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Self-supported hierarchical FeCoNi-LTH/NiCo2O4/CC electrodes with enhanced bifunctional performance for efficient overall water splitting Yuxuan Liu, Yu Bai, Yu Han, Zhou Yu, Shimin Zhang, Guohua Wang, Junhua Wei, Qibing Wu, and Kening Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12474 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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ACS Applied Materials & Interfaces

Self-supported

hierarchical

FeCoNi-LTH/NiCo2O4/CC

electrodes with enhanced bifunctional performance for efficient overall water splitting Yuxuan Liu,1,2 Yu Bai,3,* Yu Han,4 Zhou Yu,1,2 Shimin Zhang,1,2 Guohua Wang,5 Junhua Wei, 5 Qibing Wu5 and Kening Sun2,6,* 1 School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China 2 Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, China 3 Advanced Research Institute for Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, China 4 Ultra-precision Optoelectronic Instrument Engineering Center, Harbin Institute of Technology, Harbin 150080, China 5 State Key Laboratory of Advanced Chemical Power Sources, Zunyi 563000, China 6 State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150001, China

KEYWORDS: layered ternary hydroxides, hierarchical structure, transition metal electrocatalyst, alkaline media, overall water splitting. ABSTRACT: The development of advanced earth-abundant electrocatalysts for hydrogen production is highly desirable. In this paper, we report the design and synthesis of a novel and highly efficient electrode of NiCo2O4 nanoneedles decorated 1

ACS Paragon Plus Environment


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with

FeCoNi

layered

ternary

hydroxides

supported

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on

carbon

cloth

(FeCoNi-LTH/NiCo2O4/CC) by a facile and efficient two-step approach. It exhibits superior bifunctional catalytic activities for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media, due to the special structure and strong synergies. The FeCoNi-LTH/NiCo2O4/CC obtains an onset overpotential of 240 mV and an overpotential of 302 mV at the current density of 50 mA cm−2 for OER, which is superior to RuO2. It also efficiently catalyzes HER with onset overpotential of 96 mV and overpotential of 151 mV to achieve a current density of 20 mA cm−2. Serving as both cathode and anode in a two-electrode water splitting system, FeCoNi-LTH/NiCo2O4/CC only requires an overpotential of 1.65 V at current density of 50 mA cm-2. The cell exhibites outstanding stability as well, indicating that FeCoNi-LTH/NiCo2O4/CC is a befitting material to be utilized as effective bifunctional catalysts for overall water splitting.

INTRODUCTION As a way of storing and transporting sustainable energy sources (e.g., solar energy and wind energy), hydrogen production from electrolyzing water has been considered to be a promising way to address the growing energy consumption and environmental pollution.1-3 Typical complete water splitting process consists of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occurring at the cathode and anode, respectively. Catalysts are normally required to lower the barrier and to facilitate the reaction.4-6 In industrial processes, noble metal catalysts are commonly used, such as Pt-based catalysts for HER and Ru-based catalysts for OER. However, it 2


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is clear that the low reserves and high cost of noble metals impose restrictions on the further development of the water splitting industry.7 In recent years, the design of biomimetic nanostructure catalysts for OER and HER guided by redox enzymes has received extensive attention, including nanomaterials containing transition metal elements such as manganese, iron, nickel and cobalt.8-12 In terms of HER, transition metal dichalcogenides such as MoS213, WS214 and other phosphides15, selenides16 and borides17 show excellent performance in acid with high rate of hydrogen production and low energy loss. As for OER, low overpotential and high current density are achieved in alkaline media by transition metal oxides18, hydroxides/oxyhydroxide19, 20 and other transition-metal compounds21-23. However, in an ideal practical electrocatalytic system, the anode and cathode reactions should be carried out in the same electrolyte to simplify the system, which means the HER and OER catalysts are required to withstand the same environment, generally alkaline.24-26 It is more attractive to utilize the same catalyst in both electrodes for further simplifying the system and guaranteeing the matching. Therefore, developing catalysts which can be employed in the same media for bifunctional catalysis is of great importance for electrocatalysis.

