Nick Daems

Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction

Carbon materials such as graphite, graphene, carbon nanotubes and ordered mesoporous carbon have attracted a lot of attention for their use in fuel cells, due to beneficial properties like high conductivity, high mechanical and chemical stability and, for the latter, high surface area. Doping these materials with nitrogen or, less commonly, other elements alters their (electronic) properties, making them particularly suitable for application as electrocatalysts for the oxygen reduction reaction (ORR) in a fuel cell. This paper reviews the synthesis methods employed for the doping of these different types of carbon materials with various elements and the characterization techniques used to investigate their physicochemical properties such as degree of graphitization, dopant content, dopant configuration and surface area. Furthermore, their application as electrocatalysts for the oxygen reduction in a fuel cell is reviewed. Finally, the possible mechanisms for the ORR on N-doped carbon materials are critically discussed and compared to the mechanisms of commercial Pt/C electrocatalysts.

Iron-containing N-doped carbon electrocatalysts for the cogeneration of hydroxylamine and electricity in a H2–NO fuel cell

Iron-containing N-doped carbon materials were investigated as electrocatalysts for the cogeneration of hydroxylamine (NH2OH) and electricity in a H2–NO fuel cell. This electrochemical route for the production of hydroxylamine is a greener alternative to the present industrial synthesis, because it allows converting the energy released during the reaction into electricity. The studied electrocatalysts were prepared by pyrolysis of composites of activated carbon and polyaniline (PANI) incorporating Fe sites (Fe-PANI-AC). Characterisation with a combination of techniques (FT-IR and Raman spectroscopy, XRD, N2-physisorption, XPS and ToF-SIMS) showed that the materials exhibit promising features as electrocatalysts for the NO reduction reaction, as they contain the desired isolated FeNxCy sites and have a relatively high degree of graphitisation, which grants good electrical conductivity. The performance of the Fe-PANI-AC electrocatalysts was investigated by means of linear sweep voltammetry (LSV) in a half cell setup and by chronoamperometry in a H2–NO fuel cell setup and compared to that of a reference electrocatalyst consisting of iron phthalocyanine supported on activated carbon (FePc/AC). The Fe-PANI-AC electrocatalysts led to higher current density than FePc/AC under all studied conditions. At low NO concentration in the feed, FePc/AC displayed higher selectivity towards hydroxylamine, whereas the Fe-PANI-AC electrocatalysts were superior at higher NO concentration (i.e. at the industrially more relevant conditions), both in terms of production rate and of selectivity towards hydroxylamine. Moreover, the Fe-PANI-AC electrocatalysts exhibited high stability under the fuel cell operating conditions. In summary, Fe-PANI-ACs displayed very promising electrocatalytic performance in the reduction of NO to hydroxylamine and offered the additional advantage of being less expensive compared to the reference FePc/AC electrocatalyst or to a benchmark noble-metal-based electrocatalyst as Pt/AC.

High-performance membranes with full pH-stability

Following current strong demands from, among others, paper, food and mining industries, a novel type of nanofiltration membrane was developed, which displays excellent performance in terms of selectivity/flux with a unique combination of chemical stability over the full (0–14) pH-range and thermal stability up to 120 °C. The membrane consists of polyvinylidene fluoride grafted with polystyrene sulfonic acid. The optimum membrane showed water permeances of 2.4 L h−1 m−2 bar−1 while retaining NaCl, MgSO4 and Rhodamine B (479 Da) for respectively ≈60%, ≈80% and >96%.

Vapor-fed solar hydrogen production exceeding 15% efficiency using earth abundant catalysts and anion exchange membrane

Vapor-fed solar hydrogen generators can convert water vapor from the air into hydrogen using sunlight as the energy source. Hydrogen and oxygen evolution reactions are performed in the gas phase in cathode and anode compartments separated by a membrane. Anion exchange membranes show great promise for this type of solar hydrogen generator. They provide an alkaline environment enabling the use of earth abundant materials as electrocatalysts. In this work, a vapor-fed solar hydrogen generator with KOH-doped poly(vinyl alcohol) anion exchange membrane flanked with NiFe and NiMo catalysts is demonstrated. The device reached an average 15.1% solar-to-hydrogen efficiency at room temperature and 95% relative humidity. This first demonstration of gas phase water splitting with earth abundant catalysts and anion exchange membrane opens a pathway to low cost, autonomous, efficient and safe solar hydrogen generators.