Anodic catalyst design for highly efficient freshwater and seawater electrolysis

Abstract

The excessive consumption of traditional fossil fuels, driven by rapid industrialization worldwide, has resulted in severe energy crises and environmental pollution. Mitigating carbon emissions and addressing environmental degradation have emerged as critical challenges for contemporary society. In this context, the pursuit of affordable, clean, and environmentally sustainable alternative energy sources has gained considerable attention. Hydrogen energy is increasingly recognized as a viable alternative to fossil fuels and plays a pivotal role in achieving carbon neutrality. Among various hydrogen production methods, water electrolysis is particularly attractive owing to its high efficiency and zero carbon emissions. Recently, the utilization of seawater as a feedstock in place of highly purified freshwater in electrolysis has attracted considerable interest due to the widespread distribution and abundant reserves of seawater. However, the sluggish kinetics of the anodic oxygen evolution reaction (OER) significantly hinders the overall efficiency of water electrolysis. Furthermore, for seawater electrolysis, the high concentration of Cl- would compete with anodic OER, leading to the chlorine oxidation reaction (CIOR) and generating corrosive Cl2 and CIO-, which poses additional challenges. To enhance the efficiency and selectivity of OER for water/seawater electrolysis, it is crucial to develop highly efficient electrocatalysts that can facilitate the anodic OER while suppressing the CIOR. This thesis presents a comprehensive investigation into the development and optimization of efficient transition metal (TM)-based electrocatalysts for OER in practical freshwater and seawater electrolysis applications. The research aims to address the critical challenges of activity, stability, and selectivity in OER catalysis, particularly in the presence of high-concentration chloride ions in seawater. The key research findings are as follows: 1. CeO2/NiFe-based layered double hydroxides (LDH) composite with high oxygen ion diffusion rates and a high concentration of oxygen vacancies are designed to optimize OER activity under alkaline and seawater electrolytes. The partial coverage of CeO2 on LDH surfaces inhibits direct Cl- adsorption on active sites, while high proton conductivity mitigates proton accumulation within LDH interlayers, thereby preventing structural collapse and ensuring superior stability. Notably, the enhanced mixed ionic conductivities at elevated temperature contribute to significant improvements in OER performance. The chapter underscores the effectiveness of employing mixed ionic conductors for enhancing OER performance and highlights the importance of characterizing electrocatalysts and cells under service conditions for practical applications. 2. A novel nickel substituted double perovskite material, NdBa0.75Ca0.25Co1.5Fe0.4Ni0.1O5+δ (NBCCFN), has been synthesized via the electrospinning method. It has been found that the incorporation of Ni increases the concentration of oxygen vacancies in perovskite and facilitates the formation of oxyhydroxide species on the perovskite surface during OER. Density functional theory (DFT) calculations reveal that the OER process for NBCCFN proceeds via a lattice oxygen mechanism, while Ni incorporation effectively reduces the reaction energy barrier of the potential determining step. This chapter indicates that NBCCFN perovskite has remarkable activity, selectivity, and durability for water and seawater electrolysis, holding significant potential for practical applications. 3. Cobalt metaphosphate (K2Co(PO3)4) is explored as an OER catalyst in freshwater and seawater electrolysis. As indicated by various characterizations, the metaphosphate experienced a complete and irreversible structural reconstruction process during OER, in-situ forming CoOOH that acts as the real active sites. The reconstruction process is accompanied by the leaching of metaphosphate ions. Moreover, the advantageous adsorption of OH- rather than Cl- helps to achieve high selectivity of the prepared catalyst, which also prevents the negative effect of chlorine adsorption. This chapter underscores the potential of cobalt metaphosphate as a highly efficient, selective, and robust OER pre-catalyst capable of maintaining performance in challenging environments. The collective findings from these chapters advance the field of OER catalysis, particularly for seawater electrolysis applications. The developed and optimized electrocatalysts that can effectively prevent Cl- adsorption and oxidation while maintaining high OER activity and stability are crucial for the practical implementation of seawater electrolysis technologies. The strategies employed in material design have the way for the development of high-performance OER catalysts. Furthermore, the integration of in-situ and ex-situ characterization techniques provides valuable insights into understanding OER mechanisms, which can guide the design and optimization of new materials for practical applications in freshwater and seawater electrolysis.