Lars Nilausen Cleemann

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.

Synthesis and properties of poly(aryl sulfone benzimidazole) and its copolymers for high temperature membrane electrolytes for fuel cells

Poly(aryl sulfone benzimidazole) (SO2PBI) and its copolymers with poly[2,2′-p-(phenylene)-5,5′-bibenzimidazole] (pPBI), termed as Co-SO2PBI, were synthesized with varied feeding ratios of 4,4′-sulfonyldibenzoic acid (SDBA) to terephthalic acid (TPA). Incorporation of the stiff para-phenylene and flexible aryl sulfone linkages in the macromolecular structures resulted in high molecular weight copolymers with good solubility. The chemical stability towards radical oxidation was improved for SO2PBI and its copolymer membranes due to the electron-withdrawing sulfone functional groups. Upon acid doping, the membrane swelling was reduced and the mechanical strength was improved, as compared with their meta structured analogues. At an acid doping level of 11 mol H3PO4 per average molar repeat unit, the Co-20%SO2PBI membrane exhibited a tensile strength of 16 MPa at room temperature and an H2-air fuel cell peak power density of 346 mW cm−2 at 180 °C at ambient pressure. Durability tests with the membrane under a constant current density of 300 mA cm−2 at 160 °C showed a degradation rate of 6.4 μV h−1 during a period of 2400 h, which was significantly lower than that for meta PBI membranes with a similar acid doping level.

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.

Direct Synthesis of Fe3C‐Functionalized Graphene by High Temperature Autoclave Pyrolysis for Oxygen Reduction

We present a novel approach to direct fabrication of few-layer graphene sheets with encapsulated Fe3 C nanoparticles from pyrolysis of volatile non-graphitic precursors without any substrate. This one-step autoclave approach is facile and potentially scalable for production. Tested as an electrocatalyst, the graphene-based composite exhibited excellent catalytic activity towards the oxygen reduction reaction in alkaline solution with an onset potential of ca. 1.05 V (vs. the reversible hydrogen electrode) and a half-wave potential of 0.83 V, which is comparable to the commercial Pt/C catalyst.

Tetrazole substituted polymers for high temperature polymer electrolyte fuel cells

While tetrazole (TZ) has much lower basicity than imidazole and may not be fully protonated in the presence of phosphoric acid (PA), DFT calculations suggest that the basicity of TZ groups can be increased by the introduction of a 2,6-dioxy-phenyl-group in position 5 of TZ. This structure allows hydrogen bonds between TZ protons and ether oxygen atoms, and thereby establishes a resonance stabilised, co-planar structure for tetrazolium ions. Molecular electrostatic potential (MEP) calculations also indicate that tetrazolium ions possess two sites for proton hopping. This makes such materials interesting for use in a high temperature fuel cell (HT PEMFC). Based on these findings, two polymers incorporating the proposed TZ groups were synthesised, formed into membranes, doped with PA and tested for fuel cell relevant properties. At room temperature, TZ-PEEN and commercial meta-PBI showed an equilibrium uptake of 0.5 and 4.7 mol PA per mol heterocycle, respectively, indicating that PBI has higher affinity for PA than TZ-PEEN. The highest achieved PA uptake was ca. 110 wt%, resulting in a proton conductivity of 25 mS cm−1 at 160 °C with a low activation energy of about 35 kJ mol−1. In a first HT PEMFC test at 160 °C, a peak power density of 287 mW cm−2 was achieved.

Durability Issues and Status of PBI-Based Fuel Cells

This chapter briefly reviews durability and stability issues with key materials and components for HT-PEMFCs, including the polymer membrane, the doping acid, the electrocatalyst, the catalyst support and bipolar plates. Degradation mechanisms and their dependence on fuel cell operating conditions are summarized as well. To date, lifetimes of this type of fuel cells of up to 18,000 h with degradation rates of around 5 μV/h at temperatures of 150–160 °C have been demonstrated using hydrogen and air under constant moderate load. However, the degradation rate increases by a factor 10 when the cell is exposed to start-up–shutdown or load cycling.

Flexible sample environment for high resolution neutron imaging at high temperatures in controlled atmosphere

High material penetration by neutrons allows for experiments using sophisticated sample environments providing complex conditions. Thus, neutron imaging holds potential for performing in situ nondestructive measurements on large samples or even full technological systems, which are not possible with any other technique. This paper presents a new sample environment for in situ high resolution neutron imaging experiments at temperatures from room temperature up to 1100 °C and/or using controllable flow of reactive atmospheres. The design also offers the possibility to directly combine imaging with diffraction measurements. Design, special features, and specification of the furnace are described. In addition, examples of experiments successfully performed at various neutron facilities with the furnace, as well as examples of possible applications are presented. This covers a broad field of research from fundamental to technological investigations of various types of materials and components.

Catalytic Reduction of NO by Methane Using a Pt ∕ C ∕ polybenzimidazole ∕ Pt ∕ C Fuel Cell

The catalytic NO reduction by methane was studied using a (NO,CH 4 ,Ar),Pt|polybenzimidazole(PBI)-H 3 PO 4 |Pt,(H2,Ar) fuel cell at 135 and 165°C. It has been found that, without any reducing agent (like CH 4 ), NO can be electrochemically reduced in the (NO, Ar), Pt/C|PBI-H 3 PO 4 |Pt/C, (H 2 ,Ar) fuel cell with participation of H + or electrochemically produced hydrogen. When added, methane partially suppresses the electrochemical reduction of NO. Methane outlet concentration monitoring has shown the CH 4 participation in the chemical catalytic reduction, i.e., methane co-adsorption with NO inhibited the electrochemical NO reduction and introduced a dominant chemical path of the NO reduction. The products of the NO reduction with methane were N 2, C 2 H 4, and water. The catalytic NO reduction by methane was promoted when the catalyst was negatively polarized (-0.2 V). Repeated negative polarization of the catalyst increased the NO conversion. Maximum NO conversion was 48%. This effect was explained as a result of the reaction of the electrochemically produced hydrogen.

Platinum Iron Intermetallic Nanoparticles Supported on Carbon Formed In Situ by High-Pressure Pyrolysis for Efficient Oxygen Reduction

Carbon‐supported PtFe alloy catalysts are synthesized by the one‐step, high‐temperature pyrolysis of Pt, Fe, and C precursors. As a result of the high temperature, the formed PtFe nanoparticles possess highly ordered, face‐centered tetragonal, intermetallic structures with a mean size of ≈11.8 nm. At 0.9 V versus the reversible hydrogen electrode, the PtFe nanoparticles show a 6.8 times higher specific activity than the reference Pt/C catalyst towards the oxygen reduction reaction (ORR) as well as excellent stability, most likely because of the durable intermetallic structure and the preleaching treatment of the catalyst. During these preliminary syntheses, we found that a portion of the PtFe nanoparticles is buried in the in situ formed carbon phase, which limits Pt utilization in the catalyst and results in a mass‐specific activity equivalent to the commercial Pt/C catalyst. Moreover, the possible presence of other active sites, for example, FeNx, CNx, and carbon‐encapsulated metal nanoparticles, and their contribution to the ORR performance of the catalyst are also investigated.