COMBATT · COMpetition between ion insertion and hydrogen evolution reactions in aqueous BATTery systems

Aqueous batteries (ABs) hold great potential for large-scale energy storage due to their safety and environmental compatibility. However, they face a fundamental challenge as the redox potential for ion insertion overlaps with the limit of the electrochemical stability window of water. This overlap leads to severe H2 evolution reactions (HER) in ABs, reducing cell efficiency and causing side reactions. Despite its importance, the underlying mechanisms of the competition between ion insertion and HER remain poorly understood. This project aims to address this challenge by combining the experienced researcher's expertise in batteries and X-ray techniques with the host institute's expertise in interfacial electrochemistry and electrocatalysis. We will develop a novel analytical platform, named X-Mass, designed to ensure compatibility between operando X-ray absorption spectroscopy and differential electrochemical mass spectrometry. Through the X-Mass, we will track the chemical composition of the active material and H2 gas production under consistent electrochemical conditions, allowing precise quantification of ion insertion (j-inst) and HER (j-HER) current densities. This will enable us to assess how both reactions vary and compete during battery operation. To further understand the factors governing their rates and limiting steps, we will explore the effects of electrolyte and electrode modifications on j-inst and j-HER. Electrolyte properties will be controlled by additional cations (Cs+, Mg2+, Al3+, etc.), insertion ion species (Li+, H+), and salt concentrations. Electrode properties will be modified using different polymorphs of the model materials, TiO2 and WO3. This approach will deepen our understanding of their reactions particularly in relation to the double layer and electrode electronic structures. Our project will provide new insights into the critical but fundamentally unexplored competition between ion insertion and electrocatalytic surface reactions.