Toward Off-Grid Photovoltaics-Driven Hydrogen Production: A Conceptual Study on Biomass-Assisted Fe3+/Fe2+ Mediated Co-Electrolysis

As a conceptual study for low-energy hydrogen production, potentially coupled with off-grid photovoltaics, this work focuses on overcoming the constraint of the oxygen evolution reaction (OER), which features a high anode potential and significant overpotential. To reduce energy consumption, the Fe²⁺ oxidation reaction is employed to replace OER, coupled with Fe²⁺ regeneration using natural biomass. Experimental results reveal that Fe²⁺ oxidation reaction is an effective substitute, with an initial oxidation potential of 0.5 V (vs. Hg/Hg₂SO₄), much lower than that of OER. Fe²⁺ regeneration is notably influenced by both biomass type and reaction temperature. Chlorella pyrenoidosa (CP) achieves the highest Fe³⁺ reduction rate of 90.5% at 190 °C. Water-soluble organic compounds generated during biomass oxidation exert a negative impact on Fe²⁺ electrooxidation by accumulating on or coating the electrode surface, and the compounds derived from CP exert a less detrimental effect. Moreover, enhancing magnetic stirring, elevating temperature, and selecting an appropriate anode material can significantly boost the oxidation reaction. Under optimized conditions, the current density during electrolysis of CP filtrate at 1.1 V reaches 280 mA/cm², much higher than values reported in similar studies. This highlights the great potential of this co-electrolysis approach for efficient hydrogen production driven by off-grid photovoltaic power.