Niels J. Bjerrum

Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic Layers as Oxygen Reduction Catalysts

Nonprecious metal catalysts for the oxygen reduction reaction are the ultimate materials and the foremost subject for low-temperature fuel cells. A novel type of catalysts prepared by high-pressure pyrolysis is reported. The catalyst is featured by hollow spherical morphologies consisting of uniform iron carbide (Fe3 C) nanoparticles encased by graphitic layers, with little surface nitrogen or metallic functionalities. In acidic media the outer graphitic layers stabilize the carbide nanoparticles without depriving them of their catalytic activity towards the oxygen reduction reaction (ORR). As a result the catalyst is highly active and stable in both acid and alkaline electrolytes. The synthetic approach, the carbide-based catalyst, the structure of the catalysts, and the proposed mechanism open new avenues for the development of ORR catalysts.

The CO Poisoning Effect in PEMFCs Operational at Temperatures up to 200°C

The CO poisoning effect on carbon-supported platinum catalysts (at a loading of 0.5 mg Pt/cm 2 per electrode) in polymer electrolyte membrane fuel cells (PEMFCs) has been investigated in a temperature range from 125 to 200°C with the phosphoric acid-doped polybenzimidazole membranes as electrolyte. The effect is very temperature-dependent and can be sufficiently suppressed at elevated temperature. By defining the CO tolerance as a voltage loss less than 10 mV, it is evaluated that 3% CO in hydrogen can be tolerated at current densities up to 0.8 A/cm 2 at 200°C, while at 125°C 0.1% CO in hydrogen can be tolerated at current densities lower than 0.3 A/cm 2 . For comparison, the tolerance is only 0.0025% CO (25 ppm) at 80°C at current densities up to 0.2 A/cm 2 . The relative anode activity for hydrogen oxidation was calculated as a function of the CO concentration and temperature. The effect of CO 2 in hydrogen was also studied. At 175°C, 25% CO 2 in the fuel stream showed only the dilution effect.

Aluminum as anode for energy storage and conversion: a review

Aluminum has long attracted attention as a potential battery anode because of its high theoretical voltage and specific energy. The protective oxide layer on the aluminum surface is however detrimental to the battery performance, contributing to failure to achieve the reversible potential and causing the delayed activation of the anode. By developing aluminum alloys as anodes and solution additives to electrolytes, a variety of aluminum batteries have been extensively investigated for various applications. From molten salt and other non-aqueous electrolytes, aluminum can be electrodeposited and therefore be suitable for developing rechargable batteries. Considerable efforts have been made to develop secondary aluminum batteries of high power density. In the present paper, these research activities are reviewed, including aqueous electrolyte primary batteries, aluminum-air batteries and molten salt secondary batteries.

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.

Fe3C-based oxygen reduction catalysts: synthesis, hollow spherical structures and applications in fuel cells

We present a detailed study of a novel Fe3C-based spherical catalyst with respect to synthetic parameters, nanostructure formation, ORR active sites and fuel cell demonstration. The catalyst is synthesized by high-temperature autoclave pyrolysis using decomposing precursors. Below 500 °C, melamine-rich microspheres are first developed with uniformly dispersed amorphous Fe species. During the following pyrolysis at temperatures from 600 to 660 °C, a small amount of Fe3C phase with possible Fe–Nx/C active sites are formed, however, with moderate catalytic activity, likely limited by the low conductivity of the catalyst. At high pyrolytic temperatures of 700–800 °C, simultaneous formation of Fe3C nanoparticles and encasing graphitic layers occur within the morphological confinement of the microspheres. With negligible surface nitrogen or iron functionality, the thus-obtained catalysts exhibit superior ORR activity and stability. A new ORR active phase of Fe3C nanoparticles encapsulated by thin graphitic layers is proposed. The activity and durability of the catalysts are demonstrated in both Nafion-based low temperature and acid doped polybenzimidazole-based high temperature proton exchange membrane fuel cells.

Molten triazolium chloride systems as new aluminum battery electrolytes

The possibility of using molten mixtures of 1,4-dimethyl-1,2,4-triazolium chloride (DMTC) and aluminum chloride (AlCl[sub 3]) as secondary battery electrolytes was studied, in some cases extended by the copresence of sodium chloride. DMTC-AlCl[sub 3] mixtures demonstrated high specific conductivity in a wide temperature range. The equimolar system is most conductive and has [kappa] values between 4.02 [times] 10[sup [minus]5] and 7.78 [times] 10[sup [minus]2]S cm[sup [minus]1] in the range from [minus]31 to 123 C, respectively. The electrochemical window of DMTC-containing sodium tetrachloroaluminate melts varied in the region of 2.5 to 2.2 V (150--170 C) depending on melt acidity and anode material. DMTC, being specifically adsorbed and reduced on the tungsten electrode surface, had an inhibiting effect on the aluminum reduction, but this effect was suppressed on the aluminum substrate. An electrochemical process with high current density (tens of milliamperes per square centimeter) was observed at 0.344V on the acidic sodium tetrachloroaluminate background, involving a free triazolium radical mechanism. Molten DMTC-AlCl[sub 3] electrolytes are acceptable for battery performance and both the aluminum anode and the triazolium electrolyte can be used as active materials in the acidic DMTC-AlCl[sub 3] mixtures.

Brønsted acidic room temperature ionic liquids derived from N,N-dimethylformamide and similar protophilic amides

We herein describe a convenient and efficient one-pot route to a new family of cost-effective, highly proton conductive room temperature ionic liquids based on N,N-dimethylformamide and structural analogues thereof, thereby opening up potential in the fuel cell industry and other areas.

Low temperature vibrational spectroscopy. I. Hexachlorotellurates

Far infrared and Raman spectra of six hexachlorotellurate (IV) salts have been obtained at ∼100 K for the first time. In the rubidium, cesium, ammonium, and tetramethylammonium salts the Raman active T2g cation lattice translatory mode was found. In the monoclinic K2[TeCl6] a number of low frequency lattice modes were observed and interpreted in terms of a phase transition near 165 K, similar to transitions in other K2[MX6] salts. The cubic tetramethylammonium hexachlorotellurate salt undergoes a phase transition of supposed first order at a temperature near 110 K, corresponding to transitions known in analogous uranium and tin compounds. Possible reasons for the transitions are discussed. In the low temperature phases the ν4 and ν6 bendings of [TeCl6]2− have been identified with bands near ∼130 and ∼110 cm−1. No evidence seemed to favor any stereochemical distortion due to the lone pair of electrons present in hexachlorotellurates.