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Graphene, a two-dimensional layer framework of sp 2 -hybridized carbon, has attracted tremendous attentions as a promising material for oxygen reduction reactions (ORRs) due to its ideal uniqueness of high surface area and electrical conductivity. Contrary to the general belief, however, graphene catalysts have shown poor catalytic activities compared with those of other carbon-based catalysts. Herein, to understand the low ORR kinetics on the graphene catalysts, the dimensionality of graphene catalysts is sequentially tuned from sheets (2D) to ribbons (1D) and to dots (0D), and then the accompanying changes in terms of physical and electrochemical properties are investigated. In ultraviolet photoelectron spectroscopy, an increment in electropotential is measured as the dimensionality of the graphene catalysts decreases, of which the result infers the enhanced kinetics of the electron transfer from the graphene catalysts to O 2 (electropotential: 0D>1D>2D). However, ORR performance does not follow the order of electropotential, and the graphene ribbons show the best activity among the prepared graphene catalysts (ORR activity: 1D>0D>2D). Further electrochemical impedance spectroscopy studies demonstrate that ORR kinetics is primarily determined by charge transfer rates in the fabricated graphene electrodes, which are strongly related to the electrode configurations and thus also to the length-to-width ratios of the graphene catalysts. The graphene sheets and dots, of which the length-to-width ratios are almost 1, handily lay over each other, leading to an impedance of efficient charge transfers. This study suggests the importance of void channels in the fabricated graphene electrode, which have not previously been considered significantly as a factor for improving ORR activity on the graphene catalysts. [ABSTRACT FROM AUTHOR]

Abstract: As a cathode material for fuel generation from CO2 reduction in a photoelectrochemical system, layered CuO/Cu2O films were developed and their surfaces were decorated with transition metals (i.e. Ag, Au, Cd, Cu, Pb, and Sn). Deposition of the transition metals effectively enhanced CO2 conversion to fuel in terms of faradaic efficiency. In particular, Pb/CuO/Cu2O demonstrated outstanding performance among the transition metals: 0.524μmol/hcm2 for formic acid and 0.102μmol/hcm2 for methanol with 40.45% of total faradaic efficiency at −0.16V (vs. SHE), which was a higher potential than standard redox potentials of formic acid and methanol formation from CO2. Moreover, electrochemical impedance spectroscopy (EIS) showed that the deposition of the transition metals onto CuO/Cu2O electrode effectively generated photo-induced electron–hole pairs under visible light irradiation. However, fast performance degradations of the prepared electrodes were observed during the reactions as well as the disappearances of the photocurrents. X-ray photoelectron spectroscopy (XPS) results revealed that the outer CuO layer was readily reduced to Cu2O/Cu, and this compositional destruction was responsible for the degradation of the photocurrent by prohibiting transfers of the electrons (or holes) to the active sites. [Copyright &y& Elsevier]

Abstract: A novel synthesis of N-doped carbon with not only a high surface area (∼1000m2 g−1) but also with a controlled amount of N-doping is reported from the solvothermal reduction of hexachlorobenzene (HCB) and pentachloropyridine (PCP), without the emission of harmful byproducts. In the presence of metallic sodium as a reducing agent, the N-doping amount can be regulated up to 0.12 of N/C atomic ratio by simply altering the initial HCB and PCP ratios. The mechanism is proposed where the chlorine in the HCB and PCP is reacted with metallic sodium by producing NaCl, and the N-doped carbon is synthesized as the activated carbon edges of C5N and C6 rings being bonded together. The surfaces of the prepared N-doped carbons are modified through heat-treatment and this dramatically improves the mechanical and electrical properties. The dominant doping phases of N are pyridinic-N and amide or amine groups; however, the amide or amine groups are eliminated and graphitic-N is newly generated through heat-treatment. [Copyright &y& Elsevier]

