Development of transition metal catalysts for electrochemical reactions in zinc-air batteries and water-splitting hydrogen generation

Abstract

The key to realizing a carbon-neutral society is to exploit reliable, cheap, and robust electrochemical energy storage and conversion techniques (ESCTs) to confront the intermittent and unequal distributed nature of renewables. Alkaline-based Zn-air batteries (ZABs) and water electrolysis are crucial ESCTs in different scenarios, where the fundamental mechanisms involve alkaline water electrocatalysis with charge transfer and mass transport processes over an electrode structure. The overall performance is primarily underpinned by fundamental electrochemical reactions, namely, oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). Major challenges are sluggish reaction kinetics, requiring highly active, durable, and cost-effective catalysts. However, the current state-of-the-art catalysts based on scarce and expensive noble metals (such as Pt and Ir) need better activity and durability in alkaline media. Transition metal compounds (TMCs), such as oxides, phosphides, hydroxides, and carbides, are widely studied and struggling to replace noble metals. For these TMCs, either in preparation process or structure-performance relation, practical application in ESCTs, especially, remains further explored. In this dissertation, three TMCs electrode materials were fabricated to theoretically and experimentally reveal the structure-composition-performance relation underpinning their applications for ZABs and hydrogen production in alkaline conditions. Related preparation procedures, structures and composition of catalysts, and practical applications in these projects were investigated and correlated. More details include: As for ZABs, a synthesis process was simplified by one-step pyrolysis fabricating Fe2P nanoparticles embedded in the nitrogen-doped carbon (Fe2P/NC). The catalyst boosted the discharging ability of ZAB with a high specific power density and stable discharging platform under different currents, better than ZAB using commercial Pt/C. Furthermore, a bifunctional catalyst with a three-dimensional structure was developed in Chapter 3. This electrode employed two components FexNT nanotubes for ORR and NiFe(OH)x for OER, respectively, taking advantage of their respective catalytic ability by rational integration. Based on the rational-integration concept, this electrode material promises ZAB ultra-long durability for discharge and charge cycling. Finally, a self-supported RuO2-NiOx electrode was reported for alkaline hydrogen production. Nickel foam was the nickel source and underwent ruthenium etching to form monolithic heterostructure on the substrate instead of physically loading, enabling robust durability. The promoted HER performance originated from reconstructed Ru-NiOx interface, revealed by multiple characterization techniques and theoretical calculations. At last, the use of electrode material in alkaline seawater and industrial conditions (large current of 1 A cm-2 and high temperature of 65 ℃) was successfully demonstrated, highlighting the possibility of real industrial hydrogen production.