Doping of graphene with nitrogen imparted bifunctional electrocatalytic activities for efficient energy conversion and storage.

A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), the surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performance still remains a great challenge. Here we show the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (MCUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of MCUS, and thus the increase in MCUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. The electronic origin of the surface reactivity can be well explained by a new model proposed in this work, wherein the energy of the highest-occupied d-states relative to the Fermi level determines the intermediates' bonding strength by affecting the filling of the antibonding states. Our model for the first time well describes the reactivity trends among TMOs, and would initiate viable design principles for, but not limited to, OER catalysts.

To develop new OER catalysts to improve efficiency for renewable energy storage, observing oxygen intermediates is essential yet challenging. Herein, based on the electronic structure and chemical property of oxygen intermediates, we design a chemical method to probe oxygen intermediates at operating conditions of OER. Alcohols are demonstrated to be excellent probing molecules to detect oxygen intermediates over various types of catalysts at different reaction media. The general and feasible method could be widely used in every electrochemical laboratory.

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 key step for rational catalyst design in heterogeneous electrocatalysis is to reveal the distinctive energy profile of redox reactions of a catalyst that give rise to specific activity. However, it is challenging to experimentally obtain the energetics of oxygen redox in oxygen electrocatalysis because of the liquid reaction environment. Here we develop a kinetic model that constructs a quantitative relation between the energy profile of oxygen redox and electrochemical kinetic fingerprints. The detailed study here demonstrates that the kinetic fingerprints observed from experiments can be well described by different energetics of oxygen redox. On the basis of the model, a feasible methodology is demonstrated to derive binding energies of the oxygen intermediates from electrochemical data. The surface property of different catalysts derived from our model well rationalizes the experimental trends and predicts potential directions for catalyst design.