Crystalline–amorphous phase boundary engineering can be an effective strategy to develop cost-effective and high-performance electrocatalysts for water splitting.
A key issue with Na-ion batteries is the development of active materials with stable electrochemical reversibility through the understanding of their sodium storage mechanisms. We report a sodium storage mechanism and properties of a new anode material, digenite CuS, based on its crystallographic study. It is revealed that copper sulfides (Cu S) can have metal-rich formulas ( x ≥ 1.6), due to the unique oxidation state of +1 found in group 11 elements. These phases enable the unit cell to consist of all strong Cu-S bonds and no direct S-S bonds, which are vulnerable to external stress/strain that could result in bond cleavage as well as decomposition. Because of its structural rigidness, the CuS shows an intercalation/deintercalation reaction mechanism even in a low potential window of 0.1-2.2 V versus Na/Na without irreversible phase transformation, which most of the metal sulfides experience through a conversion reaction mechanism. It uptakes, on average, 1.4 Na ions per unit cell (∼250 mAh g) and exhibits ∼100% retention over 1000 cycles at 2C in a tuned voltage range of 0.5-2.2 V through an overall solid solution reaction with negligible phase separation.
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.
Borophosphate materials are promising electrocatalysts for water splitting. Their structural flexibility enable self‐adjusting of electronic structure depending on potential. The rich chemistry of borophosphate provides a huge engineering space to tune composition and structure. Herein, amorphized LiNiFe borophosphate (a‐LNFBPO) for an efficient and durable oxygen evolution reaction (OER) is first reported. Facile adsorption of oxygen intermediates on the vacancies generated by spontaneous Li dissolution during the OER and regulated electronic structure resulting from the Ni and Fe interaction can boost the OER. The amorphization of LiNiFe borophosphate modifies the electronic structure with metal‐oxygen (MO) bond contraction and the high valence state of the metal cations, which reduces the charge transfer energy between the catalyst and electrolyte. In addition, abundant defects, dangling bonds, and a disordered arrangement induced by amorphization enable an improvement in structural flexibility, facilitating a facile and entire transformation of originally inert species into the active phase during the OER process. The a‐LNFBPO@Ni foam shows excellent OER properties requiring only a 215 mV overpotential for generating 10 mA cm−2 and long‐term stability over 300 h.
Lithium sulfur (Li-S) batteries have drawn much attention as next-generation batteries because of their high theoretical capacity (1672 mAh g − 1), environmental friendliness and low cost. However, several critical issues, which are mainly associated with the polysulfide shuttling effect, result in their poor electrochemical performance. Carbon-modified separators have been introduced to attempt to address these systemic challenges. However, this approach focused only on the suppression of dissolved polysulfides on the cathodic side without considering the further entrapment of polysulfides on the anodic side. In this study, we first designed a multifunctional trilayer membrane comprising a carbon layer and a boron nitride (BN) layer to facilitate the electrochemical performance of Li-S batteries and protect the Li anode from unexpected side reactions. When a BN-carbon separator was employed, the sulfur cathode delivered stable capacity retention over 250 cycles and an excellent specific capacity (702 mAh g − 1) at a high current density (4 C). The BN-carbon separator also facilitated the uniform plating/striping of Li and, thus, suppressed the severe growth of dendritic Li on the electrode; this led to the stable operation of the Li anode with a high Coulombic efficiency and improved cycling performance.
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