The electrochemical oxygen reduction reaction in acidic media offers an attractive route for direct hydrogen peroxide (H 2 O 2 ) generation and on-site applications. Unfortunately there is still a lack of cost-effective electrocatalysts with high catalytic performance. Here, we theoretically designed and experimentally demonstrated that a cobalt single-atom catalyst (Co SAC) anchored in nitrogendoped graphene, with optimized adsorption energy of the *OOH intermediate, exhibited a high H 2 O 2 production rate, which even slightly outperformed the state-of-the-art noble-metal-based electrocatalysts. The kinetic current of H 2 O 2 production over Co SAC could reach 1 mA=cm 2 disk at 0.6 V versus reversible hydrogen electrode in 0.1 M HClO 4 with H 2 O 2 faraday efficiency > 90%, and these performance measures could be sustained for 10 h without decay. Further kinetic analysis and operando X-ray absorption study combined with density functional theory (DFT) calculation demonstrated that the nitrogen-coordinated single Co atom was the active site and the reaction was rate-limited by the first electron transfer step.

Carbon nanotubes are promising materials for various applications. In recent years, progress in manufacturing and functionalizing carbon nanotubes has been made to achieve the control of bulk and surface properties including the wettability, acid-base properties, adsorption, electric conductivity and capacitance. In order to gain the optimal benefit of carbon nanotubes, comprehensive understanding on manufacturing and functionalizing carbon nanotubes ought to be systematically developed. This review summarizes methodologies of manufacturing carbon nanotubes via arc discharge, laser ablation and chemical vapor deposition and functionalizing carbon nanotubes through surface oxidation and activation, doping of heteroatoms, halogenation, sulfonation, grafting, polymer coating, noncovalent functionalization and nanoparticle attachment. The characterization techniques detecting the bulk nature and surface properties as well as the effects of various functionalization approaches on modifying the surface properties for specific applications in catalysis including heterogeneous catalysis, photocatalysis, photoelectrocatalysis and electrocatalysis are highlighted.

Figure 1. Schematic illustration showing Pt dissolution and redeposition during the HER experiment if Pt is applied as the counter electrode.

in alkaline media. The surprisingly low OER overpotential of NiFe LDH has triggered a great deal of research attentions to reveal the reaction mechanism. [4,5] Besides, lots of work have been done to further reducing the overpotential of NiFe LDH, for example, via incorporation of a third metal, [6][7][8] hybridization with carbon materials, [9,10] and applying NiFe selenide as the templating precursor. [11] Although great attention has been paid to improve the OER activity and investigate the active site of NiFe LDHs, few works actually focus on their catalytic stability despite that stability is as important as activity in practical applications. Based on literature, the OER stability of NiFe LDHs seems satisfactory. [10][11][12][13] However, the stability of NiFe LDHs was usually assessed at room temperature with current densities of tens of milliamps per square centimeter of electrode for tens of hours. The mild evaluation condition cannot reflect the long-term stability requirement under harsh conditions for practical alkaline water electrolyzers.Herein, we reveal that the layered structure of bulk NiFe LDH is detrimental to OER stability. It has been generally accepted that the edge sites of 2D electrocatalysts (e.g., MoS 2 ) are highly active in electrocatalysis, while surface sites are usually inactive. [14] We identify that the interlayer basal plane of NiFe LDH is also able to catalyze OER, while the slow diffusion of OH − into the LDH interlayers during OER in alkaline solution induces a local acidic environment within the interlayers, which thus causes dissolution of NiFe LDH. To resolve this problem, we propose to delaminate multi-layered NiFe LDH into atomically thin nanosheets, which is able to greatly improve OER stability.NiFe LDH grown on Ni foam or carbon cloth was used to investigate the deactivation mechanism of LDH in OER. Figure S1 (Supporting Information) shows the scanning electron microscopy and transmission electron microscopy (TEM) images, in which NiFe LDH nanosheets are found to intimately and uniformly cover the entire Ni foam with NiFe LDH film thickness of ≈2.5 µm and individual sheet thickness of ≈60 nm. The high-resolution TEM image ( Figure S1d, Supporting Information) shows the lattice spacing of ≈2.5 Å, close to the theoretical interplanar spacing of NiFe LDH (009). The layered structure was further confirmed by X-ray diffraction (XRD) as shown in Figure S2 (Supporting Information).NiFe-based layered double hydroxides (LDHs) are among the most efficient oxygen evolution reaction (OER) catalysts in alkaline medium, but their long-term OER stabilities are questionable. In this work, it is demonstrated that the layered structure makes bulk NiFe LDH intrinsically not stable in OER and the deactivation mechanism of NiFe LDH in OER is further revealed. Both operando electrochemical and structural characterizations show that the interlayer basal plane in bulk NiFe LDH contributes to the OER activity, and the slow diffusion of proton acceptors (e.g., OH − ) within the NiFe LDH interl...

The binding strength of reactive intermediates with catalytically active sites plays a crucial role in governing catalytic performance of electrocatalysts. NiFe hydroxide offers efficient oxygen evolution reaction (OER) catalysis in alkaline electrolyte, however weak binding of oxygenated intermediates on NiFe hydroxide still badly limits its catalytic activity. Now, a facile ball‐milling method was developed to enhance binding strength of NiFe hydroxide to oxygenated intermediates via generating tensile strain, which reduced the anti‐bonding filling states in the d orbital and thus facilitated oxygenated intermediates adsorption. The NiFe hydroxide with tensile strain increasing after ball‐milling exhibits an OER onset potential as low as 1.44 V (vs. reversible hydrogen electrode) and requires only a 270 mV overpotential to reach a water oxidation current density of 10 mA cm−2.