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Journal Articles

Mechanisms of oxygen reduction reactions for carbon alloy catalysts via first principles molecular dynamics

Ikeda, Takashi; Hou, Z.*; Chai, G.-L.*; Terakura, Kiyoyuki*

Hyomen Kagaku, 36(7), p.345 - 350, 2015/07

Carbon alloy catalysts (CACs) are one of promising candidates for platinum-substitute cathode catalysts for polymer electrolyte fuel cells. We have investigated possible mechanisms of oxygen reduction reactions (ORRs) for CACs via first-principles-based molecular dynamics simulations. In this contribution, we review possible ORRs at likely catalytic sites of CACs suggested from our simulations.

Journal Articles

Active sites and mechanisms for oxygen reduction reaction on nitrogen-doped carbon alloy catalysts; Stone-Wales defect and curvature effect

Chai, G.-L.*; Hou, Z.*; Shu, D.-J.*; Ikeda, Takashi; Terakura, Kiyoyuki*

Journal of the American Chemical Society, 136(39), p.13629 - 13640, 2014/10

 Times Cited Count:198 Percentile:97.69(Chemistry, Multidisciplinary)

Carbon alloy catalysts (CACs) are promising catalysts for oxygen reduction reaction (ORR) to substitute Pt. However, despite extensive studies on CACs the reaction sites and mechanisms for ORR are still in controversy. Herein, we present rather general consideration on possible ORR mechanisms for various structures in nitrogen doped CACs based on the first principles calculations. Our study indicates that only a particular structure of a nitrogen pair doped Stone-Wales defect provides a good active site. The ORR activity of this structure can be tuned by the curvature around the active site, which makes its limiting potential approaching the maximum limiting potential (0.80 V) in the volcano plot for the ORR activity of CACs. The calculated results can be compared with the recent experimental ones of the half wave potential for CAC systems that range from 0.60 V to 0.80 V in the reversible-hydrogen-electrode scale.

Journal Articles

Possible oxygen reduction reactions for graphene edges from first principles

Ikeda, Takashi; Hou, Z.*; Chai, G.-L.*; Terakura, Kiyoyuki*

Journal of Physical Chemistry C, 118(31), p.17616 - 17625, 2014/08

 Times Cited Count:41 Percentile:81.59(Chemistry, Physical)

N-doped carbon-based nanomaterials are attracting a great interest as promising Pt-free electrode catalysts for polymer electrolyte fuel cells (PEFCs). In this computational study, we demonstrate that N-doped graphene edges can exhibit enhanced catalytic activity toward oxygen reduction reactions by controlling their electron-donating and -withdrawing abilities, and basicity, resulting in higher selectivity of 4e$$^{-}$$ reduction via inner and outer sphere electron transfer at edges in acidic conditions, respectively. Our simulations also show that 2e$$^{-}$$ reduction occurs selectively in the presence of pyridinic N next to carbonyl O at zigzag edges. This study thus rationalizes the roles of doped N in graphenelike materials for oxygen reduction reactions.

Journal Articles

Interplay between oxidized monovacancy and nitrogen doping in graphene

Hou, Z.*; Shu, D.-J.*; Chai, G.-L.*; Ikeda, Takashi; Terakura, Kiyoyuki*

Journal of Physical Chemistry C, 118(34), p.19795 - 19805, 2014/08

 Times Cited Count:10 Percentile:38.87(Chemistry, Physical)

In most of the N-doped graphene which attracts strong attention in the context of precious-metal free catalysts and nanoelectronics, the oxygen content is generally higher than or at least comparable to the nitrogen content. We perform density functional theory calculations to study the interplay of oxidized monovacancies and the nitrogen doping, motivated by the fact that MV is more frequently observed and more chemically active than divacancy and Stone-Wales defect. We determine the phase diagrams of un-doped and nitrogen-doped oxidized MVs as a function of temperature and partial pressure of O$$_{2}$$ and H$$_{2}$$ gases. The modification of the electronic structure of MV by oxidation and N doping is studied. Our results show that the ether group is a common component in stable configurations of oxidized MVs. Most of the stable configurations of oxidized MVs do not induce any carriers.

