Catalysts Based on Trimetallic Formulations for the Electro-Oxidation of Methanol

Abstract

Direct methanol fuel cells (DMFCs) represent a promising alternative for the energy supply in portable devices and auxiliary power units, but their commercialization is yet hindered by cost and durability issues. The use of platinum at the electrodes represents one of the main contributions to the high cost of these devices [1]. Nonetheless, there is still a room for further improvement of the electrode structure in terms of efficient noble metal utilization. At the anode the main concern is to enhance the sluggish kinetics of methanol oxidation. PtRu catalysts have shown outstanding efficiency in the electro-oxidation of methanol, i.e. when used at the anode of DMFCs [2]. Yet, high price of these catalysts remain a drawback towards the deployment of DMFC systems. In this regard, there are several routes pursued like the use of non-precious alloys, core-shell catalysts, new supports, etc. These methods, although they successfully minimize the amount of metals used, fail maximizing the catalytic activity. In the present work, composite anodes based on trimetallic formulations (Pt, Ru and a metal oxide) were developed to increase the performance of DMFCs.Fine metal oxide (MO2, M = Ir, Ti, etc.) nanoparticles were synthesized by a sulfite complex method. Some metal oxides are known for their good activity towards the water displacement reaction, producing surface OH species which can accelerate the oxidation of methanol intermediates. These metal oxides were subsequently mixed with a PtRu catalyst supported on carbon, prepared by the same procedure. A catalytic ink, composed of PtRu/C catalyst, MO2and Nafion ionomer, is then deposited on a carbon-cloth-based backing layer and used as composite anode in a DMFC. A commercial Pt catalyst was used at the cathode and a Nafion 117 polymeric membrane was used as electrolyte. Polarization curves at different temperatures (30-90ºC) and methanol concentrations (2-5 M) were carried out to investigate these composite anodes.A significantly higher performance was recorded for the composite electrode-based MEAs compared to the bare one based only on PtRu/C. The results confirm that the electrocatalytic activity is related to the characteristics of water displacement prompt by the additive, which acts as a co-catalyst for this reaction. The improvement was significantly higher by using high methanol concentration in water as the fuel, which means promising utilization in DMFC systems for prolonged operation.These results evidence that a multifunctional trimetallic catalyst can operate significantly better than PtRu for methanol oxidation since this multi-step process requires different functionalities to speed up the reaction rate. The improved electrocatalytic activity was thus attributed to the characteristics of water displacement of MO2, as occurs in the oxygen evolution process. Such a parallelism may provide new routes to design multifunctional catalysts where Pt-alloys can be combined with oxide promoters to accelerate the multiple steps of the methanol oxidation process. Acknowledgements The authors acknowledge the financial support from the European Community’s Seventh Framework Programme (FP7/2011-2014) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement DURAMET no. 278054.