Potential Dependence in the Static and Dynamic Structure of Electric Double Layers in Aqueous Batteries

The electric double layer (EDL) is a fundamental component of electrode/electrolyte interfaces in aqueous batteries that governs many key electrochemical processes, including charge transport across interfaces, passivation of solid-electrolyte interphases, and chemical stability of the electrolyte. More than a century of study has yielded general models for the ion distribution through the EDL, yet little experimental evidence for the speciation, uniformity, and dynamics of the potential-dependent EDL structure is available due, in part, to the challenges with experimentally probing buried interfaces in operando conditions. We present a study that connects applied potential to the ion distribution and double layer capacitance in the EDL formed between conductive boron-doped diamond electrodes and aqueous electrolytes spanning a diverse range of compositions, valence, and concentration. Operando synchrotron X-ray reflectivity and resonant anomalous X-ray reflectivity reveal distinct ion distributions that evolve as a function of potential. Time-dependent X-ray reflectivity during cyclic voltammetry reveals a hysteresis in the EDL structure during polarization switching that suggests an energy penalty for reconfiguring the interface. This work brings new molecular-level insight to the potential dependence in the structural, chemical, and functional properties of the EDL in aqueous batteries.