Establishing optical indicators for the state of hydrogen in MgH2

Abstract

Accurate determination of the state of hydrogen (SOH) in solid-state hydrogen storage materials is essential not only for optimizing hydrogen release kinetics and enhancing storage efficiency but also for ensuring system safety in practical applications. While most existing studies have concentrated on thermodynamics and kinetics, direct monitoring of residual hydrogen content, a parameter of critical engineering relevance, has rarely been reported. This highlights the urgent need to realize online SOH detection through new physical properties. In this study, we propose a non-invasive, real-time SOH monitoring strategy for magnesium hydride (MgH2), based on optical properties and combining density functional theory (DFT)-based optical calculations with experimental validation. Using DFT, the optical properties of MgH2 and its dehydrogenated form (Mg) were systematically calculated across the infrared, visible, and ultraviolet spectral ranges. Theoretical results revealed strong linear correlations between SOH and specific optical parameters, such as reflectance at 1200 nm and 550 nm and refractive index at 250 nm, with the coefficient of determination exceeding 0.99 and mean absolute errors below 0.05. To validate these predictions, reflectance measurements were conducted at 940 nm, a wavelength identified as highly sensitive to hydrogenation, and a consistent decrease in reflectance with increasing hydrogen uptake was observed. The underlying mechanism was attributed to band structure evolution and electron density redistribution, supported by density of states analysis and Drude model interpretations. This work establishes a robust theoretical and experimental framework for optical SOH diagnostics, emphasizes the importance of residual hydrogen detection for advancing solid-state hydrogen storage from fundamental research toward practical engineering applications, and provides new insights into the design of intelligent, optically responsive hydrogen storage systems, paving the way for the development of spectroscopic SOH sensors in next-generation hydrogen energy technologies.

1. Introduction

Hydrogen energy, with its zero emissions and high energy density, is widely recognized as a promising solution to rising global energy demand and escalating environmental challenges [1–6]. As a clean and low-carbon energy carrier, hydrogen provides a viable alternative to fossil fuels, mitigating air pollution and enhancing ecological sustainability [7–10]. Furthermore, its storage capacity allows hydrogen to mitigate the intermittency of renewable sources such as solar and wind, thereby improving the reliability and resilience of modern power systems [11]. Among the various hydrogen storage technologies, solid-state storage based on metal hydrides stands out due to its high volumetric hydrogen density, intrinsic safety, and operational stability under ambient conditions [12–17].

Solid-state hydrogen storage technology demonstrates considerable potential for efficient hydrogen retention [18–23]. Nevertheless, several technical challenges remain unresolved [24–30]. Among them, one of the most critical investigated issues is the accurate assessment of the state of hydrogen (SOH) within storage materials. Precise SOH monitoring is essential, as it directly influences hydrogen release kinetics, storage capacity, system safety, and operational durability. Conventional SOH detection techniques, including pressure–composition–temperature (PCT) measurements [31] and mass flow monitoring, remain the most commonly employed methods. However, both approaches suffer from intrinsic limitations, including system complexity, response lag, and susceptibility to external disturbances.

While PCT testing can estimate the SOH by tracking variations in pressure, temperature, and hydrogen concentration, it lacks the capability for in situ or real-time monitoring, with measurement results often subject to significant feedback delays [32–34]. This limitation becomes especially critical during the dynamic cycling of hydrogen storage materials, where environmental fluctuations can compromise data accuracy and introduce substantial deviations [35]. In contrast, mass flow meters allow real-time hydrogen flow monitoring and indirectly reflect SOH variations. However, their performance degrades over repeated cycling, resulting in increased measurement errors and reduced reliability [36]. Additionally, the deployment of mass flow meters adds to system complexity.