Hydrogen Production from Untapped Water Sources through Electro- and Photocatalysis: Key Performance Indicator (KPI) Based Benchmarking Framework

The energy-water nexus is a fundamental pillar of sustainable development. Green hydrogen (H 2 ) is highlighted as a prominent alternative energy vector. This study seeks to assess and define key performance indicators (KPIs) to evaluate and compare electro- and photocatalysis for H 2 production via water splitting using untapped water sources (seawater, groundwater, or industrial brackish wastewater) when access to use ultrapure water (UPW) is not feasible, as a zero-liquid discharge (ZLD) strategy. Energy consumption, H 2 production, H 2 purity, and geometric package density were the proposed KPIs that effectively compared electro- and photocatalysis. Results highlight that conductivity and chloride concentration significantly influence electrocatalysis, which outperformed photocatalysis across all matrices except UPW, where photocatalysis exhibits comparable performance due to its independence on ionic conductivity. Furthermore, the prospective analysis of advantages and challenges of scaling up both approaches shows a potential integration with renewable energy and H 2 oxidation to recover energy and ultrapure water.