (PBI), polyethylene oxide (PBI), polyvinidylene fluoride (PVDF), and Polytetrafluoroethylene (PTFE). The composite electrodes fabricated by drop-casting exhibit various contact angles in an extended range from 43° to 115°. They were evaluated by steady-state polarization and electrochemical impedance spectroscopy (EIS) measurements. Electrochemical behavior and mechanism were discussed towards the electrode wettability through the evaluation of electrochemical parameters including the overpotential at 100 mA/cm², kinetic parameter b (mV/dec) and charge transfer resistance (Rct). The bubble coverage, diameter before departure (lift off from the electrode surface) and residence time at the surface are greatly affected by the surface tension of the various electrodes. The comparison of the electrochemical performances revealed the importance of this characteristic for the design of an efficient hydrogen evolution electrode fabricated by a catalyst-binder approach. From Ni-PVOH to Ni-PTFE electrodes with the same catalyst loading, the overpotential at 100 mA/cm² was improved by 150 mV. The kinetic tafel slope was kept close to 116 mV/dec (Volmer-Heyvroski step) for electrodes with a contact angle between 43° to 90 °, increased for contact angles above 90° to reach up to 178 mV/dec for PTFE-based electrode showing the greater contact angle in the series at 115°. Electrochemical impedance spectroscopy (EIS) measurements at various overpotentials show that the bubbles retained at the surface increase the electrode charge transfer resistance. This behavior is assigned to the hidden active sites and justifies the observed decrease in performances of hydrophobic electrodes. This comprehensive study leads to a precise description of the interfacial phenomena at the microscopic scale during gas production by electrolysis and the evaluation of key parameters for the formulation of advanced electrodes.