Crossover permeation of catalysts and organic fuels through proton exchange membrane in liquid catalytic fuel cells

Biomass, a renewable energy source, is utilized through physical and chemical processes. Among these, liquid catalytic fuel cells (LCFCs), which commonly use Nafion proton exchange membranes to separate anolyte and catholyte, attract attention for their ability to directly utilize complex biomass wastes and generate electricity under mild conditions. However, this study reveals cross-membrane migration of catalysts and fuels in LCFCs, leading to imprecise measurement of cell performance and organic waste degradation. A typical LCFC is assembled using phosphomolybdic acid (PMo) as the anode catalyst, phosphomolybdovanadic acid (PMoV) as the cathode catalyst, and glucose as fuel. Under ambient conditions, the LCFC achieves a maximum power density of 3.57 mW/cm², but the Faradaic efficiency declines significantly over 5 days of operation, from 32 to 20%. Membrane analysis revealed that Nafion 115 possesses pores ranging from 0.98 to 4.25 nm, allowing the migration of both catalysts and organics. Permeation experiments show that concentration gradients dominate the migration of species. Glucose and mannose have permeation rates of 1.17 × 10⁻¹⁰ and 1.07 × 10⁻¹⁰ cm²/s, respectively, while PMo and PMoV exhibit lower permeation rates of 8.91 × 10⁻¹¹ and 7.44 × 10⁻¹² cm²/s. The electric field significantly accelerates the migration of vanadium, with an electric field-driven permeation rate of 1.48 × 10⁻³ cm³/(s·A). Although model predictions suggest that the cell would fail within 15 days under a 5 mA discharge, experimental results show that the cell had already nearly failed by day 8. This work highlights the need to address permeation in LCFCs through membrane or other system improvements.