Pilwon Heo

A Proton-Conducting In3 + -Doped SnP2O7 Electrolyte for Intermediate-Temperature Fuel Cells

We report proton conduction in In 3+ -doped SnP 2 O 7 in the temperature range from 100 to 300°C, and the performance of a H 2 -air fuel cell using this material as the electrolyte. The proton conductivity of In 3+ -doped SnP 2 O 7 was more than 10 -1 S cm -1 between 125 and 300°C, and a conductivity value of 1.95 × 10 -1 S cm -1 was achieved at 250°C. The resulting fuel cell exhibited a reasonable power density of 264 mW cm -2 at 250°C (electrolyte thickness = 0.35 mm), together with perfect tolerance toward 10% CO and good thermal stability in unhumidified conditions.

Proton Conduction in In3 + -Doped SnP2O7 at Intermediate Temperatures

SnP 2 O 7 -based proton conductors were characterized by Fourier transform infrared spectroscopy (FTIR), temperature-programmed desorption (TPD), X-ray diffraction (XRD), and electrochemical techniques. Undoped SnP 2 O 7 showed overall conductivities greater than 10 -2 S cm -1 in the temperature range of 75-300°C. The proton transport numbers of this material at 250°C under various conditions were estimated, based on the ratio of the electromotive force of the galvanic cells to the theoretical values, to be 0.97-0.99 in humidified H 2 and 0.89-0.92 under fuel cell conditions. Partial substitution of In 3+ for Sn 4+ led to an increase in the proton conductivity (from 5.56 X 10 -2 to 1.95 X 10 -1 S cm -1 at 250°C, for example). FTIR and TPD measurements revealed that the effects of doping on the proton conductivity could be attributed to an increase in the proton concentration in the bulk Sn 1-x In x P 2 O 7 . The deficiency of P 2 O 2 ions in the Sn 1-x In x P 2 O 7 bulk decreased the proton conductivity by several orders of magnitude, which was explained as due to a decrease in the proton mobility rather than the proton concentration. The mechanism of proton incorporation and conduction is examined and discussed in detail.

Electrodeposition and Thermoelectric Characterization of Bi2Te3

Bismuth and tellurium were electrodeposited from acidic aqueous solutions of Bi 2 (SO 4 ) 3 and K 2 TeO 3 at constant potentials in order to produce a Bi 2 Te 3 film for miniaturized thermoelectric devices. Electrodeposition behavior was examined using cathodic polarization and electrochemical quartz crystal microbalance techniques. The solution composition and the cathodic potential affected the chemical composition, preferred crystal orientation, and thermoelectric characteristics of the deposits. Films electrodeposited at potentials more positive than -100 mV vs Ag/AgCl had a dense, smooth surface and high crystallinity. The film composition depended more strongly on the solution composition than on the applied potential. A stoichiometric Bi 2 Te 3 film could be obtained from a solution containing 2 mM Bi(III) and 2.6 mM Te(IV). The Seebeck coefficient increased with Te content of the film and was inversely proportional to the electrical conductivity. The highest power factor (7.4 X 10 -4 Wm -1 K -2 ) was obtained in the case of the Bi 2 Te 3 film electrodeposited at 0 V, 293 K, pH 0.5, in a sulfuric acid solution containing 2 mM Bi(III) and 2 mM Te(IV); the film contained 57 mol % Te, corresponding to Bi-rich and a carrier concentration of 6.6 × 10 20 cm -3 .

Performance of an Intermediate-Temperature Fuel Cell Using a Proton-Conducting Sn0.9In0.1P2O7 Electrolyte

Performance of a fuel cell using Sn 0.9 In 0.1 P 2 O 7 as the electrolyte was evaluated in the temperature range of 150-300°C under unhumidified conditions. The IR drop and electrode overpotential of the cell were measured separately by the current interruption method. The dc conductivity values of the electrolyte between 150 and 300°C, estimated from the IR drop, were comparable to the ac conductivity values (1.48 × 10 -1 -1.95 X 10 -1 S cm -1 ) of the electrolyte. The cell performance was improved by forming an intermediate layer consisting of Sn 0.9 In 0.1 P 2 O 7 and Pt/C catalyst powders at the interface between the electrolyte and cathode, which significantly reduced the cathode polarization. As a result, the peak power density reached 264 mW cm -2 at 250°C using the 0.35-mm-thick electrolyte. The present fuel cell also showed high stability at low relative humidities (p H2O as 0.0075 atm) and 10% CO concentration.