Journal Articles

NMR chemical shifts of $$^{15}$$N-bearing graphene

Wang, X.*; Hou, Z.*; Ikeda, Takashi; Terakura, Kiyoyuki*

Journal of Physical Chemistry C, 118(25), p.13929 - 13935, 2014/06

 Times Cited Count:8 Percentile:32.69(Chemistry, Physical)

The NMR chemical shifts of possible N-containing moieties at edges and defects of graphene are investigated by using the first-principles method. Our computations show that pyridine-like and graphite-like N are rather easily identifiable using $$^{15}$$N NMR technique, in agreement with experiment. On the other hand, pyridinium-like N is hardly distinguished from pyrrole-like one because these $$^{15}$$N nuclei give nearly overlapping signals. However, our simulations suggest that $$^{1}$$H NMR is useful to discriminate between them; The NMR chemical shift of $$^{1}$$H directly bonded with pyridinium-like and pyrrole-like N is estimated as 0.8 and 10.8 ppm, respectively. The $$^{15}$$N NMR signals for various moieties at edges we considered are found to be similar to the corresponding ones at defects except for pyridine-like nitrogens. Conversely, the $$^{15}$$N NMR chemical shifts are altered sensitively by the degree of aggregation of pyridine-like $$^{15}$$N atoms both along armchair edges and at defect sites.

Journal Articles

Electronic structures of N-doped graphene with native point defects

Hou, Z.*; Wang, X.*; Ikeda, Takashi; Terakura, Kiyoyuki*; Oshima, Masaharu*; Kakimoto, Masaaki*

Physical Review B, 87(16), p.165401_1 - 165401_16, 2013/04

 Times Cited Count:92 Percentile:95.58(Materials Science, Multidisciplinary)

We have performed the DFT calculations to study the electronic structures of N-doped graphene with vacancies and Stone-Wales defect. Our results show that monovacancies in graphene act as hole dopants and that two substitutional N dopants are needed to compensate for the hole introduced by a monovacancy. On the other hand, divacancy does not produce any free carriers. Interestingly, a single N dopant at divacancy acts as an acceptor rather than a donor. Compared with the case of an isolated N dopant in perfect graphene, the electrons donated by substitutional N dopants would be localized significantly when a N-N pair is formed. The N-N interaction and the interference between native point defect and N dopant strongly modify the role of N doping regarding the free carrier production in the bulk $$pi$$ bands. Our results are qualitatively consistent with the experimental observation that the concentration of free electrons introduced by N dopants would be lower than that of doped N.

Journal Articles

Probing carbon edge exposure of iron phthalocyanine-based oxygen reduction catalysts by soft X-ray absorption spectroscopy

Niwa, Hideharu*; Saito, Makoto*; Kobayashi, Masaki*; Harada, Yoshihisa*; Oshima, Masaharu*; Moriya, Shogo*; Matsubayashi, Katsuyuki*; Nabae, Yuta*; Kuroki, Shigeki*; Ikeda, Takashi; et al.

Journal of Power Sources, 223, p.30 - 35, 2013/02

 Times Cited Count:17 Percentile:54.86(Chemistry, Physical)

To design non-platinum, inexpensive, but high performance carbon-based cathode catalysts for polymer electrolyte fuel cells, it is important to elucidate the active site for oxygen reduction reaction (ORR). However, it is difficult to directly identify the active site by applying conventional structural or electronic probes to such complex systems. Here, we used C 1${it s}$ X-ray absorption spectroscopy (XAS) to observe electronic structure of carbon in iron phthalocyanine-based catalysts, and found a signature of edge exposure below the $$pi^{ast}$$ edge, whose intensity is well correlated with the ORR activity. These results demonstrate that C 1${it s}$ XAS can be used to characterize the ORR activity of carbon-based cathode catalysts in terms of the edge exposure.

