The Electrochemistry of Hydrogen Titanates: From Interfacial to Confinement Phenomena

Diversifying the materials chemistry landscape is important for alleviating supply chain issues for next generation electrochemical energy storage. Titanium oxides are attractive as electrodes for energy storage applications due to the terrestrial abundance and low toxicity of Ti. Hydrogen titanates (HTOs) form a diverse group of metastable, layered titanium oxides with an interlayer containing both water molecules and structural protons. We investigated how the chemistry of this interlayer environment influences electrochemical ion insertion from aqueous and non-aqueous electrolytes and correlated the electrochemical response with the physical and chemical properties of HTOs using a suite of characterization tools, including operando and in situ methods. In aqueous acidic electrolytes, we found that the nature of the acid (inorganic vs. organic) dictates the selectivity of the proton-coupled electrochemical reactions and that the primary degradation mechanism is via Ti3+ dissolution. In the case of non-aqueous aprotic electrolytes, we found that structural protons were necessary for activating HTOs towards Li+ insertion, and that the potential of the first reduction reaction trended with the relative acidity of the structural protons. We propose that the initial electrochemical reaction in the non-aqueous electrolyte involves reduction of the structural protons to yield hydrogen gas and formation of a lithium titanate. The hydrogen gas is initially confined within the HTO lattice until the titanate structure expands upon oxidation. Our work has broad implications for the electrochemical behavior of insertion materials containing hydrogen and structural water molecules, where hydrogen evolution is expected at potentials below the hydrogen reduction potential and in the absence of water or protons. This behavior is distinct to the aqueous electrochemistry of these materials, and provides an example of electrochemical electron transfer to a non-metallic element in a solid-state host, in analogy to anion redox.

Veronica Augustyn is the Jake and Jennifer Hooks Distinguished Scholar in Materials Science and Engineering and Associate Professor in the Department of Materials Science and Engineering at North Carolina State University. Her research focuses on the electrochemistry of materials, and especially transition metal oxides, for energy and environmental applications, including understanding interfacial phenomena, insertion mechanisms, and confinement effects. She is the founder and faculty advisor of an award-winning international project, SciBridge, a student-led group that develops renewable energy research and education collaborations between universities in Africa and the U.S. She supports the materials electrochemistry community as an Associate Editor of the Journal of Materials Chemistry A and Materials Advances, the editorial advisory boards of ACS Energy Letters, ACS Electrochemistry, Physical Review Materials, Energy Storage Materials, and ACS Nanoscience Au, and as a Member-at-Large of the Electrochemical Society Battery Division.

Host: Prof. Chong Liu, Neubauer Family Assistant Professor of Molecular Engineering in the UChicago Pritzker School of Molecular Engineering