Suenghoon Han

Highly active and stable hydrogen evolution electrocatalysts based on molybdenum compounds on carbon nanotube-graphene hybrid support.

Highly active and stable electrocatalysts for hydrogen evolution have been developed on the basis of molybdenum compounds (Mo2C, Mo2N, and MoS2) on carbon nanotube (CNT)-graphene hybrid support via a modified urea-glass route. By a simple modification of synthetic variables, the final phases are easily controlled from carbide, nitride to sulfide with homogeneous dispersion of nanocrystals on the CNT-graphene support. Among the prepared catalysts, Mo2C/CNT-graphene shows the highest activity for hydrogen evolution reaction with a small onset overpotential of 62 mV and Tafel slope of 58 mV/dec as well as an excellent stability in acid media. Such enhanced catalytic activity may originate from its low hydrogen binding energy and high conductivity. Moreover, the CNT-graphene hybrid support plays crucial roles to enhance the activity of molybdenum compounds by alleviating aggregation of the nanocrystals, providing a large area to contact with electrolyte, and facilitating the electron transfer.

A highly efficient transition metal nitride-based electrocatalyst for oxygen reduction reaction: TiN on a CNT–graphene hybrid support

Transition metal nitrides of group 4–6 (Mo2N, W2N, NbN, Ta3N5, and TiN) were synthesized by the urea-glass route and screened for oxygen reduction reaction (ORR) electrodes in PEMFCs. In terms of electrochemical stability and activity, TiN was selected as the most promising candidate as a catalyst for ORR. To further enhance the activity for ORR, TiN was modified with nanostructured carbon supports including CNTs, graphene (GR), and CNT–GR hybrid. The obtained nanocarbon-supported TiN catalysts exhibited small particle sizes of TiN (<7 nm) and a good TiN–support interaction with reduced aggregation and no free-standing TiN particles away from the supports compared to bare TiN. In particular, TiN supported on the CNT–GR hybrid (TiN/CNT–GR) showed greatly enhanced ORR activity than bare TiN and other supported TiN catalysts. It exhibited a high onset potential (0.83 V) and the highest current density among the reported nitride-based electrocatalysts. The enhancement was ascribed to a synergistic effect between TiN nanoparticles (NPs) and CNT–GR hybird support, roles of which were to provide active sites for ORR and a facile electron pathway to NPs, respectively. Besides, TiN/CNT–GR exhibited large mesopores that could allow easy access of the electrolyte due to the formation of a 3-D CNT–GR structure assembled between 2-D graphene and 1-D CNTs. Further, it showed an excellent methanol tolerance compared to the commercial Pt/C catalyst. Thus, our TiN/CNT–GR could be a promising ORR electrocatalyst for PEMFCs and DMFCs.

Enhanced activity of carbon-supported PdCo electrocatalysts toward electrooxidation of ethanol in alkaline electrolytes

Carbon-supported Pd and PdCo (1: 2, 1: 1, 2: 1 and 3: 1) catalysts were synthesized by chemical reduction with NaBH4. Their electrochemical properties were investigated by cyclic voltammetry, chronoamperometry and CO stripping voltammetry in alkaline electrolytes, and compared with commercial Pt/C and PtRu(1: 1)/C catalysts. In electrochemical oxidation of ethanol in an alkaline electrolyte, marked improvements in the current density and onset potential were observed by incorporating Co into Pd/C to form PdCo/C alloy electrocatalysts. The best catalyst PdCo (1: 1)/C showed performance superior to the commercial Pt/C or PtRu/C catalysts. It is shown that the incorporated Co facilitates the oxidation of strongly-adsorbed carbonaceous intermediate species on the surface of Pd by forming OH− group and reacts away the intermediates from Pd surface. Thus, PdCo(1: 1)/C catalyst is a promising anode catalyst for direct ethanol fuel cells with alkaline electrolytes.

Tungsten Carbide and CNT-Graphene-Supported Pd electrocatalyst toward electrooxidation of hydrogen

Development of platinum‐free catalysts is urgently needed to make fuel cells a competitive technology for power generation in various fields. Here we report that combination of palladium and tungsten carbides supported on carbon nanotubes (CNT) graphene (GR) composite exhibits high activity for electrochemical hydrogen oxidation reaction. In the cyclic voltammetry, the electrochemical active surface area of Pd/WC/CNT–GR is much larger than that of other synthesized catalysts and even commercial Pt/C catalyst. In single cell operation, Pd/WC/CNT–GR shows 65 % of the maximum power density of the state‐of‐the‐art Pt/C catalyst and loses only 5 % of maximum power density after 50 h. Beneficial roles of WC and CNT–GR are discussed for the improved performance of palladium catalysts.

A precious metal-free solar water splitting cell with a bifunctional cobalt phosphide electrocatalyst and doubly promoted bismuth vanadate photoanode

A bifunctional cobalt phosphide (CoP) electrocatalyst is applied to a doubly promoted BiVO4 photoanode as an oxygen evolution as well as to a cathode as a hydrogen evolution reaction (HER) catalyst to establish a photoelectrochemical (PEC) water splitting cell made of only earth abundant elements without any precious metals. Although the intrinsic HER activity of CoP is lower than Pt/C, CoP can replace Pt as the cathode of PEC water splitting cells without any significant loss in performance. During the water splitting reaction, the cathode remains as CoP, but CoP loaded on the BiVO4 photoanode turns into amorphous CoOx–HPOy with its chemical state and performance very similar to those of well-known cobalt phosphate electrocatalysts. A tandem cell composed of CoP/hydrogen-treated, 1% Mo-doped BiVO4 as the photoanode, a CoP/Ni foam as the cathode and a 2-piece, series connected crystalline Si solar cell as the bias power generator demonstrates stable unassisted water splitting with a solar-to-hydrogen conversion efficiency of 5.3% under simulated solar light, which represents the highest among reported precious metal-free unassisted solar water splitting devices.