Recent Progresses in Engineering of Ni and Co based Phosphides for Effective Electrocatalytic Water Splitting

Producing pure hydrogen (H-2) is presumably an alternative way for replacing the fossil fuels. Since the last few decades finding alternatives for state-of-the-art catalysts such as IrO2 and RuO2 for oxygen evolution reaction (OER) and platinum for hydrogen evolution reaction (HER) has been a major challenge in the field of water electrolysis. To replace such precious metals, many efforts have been made to develop transition metal based bifunctional catalyst for alkaline water electrolysis. Among them, transition metal phosphides (TMPs) show superior activity in both OER and HER with varying pH conditions. Moreover, the different phases of phosphides, P-rich or P deficient metal composites (Ni/Co-P) offer a great opportunity in total water splitting (TWS) applications. There is no detailed review in the literature outlining the possible combinations, synthesis methods and morphologies of Ni and Co based phosphides. Based on the above shortage about TMPs as a bi-functional catalyst for water electrolysis, this review summarizes the activity and stability Ni and Co phosphides. Furthermore, the review highlights monometallic phosphides such as NiP and CoP, bi-metallic phosphides such as NiCoP, and the synergistic effect of doping Fe into NiP/CoP or Fe -NiCoP systems while detailing their preparation methods. Overall, this review extends the role of designing Ni and Co phosphides for efficient large scale electrocatalysis of hydrogen production.