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John Baniecki1 Catalin Harnagea2 Dan Ricinschi3 Takashi Yamazaki5 Yoshihiko Imanaka4 Hiroyuki Aso1

1, Next Generation Materials Project, Fujitsu Laboratories, Atsugi shi, Kanagawa, Japan
2, INRS - Énergie Matériaux et Télécommunications, Varennes, Quebec, Canada
3, Innovator and Inventor Development Platform, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan
5, Fujitsu Laboratories Ltd., Atsugi shi, Kanagawa, Japan
4, Digital Annealing Project, Fujitsu Laboratories Ltd., Atsugi shi, Kanagawa, Japan

Fuel produced from the electrochemical splitting of water can be used to power a wide variety of technologies including information communication technologies infrastructure. The slow kinetics of the oxygen evolution reaction (OER) is one of the performance-limiting factors for hydrogen production through electrolysis. OER catalysts are often unstable in alkaline environments exhibiting deactivation and structural transformation causing a significant challenge for use in photoeletrochemical cells and water electrolyzers. Moreover, in nanoscopically thin catalyst layers, OER activity also decreases due to inefficient charge transfer to the electrolyte-catalyst interface. Epitaxial heterostructures are promising to solve these issues, though recent attempts yielded improved stability only at the expense of greatly reduced OER activity. In this presentation, we elucidate the competing factors for deactivation of LaxSr1-xCoO3 (LSCO) in nanoscopically thin layers supported on conducting perovskite substrates, and demonstrate heterostructured anodes with simultaneously high activity and stability during electrochemical water splitting in alkaline environments (pH = 13).
Epitaxial thin films of La1-xSrxCoO3, Ba1-xLaxSnO3, and Sr1-xLaxTiO3 were grown by pulsed laser epitaxy. Interface energetics were characterized using in situ X-ray and ultraviolet photoelectron spectroscopies. Scanning transmission electron microscopy and electron energy loss spectroscopy were used to resolve the atomic structures, and scanning nonlinear dielectric microscopy used to probe the nature of the charge carrier character on the heterostructured catalyst surface. Density functional theory calculations were used to assess the impact of the electronic structure of the heterostructured catalyst layers on the overpotential and OER catalytic activity.
While the LSCO undergoes dramatic structural and electronic changes during electrolysis, including leaching of La and Sr from the film to yield a layer of cobalt oxyhydroxide, the thickness dependence of the OER activity will be revealed to be due to inefficiency of charge carrier transport to active sites. We demonstrate engineering of depletion layers widths and chemical stability using heterostructures comprised of nanoscopically thin epitaxial layers of degenerately doped stannate and titanate perovskite structure oxides to yield low overpotentials ~ 300 mV at current densities (~10 mA/cm2) relevant for hydrogen production in electrolyzers and photo-electrochemical cells, at hundreds of hours operations in nanoscopically thin active layers. Implications of the results for applications of nanoscopically thin oxide heterostructures for designs of high activity and stable anodes for carbon neutral energy production via the electrochemical splitting of water will be discussed.

Acknowledgement
C.H. would like to thank the Japan Trust program of the National Institute of Information and Communication Technologies (NICT) for funding.

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