Electrochemical phase transformation–driven shifts in charge-storage mechanisms of Bi redox films

Electrochemically induced phase transformations often dictate the charge-transfer behavior of metal-oxide electrodes; however, their impact on the charge-storage remains largely unexplored, particularly for Bi-based systems. In this work, we investigate the charge-storage mechanisms governing the battery-like behavior of films based on the Bi-oxide/Bi0 redox couple, prepared without any carbon-based additives or binders. In particular, we compared the performances of electrochemically activated BiOx(NO3)y(OH)z xerogel—used without thermal treatment—with those of materials annealed in forming gas, consisting of β-Bi2O3/Bi0 crystalline domains. The electro-activated xerogels exhibit nearly constant specific capacitances across the investigated current-density range (∼115 F g−1 at 0.5 A g−1 to ∼80 F g−1 at 3 A g−1) in 1 M KOH. Annealed samples show higher specific capacitances at low current-density values (∼320 F g−1 at 0.5 A g−1) with lower values (∼50 F g−1 at 3 A g−1) at higher current densities. A multitechnique characterization reveals that charge storage in the electro-activated xerogel is predominantly governed by the surface Bi ↔ BiO2− reaction. Conversely, in the annealed films, the behavior is largely diffusion-controlled. In these systems, the initial β-Bi2O3 phase undergoes an electrochemically driven transformation into monoclinic α-Bi2O3, giving rise to an additional charge-storage pathway associated with the internal α-Bi2O3 ↔ Bi0 redox process.