Graphdiyne Electrocatalyst

Electrochemical oxygen reduction with high intrinsic activity was demonstrated on a few-layer sp-hybridized N doped graphdiyne nanosheet in a new Nature Chemistry article. The synthesis mechanism of doped graphdiyne and intrinsic role of sp-hybridized N in the ORR process were elucidated, advancing development of high-performance metal-free carbon electrocatalysts for oxygen reduction. Electrochemical oxygen reduction with high intrinsic activity was demonstrated on a few-layer sp-hybridized N doped graphdiyne nanosheet in a new Nature Chemistry article. The synthesis mechanism of doped graphdiyne and intrinsic role of sp-hybridized N in the ORR process were elucidated, advancing development of high-performance metal-free carbon electrocatalysts for oxygen reduction. Growing energy demands and environmental concerns inspire scientists to search for sustainable and environmentally friendly energy technologies. Due to their high specific energies, fuel cells and metal-air batteries are two such promising technologies, where the electrochemical oxygen reduction reaction (ORR) occurs on the cathodes.1Debe M.K. Electrocatalyst approaches and challenges for automotive fuel cells.Nature. 2012; 486: 43-51Crossref PubMed Scopus (4231) Google Scholar Unfortunately, the electrocatalytic ORR process is, in practice, extremely sluggish owing to the large kinetic energy barriers caused by its multi-electron reaction pathways.2Seh Z.W. Kibsgaard J. Dickens C.F. Chorkendorff I. Nørskov J.K. Jaramillo T.F. Combining theory and experiment in electrocatalysis: Insights into materials design.Science. 2017; 355https://doi.org/10.1126/science.aad4998Crossref PubMed Scopus (5259) Google Scholar In this regard, to enhance the energy conversion efficiencies of fuel cells and metal-air batteries, electrocatalysts are of pivotal significance for accelerating the sluggish ORR kinetic process. Currently, platinum-based catalysts have been widely reported as high-activity ORR electrocatalysts.3Huang X. Zhao Z. Cao L. Chen Y. Zhu E. Lin Z. Li M. Yan A. Zettl A. Wang Y.M. et al.ELECTROCHEMISTRY. High-performance transition metal-doped Pt3Ni octahedra for oxygen reduction reaction.Science. 2015; 348: 1230-1234Crossref PubMed Scopus (1389) Google Scholar However, the high cost and scarcity of platinum seriously hamper its large-scale utilization in fuel cells and metal-air batteries. In 2009, nitrogen-doped carbon nanotube arrays (NCNT) were first reported as a metal-free ORR electrocatalyst in alkaline electrolytes.4Gong K. Du F. Xia Z. Durstock M. Dai L. Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction.Science. 2009; 323: 760-764Crossref PubMed Scopus (6056) Google Scholar Afterward, owing to their low costs, large specific surface areas, abundant porous structures, excellent electrical conductivities, and superior chemical and structural stabilities, various metal-free carbon nanomaterials have been developed as ORR electrocatalysts.5Liu R. Wu D. Feng X. Müllen K. Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction.Angew. Chem. Int. Ed. 2010; 49: 2565-2569Crossref PubMed Scopus (1194) Google Scholar In principal, defect-free carbon skeleton is inert for electrocatalytic ORR, whereas partially substituting carbon atoms with heteroatoms (N, S, P, or B) is a viable strategy for modulating the electronic properties of the carbon skeleton.6Dai L. Xue Y. Qu L. Choi H.-J. Baek J.-B. Metal-free catalysts for oxygen reduction reaction.Chem. Rev. 2015; 115: 4823-4892Crossref PubMed Scopus (1857) Google Scholar As a result of electronegativity difference between heteroatoms and neighboring carbon atoms in the sp2-carbon frameworks, doping heteroatoms can cause the re-arrangement of charge distribution, thus activating the neighboring carbon atoms into active centers for catalyzing the oxygen reduction. Among diversified heteroatoms, nitrogen dopants render carbon materials with the highest ORR activity, which is superior even to those for commercial Pt/C catalysts.5Liu R. Wu D. Feng X. Müllen K. Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction.Angew. Chem. Int. Ed. 2010; 49: 2565-2569Crossref PubMed Scopus (1194) Google Scholar Generally, the nitrogen dopants in the carbon skeletons exist as pyridinic N, graphitic N, pyrrolic N, and oxidized N. In 2016, well-tailored graphite model electrocatalysts with defined N species (pyridinic N, pyrrolic N, or graphitic N) were fabricated, which revealed that pyridinic N is responsible for activating the neighboring carbon into active sites for the ORR.7Guo D. Shibuya R. Akiba C. Saji S. Kondo T. Nakamura J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts.Science. 2016; 351: 361-365Crossref PubMed Scopus (2928) Google Scholar Nevertheless, developing carbon electrocatalysts with dense pyridinic N or a new type of N with large negative charge, to date, still remains a grand challenge. In a recent work published in Nature Chemistry, Yang, Zhu, Li, Wang, and co-workers first reported a novel sp-hybridized nitrogen (sp-N) doped graphdiyne sheet (NFLGDY) as the metal-free carbon electrocatlyast.8Zhao Y. Wan J. Yao H. Zhang L. Lin K. Wang L. Yang N. Liu D. Song L. Zhu J. et al.Few-layer graphdiyne doped with sp-hybridized nitrogen atoms at acetylenic sites for oxygen reduction electrocatalysis.Nat. Chem. 2018; (Published August 6, 2018)https://doi.org/10.1038/s41557-018-0100-1Crossref Scopus (428) Google Scholar, 9Jia Z. Li Y. Zuo Z. Liu H. Huang C. Li Y. Synthesis and properties of 2D carbon-graphdiyne.Acc. Chem. Res. 2017; 50: 2470-2478Crossref PubMed Scopus (354) Google Scholar, 10Zhang T. Hou Y. Dzhagan V. Liao Z. Chai G. Löffler M. Olianas D. Milani A. Xu S. Tommasini M. et al.Copper-surface-mediated synthesis of acetylenic carbon-rich nanofibers for active metal-free photocathodes.Nat. Commun. 