Transition

metal layered

double hydroxides

(LDH) are

widely used in

electrochemistry due to the special property of structure and the selectivity of the elements on layer or inter layer composition.27-29 Researchers have demonstrated that LDH materials have substantial application prospects in water splitting,30, 31 and some 3



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of them even exhibit bifunctional properties.32, 33 In recent years, it is found that the introduction of a third metal ion into the layers to form layered ternary hydroxides (LTH) would play a regulatory role in the structure and properties of LDH materials.7, 34

This is likely to greatly improve the intrinsic properties of LDH, making it more

suitable for water splitting. In addition, the construction of advanced structures to support LDH and LTH materials is also an important way to improve the performance of the catalyst. A suitable three-dimensional hierarchical structure can increase the surface area of the electrode and maximize the exposure of the catalytical active sites.35, 36 NiCo2O4 is a suitable candidate for employing as substrate for hierarchical structure because it can be easily synthesized and controlled to form a variety of morphologies.37, 38 Moreover, NiCo2O4 has been proved to be an excellent catalyst for splitting water,39, 40 which is more conducive to promote the performance optimization of its composite materials. In addition, the earth-abundant and environmentally friendly features make it applicable for large-scale industrial applications.41

Herein, a self-supported hierarchically structured composite electrode of NiCo2O4 nanoneedles

decorated

with

FeCoNi-LTH

on

carbon

cloth

(FeCoNi-LTH/NiCo2O4/CC) is designed and fabricated for high-performance overall water splitting. In this structure, NiCo2O4 nanosneedle arrays are vertically grown on CC by hydrothermal and annealing method followed by electrodeposition to load FeCoNi-LTH nanosheets on NiCo2O4 nanosneedles. Due to the especial structure and intrinsic catalytic properties of FeCoNi-LTH/NiCo2O4/CC, the electrode can catalyze 4


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both HER and OER in alkaline electrolyte with superior activity and stability, which makes the material incontrovertibly one of the state-of-the-art bifunctional catalysts for effective electrocatalysis or photoelectrocatalysis.

RESULTS AND DISCUSSION

The schematic diagrams in Scheme 1 illustrate the two-step procedure for preparing FeCoNi-LTH/NiCo2O4/CC. CC is selected as the freestanding substrate, which can provide high surface area and serve as a current collector due to its excellent conductivity. Vertically aligned NiCo2O4 nanoneedle arrays were synthesized on the surface of CC through 6 h hydrothermal reactions at 120 ℃ followed by another 2 h annealing at 350 ℃ in air to crystallize the metal oxide composites.42 After hydrothermal and annealing reaction, a black sample of NiCo2O4 nanoneedle arrays grown on CC (denoted as NiCo2O4/CC) is obtained and further used as the substrate for the next step. The ultrathin FeCoNi-LTH nanosheets were then decorated on the NiCo2O4/CC using a facile electrodeposition method. Upon completing the above steps, a self-supported electrode with hierarchical structure is prepared successfully. In this construction, NiCo2O4 nanosneedles provide substantial surface area for loading FeCoNi-LTH nanosheets, and a large number of porous channels for electrolyte permeation and gas products diffusion can be formed to accelerate mass transfer, which is expected to improve the electrocatalytic performance.

The scanning electron microscope (SEM) images in Fig.1a show that the NiCo2O4

5



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nanoneedles grow uniformly and vertically on the surface of the carbon fibers. After electrodeposition, the nanoneedles still keep their integrity and uniformity, as shown in Fig. 1b. Ultrathin nanosheets grow homogeneously on the NiCo2O4 nanoneedles and the thickness of the FeCoNi-LTH nanosheets is several nanometers. Porous open-space structures formed by hierarchical nanoneedles and nanosheets would accelerate the electrolyte penetration and ion diffusion.43 More detailed structure can be observed in Fig. S3. Transmission electron microscopy (TEM) is further used to characterize the morphology of individual nanostructure. As shown in Fig. 1c, the NiCo2O4 nanoneedle is stacked by a number of nanoparticles. Irregular-shape nanosheets distributed on the nanoneedle appear nearly transparent under the electron beam, due to their ultrathin feature. Fig. 1d shows the high resolution transmission electron microscopy (HRTEM) at the interface of NiCo2O4 and FeCoNi-LTH, which clearly shows the different domains and crystallites of NiCo2O4 and FeCoNi-LTH. The lattice fringes with widths of 0.24 nm and 0.28 nm in the lower left correspond to the (311) plane and the (220) plane of cubic NiCo2O4 spinel phase, respectively.44 The lattice with widths of 0.27 nm in the upper right corresponds to the (012) plane of FeCoNi-LDH. The conjoint interface between the two domains confirms that FeCoNi-LTH nanosheets are well attached onto the NiCo2O4 nanoneedles. The tightly combination may cause strong synergistic effect between the two materials, which is beneficial to enhance the electron transport and catalysis stability.45 The selected area electron diffraction (SAED) pattern of FeCoNi-LTH/NiCo2O4/CC indicates the polycrystalline structure of NiCo2O4 and FeCoNi-LTH (inset of Fig. 1d). 6


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The presence and content of component elements in FeCoNi-LTH/NiCo2O4/CC are probed by energy dispersive spectrometer (EDS) elemental mappings. As shown in Fig.

