Heterointerface Engineering of Hierarchically Assembling Layered Double Hydroxides on Cobalt Selenide as Efficient Trifunctional Electrocatalysts for Water Splitting and Zinc‐Air Battery

Abstract Engineering of structure and composition is essential but still challenging for electrocatalytic activity modulation. Herein, hybrid nanostructured arrays (HNA) with branched and aligned structures constructed by cobalt selenide (CoSe2) nanotube arrays vertically oriented on carbon cloth with CoNi layered double hydroxide (CoSe2@CoNi LDH HNA) are synthesized by a hydrothermal‐selenization‐hybridization strategy. The branched and hollow structure, as well as the heterointerface between CoSe2 and CoNi LDH guarantee structural stability and sufficient exposure of the surface active sites. More importantly, the strong interaction at the interface can effectively modulate the electronic structure of hybrids through the charge transfer and then improves the reaction kinetics. The resulting branched CoSe2@CoNi LDH HNA as trifunctional catalyst exhibits enhanced electrocatalytic performance toward oxygen evolution/reduction and hydrogen evolution reaction. Consequently, the branched CoSe2@CoNi LDH HNA exhibits low overpotential of 1.58 V at 10 mA cm−2 for water splitting and superior cycling stability (70 h) for rechargeable flexible Zn‐air battery. Theoretical calculations reveal that the construction of heterostructure can effectively lower the reaction barrier as well as improve electrical conductivity, consequently favoring the enhanced electrochemical performance. This work concerning engineering heterostructure and topography‐performance relationship can provide new guidance for the development of multifunctional electrocatalysts.

under stirring. Then HMT (0.1 g) was added and the mixed solution was poured into a 50 mL reaction kettle. Subsequently, B-CoSe 2 were added and the reaction kettle was heated at 80 °C for 10 h. After cooling to room temperature, moist CC were washed with deionized water and ethanol respectively, then dried at 60 °C. The synthesized CoSe 2 @CoNi LDH HNA with branched structure was denoted as B-CoSe 2 @CoNi LDH HNA. While V-CoSe 2 was added, the obtained CoSe 2 @CoNi LDH with aligned structure was denoted as V-CoSe 2 @CoNi LDH HNA. CoNi LDH sample as reference was prepared in the same way without adding CoSe 2 .
Materials Characterizations: Morphology and microstructure of samples were observed via field-emission scanning electron microscope (FESEM, Regulus 8100) and transmission electron microscope (TEM, FEI Tecnai G2 F20) as well as high resolution TEM (HRTEM).
TEM mapping was also conducted on FEI Tecnai G2 F20. The Brunauer-Emmett-Teller (BET) surface area were determined using nitrogen adsorption-desorption carried by the Surface Area (V-Sorb 2800P, Gold APP Instruments, China). X-ray diffraction (XRD) data acquired from Bruker D8 Advance instrument was used to explore the crystal structure. X-ray photoelectron spectrometer (XPS, Escalab 250Xi) with Al Kα X-rays as the excitation source were performed to investigate the surface composition of the samples. The extended X-ray absorption fine structure (EXAFS) was measured at Taiwan Photon Source (TPS44A1, TLS17C1) beam line, 44A Quick-scanning X-ray absorption spectroscopy (XAS), in National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. WT EXAFS software was applied to calculate the wavelet transformations. The in-suit Raman spectra were collected by RENISHAW equipped with an excitation wavelength of 633 nm.
Assembly of Solid-State Zn-Air Battery: Firstly, Acrylic acid (2.20 g) and N, N′methylene-bisacrylamide (0.01 g) were dispersed by ultrasound to form a homogeneous solution. Then, 8.4 moL.L -1 KOH solution was added dropwise and stirred continuously under ice bath. Subsequently, 0.3 M K 2 S 2 O 8 solution (80 μL) was added and stirred for 30 s. Finally, the mixture was poured into a glass mold to obtain the thick gel thin film. The carbon cloth covered with CoSe 2 @CoNi LDH-1 HNA catalysts (effective area of 1 cm 2 ) was directly used as air electrodes. The solid-state zinc-air battery was assembled via a layer-by-layer method.
Zinc foil and carbon cloth covered with catalyst were placed on the two sides of the gel thin film. Finally, it was packed with aluminum film.
Calibration of RHE electrode: Ag/AgCl was used as reference electrode for all electrochemical measurements. However, all measured potentials in this work referred to the reversible hydrogen electrode (RHE). The calibration was performed in the high purity hydrogen saturated electrolyte with a Pt wire as the working electrode. CVs were run at a scan rate of 1 mV s−1, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions.

Computational Calculations:
We have the first-principles [3] were employed to perform all spin-polarization density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) [4] formulation. We have chosen the projected augmented wave (PAW) potentials [5,6] to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10 −4 eV. A geometry optimization was considered convergent when the energy change was smaller than 0.05 eV Å −1 . The vacuum spacing in a direction perpendicular to the plane of the structure is 18 Å. The Brillouin zone integration is performed using 2×2×1 Monkhorst-Pack k-point sampling for a structure. Finally, the adsorption energies (Eads) were calculated as E ads = E ad/sub -E ad -E sub , where E ad/sub , E ad , and E sub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively. The free energy was calculated using the equation: where G, E, ZPE and TS are the free energy, total energy from DFT calculations, zero-point energy and entropic contributions, respectively. In our structure, the NiCo LDH (001) surface and CoSe 2 (211) surface had been established using first-principles from the bulk structure.

G E ZPE TS
   Figure S1. SEM of branched Co precursor. Figure S2. SEM of aligned Co precursor. Co precursors with two different morphologies were obtained through hydrothermal reaction. One is aligned nanoarray, and the other is branched array ( Figure S1).