Two methoxy fuels, dimethoxymethane ͑DMM͒ and trimethoxymethane ͑TMM͒, were characterized in terms of stability in aqueous solutions and electro-oxidation. TMM was unstable in water even at room temperature, whereas DMM was stable except in an acidic electrolyte or at elevated temperatures. The temperature dependence of the electrochemical oxidation of DMM at Pt and PtRu electrodes was investigated by hydrodynamic voltammetry using a thin-layer flow cell. DMM itself was electrochemically inactive, but a noticeable oxidation current was observed after hydrolysis into methanol and formaldehyde. The oxidation current and the onset potential for the oxidation were comparable to those of methanol in the temperature region from 17 to 120°C.Polymer electrolyte fuel cells have attracted great interest as a primary power source for vehicles and portable devices. Direct oxidation-type fuel cells ͑DOFCs͒ have significant advantages over reforming-type fuel cells in weight, volume, and simplicity of the system or its maintenance. The direct methanol fuel cell ͑DMFC͒ is a typical example of a DOFC. However, methanol largely crosses over a proton-exchange membrane such as Nafion, resulting in lowered performance and fuel utilization efficiency. Alternative organic fuels for the DOFC have been investigated to improve cell performance as well as fuel efficiency, because the larger the molecular size the less crossover can be expected, e.g., ethanol, 1 1-propanol, 1 2-propanol, 1 dimethoxymethane ͑DMM͒, 2,3 trimethoxymethane ͑TMM͒, 2-4 trioxane, 2 dimethyl ether, 3 oxalic acid, 5 ethylene glycol, 5 dimethyl oxalate, 5 and 1-methoxy-2-propanol. 6 Unfortunately, most of these fuels are electrochemically less reactive than methanol. However, there have been reports that the performance of DOFCs using DMM 2,3 and TMM 2-4 is better than or comparable to that of the DMFC and these fuels have higher energy densities than methanol, i.e., 4.89 Ah/mL ͑5.62 Ah/g͒ for DMM, 4.90 Ah/mL ͑5.05 Ah/g͒ for TMM, and 3.97 Ah/mL ͑5.02 Ah/g͒ for methanol. The acidcatalyzed hydrolysis of these fuels during the cell operation was pointed out in the literature 2-4However, a systematic study has not been reported regarding the stability of these fuels in various DOFC operation conditions. Inconsistency about the reactivity of DMM or TMM in previous works may be related to unstable chemical properties of these fuels. Therefore, it is important to elucidate their chemical and electrochemical properties as a fuel for the DOFC.Here we report the details of the chemical stability of DMM and TMM in pure water and acidic solution with/without Pt catalyst and also the ''real'' electrochemical reactivity of DMM at Pt and PtRu electrodes from 17 to 120°C.
ExperimentalAnalysis of solutions.-Concentrations of DMM and its hydrolyzed species in pure water and acidic solution were monitored by high performance liquid chromatography ͑HPLC͒ using a refractive index detector ͑RI-2031, JASCO Co. Ltd., Tokyo, Japan͒ and an ion-exchange column ͑Ionpak KC-811, Shodex, Showa Den...