2a,

Fe,

Co,

Ni

and

Cl

elements

are

uniformly

distributed

in

FeCoNi-LTH/NiCo2O4/CC, which reveals that Fe, Co and Ni elements participate in the reaction evenly and produce a homogeneous reactant. Since the NiCo2O4 contains no Fe element, it can be speculated that Fe element is derived from FeCoNi-LTH. According to the data given in Fig. 2b, the content of Ni and Co is relatively high, for the NiCo2O4 substrate and FeCoNi-LTH both contain plentiful Ni and Co elements. In addition, chlorine is also detected in FeCoNi-LTH/NiCo2O4/CC. The chlorine element is introduced by raw materials, and meets the need of anions in the interlayer of LTH to maintain the electric balance.

Fig. 2c shows the X-ray diffraction (XRD) patterns of FeCoNi-LTH/NiCo2O4/CC, NiCo2O4/CC and CC directly decorated with FeCoNi-LTH (FeCoNi-LTH/CC). The obtained diffraction peaks of NiCo2O4/CC match well with the standard patterns of spinel NiCo2O4 phase (JCPDS card no. 20-0781), where the typical diffraction peaks assign to the (220), (311) and (440) crystal facets are observed at 31.3°, 36.8°and 64.9°, respectively. They are consistent with the previous lattice spacing observed under HRTEM. In order to demonstrate the successful synthesis of FeCoNi-LTH, FeCoNi-LTH directly grown on CC was synthetized using the same electrodeposition method and tested by XRD. Its XRD pattern (black line in Fig. 2c) manifests three 7



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landmark diffraction peaks of the LDH materials, indicating that FeCoNi-LTH still retains the layered structure of LDH materials. Additionally, the microstructure of FeCoNi-LTH/CC was characterized by SEM, showing the homogeneous and dense construction of FeCoNi-LTH/CC nanosheets (Fig. S4). They are similar to those grown on NiCo2O4 nanoneedle arrays in appearance. Specifically, the XRD pattern of FeCoNi-LTH/NiCo2O4/CC shows both the diffraction peaks of NiCo2O4 and the diffraction peaks of LTH material, indicating that spinel NiCo2O4 and FeCoNi-LTH have been successfully synthesized.

X-ray photoelectron spectroscopy (XPS) measurement was carried out to further explore the element composition and valence states in FeCoNi-LTH/NiCo2O4/CC. An elemental survey of XPS indicates the presence of Fe, Co, Ni, Cl, O and C elements in FeCoNi-LTH/NiCo2O4/CC (Fig. S5). Co 2p spectra are deconvoluted into two spin-orbit doublets of Co2+ and Co3+ and two shakeup satellites (Fig. 3a). The two main peaks are located at 780.46 eV and 795.70 eV, respectively, corresponding to Co 2p1/2 and Co 2p3/2, where the two peaks has a spin orbital splitting of ~15 eV. The two detected shakeup satellite peaks are located at 785.66 eV and 802.20 eV, respectively. Detailed deconvolution results show that FeCoNi-LTH/NiCo2O4/CC contains two Co species, which can be identified as Co2+ (corresponding to the 781.54 eV peak in Co 2p3/2 and the 796.82 eV peak in Co 2p1/2) and Co3+ (corresponding to the 780.33 eV peak in Co 2p3/2 and the 795.47 eV peak in Co 2p1/2). Similar results are observed in the spectra of Ni 2p (Fig. 3b), which consist of two spin orbit doublets 8


