NiCoFe Layered Triple Hydroxides with Porous Structures as High

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NiCoFe Layered Triple Hydroxides with Porous Structures as HighPerformance Bifunctional Electrocatalysts for Overall Water Splitting An-Liang Wang, Han Xu, and Gao-Ren Li ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00219 • Publication Date (Web): 20 Jul 2016 Downloaded from http://pubs.acs.org on July 24, 2016

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ACS Energy Letters

NiCoFe Layered Double Hydroxides with Porous Structures as HighPerformance Electrocatalysts for Overall Water Splitting An-Liang Wang, Han Xu, Gao-Ren Li* MOE Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China

E-mail: [email protected]

Abstract Here we report a type of NiCoFe layered double hydroxides (ALDHs) supported on carbon fiber cloth (CFC) (NiCoFe ALDHs/CFC) as high-performance electrocatalysts for overall water splitting in alkaline media. The NiCoFe ALDHs/CFC as an oxygen evolution reaction (OER) electrocatalyst shows excellent catalytic activity and durability, such as low overpotential of ~239 mV at 10 mA cm−2, small Tafel slope of ~32 mV/dec, and conservation rate of catalytic activity (~99%) after 12 h continuous electrolysis at 20 mA/cm2. As a hydrogen evolution reaction (HER) electrocatalyst, NiCoFe ALDHs/CFC also shows low onset potential, small Tafel slope and superior durability. The NiCoFe ALDHs/CFC-based overall water splitting exhibits a low onset potential (~1.51 V), a low splitting potential (~1.55 V) at 10 mA cm-2 and excellent durability, and the performance is comparable to or even better than that of IrO2/Pt-based overall water splitting. This work opens a new avenue toward the development of high-performance and inexpensive bifunctional electrocatalysts.

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Hydrogen has been considered as the clean and renewable energy source of the next generation by virtue of its high energy density, lightweight, highly abundant and zero environmental impact of the combustion products.1-3 Electrochemically evolving hydrogen by splitting water is considered to be an efficient method for hydrogen generation.4-5 However, two half reactions of water splitting, namely hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), both require efficient electrocatalysts to enhance sluggish kinetic processes and improve efficiency of water splitting. So far, the state-of-art electrocatalysts are noble metals such as Pt for HER and IrO2 for OER.6-7 However, the high price and scarcity of Pt and IrO2 greatly hinder the development of water splitting. Therefore, the earth-abundunt electrocatalysts with low-cost and high-performance are highly desirable for water splitting.8-12 Owing to the thermodynamic convenience and potential application in proton-exchange membrane or alkaline electrolyzers, great efforts have been devoted to developing HER catalysts for strongly acidic conditions and OER catalysts for strongly basic conditions. The strongly acidic condition and alkaline condition will obviously decrease the overpotentials and accelerate the rates of HER and OER, respectively.12 Many HER catalysts based on the transition-metal sulfides (MoS2, NiS2, FeS2, CoS2, etc.),13-18 selenides (FeSe2, CoSe2, FeSe2, etc.),19-20 phosphides (CoP, Ni2P, FeP, Cu3P, etc.),21-24 carbides (Mo2C, WC, etc.),25-27 borides (MoB, etc.)28 and even non-metal materials (C3N4, N or P-doped carbon, etc.)29-31 have shown superior catalytic performance for HER in strong acidic electrolytes. In addition, the oxides/hydroxides of iron,32-33 cobalt,34-35 nickel,36-37 manganese,38 and copper39 have been reported as high-performance OER electrocatalysts in strongly alkaline solution. However, to accomplish highefficient overall water splitting, the coupling of HER and OER electrocatalysts in the same electrolyte is a huge challenge as the current prevailing approaches often result in incompatible integration of HER and OER electrocatalysts and lead to low-efficient water splitting.40 Recently, the appearance of bifunctional electrocatalysts provides a new revene for water splitting and can avoid these problems. In addition, the bifunctional electrocatalysts used in the same media can simplify the system and reduce the cost of water splitting device. However, up to now, the development of bifunctional electrocatalysts with 2 Plus Environment ACS Paragon

