Self-Supported Hierarchical Nanostructured NiFe ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 380−388

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Self-Supported Hierarchical Nanostructured NiFe-LDH and Cu3P Weaving Mesh Electrodes for Efficient Water Splitting Heng Wang, Tingting Zhou, Pengli Li, Zhen Cao, Wei Xi, Yunfeng Zhao,* and Yi Ding Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, 391 West Binshui Road, Tianjin 300384, China S Supporting Information *

ABSTRACT: Highly efficient low-cost electrocatalysts for water splitting have attracted increasing interest in the development of energy storage and conversion. Here, we utilized copper (Cu) weaving mesh to in situ fabricate earthabundant elements-based integrated electrodes for high performance water splitting, where NiFe layered double hydroxide (NiFe-LDH) ultrathin nanoarrys for oxygen evolution reaction (OER) and Cu3P nanowires for hydrogen evolution reaction (HER) were successfully constructed on the Cu mesh. Notably, large stable current densities are obtained for both OER (600 mA cm−2) and HER (200 mA cm−2) electrodes under low overpotential, which is superior to most of the nanoparticle-based electrodes. The large current density is mainly because of the excellent conductivity and clean surface (binderfree) of the Cu mesh-based electrode, and which is extremely important for industrial application. The prepared integrated electrodes are coupled with a macroscopic porous sieve and microscopic nanostructures. The assembled NiFe-LDH∥Cu3P electrolyzer exhibits a small cell working voltage of 1.72 V under the current density of 10 mA cm−2 at room temperature, as well as long-term stabilities (>10 h) in 1 M KOH. These excellent performances of our earth-abundant elements-based weaving mesh electrode result from their improved charge transfer, surface area, mass transport, and faster kinetics of catalytic reactions. KEYWORDS: Oxygen evolution reaction, Hydrogen evolution reaction, Water splitting, Cu3P, NiFe-LDH

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INTRODUCTION Developing efficient hydrogen gas (H2) production technologies is very important and urgent for sustainable development for human beings due to the intrinsic advantages of the highest power density and zero carbon emissions for hydrogen energy. Electrochemical water splitting is regarded as one of the most promising approaches for hydrogen evolution owing to its characterizations of low working potential and simple setup.1−6 Generally, electrochemical water splitting can be simply classified into two half-reactions: hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode.2,4,5,7,8 Nevertheless, both reactions (especially for OER) have been mitigated by the intrinsic activation barriers (e.g., activation energy, mass transfer, and conductivity), which inevitably lead to the larger overpotentials of both electrode reactions.9−13 To satisfy the industrial demand, fabrication strategy of various electrodes must be considered in terms of both material and structural aspects.14,15 Particularly, the cost of materials is strongly associated with the process of design and preparation of high intrinsic activity catalysts. Up to now, the state-of-the-art catalysts for water electrolysis are still the noblemetal-based materials, for instance, Pt-based materials for HER and Ru- or Ir-based materials for OER.2,16,17 However, the scarcity, high cost, and unsatisfactory stability of these precious metals seriously impede their large-scale applications. Further research efforts have been committed to inexpensive, highly © 2017 American Chemical Society

efficient, and durable nonprecious metal electrocatalysts for the two half-reactions. For OER catalysis, layered double hydroxide (LDH) materials exhibited robust electroactivity as OER catalysts, among which LDH materials composed of the earth-abundant 3d transition elements have demonstrated its superior performance,18−21 examples include ZnCo-LDH,22 NiCo-LDH,23 CoFe-LDH,24 and NiFe-LDH.25,26 For the other half-reaction, HER catalysis, considerable efforts have developed many high-performance yet low-cost electrocatalysts, such as metal sulfides, metal carbides, metal nitrides, and metal phosphides.27−32 However, the single half-reaction, as a standalone entity, cannot satisfy the sustained extension of water electrolysis in practical applications. Hence, it is definitely imperative to develop earth-abundant and well-coupled OER/ HER catalysts as a cooperating integration of two electrodes to achieve highly efficient water splitting. Recently, an in situ constructed nanostructured catalysis electrode (integrated electrode) based on porous conductive substrates, such as Ni foam, Cu foam, and carbon cloth, has attracted great attention. Most importantly, it has been demonstrated that these integrated electrodes can effectively split water into H2 and O2 under low potential and large Received: August 2, 2017 Revised: October 26, 2017 Published: November 22, 2017 380

