Near-infrared imaging of buoyancy plumes in diluted ionic solutions during electrodeposition on electrode surfaces

Abstract Understanding the development of buoyancy-driven plumes generated by electrochemical reactions is crucial for revealing how flow behavior can be influenced and optimized through electrode architecture and electrolyte formulation. However, their roles in transparent aqueous electrolyte systems remain underexplored. To address this, this study develops a near-infrared (NIR) imaging method for the in-situ visualization of buoyancy-driven plumes generated from a zinc acetate electrolyte/electrode interface during electrochemical reduction. The method uses the concentration-dependent absorbance of zinc acetate at a primary wavelength of 1579 nm, corresponding to the isosbestic point for acetate with supplementary wavelengths of 1308 and 1445 nm. Two-dimensional absorbance images of the zinc aqueous solution were acquired using a condenser-illuminated microscope coupled with an NIR camera. A custom microreactor cell was employed, which modeled a zinc–air battery configuration, and incorporated a 1 mm optical path length along with a circular, 1 mm-diameter glassy carbon working electrode. Potentiostatic reductions were performed at −1.5, −2.0, and −2.5 V using bulk zinc acetate concentrations of 0.2, 0.4, 0.6, 0.8, and 1.0 M. The quality of intensity images was enhanced through baseline correction, Wave-U-Net convolutional neural network denoising, and second-stage smoothing. The resulting images and absorbance profiles exhibited edge-initiated depletion plumes of reduced Zn 2+ that subsequently merged and redistributed species toward the central axis. In terms of potential dependence, the more cathodic potentials yielded earlier onsets with greater plume penetration. In terms of concentration dependence, lower bulk concentrations (0.2–0.4 M) displayed earlier and stronger plumes than higher concentrations (0.8–1.0 M). These findings establish NIR imaging as a practical, minimally perturbative modality for quantifying reaction-coupled concentration fields and buoyancy-driven transport, providing a guide for future electrode design, electrolyte formulation, and magnetization effects in electrochemical systems.