Electrocatalysts for Ethanol Oxidation in Direct Ethanol Fuel Cell (DEFC)
Abstract
<p>Ethanol, with its high energy density, likely production from renewable sources, ease of storage and distribution, is almost the ideal combustible for fuel cells wherein its chemical energy can be converted directly into electrical energy. However, commercialization of direct ethanol fuel cells (DEFC) has been impeded by ethanol's slow, inefficient oxidation even at the best electrocatalysts.</p><p> We synthesized a ternary Pt-Rh-SnO<sub>2</sub> electrocatalyst that is capable of splitting C-C bond and oxidizing ethanol to CO<sub>2</sub> with high efficiency. A model catalyst, RhSnO<sub>2</sub>/Pt(111), was first prepared by depositing Rh and SnO<sub>2</sub> nanoclusters on Pt(111) single crystal surface; and then carbon-supported PtRhSnO<sub>2</sub> nanoparticle catalysts were synthesized by a seeded growth approach. Both showed unprecedented activity for ethanol oxidation reaction (EOR) with the onset of reaction occurring at low overpotentials. In situ infrared reflection-absorption spectra (IRRAS) obtained during EOR with both RhSnO<sub>2</sub>/Pt(111) and PtRhSnO<sub>2</sub>/C indicate CO<sub>2</sub> is the major product and it also demonstrate that we successfully split C-C bond at room temperature. A density functional theory (DFT) investigation of ethanol decomposition was carried out over a model RhPt/SnO<sub>2</sub>(110) catalyst, and results suggest the optimal pathway leading to C-C bond breaking is: CH<sub>3</sub>CH<sub>2</sub>OH → *CH<sub>3</sub>CH<sub>2</sub>O+H* → *CH<sub>2</sub>CH<sub>2</sub>O+2H* → *CH<sub>2</sub>+*CH<sub>2</sub>O+2H*. In situ X-ray absorption spectroscopy (XAS) study was conducted and the results indicate that the PtRh surface is only slightly oxidized. EXAFS fitting results revealed structure information like the particle size and bond distance. These results were corroborated by those obtained using XRD, HADDF-STEM, EDS, and ICP-OES. PtRhSnO<sub>2</sub>/C electrocatalysts with a moderate Rh content, i.e. Pt/Rh = 2/1 and 3/1, showed highest EOR activity and selectivity towards C-C bond splitting. </p><p> Pt-Ir-SnO<sub>2</sub>/C electrocatalyst with atomic ratio Pt:Ir:Sn of 1:1:1 demonstrated a moderately enhanced capability in C-C bond cleavage. Ir-based electrocatalysts (Ir, Ir-Sn, Ir-Ru) were prepared using a simple thermal decomposition method and Ir-Sn/C exhibited high EOR activity at low overpotentials. Pt monolayer deposited on Au(111) substrate and carbon-supported Au@Pt core-shell nanoparticle electrocatalyst both demonstrated enhanced activity in the electro-oxidation of methanol and ethanol.</p><p> In summary, our findings potentially resolve the major impediment hindering the development of practical DEFCs and open new possibilities for studies of C-C bond splitting in variety of important reactions.</p>