DOI: 10.1021/acssuschemeng.7b02654 ACS Sustainable Chem. Eng. 2018, 6, 380−388

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Illustration of Synthesis Process of NiFe-LDH Electrode as Anode for Oxygen Evolutions and Cu3P Electrode as Cathode for Hydrogen Evolution Applied to Water Electrolysis

electronic structures. The water electrolysis performance of the as-prepared electrodes was investigated in a solution of 1 M KOH. The results confirmed that the NiFe-LDH and Cu3P electrodes, as direct OER and HER catalysts, achieved a very high current density of 100 mA cm−2 at an overpotential of 300 and 382 mV and a low Tafel slope of 61 and 107 mV/dec, respectively. Furthermore, a water electrolyzer assembled by NiFe-LDH as the anode and Cu3P as the cathode demonstrated that a cell voltage of 1.71 V was achieved at a current density of 10 mA cm−2 with a significant increase in performance after 12 h of continuous testing at a constant cell voltage of 1.75 V.

current density. Dai et al. reported that a nickel oxide/nickel (NiO/Ni) heterojunction-like material attached to a mildly oxidized carbon nanotube exhibiting high HER catalytic activity enables a high-performance electrolyzer with ∼20 mA cm−2 at a voltage of 1.5 V.33 Qiao et al. used Ni foam substrates to prepare the bifunctional hollow Co3O4 microtubule arrays electrode displaying efficient overall water splitting, even competing with the performance of the commercial catalysts.34 Li et al. described that a nickel−phosphorus nanoparticles film electrodeposited on copper foam (Ni−P/CF), as a Janus electrocatalyst, showed high catalytic performance for both OER and HER and produced 10 mA cm−2 at a cell voltage of 1.68 V in an alkaline water electrolyzer.35 In addition, Yang et al. synthesized Mn−Co−borate nanowires arrays and MnCo2O4 nanowires arrays grown on carbon cloth (Mn− Co−Bi/CC, MnCo2O4/CC), respectively, achieving 10 mV cm−2 at a cell voltage of 1.97 V in near-neutral media.36 In our reported study, we have developed a method to prepare metaldoped hierarchical Co-based and Ni-based hydroxide electrocatalysts integrated on a three-dimensional Cu foam electrode, which exhibited high OER activity.37,38 Furthermore, the “integrated electrode” fabricated by the in situ procedure possesses several advantages that can effectively overcome the drawbacks of a “nanoparticles-based electrode”. Self-supported materials directly employed as electrodes create a variety of advantages to improve the performance of electrocatalysts, including enlarging the electrochemical surface area, expediting the diffusion of electrolyte, and gas bubbles transport from the catalyst surface.39−41 Moreover, the in situ grown nanostructure catalysts on current collectors can effectively improve the electron transformation due to better interface contacts and binder-free characteristics.42,43 Therefore, the supported homologous electrocatalysts for water splitting are uniquely valuable for extensive attention and indepth research. In this contribution, we take advantage of commercially available copper weaving mesh as both support and current conductors to promote growth of low-cost nanostructured electrochemical catalysts on it. After simple treatment, Cu2O nanowires are grown on the surface of Cu mesh, which acts as structure-directing templates to synthesize highly efficient NiFe-LDH catalysts for OER and Cu3P nanowires catalysts for HER. The homologous OER and HER catalysts derived from the same precursor were proposed to facilitate the preparation process and to optimize the morphology and