Abstract: N-doped carbon was prepared from dicyandiamide as the source for both carbon and nitrogen. Dicyandiamide was pyrolyzed at 900°C on various metal chlorides (MeCl2·xH2O, Me=Co, Fe, and Ni) to clarify the effects of metal type on the properties of the N-doped carbon. The results showed that electrochemical and physical characteristics of the N-doped carbon were altered by the type of metal seed. The N-doped carbon from Co showed the best performance in oxygen reduction reactions (ORRs) and the order of ORR activity was N-doped carbon from Co>Fe>Ni. Raman and XPS studies revealed that the metal type during pyrolysis step determined degree of sp 2-carbon network, and ORR activity of N-doped carbon was improved as the degree of sp 2-carbon network increased. The effects of the amount of metal precursor on activity of ORRs and rate of H2O2 formation were faint but the carbonization yield of dicyandiamide increased as the amount of metal precursor increased. The catalysts optimized by altering the type and amount of metal seed showed a 0.60V (vs. Ag/AgCl) onset potential and a 42% oxygen reduction reactivity at 0.40V (vs. Ag/AgCl) compared to that of a commercial Pt/C catalyst. [Copyright &y& Elsevier]

Abstract: In order to develop cheap electrochemical oxygen reduction reaction (ORR) catalysts, N-doped CNTs grafted on Vulcan carbon were synthesized via pyrolysis of dicyandiamide on Fe2O3/C. Various contents of iron in Fe2O3/C (0, 20, 40 and 60 wt. %) were used as supports to investigate the effects and roles of iron content on ORR. It was shown that the iron acted as a promoter for doping nitrogen into carbon; as the iron content increased, the amount of nitrogen doping also increased. TEM and element analysis results indicated that iron induced growth of CNTs and facilitated N-doping in carbon. However, further increase in iron content higher than 20 wt. % showed negative effects on the ORR activity due to a decrease of the surface area of the prepared catalysts. Hence, the catalyst with the highest performance was observed when dicyandiamide was pyrolyzed with Fe2O3/C 20 wt. % (Fe-N-C-20) and the order of activity towards ORR was Fe-N-C-20 > Fe-N-C-40 > Fe-N-C-60 > Fe-N-C-0 > Vulcan XC-72R. [Copyright &y& Elsevier]

Abstract: Non-noble electro-catalysts were synthesized by pyrolyzing dicyandiamide (DCDA) on carbon supported iron oxide at 700°C, 800°C and 900°C. The effects of pyrolysis temperature on electrochemical properties towards oxygen reduction reaction (ORR) were investigated. After pyrolysis, newly generated carbon arising from DCDA was observed in all the catalysts. Especially at 900°C, carbon nanotubes (CNTs) were grown catalyzed by Fe2O3/C. Pyrolysis of DCDA at higher temperatures increased ORR activity significantly. The catalyst exhibiting the best performance towards ORR was Fe2O3/C pyrolyzed with DCDA at 900°C. EDX and XPS experiments revealed that nitrogen atoms from DCDA were doped into carbon and the amount of doped nitrogen increased with increasing temperature. The reason for high reactivity towards oxygen reduction reaction is due to nitrogen doped CNTs grown on Vulcan carbon support. The reactivity increased as the amount of doped nitrogen increased. [Copyright &y& Elsevier]

The electrochemical reduction of CO 2 to ethylene has the potential to reduce greenhouse gas emissions while producing commodity chemicals for plastics; however, a scalable and feasible system for this remains a challenge. Herein, we report an efficient and stackable electrode design for the electrolysis of CO 2 to ethylene. Using KOH-incorporated Cu nanoparticle (Cu-KOH) as the cathode in a zero-gap electrolyzer, Faradaic efficiency of 78.7% for C 2 products was achieved at a current density of 281 mA cm–2. Among C 2 products, ethylene with a 54.5% FE was dominant product. For mass production, three membrane electrode assemblies (MEAs) were stacked and operated. Operando X-ray absorption spectroscopy under the zero-gap electrolyzer suggested mainly metallic Cu state with some persistent oxide-derived Cu species in Cu-KOH, including Cu 2 O and Cu(OH) 2 , which expected a synergistic effect for the conversion of CO 2 to C 2 H 4. Our findings provide a new strategy for converting CO 2 to C 2 H 4 , which is expected to accelerate the commercialization of high-value chemical production through electrochemical CO 2 reduction. KOH incorporated Cu electrode in zero-gap electrolyzer achieved a current density of 281 mA cm–2 with a Faraday efficiency of 54.5% toward C 2 H 4. Operando X-ray Absorption Spectroscopy reveals that the presence of Cu 2 O and Cu(OH) 2 species during CO 2 RR condition exhibit synergistic effect to improve selectivity and kinetics for CO 2 conversion to C 2 H 4. Furthermore, 3-cell stack operation result suggests the possibility of industrial development. [Display omitted] • KOH incorporated Cu in zero-gap electrolyzer achieved a current density of 281 mA cm–2 with a FE of 54.5% toward C 2 H 4. • Operando X-ray Absorption Spectroscopy suggests the presence of Cu 2 O and Cu(OH) 2 species during CO 2 RR condition. • Synergistic effect of Cu 2 O and Cu(OH) 2 species improve selectivity and kinetics for CO 2 RR to C 2 H 4. • C 2 H 4 FE of 50% at a current density of 200 mA cm2 for 6 h was achieved in a 3-cell stack. • 3-cell stack operation result suggests the possibility of industrial development. [ABSTRACT FROM AUTHOR]