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high-performance for HER and OER in the same electrolyte still is a huge challenge. In this work, we developed NiCoFe layered double hydroxides (ALDHs) supported on carbon fiber cloth (CFC) (NiCoFe ALDHs/CFC) as high-performance bifunctional electrocatalysts for overall water splitting in the same alkaline solution. To the best of our knowledge, this is the first study of triple transition-metal hydroxide electrocatalysts for overall water splitting. Owing to the large specific surface area, nanosheets, porous structures and triple hydroxides, the optimal NiCoFe ALDHs/CFC will provide efficient exposure of the electrocatalytic active sites, free diffusion of gas (H2 and O2), fast electrolyte penetration/diffusion, and strongly synergistic effects among the triple hydroxides. The resulting NiCoFe ALDHs/CFC exhibited remarkable electrocatalytic performance for both HER and OER in the same solution of 1.0 M KOH, such as low onset potential, small Tafel slope and excellent durability. Furthermore, when the NiCoFe ALDHs/CFC was utilized for both anode and cathode electrocatalysts in an overall water electrolyzer in 1.0 M KOH, the high performance such as low onset potential, low splitting potential and excellent durability were achieved, indicating a promising bifunctional electrocatalyst for overall water splitting. The fabrication procedures of the NiCoFe ALDHs/CFC are shown in Figure 1a. The procedure details are described in the Experimental section. SEM images of NiCoFe ALDHs/CFC with different magnifications are shown in Figure 1b-c, which shows that the NiCoFe triple hydroxide nanosheets are uniformly wrapped on the surfaces of CFC. The magnified SEM image of NiCoFe nanosheets is shown in inset in Figure 1c, which clearly shows the porous structure and the ultrathin thickness of 3~5 nm. These nanosheets are linked with each other and the networks are formed. The networks with porous structures will provide enough space for the transportation/diffusion of resultants and reactants. Besides, the nanosheets grown on the CFC directly will guarantee the fast transportation of electron, leading to the reduced resistance of NiCoFe ALDHs/CFC electrocatalysts. EDX data are shown in Figure S1, which shows the existences of Ni, Co, Fe and O elements. Quantitative analysis shows Ni:Co:Fe of 8.7:10.5:1.0 (mole ratio) in the NiCoFe ALDHs/CFC. TEM image of NiCoFe triple hydroxide nanosheets is shown in Figure 1d, which shows the almost transparent nanosheets. High-resolution TEM 3 Plus Environment ACS Paragon

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(HRTEM) image of the NiCoFe triple hydroxide nanosheet is shown in the Figure 1e and almost no lattice fringe is seen, indicating the amorphous structure of NiCoFe nanosheets. Selected area electron diffraction (SAED) pattern is shown in inset in Figure 1e, which only shows amorphous halos and also reveals the amorphous structure of NiCoFe ALDHs/CFC. The scanning TEM (STEM) image of NiCoFe nanosheets is shown in Figure 2a, which clearly shows that the nanosheets own porous structure that will further enhance the surface area. Here the distributions of Ni, Co and Fe in samples were studied by scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) elemental mapping in the marked area in Figure 2a, and the elemental mappings of Co, Fe and Ni are shown in Figure 2b, 2c, and 2d, respectively. It is clear that the elements Ni, Fe and Co are distributed uniformly and the uniform distributions will be highly beneficial for synergistic effects among Ni, Co and Fe. The corresponding electron-energy-loss-spectra (EELS) analyses of NiCoFe nanosheets were also carried out to verify the compositions of samples as shown in Figure 2e. Three pairs of peaks at 716.4/718.3 eV, 787.5/802.8 eV and 863.2/881.7 eV correspond to Fe L2,3 edges, Co L2,3 edges and Ni L2,3 edges, respectively.41-43 The above results further demonstrate the existences of Fe, Co and Ni in the sample. XRD pattern of the NiCoFe ALDHs/CFC is shown in Figure 2f, which shows no diffraction peaks besides the peaks of CFC at 260 and 430. This result also indicates the amorphous structure of NiCoFe nanosheets. In order to investigate the chemical states of NiCoFe ALDHs/CFC, the XPS measurements were carried out. XPS spectrum of Ni 2p region is shown in Figure 3a. Besides two shake-up satellites at 787.53 and 804.28 eV, there are a pair of peaks at 856.35 and 873.90 eV, corresponding to Ni 2p3/2 and 2p1/2, respectively. The energy separation between Ni 2p3/2 and 2p1/2 is ~17.55 eV, and this indicates the existence of Ni(OH)2 phase.44-45 In Co 2p region, the XPS spectrum is fitted with one spin-orbit doublet and two shakeup satellites as shown in Figure 3b. The peaks of Co 2p1/2 at 797.26 eV and Co 2p3/2 at 781.81 eV indicate that the Co element exists as Co(OH)2.46 XPS spectrum of Fe 2p is shown in Figure 3c. The peaks at 712.06 and 726.89 eV correspond to Fe 2p3/2 and 2p1/2, respectively, and they are the characteristic peaks of Fe3+. In addition, the energy separation between Fe 2p1/2 and 2p3/2 is 14.8 eV, 4 Plus Environment ACS Paragon

