Electrochemical Hydrogen Separation from Reformate Using High-Temperature Polybenzimidazole (PBI) Membranes: The Role of Chemistry

Various phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes, para-PBI, m/p-PBI, and meta-PBI, were prepared via the poly­(phosphoric acid) (PPA) process. These three membranes showed high levels of PA (10–32 PA/PBI repeat unit (r.u.)) and proton conductivity (0.14–0.26 S/cm at 180 °C) as compared with a conventionally imbibed meta-PBI membrane (6 PA/PBI r.u. and 0.08 S/cm at 180 °C). By controlling chemistry and increasing the polymer solid content to ∼18 wt %, m/p-PBI and meta-PBI membranes exhibited significantly improved creep resistance (<2 × 10^–6 Pa^–1) compared to para-PBI (10 × 10^–6 Pa^–1). In this work, various chemistries of PBI have been investigated to understand how the chemistry affected the electrochemical hydrogen separation (EHS) performance, including voltage requirement, power consumption, efficiency, hydrogen purities, and also long-term durability of the MEAs. Reformate streams containing H₂, N₂, and CO were used to validate the increased utility of this technique when operating at 160–200 °C due to the increased Pt tolerance to CO. The EHS device based on PBI membranes synthesized via the PPA process can be operated using dilute hydrogen feed streams with large amounts of CO (1–3%), producing fairly pure hydrogen products (>99.6% with <0.4% nitrogen crossover and ppm levels of CO) with very high power efficiencies of up to ∼72%.