Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst

Electrocatalysts for oxygen reduction are a critical component that may dramatically enhance the performance of fuel cells and metal-air batteries, which may provide the power for future electric vehicles. Here we report a novel bio-inspired composite electrocatalyst, iron phthalocyanine with an axial ligand anchored on single-walled carbon nanotubes, demonstrating higher electrocatalytic activity for oxygen reduction than the state-of-the-art Pt/C catalyst as well as exceptional durability during cycling in alkaline media. Theoretical calculations suggest that the rehybridization of Fe 3d orbitals with the ligand orbitals coordinated from the axial direction results in a significant change in electronic and geometric structure, which greatly increases the rate of oxygen reduction reaction. Our results demonstrate a new strategy to rationally design inexpensive and durable electrochemical oxygen reduction catalysts for metal-air batteries and fuel cells.close34

Quantifying the density and utilization of active sites in non-precious metal oxygen electroreduction catalysts

Carbon materials doped with transition metal and nitrogen are highly active, non-precious metal catalysts for the electrochemical conversion of molecular oxygen in fuel cells, metal air batteries, and electrolytic processes. However, accurate measurement of their intrinsic turn-over frequency and active-site density based on metal centres in bulk and surface has remained difficult to date, which has hampered a more rational catalyst design. Here we report a successful quantification of bulk and surface-based active-site density and associated turn-over frequency values of mono- and bimetallic Fe/N-doped carbons using a combination of chemisorption, desorption and (57)Fe Mössbauer spectroscopy techniques. Our general approach yields an experimental descriptor for the intrinsic activity and the active-site utilization, aiding in the catalyst development process and enabling a previously unachieved level of understanding of reactivity trends owing to a deconvolution of site density and intrinsic activity

Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

International audienceWhile platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe–N–C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe–N–C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe–N–C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe–N–C materials. These new insights open the path to bottom-up synthesis approaches and studies on site–support interactions

Strategies for Enhancing the Electrocatalytic Activity of M-N/C Catalysts for the Oxygen Reduction Reaction

The development of highly active and durable nonprecious metal catalysts that can replace expensive Pt-based catalysts for the oxygen reduction reaction (ORR) is of pivotal importance in polymer electrolyte membrane fuel cells. In this line of research, metal and nitrogen codoped carbon (M-N/C) catalysts have emerged as the most promising alternatives to Pt-based catalysts. This review provides an overview of recently developed synthetic strategies for the preparation of M-N/C catalysts to enhance the catalytic activity of the ORR. We present five major strategies, namely the use of metal-organic frameworks as hosts or precursors, the use of sacrificial templates, the addition of heteroelements, the preferential generation of active sites, and a biomimetic approach. For each strategy, the advantages capable of boosting catalytic activity in the ORR are summarized, and notable examples and their catalytic performances are presented. The ORR activities and measurement conditions of high-performing M-N/C catalysts are also tabulated. Finally, we summarize this review with some suggestions for future studie

Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction

Nitrogen-doped carbon materials with atomically dispersed iron or cobalt are promising for catalytic use. Here, the authors show that cobalt moieties have a higher redox potential, bind oxygen more weakly and are less active toward oxygen reduction than their iron counterpart, despite similar coordination