The direct electroreduction of CO 2 to pure CO streams has attracted much attention for both academic research and industrial polymer synthesis development. Here, we explore catalytically very active, coral-structured Ag catalyst for the generation of pure CO from CO 2 -feeds in lab-bench scale zero-gap CO 2 electrolyzer. Coral-shaped Ag electrodes achieved CO partial current densities of up to 312 mA cm−2, EE CO of 38%, and FE CO clearly above 90%. In-situ/operando X-ray Absorption Spectroscopy revealed the sustained presence of Ag+ subsurface species, whose local electronic field effects constitute likely molecular origins of the favorable experimental kinetics and selectivity. In addition, we show how electrode flooding in zero-gap CO 2 electrolyzer compromises efficient CO 2 mass transfer. Our studies highlight the need for a concomitant consideration of factors related to intrinsic catalytic activity of the active phase, its porous structure and its hydrophilicity/phobicity to achieve a sustained high product yield in AEM zero-gap electrolyzer. Coral-shaped Ag for electrochemical CO 2 reduction was measured for catalytic activity in zero-gap CO 2 flow electrolyzer and in-situ/operando X-ray absorption spectroscopy under real device conditions. Ag coral achieved a high current density and Faradaic efficiency for CO owing to the porous structure and Cl-doped-Ag coral surface with abundant defects. Furthermore, the role of electrode flooding owing to increased hydrophilicity is discussed. Image 1 • Coral-structured Ag electrode was prepared for generation of CO in lab-bench scale zero-gap CO 2 electrolyzer. • Ag coral electrodes achieved CO partial current densities of up to 312 mA cm−2 and FE CO above 90%. • Ag electronic structure was observed using operando X-ray absorption spectroscopy under real device conditions. • Cl-doped-Ag coral possess enhanced negative electric fields, leading low overpotential for CO 2 reduction to CO. • Hydrophobicity of the electrode was important to prevent the flooding by improving mass transfer of CO 2. [ABSTRACT FROM AUTHOR]

Abstract: The photocatalytic activity in hydrogen production from methanol reforming can be significantly enhanced by Pt/MoO3/TiO2 photocatalysts. Compared with Pt/P25, the photocatalytic activity of optimized Pt/MoO3/TiO2 shows an evolution rate of 169 μmol/h/g of hydrogen, which is almost two times higher than that of Pt/P25. XRD and Raman spectra show that MoO3 are formed on the surface of TiO2. It is found that with the bulk MoO3 just formed, the catalyst shows the highest activity due to a large amount of heterojunctions and the high crystallinity of MoO3. The HRTEM image showed a close contact between MoO3 and TiO2. It is proposed that the Z-scheme type of heterojunction between MoO3 and TiO2 is responsible for the improved photocatalytic activity. The heterojunction structure of MoO3/TiO2 does not only promote the charge separation, but also separates the reaction sites, where the oxidation (mainly on MoO3) and reduction (on TiO2) reactions occurred. [Copyright &y& Elsevier]