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characteristic of Ni2+ and Ni3+ and two satellite peaks. Two main peaks observed at 854.80 eV (Ni3+ at 854.69 eV and Ni2+ at 855.94 eV) and 872.41 eV (Ni3+ at 872.29 eV and Ni2+ at 873.53 eV) with a spin orbit splitting of ~17 eV are attributed to Ni 2p1/2 and Ni 2p3/2. Additionally, there are two shake-up satellite peaks at 861.14 eV and 879.31 eV. The deconvolution results of Fe 2p indicate two split peaks corresponding to Fe 2p3/2 and Fe 2p1/2, which are located at 713.65 eV and 724.92 eV, respectively (Fig. 3c). This implies that Fe exists in the form of Fe3+. Literature has reported that the peak of Fe2+ should be located at 708-711.5 eV,46 while this is not found in Fe 2p3/2, confirming the dominance of Fe3+. The O 1s spectra (Fig. 3d) are very complicated because of the complexity of the bondings between cations and O in FeCoNi-LTH/NiCo2O4/CC. Basically, O 1s can be divided into the following four peaks: the characteristic peak of Fe-O-H at 530.17 eV, the characteristic peak of Ni-O-H at 530.63 eV, the characteristic peak of Co-O-H at 531.16 eV and the characteristic peak of physisorbed or chemisorbed water on the surface of sample at 531.89 eV. Above analysis of the XPS results effectively indicates that the anticipated FeCoNi-LTH/NiCo2O4/CC has been successfully synthesized.

The

OER electrocatalytic

performances of FeCoNi-LTH/NiCo2O4/CC were

investigated using a three-electrode system with a scan rate of 2 mV s−1. Fig. 4a shows the linear sweep voltammetry (LSV) curves for FeCoNi-LTH/NiCo2O4/CC, NiCo-LDH

grown

FeCoNi-LTH/CC,

on

NiCo2O4/CC

NiCo2O4/CC,

bare

(denoted

as

CC

RuO2/CC.

and

NiCo-LDH/NiCo2O4/CC),

9


The

as-prepared


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FeCoNi-LTH/NiCo2O4/CC shows an onset overpotential of 240 mV, which is smaller than that of RuO2 (290 mV). Specifically, only an overpotential of 302mV at the current density of 50 mA cm−2 is required for OER, which is much lower than that for RuO2 (405 mV). This superior OER performance at large current density with small overpotential indicates that FeCoNi-LTH/NiCo2O4/CC is an extraordinarily efficient OER catalyst. As expected, bare CC exhibits poor OER catalysis activity, while NiCo2O4/CC and FeCoNi-LTH/CC exhibit marked higher catalytic activity with onset overpotential of ~330 mV and ~350 mV, respectively. This indicates that both NiCo2O4 and FeCoNi-LTH can act as active centers to boost the OER, as reported in literatures.39,

43

Moreover, the OER performance of FeCoNi-LTH/NiCo2O4/CC is

much better than that of NiCo2O4/CC and FeCoNi-LTH/CC. The superiority would derive from the higher surface area and the more active sites exposed caused by the fabricated hierarchical structure as well the intrinsic catalytic activity of NiCo2O4/CC and FeCoNi-LTH/CC. Lacking of Fe element, NiCo-LDH/NiCo2O4/CC shows similar morphology to that of FeCoNi-LTH/NiCo2O4/CC, as shown in Fig. S6. Although it is reported that NiCo-LDH is an appropriate material to catalyze OER,47,

48

FeCoNi-LTH/NiCo2O4/CC presents much higher catalytic activity than it, which illustrates that the insertion of Fe can improve the catalysis activity of NiCo-LDH material. Actually, several literatures have reported that the insertion of Fe element can adjust layer spacing and conductivity of LDH material containing Ni, thus improving the OER performance.49-51 The OER kinetics of the above catalysts was assessed by Tafel plots, and more favorable kinetics and better catalytic activity can 10


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be seen from the relatively low Tafel slope. As shown in Fig. 4b, the Tafel slope for FeCoNi-LTH/NiCo2O4/CC is 71.5 mV dec−1, much lower than those of NiCo-LDH/NiCo2O4/CC, FeCoNi-LTH/CC, NiCo2O4/CC and bare CC, indicating the outstanding kinetics of FeCoNi-LTH/NiCo2O4/CC. Furthermore, it is extremely close to that of RuO2 (67.9 mV dec−1), which means FeCoNi-LTH/NiCo2O4/CC can be a ready substitute for noble-metal-based electrocatalyst.

