An 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. HIGHLIGHTSAtomic Ni trapped in carbon defects serves as an active site for superb OER and HER Direct identification of the atomic Ni in defects is provided by probecorrected TEM DFT simulations suggest that tuning the electronic structure enhances OER and HER Zhang et al., Chem 4,[285][286][287][288][289][290][291][292][293][294][295][296][297] February 8, 2018 ª SUMMARY Downsizing the catalyst to atomic scale provides an effective way to maximize the atom efficiency and enhance activity for electrocatalysis. Here, we report a concept whereby graphene defects trap atomic Ni species (aNi) inside to form an integrity (aNi@defect). X-ray adsorption characterization and density-functional-theory calculation revealed that the diverse defects in graphene can induce different local electronic densities of state (DOSs) of aNi, which suggests that aNi@defect serves as an active site for unique electrocatalytic reactions. As examples, aNi@G585 is responsible for the oxygen evolution reaction (OER), and aNi@G5775 activates the hydrogen evolution reaction (HER). The derived catalyst exhibits exceptionally good activity for both HER and OER, e.g., an overpotential of 70 mV at 10 mA/cm 2 for HER (analogous to the commercial Pt/C) and 270 mV at 10 mA/cm 2 for OER (much superior to that of Ir oxide).

A new efficient photocatalyst structure, a shell of anatase nanocrystals on the fibril core of a single TiO(2)(B) crystal, was obtained via two consecutive partial phase transition processes. In the first stage of the process, titanate nanofibers reacted with dilute acid solution under moderate hydrothermal conditions, yielding the anatase nanocrystals on the fiber. In the subsequent heating process, the fibril core of titanate was converted into a TiO(2)(B) single crystal while the anatase crystals in the shell remained unchanged. The anatase nanocrystals do not attach to the TiO(2)(B) core randomly but coherently with a close crystallographic registry to the core to form a stable phase interface. For instance, (001) planes in anatase and (100) planes of TiO(2)(B) join together to form a stable interface. Such a unique structure has several features that enhance the photocatalytic activity of these fibers. First, the differences in the band edges of the two phases promote migration of the photogenerated holes from anatase shell to the TiO(2)(B) core. Second, the well-matched phase interfaces allow photogenerated electrons and holes to readily migrate across the interfaces because the holes migrate much faster than excited electrons, more holes than electrons migrate to TiO(2)(B) and this reduces the recombination of the photogenerated charges in anatase shell. Third, the surface of the anatase shell has both a strong ability to regenerate surface hydroxyl groups and adsorb O(2), the oxidant of the reaction, to yield reactive hydroxyl radicals (OH(.)) through reaction between photogenerated holes and surface hydroxyl groups. The adsorbed O(2) molecules can capture the excited electrons on the surface, forming reactive O(2)(-) species. The more reactive species generated on the external surface, the higher the photocatalytic activity will be, and generation of the reactive species also contributes to reducing recombination of the photogenerated charges. Indeed, the mixed-phase nanofibers exhibited superior photocatalytic activity for degradation of sulforhodamine B under UV light to the nanofibers of either pure phase alone or mechanical mixtures of the pure phase nanofibers with a similar phase composition. Finally, the nanofibril morphology has an additional advantage that they can be separated readily after reaction for reuse by sedimentation. This is very important because the high cost for separating the catalyst nanocrystals has seriously impeded the applications of TiO(2) photocatalysts on an industrial scale.

Platinum (Pt) is the state-of-the-art catalyst for oxygen reduction reaction (ORR), but its high cost and scarcity limit its large-scale use. However, if the usage of Pt reduces to a sufficiently low level, this critical barrier may be overcome. Atomically dispersed metal catalysts with high activity and high atom efficiency have the possibility to achieve this goal. Herein, we report a locally distributed atomic Pt-Co nitrogen-carbon-based catalyst (denoted as A-CoPt-NC) with high activity and robust durability for ORR (267 times higher than commercial Pt/C in mass activity). The A-CoPt-NC shows a high selectivity for the 4e pathway in ORR, differing from the reported 2e pathway characteristic of atomic Pt catalysts. Density functional theory calculations suggest that this high activity originates from the synergistic effect of atomic Pt-Co located on a defected C/N graphene surface. The mechanism is thought to arise from asymmetry in the electron distribution around the Pt/Co metal centers, as well as the metal atoms' coordination with local environments on the carbon surface. This coordination results from N8V4 vacancies (where N8 represents the number of nitrogen atoms and V4 indicates the number of vacant carbon atoms) within the carbon shell, which enhances the oxygen reduction reaction via the so-called synergistic effect.