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indicating the existence of Fe(OH)3.47 XPS spectrum of NiCoFe ALDHs/CFC in O 1s region is shown in Figure 3d, which shows four oxygen peaks. The peak at 532.54 eV is the characteristic peak of cobaltoxygen-hydrogen (Co-O-H) bond. The peak at 531.70 eV is the characteristic peak of nickel-oxygenhydrogen (Ni-O-H) bond. The peak at 531.07 eV is the characteristic peak of iron-oxygen-hydrogen (Fe-O-H) bond. 48-52 The peak at 533.6 eV can be ascribed to the physisorbed or chemisorbed water on the surface of sample.48,50 Therefore, the above results demonstrated that the NiCoFe ALDHs/CFC was successfully fabricated. OER electrocatalytic property of NiCoFe ALDHs/CFC (Ni:Co:Fe=8.7:10.5:1.0, atomic/molar ratio) was investigated in the three-electrode system in O2-saturated 1.0 M KOH solution at the scan rate of 2.0 mV s-1, and the polarization curve is shown in Figure 4a. For comparision study, the OER electrocatalytic properties of NiFe double hydroxides nanosheet arrays(DHNAs)/CFC (Ni:Fe=7.3:1, atomic ratio), CoFe DHNAs/CFC (Co:Fe=8.7:1), NiCo DHNAs/CFC (Ni:Co=1:1.3), Ni single hydroxides nanosheet arrays (SHNAs)/CFC, Co SHNAs/CFC and Fe SHNAs/CFC were also studied in the same conditions. The NiCoFe ALDHs/CFC shows a small onset potential of 1.45 V (the overpotential is only ~0.22 V), which is much smaller than those of NiCo DHNAs/CFC (1.57 V), NiFe DHNAs/CFC (1.50 V), CoFe DHNAs/CFC (1.52 V), Ni SHNAs/CFC (1.51 V), Co SHNAs/CFC (1.51 V) and Fe SHNAs/CFC (1.65 V). As we all know, it is simple and convenient to evaluate the OER catalytic activity of catalyst by the potential at the current density of 10 mA cm-2.53 Herein, when the current density is 10 mA cm-2, the potential of NiCoFe ALDHs/CFC is only ~1.46 V, which is significantly lower than those of CoFe DHNAs/CFC (1.52 V), NiFe DHNAs/CFC (1.49 V), NiCo DHNAs/CFC (1.56 V), Ni SHNAs/CFC (1.58 V), Co SHNAs/CFC (1.54 V) and Fe SHNAs/CFC (1.72 V). So the above results show that the NiCoFe ALDHs/CFC owns much higher electrocatalytic activity than CoFe DHNAs/CFC, NiFe DHNAs/CFC, NiCo DHNAs/CFC, Ni SHNAs/CFC, Co SHNAs/CFC and Fe SHNAs/CFC, and the activity enhancement can be attributed to the synergistic effect of NiCoFe ALDHs/CFC. Compared with other non-precious metal-based electrocatalysts in alkaline media, the NiCoFe ALDHs/CFC also shows much enhanced catalytic activity as shown in Table S1. Furthermore, the NiCoFe ALDHs/CFC gives a small 5 Plus Environment ACS Paragon

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Tafel slope of ~32 mV dec-1, which is much smaller than those of NiFe DHNAs/CFC (65 mV dec-1), CoFe DHNAs/CFC (38 mV dec-1), NiCo DHNAs/CFC (45 mV dec-1), Ni SHNAs/CFC (274 mV dec-1), Co SHNAs/CFC (136.65 mV dec-1) and Fe SHNAs/CFC (130 mV dec-1) as shown in Figure 4b and is even smaller than the well-established OER electrocatalysts, such as IrOx (~49 mV/dec)54 and Ir/C (~40 mV/dec).55 The electrocatalytic durability of the NiCoFe ALDHs/CFC is tested in 1.0 M KOH by chronopotentiometry method at the current density of 20 mA cm-2 as shown in Figure 4c, which shows the excellent durability and the operating potential is very stable for 12 h polarization (~1% fluctuation). The surface morphology (nanosheet networks) of NiCoFe ALDHs/CFC was still maintained very well after chronoamperometry test of 12 h for OER as shown in Figure 4d, indicating excellent structural stability of NiCoFe ALDHs/CFC. To optimize the composition of electrocatalyst, the polarization curves of NiCoFe ALDHs/CFCs with different ion mole ratios of Ni:Co:Fe in the deposition solution were measured in 1.0 M KOH solution as shown in Figure S8, which indicates that the NiCoFe ALDHs/CFC with Ni2+:Co2+:Fe3+ of 1:1:1 (ion mole ratio) owns the highest OER electrocatalytic activity. HER electrocatalytic property of the NiCoFe ALDHs/CFC is tested in N2-saturated alkaline media (1.0 M KOH) at 2 mV s-1 as shown in Figure 5a. The NiCoFe ALDHs/CFC exhibits a low onset potential of ~0.180 V, which is much smaller than those of NiFe DHNAs/CFC (0.266 V), CoFe DHNAs/CFC (0.260 V), NiCo DHNAs/CFC (0.296 V), Fe SHNAs/CFC (0.284 V), Co SHNAs/CFC (0.259 V) and Ni SHNAs/CFC (0.293 V). Here the HER catalytic activity of catalyst is evaluated by the overpotential at the current density of 10 mA cm-2. The NiCoFe ALDHs/CFC affords a current density of 10 mA cm-2 at a small overpotential of 0.20 V, which is much smaller than those of NiFe DHNAs/CFC (0.33 V), CoFe DHNAs/CFC (0.29 V), NiCo DHNAs/CFC (0.35 V), Fe SHNAs/CFC (0.37 V), Co SHNAs/CFC (0.27 V) and Ni SHNAs/CFC (0.40 V). In addition, compared with other non-precious metal-based catalysts in alkaline media, the electrocatalytic activity of NiCoFe ALDHs/CFC shows much enhanced electrocatalytic activity as shown in Table S2. As shown in the Figure 5b, the NiCoFe ALDHs/CFC exhibits a Tafel slope of ~70 mV dec-1 in 1.0 M KOH solution, 6 Plus Environment ACS Paragon

