Advancing Electrodes and Electrolytes for High Capacity and Rate Proton Batteries

Abstract

To develop faradaic electrodes that integrate advantages of both battery (high capacity), and supercapacitor (fast rate and excellent cycle life) is revolutionary to transform our use of electrical energy, but presents a grand challenge in energy storage. Because their origins are intrinsically conflicting: high capacity requires in-depth redox reactions and longer diffusion distance of ion-charge-carriers, but the corresponding kinetics typically slow down as consequences. Recently, the emerging proton electrochemistry facilitates developments of batteries where proton/hydronium serve as ion-charge-carrier (named as proton batteries) and offers new opportunities for the long pursuit. This thesis aims to understand fundamental working principles of proton electrochemistry, design novel electrode-materials and electrolytes, and correspondingly develop full batteries. Charge storage controlled by diffusion and phase transformation of electrodes are representative indicators of sluggish kinetics in battery chemistries. However, fast rate-capabilities are reached in a α-MoO3 electrode that presents both features associating with protons. In Chapter 2 and 3, this unique topochemistry is disclosed to involve sophisticated ion-electrode-interactions and contains two key steps: hydronium adsorption on surface, and the subsequent naked proton insertion in bulk electrode lattices to trigger irreversible structure transformation to hydrogen molybdenum bronzes (HMBs) from the parent MoO3. Following rearrangements take place only among HMBs phases and present high reversibility and kinetics, therefore structural explanations to the fast rate-capability are provided. At electrode-electrolyte-interfaces, hydronium is determined as the active ion-charge-carrier to initiate charge transfer as well as surface hydration, where water adsorption/expelling with reduced polarizations and enhanced kinetics is accompanied in the meantime. Otherwise, water activities influence the basic electrochemical properties and induce material-dissolutions during functioning. Material pseudocapacitance is an alternative strategy to achieve faradaic electrodes of both high capacity and rate, where the intercalation pseudocapacitance is considered the most promising because of the full involvement of bulk reactions. In Chapter 4, the advantages of integrated intercalation pseudocapacitance and proton electrochemistry are disclosed, via the studies of proton redox chemistry in hexagonal MoO3 (h-MoO3). As a structure isomer to α-MoO3, similar properties can be found in h-MoO3 such as similar maximum proton exchange amount and an initial process out of structure rearrangements, etc. Interestingly, though surface ion de-solvation phenomena are also observed, it is identified increased crystal water in h-MoO3 after electrochemistry, which is attributed to certain hydronium intercalation into the intracrystalline tunnels. After the initial process, solid-solution proton intercalation is demonstrated in h-MoO3, accompanied with fast rate capabilities almost independent on particle sizes. Surprisingly, fast rate capabilities from proton intercalation pseudocapacitance has been demonstrated in electrodes of monocrystals over 100 μm scale. The high-potential MnO2/Mn2+ redox couple (MnO2 + 4H+ + 2e ↔ Mn2+ + 2H2O, Eθ=1.23 V, v.s. SHE) has recently attracted attentions in developing aqueous batteries, typically via electrodepositing solids on substrates for energy storage. This electrolysis reaction provides a facile and competitive cathode choice for the emerging proton batteries, because most of known electrodes either function at low potentials (due to proton reduction thermodynamics) or overlap in potentials with each other to restrict the cell voltage. However, its redox behavior is little understood in concentrated acids, and current full-cells often suffer limited cycle life (e.g., tens-of-hours). In Chapter 5, we show a homogeneous and stable MnO2 colloid electrolytes prepared by electrolysis in H2SO4 (≥ 1.0 M), and their application to achieve long life proton batteries. The colloid electrolytes enable prolonged cycling of a MnO2//MoO3 cell from 11.7 h to 33 days, and a MnO2//pyrene-4,5,9,10-tetraone cell for 489 days, which is the longest duration ever reported for proton batteries, to the best of the authors’ knowledge. Through water dilution colloids precipitate into hierarchical nanosheet spheres; Further characterizations together with deposited substrates reveal major electrolytic products as ε-MnO2 regardless of electrolytes. Colloids reform from precipitates differently with Mn2+ present/absent acids, suggesting colloid balances include both physical and chemical interactions. The results achieved in this thesis offer new fundamental insights in proton electrochemistry, introduce findings of novel electrodes and electrolytes, and demonstrate proof-of-concept application of proton batteries. The findings will guide the further search for electrode materials, exploitation of electrode performances, electrolytes, and advanced full-cell designs. It is my hope these would contribute to future energy storage techniques of high rate and capacity and beyond.