High-Performance Direct Dimethyl Ether Fuel Cell Operating with an Advanced Activation Process

The direct methanol fuel cell (DMFC) has been of interest due to the absence of a difficult to split carbon-carbon bond in methanol, high energy content of the liquid fuel and facile handling. Other established high energy-density liquid fuels, such as ethanol, are not yet amenable to direct electrochemical oxidation due to the lack of electrocatalysts for an efficient carbon-carbon bond scission.

A related fuel to methanol, with strong direct feed fuel cell potential is dimethyl ether (DME). DME possesses many of the same physical advantages that methanol does.^1-3 It is similar to methanol in terms of theoretical open cell voltage (1.18 vs. 1.21 V at 25°C) and does not require a carbon-carbon bond scission. DME is also a clean and, unlike methanol, nontoxic fuel that undergoes facile biodegradation and can be easily synthesized from recycled carbon dioxide and renewable energy.⁴ Due to its lower dipole moment, DME is less subject to membrane crossover than methanol; it also has a higher energy content than methanol (8.2 vs. 6.1 kWh/kg).^5,6Complete electro-oxidation of DME consumes 3 equivalents of water and yields 12 equivalents of electrons:

CH₃ OCH₃+3H₂O→2CO₂+12H⁺+12e^-      (1)

While the direct DME fuel cell (DDMEFC) performance has by now been demonstrated to match that of the state-of-the-art DMFC,⁷ DDMEFC still requires development to become fully viable. In addition to the needed improvements in the anode catalyst activity, the catalyst deactivation and its durability continue to pose a challenge. Several aspects of optimization have been explored of which break-in has been one area of attention.⁸

In this work, membrane electrode assembly (MEA) preparation and initial break-in were carried out according to standard procedure.¹ Prior to operation in the DDMEFC mode, the fuel cell was operated on humidified hydrogen/air to condition the membrane. When the current at 0.7 V plateaued the break-in was stopped, fuel cell polarization recorded on H₂/air, and the anode feed switched to preheated, humidified DME under a pressure of approximately 1.75 bar gauge (2.56 bar absolute).¹  Performance at this point reproduced previously observed performance on DME under similar conditions. Several DDMEFC polarization plots were recorded, leading a gradual performance loss. In an attempt to recover lost performance, the cell was switched back to break-in conditions on H₂/air and after break in, DDMEFC polarization plots were recorded again. With each additional iteration of this procedure, the fuel cell performance increased, up to ~0.3 A cm^-2 at 0.5 V after a thirteenth iteration (Figure 1). As a result of this conditioning, the DDMEFC power density achieved reached 0.18 W cm^-2 – a 50% increase over the highest power density value reported in the literature with a commercial PtRu catalyst (0.12 W cm^-2).⁷

In this presentation, we will provide details of the procedure used and discuss the causes of such a significant performance improvement. We will also comment on the DDMEFC performance durability in the context of the conditioning approach used in this study.

Acknowledgment

Financial support from the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy through Fuel Cell Technologies Office is gratefully acknowledged.

REFERENCES

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2              Jensen, J. O., Vassiliev, A., Olsen, M., Li, Q., Pan, C., Cleemann, L. N., Steenberg, T., Hjuler, H. A., Bjerrum, N. "Direct dimethyl ether fueling of a high temperature polymer fuel cell." Journal of Power Sources 211, 173-176 (2012).

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7              Chung, H. T., Dumont, J. H., Martinez, U., Zelenay, P. Catalyst Development for Dimethyl Ether Electrooxidation. in Meeting abstracts.   929-929p (The Electrochemical Society).

8              Qi, Z., Kaufman, A. "Quick and effective activation of proton-exchange membrane fuel cells." Journal of Power Sources 114, 21-31 (2003).