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EXPERIMENTAL SECTION

Synthesis of Cu2O Nanowires. Cu mesh (100 mushes, Hebei Shijiazhuang Yuanpeng Metal Co., Ltd.) was first cut into 1 cm × 2 cm pieces and cleaned by water (18.2 MΩ cm) and absolute ethanol before using and then anodized in 3 M KOH aqueous solution for 3 min under 40 mA cm−2 to form Cu(OH)2 nanowires arrays. The asanodized Cu(OH)2 nanowires on Cu mesh were treated under Ar atmosphere at 400 °C for 2 h with a heating rate of 1 °C min−1; then, Cu2O nanowires arrays were fabricated. Synthesis of NiFe-LDH. The NiFe-LDH was fabricated through the cation exchange method as described before.44 Typically, a 1.36 g mixture of NiCl2·6H2O and FeCl2·4H2O (mole ratio of 1:1) were added into 70 mL of an ethanol/water mixed solvent (volume ratio of 3:4) to form a suspension. Then, 40 mL of Na2S2O3·5H2O (1.5 M) was added into the suspension. After 30 min of continuous stirring, Cu mesh covered with Cu2O nanowires arrays was immersed into the above suspension at room temperature for 1 h. Eventually, the resulting mesh was washed by water and ethanol repeatedly, followed by drying at 80 °C in vacuum overnight. Synthesis of Cu3P Nanowires. In order to prepare Cu3P, the asobtained Cu2O nanowires arrays were placed at the downstream side of the tube furnace, with 10 mg of NaH2PO2 at the upstream side. Then, the tube furnace was heated to 300 °C with a heating rate of 2 °C min−1 under an Ar atmosphere and held at this temperature for 120 min. After being naturally cooled to room temperature, the Cu3P nanowires grown on Cu mesh were obtained. Next, the effect of the different amounts of phosphide on the structure and electrocatalytic activity was studied. A lower amount (5 mg) and a higher amount (30 mg) were applied, and the obtained samples were tagged as Cu3P-L and Cu3P-H, respectively. Characterization. X-ray diffraction patterns (XRD) for qualitative phase analysis were recorded on a Rigaku Ultima Iv X-ray diffractometer (Cu Kα, λ = 1.5406 Å) at a scan rate of 10° min−1 in the 2θ range from 5° to 80°. The morphologies and EDAX mapping of synthesized catalysts were investigated using a Verios 460L scanning 381


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Figure 1. (a) XRD patterns of the NiFe-LDH (peaks marked with “cloverleaf”, “⧫”, and “#” denote NiFe-LDH, Cu2O precursor, and Cu substrate, respectively). (b, c) SEM images of NiFe-LDH. (d) EDS mapping. electron microscope (SEM). HR-TEM was performed on FEI TalosF200X at 200 kV to investigate their structural features. The X-ray photoelectron spectrum (XPS) measurement was performed on ESCALAB 250Xi, and all the spectra were referenced to the C 1s at binding energy of 284.8 eV. The genuine contents of the elements in electrocatalysts were evaluated by inductively coupled plasma (ICP) emission spectrometry (VISTA-MPX). Electrochemical Measurements. All electrochemical measurements were performed in a three-electrode system controlled by an electrochemistry workstation. The as-prepared samples were used as the working electrode directly, a platinum wire as the counter electrode, and saturated Hg/HgO as the reference electrode. Linear sweep voltammetry with scan rates of 5 mV s−1 were conducted at room temperature in 1 M KOH solution. The Cu mesh loaded with IrO2 or Pt/C (20%) (0.64 mg cm−2) was used as the electrode for comparison under the same conditions. There were several iR drops for compensation, and no stirring was used for the polarization curve. All experiments were conducted in an aqueous solution of 1 M KOH (pH 14), and the potentials reported here were calibrated with respect to the reversible hydrogen electrode (RHE): E (RHE) = E (Hg/HgO) + 0.098 + 0.0591 × pH. In order to determine the electrochemical surface area, cyclic voltammetry at the non-Faradaic region for OER and HER was performed in a 1 M KOH solution at various scan rates (10−100 mV s−1) to obtain the double-layer capacitance (Cdl), which was used to evaluate the electrochemical surface area (ECSA). For this, Cdl was estimated by plotting the linear curve of Δj at a given potential against the scan rate. The slope is twice the double-layer capacitance Cdl.

