Inez M. Weidinger

Ruthenium/nitrogen-doped carbon as an electrocatalyst for efficient hydrogen evolution in alkaline solution

For all electrocatalysts (even Pt), the kinetics of the hydrogen evolution reaction (HER) in alkaline environments are more sluggish by a factor of two to three compared to those in acidic solutions. Here, we demonstrated the preparation of a novel ruthenium/nitrogen-doped carbon (Ru/NC) electrocatalyst supported by graphite foam, in which abundant, singly dispersed Ru atoms were chelated to a nitrogen-doped carbon matrix. In a 1 M KOH aqueous solution, the resultant Ru/NC electrocatalyst exhibited excellent electrocatalytic HER activity with an extremely low overpotential of only 21 mV at 10 mA cm−2 and an excellent mass current density as high as 8 A mgRu−1 at 100 mV, which is superior to the values for reported electrocatalysts (overpotentials of >50 mV at 10 mA cm−2), even Pt catalysts (overpotential of ∼36 mV at 10 mA cm−2). Importantly, the inherent turnover frequency (TOF) value (per Ru atom) of the Ru/NC electrocatalyst reaches 4.55 s−1, which is 3.2 times higher than that of the Pt catalyst (1.41 s−1). Electrochemical analyses and structural characterization revealed that atomically dispersed Ru is responsible for the outstanding HER activity of the Ru/NC electrocatalyst because of a substantially accelerated Volmer step. The outstanding HER performance gives the Ru/NC electrocatalyst promising potential for practical hydrogen production applications.

Molecular LEGO by domain-imprinting of cytochrome P450 BM3

HYPOTHESISnElectrosynthesis of the MIP nano-film after binding of the separated domains or holo-cytochrome BM3 via an engineered anchor should result in domain-specific cavities in the polymer layer.nnnEXPERIMENTSnBoth the two domains and the holo P450 BM3 have been bound prior polymer deposition via a N-terminal engineered his6-anchor to the electrode surface. Each step of MIP preparation was characterized by cyclic voltammetry of the redox-marker ferricyanide. Rebinding after template removal was evaluated by quantifying the suppression of the diffusive permeability of the signal for ferricyanide and by the NADH-dependent reduction of cytochrome c by the reductase domain (BMR).nnnFINDINGSnThe working hypothesis is verified by the discrimination of the two domains by the respective MIPs: The holoenzyme P450 BM3 was ca. 5.5 times more effectively recognized by the film imprinted with the oxidase domain (BMO) as compared to the BMR-MIP or the non-imprinted polymer (NIP). Obviously, a cavity is formed during the imprinting process around the his6-tag-anchored BMR which cannot accommodate the broader BMO or the P450 BM3. The affinity of the MIP towards P450 BM3 is comparable with that to the monomer in solution. The his6-tagged P450 BM3 binds (30 percent) stronger which shows the additive effect of the interaction with the MIP and the binding to the electrode.

Robust electrografted interfaces on metal oxides for electrocatalysis – an in situ spectroelectrochemical study

Diazonium salts were electrografted onto transparent conductive oxides as stable alternatives to conventional anchoring groups for the surface immobilisation of molecular species, such as molecular electrocatalysts and/or photosensitisers. Surface sensitive in situ ATR-IR spectroscopy, in combination with spectroelectrochemistry, was used as tool to provide unprecedented information on the interface formation, structure and stability, which is not possible using conventional approaches. Electrografted interfaces deposited on model oxides were thereby shown to be stable in an extraordinarily wide pH range (2.5–12) and electrochemical potential window (−0.73 to 2.23 V vs. RHE in aqueous media, and −1.3 V to 1.6 V vs. Fc/Fc+ in organic media). As a model electrocatalyst, an Fe porphyrin active for the oxygen reduction reaction (ORR) was immobilised on both mesoporous antimony doped tin oxide (me-ATO) and planar tin doped indium oxide (pl-ITO) using this electrografting approach. Surface coverages comparable to those previously reported using phosphonate anchoring groups were achieved. In addition, fast electron transfer rates were demonstrated, while in situ resonance Raman spectroelectrochemistry proved the co-ordination of the immobilised species and demonstrated its excellent electrochemical accessibility.

Enhancement of the Electrocatalytic Activity of Thienyl‐Substituted Iron Porphyrin Electropolymers by a Hangman Effect

The thiophene‐modified iron porphyrin FeT3ThP and the respective iron Hangman porphyrin FeH3ThP, incorporating a carboxylic acid hanging group in the second coordination sphere of the iron center, were electropolymerized on glassy carbon electrodes using 3,4‐ethylenedioxythiophene (EDOT) as co‐monomer. Scanning electron microscopy images and Resonance Raman spectra demonstrated incorporation of the porphyrin monomers into a fibrous polymer network. Porphyrin/polyEDOT films catalyzed the reduction of molecular oxygen in a four‐electron reaction to water with onset potentials as high as +0.14u2005V vs. Ag/AgCl in an aqueous solution of pHu20057. Further, FeT3ThP/polyEDOT films showed electrocatalytic activity towards reduction of hydrogen peroxide at highly positive potentials, which was significantly enhanced by introduction of the carboxylic acid hanging group in FeH3ThP. The second coordination sphere residue promotes formation of a highly oxidizing reaction intermediate, presumably via advantageous proton supply, as observed for peroxidases and catalases making FeH3ThP/polyEDOT films efficient mimics of heme enzymes.

Hydrogen evolution by cobalt hangman porphyrins under operating conditions studied by vibrational spectro-electrochemistry

Cobalt hangman complexes are promising catalysts for dihydrogen production, yet their electrocatalytic performance in aqueous environment is still a topic of dispute. Surface-enhanced resonance Raman (SERR) spectro-electrochemistry has a great potential to give insight into the reaction mechanism of such molecular catalysts attached to electrodes under turnover conditions. However, the intrinsic catalytic activity of plasmonic supports and photoinduced side-reactions make the in situ analysis of their structures very challenging. In this work, the structure of hangman complexes attached to electrodes via dip-coating was investigated during catalytic turnover by electrochemistry and SERR spectroscopy. In order to explore the relevance of the hanging group for proton supply, complexes bearing a carboxylic acid and an ester hanging group were compared. For the former, SERR spectra recorded under turnover conditions indicate the reductive formation of a CoIII–H species, followed by laser-induced translocation of a proton to the carboxylic hanging group and the associated formation of the CoI state. Due to the lack of a proton accepting group, hangman complexes with an ester group could not be trapped in the CoI intermediate state and as a consequence SERR spectra solely reflected the (photo-enriched) CoII resting state under turnover conditions. These results represent the first Raman spectroscopic insights into intermediates of dihydrogen evolution catalysed by cobalt hangman complexes on electrodes and support the direct involvement of the hanging group as a proton shuttle.

High Electromagnetic Field Enhancement of TiO2 Nanotube Electrodes

We present the fabrication of TiO2 nanotube electrodes with high biocompatibility and extraordinary spectroscopic properties. Intense surface-enhanced resonance Raman signals of the heme unit of the redox enzyme Cytochromeu2005b5 were observed upon covalent immobilization of the protein matrix on the TiO2 surface, revealing overall preserved structural integrity and redox behavior. The enhancement factor could be rationally controlled by varying the electrode annealing temperature, reaching a record maximum value of over 70 at 475u2009°C. For the first time, such high values are reported for non-directly surface-interacting probes, for which the involvement of charge-transfer processes in signal amplification can be excluded. The origin of the surface enhancement is exclusively attributed to enhanced localized electric fields resulting from the specific optical properties of the nanotubular geometry of the electrode.