Enhancing the Methanol Oxidation Reaction of Precious Metal Catalysts on Carbon Nanotube Support with a Thin Hydrogenated TiO₂ Shell

Abstract

Direct Methanol Fuel Cells (DMFCs) has drawn widespread research interests due to the low cost, higher energy density (6.08 kWh/kg) and ease in handling of methanol. In DMFCs, methanol is oxidized to CO2 at the anode and oxygen is reduced to water at the cathode to produce electricity. Platinum (Pt) is the state-of-the-art electro catalyst for methanol oxidation reaction (MOR). However, the high cost of Pt, sluggish MOR kinetics and CO poisoning on Pt surface impedes the commercialization of DMFCs. In addition, the methanol crossover from the anode side through polymer electrolyte membrane to the cathode side prevents the high concentration or pure methanol from being used. This causes the deliverable power density substantially lower than the theoretical value. Therefore, proper design of MEA and development of more efficient anode catalysts are necessary to achieve higher power density in DMFCs using high-concentration methanol fuels. General strategies adopted to overcome the issues of CO poisoning is to alloy oxophilic metals such as Ru, Sn and Rh with Pt or incorporate metal oxides such as TiO2, CeO2, MnO2, etc. as catalyst supports. These components provide abundant -OH species to efficiently oxidize CO to CO2 and improve the MOR kinetics. However, these strategies have been explored and are proven to work well in half-cell studies and their application in a full DMFC cell is lacking.Here in, we demonstrate a promising MOR catalyst based on a novel anode architecture with dual roles as the catalyst support and the microporous layer (MPL) in enhancing the MOR kinetics. A conformal thin layer of TiO2 shell has been successfully deposited on the oxygen functionalized nitrogen-doped carbon nanotube (O-NCNT) support to facilitate the formation of stable PtRu nanoparticles. Our synthesis approach involves the utilization of microwave heating and post-synthesis thermal annealing in H2 environment to form alloyed PtRu nanoparticles, denoted as PtRu/TiO2/ONCNTs. The synthesized catalyst shows improved MOR kinetics, enhanced CO oxidation and excellent stability compared to commercial PtRu/C anode catalyst in half-cell studies. The thin layer of hydrogenated TiO2 provide improved stability due to strong metal-support interaction, which reduces the nanoparticle agglomeration during high temperature annealing and further increases the oxophilicity of the catalyst, leading to enhanced CO oxidation. Alongside, this catalyst is used as an anode in a DMFC prototype single cell (5 cm2 active area MEA) to study the parameters that influence the peak power density of the catalyst. Our preliminary full cell results show that the catalysts synthesized using the NCNT support has very different mass transport comparing to the commercial PtRu/C anode. This new anode architecture requires different optimization in DMFCs before realizing the highly catalytic activities at high-concentration methanol to achieve high power densities.