Mati Kook

Graphene functionalised by laser-ablated V2O5 for a highly sensitive NH3 sensor

Graphene has been recognized as a promising gas sensing material. The response of graphene-based sensors can be radically improved by introducing defects in graphene using, for example, metal or metal oxide nanoparticles. We have functionalised CVD grown, single-layer graphene by applying pulsed laser deposition (PLD) of V2O5 which resulted in a thin V2O5 layer on graphene with average thickness of ≈0.6 nm. From Raman spectroscopy, it was concluded that the PLD process also induced defects in graphene. Compared to unmodified graphene, the obtained chemiresistive sensor showed considerable improvement of sensing ammonia at room temperature. In addition, the response time, sensitivity and reversibility were essentially enhanced due to graphene functionalisation by laser deposited V2O5. This can be explained by an increased surface density of gas adsorption sites introduced by high energy atoms in laser ablation plasma and formation of nanophase boundaries between deposited V2O5 and graphene.

Synthesis of highly-active Fe–N–C catalysts for PEMFC with carbide-derived carbons

Proton exchange membrane fuel cells (PEMFC) offer a viable alternative to internal combustion engines, but highly performing stacks still require large amounts of platinum-based catalysts. Fe–N–C catalysts have recently emerged as potential substitutes. Carbide-derived carbon (CDC) can be designed to have various pore size distributions (PSD), in the microporous and/or mesoporous domains, which can be used for defining the number and/or accessibility of active sites in Fe–N–C catalysts based on the CDC. In this work, we compare two sets of Fe–N–C catalysts derived from two different CDCs, one with most frequent pore size of 8.5 A, (CDC-2) and another one with most frequent pore sizes at 7.8 and 30 A (CDC-1). The CDC-based Fe–N–C catalysts show excellent half-wave potential for oxygen reduction reaction (ORR) of 0.81 V vs. RHE in 0.5 M H2SO4. This work presents the first study of CDC-based catalysts in a PEMFC, where the performance of the CDC-2 based catalyst rivaled that of the best Fe–N–C materials in the literature. The catalyst derived from CDC-2 showed ca. 5 times higher activity at 0.8 V vs. RHE than the one derived from CDC-1. We show that the residual presence of boron in CDC-1 is the main reason for the lower activity of CDC-1 derived catalysts, leading to the formation of iron boride instead of ORR-active FeNxCy moieties. Higher Fe contents were investigated for CDC-2, but lead to unmodified activity, which is explained from Mossbauer spectroscopy measurements by the increasing formation of ORR-inactive Fe species at high Fe content. In summary, we demonstrate the excellent potential for CDC materials to be used in catalyst design and also identify some key issues that may arise from the possible residual presence of secondary atoms from the starting carbide.

Oxygen Reduction on Catalysts Prepared by Pyrolysis of Electrospun Styrene–Acrylonitrile Copolymer and Multi-walled Carbon Nanotube Composite Fibres

For the first time, the oxygen reduction reaction (ORR) is studied on pyrolysed electrospun multi-walled carbon nanotube (MWCNT) and styrene–acrylonitrile (SAN) composite fibre catalysts in alkaline medium. Scanning electron microscopy images revealed that the prepared catalysts mainly consist of MWCNTs, while nitrogen doping of the catalysts is confirmed by X-ray photoelectron spectroscopy. This indicates that SAN can be used as a nitrogen precursor. The ORR studies carried out by rotating disc electrode (RDE) and rotating ring-disc electrode (RRDE) methods showed that the prepared catalysts were considerably more active towards the ORR than the pristine MWCNTs.Graphical Abstract