Journal Articles

Theoretical characterization of X-ray absorption, emission, and photoelectron spectra of nitrogen doped along graphene edges

Wang, X.*; Hou, Z.*; Ikeda, Takashi; Oshima, Masaharu*; Kakimoto, Masaaki*; Terakura, Kiyoyuki*

Journal of Physical Chemistry A, 117(3), p.579 - 589, 2013/01

 Times Cited Count:29 Percentile:76.5(Chemistry, Physical)

${it K}$-edge X-ray absorption (XAS), emission (XES), and photoelectron (XPS) spectra of nitrogen doped along graphene edges are systematically investigated by using first-principles methods. In this study we considered pyridinium-like, pyridine-like, cyanide-like, and aminelike nitrogens at armchair and zigzag edges and pyrrole-like nitrogen at armchair edge as well as graphite-like nitrogen at graphene interior site. Our results indicate that nitrogen configuration and its location (armchair or zigzag edge) in nitrogen-doped graphene can be identified via the spectral analysis. Furthermore, some controversial spectral features observed in experiment for N-doped graphene-like materials are unambiguously assigned. The present analysis gives an explanation to the reason why the peak assignment is usually made differently between XPS and XAS.

Journal Articles

Nitrogen doping in graphene; Structure and electronic states

Terakura, Kiyoyuki*; Hou, Z.*; Wang, X.*; Ikeda, Takashi

Gurafen No Kino To Oyo Tembo, 2, p.26 - 39, 2012/12

no abstracts in English

Journal Articles

Study on the oxygen adsorption property of nitrogen-containing metal-free carbon-based cathode catalysts for oxygen reduction reaction

Kiuchi, Hisao*; Niwa, Hideharu*; Kobayashi, Masaki*; Harada, Yoshihisa*; Oshima, Masaharu*; Chokai, Masayuki*; Nabae, Yuta*; Kuroki, Shigeki*; Kakimoto, Masaaki*; Ikeda, Takashi; et al.

Electrochimica Acta, 82(1), p.291 - 295, 2012/10

 Times Cited Count:12 Percentile:34.28(Electrochemistry)

We study the characteristics of oxygen adsorption on metal-free carbon-based cathode catalysts derived from nitrogen-containing polyamide (PA) and nitrogen-free phenolic resin (PhRs). Electrochemical analysis and Raman spectroscopy showed higher 2-electron oxygen reduction reaction (ORR) activity and more defect sites in PA than PhRs. The increase in the amount of adsorbed oxygen in PA was also identified by oxygen adsorption isotherms. ${it In-situ}$ X-ray photoelectron spectroscopy reveals that graphite-like nitrogen contributes to oxygen adsorption and C=O components are dominant in PA. These experimental results indicate that the adsorbed C=O components near the graphite-like nitrogen can be assigned as active sites for 2-electron ORR.

Journal Articles

Indirect contribution of transition metal towards oxygen reduction reaction activity in iron phthalocyanine-based carbon catalysts for polymer electrolyte fuel cells

Kobayashi, Masaki*; Niwa, Hideharu*; Saito, Makoto*; Harada, Yoshihisa*; Oshima, Masaharu*; Ofuchi, Hironori*; Terakura, Kiyoyuki*; Ikeda, Takashi; Koshigoe, Yuka*; Ozaki, Junichi*; et al.

Electrochimica Acta, 74, p.254 - 259, 2012/07

 Times Cited Count:48 Percentile:82.87(Electrochemistry)

The electronic structure of the residual metal atoms in FePc-based carbon catalysts, prepared by pyrolyzing a mixture of FePc and phenolic resin polymer at 800$$^{circ}$$C, before and after acid washing have been investigated using XAFS spectroscopy to clarify the role of Fe in the ORR activity. The decomposition analyses for the XAFS spectra reveal that the composition ratio of each Fe component is unaltered by the acid washing, indicating that the residual Fe components were removed by the acid washing irrespective of their chemical states. Because the oxygen reduction potential was approximately unchanged by the acid washing, the residual Fe itself does not seem to contribute directly to the ORR activity. The residual Fe can act as a catalyst to accelerate the growth of the sp$$^{2}$$ carbon network during pyrolysis. The results imply that light elements are components of the ORR active sites in the FePc-based carbon catalysts.

Journal Articles

Interplay between nitrogen dopants and native point defects in graphene

Hou, Z.*; Wang, X.*; Ikeda, Takashi; Terakura, Kiyoyuki*; Oshima, Masaharu*; Kakimoto, Masaaki*; Miyata, Seizo*

Physical Review B, 85(16), p.165439_1 - 165439_9, 2012/04

 Times Cited Count:103 Percentile:96.03(Materials Science, Multidisciplinary)

To understand the interaction between nitrogen dopants and point defects in graphene, we have studied energetic stability of N-doped graphene with vacancies and Stone-Wales defect by performing the density functional theory calculations. Our results show that N substitution energetically prefers to occur at the carbon atoms near the defects, especially for those sites with larger bond shortening, indicating that the defect-induced strain plays an important role in the stability of N dopants. In the presence of mono-vacancy, the most stable position for N is the pyridine-like configuration while for other point defects studied N prefers a site in the pentagonal ring. While the N doping is endothermic in defect-free graphene, it becomes exothermic for defective one. Our results imply that the point defect and N dopant attract each other, which means that substitutional N dopants would increase the probability of point defect generation and vice versa.

