CdTe is widely applied in thin film solar cells as a p-type layer, which is usually in contact with a metal back electrode. Using ab initio energy band calculations, here we study the interfacial properties of CdTe (110)–metal interfaces (metals = Al, Ag, Au, Cu, and Ni) systematically. Weak chemisorption and large interfacial distances are found between CdTe and Al, Ag and Cu surfaces, while medium or strong chemisorption and small interfacial distances are found between CdTe and Au and Ni surfaces. After GW correction, it is found that CdTe forms n-type Schottky contacts with Ag, Al and Cu and p-type Schottky contacts with Au and Ni at the interface between metalized CdTe and semiconductive CdTe, consistent with previous experimental values. Besides the Schottky barrier, tunneling barriers also exist at the CdTe–metal contact interface. The potential profiles at the vertical CdTe–metal interfaces reveal that due to the medium or strong chemisorption, tunneling barrier is absent at CdTe–Au and CdTe–Ni contacts, while the weak bonding interfaces (Ag, Al and Cu) have obvious tunneling barriers. Finally, methods to optimize the interface of the CdTe–metal contact to further decrease the Schottky barrier at the CdTe–metal contact are discussed.

Non-noble metal catalysts with catalytic activity toward oxygen reduction reaction (ORR) comparable or even superior to that of Pt/C are extremely important for the wide application of metal–air batteries and fuel cells. Here, we develop a simple and controllable strategy to synthesize Fe-cluster embedded in Fe3C nanoparticles (designated as Fe3C(Fe)) encased in nitrogen-doped graphitic layers (NDGLs) with graphitic shells as a novel hybrid nanostructure as an effective ORR catalyst by directly pyrolyzing a mixture of Prussian blue (PB) and glucose. The pyrolysis temperature was found to be the key parameter for obtaining a stable Fe3C(Fe)@NDGL core–shell nanostructure with an optimized content of nitrogen. The optimized Fe3C(Fe)@NDGL catalyst showed high catalytic performance of ORR comparable to that of the Pt/C (20 wt %) catalyst and better stability than that of the Pt/C catalyst in alkaline electrolyte. According to the experimental results and first principle calculation, the high activity of the Fe3C(Fe)@NDGL catalyst can be ascribed to the synergistic effect of an adequate content of nitrogen doping in graphitic carbon shells and Fe-cluster pushing electrons to NDGL. A zinc–air battery utilizing the Fe3C(Fe)@NDGL catalyst demonstrated a maximum power density of 186 mW cm–2, which is slightly higher than that of a zinc–air battery utilizing the commercial Pt/C catalyst (167 mW cm–2), mostly because of the large surface area of the N-doped graphitic carbon shells. Theoretical calculation verified that O2 molecules can spontaneously adsorb on both pristine and nitrogen doped graphene surfaces and then quickly diffuse to the catalytically active nitrogen sites. Our catalyst can potentially become a promising replacement for Pt catalysts in metal-air batteries and fuel cells.Keywords: Fe3C(Fe) cluster; N-doped graphitic layers; Oxygen reduction reaction (ORR); Synergistic effect; Zn−air batteries;

N-doped graphene (NDG) was investigated for oxygen reduction reaction (ORR) and used as air-electrode catalyst for Zn–air batteries. Electrochemical results revealed a slightly lower kinetic activity but a much larger rate capability for the NDG than commercial 20% Pt/C catalyst. The maximum power density for a Zn-air cell with NDG air cathode reached up to 218 mW cm–2, which is nearly 1.5 times that of its counterpart with the Pt/C (155 mW cm–2). The equivalent diffusion coefficient (DE) of oxygen from electrolyte solution to the reactive sites of NDG was evaluated as about 1.5 times the liquid-phase diffusion coefficient (DL) of oxygen within bulk electrolyte solution. Combined with experiments and ab initio calculations, this seems counterintuitive reverse ORR of NDG versus Pt/C can be rationalized by a spontaneous adsorption and fast solid-state diffusion of O2 on ultralarge graphene surface of NDG to enhance effective ORR on N-doped-catalytic-centers and to achieve high-rate performance for Zn–air batteries.Keywords: nitrogen-doped graphene; oxygen reduction reaction; oxygen transport; spillover; Zn−air battery;

•EQCM is applied to study the Li(Na)FePO4/electrolyte interfacial reactions and surface redox potential.•The mass-potential curve for LiFePO4 in aqueous electrolyte shows an anomalous mass change interval around 3.42 V (vs. Li/Li+).•DFT study presents a microscopic picture of the LFP/H2O, NFP/H2O interface structures and surface redox potentials.•The electrochemical process of LFP/H2O interface is obtained by combining experimental and DFT study.In this work, an in-situ experimental mass-electrochemical investigation of the LiFePO4 (LFP) and NaFePO4 (NFP)electrolyte interfacial chemical reactions and surface redox potential is achieved by adopting electrochemical quartz crystal microbalance (EQCM) to monitor the mass change trend. In organic electrolyte, LFP (NFP) cathode's mass decreases/increases during the charge/discharge process because of deintercalation/intercalation of Li (Na) ions, which is an normal phenomenon which is generally known. However, the mass-potential curve for LFP nanocrystals in aqueous electrolyte show an anomalous mass change interval (AMCI) around 3.42 V (vs. Li/Li+) where the cathode's mass increase in the charging process and mass decrease in the discharging process, which doesn’t obey the normal law of mass change. Through density functional theory (DFT) calculations, we gain a microscopic picture of the solid-liquid interface structure with a reconstructed LFP (010)/H2O and NFP (010)/H2O interface. Taken together, it's concluded that the surface redox potential of LFP is around 3.31 V, which is lower than the bulk potential (3.42 V) and the desolvation/solvation rate of surficial Li-ion is lower than the bulk Li-ion diffusion rate. While for NFP, it's surface redox potential is almost the same as the bulk one.Download high-res image (164KB)Download full-size image