Proton Conductivity and Solid Acidity of Mg-, In-, and Al-Doped SnP2O7

Mg-, In-, and Al-doped SnP 2 O 7 were synthesized, and their potential as electrolytes for fuel cells was evaluated in terms of their performance and durability. The overall conductivity of Mg-doped SnP 2 O 7 increased with increasing Mg 2+ content and reached a maximum at 10 mol %. However, this was smaller than that obtained for doping with 10 mol % In 3+ and 5 mol % Al 3+ , especially at temperatures below 300°C. This result was interpreted to be a consequence of lower proton mobility rather than of smaller proton concentration in Mg-doped SnP 2 O 7 , probably due to the electrostatic restriction by Mg 2+ against proton transport. Moreover, to investigate the influence of the Mg 2+ doping on the fuel-cell properties, fuel-cell tests were conducted using Sn 0.9 Mg 0.1 P 2 O 7 , Sn 0.9 In 0.1 P 2 O 7 , and Sn 0.95 Al 0.05 P 2 O 7 as electrolytes at a temperature of 150°C. Compared with Sn 0.9 In 0.1 P 2 O 7 and Sn 0.95 Al 0.05 P 2 O 7 cells, the Sn 0.9 Mg 0.1 P 2 O 7 cell showed a somewhat higher Ohmic resistance but exhibited a comparable polarization resistance, which is the main component of the total electrical resistance. More importantly, the Mg 2+ doping led to an improved stability of the Pt catalyst and carbon support at high potentials. This beneficial effect was attributed to a large reduction in the acidity of SnP 2 O 7 resulting from the basicity of MgO.

Sn0.9In0.1P2O7-Based Organic/Inorganic Composite Membranes Application to Intermediate-Temperature Fuel Cells

2by reducing the electrolyte thickness to 60 m. The peak power densities achieved with unhumidified H2 and air were 109 mW cm �2

Proton Conduction in Sn0.95Al0.05P2O7 – PBI – PTFE Composite Membrane

Proton-conducting composite membranes were fabricated by blending S n0.95 Al 0.05 P 2 O 7 having an excess of phosphates with polybenzimidazole (PBI) and polytetrafluoroethylene (PTFE). The addition of PBI to Sn 0.95 Al 0.05 P 2 O 7 -P x O 7 powder stabilized the conductivity of the composite, providing higher conductivities than those of stoichiometric Sn 0.05 Al 0.05 P 2 O 7 The addition of PTFE to Sn 0.95 Al 0.05 P 2 O 7 -P x O y -PBI powder reduced the conductivity but increased the tensile strength. The resulting composite membrane exhibited a conductivity of 0.04 S cm -1 at 200°C and a tensile strength of 2.30 MPa. Moreover, a fuel cell made with this composite membrane yielded high power densities exceeding 200 mW cm -2 above 100°C and good durability under unhumidified conditions.

A High-Performance Mo2C-ZrO2 Anode Catalyst for Intermediate-Temperature Fuel Cells

Anode performance of non-Pt catalysts for hydrogen oxidation was investigated in intermediate-temperature proton exchange membrane fuel cells. Molybdenum carbide (Mo 2 C) showed the highest catalytic activity among the transition metal carbides tested. Furthermore, the catalytic activity of Mo 2 C was significantly improved by the addition of ZrO 2 to the anode. Transmission electron microscopy and X-ray diffraction measurements revealed that Mo 2 C was more highly dispersed in the MO 2 C-ZrO 2 /C than in the Mo 2 C/C, suggesting that the particle growth of Mo 2 C was suppressed by the addition of ZrC 2 . We also tested the performance of a fuel cell using Mo 2 C-ZrC 2 /C and Sn 0.9 In 0.1 P 2 O 7 as the anode and electrolyte materials, respectively, between 150 and 300°C. At 250°C or higher, the Mo 2 C-ZrO 2 /C anode showed a cell performance comparable to that of the Pt/Canode. However, cell performance was strongly dependent on the operating temperature, reflecting that the catalytic activity of Mo 2 C-ZrO 2 was greatly lowered by the decrease in operating temperature. Thus it was concluded that the MO 2 C-ZrO 2 catalyst is a promising alternative anode material to Pt, especially at intermediate temperatures.

Direct Dimethyl Ether Fuel Cells at Intermediate Temperatures

Direct dimethyl ether fuel cells (DDMEFCs) were investigated using proton-conducting Sn 0.9 In 0.1 P 2 O 7 as an electrolyte at intermediate temperatures between 150 and 300°C. Fuel cell operation at intermediate temperatures allowed a Pt/C anode to achieve excellent CO tolerance and high catalytic activity for the anode reaction of DME. The catalytic activity of the Pt/C anode was further improved by the addition of Ru to Pt, especially at elevated temperatures. The anodic overpotential and anode product measurements revealed that the H 2 produced by the reforming reaction of DME was electrochemically oxidized at the PtRu/C anode, which proceeded in parallel with the direct oxidation reaction of DME. As a result, DDMEFCs with the PtRu/C anode achieved a peak power density of 31, 52, and 78 mW cm -2 at 200, 250, and 300°C, respectively.

Nanostructured Pt – Sn1 − x In x P2O7 Cathodes for High-Temperature Proton Exchange Membrane Fuel Cells

Nanosized proton-conducting Sn 0.95 In 0.05 P 2 O 7 ionomers were grown on carbon supports by the coprecipitation of tin and indium chlorides with excess ammonia water and subsequent solid-state reaction with phosphoric acid. The resulting homogeneous networks of Sn 0.95 In 0.05 P 2 O 7 ionomers enhanced the activity of a Pt catalyst for the oxygen reduction reaction in the temperature range of 150-250°C. In addition, the Pt catalyst showed high corrosion resistance in the cathode environment while maintaining high performance levels. A comparison of the cathode performance between Sn 0.95 In 0.05 P 2 O 7 and H 3 PO 4 ionomers further demonstrated that the Sn 0.95 In 0.05 P 2 O 7 nanoparticles are promising candidates for cathode ionomer materials at high temperatures.