Progress and perspectives on metal-free biochar-derived carbon nanostructures for efficient water splitting: mechanistic insights and strategies toward sustainable hydrogen production

The global shift toward net-zero emissions and sustainable energy systems has elevated hydrogen as a clean and green energy source, with water splitting recognized as one of the most promising routes for large-scale hydrogen production. However, the slow kinetics of the hydrogen evolution (HER) and oxygen evolution (OER) reactions necessitate highly efficient catalysts. Typically, platinum group metal (PGM)-based catalysts (Pt, Ir and Ru) have been extensively studied, which performed as benchmark catalysts. Though their poor abundance and expensive limit commercial viability for day-to-day applications. In this review, we have comprehended the critical role of biomass-derived carbon nanostructures as metal-free and sustainable alternative electrocatalysts to PGM. In addition, the review emphasizes recent advancements in tuning physicochemical properties through various approaches to achieve a competitive electrocatalytic performance for both OER and HER. Critically, we highlight three strategic hydrogen production pathways: (i) electrocatalytic, (ii) photocatalytic, and (iii) photoelectrochemical, where rational catalyst design regulates surface adsorption energy guided by modulating the electronic properties. In this comprehensive analysis, we converge the need for rational catalyst design that balances an improved catalytic performance and economic viability to achieve a sustainable alternative to existing PGM-based counterparts. As well as the critical insight and the future perspective into the metal-free carbon-based catalysts derived from biomass materials and their mechanistic pathways during OER and HER across all three water-splitting approaches for efficient, cost-effective and scalable hydrogen production in real-time applications.