Facilitated Direct Dimethyl Ether Fuel Cells through High Temperature Membrane Electrode Assemblies

Abstract

In searching for an exemplary carbon neutral fuel, dimethyl ether (DME) may be the most appealing. This simplest of the ethers can be readily produced from renewably sourced hydrogen and CO2, making it essentially a hydrogen carrier (1). Both nontoxic and easy to liquefy under moderate pressure, DME closely matches diesel and has been run in trucks (2). The ubiquitous propane grill tanks can store DME without modification. Recently, Los Alamos National Laboratory (LANL) demonstrated the potential for direct oxidation of DME in a fuel cell.The breakthrough of our work is LANL’s highly active catalyst for direct oxidation of DME that in the early phase of development shows similar performance to direct methanol when using typical low-temperature membranes. However, the output is not sufficient to approach commercial acceptance targets for higher power applications or precious metal cost. High-temperature membrane electrode assemblies (HT-MEAs), based on phosphoric-acid-imbibed membranes, operate at 160 °C to 180 °C without additional water and are highly tolerant to carbon monoxide – an intermediate of DME oxidation. This work exploits a novel ternary LANL anode catalyst with the features of high-temperature operation to produce high-power, low-cost direct DME MEAs.In support of direct DME oxidation, scientists at LANL have recently shown a direct liquid fuel cell operating with DME as part of a program to develop technology for direct methanol (MeOH), ethanol (EtOH), and DME fuel cells (3). In this work, it has been shown that DME can be directly oxidized via a PtRuPd ternary catalyst that not only activates the ether bonds but facilitates the removal of CO as a reaction by product (Figure 1). Even at this early stage, the team led by LANL has shown results that approach a highly developed DMFC system with regards to performance and precious metal loading (3). One highly relevant aspect of this work is the sharp temperature dependency of the current-voltage behavior, with an increase in the cell temperature from 80 °C to 90 °C leading to an increase in the current density at 0.5 V by ca. 50%, from 140 mA cm-2 to 220 mA cm- 2 High-temperature PEM MEAs developed by BASF for the polybenzylimide (PBI) system or pyridine polymer based TPS®system of Advent Technologies, Inc. have demonstrated utility and robustness when operating with highly impure reformates of around 1-3% CO (4). Similarly, cathode sensitivity to air contaminants is also ameliorated. The electrolyte in both systems is phosphoric acid imbibed into a stable and inert polymer matrix. Phosphoric acid is notable in that it does not need water to conduct protons, and can do so over a wide temperature range from ~120 °C to more than 200 °C. Advent is a licensee of BASF’s MEA and gas-diffusion-electrode technology.This presentation will summarize our on-going efforts at facilitating direct oxidation of DME by using high temperature PEM MEAs with both PBI and TPS combined with the LANL catalyst operating at 160 oC to 180 oC. Olah, G.A., Goeppert, A. and Prakash, G.K. Surya in J. Org. Chem., 2009, 74, 487-498. Hutchinson, H., 2013 in ASME web article www.asme.org/engineering-topics/articles/transportation/diesel-alternative-hits-the-road. Advanced Materials and Concepts for Portable Power Fuel Cells, 2014 Hydrogen and Fuel Cells Program AMR presentation. Technical Brochure, 2014 Advent Technologies, Inc. at www.Advent-Energy.com. Figure 1