Measurement and Dynamic Modeling of Hydrogen Separation from Hydrogen Enriched Natural Gas Using Phosphoric Acid Doped Polybenzimidazole Hydrogen Pump

Abstract

The blending of hydrogen into pre-existing natural gas pipelines as a means of transporting hydrogen to consumers is an attractive strategy with regards to cost and efficiency [1]. In many jurisdictions hydrogen blending has been determined to be technically feasible today when considering concentrations of hydrogen up to 15-20% by volume [2]. Fuel cell electric vehicle fueling, among other desirable hydrogen end-uses, requires high-purity hydrogen feedstock and thus the capability to efficiently extract high purity hydrogen from hydrogen blended natural gas is identified as a critical component in adopting hydrogen natural gas blends as an energy transition strategy [3].Proton-conducting membranes can be utilized to separate hydrogen electrolytically from blended gas streams efficiently while producing a high purity product. Perfluorosulfonic acid (PFSA) membranes such as Nafion are the most employed due to their high proton conductivity as well as their high mechanical strength but are complicated by the need for membrane humidification. High-temperature proton exchange membranes (HT-PEM) offer attractive characteristics relative to the PFSA-based cells, particularly in their application to hydrogen separation. Phosphoric-acid (PA) doped polybenzimidazole (PBI) membranes have high proton conductivity at temperatures of 160oC without the presence of water and due to these higher operating temperatures have improved reaction kinetics and catalyst tolerance to impurities such as CO and sulfur. Their ability to separate hydrogen from nitrogen, CO2, and low concentrations of CO (~1% vol) was shown by Perry et al. [4]. Recent developments in PA-doped polymeric materials synthesis has led to improved durability and performance [5] and even higher operating temperatures offering further performance benefits [6].This study examines the use of a single cell HT-PEM H2 pump based on PA-doped PBI membrane for the separation of hydrogen from hydrogen blended methane and natural gas. The experiments are performed on 5 cm2 cells with in-house gas diffusion electrodes. Freudenberg H23C2 is used as a gas diffusion layer. The catalyst layer consists of PTFE binder and 40% wt Pt/C catalyst with loading of 1 mg Pt/cm2 . The PBI membranes (Fumapem AP-30) are soaked in 85%wt H3PO4 at room temperature for 2 days. Hydrogen concentration in the feed gas (2-20% molar concentration), hydrogen recovery rate, and other operating conditions are varied to characterize cell performance in terms of hydrogen separation efficiency. We show a feasibility to separate hydrogen from various blends from 20% down to 2 % molar concentration (Figure 1). For the most constrained case of 2 % H2 in methane an overpotential of 240 mV at 0.2 A/cm2 was observed. Gas chromatography is used to provide outlet gas composition. Electrochemical measurements from cell operation are used to modify and validate existing 2-D non-isothermal dynamic PEM cell model [7].