structures and components of the synthesized electrodes were immediately characterized. It can be seen that the aligned Cu(OH)2 nanowires vertically grow on Cu mesh and kept their original morphology after being transferred to Cu2O nanowires (Figure S1a,b). The loading amount of active materials supported on electrodes were measured by ICP (Table S1). The results show the atomic ratios of the content of Ni and Fe in NiFe-LDH is approximately 1:1, and the total loading amount of NiFe-LDH is 0.85 mg cm−2 calculated by combining the ICP with EDS data. In addition, the content of the Cu element in Cu3P loaded on the catalyst for the cathode was calculated based on the content of the P element due to the Cu basement. The mass loading of Cu3P was about 1.8, 3.8, and 7.9 mg cm−2 depending the phosphating condition. Correspondingly, the XRD patterns (Figure S1c,d) clearly indicated that the Cu(OH)2 and Cu2O phase are in conformity with the standard diffraction patterns of orthorhombic Cu(OH)2 (JCPDS card No. 35-0505) and cubic structured Cu2O (JCPDS card No. 77-0199), respectively. Besides, three strong peaks at 43.3°, 50.4°, and 74.1° originate from the Cu mesh substrate (JCPDS card No. 04-0836), and no other impurities, such as CuOx, could be detected in the as-prepared samples. Immediately, NiFe-LDH was fabricated by templating against Cu2O nanowires precursor with a cation exchange method (see Experiment Section). The XRD result demonstrates that NiFeLDH in the as-prepared samples is well crystallized, and the characteristic peaks at 9.9°, 19.9°, and 34.5° are consistent with the typical profile of LDH materials (Figure 1a).46,47 SEM images of the as-synthesized NiFe-LDH displays an open 3D architecture grown vertically on the Cu2O precursor (Figure 1b,c). Furthermore, the LDH materials are composed of interconnected thin nanoflakes with a thickness of approximately 15 nm, which could result in a rougher surface to provide more active sites as compared with the original Cu2O nanowires template. The corresponding elemental mapping analysis confirms a uniform intensity distribution of all elements proving a homogeneous chemical composition of

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RESULTS AND DISCUSSION As shown in Scheme 1, first, the Cu(OH)2 nanowires arrays are directly grown on Cu mesh by a simple anodic oxidation process, with a subsequent thermal annealing to obtain a homogeneous Cu2O nanowires precursor with a growth length of about 8 μm as reported before.45 For the fabrication of the OER electrode, Cu2O nanowires acted as a self-sacrificial template to direct the growing of crystalline NiFe-LDH nanostructured catalysts by a solution-phase cation exchange method. On the other hand, Cu3P nanowires employed as the HER catalyst were facilely synthesized by phosphating Cu2O nanowires in a tube furnace by heating to 300 °C. The 382


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Figure 2. (a) XRD patterns of Cu3P electrode. (b) SEM images of Cu2O precursor. (c, d) SEM images of Cu3P electrode.

Figure 3. High-resolution XPS spectra of (a) Ni 2p, (b) Fe 2p, (c) O 1s of NiFe-LDH, (d) Cu 2p, and (e) P 2p of Cu3P.

compositions of as-prepared samples, XRD patterns of Cu3P nanowires are revealed in Figure 2a, in which the characteristic diffraction peaks at 2θ = 36.2°, 39.3°, 41.8°, 45.1°, 46.5°, and 47.3° corresponding with the crystal faces of (112), (202), (211), (300), (113), and (212), respectivey, are obviously stronger in all the reflections, which are consistent with those of the standard values (JCPDS card No. 02-1263). It can be identified that the Cu3P nanowires are successfully synthesized by phosphating the Cu2O precursor. SEM images (Figure 2c,d) clearly illustrate that the Cu3P nanowires supported on the copper mesh dendritically interlace with each other, just as a

the LDH lay (Figure 2d and Figure S2). The residual Cu (4%) may originate from the Cu2O template and Cu mesh, and Ni and Fe are equal in proportion with the total contents of 28.8%, which is well verified with the ICP result (Table S1). The Cu3P nanowires grown on copper mesh were facilely synthesized by the phosphorization of Cu2O nanowires. Then, a series of qualities of different phosphorization were compared to optimize the optimal samples. It can be clearly seen that the morphologies of Cu3P samples have changed from original nanowires to irregular bulges with increasing amounts of phosphide (Figure S3). In order to further confirm the 383


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Figure 4. (a) OER polarization curves of Cu2O precursor, IrO2, and NiFe-LDH electrode in 1 M KOH. (b) Tafel slopes derived from OER polarization curves. (c) Stability test for commercial IrO2 and NiFe-LDH. (d) HER polarization curves of Cu2O precursor, Pt/C, and Cu3P electrode in 1 M KOH. (e) Tafel slopes derived from HER polarization curves. (f) Stability test for commercial Pt/C and Cu3P electrodes.

