Metal hydrides are a critical component in hydrogen storage systems due to their ability to absorb and release hydrogen reversibly. However, their performance degrades over time due to several mechanisms, including cycling fatigue, impurity poisoning, and phase segregation. Understanding these degradation pathways and developing mitigation strategies is essential for improving the longevity and reliability of metal hydride storage systems.

**Cycling Fatigue**
Repeated hydrogen absorption and desorption cycles induce mechanical stress in metal hydrides, leading to microstructural changes such as cracking, pulverization, and grain boundary decohesion. These structural defects reduce hydrogen storage capacity and kinetics by decreasing the effective surface area for hydrogen exchange and increasing diffusion path lengths. Cycling fatigue is particularly pronounced in intermetallic hydrides and complex hydrides, where lattice expansion and contraction during cycling exacerbate material fatigue.

Mitigation strategies focus on enhancing material durability through compositional modifications and structural engineering. Alloying elements can be introduced to improve mechanical resilience, while nanostructuring the material reduces crack propagation by limiting grain size. Composite approaches, where metal hydrides are embedded in a ductile matrix, also help absorb mechanical stresses. Additionally, optimizing cycling conditions—such as temperature, pressure, and hydrogen flow rates—minimizes abrupt phase transitions that contribute to fatigue.

**Impurity Poisoning**
Impurities in hydrogen gas, such as oxygen, carbon monoxide, sulfur compounds, and moisture, can irreversibly adsorb onto metal hydride surfaces, blocking active sites and hindering hydrogen absorption. Some impurities react with the hydride material, forming stable oxides or sulfides that permanently reduce storage capacity. For example, even trace amounts of oxygen can form passivation layers on rare-earth-based hydrides, severely degrading performance.

Gas purification is the primary defense against impurity poisoning. Pre-filtration systems, including pressure swing adsorption and membrane separation, remove contaminants before hydrogen interacts with the hydride. Surface coatings that resist chemisorption of impurities can also be applied. Another approach involves designing hydrides with sacrificial secondary phases that preferentially react with impurities, preserving the primary hydrogen storage phase.

**Phase Segregation and Inhomogeneity**
During cycling, some metal hydrides undergo phase segregation, where elemental constituents separate due to differing thermodynamic stabilities. This leads to compositional inhomogeneity, reducing overall hydrogen capacity and altering absorption-desorption kinetics. For instance, multicomponent hydrides like Mg-based systems often experience magnesium-rich and alloy-rich domain formation after prolonged cycling.

Homogenization techniques, such as rapid solidification or mechanical alloying, help maintain uniform phase distribution. Post-cycling thermal treatments can also restore homogeneity by promoting atomic diffusion. Advanced synthesis methods, including high-energy ball milling and thin-film deposition, produce metastable phases that resist segregation.

**Lifetime Predictions**
Predicting the operational lifespan of metal hydrides requires understanding degradation kinetics under realistic conditions. Accelerated aging tests simulate long-term cycling by subjecting materials to extreme temperatures and pressures. Data from these tests are used to develop empirical models correlating cycle number with capacity loss.

Statistical methods, such as Weibull analysis, estimate failure probabilities based on degradation trends. More sophisticated approaches integrate first-principles calculations with continuum models to predict microstructural evolution over time. These models account for factors like stress accumulation, impurity concentrations, and environmental conditions.

**Mitigation Strategies Summary**
- **Material Design:** Alloying, nanostructuring, and composite formation improve mechanical and chemical stability.
- **Process Optimization:** Controlled cycling conditions minimize stress and phase segregation.
- **Gas Purification:** Removing impurities prevents surface poisoning and side reactions.
- **Thermal Management:** Maintaining optimal temperatures avoids detrimental phase transitions.
- **Reconditioning:** Periodic thermal or chemical treatments restore performance in degraded materials.

**Conclusion**
Degradation in metal hydrides is a multifaceted challenge influenced by mechanical, chemical, and thermal factors. While no single solution eliminates all degradation pathways, a combination of material engineering, system design, and operational control can significantly extend service life. Continued research into degradation mechanisms and predictive modeling will further enhance the viability of metal hydrides for large-scale hydrogen storage applications.