2018; 9: 1140Crossref PubMed Scopus (92) Google Scholar In comparison with pyridinic N, the stronger electronegativity of the sp-N atoms renders their neighboring carbon sites with more positive charge, which facilitates the chemisorption of O2 molecules during the electrochemical ORR. Under alkaline condition, the sp-N doped graphdiyne exhibited excellent ORR performance with a high half-wave potential of 0.87 V, which was higher than 0.86 V for Pt/C catalyst and comparable to those values for the state-of-the-art metal-free carbon electrocatalysts (Figures 1A and 1B ). The electron transfer number of the sp-N doped graphdiyne reached ∼3.9 at 0.65–0.8 V, presenting a 4e− oxygen reduction kinetics (Figure 1C). To fabricate the sp-N doped graphdiyne, the authors used few-layer oxidized graphdiyne (FLGDYO) as the carbon skeletons (Figure 2). The synthesis mechanism of the NFLGDY was proposed as follows: when a mixture of FLGDYO and melamine was thermally treated, the sp-N atoms were doped into specific sites of few-layer oxidized graphdiyne (FLGDYO) through a pericyclic replacement of the acetylene groups. Specially, (i) upon calcination at 900°C, the NHCNH2+ fragments generated from melamine bonded with the acetylene groups of FLGDYO and thus a five-membered carbon–nitrogen heterocycle was formed; (ii) induced by high temperature, the five-membered carbon–nitrogen heterocycle rearranged; and (iii) as a result of intense thermal vibrations and the existence of radicals, the H2C = N = CH2+ fragment was dissociated from the five-membered carbon–nitrogen heterocycle. Eventually, the sp-N doped graphdiyne was achieved. X-ray absorption near-edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) indicated the co-existence of sp-N atoms with the pyridinic N, amino N, and graphitic N atoms in NFLGDY. In order to reveal the correlation of various N types with electrochemical ORR activity, the authors tuned the contents of pyridinic N and sp-N atoms in the FLGDY by varying calcination temperature from 700°C, 800°C, and 900°C. Accordingly, the content of sp-N atoms increased from 0 at% for NFLGDY-700 to 0.35 at% for NFLGDY-800 and 1.36 at% for NFLGDY-900, while the content of pyridinic N atoms correspondingly decreased from 3.84 at% for NFLGDY-700 to 2.45 at% for NFLGDY-800 and 2.12 at% for NFLGDY-900. Under alkaline condition, along with increased content of sp-N atoms, the ORR peak potential of the NFLGDY positively shifted, suggesting an enhanced ORR activity even while the content of pyridinic N atoms decreased. Therefore, the sp-N atoms rather than pyridinic N atoms were claimed to play a dominant role in catalyzing the ORR. To clearly compare the ORR activities of NFLGDY samples with different proportions of sp-N atoms, the cathodic current density at a potential of 0.65 V was analyzed. As revealed in Figure 1D, the ORR current density augmented with increased concentration of sp-N atoms. In addition, FLGDY-900, FLGDYO treated with ammonia, and N-doped graphene showed lower catalytic activity than NFLGDY. These results indicated that the sp-N acted as the most active N type for enhancing the ORR activity of metal-free carbon materials. Next, the authors employed theoretical calculations to probe the underlying mechanism that the sp-N atoms improved the ORR activity of NFLGDY. The calculated Bader effective charges of various N species in NFLGDY confirmed that sp-N atoms were the most beneficial for O2 adsorption. Meanwhile, in comparison with the pyridinic N (3.29 eV) and graphitic N atom (3.96 eV), the oxygen binding energies (Δ EO) of the sp-N atoms were 2.13 and 2.51 eV, which is well located at the optimal ranges from ∼1.81 to 2.88 eV. This result suggests a modest bonding strength for the adsorption/dissociation/desorption of O-containing species during the ORR process. The authors have highlighted a novel sp-hybridized nitrogen type in 2D FLGDY. Theoretical and experimental investigations demonstrated that the sp-N atom is the most beneficial active site for O2 adsorption and showed the highest ORR performance in alkaline solution among different N types of metal-free carbon electrocatalysts. The synthesis strategy for doping sp-N atoms into carbon skeletons and the fundamental understandings of the doping mechanism may open up a new avenue for pursuing novel carbon and carbon-rich electrocatalysts with defined molecular structures and high-activity centers. Toward these targets, four major challenges remain: (1) for revealing the intrinsic role of N dopants and exploring carbon nanomaterials with single N types, e.g., pyridinic N, graphitic N, or sp-hybridized N is essential; (2) experimentally unveiling the ORR pathway using operando characterizations; (3) enhancing the ORR activity in acidic solution by design of well-defined structures and active centers; and (4) evaluating the practical applications of carbon electrocatalysts in the real fuel cells and metal-air batteries.