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which is much smaller than those of NiFe DHNAs/CFC (200 mV dec-1), CoFe DHNAs/CFC (88 mV dec-1), NiCo DHNAs/CFC (128 mV dec-1), Fe SHNAs/CFC (180 mV dec-1), Co SHNAs/CFC (148 mV dec-1) and Ni SHNAs/CFC (221 mV dec-1). In addition, the stability of the NiCoFe ALDHs/CFC is tested by chronopotentiometry at the current density of 20 mA cm-2 as shown in Figure 5c, which shows high durability and the operating potential is very stable for 12 h (~5% fluctuation). The surface morphology of NiCoFe ALDHs/CFC was still maintained very well after the chronoamperometry test of 12 h for HER as shown in Figure 5d, indicating the excellent structural stability of NiCoFe ALDHs/CFC. The composition effect of NiCoFe ALDHs/CFCs on the electrocatalytic activity was studied in 1.0 M KOH solution as shown in Figure S9, which also shows that the NiCoFe ALDHs/CFC with Ni2+:Co2+:Fe3+ of 1:1:1 (ion mole ratio) in the deposition solution owns the best HER activity. The superior OER and HER electrocatalytic performances of NiCoFe ALDHs/CFC can be attributed to the following factors: i) the synergistic effect of NiCoFe ALDHs realizes smaller onset potential, smaller overpotential, smaller Tafel slope and higher durability compared with the double hydroxides and single hydroxies; ii) the uniform distributions of Ni, Co and Fe will be in favor of the synergistic effect of NiCoFe ALDHs and accordingly this will further promote the improvement of performance; iii) the direct integration of NiCoFe ALDHs onto CFC enables efficient mass and electron transfer and good mechanical adhesion, which will be beneficial for the improvement of electrocatalytic activity and stability; iv) the open spaces between nanosheets can facilitate the transportations and diffusions of the active species and resultant species, thus enhancing the electroactive of electrocatalyst; v) the NiCoFe ALDHs/CFC will be much less vulnerable to dissolution, Ostwald ripening and aggregation because of the nanosheet arrays, which will be highly beneficial to the improvement of stability of electrocatalysts; vi) the self-standing feature of NiCoFe ALDHs/CFC will avoid the use of binder that may block active sites and inhibit diffu-sion and have an excellent electrical contact with the current collectors, and this will let NiCoFe ALDHs effectively participate in the catalytic reactions and almost no “dead” volume. Considering the excellent OER and HER performances, the NiCoFe ALDHs/CFC was utilized as the bifunctional electrocatalyst for overall water splitting in alkaline solution. The two-electrode system 7 Plus Environment ACS Paragon

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employing NiCoFe ALDHs/CFC as both anode and cathode electrocatalysts was constructed, and the polarization curves were measured in N2- saturated 1.0 M KOH solution at 2.0 mV s-1. In addition, the Pt/C as both anode and cathode catalysts with the same loadings were also tested for comparison study. As shown in Figure 6a, the NiCoFe ALDHs/CFC-based overall water electrolysis shows the onset potential of ~1.51 V, which is much lower than 1.62 V of Pt/C-based overall water electrolysis. When the current density is 10 mA cm-2, the potential of NiCoFe ALDHs/CFC-based water splitting is 1.55 V, which is also much smaller than that of Pt/C (> 2.0 V). In addition, the potentials of overall water splitting of NiCoFe ALDHs/CFC bifunctional electrocatalyst at 10 and 20 mA cm-2 are compared with those of other bifunctional electrocatalysts reported in literatures as shown in Figure 6b, 6c and Table S3, which all show that the potentials of NiCoFe ALDHs/CFC-based overall water splitting at 10 and 20 mA cm-2 are much lower than those of other bifunctional electrocatalysts. Furthermore, the NiCoFe ALDHs/CFC bifunctional electrocatalyst shows lower splitting potential than IrO2/Pt as shown in Figure 6b and 6c. In order to further study the durability of NiCoFe ALDHs/CFC as functional electrocatalysts, the chronopotentiometry of overall water splitting was performed at the current density of 10 mA cm-2 as shown in the Figure 6d, which shows that the water splitting potential is very stable for 19 h durative electrolysis (~2% fluctuation). Therefore, the NiCoFe ALDHs/CFC as functional electrocatalysts showed high electro-catalytic activity and durability for overall water splitting in alkaline solution. In conclusion, we have reported NiCoFe ALDHs/CFC as high-performance electrocatalysts for overall water splitting. The NiCoFe ALDHs/CFC shows excellent performance for OER and HER, such as low onset potential and excellent durability, and it only requires the potentials of 1.47 V for OER and 0.20 V for HER to reach 10 mAcm-2 with Tafel slopes of 32 and 70 mV dec-1, respectively, in alkaline media. The NiCoFe ALDHs/CFC-based overall water splitting exhibits a low onset potential of ~1.51 V, a low splitting potential of ~1.55 V at 10 mA cm-2 and excellent durability for more than 19 h durative electrolysis (~2% fluctuation). This study will open up a new route for the design and fabrication of ternary transition-metal hydroxides as bifunctional electrocatalysts for overall water splitting.