materials.50 The single peak (531.3 eV) of O 1s XPS spectrum (Figure 3c) further demonstrates the existent of the hydroxide of Ni and Fe in the LDH materials.51 The atomic ratio of Ni and Fe was measured to be 1:1, signifying the composition of NiFe-LDH in accord with the corresponding EDS and ICP data. On the other hand, it is clearly shown that the characteristic peaks of Cu 2p appear at 933.1 (2p3/2) and 952.8 eV (2p1/2) (Figure 3d and Figure S5), and the binding energies of 934.6 and 954.4 eV correspond to the Cu 2p peaks in oxidized copper.52 The satellite peaks at 943.8 and 962.5 eV also show the existence of the oxidation state of copper. For the highresolution P 2p spectrum (Figure 3e), the peak of P 2p3/2 in the spectrum is located at 129.4 eV with a spin energy separation of around 0.8 eV, and the other peak at 133.7 eV is assigned to the metal phosphate due to the electrode surface exposed to air.53,54 The peaks at 934.6 eV for Cu 2p3/2 and 129.4 eV for P 2p3/2 are close to the binding energy for Cu 2p and P 2p in Cu3P, respectively. The electrochemical activity of NiFe-LDH toward OER was evaluated using a three-electrode system in 1 M KOH solution,

secondary growth. According to the previous reports, the phosphating process of cubic Cu2O can be illuminated following that Cu2O is reduced to zerovalent Cu by PH3 produced in situ from the thermal decomposition of NaH2PO2,48 and the resulting Cu subsequently catalyzes the decomposition of PH3 into elemental P, which further combines with surficial Cu to form hexagonal Cu3P.49 XPS analysis was performed to study the surface elemental compositions and valence states of NiFe-LDH and Cu3P electrodes. The full survey XPS spectrum (Figure S4) shows that NiFe-LDH comprises Ni, Fe, O, and C elements. The high resolution XPS results (Figure 3a) show that the binding energy peaks of Ni 2p3/2 and Ni 2p1/2 are located at 856.1 and 873.6 eV, respectively, indicating the presence of Ni2+ species in NiFe-LDH. Additionally, the intense signals located at 862.1 and 880.1 eV corresponding to the satellite peaks of Ni 2p3/2 and Ni 2p1/2, respectively, certainly indicate the main compositions of Ni2+. The high-resolution Fe 2p XPS spectra (Figure 3b) displays that the peak of 2p3/2 is located at 712.3 eV, and the spin−orbit splitting value of 2p1/2 and 2p3/2 reaches 13.5 eV, suggesting the +3 oxidation states of Fe in the LDH 384


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Figure 5. (a) Linear sweep voltammetry of water electrolysis using NiFe-LDH as the anode and Cu3P as the cathode and in 1 M KOH. Inset: Photograph showing H2 and O2 bubbles generation during water electrolysis. (b) Stability test for water electrolysis at a cell voltage of 1.75 V.

