Steam-assisted electro-reduction of NiO: a sustainable alternative to conventional hydrogen reduction

Abstract

Conventional methods of hydrogen reduction employed for green nickel production face challenges due to the high potentials required for water splitting at room temperature (>1.8 V) and/or the necessity for expensive catalysts. Additionally, the transportation of hydrogen from the production unit to the reduction unit (generally at 900 °C) presents hazards due to potential hydrogen leakage. Our findings address these challenges by proposing an innovative approach that utilises hydrogen generated via electrolysis at high temperatures for the in situ reduction of NiO in molten salt electrolytes. This process involves the hydrolysis of molten salts, generating protons which are then cathodically discharged to produce hydrogen at a low cell voltage, ranging from 1.0 to 1.4 V. Two different setups are investigated, utilising either NiO cathode pellets or NiO powder immersed in the melt to explore the hydrogen evolution and subsequent reduction of the oxide phase. Various parameters, including cell voltage, cell configuration, electrolyte chemical composition and temperature are examined for their impact on the reduction process. It is observed that the composition of the molten salt electrolyte significantly influences the reduction kinetics. The addition of KCl to LiCl electrolyte aids the reduction process by improving the wettability of electrodes, while CaCl₂ enhances the hydrolysis of the molten electrolyte. In particular, NiO pellets demonstrate efficient reduction to Ni in LiCl–30 wt% KCl, achieving a remarkable current efficiency of 97.4% with an energy consumption of 1.28 kW h per kilogram of produced Ni.