Recent development in catalysts for the production, storage and release of hydrogen.

The rising demand for energy and growing environmental concerns have intensified the search for sustainable alternatives to fossil fuels, establishing hydrogen as a promising clean energy carrier. However, its widespread adoption is currently limited by its reliance on a fossil-based production method, as well as by persistent challenges in storage and transport. Recent advancements in CO2 hydrogenation, catalytic design, and machine learning-driven optimisation offer ways to enhance both the sustainability and efficiency of hydrogen production. Among these advancements, formic acid (FA) is emerging as a sustainable energy source due to its ease of handling compared to solid or gaseous materials. This review explores the development of state-of-the-art catalysts for CO2 hydrogenation and FA dehydrogenation, contributing to the advancement of a hydrogen-based economy. It also delves into the methanol economy as an alternative strategy. Particular emphasis has been placed on transition metal-based complexes, which are recognised for their high catalytic activity associated with a well-defined mechanism, including Pd-based heterogeneous and non-noble metal homogeneous catalysts. Furthermore, this review demonstrates how machine learning can accelerate catalyst innovation. By addressing the key challenges of hydrogen storage, efficiency, and scalability, this review contributes to the development of practical, cost-effective, and environmentally friendly hydrogen energy solutions.