Parallelized Reaction Pathway and Stronger Internal Band Bending by Partial Oxidation of Metal Sulfide–Graphene Composites: Important Factors of Synergistic Oxygen Evolution Reaction Enhancement

Abstract

The electrocatalytic performance of transition metal sulfide (TMS)− graphene composites has been simply regarded as the results of high conductivity and the large surface/volume ratio. However, unavoidable factors such as degree of oxidation of TMSs have been hardly considered for the origin of this catalytic activity of TMS−graphene composites. To accomplish the reliable application of TMS-based electrocatalytic materials, a clear understanding of the thermodynamic stability of TMS and effects of oxidation on catalytic activity is necessary. In addition, the mechanism of charge transfer at the TMS−graphene interface must be studied in depth to properly design composite materials. Herein, we report a comprehensive study of the physical chemistry at the junction of a Co 1−x Ni x S 2 −graphene composite, which is a prototype designed to unravel the mechanisms of charge transfer between TMS and graphene. Specifically, the thermodynamic stability and the effects of oxidation of TMSs during the oxygen evolution reaction (OER) on the reaction mechanism are systematically investigated using density functional theory (DFT) calculations and experimental observations. Cobalt atoms anchored on pyridinic N sites in the graphene support form metal−semiconductor (SC) junctions, and the internal band bending at these junctions facilitates electron transfer from TMSs to graphene. The junction enables fast sinking of the excess electron from OH − adsorbate. Partially oxidized amorphous TMS layers formed during the OER can facilitate adsorption and desorption of OH and H atoms, boosting the OER performance of TMS−graphene nanocomposites. From the DFT calculations, the enhanced electrocatalytic activity of TMS−graphene nanocomposites originates from two important factors: (i) increased internal band bending and (ii) parallelized OER pathways at the interface of pristine and oxidized TMSs.

Zeolitic Imidazolate Framework‐Derived Core‐Shell‐Structured CoS2/CoS2‐N‐C Supported on Electrochemically Exfoliated Graphene Foil for Efficient Oxygen Evolution

“…The formed Co−O groups was further transformed to Co‐OOH species with the increase of potential, in which the Co‐OOH species could contribute to the superb catalytic performance ,. This result is well consistent with the previously reported phenomenon that the Co‐OOH would appear in high potential range during OER , . It is also notable that beyond 1.7 V, the peak intensities of both Co−O and Co−OOH bonds were diminished.…”

Natural‐Cellulose‐Nanofibril‐Tailored NiFe Nanoparticles for Efficient Oxygen Evolution Reaction

“…The lower overpotential and smaller Tafel slope of NiFeNSC catalyst indicated that NiFeNSC presented better electrical transport and faster reaction kinetics than other catalysts in this study. [18] The turnover frequency (TOF) value of NiFeNSC was higher than that of NiNSC and NiFeC, suggesting the superior catalytic activity toward OER (Table S1).…”

“…[16,17] For instance, Han et al synthesized Co 1-x Ni x S 2graphene composite, and the CNS-NGA (Ni-doped CoS 2 integrated with nitrogen-doped reduced graphene) showed a much lower overpotential (330 mV at 10 mA cm À 2 ) than CNS (Ni-doped CoS 2 ), which benefited from the three-dimensional (3D) porous structure with a large surface area. [18] However, for graphene-based catalyst, since the number of active groups is limited, the bonding of the metal catalysts with graphene is weak, which prevented the metal catalyst from dispersing homogeneously, and give the resulted electrocatalyst poor stability. In addition, graphene-based composite electrodes often suffer from a low mass activity because the two-dimensional (2D) graphene sheets tend to exhibit ineffective packing.…”

Porosity‐Engineering of MXene as a Support Material for a Highly Efficient Electrocatalyst toward Overall Water Splitting

“…[1][2][3][4][5] Precious-metal-based electrocatalysts exhibit the highest catalytic activity to reduce the overpotential requirement, but the high cost, scarcity and low stabilityo ft hesec atalysts have hindered their widespread application greatly. [6][7][8][9][10][11][12] Therefore, substantial efforts have been devotedt od evelop highly efficient and inexpensive materials by reducing the noblem etal usage or by exploring viable alternatives. [7,8,[13][14][15][16][17][18][19][20] There are two common approaches to enhance the activity of an electrocatalyst: ( 1) by increasing the quality of each active site (e.g.,n ormally by modulating the electronic configuration) and (2) by increasing the number of active sites at as pecific mass loading and electrode area (e.g., by engineering the structure and morphology to expose more active sites).…”