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Supporting information SEM images, EDS spectrum and CVs. The authors declare no competing financial interest.

Acknowledgements This work was supported by National Basic Research Program of China (2016YFA0202603 and 2015CB932304), Natural Science Foundation of Guangdong Province (S2013020012833 and 2016A010104004), and Fundamental Research Fund for the Central Universities (16lgjc67).

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Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299. (14) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nat. Mater. 2012, 11, 963-969. (15) Yu, X. Y.; Yu, L.; Wu, H. B.; Lou, X. W. D. Formation of Nickel Sulfide Nanoframes from Metal–Organic Frameworks with Enhanced Pseudocapacitive and Electrocatalytic Properties. Angew. Chem., Int. Ed. 2015, 54, 5331-5335. (16) Faber, M. S.; Lukowski, M. A.; Ding, Q.; Kaiser, N. S.; Jin, S. Earth-Abundant Metal Pyrites (FeS2, CoS2, NiS2, and Their Alloys) for Highly Efficient Hydrogen Evolution and Polysulfide Reduction Electrocatalysis. J. Phys, Chem. C 2014, 118, 21347-21356. (17) Wang, D.-Y.; Gong, M.; Chou, H.-L.; Pan, C.-J.; Chen, H.-A.; Wu, Y.; Lin, M.-C.; Guan, M.; Yang, J.; Chen, C.W. Highly Active and Stable Hybrid Catalyst of Cobalt-Doped FeS2 Nanosheets–Carbon Nanotubes for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 1587-1592. (18) Faber, M. S.; Dziedzic, R.; Lukowski, M. A.; Kaiser, N. S.; Ding, Q.; Jin, S. High-Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS2) Micro- and Nanostructures. J. Am. Chem. Soc. 2014, 136, 10053-10061. (19) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 4897-4900. (20) Liang, J.; Yang, Y.; Zhang, J.; Wu, J.; Dong, P.; Yuan, J.; Zhang, G.; Lou, J. Metal Diselenide Nanoparticles as Highly Active and Stable Electrocatalysts for the Hydrogen Evolution Reaction. Nanoscale 2015, 7, 14813-14816. (21) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 53, 6710-6714. (22) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc.2013, 135, 9267-9270. (23) Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X. A Cost-Effective 3D Hydrogen Evolution Cathode with High Catalytic Activity: FeP Nanowire Array as the Active Phase. Angew. Chem., Int. Ed. 2014, 53, 1285512859. (24) Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Self-Supported Cu3P Nanowire Arrays as an Integrated HighPerformance Three-Dimensional Cathode for Generating Hydrogen from Water. Angew. Chem., Int. Ed. 2014, 53, 9577-9581. (25) Ma, F. X.; Wu, H. B.; Xia, B. Y.; Xu, C. Y.; Lou, X. W. D. Hierarchical β-Mo2C Nanotubes Organized by Ultrathin Nanosheets as a Highly Efficient Electrocatalyst for Hydrogen Production. Angew. Chem., Int. Ed. 2015, 54, 15395-15399. (26) Liu, Y.; Yu, G.; Li, G. D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. Coupling Mo2C with Nitrogen-Rich Nanocarbon Leads to Efficient Hydrogen-Evolution Electrocatalytic Sites. Angew. Chem., Int. Ed. 2015, 54, 10752-10757. (27) Fan, X.; Zhou, H.; Guo, X. WC Nanocrystals Grown on Vertically Aligned Carbon Nanotubes: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. ACS Nano 2015, 9, 5125-5134. (28) Vrubel, H.; Hu, X. Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in both Acidic and Basic Solutions. Angew. Chem., Int. Ed. 2012, 51, 12703-12706. (29) Han, Q.; Wang, B.; Zhao, Y.; Hu, C.; Qu, L. A Graphitic-C3N4 "Seaweed" Architecture for Enhanced Hydrogen


