Understanding Electrode-Electrolyte Interfaces with Metal Dissolution and Redeposition Chemistry
The fundamental understanding of the dynamic characteristics of metal dissolution and redeposition behavior at the electrode-electrolyte interface is essential, which provides the basis for the development of advanced energy and conversion devices (such as electrochromic devices, electrocatalysts, and batteries) with superior electrochemical performances. We firstly demonstrate the feasibility of resynthesizing the electrode surface chemistry and tuning the electrochemical reactions at the solid-liquid interface by selectively changing the electrolyte composition and electrochemical cycling conditions. Amorphous TiO2 surface layers can be formed on WO3 electrodes by adding exotic Ti cations to the electrolyte, and slow electrochemical cycling. The dissolution and redeposition of electrodes and surface coatings are intertwined, helping to establish a dissolution-redeposition equilibrium at the interface, which can inhibit metal dissolution, stabilize electrode morphology, and promote electrochemical performance. Since the diffusion layer generated by the dissolution of transition metals is ubiquitous at the electrochemical solid-liquid interface, by combining in situ three-electrode electrochemical reaction cell with advanced spatially resolved synchrotron X-ray fluorescence microscopy and micro-X-ray absorption spectroscopy, we then successfully demonstrate the formation and chemical identification of the diffusion layer. By studying the evolution of diffusion layers(tens of micrometers thick) when using WO3 electrodes in acidic electrolytes, we find that with increasing distance of the dissolved species from the electrode surface, the oxidation state remains largely unchanged, but the local electronic environment of the dissolved W species becomes more distorted. We subsequently report a systematic experimental approach by collecting a series of twodimensional fluorescence images at the electrodes to study electrode dissolution and redeposition under different electrochemical conditions. The results show that (1) metal dissolution and redeposition behaviors greatly evolve under different electrode polarization and electrolyte compositions; (2) metal dissolution and redeposition behaviors are independent of bulk electrolyte pH but depend on interfacial pH; and (3) the accumulation of interfacial dissolved species promotes the formation of polytungstate interfacial networks, which ultimately manifest as temporal heterogeneity of redeposition. Lastly, we provide an in-depth study of the underlying mechanism of electrochemicalcycling induced crystallization at the electrode-electrolyte interface through a combination of advanced synchrotron radiation characterization techniques and an in situ electrochemical reaction setup. We have discovered that (1) foreign cations from the electrolyte engender both tensile and compressive strains inside the crystal; (2) repeated electrode dissolution and redeposition promote crystal growth through a non-classical crystallization pathway of particle attachment, but the initial growth of crystals is inhibited by internal strains; and (3) as the strain accumulates, the crystal rotates or moves, which is the fundamental reason for the dynamic structure evolution of the crystal during electrochemical cycling. To our knowledge, this is the first study of electrochemical-cycling-induced crystallization and its strain evolution. These new findings reveal a previously unknown relationship between crystal growth and its internal strain at the electrode-electrolyte interface.