Thomas Steenberg

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.

Electrolytic deposition of amorphous and crystalline zinc-calcium phosphates

Amorphous Zn–Ca phosphates and crystalline Zn3(PO4)2·4H2O conversion layers on cathode substrates were prepared by electrolysis of mixtures of acidic solutions saturated with metal phosphates. The solutions contained tricalcium phosphate (Ca3(PO4)2) and/or zinc phosphate dihydrate (Zn3(PO4)2·2H2O). The depositions was carried out with constant or pulsating cathode current densities in the range 20–70 mA cm-2 at 20–70 °C. The deposition of the uniform crystalline Zn3(PO4)2·4H2O was performed at a pulsating cathode current density of 70 mA cm-2 at 70 °C for periods up to 10 min. Amorphous deposits of Zn–Ca phosphates containing 20 wt% H2O with variable Zn-to-Ca ratios were deposited at a constant cathodic current of 30 mA cm-2 at 20 °C for 3 min. Surface areas of the amorphous deposits were of the order of 28 m2 g-1. X-ray diffraction, differential thermal analysis and thermogravimetry were used to investigate phase formation and transitions at increasing temperatures. The amorphous Zn–Ca phosphate deposit was after calcination at 900 °C transformed to crystalline phosphates containing the β-Ca3(PO4)2 or Ca3-xZnx(PO4)2 and α-CaZn2(PO4) phases.

Estimation of temperature in the lubricant film during cold forging of stainless steel based on studies of phase transformations in the film

Abstract Changes in friction during backward can extrusion of stainless steel was observed for four different lubricant systems. The observed changes in friction are believed to be caused by phase transitions in the lubricant. The lubricant systems consisted of two different carrier coatings (crystalline Zn 3 (PO 4 ) 2 ·4H 2 O and amorphous Zn 1.5 Ca 1.5 (PO 4 ) 2 ) lubricated with soap or MoS 2 . The temperature in the lubricant film during the process was estimated from changes in friction in correlation with observed phase transitions in the lubricant. Phase transitions in the carrier coatings as a function of pressure and temperature were investigated. Several previously unknown phase transitions were discovered. Four different crystal structures of Zn 3 (PO 4 ) 2 · x H 2 O were observed as a function of temperature. At 700–900°C Zn 3 (PO 4 ) 2 becomes amorphous (depending on the pressure). Amorphous Zn 1.5 Ca 1.5 (PO 4 ) 2 crystallises at 200–300°C and a second phase transition is observed at 400–700°C (depending on the pressure). It is estimated on the basis of the results from the backward can extrusion and analysis of the structure as a function of temperature that the temperature may reach in the excess of 450°C at a height/diameter ratio of two.