Hans Aage Hjuler

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.

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.

Roll-to-roll coated PBI membranes for high temperature PEM fuel cells

We employed roll-to-roll coating in the preparation of 40 μm thick poly[2,2′(m-phenylene)-5,5′bibenzimidazole] (PBI) films for fuel cells using both knife-coating (KC) and slot-die (SD) coating. The films were coated directly from a 9% (w/w) solution of PBI in dimethylacetamide onto a sacrificial low cost paper or plastic based carrier substrate and dried using a hot air oven with a length of 1 m at 140 °C. A web width of 305 mm, a working width of 250 mm and a web speed of 0.2 m min−1 were explored to ensure efficient drying of the thick wet film. A large air flow was found to efficiently avoid skinning. Films were prepared by a single coating step and by two subsequent coating steps in order to explore whether two coating steps gave films with fewer defects. A significant development towards upscaling the PEM fuel cell technology was that the PBI membrane was coated onto a sacrificial carrier substrate allowing for easy recoating on top of the firstly prepared film. It was thus possible to prepare free-standing films by a simple coating procedure followed by delamination from the carrier substrate post-film formation and drying. We finally carried out systematic membrane characterization with respect to solubility, phosphoric acid doping and fuel cell performance. Our results showed that the PBI membranes prepared in this work have identical properties compared to traditionally cast membranes while enabling an increase of a factor of 100 in manufacturing speed.

High temperature polymer electrolyte membrane fuel cells: Approaches, status, and perspectives

perspectives DTU Orbit (08/11/2019) High temperature polymer electrolyte membrane fuel cells: Approaches, status, and perspectives This book is a comprehensive review of high-temperature polymer electrolyte membrane fuel cells (PEMFCs). PEMFCs are the preferred fuel cells for a variety of applications such as automobiles, cogeneration of heat and power units, emergency power and portable electronics. The first 5 chapters of the book describe rationalization and illustration of approaches to high temperature PEM systems. Chapters 6 13 are devoted to fabrication, optimization and characterization of phosphoric acid-doped polybenzimidazole membranes, the very first electrolyte system that has demonstrated the concept of and motivated extensive research activity in the field. The last 11 chapters summarize the state-of-the-art of technological development of high temperature-PEMFCs based on acid doped PBI membranes including catalysts, electrodes, MEAs, bipolar plates, modelling, stacking, diagnostics and applications.

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.

A Novel Inorganic Low Melting Electrolyte for Secondary Aluminum‐Nickel Sulfide Batteries

A new, inorganic low melting electrolyte with the composition LiAlCl/sub 4/-NaAlCl/sub 4/-NaAlBr/sub 4/-KAlCl/sub 4/ (3:2:3:2) (or equivalently LiAlBr/sub 4/-NaAlCl/sub 4/-KAlCl/sub 4/ (3:5:2)) has been developed. The melting point for this neutral melt is 86/sup 0/C; the decomposition potential is approximately 2.0V; the ionic conductivity is measured in the range 97/sup 0/-401/sup 0/C and is 0.142s cm/sup -1/ at 100/sup 0/C, and the density is 2.07g cm/sup -3/. The conductivity seems to be an almost linear combination of the conductivities of the four individual halo salts which form the melt. Other examined higher melting mixtures exhibit conductivities deviating less than +-10% from their combination expectations. The low melting electrolyte is employed in the rechargeable battery system Al/electrolyte/Ni/sub 3/S/sub 2/ at 100/sup 0/C. The open-circuit voltage of this system is from 0.82 to 1.0V. Dendrite-free aluminum deposits are obtained. The cycling behavior of the battery system is reported.

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.