Electrocatalysis over Graphene-Defect-Coordinated Transition-Metal Single-Atom Catalysts

The family of single-atom catalysts dispersed in defective graphene matrices are now of increasing interest for a wide range of reactions given their tunable coordination environments and electronic structures. In this issue of Chem, Yao and co-workers demonstrate atomic [email protected] catalysts for water splitting via different active motifs. The family of single-atom catalysts dispersed in defective graphene matrices are now of increasing interest for a wide range of reactions given their tunable coordination environments and electronic structures. In this issue of Chem, Yao and co-workers demonstrate atomic [email protected] catalysts for water splitting via different active motifs. Transition-metal single-atom catalysts (SACs) are well known for their superb specific activity arising from the maximum atom efficiency, as well as their low-coordinated nature for the investigation of surface chemistry. Because a single atom typically represents a large surface free energy, metal oxide supports that have strong interactions with single atom species are generally used to better anchor and/or disperse SACs from aggregation. So far, these SACs have been widely reported in CO preferential oxidation,1Qiao B. Wang A. Yang X. Allard L.F. Jiang Z. Cui Y. Liu J. Li J. Zhang T. Single-atom catalysis of CO oxidation using Pt1/FeOx.Nat. Chem. 2011; 3: 634-641Crossref PubMed Scopus (3934) Google Scholar aldehyde hydrogenation,2Liu P. Zhao Y. Qin R. Mo S. Chen G. Gu L. Chevrier D.M. Zhang P. Guo Q. Zang D. et al.Photochemical route for synthesizing atomically dispersed palladium catalysts.Science. 2016; 352: 797-801Crossref PubMed Scopus (1229) Google Scholar methane partial oxidation,3Shan J. Li M. Allard L.F. Lee S. Flytzani-Stephanopoulos M. Mild oxidation of methane to methanol or acetic acid on supported isolated rhodium catalysts.Nature. 2017; 551: 605-608Crossref PubMed Scopus (392) Google Scholar and so on. Despite these continuous efforts with metal-oxide-supported SACs, defective graphene matrices have recently been of particular interest as SAC hosts in electrocatalysis because of their large surface area, high electron conductivity, chemical stability, and abundance of defect configurations with or without alien metalloid dopants (B, N, P, S, etc.) for potential metal-support coordination. Take the Fe–N–C SAC for example: it is widely studied as the alternative oxygen reduction reaction (ORR) catalyst in place of Pt, and the latest aberration-corrected scanning transmission electron microscopy (STEM) technique directly visualized and identified Fe-N4 species embedded in graphene layers as the active center.4Chung H.T. Cullen D.A. Higgins D. Sneed B.T. Holby E.F. More K.L. Zelenay P. Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst.Science. 2017; 357: 479-484Crossref PubMed Scopus (1004) Google Scholar Similar Co–Nx–C5Fei H. Dong J. Arellano-Jiménez M.J. Ye G. Dong Kim N. Samuel E.L. Peng Z. Zhu Z. Qin F. Bao J. et al.Atomic cobalt on nitrogen-doped graphene for hydrogen generation.Nat. Commun. 2015; 6: 8668Crossref PubMed Scopus (1189) Google Scholar and Ni–N4–C6Ju W. Bagger A. Hao G.-P. Varela A.S. Sinev I. Bon V. Roldan Cuenya B. Kaskel S. Rossmeisl J. Strasser P. Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO2.Nat. Commun. 2017; 8: 944Crossref PubMed Scopus (666) Google Scholar sites have also been demonstrated to effectively catalyze electrochemical hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR), respectively. Extended X-ray absorption fine structure (EXAFS) and microscopic analyses assigned direct metal–N rather than metal–C bonding as the predominant chemical environment within these SACs. In a recent Chem paper,7Jiang K. Siahrostami S. Akey A.J. Li Y. Lu Z. Lattimer J. Hu Y. Stokes C. Gangishetty M. Chen G. et al.Transition metal atoms in a graphene shell as active centers for highly efficient artificial photosynthesis.Chem. 2017; 3: 950-960Abstract Full Text Full Text PDF Scopus (273) Google Scholar our group reported single Ni atomic sites embedded in graphene vacancies as highly active and selective electrocatalysts for CO2-to-CO conversion. Three-dimensional atomic probe