Phase Stabilization of the Metastable Piezoelectric Phase of Barium Nickelate for Oxygen Evolution

Oxygen evolution reaction (OER) is the rate limiting step limiting the efficient production of fuel from water.^1,2 The potential generated in a piezoelectric material under stress has long been proposed as a route to increase OER catalytic activity, potentially decreasing the required overpotential. A lack of suitable materials has slowed progress in piezocatalysis due to surface instability. Barium nickelate (BaNiO₃) is an ideal piezocatalytic candidate because it has been shown to have an order of magnitude higher OER activity than the current benchmark rare earth catalyst iridium oxide (IrO₂).¹ However, the ground state <i>P</i>6₃/<i>mmc</i> phase of BaNiO₃ is centrosymmetric which cannot be piezoelectric, but the piezoelectric <i>P</i>6₃<i>mc</i> phase of BaNiO₃ is predicted to be close in energy to the ground state and thus likely accessible.³<br/>In this work, we describe the stabilization of the metastable piezoelectric <i>P</i>6₃<i>mc</i> phase of BaNiO₃. Barium nickelate powders were synthesized via an adapted sol-gel solution processing method described by Lee et al.¹ The sol-gel solution pH and calcination temperature were varied to reduce secondary phase formation. A calcination atmosphere of flowing ultra-high purity oxygen was used to control the final oxidation state of the product. The addition of barium carbonate (BaCO₃) during the calcination process was found to influence the phase formation pathway leading to the <i>P</i>6₃<i>mc</i> phase of BaNiO₃. In-situ x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) were used to elucidate the differences in the phase formation pathways of the <i>P</i>6₃/<i>mmc</i> and the <i>P</i>6₃<i>mc</i> phases. Both in-situ XRD and XPS suggest that the addition of BaCO₃ preferentially forms the BaNi_0.83O_2.5 <i>R</i>32 phase which is more structurally equivalent to the <i>P</i>6₃<i>mc</i> phase of BaNiO₃. XPS and energy-dispersive x-ray spectroscopy (EDS) were used to determine the final oxidation states after calcination. XRD, Rietveld refinements, Raman spectroscopy, and transition electron microscopy (TEM) were used to verify the phase of the resulting powders after calcination. These results provide insights into new pathways for the stabilization of metastable materials via targeting polymorphs that are structurally similar to the desired metastable phase. Furthermore, these results allow for the future development of piezoelectric catalysts for OER, potentially enabling the production of fuel from water.<br/><br/>1. Lee, J. G. <i>et al.</i> A New Family of Perovskite Catalysts for Oxygen-Evolution Reaction in Alkaline Media: BaNiO ₃ and BaNi _0.83 O _2.5. <i>J. Am. Chem. Soc.</i> <b>138</b>, 3541–3547 (2016).<br/>2. Plevová, M., Hnát, J. & Bouzek, K. Electrocatalysts for the oxygen evolution reaction in alkaline and neutral media. A comparative review. <i>Journal of Power Sources</i> <b>507</b>, 230072 (2021).<br/>3. Jain, A. <i>et al.</i> Commentary: The Materials Project: A materials genome approach to accelerating materials innovation. <i>APL Materials</i> <b>1</b>, 011002 (2013).