Fundamental aspects and electrochemical investigation of metal hydride electrodes: Principles, methods, and practical insights

In the evolving landscape of energy storage technologies, nickel-metal hydride (Ni-MH) batteries maintain significant market relevance despite the prominence of lithium-ion systems. The global Ni-MH market continues to grow steadily, reflecting their continued importance in applications demanding safety, environmental compatibility, and reliability. This review provides a critical analysis of electrochemical techniques used to investigate metal hydride electrodes for Ni-MH batteries. We examine the thermodynamic and kinetic phenomena of hydrogen storage in intermetallic compounds, focusing on the correlation between gas-phase hydrogen interactions and electrochemical processes. The review systematically analyzes various methods including cyclic voltammetry, chronopotentiometry, chronoamperometry, galvanostatic and potentiostatic intermittent titration techniques (GITT and PITT), and electrochemical impedance spectroscopy (EIS). For each method, we discuss theoretical foundations, practical applications, and fundamental limitations when applied to metal hydride systems. Our analysis reveals significant challenges in the direct application of methodologies originally developed for liquid-phase electrochemical systems to solid-state metal hydride electrodes. This methodological transfer without appropriate modifications has resulted in inconsistent data interpretation and discrepancies in reported parameters, particularly for hydrogen diffusion coefficients determined by different techniques. We identify common methodological errors and provide guidelines for more accurate experimental design and data analysis. The critical examination presented in this review demonstrates the necessity for researchers to carefully consider the fundamental differences between liquid-phase and solid-state electrochemical processes when selecting and applying characterization methods. Given the continued importance of Ni-MH batteries in energy infrastructure, establishing reliable characterization protocols is important for advancing the fundamental understanding and technological development of metal hydride electrode materials.