• Surface of Me-N-C catalysts (Me = Co, Mn and Fe) was oxidized by H 2 O 2 treatment. • Oxidized Me-N-C catalysts promoted a 2e−-pathway in oxygen reduction reactions. • Dissociation of O O bond was suppressed by the introduced oxygen functionalities. • Among the three metal candidates, Co-N-C showed the highest H 2 O 2 selectivity. The electrochemical synthesis of hydrogen peroxide (H 2 O 2) can provide an attractive alternative to the current anthraquinone redox process, as it combines on-site chemical and electricity productions. A major challenge in the electrochemical H 2 O 2 synthesis is the catalyst design which leads to a selective two-electron pathway in the oxygen reduction reaction (ORR) without dissociation of the O O bond. In the present work, we report that the partial oxidation of metal-nitrogen-carbon catalysts (Me-N-C, Me = iron, cobalt and manganese) can modify their ORR mechanisms from a four- to a two-electron pathway. Spectroscopic measurements reveal that ex situ H 2 O 2 treatment introduces abundant oxygen functionalities on the Me-N-C surface without considerable changes to their bulk properties, such as crystallinity, degree of defects, surface area, and metal contents. Decreased H 2 O 2 reduction kinetics on the oxidized catalysts confirm that the dissociation of the O O bond is strongly suppressed by the newly introduced oxygen functionalities. Among the three central metal candidates, the cobalt-nitrogen-carbon catalyst shows the highest H 2 O 2 selectivity of >85%. This work provides a new simple guideline for designing Me-N-C catalysts for the efficient electrochemical synthesis of H 2 O 2. [ABSTRACT FROM AUTHOR]

• Fe-N-C catalyst was prepared for electrochemical oxygen reduction reactions. • Wetting with water and drying treatments introduced non-negligible activity loss. • Extend of the activity loss was affected by the treatment conditions. • Subtle changes in Fe-N-C surface were observed after the treatments. The membrane electrode assembly (MEA) is a core component of low-temperature fuel cells. The first step of MEA manufacturing is the preparation of a catalyst ink suspension in which the catalyst powder is homogeneously dispersed in a liquid solvent through mechanical or sonic agitation. In this work, we have studied the effects of catalyst dispersion in water or alcohol solutions and subsequent drying processes on the physicochemical properties of Fe-N-C catalysts and their electrocatalytic oxygen reduction activities. We find that dispersing the model Fe-N-C catalyst comprising only FeN x C y moieties in water and subsequent drying treatment change neither its bulk structure nor surface composition, as indicated by various spectroscopic measurements before and after treatment. However, zeta potential measurements, which are very sensitive to the chemistry of functionalities present on the carbon surface, reveal that the Fe-N-C catalyst becomes slightly more acidic, and that the change in their acido-basicity is magnified with a) increasing treatment temperature and b) repetitions of a same wetting/drying treatment. This small change in the surface acido-basicity of the Fe-N-C catalyst results in a measurable and reproducible decrease in its electrocatalytic activity, which shows a positive correlation with the zeta potential changes measured at pH = 1. Observed on the Fe-N-C catalyst but not on Pt/C, it is surmised that the electrocatalytic activities of the oxygen-reducing FeN x C y moieties are influenced by the surface chemistry of the carbonaceous support. Since catalyst wetting and drying processes are essential for MEA fabrication for fuel cells, these results suggest that careful attention should be paid to the conditions employed to prepare and dry catalytic inks for the family of Fe-N-C catalysts in order to obtain their highest possible ORR activity. [ABSTRACT FROM AUTHOR]

• MeNC catalysts (Me = Cr, Mn, Fe, Co, Ni) with abundant Me-N 4 sites were prepared. • In experiments, hydrogen evolution reaction (HER) on MeNCs was measured. • In theoretical works, hydrogen binding energy (HBE) on Me-N 4 sites was predicted. • A great correlation was found between the HER activity and HBE except for Ni-N 4. • Ni-N 3 V 1 geometry was identified as the main catalytic site of NiNC. NiNC catalysts, in which Ni ions are atomically dispersed by ligation with nitrogen functionalities on a carbon support, have received considerable attention owing to their promising activity in the electrochemical reduction of CO 2 to CO. However, a poor catalytic activity has been theoretically predicted for the conventional metalloporphyrin-like catalytic moieties, symmetric Ni-N 4. This has led to extensive debate regarding the nature of the actual catalytic site. In this study, we prepare five MeNC catalysts (Me = Cr, Mn, Fe, Co, or Ni) with predominant Me-N 4 moieties and measure their hydrogen evolution reaction (HER) activities. A linear correlation is identified between the HER activity and the theoretically predicted hydrogen binding energy (HBE) of all the symmetric Me-N 4 models, except for Ni-N 4. Thus, various coordination structures are further considered as catalytic Ni sites. Based on the combination of experimental and theoretical approaches, we conclude that coordinately unsaturated Ni sites can be present, which are responsible for the considerable electrocatalytic activity of the NiNC. [Display omitted] [ABSTRACT FROM AUTHOR]