Kinetics of hydrogen evolution reaction on Ni-Me-P electrodes

Abstract

Hydrogen, a wholly non-polluting and renewable fuel, may be obtained by the electrocatalytic hydrogen evolution reaction (HER). Electrocatalytic hydrogen production calls for efficient and durable electrodes by finding the origin of the electrocatalytic activity, and improving the properties. The candidate materials for cathodes are limited. Those that have high surface area suffer from low physical stability and deterioration in long-term operation. If physical stability is improved the activity is decreased. When both properties are improved the production expense is increased. The main purpose of this study is to prepare new electrode materials and study their activity and reaction mechanism for the HER. Three categories of the electrodes of nickel phosphorous family were studied, Ni-P, Ni-Mo-P, and Ni-Zn-P. Ni-P materials are very stable in alkaline solutions. Their stability is better than Ni. Phosphorous is not removed by means of leaching the electrodes in HF, alkaline solutions, heating, and/or oxidation to produce a rough surface. Electrodes prepared at low current densities and temperature were active for the HER due to increase in the surface roughness. The most active Ni-P electrode prepared in this study had a surface roughness of 10[subscript 3] XRD patterns of Ni-P electrodes have amorphous structure. After heating at 400 [degree Celsius] the structure of electrodes changed to crystalline with domination of Ni[subscript 3]P phase (for high phosphorous content electrodes) or Ni[subscript 3]P and Ni (for low phosphorous content electrodes). Ni-Mo-P electrodes were prepared by"multi-step" electrodeposition. The activity of the electrodes was increased through increase in surface roughness as well as intrinsic activity. Cycling of the electrodes between the HER and oxygen evolution reaction (OER) deactivated them. Mo was mostly dissolved during electrooxidation of these materials. Ni-Zn-P electrodes were prepared by subsequent electrodeposition of Ni, Ni-P, and Ni-Zn-P. The top-most layer was obtained by gradual addition of zinc to the plating bath. After leaching the electrodes in 30% KOH, about 80% of Zn was removed leading to a porous surface with a large area with excellent stability. Those prepared at high current densities are the most active ones. They are characterized by low Tafel slopes and large surface area with roughness factor of 10[subscript 4]. Electrochemical impedance spectroscopy (EIS) and steady-state polarization are mainly used to explain activity of the electrodes using real surface area. A new technique for the surface roughness determination i.e.,"CO molecular probe" was developed. Ni-Zn-P porous electrode surface roughness was measured by the EIS and compared with results from surface oxidation, cyclic voltammetry (cv), ratio of the polarization current densities, and CO molecular probe showing good agreement with those obtained by the EIS. Besides, the results show that dissolved CO in NaOH can be oxidized on Ni and Ni-Zn-P electrodes. This is a new observation of CO behavior on nickel based electrodes."--Résumé abrégé par UMI