Lithium iron phosphate, a widely used cathode material, crystallizes typically in olivine-type phase, α-LiFePO4 (αLFP). However, the new phase β-LiFePO4 (βLFP), which can be transformed from αLFP under high temperature and pressure, is originally almost electrochemically inactive with no capacity for Li-ion battery, because the Li-ions are stored in the tetrahedral [LiO4] with very high activation barrier for migration and the one-dimensional (1D) migration channels for Li-ion diffusion in αLFP disappear, while the Fe ions in the β-phase are oriented similar to the 1D arrangement instead. In this work, using experimental studies combined with density functional theory calculations, we demonstrate that βLFP can be activated with creation of effective paths of Li-ion migration by optimized disordering. Thus, the new phase of βLFP cathode achieved a capacity of 128 mAh g–1 at a rate of 0.1 C (1C = 170 mA g–1) with extraordinary cycling performance that 94.5% of the initial capacity retains after 1000 cycles at 1 C. The activation mechanism can be attributed to that the induced disorder (such as FeLiLiFe antisite defects, crystal distortion, and amorphous domains) creates new lithium migration passages, which free the captive stored lithium atoms and facilitate their intercalation/deintercalation from the cathode. Such materials activated by disorder are promising candidate cathodes for lithium batteries, and the related mechanism of storage and effective migration of Li-ions also provides new clues for future design of disordered-electrode materials with high capacity and high energy density.

Specific capacity and cyclic performance are critically important for the electrode materials of rechargeable batteries. Herein, a capacity boost effect of Li- and Na-ion batteries was presented and clarified by nitrogen-doped graphene sheets. The reversible capacities progressively increased from 637.4 to 1050.4 mAh g–1 (164.8% increase) in Li-ion cell tests from 20 to 185 cycles, and from 187.3 to 247.5 mAh g–1 (132.1% increase) in Na-ion cell tests from 50 to 500 cycles. The mechanism of the capacity boost is proposed as an electrochemical induced cascading evolution of graphitic N to pyridinic and/or pyrrolic N, during which only these graphitic N adjacent pyridinic or pyrrolic structures can be taken precedence. The original and new generated pyridinic and pyrrolic N have strengthened binding energies to Li/Na atoms, thus increased the Li/Na coverage and delivered a progressive capacity boost with cycles until the entire favorable graphitic N transform into pyridinic and pyrrolic N.Keywords: capacity boost; cyclic performance; first-principles calculation; lithium ion battery; nitrogen-doped graphene; sodium ion battery

Using first-principles calculations, we study the structural and electronic properties of a new layered hydrogen-bonded 2D material Fe3(PO4)2·8H2O. Interestingly, unlike other common 2D materials, such as layered van der Waals 2D materials, the band gap of 2D Fe3(PO4)2·8H2O-(010)-(1 × 1) is smaller than bulk Fe3(PO4)2·8H2O, which does not obey the normal quantum confinement effect and can be attributed to the edge states and the hydrogen bonds between the layers. We also find that the band-gap variation with the reduced layers depends on the length of the interlayer hydrogen bond and the stronger interlayer hydrogen bond leads to the larger band gap.

CdTe is widely applied in thin film solar cells as a p-type layer, which is usually in contact with a metal back electrode. Using ab initio energy band calculations, here we study the interfacial properties of CdTe (110)–metal interfaces (metals = Al, Ag, Au, Cu, and Ni) systematically. Weak chemisorption and large interfacial distances are found between CdTe and Al, Ag and Cu surfaces, while medium or strong chemisorption and small interfacial distances are found between CdTe and Au and Ni surfaces. After GW correction, it is found that CdTe forms n-type Schottky contacts with Ag, Al and Cu and p-type Schottky contacts with Au and Ni at the interface between metalized CdTe and semiconductive CdTe, consistent with previous experimental values. Besides the Schottky barrier, tunneling barriers also exist at the CdTe–metal contact interface. The potential profiles at the vertical CdTe–metal interfaces reveal that due to the medium or strong chemisorption, tunneling barrier is absent at CdTe–Au and CdTe–Ni contacts, while the weak bonding interfaces (Ag, Al and Cu) have obvious tunneling barriers. Finally, methods to optimize the interface of the CdTe–metal contact to further decrease the Schottky barrier at the CdTe–metal contact are discussed.