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1. Atomic-level active sites of efficient imidazolate framework-derived nickel catalysts for CO ₂ reduction

   https://doi.org/10.1039/C9TA08862H  

   Pan, Fuping ; Zhang, Hanguang ; Liu, Zhenyu ; Cullen, David ; Liu, Kexi ; More, Karren ; Wu, Gang ; Wang, Guofeng ; Li, Ying ( November 2019 , Journal of Materials Chemistry A)

   Nickel and nitrogen co-doped carbon (Ni–N–C) has emerged as a promising catalyst for the CO 2 reduction reaction (CO 2 RR); however, the chemical nature of its active sites has remained elusive. Herein, we report the exploration of the reactivity and active sites of Ni–N–C for the CO 2 RR. Single atom Ni coordinated with N confined in a carbon matrix was prepared through thermal activation of chemically Ni-doped zeolitic imidazolate frameworks (ZIFs) and directly visualized by aberration-corrected scanning transmission electron microscopy. Electrochemical results show the enhanced intrinsic reactivity and selectivity of Ni–N sites for the reduction of CO 2 to CO, delivering a maximum CO faradaic efficiency of 96% at a low overpotential of 570 mV. Density functional theory (DFT) calculations predict that the edge-located Ni–N 2+2 sites with dangling bond-containing carbon atoms are the active sites facilitating the dissociation of the C–O bond of the *COOH intermediate, while bulk-hosted Ni–N 4 is kinetically inactive. Furthermore, the high capability of edge-located Ni–N 4 being able to thermodynamically suppress the competitive hydrogen evolution is also explained. The proposal of edge-hosed Ni–N 2+2 sites provides new insight into designing high-efficiency Ni–N–C for CO 2 reduction.
2. Understanding activity origin for the oxygen reduction reaction on bi-atom catalysts by DFT studies and machine-learning

   https://doi.org/10.1039/d0ta08004g  

   Deng, Chaofang ; Su, Yang ; Li, Fuhua ; Shen, Weifeng ; Chen, Zhongfang ; Tang, Qing ( December 2020 , Journal of Materials Chemistry A)

   Bi-atom catalysts (BACs) have attracted increasing attention in important electrocatalytic reactions such as the oxygen reduction reaction (ORR). Here, by means of density functional theory simulations coupled with machine-learning technology, we explored the structure–property correlation and catalytic activity origin of BACs, where metal dimers are coordinated by N-doped graphene (NC). We first sampled 26 homonuclear (M 2 /NC) BACs and constructed the activity volcano curve. Disappointingly, only one BAC, namely Co 2 /NC, exhibits promising ORR activity, leaving considerable room for enhancement in ORR performance. Then, we extended our study to 55 heteronuclear BACs (M 1 M 2 /NC) and found that 8 BACs possess competitive or superior ORR activity compared with the Pt(111) benchmark catalyst. Specifically, CoNi/NC shows the most optimal activity with a very high limiting potential of 0.88 V. The linear scaling relationships among the adsorption free energy of *OOH, *O and *OH species are significantly weakened on BACs as compared to a transition metal surface, indicating that it is difficult to precisely describe the catalytic activity with only one descriptor. Thus, we adopted machine-learning techniques to identify the activity origin for the ORR on BACs, which is mainly governed by simple geometric parameters. Our work notmore »only identifies promising BACs yet unexplored in the experiment, but also provides useful guidelines for the development of novel and highly efficient ORR catalysts.« less
3. High-performance fuel cell cathodes exclusively containing atomically dispersed iron active sites

   https://doi.org/10.1039/C9EE00877B  

   Zhang, Hanguang ; Chung, Hoon T. ; Cullen, David A. ; Wagner, Stephan ; Kramm, Ulrike I. ; More, Karren L. ; Zelenay, Piotr ; Wu, Gang ( August 2019 , Energy & Environmental Science)

   Platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) with atomically dispersed FeN 4 sites have emerged as a potential replacement for low-PGM catalysts in acidic polymer electrolyte fuel cells (PEFCs). In this work, we carefully tuned the doped Fe content in zeolitic imidazolate framework (ZIF)-8 precursors and achieved complete atomic dispersion of FeN 4 sites, the sole Fe species in the catalyst based on Mößbauer spectroscopy data. The Fe–N–C catalyst with the highest density of active sites achieved respectable ORR activity in rotating disk electrode (RDE) testing with a half-wave potential ( E 1/2 ) of 0.88 ± 0.01 V vs. the reversible hydrogen electrode (RHE) in 0.5 M H 2 SO 4 electrolyte. The activity degradation was found to be more significant when holding the potential at 0.85 V relative to standard potential cycling (0.6–1.0 V) in O 2 saturated acid electrolyte. The post-mortem electron microscopy analysis provides insights into possible catalyst degradation mechanisms associated with Fe–N coordination cleavage and carbon corrosion. High ORR activity was confirmed in fuel cell testing, which also divulged the promising performance of the catalysts at practical PEFC voltages. We conclude that the key factor behind the high ORR activity ofmore »the Fe–N–C catalyst is the optimum Fe content in the ZIF-8 precursor. While too little Fe in the precursors results in an insufficient density of FeN 4 sites, too much Fe leads to the formation of clusters and an ensuing significant loss in catalytic activity due to the loss of atomically dispersed Fe to inactive clusters or even nanoparticles. Advanced electron microscopy was used to obtain insights into the clustering of Fe atoms as a function of the doped Fe content. The Fe content in the precursor also affects other key catalyst properties such as the particle size, porosity, nitrogen-doping level, and carbon microstructure. Thanks to using model catalysts exclusively containing FeN 4 sites, it was possible to directly correlate the ORR activity with the density of FeN 4 species in the catalyst.« less
4. Atomically dispersed single Ni site catalysts for high-efficiency CO ₂ electroreduction at industrial-level current densities

   https://doi.org/10.1039/D2EE00318J  

   Li, Yi ; Adli, Nadia Mohd ; Shan, Weitao ; Wang, Maoyu ; Zachman, Michael J. ; Hwang, Sooyeon ; Tabassum, Hassina ; Karakalos, Stavros ; Feng, Zhenxing ; Wang, Guofeng ; et al ( May 2022 , Energy & Environmental Science)

   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. The 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 alsomore »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
5. Non-covalent assembly of proton donors and p- benzoquinone anions for co-electrocatalytic reduction of dioxygen

   https://doi.org/10.1039/D1SC01271A  

   Hooe, Shelby L. ; Cook, Emma N. ; Reid, Amelia G. ; Machan, Charles W. ( July 2021 , Chemical Science)

   The two-electron and two-proton p -hydroquinone/ p -benzoquinone (H 2 Q/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic aerobic oxidation reactions, redox flow batteries, and co-electrocatalytic oxygen reduction. Under nominally aprotic conditions in non-aqueous solvents, BQ can be reduced by up to two electrons in separate electrochemically reversible reactions. With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH) m ]˙ − . The solvation shell created by these interactions can mediate a second one-electron reduction coupled to proton transfer at more positive potentials ([BQ(AH) m ]˙ − + n AH + e − ⇌ [HQ(AH) (m+n)−1 (A)] 2− ), resulting in an overall two electron reduction at a single potential at intermediate acid concentrations. Here we show that hydrogen-bonded adducts of reduced quinones and the proton donor 2,2,2-trifluoroethanol (TFEOH) can mediate the transfer of electrons to a Mn-based complex during the electrocatalytic reduction of dioxygen (O 2 ). The Mn electrocatalyst is selective for H 2 O 2 with only TFEOH and O 2 present, however, with BQ present under sufficientmore »concentrations of TFEOH, an electrogenerated [H 2 Q(AH) 3 (A) 2 ] 2− adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H 2 O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded quinone anions can function in an analogous co-electrocatalytic manner to H 2 Q.« less