Li Qingfeng

Phosphoric acid doped polybenzimidazole membranes: Physiochemical characterization and fuel cell applications

A polymer electrolyte membrane fuel cell operational at temperatures around 150–200 °C is desirable for fast electrode kinetics and high tolerance to fuel impurities. For this purpose polybenzimidazole (PBI) membranes have been prepared and H3PO4-doped in a doping range from 300 to 1600 mol %. Physiochemical properties of the membrane electrolyte have been investigated by measurements of water uptake, acid doping level, electric conductivity, mechanical strength and water drag coefficient. Electrical conductivity is found to be insensitive to humidity but dependent on the acid doping level. At 160 °C a conductivity as high as 0.13 S cm−1 is obtained for membranes of high doping levels. Mechanical strength measurements show, however, that a high acid doping level results in poor mechanical properties. At operational temperatures up to 190 °C, fuel cells based on this polymer membrane have been tested with both hydrogen and hydrogen containing carbon monoxide.

Development and Characterization of Acid-Doped Polybenzimidazole/Sulfonated Polysulfone Blend Polymer Electrolytes for Fuel Cells

Polymeric membranes from blends of sulfonated polysulfones (SPSF) and polybenzimidazole (PBI) doped with phosphoric acid were developed as potential high-temperature polymer electrolytes for fuel cells and other electrochemical applications. The water uptake and acid doping of these polymeric membranes were investigated. Ionic conductivity of the membranes was measured in relation to temperature, acid doping level, sulfonation degree of SPSF, relative humidity, and blend composition. The conductivity of SPSF Was of the order of 10 3- S cm 1 . In the case of blends of PBI and SPSF it was found to be higher than 10 -2 S cm -1 . Much improvement in the mechanical strength is observed for the blend polymer membranes, especially at higher temperatures. Preliminary work has demonstrated the feasibility of these polymeric membranes for fuel-cell applications.

Oxygen reduction on carbon supported platinum catalysts in high temperature polymer electrolytes

Abstract Oxygen reduction on carbon supported platinum catalysts has been investigated in H 3 PO 4 , H 3 PO 4 -doped Nafion and polybenzimidazole (PBI) polymer electrolytes in a temperature range up to 190°C. Compared with pure H 3 PO 4 , the combination of H 3 PO 4 and polymer electrolytes can significantly improve the oxygen reduction kinetics due to increased oxygen solubility and suppressed adsorption of phosphoric acid anions. Further enhancement of the catalytic activity can be obtained by operating the polymer electrolytes at higher temperatures. Efforts have been made to develop a polymer electrolyte membrane fuel cell based on H 3 PO 4 -doped PBI for operation at temperatures between 150 and 200°C.

A Quasi-Direct Methanol Fuel Cell System Based on Blend Polymer Membrane Electrolytes

On the basis of blend polymer electrolytes of polybenzimidazole and sulfonated polysulfone, a polymer electrolyte membrane fuel cell was developed with an operational temperature up to 200°C. Due to the high operational temperature, the fuel cell can tolerate 1.0-3.0 vol % CO in the fuel, compared to less than 100 ppm CO for the Nafion-based technology at 80°C. The high CO tolerance makes it possible to use the reformed hydrogen directly from a simple methanol reformer without further CO removal. That both the fuel cell and the methanol reformer operate at temperatures around 200°C opens the possibility for an integrated system. The resulting system is expected to exhibit high power density and simple construction as well as efficient capital and operational cost.

Hydrogen Oxidation on Gas Diffusion Electrodes for Phosphoric Acid Fuel Cells in the Presence of Carbon Monoxide and Oxygen

Hydrogen oxidation has been studied on a carbon-supported platinum gas diffusion electrode in a phosphoric acid electrolyte in the presence of carbon monoxide and oxygen in the feed gas. The poisoning effect of carbon monoxide present in the feed gas was measured in the temperature range from 80 to 150 C. It was found that throughout the temperature range, the potential loss due to the CO poisoning can be reduced to a great extent by the injection of small amounts of gaseous oxygen into the hydrogen gas containing carbon monoxide. By adding 5 volume percent (v/o) oxygen, an almost CO-free performance can be obtained for carbon monoxide concentrations up to 0.5 v/o CO at 130 C, 0.2 v/o CO at 100 C, and 0.1 v/o CO at 80 C, respectively.

