Catalyst Development for Dimethyl Ether Electrooxidation

Dimethyl ether (DME) has several advantages over the alternative fuels for direct-feed fuel cells. Unlike ethanol but similarly to methanol DME does not require C-C bond scission to be oxidized to CO­₂. DME is less toxic than methanol though and can be conveniently stored and transported using existing infrastructure and storage technologies. Furthermore, DME has a higher energy density than that of methanol: 8.2 vs. 6.1 kWh kg^-1.^{1, 2} However, the performance of the direct DME fuel cell (DDMEFC) is generally much lower than that of the direct methanol fuel cells (DMFC).³Given relatively low crossover of DME relative to methanol, the reason of the low DDMEFC performance is quite clearly the sluggish electrocatalysis of DME oxidation at the anode.

Recently, our group has developed a new ternary PtRuPd/C catalyst that not only outperforms ME oxidation catalysts reported to date^4,5 but also allows DDMEFC to reach performance similar to that of the DMFC, with the measured current density reaching ca. 75% of that measured in the DMFC at 0.5 V.² Based on that result, we assumed that further improvement in the DDMEFC anode performance could potentially be accomplished by developing catalysts designed specifically for DME, i.e. by accounting in particular for the differences in DME oxidation relative to methanol. The mechanism of DME oxidation has been proposed as follows:^{6, 7}

CH₃OCH₃ + Pt → Pt(C-O-CH₃)_ads + 3H⁺ + 3e^-             (1)

Pt(C-O-CH₃)_ads + H₂O → Pt(HCO)_ads + CH₃OH           (2)

Pt(HCO)_ads → Pt(CO)_ads + H⁺ + e^-                              (3)

Pt + H₂O → Pt-OH + H⁺ + e^-                                      (4)

Pt(CO)_ads + Pt-OH → 2Pt + CO₂ + H⁺ + e^-                  (5)

According to the above mechanism, the following catalyst requirements need to be met for efficient oxidation of DME: (i) optimum adsorption of DME on the catalyst (eq. 1); (ii) facile O-C bond cleavage (eq. 2); and (iii) fast and complete oxidation of the adsorbed CO intermediate (eq. 5). In the ternary PtRuPd/C catalyst, Pt atoms likely act as adsorption and dehydrogenation sites; as in the case of methanol oxidation, Ru aids in the oxidation of adsorbed CO by providing surface oxidant, possibly hydroxide species. The rationale for introducing Pd has been to enhance the cleavage of the O-C bond (eq. 2). Preliminary electrochemical and fuel cell data suggests that Pd does indeed activate the O-C bond cleavage.²The optimization of the Pt:Ru:Pd ratio has been ongoing at LANL. In an effort to develop even more efficient catalyst for DME oxidation, part of the research effort has also concentrated on Pt alloys with other precious metals, including Rh and Ir.

In this talk, we will present our efforts to facilitate the (i) adsorption of DME, (ii) the cleavage of the O-C bond, and (iii) oxidation and removal of CO intermediates through the optimization of the ternary PtRuPd catalyst and introduction of other precious metals into both binary and ternary alloys with Pt.

 

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

1. Q. Li, G. Wu, C. M. Johnston, P. Zelenay, ECS Trans.  41, 1969 (2011).
2. Q. Li, G. Wu, Z. Bi, C. M. Johnston, P. Zelenay, ECS Trans. 50, 1933 (2012).
3. A. Serov, C. Kwak, Appl. Catal. B-Environ., 91, 1 (2009).
4. J.-Y. Im, B.-S. Kim, H.-G. Choi, and S. Cho, J. Power Sources, 179, 301 (2008).
5. J.O. Jensen, A. Vassiliev, M. I. Olsen, Q. Li, C. Pan, L. N. Cleemann, T. Steenberg, H. A. Hjuler, N. J. Bjerrum, J. Power. Sources, 211, 173 (2012).
6. J. T. Muller, P. M. Urban, W. F. Holderich, K. M. Colbow, J. Zhang, D. P. Wilkinson, J. Elctrochem. Soc., 147, 4058 (2000).
7. E. Y. Votchenko, M. S. Kubanova, N. V. Smirnova, O. A. Petrii, Russ. J. Electrochem., 46, 212 (2010).