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ACS Energy Letters

Evolution. Angew. Chem., Int. Ed. 2015, 54, 11433-11437. (30) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@N-Graphene Films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931-940. (31) Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. Toward Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution. ACS Nano 2014, 8, 5290-5296. (32) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni–Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2013, 135, 12329-12337. (33) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R. Identification of Highly Active Fe Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting. J. Am. Chem. Soc. 2015, 137, 1305-1313. (34) Sa, Y. J.; Kwon, K.; Cheon, J. Y.; Kleitz, F.; Joo, S. H. Ordered Mesoporous Co3O4 Spinels as Stable, Bifunctional, Noble Metal-free Oxygen Electrocatalysts. J. Mater. Chem. A 2013, 1, 9992-10001. (35) Du, S.; Ren, Z.; Zhang, J.; Wu, J.; Xi, W.; Zhu, J.; Fu, H. Co3O4 Nanocrystal Ink Printed on Carbon Fiber Paper as a Large-area Electrode for Electrochemical Water Splitting. Chem. Commun. 2015, 51, 8066-8069. (36) Xiao, M.; Tian, Y.; Yan, Y.; Feng, K.; Miao, Y. Electrodeposition of Ni(OH)2/NiOOH in the Presence of Urea for the Improved Oxygen Evolution. Electrochim. Acta 2015, 164, 196-202. (37) Zhou, X.; Xia, Z.; Zhang, Z.; Ma, Y.; Qu, Y. One-step Synthesis of Multi-walled Carbon Nanotubes/ Ultrathin Ni(OH)2 Nanoplate Composite as Efficient Catalysts for Water Oxidation. J. Mater. Chem. A 2014, 2, 1179911806. (38) Guo, C. X.; Chen, S.; Lu, X. Ethylenediamine-mediated Synthesis of Mn3O4 Nano-octahedrons and Their Performance as Electrocatalysts for the Oxygen Evolution Reaction. Nanoscale 2014, 6, 10896-10901. (39) Jahan, M.; Liu, Z.; Loh, K. P. A Graphene Oxide and Copper-Centered Metal Organic Framework Composite as a Tri-Functional Catalyst for HER, OER, and ORR. Adv. Funct. Mater. 2013, 23, 5363-5372. (40) Jiang, N.; You, B.; Sheng, M.; Sun, Y. Electrodeposited Cobalt-phosphorous-derived Films as Competent Bifunctional Catalysts for Overall Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 6251-6254. (41) Ma, X.; Liu, J.; Liang, C.; Gong, X.; Che, R. A Facile Phase Transformation Method for the Preparation of 3D Flower-like β-Ni(OH)2/GO/CNTs Composite with Excellent Supercapacitor Performance. J. Mater. Chem. A 2014, 2, 12692-12696. (42) Wender, H.; Gonçalves, R. V.; Dias, C. S. B.; Zapata, M. J.; Zagonel, L. F.; Mendonça, E. C.; Teixeirab, S. R.; Garcia, F. Photocatalytic Hydrogen production of Co(OH)2 Nanoparticle-coated α-Fe2O3 Nanorings. Nanoscale 2013, 5, 9310-9316. (43) Abellán, G.; Carrasco, J. A.; Coronado, E.; Romero, J.; Varela, M. Alkoxide-intercalated CoFe-Layered Double Hydroxides as Precursors of Colloidal Nanosheet Suspensions: Structural, Magnetic and Electrochemical Properties. J. Mater. Chem. C 2014, 2, 3723-3731. (44) Yan, J.; Fan, Z.; Sun, W.; Ning, G.; Wei, T.; Zhang, Q.; Zhang, R.; Zhi, L.; Wei, F. Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density. Adv. Funct. Mater. 2012, 22, 2632-2641. (45) Lee, J. W.; Ahn, T.; Soundararajan, D.; Ko, J. M.; Kim, J.-D. Non-aqueous Approach to the Preparation of Reduced Graphene Oxide/α-Ni(OH)2 Hybrid Composites and Their High Capacitance Behavior. Chem. Commun.


ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2011, 47, 6305-6307. (46) Yang, J.; Liu, H.; Martens, W. N.; Frost, R. L. Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs. J. Phys, Chem. C 2009, 114, 111-119. (47) Wang, D.; Zhou, J.; Hu, Y.; Yang, J.; Han, N.; Li, Y.; Sham, T.-K. In Situ X-ray Absorption Near-Edge Structure Study of Advanced NiFe(OH)x Electrocatalyst on Carbon Paper for Water Oxidation. J. Phys, Chem. C 2015, 119, 19573-19583. (48) Zhao, Z.; Wu, H.; He, H.; Xu, X.; Jin, Y. A High-Performance Binary Ni–Co Hydroxide-based Water Oxidation Electrode with Three-Dimensional Coaxial Nanotube Array Structure. Adv. Funct. Mater. 2014, 24, 4698-4705. (49) Yu, X.; Zhang, M.; Yuan, W.; Shi, G. A High-performance Three-Dimensional Ni–Fe Layered Double Hydroxide/Graphene Electrode for Water Oxidation. J. Mater. Chem. A 2015, 3, 6921-6928. (50) Shang, C.; Dong, S.; Wang, S.; Xiao, D.; Han, P.; Wang, X.; Gu, L.; Cui, G. Coaxial NixCo2x(OH)6x/TiN Nanotube Arrays as Supercapacitor Electrodes. ACS Nano 2013, 7, 5430-5436. (51) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni–Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2013, 135, 12329-12337. (52) Lee, S.; Cheon, J. Y.; Lee, W. J.; Kim, S. O.; Joo, S. H.; Park, S. Production of Novel FeOOH/Reduced Graphene Oxide Hybrids and Their Performance as Oxygen Reduction Reaction Catalysts. Carbon 2014, 80, 127-134. (54) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution. J. Am. Chem. Soc. 2012, 134, 17253-17261. (55) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H., An Advanced Ni–Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452-8455. (56) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Grätzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-abundant Catalysts. Science 2014, 345, 1593-1596. (57) Ledendecker, M.; Krick Calderón, S.; Papp, C.; Steinrück, H. P.; Antonietti, M.; Shalom, M. The Synthesis of Nanostructured Ni5P4 Films and their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting. Angew. Chem. 2015, 127, 12538-12542. (58) Liu, D.; Lu, Q.; Luo, Y.; Sun, X.; Asiri, A. M. NiCo2S4 nanowires array as an efficient bifunctional electrocatalyst for full water splitting with superior activity. Nanoscale 2015, 7, 15122-15126. (59) Liu, T.; Liu, Q.; Asiri, A. M.; Luo, Y.; Sun, X. An Amorphous CoSe Film Behaves as an Active and Stable Full Water-splitting Electrocatalyst under Sstrongly Alkaline Conditions. Chem. Commun. 2015, 51, 16683-16686. (60) Shi, J.; Hu, J.; Luo, Y.; Sun, X.; Asiri, A. M. Ni3Se2 Film as a Non-precious Metal Bifunctional Electrocatalyst for Efficient Water Splitting. Catal. Sci. Technol. 2015, 5, 4954-4958. (61) Zhu, W.; Yue, X.; Zhang, W.; Yu, S.; Zhang, Y.; Wang, J.; Wang, J. Nickel Sulfide Microsphere Film on Ni Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Commun. 2015, 52, 1486-1489. (62) Tian, J.; Cheng, N.; Liu, Q.; Sun, X.; He, Y.; Asiri, A. M. Self-supported NiMo Hollow Nanorod Array: an efficient 3D Bifunctional Catalytic Electrode for Overall Water Splitting. J. Mater. Chem. A 2015, 3, 2005620059. (63) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew.Chem. 2015, 127, 9483-9487.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(64) Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Ni2P as a Janus Catalyst for Water Splitting: the Oxygen Evolution Activity of Ni2P Nanoparticles. Energy Environ. Sci. 2015, 8, 2347-2351. (65) Jiang, N.; You, B.; Sheng, M.; Sun, Y. Electrodeposited Cobalt-Phosphorous-Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 6251-6254. (66) Zhu, Y. P.; Liu, Y. P.; Ren, T. Z.; Yuan, Z. Y. Self-Supported Cobalt Phosphide Mesoporous Nanorod Arrays: A Flexible and Bifunctional Electrode for Highly Active Electrocatalytic Water Reduction and Oxidation. Adv. Funct. Mater. 2015 , 25, 7337-7347.


ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 20

(a)

Figure 1. (a) Representation of the fabrication process of the NiCoFe ALDHs/CFC; (b-c) SEM images of NiCoFe ALDHs/CFC with different magnifications; (d) TEM image of the NiCoFe triple hydroxide nano-sheets; (e) HRTEM image and SAED (inset) of a typical NiCoFe triple hydroxide nanosheet.


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L3 L2

Co 710

715

Fe L3 L 2

700

720

725

Ni

2+

800

850

Loss energy (eV)

C

NiCoFe ALDHs/CFC CFC

2+

L2

L2

3+

750

(f)

L3

L3

Intensity (a.u.)

(e) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

10

C

20

30

40

50

60

2 Theta (degree)

70

80

Figure 2. (a) HAADF-STEM image of a typical NiCoFe triple hydroxide nanosheet and the correspond-

ing EDX elemental mapping images of (b) Co, (c) Fe, (d) Ni; (e) EELS spectrum of NiCoFe ALDHs/CFC; (f) XRD pattern of NiCoFe ALDHs/CFC.


ACS Energy Letters

(a) Intensity (a.u.)

858

864

Ni

Co 2p5/2 shake-up

870

876

882

775

3+

780

Intensity (a.u.)

Fe 2p5/2

715

720

725

785

790

795

800

(d)

Fe

Binding energy (eV)

Shake-up

805

810

Binding energy (eV)

Fe 2p3/2

710

Co 2p3/2

Shake-up

Binding energy (eV)

(c)

Co2+

Intensity (a.u.)

Ni 2p3/2

shake-up

852

(b)

2+

Ni 2p5/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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730

O 1s Co-O-H

Ni-O-H

H2O

Fe-O-H

528

530

532

534

536

Binding energy (eV)

Figure 3. XPS spectra of NiCoFe ALDHs/CFC in (a) Ni 2p, (b) Co 2p, (c) Fe 2p, and (d) O 1s regions.


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NiCoFe ALDHs/CFC NiFe DHNAs/CFC CoFe DHNAs/CFC NiCo DHNAs/CFC Co SHNAs/CFC Fe SHNAs/CFC Ni SHNAs/CFC

200 150

(a)

1.8

NiFe DHNAs/CFC (65 mV/dec) CoFe DHNAs/CFC (38 mV/dec) NiCo DHNAs/CFC (45 mV/dec) Co SHNAs/CFC (137 mV/dec) Fe SHNAs/CFC (130 mV/dec) Ni SHNAs/CFC (274 mV/dec)

1.7

1.6

100

(b)

NiCoFe ALDHs/CFC (32 mV/dec)

E / V (vs.RHE)

-2

Current density (mA cm )

250

1.5

50 0 1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.4 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

-2

log (j, mA cm )

E / V (vs.RHE) 1.7

(c)

(d)

(d)

1.6

E / V (vs.RHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

1.5

99 %

1.4 1.3 1.2 1.1 1.0

0

2

4

6

8

10

12

Time / h Figure 4. (a) Polarization curves and (b) the corresponding Tafel curves of NiCoFe ALDHs/CFC, NiFe

DHNAs/CFC, CoFe DHNAs/CFC, NiCo DHNAs/CFC, Ni SHNAs/CFC, Co SHNAs/CFC and Fe SHNAs/CFC; (c) Chronopotentiometry curves of NiCoFe ALDHS/CFC at the current density of 20 mA/cm2. (d) SEM image of NiCoFe ALDHs/CFC after chronopotentiometry test of 12 h.