Subsequently, Cu3P was applied as the HER electrode with a scan rate of 5 mV s−1 in 1 M KOH solution. For comparison, the Cu2O precursor and commercial Pt/C were also measured. From the LSV polarization curves (Figure 4d), Pt/C shows the highest activity with an onset overpotential near zero, while the Cu2O precursor shows the inactive electroactivity with an onset overpotential of 340 mV. In contrast, the Cu3P electrode affords good performance and the current density increases rapidly from a small overpotential. It can be seen (Figure S9a) that Cu3P, Cu3P-L, and Cu3P-H reach a catalytic current density of 10 mA cm−2 at overpotentials of 266, 328, and 275 mV, respectively. Therefore, as-prepared Cu3P is an optimized electrocatalyst, which also achieves a high current density at the same overpotential. A Tafel slope of 107 mV/dec for Cu3P is superior to the reported value for three-dimensional Cu3P catalysts.52 In contrast, the inadequate or excessive phosphating shows relatively sluggish kinetics corresponding to Cu3P-L (124 mV/dec) and Cu3P-H (120 mV/dec), respectively, which can be attributed to the unfavorable structure mitigating HER on the electrode (Figure S9b). After 400 consecutive polarization scans, the XRD and XPS patterns and lattice spacing of HRTEM of the Cu3P electrode are almost consistent with that of the initial samples (Figures S10 and S11), which indicate that the composition and element valence of the HER electrode is stable after polarization scans. Besides, it is indispensable to measure the long-term durability of the electrode for practical applications. The Cu3P electrode was investigated at a constant overpotential in 1 M KOH solution. It can be seen that the current density shows no noticeable degradation (loss of 10%) after 12 h of testing (Figure 4f). In sharp contrast, the current density of commercial Pt/C decreases gradually under the same conditions. For the corresponding ECSA, the calcuated Cdl value of Cu3P electrode is 16.3 mF cm−2 extracted by plotting the Δj (ja − jb) at 0.25 V vs RHE against the scan rate (Figure S8). The ECSAs of NiFe-LDH and Cu3P have decreased little after a 12 h durability test, exhibiting excellent stability and continually exposed active sites during the catalytic reaction (Figure S8). The TOF of Cu3P is very promising, which is close to 0.279 s−1 at an overpotential of 300 mV. Encouraged by the promising results, we subsequently used the NiFe-LDH electrode as the anode and the Cu3P electrode as the cathode to build an electrolyzer for overall water splitting in 1 M KOH at room temperature. As shown in Figure 5a, one can see that the onset potential is as low as 1.61 V (overpotential of ∼380 mV), and a catalytic current density of 10 mA cm−2 is observed at a voltage of 1.72 V for water electrolysis using the noble-metal-binder-free catalyst, signifying an overpotential of ∼490 mV to achieve overall water splitting.

as shown in Figure 4a. Linear sweep voltammetry (LSV) was carried out at 5 mV/s for the polarization curves. The Cu2O precursor and IrO2 coated on copper mesh as two references were also examined for comparison. The anodic current recorded with the NiFe-LDH catalyst shows a sharp onset of the greatest OER current at ∼1.52 V vs the reversible hydrogen electrode (RHE). In addition, a shoulder peak at around 1.50 V of NiFe-LDH is caused by the transformation of Ni2+ to Ni3+ species (Figure 4a).55 Consequently, a high OER rate (e.g., 100 mA cm−2) can be achieved only by having a small overpotential (300 mV), which is far less than the IrO2 catalyst (498 mV). By contrast, the Cu2O precursor shows very little OER activity, indicating that the excellent electrocatalytic activity of NiFeLDH has nothing to do with the precursor. Additionally, the Tafel slopes for all electrocatalysts have been further calculated to evaluate the reaction kinetics (Figure 4b). A small Tafel slope of NiFe-LDH was observed to be 61 mV/dec, which is smaller than that of the Cu2O (149 mV/dec) precursor and IrO2 (72 mV/dec). This high electrochemical activity of NiFeLDH is attributed to abundant active metal sites acting on the OER. In addition to high catalyst activity, the NiFe-LDH catalyst exhibits a long-term durability for OER in 1 M KOH solution, without a significant degradation (loss of 11.8%) of current density after 12 h (Figure 4c), indicating its outstanding electrocatalytic durability. While the current density remarkably decreases (loss of 50.2%) for the IrO2 catalyst under the same condition. As shown in Figure S6a, after polarization scans, the diffraction peaks of NiFe-LDH were very weak, which could be attributed to the decrease in crystallinity caused by catalysts oxidized to oxyhydroxide.56 Furthermore, the corresponding TEM and HRTEM images (Figure S7c,d) show poor crystallinity, which is due to the generation of oxyhydroxide during OER. To further verify this, we carried out the XPS characterization for them. As shown in Figure S6b−d, shifting toward lower energy was observed for Ni 2p, Fe 2p, and O 1s, resulting from formation of OOH on the surface of NiFe-LDH during OER.57 The TOF of NiFe-LDH at overpotential of 300 mV is calculated as 0.026 s−1, implying an efficient electrocatalytic performance for OER. Moreover, ECSA is a significant factor to evaluate the electrochemical activity of catalysts. According to the reported literature,58 the linear slope of capacitive current against scan rate, equivalent to twice of the double-layer capacitance Cdl, was used to represent the ECSA. It can be seen (Figure S8) that the capacitance (21.6 mF cm−2) of NiFe-LDH is much higher than the previously reported NiCo-LDH59 and NiFe-LDH,60 manifesting a larger ECSA for our as-prepared NiFe-LDH catalyst. 385