As shown in Fig 4c, the durability of FeCoNi-LTH/NiCo2O4/CC for OER was assessed using continuous CV sweeps from 1.0 to 2.0 V vs. RHE at a scan rate of 100 mV s−1. The polarization curve shows little decay after 500 CV scanning cycles. Time-dependent current density curve at a fixed overpotential of 280 mV also confirmed the superb durability of FeCoNi-LTH/NiCo2O4/CC, since the current only show a slight decrease of 1.7% after 24 hours of uninterrupted catalytic reaction (see Fig. 4d). After 24 hours of reaction, we re-characterized the morphology of sample by SEM. It is shown that the electrode maintains its integrity of configuration and morphology, indicating that the catalyst owns excellent long-time stability for OER. Electrochemical active surface area (EASA) is one of the most effective evidences to represent the intrinsic activity of the catalyst because it reflects the amount of exposed active sites of the catalyst.52 We valued the EASA of FeCoNi-LTH/NiCo2O4/CC by measuring the double layer capacitance (Cdl) via CV scanning method. The charging currents measured in the non-Faradaic potential from 0.87 V to 1.07 V vs RHE at different scan rates for FeCoNi-LTH/NiCo2O4/CC, FeCoNi-LTH/CC, NiCo2O4/CC 11



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and NiCo-LDH/NiCo2O4/CC are presented in Fig. S7. The plots of the charging current density differences (∆j) against the scan rates have a linear relationship as shown in Fig. 5, where Cdl can be obtained by reducing the slope by half. Compared with the control groups, the Cdl of FeCoNi-LTH/NiCo2O4/CC is 1.36, 1.90 and 2.21 folds higher than that of NiCo-LDH/NiCo2O4/CC, NiCo2O4/CC and FeCoNi-LTH/CC, respectively, indicating that the hierarchical structure could dramatically increase the surface area of the electrode as well the exposure of active sites, which play important roles in enhancing catalytic activity.53

The HER electrocatalytic property of the as-prepared samples were further investigated and commercial Pt/C and bare CC were also examined for comparison. As shown in Fig. 6a, the NiCo-LDH/NiCo2O4/CC, NiCo2O4/CC and FeCoNi-LTH/CC present

characteristic

OER

catalytic

behavior.

The

obtained

FeCoNi-LTH/NiCo2O4/CC exhibits superior electrocatalytic activity towards HER, showing an onset overpotential of ~96 mV, which is closed to the most advanced commercial catalyst Pt/C (62 mV). Furthermore, the overpotential required to drive a cathodic

current

density

of

20

mA

cm-2

is

only

151

mV

for

FeCoNi-LTH/NiCo2O4/CC, verifying that it is an outstanding catalyst for HER in alkaline media. The specific hierarchical structure formed by nanoneedles and nanosheets endows the FeCoNi-LTH/NiCo2O4/CC with increased surface area and abundant diffusion channels, facilitating the penetration of the electrolyte and the removal of hydrogen bubbles. 12


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The kinetics study of HER catalysts is of great significance for a deeper understanding of the HER catalytic process. The Tafel plots are measured in order to characterize the kinetics (Fig. 6b). The Pt/C loaded on CC shows the lowest Tafel slope of 53.7 mV dec−1, and FeCoNi-LTH/NiCo2O4/CC (114.2 mV dec−1) owns the smallest slope except Pt/C, indicating that its kinetics performance is relatively outstanding in HER. The slightly higher Tafel slope of FeCoNi-LTH/NiCo2O4/CC indicates that the HER process of the catalyst may have undergone the Volmer-Heyrovsky

process.54

Although

the

hierarchical

feature

of

FeCoNi-LTH/NiCo2O4/CC may somewhat influence the kinetics, making it not so excellent as Pt/C, the much lower Tafel slope compared with other control groups discloses that FeCoNi-LTH/NiCo2O4/CC has guaranteed relatively strong electron transport between the substrate and the topmost structure.

The durability of FeCoNi-LTH/NiCo2O4/CC in HER was also tested using continuous CV sweeps from -0.5 to 0.2 V (vs. RHE) at a scan rate of 100 mV s−1. As shown in Fig. 6c, the polarization curve shows a little decay after 500 CV scanning cycles. Time-dependent current density curve at a fixed overpotential of 200 mV for 24 hours reveals a decrease of ~20% in term of the current density (Fig. 6d), which is moderately stable, although not as stable as it is in the OER process. And SEM result shows that the morphology of the catalyst hardly changes after 24 hours of catalysis, indicating that the catalyst still maintains robust in its configuration. 13



