The electrochemical subsurface treatment of Ni-based catalytic materials for efficient alkaline water electrolysis Gwangju Institute of Science and Technology School of Materials Science and Engineering Seunghyun Jo

Abstract

Due to the high cost of noble metal catalysts such as platinum (Pt) and iridium (Ir), which are commonly used in catalytic material for water electrolysis, the competitiveness of green hydrogen has been evaluated as lacking. In response, nickel-based compounds have received significant attention as catalyst for hydrogen evolution reactions (HER) and oxygen evolution reactions (OER), owing to their low cost and relatively high reactivity. However, homogeneous nickel (Ni) or nickel oxide (NiO) face several challenges in competing with noble metal catalysts. First, as a hydrogen evolution catalyst, Ni shows lower performance compared to the noble metal catalyst, Pt. Then, as an oxygen evolution catalyst, Ni oxidizes to nickel oxyhydroxide (NiOOH) via NiO, but the balance between the reaction steps is disrupted, resulting in lower performance compared to the noble metal catalyst IrO2. Additionally, Ni suffers from rapid corrosion under OER conditions, leading to shorter lifespans. To address these issues, various Ni-based compound catalysts are being developed, but to date, the gap between the theoretically optimized performance and the practical challenges in synthesis and application has prevented commercialization. In this point, electrochemical surface modification techniques offer a potential solution to solve these problems. Because the catalytic reactions occur on the electrochemical active surface area (ECSA) of the catalyst, the surface properties of the material critically affect the catalytic performance. In this contents, by using electrochemical methods to modify the catalyst surface, it is possible to minimize side effects by equalizing the reactive surface area with the modified area, and providing engineering advantages for practical applications such as water electrolysis single cell. In this thesis, I analyze the mechanisms by which Ni-based compounds enhance catalytic performance and suggest two electrochemical surface modification methods to solve the identified research gap. At first, it involves doping bismuth (Bi) additive to NiO catalysts and analyzing its effects on the activity and stability of alkaline HER and OER. Because Bi has a higher electronegativity than Ni, Bi doping is able to control the electronic structure of Ni by transferring electrons. Additionally, Bi is a suitable metal for catalysis due to reversible redox reactions under alkaline hydrogen and oxygen evolution conditions. Through chemical and electrochemical analysis, this study aims to enhance the hydrogen and oxygen evolution rates of NiO catalysts via Bi doping and to use computational calculations to investigate how Bi doping affects the mechanisms of each reaction. Then, the first approach involves optimizing the hydrogen evolution performance of Ni-P catalysts through electrochemical surface engineering to create a P-rich surface. By analyzing the influence of additives, the study identifies the optimal composition between Ni and P. Then, by adjusting the composition on the electrochemical active surface through electrochemical surface modification, it optimizes the activity and durability as basis on each mechanism of HER in acidic and alkaline media. Finally, the second approach introduces an in-situ electrochemical surface modification to control the surface crystallinity of NiFe-LDH catalyst, accelerating the oxygen evolution rate in alkaline media. NiFe-LDH catalyst possess a 2D layered structure, where the high resistance encountered by OH- ions moving into the narrow 2D layers and contributed to performance degradation. The in-situ surface modification reduces the pathway resistance for OH- ions, thereby increasing the oxygen evolution rate and enhancing the durability of the catalyst.