(Invited) Neutral Seawater Splitting of Perovskite Oxynitrides Under Sunlight via Alternative Oxidation Reactions
Authors: Seo Jeongsuk 
Published: December 2024

Photo-electrochemical (PEC) seawater splitting using semiconductors is a promising means of directly producing hydrogen from seawater occupying approximately 97% of Earth’s water. The n-type perovskite oxynitride semiconductors with a general formula AB(O,N)3 (A=Ca, Sr, Ba or La; B=Ti, Nb or Ta) are capable of absorbing intensive visible light up to 750 nm wavelength, thermodynamically leading to high solar-to-hydrogen conversion efficiency of 10% or above.1 Also, their band positions straddle various oxidation and reduction potentials including the water redox potential to produce value-added energy resources such as H2, HCOOH, and NH3.2 Based on the favorable optical properties, the water splitting activity using the oxynitrides has been significantly increased in strong alkaline electrolytes, although it is still low in neutral electrolytes.2-3

Oxygen evolution reaction (OER) is driven via four-electron oxidation. Its kinetics is commonly very sluggish in neutral aqueous solutions, which the significant overpotential is required to catalyze it. Thus, applying alternative oxidations to OER, such as chlorine evolution reaction (CER), may be a feasible approach to efficient hydrogen production in neutral seawater electrolytes.4 In terms of thermodynamic potential, OER (E
0 = 1.23 VNHE) is more oxidative than CER (E
0 = 1.36 VNHE). However, CER driven via two-electron oxidation can be catalyzed faster than OER kinetically in neutral electrolytes.
Herein we first present sunlight-driven seawater splitting activity of perovskite AB(O,N)3 largely improved in neutral environments and also discuss CER activity depending on various pH values as an alternative oxidation to OER. Highly-crystalline, porous SrNbO2N particles grown on Nb substrate was prepared by bottom-up fabrication including oxidation of Nb substrate and flux-assisted nitridation.5 The advanced fabrication caused less-defective oxynitride with a large surface area. The resulting SrNbO2N/Nb photoanode produced significant OER photocurrent in 0.2 M NaPi electrolyte at a neutral pH 6.4, and the photocurrent was three times increased by adding 0.5 M NaCl to catalyze CER simultaneously. Moreover, the onset potential for seawater splitting including CER was shifted to a lower potential by approximately 0.1 VRHE than that for water splitting leading to OER exclusively. These results clearly demonstrate that CER was preferentially catalyzed during neutral seawater splitting compared with competitive OER. It also implies that CER kinetics over the SrNbO2N was faster than OER kinetics despite its unfavorable thermodynamic potential.
The BaTaO2N/Ta photoanode, more photoactive and stable than the Nb-based oxynitrides, was synthesized in the similar manner. The seawater splitting of the oxynitride at different pH values was further investigated, and the quantitative analysis of the resulting products, namely, H2, O2, and ClO− was also performed to determine the operating conditions suitable for efficient and stable seawater splitting. Moreover, different electro-catalysts to improve the selectivity to CER were evaluated in the artificial seawater. The enhanced seawater splitting activity and selectivity to CER of the AB(O,N)3 in artificial seawater will be discussed in detail in a presentation.

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