tomography of over ∼10,000 individual Ni single atoms clearly revealed that direct Ni–C coordination predominates within the graphene shell structure. Further density functional theory (DFT) calculations showed that the atomic Ni trapped in a graphene single vacancy (SV) and/or double vacancy (DV) (see Figure 1A) is responsible for weakening CO adsorption on Ni sites (leading to a more facile desorption and evolution) and suppressing HER side reactions. Given the wide variety of graphene defects,8To J.W.F. Ng J.W.D. Siahrostami S. Koh A.L. Lee Y.J. Chen Z.H. Fong K.D. Chen S.C. He J.J. Bae W.G. et al.High-performance oxygen reduction and evolution carbon catalysis: From mechanistic studies to device integration.Nano Res. 2017; 10: 1163-1177Crossref Scopus (59) Google Scholar more diverse metal–C moieties are expected to be employed in different catalytic applications. In this issue of Chem, Yao and co-workers first employed an impregnation method with acid leaching to prepare defective-graphene-supported Ni SACs with Ni loading up to 1.24%.9Zhang L. Jia Y. Gao G. Yan X. Chen N. Chen J. Soo M.T. Wood B. Yang D. Du A. et al.Defects on graphene trapping atomic Ni species for hydrogen and oxygen evolution reactions.Chem. 2018; 4: 285-297Abstract Full Text Full Text PDF Scopus (505) Google Scholar By calculating the formation energy, the authors identified several stable Ni–C coordination structures, including those trapped in graphene DVs (5775 defects [D5775] and perfect hexagons, as shown in Figure 1A), which were further reinforced by high-angle annular dark field (HAADF)-STEM imaging (Figures 1B–1D, [email protected]) plus X-ray adsorption spectroscopic characterization and fitting analysis. Electrochemical measurements demonstrated that the present Ni SAC exhibits superb full water-splitting performance (ηHER = 70 mV and ηOER = 270 mV, where OER is the oxygen evolution reaction) at a current density of 10 mA/cm2, analogous or even superior to that of benchmark Pt/C and IrO2 catalysts. Although both the earlier work7Jiang K. Siahrostami S. Akey A.J. Li Y. Lu Z. Lattimer J. Hu Y. Stokes C. Gangishetty M. Chen G. et al.Transition metal atoms in a graphene shell as active centers for highly efficient artificial photosynthesis.Chem. 2017; 3: 950-960Abstract Full Text Full Text PDF Scopus (273) Google Scholar and the current report9Zhang L. Jia Y. Gao G. Yan X. Chen N. Chen J. Soo M.T. Wood B. Yang D. Du A. et al.Defects on graphene trapping atomic Ni species for hydrogen and oxygen evolution reactions.Chem. 2018; 4: 285-297Abstract Full Text Full Text PDF Scopus (505) Google Scholar on [email protected] and [email protected] suggest suppressed HER activity over these two Ni–C sites, the presence of [email protected] (ca. 44 atom % by linear combination fitting of Ni near-edge structure) favors thermodynamic H2 evolution close to that on the Pt surface. Meanwhile, [email protected] sites (ca. 36 atom % content) exhibit an optimized binding strength for *O and *OOH active intermediates, thus boosting the overall OER activity. Last but not least, Ni SACs maintain good catalytic and structural stability even after long-term operation in an acidic solution of 0.5 M H2SO4, suggesting a strong interaction between Ni and its neighboring C atoms, which is quite similar to the well-studied stabilizing effect of metal-metal oxide. This is a vivid demonstration that the electronic structure of atomic Ni can be fine-tuned by bonding to diverse types of graphene defects through different target reactions. In fact, more two-dimensional materials beyond graphene can be employed as the proper host matrix for incorporating transition-metal single atoms, giving rise to an important platform for both fundamental mechanism studies in catalysis and a variety of applications in industry and the renewable energy field. Graphene Defects Trap Atomic Ni Species for Hydrogen and Oxygen Evolution ReactionsZhang et al.ChemJanuary 23, 2018In BriefAn integrated coordination structure composed of atomic Ni trapped in graphene defects is directly identified by probe-corrected TEM. The tuned electronic structure is considered to be the origin of enhanced oxygen evolution reaction and hydrogen evolution reaction. Full-Text PDF Open Archive