Carbon‐supported NiII single‐atom catalysts with a tetradentate Ni‐N2O2 coordination formed by a Schiff base ligand‐mediated pyrolysis strategy are presented. A NiII complex of the Schiff base ligand (R,R)‐(−)‐N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni‐N2O2/C catalyst showed excellent performance for the electrocatalytic reduction of O2 to H2O2 through a two‐electron transfer process in alkaline conditions, with a H2O2 selectivity of 96 %. At a current density of 70 mA cm−2, a H2O2 production rate of 5.9 mol gcat.−1 h−1 was achieved using a three‐phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni‐N2O2/C catalyst could electrocatalytically reduce O2 in air to H2O2 at a high current density, still affording a high H2O2 selectivity (>90 %). A precise Ni‐N2O2 coordination was key to the performance.
However, the large-scale commercialization of PEMFCs is currently hindered by the high cost of platinumgroup-metal (PGM) electrocatalysts used for the hydrogen oxidation reaction and oxygen reduction reaction (ORR) in these devices. Presently, PGM metal catalysts (mainly Pt) comprise around 30% of the overall cost of PEMFC systems. [2] The overall kinetics of PEMFCs are generally limited by the sluggish ORR at the cathode, demanding a high loading (up to 20 wt%) of PGM electrocatalysts to overcome this kinetic barrier. For the widespread implementation of PEMFCs in the transportation sector (especially for medium to heavy vehicles), high-performance PGM-free ORR electrocatalysts are demanded.Among the various PGM-free electrocatalysts explored to date, nitrogen-doped carbon-supported iron single atom electrocatalysts (Fe-N-C) afford the highest ORR performance. [3] Both theoretical and experimental studies have confirmed that carbon-supported FeN 4 moieties possess high intrinsic ORR activity. [4] Fe-N-C electrocatalysts for ORR are typically prepared by the pyrolysis of precursors containing carbon, nitrogen, and iron at high temperatures (>600 °C). [3g,5] However, such pyrolysis routes Iron single atom catalysts (FeN 4 ) hosted in the micropores of N-doped carbons offer excellent performance for the oxygen reduction reaction (ORR). Achieving a high density of FeN 4 sites accessible for ORR has proved challenging to date. Herein, a simple surface NaCl-assisted method towards microporous N-doped carbon electrocatalysts with an abundance of catalytically accessible FeN 4 sites is reported. Powder mixtures of microporous zeolitic imidazolate framework-8 and NaCl are first heated to 1000 °C in N 2 , with the melting of NaCl above 800 °C creating a highly porous N-doped carbon product (NC-NaCl). Ferric (Fe 3+ ) ions are then adsorbed onto NC-NaCl, with a second pyrolysis stage at 900 °C in N 2 yielding a porous Fe/NC-NaCl electrocatalyst (Brunauer-Emmett-Teller surface area, 1911 m 2 g −1 ) with an excellent dispersion and high density of accessible surface FeN 4 sites (26.3 × 10 19 sites g −1 ). The Fe/NC-NaCl electrocatalyst exhibits outstanding ORR performance with a high half-wave potential of 0.832 V (vs reversible hydrogen electrode) in 0.1 m HClO 4 . When used as the ORR cathode catalyst in a 1.0 bar H 2 -O 2 fuel cell, Fe/NC-NaCl offers a high peak power density of 0.89 W cm −2 , ranking it as one of the most active M-N-C materials reported to date.
Photocatalytic CO2 reduction reaction (CO2RR) is an attractive process to convert CO2 into valuable chemicals. But this reaction is often restricted by the poor mass transfer of CO2 in the liquid phase. Here, we have developed a triphase photocatalytic CO2RR system by supporting Ag‐decorated TiO2 nanoparticles at a gas–water boundary with hydrophobic–hydrophilic abrupt interfacial wettability. Such a triphase system allows the rapid delivery of gas‐phase CO2 to the surface of photocatalysts while maintaining an efficient water supply and uncovered active sites. Ag‐TiO2 supported at the gas–water boundary showed a CO2 reduction rate of 305.7 μmol g−1 h−1, without hole scavengers, approximately 8 times higher than the nanoparticles dispersed in the liquid phase. Even using diluted CO2 (10 %) as the reactant, the CO2RR activity was superior to most reported Ag‐TiO2 based photocatalysts using pure CO2. The findings provide a general strategy to promote the interfacial CO2 mass transfer to improve photoactivity and selectivity.
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