Journal Articles

Selective nitrogen doping in graphene; Enhanced catalytic activity for the oxygen reduction reaction

Wang, X.*; Hou, Z.*; Ikeda, Takashi; Huang, S.-F.*; Terakura, Kiyoyuki*; Boero, M.*; Oshima, Masaharu*; Kakimoto, Masaaki*; Miyata, Seizo*

Physical Review B, 84(24), p.245434_1 - 245434_7, 2011/12

 Times Cited Count:30 Percentile:77.86(Materials Science, Multidisciplinary)

The structural and electronic properties of N-doped zigzag graphene ribbons with various ratios of di- to monohydrogenated edge carbons are investigated within the density functional theory framework. We find that the stability of graphitic N next to the edge, which is claimed to play important roles in the catalytic activity in our previous work, will be enhanced with increasing the concentration of di-hydrogenated carbons. Furthermore, the di-hydrogenated edge carbons turn out to be easily converted into mono-hydrogenated ones in the presence of oxygen molecules at room temperature. Based on our results, we propose a possible way to enhance the oxygen reduction catalytic activity of N-doped graphene by controlling the degrees of hydrogenation of edge carbons. The characteristic features in the X-ray absorption and emission spectra for each specific N site considered here will also be given.

Journal Articles

Role of residual transition-metal atoms in oxygen reduction reaction in cobalt phthalocyanine-based carbon cathode catalysts for polymer electrolyte fuel cell

Kobayashi, Masaki*; Niwa, Hideharu*; Harada, Yoshihisa*; Horiba, Koji*; Oshima, Masaharu*; Ofuchi, Hironori*; Terakura, Kiyoyuki*; Ikeda, Takashi; Koshigoe, Yuka*; Ozaki, Junichi*; et al.

Journal of Power Sources, 196(20), p.8346 - 8351, 2011/10

 Times Cited Count:32 Percentile:70.76(Chemistry, Physical)

The electronic structure of Co atoms in CoPc-based carbon catalysts, which were prepared by pyrolyzing a mixture of CoPc and phenol resin polymer up to 1000$$^circ$$C, has been investigated using XAFS analysis and HXPES. The Co K XAFS spectra show that most of the Co atoms are in the metallic state and small quantities of oxidized Co components are present in the samples even after acid washing to remove Co atoms. Based on the difference in probing depth between XAFS and HXPES, it was found that after acid washing, the surface region with the aggregated Co clusters is primarily composed of metallic Co. Since the electrochemical properties remain almost unchanged even after the acid washing process, the residual metallic and oxidized Co atoms themselves will hardly contribute to the ORR activity of the CoPc-based carbon cathode catalysts, implying that the active sites of the CoPc-based catalysts primarily consist of light elements such as C and N.

Journal Articles

Effect of hydrogen termination on carbon $$K$$-edge X-ray absorption spectra of nanographene

Hou, Z.*; Wang, X.*; Ikeda, Takashi; Huang, S.-F.*; Terakura, Kiyoyuki*; Boero, M.*; Oshima, Masaharu*; Kakimoto, Masaaki*; Miyata, Seizo*

Journal of Physical Chemistry C, 115(13), p.5392 - 5403, 2011/03

 Times Cited Count:36 Percentile:74.9(Chemistry, Physical)

Carbon ${it K}$-edge X-ray absorption spectra of nanographene have been simulated by density functional theory calculations in order to obtain the information on the edge termination by hydrogen. Our results show that different edge terminations significantly affect the binding energy of 1s core-level of C atoms in the vicinity of edges because of the change in chemical bonding and the localized edge states. We find that a shoulder or a peak appears below the $$pi^{ast}$$ peak at relatively different positions with respect to the $$pi^{ast}$$ peak position in the theoretical spectra of zigzag graphene nano-ribbons, depending on the ratio of mono-hydrogen- to di-hydrogen-terminations. We also point out that the two additional features observed between the $$pi^{ast}$$ and $$sigma^{ast}$$ peaks of an ideal graphene originate from the $$sigma^{ast}$$ states of C-H bonding and C-H$$_{2}$$ bonding at the edges.

