Elettra | SciQC, TwinMic Revealing the Early-Stage Reduction Mechanism of ZnO Nanorods for Zinc-Ion Batteries Zinc-based batteries are gaining recognition as promising candidates for next-generation energy storage, owing to their intrinsic safety, raw material abundance, and environmental friendliness. Nevertheless, the reversibility of Zinc anodes in aqueous ambient is hindered by shape change and passivation, severely limiting their cycle life and active material utilization. A promising strategy to overcome these limitations is nanostructuring the active material to hinder the formation of compact insulating ZnO layers and to confine soluble Zn(II) species. Zn nanostructuring mandates anode fabrication in the form of ZnO, requiring a formation step for battery activation, but understanding of the reduction mechanism of nanostructured ZnO under electrochemical conditions remains lacking. This work addresses this knowledge gap, focusing on the electrochemical and morphochemical evolution of ZnO nanorods grown directly on carbon cloth (ZnO/CC) during the initial formation cycle, wherein ZnO is reduced to metallic Zn, elucidating the reduction mechanism and the associated structural transformations. Vertically aligned ZnO nanorods were synthesized using a hydrothermal synthesis route employing hexamethylenetetramine (HMTA) to grow vertically aligned ZnO nanorods directly onto carbon cloth. The synthesis steps are reported in the schematic representation of Fig. 1 (a). With this approach, a uniform, well-aligned array of ~1 µm long nanorods can be grown, with a high aspect ratio of ca. 10. The carbon cloth acts both as a mechanical scaffold and as an integrated current collector, providing intimate electrical contact with the active material. This binder-free architecture eliminates the need for polymeric binders and favors better electrochemical access to the active material. The study combines advanced morphological and spectroscopic characterization with galvanostatic electrochemical reduction, providing a detailed picture of the first charge impact on the nanostructured anode. Electrochemical reduction of ZnO/CC electrodes was performed in a neutral Na2SO4 electrolyte, chosen to suppress Zn2+ deposition from other Zn-containing species and allowing to isolate the observation of the solely ZnO-to-Zn conversion. Reduction to 50% depth of charge (DOC) was achieved via chronopotentiometry, during which the working potential dropped to values typical of the Zn/Zn2+ redox couple, confirming the formation of metallic Zn. Scanning and Transmission Electron Microscopy (SEM and TEM) microscopy disclosed that electrochemical formation notably affects the structure of pristine nanorods, Specifically, they become shorter and stubbier by factors of ca. 30%, respectively, and a porous matrix forms in the intervening space among them. Moreover, TEM diffraction patterns showed the appearance of amorphous regions that clearly result from the electrochemical treatment. Unique information about morphochemical transformations brought about by electrochemical formation could be obtained by soft x-ray spectromicroscopy in the direct (STXM) and Fourier spaces (ptychography) carried out at the TwinMic beamline at Elettra. In particular, we could visualize the spatial distribution of Zinc oxidation states within nanorods down to nanometer resolution (Fig. 1 c). In fact, Zn L-edge maps revealed that reduced Zn does not form a uniform front, progressing along the rod length, but rather it yields, a granular distribution of reduced domains. In shorter rods, the dimension of which are close to the space resolution of STXM, ptychographic imaging confirmed the same granular pattern. The assessed lack of axial dependence of the reduction progress thus allowed to exclude a solid-state reduction mechanism. Based on these observations, we propose a formation mechanism, schematized in Fig. 1 (b), whereby reduction begins at the triple-phase boundary (TPB) among, the carbon substrate, ZnO, and electrolyte. Metallic Zn starts to grow from the TPBs, forming electronically conductive paths, in turn yielding fresh TPBs that trigger further reduction along the nanorod surface, giving rise to a porous Zn shell. Further, this porous layer allows water to diffuse toward the rod core, enabling the reaction front to progress radially inward. Eventually this interplay of electronic and ionic transport through a dynamic porous network governs the conversion process. This study offers, for the first time, a combined morphological and chemicalstate mapping of ZnO nanorods undergoing electrochemical reduction, enabling a mechanistic understanding of the electroreduction process that governs active material formation. These insights may pave the way for rationally designing more stable and efficient Zinc-ion battery anodes, by targeting architectures that support controlled and reversible redox transformations. Elettra Highlights 2024–2025 28
RkJQdWJsaXNoZXIy MjQ4NzI=