Exposing Fe–N–C catalysts to H2O2-byproduct leaves their catalytic sites untouched but decreases the turnover frequency via oxidation of the carbon surface.

We demonstrate the direct electrochemical conversion of CO2 to CO using solid state Ni–N–C carbon catalysts characterized by a coordinative molecular Ni–Nx active moiety at industrial current densities of up to 700 mA cm−2 with faradaic efficiencies superior to those of the state-of-the-art AgOx electrocatalysts.

This contribution reports the discovery and analysis of the first PGM-free, p-block Sn-based single metal and nitrogen-doped carbon (MNC) catalysts for the electroreduction of molecular oxygen (ORR) in acidic conditions at fuel cell cathodes. The prepared SnNC catalysts meet and exceed state of art FeNC catalysts in terms of intrinsic catalytic turn over frequency (TOF) and hydrogenair fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Added benefits include a high favorable selectivity for the 4-electron reduction pathway and a Fenton-inactive character of Sn.A range of analytical techniques, combined with DFT calculations indicate that stannic Sn(IV)-Nx single metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytic active moieties. The superior PEMFC performance of SnNC cathode catalysts under realistic, hydrogen-air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising replacement of today's state-of-art Fe-based catalysts. 4Growing concerns over fossil energy and the environment are incentives to develop new energy technologies. Low-temperature hydrogen/air proton-exchange membrane fuel cell (PEMFC) is one such technology, converting hydrogen into electrical energy 1, 2 . For catalyzing the oxygen reduction reaction (ORR) and hydrogen oxidation reaction at the electrodes, PEMFCs rely however on precious, in particular platinum-based catalysts 3, 4 , a scarce and expensive metal.Research to replace precious group metals (PGMs) has led to a class of bio-inspired catalysts, labelled MNC, that involve non-precious 3d transition metal cations stabilized by nitrogen atoms (Metal-Nx moieties), themselves incorporated in conductive carbon matrices. Fe, Co and Mn are hitherto the only three metals that result in ORR-active Metal-Nx moieties in acidic reaction environments 5,6,7,8,9,10 . While the number and utilization of such moieties embedded in carbon are being improved 9 , the fundamental nature of such sites is not so new. Indeed, the large body of experimental research on pyrolyzed FeNC and CoNC materials identifies Metal-N4 motifs as the most active sites for catalyzing ORR in acid 11,12,13 . Such sites are akin to square-planar Metal-N4 sites in Fe or Co macrocycles, identified in 1964 to be ORR active 14 Here, we report the discovery of the first p-block single metal site catalyst, SnNC, exhibiting catalytic ORR reactivities in acidic environments that meet and exceed all state-of-art PGM-free catalyst concepts, while adding important benefits in terms of catalyst stability. The catalytically active single-metal SnNx moieties embedded in the surface of the SnNC catalyst were characterized by high-resolution scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), extended X-ray absorption fine structure (EXAFS), Xray photoelectron spectroscopy (XPS) and 119 Sn Mössbauer spectroscopy, com...

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We report novel structure–activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO2 reduction reaction (CO2RR) catalyzed by a series of porous iron–nitrogen–carbon (FeNC) catalysts.

We report a joint experimental−computational mechanistic study of electrochemical reduction of CO 2 to CH 4 , catalyzed by solid-state Fe−N−C catalysts, which feature atomically dispersed, catalytically active Fe−N x sites and represent one of the very rare examples of solid, non-Cu-based electrocatalysts that yield hydrocarbon products. Work reported here focuses on the identification of plausible mechanistic pathways from CO 2 to various C 1 products including methane. It is found that Fe−N x sites convert only CO 2 , CO, and CH 2 O into methane, whereas CH 3 OH appears to be an end product. Distinctly different pH dependence of the catalytic CH 4 evolution from CH 2 O in comparison with that of CO 2 and CO reduction indicates differences in the proton participation of ratedetermining steps. By comparing the experimental observations with density functional theory derived free energy diagrams of reactive intermediates along the CO 2 reduction reaction coordinates, we unravel the dominant mechanistic pathways and roles of CO and CH 2 O during the catalytic CO 2 -to-CH 4 cascades and their rate-determining steps. We close with a comprehensive reaction network of CO 2 RR on single-site Fe−N−C catalysts, which may prove useful in developing efficient, non-Cubased catalysts for hydrocarbon production.

