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  1. Free, publicly-accessible full text available July 11, 2023
  2. Atomically dispersed and nitrogen-coordinated single Ni sites ( i.e. , NiN x moieties) embedded in partially graphitized carbon have emerged as effective catalysts for CO 2 electroreduction to CO. However, much mystery remains behind the extrinsic and intrinsic factors that govern the overall catalytic CO 2 electrolysis performance. Here, we designed a high-performance single Ni site catalyst through elucidating the structural evolution of NiN x sites during thermal activation and other critical external factors ( e.g. , carbon particle sizes and Ni content) by using Ni–N–C model catalysts derived from nitrogen-doped carbon carbonized from a zeolitic imidazolate framework (ZIF)-8. Themore »N coordination, metal–N bond length, and thermal wrinkling of carbon planes in Ni–N–C catalysts significantly depend on thermal temperatures. Density functional theory (DFT) calculations reveal that the shortening Ni–N bonds in compressively strained NiN 4 sites could intrinsically enhance the CO 2 RR activity and selectivity of the Ni–N–C catalyst. Notably, the NiN 3 active sites with optimal local structures formed at higher temperatures ( e.g. , 1200 °C) are intrinsically more active and CO selective than NiN 4 , providing a new opportunity to design a highly active catalyst via populating NiN 3 sites with increased density. We also studied how morphological factors such as the carbon host particle size and Ni loading alter the final catalyst structure and performance. The implementation of this catalyst in an industrial flow-cell electrolyzer demonstrated an impressive performance for CO generation, achieving a current density of CO up to 726 mA cm −2 with faradaic efficiency of CO above 90%, representing one of the best catalysts for CO 2 reduction to CO.« less
    Free, publicly-accessible full text available May 18, 2023
  3. Free, publicly-accessible full text available March 14, 2023
  4. Low-temperature direct ammonia fuel cells (DAFCs) use carbon-neutral ammonia as a fuel, which has attracted increasing attention recently due to ammonia's low source-to-tank energy cost, easy transport and storage, and wide availability. However, current DAFC technologies are greatly limited by the kinetically sluggish ammonia oxidation reaction (AOR) at the anode. Herein, we report an AOR catalyst, in which ternary PtIrZn nanoparticles with an average size of 2.3 ± 0.2 nm were highly dispersed on a binary composite support comprising cerium oxide (CeO 2 ) and zeolitic imidazolate framework-8 (ZIF-8)-derived carbon (PtIrZn/CeO 2 -ZIF-8) through a sonochemical-assisted synthesis method. The PtIrZnmore »alloy, with the aid of abundant OH ad provided by CeO 2 and uniform particle dispersibility contributed by porous ZIF-8 carbon (surface area: ∼600 m 2 g −1 ), has shown highly efficient catalytic activity for the AOR in alkaline media, superior to that of commercial PtIr/C. The rotating disk electrode (RDE) results indicate a lower onset potential (0.35 vs. 0.43 V), relative to the reversible hydrogen electrode at room temperature, and a decreased activation energy (∼36.7 vs. 50.8 kJ mol −1 ) relative to the PtIr/C catalyst. Notably, the PtIrZn/CeO 2 -ZIF-8 catalyst was assembled with a high-performance hydroxide anion-exchange membrane to fabricate an alkaline DAFC, reaching a peak power density of 91 mW cm −2 . Unlike in aqueous electrolytes, supports play a critical role in improving uniform ionomer distribution and mass transport in the anode. PtIrZn nanoparticles on silicon dioxide (SiO 2 ) integrated with carboxyl-functionalized carbon nanotubes (CNT–COOH) were further studied as the anode in a DAFC. A significantly enhanced peak power density of 314 mW cm −2 was achieved. Density functional theory calculations elucidated that Zn atoms in the PtIr alloy can reduce the theoretical limiting potential of *NH 2 dehydrogenation to *NH by ∼0.1 V, which can be attributed to a Zn-modulated upshift of the Pt–Ir d-band that facilitates the N–H bond breakage.« less