Unbiased Tandem System for Simultaneous Methane and Glycerol Upgrading to Formic Acid Under Ambient Conditions

Direct methane conversion to liquid products under mild conditions remains a long-standing challenge, often requiring expensive oxidants or external bias that hinder practical viability. Here, we report a bias-free photovoltaic-photoelectrochemical (PV-PEC) tandem system that co-upgrades methane and glycerol into formic acid at both electrodes under ambient conditions. The cathodic process employs oxidized carbon black to drive in situ generation of hydrogen peroxide, which in turn mediates methane oxidation via electro-Fenton reactions in the presence of Fe²⁺. The photoanodic process uses nanoporous BiVO₄ to oxidize glycerol with nearly 100% Faradaic efficiency toward a mixture of glycerol oxygenates. Coupling these reactions in a PV-PEC configuration enables bias-free operation at 1.7 mA cm^{- 2} under standard solar illumination (Air Mass 1.5 Global, AM1.5G), yielding 2.87 mmol g_cat ^-1 h^-1 formic acid from methane oxidation (with a total production rate of 7.99 µmol cm^-2 h^-1) and achieving Faradaic efficiencies of 4.96% for the cathode and 28.5% for the anode. Life cycle assessment reveals over 66% reduction in carbon footprint compared to conventional syngas-based processes. This work demonstrates the first bias-free, full-cell platform for simultaneous methane and glycerol upgrading, advancing sustainable chemical production and efficient biomass utilization.