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Table 1. Electrocatalytic Performance List of Low-Cost Metal-Based-Integrated Electrodes for Full Water Splitting OER OER||HER Cat. NiCo2O4||Ni0.33Co0.67S2 CoNi(OH)x||NiNx NiFe-LDH||Ni/NiO NiFe/Ni(OH)2/NiAl||NiMo/Ni(OH)2/NiAl Ni(OH)2||NiSe2 NiFe-LDH||NiO/Ni-CNT NiFe-LDH||NiFe-LDH CoP-MNA||CoP-MNA Ni5P4||Ni5P4 NiFe-LDH||Cu3P

Electrolyte 1 1 1 1 1 1 1 1 1 1

M M M M M M M M M M

KOH KOH NaOH KOH KOH KOH NaOH KOH KOH KOH

HER

Full water splitting

ηonseta

η10/100b

TSc

ηonseta

η10/100b

TSc

ηworkingd

Stability

Ref

156 148 146 146 158 NA 140 150 150 152

160/171 151/157 NA/260 149/164 164/NA NA 147/168 152/159 1.54/NA NA/153

60 77 31 NA 60 NA NA 65 40 61

−50 −150 ∼0 ∼0 −110 ∼0 −100 −30 −100 −228

88/244 NA/325 50/137 78/NA 184/NA 31/100 210/NA 54/121 150/NA 266/382

118 127 65 NA 76.6 51 NA 51 40 107

1.72 1.65 1.48 1.59 1.78 1.5 (20) 1.70 1.62 1.70 1.72

20 h 600 s 24 h 10 h 2h 24 h 10 h 32 h NA 20 h

61 45 62 63 64 33 65 66 67 this study

ηonset represents the onset potential with the unit of mV. bη10/100 represents the working potential at the current density of 10 and 100 mA cm−2 with the unit of mV, respectively. cTS means Tafel slope of electrode with the unit of mV/dec. dηworking represents of the working potential at the current density of 10 mA cm−2 of full water splitting in a two-electrode cell with the unit of V. a

Next, long-term stability was further measured at a voltage of 1.75 V for 20 h at room temperature to evaluate a practical electrolyzer. Impressively, the water electrolysis voltage is not reduced but instead continuously increased in value within 6 h and exceeded the initial value of 21.5% after running 20 h (Figure 5b). The Faradaic efficiency (FE) for water electrolysis was calculated by comparing the amount of experimentally quantified gas with theoretically calculated gas (assuming 100% FE) under a steady-state potential of 1.75 V. The agreement of both values suggests 100% FE for both HER and OER with the ratio of H2 and O2 being close to 2:1 (Figure S12). These results suggest that our as-prepared NiFe-LDH and Cu3P electrodes reveal an excellent activity and stability for electrocatalytic water splitting, and this durability is comparable with the reported full water splitting device fabricated from the catalysts composed of earth-abundant elements (Table 1).

ORCID

CONCLUSIONS In summary, we successfully designed and fabricated weaving mesh electrodes for efficient water splitting. The NiFe-LDH decorated electrode gives a low working potential (1.53 V at η100) and Tafel slop (61 mV/dec) in the oxygen evaluation reaction (OER), and Cu3P nanoarrays supported on copper mesh, assembled together for the first time as water electrolysis catalysts possess the properties of large surface area and excellent electrochemical activity and stability in alkaline medium. Thus, our work is marked by a self-supported and binder-free hybridizing system between a double hydroxide and phosphide of a low-cost transition metal to motivate further study with highly effective water electrolysis catalysts.

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Yunfeng Zhao: 0000-0002-1442-992X Yi Ding: 0000-0002-1347-2811 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC, 21402136), Natural Science Foundation of Tianjin City (16JCYBJC17000, 16ZXCLGX00120), and joint research fund of National Natural Science Foundation of China and Macau Science and Technology Fund (FDCT) (NSFC-FDCT, 5171101212). Y.Z. acknowledges support from the “Talent Program” of Tianjin University of Technology and “Youth Thousand Talents Program” and “131 Project “of Tianjin City.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02654. Experimental details of electrochemical measurements and calculations, SEM images, HRTEM images, and EDXS, ICP, XPS, ECSA, XRD data of electrodes. (PDF)

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REFERENCES

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