Ultrafine Ni-Based Nanomaterials on Hierarchically Porous Carbon from Biomass: An Efficient Bifunctional Electrocatalyst for Water Splitting

Abstract

A highly efficiency bifunctional electrocatalyst, consisting of Ni/NiO nanoparticles decorated on biomass-derived hierarchically porous carbon (BHPC) was fabricated via one-step pyrolysis. Cauliflower leaf waste was used as a natural biomass carbon/nitrogen source for our cost-neutral BHPC. The catalytic activities of Ni/NiO/BHPC towards oxygen evolution reaction (OER) and hydrogen evolution reduction (HER) were investigated with variable pyrolysis temperatures ranging from 400 to 600 oC, which triggered changes in the crystallography and composition ratio between Ni and NiO in the resultant composite. For the optimal temperature of 500 oC, the resulting hybrid had 20.8 at.% Ni0, exhibiting modest Tafel slopes of 70 and 121 mV dec-1, while requiring 346 and 180 mV overpotentials to reach 10 mA cm-2 for OER and HER in 0.1 M KOH solution, respectively. When used for total water electrolyser, the Ni/NiO/BHPC electrodes achieved a current density of 10 mA cm-2 at 1.688 V only, and demonstrated superior cycling stability and rate capability over 20 h between 10 and 30 mA cm-2. The catalytic activity of the NiOx/BHPC-500||NiOx/BHPC-500 electrolyser is comparable to that of the recently reported cells assembled with Ni-based electrocatalysts. The excellent catalytic properties of the designed electrocatalyst are ascribed to the synergistic effect of Ni/NiO heterostructure and hierarchically porous carbon framework, which generates large electrochemical active surface area, numerous catalytic active sites and fast electron transport. This work provides strategies for the sustainable design of bifunctional electrocatalysts for efficient total water splitting. Keywords: hierarchically porous carbon, Ni/NiO heterostructure, biomass, water splitting