A review of the effects of morphology and surface characteristics of mixed metal oxide nanostructures on electrochemical hydrogen storage capacity

The industrial revolution has led to an increase in global energy demand, making unsustainable reliance on limited fossil fuels owing to environmental risks such as CO2 emissions. Hydrogen has emerged as a promising renewable alternative with environmentally friendly solutions and high energy density. Solid-state hydrogen storage has great potential for application in batteries, supercapacitors, and fuel cells. Metal oxides with robust structure are durable for hydrogen uptake and release; however, they suffer from low specific capacity and rate performance owing to their limited surface area and reliance on solid-state diffusion. Mixed metal oxides (MMOs) offer improved hydrogen storage capacity. Morphology plays a vital role in enhancing the electrochemical performance, as nanostructures provide a higher surface area, more active sites, and efficient ion diffusion pathways. This review examines the impact of various morphologies of the MMOs on hydrogen storage capacity. The morphologies included spherical/quasi-spherical nanostructures, rod/moss-like structures, plate-shaped structures, bean-shaped nanoparticles, polyhedral structures, nanoribbons, flower-like, porous/sponge-like architectures, and stacked/aggregated shapes. These structures significantly enhance hydrogen storage performance by altering the surface area, porosity, accessibility of active sites, and hydrogen diffusion pathways. After reviewing the published literature in this field and comparing the hydrogen storage capacity values across different morphologies, the most optimal morphologies were identified and presented. Rationally designed MMOs having diverse morphologies can be further explored in energy storage devices.