Owing to the difficulty in controlling the dopant or defect types and their homogeneity in carbon materials, it is still a controversial issue to identify the active sites of carbon-based metal-free catalysts. Herein, we report a proof of concept study on the active-site evaluation for a highly oriented pyrolytic graphite catalyst with specific pentagon carbon defective patterns (D-HOPG).It is demonstrated that specific carbon defect types (edged pentagon in this work) could be selectively created via controllable N-doping. Work function analyses coupled with macro/microelectrochemical performance measurements suggest that the pentagon defects in D-HOPG served as major active sites for acidic oxygen reduction reaction (ORR), even much superior to the pyridinic nitrogen sites in N-doped highly oriented pyrolytic graphite (N-HOPG). This work enables us to elucidate the relative importance of the specific carbon defects vs N-dopant species and their respective contributions to the observed overall acidic ORR activity.

It is vitally essential to design highly efficient and cost-effective bifunctional electrocatalysts toward water splitting. Herein, we report the development of P-doped Co3O4 nanowire array on nickel foam (P-Co3O4/NF) from Co3O4 nanowire array through low-temperature annealing, using NaH2PO2 as the P source. As a 3D catalyst, such P-Co3O4/NF demonstrates superior performance for oxygen evolution reaction with a low overpotential (260 mV at 20 mA cm–2), a small Tafel slope (60 mV dec–1), and a satisfying durability in 1.0 M KOH. Density functional theory calculations indicate that P-Co3O4 has a reaction free-energy value that is much smaller than that of pristine Co3O4 for the potential determining step of the oxygen evolution reaction. Such P-Co3O4/NF also performs efficiently for hydrogen evolution reaction, and a two-electrode alkaline electrolyzer assembled by P8.6-Co3O4/NF as both anode and cathode needs only 1.63 V to reach a water-splitting current of 10 mA cm–2.

Searching for the highly active, stable, and high-efficiency bifunctional electrocatalysts for overall water splitting, e.g., for both oxygen evolution (OER) and hydrogen evolution (HER), is paramount in terms of bringing future renewable energy systems and energy conversion processes to reality. Herein, three-dimensional (3D) NiFeN nanoparticles/reduced graphene oxide (r-GO) aerogel electrocatalysts were fabricated using precursors of (Ni,Fe)/r-GO alginate hydrogels through an ion-exchange process, followed by a convenient one-step nitrogenization treatment in NH at 700 °C. The resultant materials exhibited excellent electrocatalytic performance for OER and HER in alkaline media, with only small overpotentials of 270 and 94 mV at a current density of 10 mA cm, respectively. The good performance was attributed to abundant active sites and high electrical conductivity of the bimetallic nitrides and efficient mass transport of the 3D r-GO aerogel framework. Furthermore, an alkaline electrolyzer was set up using NiFeN/r-GO as both the cathode and the anode, which achieved a 10 mA cm current density at 1.60 V with durability of 100 h for overall water splitting. Density functional theory calculations support that NiFeN (111)/r-GO is more favorable for overall water splitting since the surface electronic structure of NiFeN is tuned by transferring electrons from NiFeN cluster to the r-GO through interaction of two metal species. Thus, the currently developed NiFeN/r-GO with superior water-splitting performance may potentially serve as a material for use in industrial alkaline water electrolyzers.

The oxygen evolution reaction (OER) is a key reaction for many electrochemical devices. To date, many OER electrocatalysts function well in alkaline media, but exhibit poor performances in neutral and acidic media, especially the acidic stability. Herein, sodium‐decorated amorphous/crystalline RuO2 with rich oxygen vacancies (a/c‐RuO2) was developed as a pH‐universal OER electrocatalyst. The a/c‐RuO2 shows remarkable resistance to acid corrosion and oxidation during OER, which leads to an extremely high catalytic stability, as confirmed by a negligible overpotential increase after continuously catalyzing OER for 60 h at pH=1. Besides, a/c‐RuO2 also exhibits superior OER activities to commercial RuO2 and most reported OER catalysts under all pH conditions. Theoretical calculations indicated that the introduction of Na dopant and oxygen vacancy in RuO2 weakens the adsorption strength of the OER intermediates by engineering the d‐band center, thereby lowering the energy barrier for OER.

Newly N-S-C coordination-structured active sites, originating from the integrity of edged thiophene S, graphitic N, and pentagon defects, were reconstructed by N-modified S defects in carbon aerogel with a 3D hierarchical macro-mesomicroporous structure. This metal-free material exhibited outstanding oxygen reduction reaction (ORR) activity (e.g., half-wave potentials of 0.76 V in 0.5 M H 2 SO 4 and 0.1 M HClO 4 ; 0.85 V in 0.1 M KOH) with good stability and high current density in both acidic and alkaline electrolytes. The experimental and computational results reveal that the pentagon S defect is essential for the ORR in acidic electrolytes because it provides a remarkable increase in reactivity. One graphitic-type N in the meta-position of the pentagon defect further significantly improves the reactivity as a result of locally precise control of the electronic structure, thus forming highly active sites for ORR in acid.