We report on a non-precious, two-phase bifunctional oxygen reduction and evolution (ORR and OER) electrocatalyst with previously unachieved combined roundtrip catalytic reactivity and stability for use in oxygen electrodes of unitized reversible fuel cell/electrolyzers or rechargeable metal-air batteries. The combined OER and ORR overpotential, total, at 10 mA cm À2 was a record low value of 0.747 V. Rotating Ring Disk Electrode (RRDE) measurements revealed a high faradaic selectivity for the 4 electron pathways, while subsequent continuous MEA tests in reversible electrolyzer cells confirmed the excellent catalyst reactivity rivaling the state-of-the-art combination of iridium (OER) and platinum (ORR).Electrochemical energy storage based on the interconversion of renewable electricity and molecular fuels (solar fuels) and solid state structures (aqueous metal-air cells) invariably involves the oxygen/water redox system supplying and consuming water, protons, electrons and oxygen. This is why efficient catalysts for the oxygen evolution reaction (OER:are critical. [1][2][3][4] Combining the two functionalities in one single bifunctional oxygen redox electrode would greatly simplify the design of energy conversion devices or enhance the mobility and power-to-weight ratio. This plays an important role in spacecraft, aircraft, and ground transportation applications. Active oxygen redox catalysts such as IrO 2 or Pt are rare and expensive, which is why the development of efficient non-precious oxygen catalysts is of interest. 5-10 The layered double hydroxide of Ni and Fe (''NiFe-LDH'') is known to be one of the most active non-noble OER catalysts in alkaline solution. 5,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] In contrast, nitrogendoped carbon materials are promising non-precious candidates for the ORR. [27][28][29][30] Rather than exploring suitable bifunctional catalytic surface sites, or designing two distinct active sites on the same substrate, we propose the facile heterogeneous mixing of either material to obtain a two-phase bifunctional catalyst. This was shown for noble metal catalysts of iridium and platinum. 31,32 Recently, non-precious metal mixtures of Mn-Co oxides and carbon nanotubes have been tested. 33 Realizing that a twocomponent surface is necessary for highly active bifunctional catalysts, 34,35 in this contribution, we designed two-component NiFe-LDH -Fe-N-C catalysts resulting in today's most efficient bifunctional oxygen electrodes in 0.1 M KOH. A mutual improving effect between the two components in the two-phase structure with distinct neighbouring active sites appears key to the observed performance. Using a fast microwave-assisted solvothermal one-pot synthesis route (Fig. S1, ESI †), we prepared a carbon-supported crystalline NiFe-LDH catalyst material in a Ni/Fe ratio of B3.6 (Ni 0.78 Fe 0.22 (OH) x ) and a metal loading of B37 wt%.The X-ray diffraction (XRD) pattern (Fig. 1) is consistent with the data-based reflections of layered double hydroxides (JCPDS: 00-01...

Metal−nitrogen−carbon (MNC) catalysts represent a potential means of reducing cathode catalyst costs in low temperature fuel cell cathodes. Knowledge-based improvements have been hampered by the difficulty to deconvolute active site density and intrinsic turnover frequency. In the present work, MNC catalysts with a variety of secondary nitrogen precursors are addressed. CO chemisorption in combination with Mossbauer spectroscopy are utilized in order to unravel previously inaccessible relations between active site density, turnover frequency, and active site utilization. This analysis provides a more fundamental description and understanding of the origin of the catalytic reactivity; it also provides guidelines for further improvements. Secondary nitrogen precursors impact quantity, quality, dispersion, and utilization of active sites in distinct ways. Secondary nitrogen precursors with high nitrogen content and micropore etching capabilities are most effective in improving catalysts performance.