Operando Optical Imaging of Phase Transition During Charging/Discharging of Nickel (Oxy)Hydroxide Electrodes

Nickel (oxy)hydroxide electrodes capacity is greatly reduced at high charging rates due to early onset of oxygen evolution before fully charging. Many modeling efforts were undertaken to understand the charging dynamic and design electrodes which balance the capacity-charging rate tradeoff. Most of these models employed a homogeneous solid-solution approach that ignores the phase transition that occurs upon charge/discharge cycles, and failed to explain phenomena such as hysteresis and negative differential resistance. Other models assumed a uniform phase transition propagating from the substrate to the surface of the electrode, i.e., along the direction of the electric field. These models overlook nonuniform spatio-temporal dynamics that are common in battery electrodes and other electrochemical systems.
In this study, the β-Ni(OH)₂/β-NiOOH phase transition, which dominates the charge/discharge dynamics and features a color change, was optically monitored during charge/discharge cycles. For this purpose, thin films of nickel hydroxide were deposited in a variety of methods on a transparent and conducive substrate. Successive ionic layer adsorption and reaction (SILAR) process allowed for the deposition of a layer with uniformity and thickness appropriate for optical imaging and modelling of spatio-temporal charging dynamics.
We observed seeding and lateral growth of large, macroscopic phase domains during charging. Moreover, we show that relatively simple image analysis of the movie frames yields surprisingly precise quantitative information about the electrochemical processes at play. On a global level, we demonstrate the ability for fast, potential-dependent Faradaic efficiency curves. Locally, we extract the state-of-charge profiles of the phase boundary region with exponential profile. We present a dynamical model of the system to elucidate the observed behavior. To our knowledge this is the first time where an electrochemical reaction front propagation perpendicular to the electric field is observed on the macro scale.