A pH-differential microfluidic reactor for hydrogen energy conversion

Abstract

H2/O2 unitized regenerative fuel cells (URFCs) are a promising option for electrical energy storage & conversion because of their high energy density, but have not been widely considered due to concerns over power density and round-trip efficiency. Here, we present a switchable pH-differential unitized regenerative fuel cell (s-URFC) that raises the electrochemical performance and efficiency substantially by thermodynamic approach. Relying on a membraneless laminar flow-based design, pH environments in the cell are optimized independently for different electrode reactions and are switchable together with the cell process to ensure always favorable thermodynamics for each electrode reaction. Under room conditions, the cell achieves a fuel cell open circuit voltage of 1.89 V and a peak power density of 1.31 W/cm2, 1.5 times as much as conventional URFCs operating at elevated temperature and pressure. Freed from the thermodynamic barrier, a round-trip efficiency of 74% is recorded at 40 mA/cm2 with the onset electrolysis voltage halved to 0.57 V. This work also conducts the first systematic investigation and analysis on the heat generation mechanism and acid/alkaline mixing layer in microchannels when implementing pH-differential technique. Examination of the catalyst durability in different scales shows the feasibility of future industrialization. With the membrane-less architecture and enhanced thermodynamic property, s-URFCs not only help address the most pressing hindrances of URFCs, but also benefits the future development of compact yet robust energy conversion systems.