Electrochemical water splitting for green hydrogen: Key material roles, persistent challenges, and outlook

Electrochemical water splitting offers a compelling pathway to large-scale green hydrogen production, yet renewable sources account for merely 1% of global hydrogen supply, with production costs (USD 2.28–7.39 kg−1) remaining far above the fossil-derived benchmark (USD 0.67–1.31 kg−1). This review systematically maps the convergence of substrate engineering, catalyst design, real-water degradation, and forward osmosis (FO) integration, moving beyond isolated catalyst-centric metrics. Three-dimensional (3D) binder-free substrates, including nickel foam, carbon cloth, and stainless-steel mesh, reduce mass-transport overpotentials by 100–270 mV relative to planar electrodes. Earth-abundant Fe- and Ni-doped catalysts deliver hydrogen evolution reaction (HER) overpotentials as low as 50 mV at 10 mA cm−2 and oxygen evolution reaction (OER) overpotentials of 211–332 mV at 10–1000 mA cm−2, with operational lifetimes of up to 1000 h. Heterostructure engineering further extends stability, with a FeCoP/TiN electrode sustaining 2000 h in alkaline freshwater and 340 h in alkaline seawater at 500 mA cm−2. Noble metal loadings have been reduced to as low as 2.6 μg cm−2 for Ru (achieving an overpotential of 10 mV at 10 mA cm−2 in 1.0 M KOH) and to 50 μg cm−2 for both Ru (in AEMWE at 1000 mA cm−2) and Pt (in PEMWE at 100 mA cm−2). At these ultra-low loadings, Ru and Pt achieve degradation rates of 0.43 mV h−1 and 8.3 × 10−3 mV h−1, respectively, over 500–600 h, demonstrating up to 14-fold greater stability to commercial benchmarks. Moreover, FO pretreatment with phosphate buffer draw solutions achieves 99.6% rejection of divalent scaling ions and 97.8% rejection of Cl−/Br−, enabling stable operation beyond 360 h at 360 mA cm−2 in alkaline seawater, with life-cycle emissions of only 0.3–2.4 kg CO2-eq. per kg H2, compared to 27.5 kg CO2-eq. per kg H2 for fossil-based routes. The findings establish FO-electrolysis integration, bifunctional heterostructure catalysts, and superaerophobic 3D electrode architectures as the essential foundations for commercially viable, freshwater-independent green hydrogen production.