Exploring and resolving the effects of environment on perovskite oxide catalysts for oxygen evolution in water electrolysis

• Main Author: Seow, Justin Zhu Yeow
• Other Authors: Xu Zhichuan, Jason
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Energy materials; Science::Chemistry::Physical chemistry::Electrochemistry
• Online Access: https://hdl.handle.net/10356/170930
• Institution: Nanyang Technological University

While integration of renewable energy sources can increase the sustainability of electricity generation process, the intermittence of some of these energy sources has affected the ability of the electrical grid to fulfill the time-dependent demand. Energy storage system will be beneficial in resolving this issue, and since hydrogen has high specific energy and compressibility, it has become an attractive large-scale energy storage medium. The greenest way to produce hydrogen is through water electrolysis, but its lack of energy efficiency, mainly limited by oxygen evolution reaction (OER), has reduced its feasibility for commercialization. In various works improving the efficiency of OER, it has become evident that active-site-rich oxyhydroxides formed from surface reconstruction of perovskite oxides are competitive OER catalysts compared to precious metal catalysts. Nonetheless, the limitations of OER catalysis have been compounded by various environmental factors. For instance, proton exchange membranes in water electrolyzers could undergo chemical degradation that releases sulfate ions. While it has been known that the strong adsorption of sulfate ions on OER active sites would reduce in OER activity and that the interaction between cations leached from perovskite oxide and sulfate ions could cause sulfate precipitation that physically blocks OER active sites, the understanding on the extent that a perovskite-oxide-derived active-site-rich oxyhydroxide would be affected by sulfate ions in acid was lacking. A study in this thesis has compared the OER activity and performance stability between IrOxHy/SrCo0.9Ir0.1O3-δ (IrOxHy/SCIO) and a commercial IrO2 in both 0.1 M HClO4 and 0.1 M H2SO4, and has found that while the two catalysts have similar OER performance stability in 0.1 M HClO4, the former has an inferior OER performance stability compared to IrO2 due to formation of SrSO4 and subsequent loss of accessibility of Ir active sites to OER reactants through the formation of dehydrated region despite the Sr-deficient catalytic surface. This has highlighted the additional adverse effect of sulfate on oxyhydroxides derived from perovskite oxides. Another instance where environmental factors would affect OER catalysis is the introduction of seawater. There has been plenty of studies suggesting the inferior OER activity in seawater compared to that in saline water (where only 0.5 – 0.6 mol/L NaCl was added) without satisfactory explanation for the difference. Hence, the model Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) was studied at pH 13 to screen the effects of each major ion in alkalinized seawater. None of the major anions in seawater have individually posed significant OER activity reduction to BSCF, while the presence of Ca2+ at concentration of as low as 0.5 mmol/L could cause CaCO3 precipitation layering on the catalytic surface, inhibiting surface reconstruction of BSCF and reducing its peak OER activity. An attempt to remove calcium through chemical precipitation through salt addition into artificial seawater (pH 13) electrolyte has resulted in only partial recovery of OER activity loss due to either residual calcium in the electrolyte or introduction of a new active-site-adsorbing species. This study highlights the importance of calcium removal from the catalyst-electrolyte interface in seawater splitting. The lack of CO2 tolerance of various perovskite oxides has increased the tendency of carbonate formation on the surface of the oxide. The effect of carbonate formation on OER catalysis of perovskite oxides is rarely studied, especially when surface reconstruction is involved. Using SCIO and BSCF as model perovskite oxides for OER in acid (0.1 M HClO4) and alkali (0.1 M KOH) respectively, the OER activity and OER performance stability of a fresh sample and an old sample stored for more than 3 years have been compared. Interestingly, in the case of SCIO, despite both forming IrOxHy/SCIO with carbonate accumulation on the catalytic surface after potential cycling, old SCIO still has significantly lower OER activity and OER stability due to incomplete surface reconstruction. In the case of BSCF, old BSCF has demonstrated a lower surface reconstruction rate compared to fresh BSCF due to physical blockage of carbonates. The removal of carbonates through heat treatment from both old samples has not succeeded in recovering lost OER activity, possibly due to particle enlargement. This series of studies have highlighted the effect of environment-facilitated precipitation on the surface reconstruction of perovskite oxides and its subsequent OER catalytic performance, which needs to be resolved to improve the applicability of the water splitting technology.