Journal Articles

X-ray photoemission spectroscopy analysis of N-containing carbon-based cathode catalysts for polymer electrolyte fuel cells

Niwa, Hideharu*; Kobayashi, Masaki*; Horiba, Koji*; Harada, Yoshihisa*; Oshima, Masaharu*; Terakura, Kiyoyuki*; Ikeda, Takashi; Koshigoe, Yuka*; Ozaki, Junichi*; Miyata, Seizo*; et al.

Journal of Power Sources, 196(3), p.1006 - 1011, 2011/02

 Times Cited Count:80 Percentile:92(Chemistry, Physical)

We report on the electronic structure of three different types of N-containing carbon-based cathode catalysts for polymer electrolyte fuel cells observed by hard X-ray photoemission spectroscopy. C 1s spectra show the importance of $$sp^{2}$$ carbon network formation for the oxygen reduction reaction (ORR) activity. Samples having high oxygen reduction reaction activity in terms of oxygen reduction potential contain high concentration of graphite-like nitrogen. Based on a quantitative analysis of our results, the oxygen reduction reaction activity of the carbon-based cathode catalysts will be improved by increasing concentration of graphite-like nitrogen in a developed $$sp^{2}$$ carbon network.

Journal Articles

Enhanced catalytic activity of Carbon Alloy Catalysts codoped with boron and nitrogen for oxygen reduction reaction

Ikeda, Takashi; Boero, M.*; Huang, S.-F.*; Terakura, Kiyoyuki*; Oshima, Masaharu*; Ozaki, Junichi*; Miyata, Seizo*

Journal of Physical Chemistry C, 114(19), p.8933 - 8937, 2010/05

 Times Cited Count:58 Percentile:84.82(Chemistry, Physical)

Carbon Alloy Catalysts (CACs) have been attracting a growing interest as potential Pt-free electrode catalysts for polymer electrolyte fuel cell. In this computational study, we inspect possible oxygen adsorption and reduction processes on various models for exposed edges of these catalysts via first principles molecular dynamics. Our simulations suggest that the codoping of boron and nitrogen in CACs is a promising route to further enhancement of their catalytic activity with respect to both stability and reactivity.

Journal Articles

Mechanisms of catalytic activity for carbon alloy catalysts

Ikeda, Takashi; Huang, S.-F.*; Boero, M.*; Terakura, Kiyoyuki*

Hakkin Daitai Kabon Aroi Shokubai, p.121 - 138, 2010/04

no abstracts in English

Journal Articles

First-principles calculation of the electronic properties of graphene clusters doped with nitrogen and boron; Analysis of catalytic activity for the oxygen reduction reaction

Huang, S.-F.*; Terakura, Kiyoyuki*; Ozaki, Taisuke*; Ikeda, Takashi; Boero, M.*; Oshima, Masaharu*; Ozaki, Junichi*; Miyata, Seizo*

Physical Review B, 80(23), p.235410_1 - 235410_12, 2009/12

 Times Cited Count:159 Percentile:97.5(Materials Science, Multidisciplinary)

Recent studies suggest that the carbon-alloy catalyst with doped nitrogen may be a powerful candidate for cathode catalyst of fuel cell. In this paper, we aim to clarify the microscopic mechanisms of the enhancement in the catalyst activity caused by nitrogen doping using a simple graphene cluster model. We analyze modifications in the electronic structures and the energetical stability for some different configurations of N doping. We extend the analysis to the case of co-doping of nitrogen and boron and propose two possible scenarios explaining the further enhancement of catalytic activity by N and B co-doping.

Journal Articles

Electronic states and catalytic activity of doped graphene

Ikeda, Takashi; Huang, S.-F.*; Boero, M.*; Terakura, Kiyoyuki*

Gurafen No Kino To Oyo Tembo, p.46 - 59, 2009/07

no abstracts in English

39 (Records 1-20 displayed on this page)