Limiting Current of Oxygen Reduction on Gas‐Diffusion Electrodes for Phosphoric Acid Fuel Cells

Various models have been devoted to the operation mechanism of porous diffusion electrodes. They are, however, suffering from the lack of accuracy concerning the acid-film thickness on which they are based. In the present paper the limiting current density has been measured for oxygen reduction on polytetrafluorine-ethyl bonded gas-diffusion electrodes in phosphoric acid with and without fluorinated additives. This provides an alternative to estimate the film thickness by combining it with the acid-adsorption measurements and the porosity analysis of the catalyst layer. It was noticed that the limiting current density can be accomplished either by gas-phase diffusion or liquid-phase diffusion, and it is the latter that can be used in the film-thickness estimation. It is also important to mention that at such a limiting condition, both the thin-film model and the filmed agglomerate model reach the same expression for the limiting current density. The acid-film thickness estimated this way was found to be of 0.1 [mu]m order of magnitude for the two types of electrodes used in phosphoric acid with and without fluorinated additives at 150 C.

Complex Formation during Dissolution of Metal Oxides in Molten Alkali Carbonates

Dissolution of metal oxides in molten carbonates relates directly to the stability of materials for electrodes and construction of molten carbonate fuel cells. In the present work the solubilities of PbO, NiO, Fe{sub 2}O{sub 3}, and Bi{sub 2}O{sub 3} in molten Li/K carbonates have been measured at 650 C under carbon dioxide atmosphere. It is found that the solubilities of NiO and PbO decrease while those of Fe{sub 2}O{sub 3} and Bi{sub 2}O{sub 3} remain approximately constant as the lithium mole fraction increases from 0.43 to 0.62 in the melt. At a fixed composition of the melt, NiO and PbO display both acidic and basic dissolution as the partial pressure of carbon dioxide varies. By combination of solubility and electromotive force measurements, a model is constructed assuming the dissolution involves complex formation. The possible species for lead are proposed to be [Pb(CO{sub 3}){sub 2}]{sup {minus}2} and/or [Pb(CO{sub 3}){sub 3}]{sup {minus}4}. A similar complex chemistry for nickel oxide dissolution might be expected.

Oxygen Reduction on Gas‐Diffusion Electrodes for Phosphoric Acid Fuel Cells by a Potential Decay Method

The reduction of gaseous oxygen on carbon-supported platinum electrodes has been studied at 150 C with polarization and potential decay measurements. The electrolyte was either 100 weight percent phosphoric acid or that acid with a fluorinated additive, potassium perfluorohexanesulfonate (C{sub 6}F{sub 13}SO{sub 3}K). The pseudo-Tafel curves of the overpotential vs log (ii{sub L}/(i{sub L}{minus}i)) show a two-slope behavior, probably due to different adsorption mechanisms. The potential relaxations as functions of log (t+r) and log({minus}d{eta}/dt) have been plotted. The variations of these slopes and the dependence of the double-layer capacitance on the overpotential depended on the electrode manufacture and the kind of electrolyte (whether containing the fluorinated additive or not).

Electrochemical Deposition of Aluminum from NaCl ‐ AlCl3 Melts

Electrochemical deposition of aluminum from melts saturated with onto a glassy carbon electrode at 175°C has been studied by voltammetry, chronoamperometry, and constant current deposition. The deposition of aluminum was found to proceed via a nucleation/growth mechanism, and the nucleation process was found to be progressive. The morphology of aluminum deposits was examined with photomicroscopy. It was shown that depending on the current densities (c.d.) applied, three types of aluminum deposits could be obtained, namely, spongy deposits formed at lower c.d. (below 0.7 mA/cm2), smooth layers deposited at intermediate c.d. (between 2 and 10 mA/cm2), and dendritic or porous deposits obtained at high c.d. (above 15 mA/cm2). However, the smooth aluminum deposits were about five times more voluminous than the theoretical value. The spongy deposits were formed due to difficulties in electronucleation and could be inhibited by application of pulsed currents and/or addition of manganese chloride into the melt.

Electrochemical Deposition and Dissolution of Aluminum in NaAlCl4 Melts Influence of and Sulfide Addition

Effects of the additives MnCl 2 , sulfide, and their combined influence on aluminum deposition and dissolution in NaAlCl 4 saturated with NaCl have been studied by polarization measurements, galvanostatic deposition, and current reversal chronopotentiometry