0.40

(a)

Page 18 of 20

(b)

0.36

-20 -40

NiCoFe ALDHs/CFC NiFe DHNAs/CFC

-60

NiCo DHNAs/CFC

0.24

Fe SHNAs/CFC

-100

Co SHNAs/CFC

-120

0.20

Ni SHNAs/CFC

-0.5

-0.4

-0.3

-0.2

0.32 0.28

CoFe DHNAs/CFC

-80

E / V (vs.RHE)

-2

0

-0.1

0.0

NiCoFe ALDHs/CFC (70 mV/dec) NiFe DHNAs/CFC (200 mV/dec) CoFe DHNAs/CFC (88 mV/dec) NiCo DHNAs/CFC (128 mV/dec) Fe SHNAs/CFC (221 mV/dec) Co SHNAs/CFC (148 mV/dec) Ni SHNAs/CFC (180 mV/dec)

-0.4

0.0

0.0

0.4

0.8

1.2

1.6

2.0

2.4

-2

Log (j, mA cm )

E / V (vs RHE)

(c)

-0.1 -0.2

E / V (vs.RHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACS Energy Letters

95%

-0.3 -0.4 -0.5 -0.6 -0.7

0

2

4

6

8

10

12

Time / h Figure 5. (a) Polarization curves and (b) the corresponding Tafel curves of NiCoFe ALDHs/CFC, NiFe

DHNAs/CFC, CoFe DHNAs/CFC, NiCo DHNAs/CFC, Ni SHNAs/CFC, Co SHNAs/CFC and Fe SHNAs/CFC; (c) Chronopotentiometry curves of NiCoFe ALDHS/CFC at the current density of 20 mA/cm2. (d) SEM image of NiCoFe ALDHs/CFC after chronopotentiometry test of 12 h.


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(b)

NiCoFe ALDHs/CFC Pt/C

Ni(OH)2/NF (Ref 55)

1.80

15

Splitting potential (V vs RHE)

-2


(a) 20

1.76 1.72

10

Ni5P4 film (Ref 52)

NiFe LDH/NF (Ref 55)

1.68

NiCo2S4 NA (Ref 57)

CoSe film (Ref 54) NiS/Ni foam (Ref 62) NiSe NW/Ni foam (Ref 62)

1.64

5

Ni3Se2/CF (Ref 59) NiMo HNRs Ref 61 Ni2P (Ref 63), CoP film (Ref 64) CoP MNA (Ref 65) IrO2/Pt (Ref 65)

1.60 1.56 NiCoFe ALDHs/CFC (our work)

0 1.35

1.50

1.65

1.80

1.95

2.10

Voltage (V)

(c) 1.85

8

RuO2/TiM//Pt/C/TiM (Ref 61)

10

9

12

11 -2


(d)2.0

Ni(OH)2/NF (Ref 55) NiCo2S4 NA (Ref 57)

Potential (V vs RHE)

Splitting potential (V vs RHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

1.80 NiFe LDH/NF (Ref 55)

1.75

NiSe NW/Ni foam (Ref 62)

1.70

NiMo HNRs (Ref 61) NiS/Ni foam (Ref 60) Ni2P (Ref 63) CoSe film (Ref 58) IrO2/Pt (Ref 65) CoP MNA (Ref 65)

Ni3Se2/CF (Ref 59) CoP film (Ref 64)

1.65

18

19

98% 1.2

NiCoFe ALDHs/CFC 0.8 0.4

NiCoFe ALDHs/CFC (our work)

RuO2/TiM//Pt/C/TiM (Ref 61)

17

1.6

20

21

22

-2

23

0.0 0

4


8

12

16

20

Time / h

Figure 6. (a) Polarization curves of the overall water splitting using NiCoFe ALDHs/CFC and Pt/C as

functional electrocatalysts (same loading) in a two-electrode system at the scan rate of 2.0 mV s-1; (b) The comparisons of splitting potential of NiCoFe ALDHs/CFC as bifunctional electrocatalysts with those of other bifunctional electrocatalysts at 10 mA/cm2; (c) The comparisons of splitting potential of NiCoFe ALDHs/CFC as bifunctional electrocatalysts with those of other bifunctional electrocatalysts at 20 mA/cm2; (d) Chronopotentiometric curve of the water electrolysis using NiCoFe ALDHs/CFC as bifunctional elec-trocatalysts in a two-electrode configuration at the current density of 10 mA cm-2.


ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC


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NiCoFe Layered Triple Hydroxides with Porous Structures as High

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