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The excellent catalytic activity of FeCoNi-LTH/NiCo2O4/CC for water splitting, especially for OER, may come from the following aspects. First, the insertion of Fe atoms has a regulatory effect on LDH materials. The modulatory effect of the third element makes the properties of LTH materials better than LDH materials. Secondly, NiCo2O4 nanoneedle arrays and FeCoNi-LTH own intrinsic OER and HER catalytic performance. The synergistic effect between NiCo2O4 and FeCoNi-LTH ensures the rapid electron transport within the electrode, enhancing the bifunctional performance significantly. Thirdly, NiCo2O4 nanoneedle arrays provide large catalytic surface areas for NiCo2O4 and FeCoNi-LTH, increasing the exposure of active sites. Fourthly, the hierarchical structure of FeCoNi-LTH/NiCo2O4/CC is beneficial to accelerate the mass transfer process on the surface of the catalyst, and it is also beneficial to the rapid release of the gas bubbles from the catalyst and prevents the formation of the gas film from hindering further reaction. Finally, the self-supported construction can avoid the use of the binder, which allows the structure to expose more active sites and does not hinder the ion transport. Table S1 provides OER electrocatalytic parameters for FeCoNi-LTH/NiCo2O4/CC and other structured catalysts, confirming the advantage of this hierarchical structure.

Additionally, we found that the deposition time and concentration of metal ions during the electrodeposition process have a great influence on the morphology and properties of the samples (Fig. S8-S9). Fig. S8a illustrates that electrodepositing for a 14


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short time will lead to insufficient growth of nanosheets. In fact, only a small portion of nanosheets grow at the tips of the nanoneedles when the electrodeposition time is only 300 seconds, leading to reduced surface area and active sites, thus deteriorating the OER and HER performance(Fig. S8d and e). However, electrodepositing for a long time could cause the accumulation of nanosheets, as shown in Fig. S8c, which would greatly reduce the gaps between the nanoneedle arrays. This will certainly impede the mass transfer process during the OER and HER process. By properly controlling the deposition time, the product in desirable morphology with excellent performance can be obtained. On the other hand, the concentration of Fe3+ ions also plays

great

role

in

controlling

the

morphologies

and

properties

of

FeCoNi-LTH/NiCo2O4/CC. As the concentration of Fe3+ ions increases, the nanosheets formed by electrodeposition tend to be disordered and become wrinkled (Fig.S8a-c). This may be due to the excessive Fe atoms inserted in the LDH structure damage the structure seriously. We also investigated the OER and HER properties of samples prepared in the electrodeposition bath with different concentrations of Fe (Fig. S9d and e). Ultimately, we found that the optimum concentration was 0.04 M, with element ratio of Fe : Co : Ni = 2 mmol : 2 mmol : 2 mmol in solution with 50mL ultrapure water, and the best electrodeposition time is 500s under the negative potential of -1 V versus Ag/AgCl (3M KCl). It can be indicated that the material is perfectly matched for OER and HER, which makes it practical to be employed in the same electrocatalysis system with same media.

15



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Since FeCoNi-LTH/NiCo2O4/CC exhibits excellent bifunctional properties, we utilize a two-electrode system to evaluate its performance for overall water splitting (Fig. 7a). In this electrolytic cell, FeCoNi-LTH/NiCo2O4/CC is used as both cathode and anode, and the electrolyte is 1.0M KOH. When the electrolysis is carried out, a large amount of hydrogen and oxygen is generated at the cathode and anode, respectively. Gas products are quickly separated from the electrode, without forming large bubbles. Polarization curve of overall water splitting is measured in electrolyte of 1M KOH, with a scan rate of 5 mV s-1, as shown in Fig. 7b. The cell only needs potential of 1.65 V

to

reach

a

large

current

density

of

50

mA cm-2,

which

makes

FeCoNi-LTH/NiCo2O4/CC one of the best bifunctional materials. In order to evaluate the stability of the catalysts for overall water splitting, we measured the chronoamperometry curves under different overpotentials. As shown in Fig. 7c, after 24 hours of uninterrupted catalytic reaction, the current densities decrease by 6.4%, 5.8% and 4.8% at the overpotentials of 1.8 V, 1.75 V and 1.65 V, respectively. This indicates that the FeCoNi-LTH/NiCo2O4/CC electrode is quite stable in overall water splitting system. Excellent electrolytic activity and super-stable characteristic make FeCoNi-LTH/NiCo2O4/CC a potential catalytic material for water splitting. CONCLUSIONS In summary, a hierarchical and self-supported FeCoNi-LTH/NiCo2O4/CC electrode has been designed and prepared through a facile two-step method for highly efficient overall water splitting. Carbon cloth is employed as an efficient conductive network, on which NiCo2O4 nanoneedle arrays and ultrathin FeCoNi-LTH nanosheets are 16


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decorated hierarchically to provide a large number of catalytical active sites. Additionally, the hierarchical structure offers a large number of porous channels and significantly enhanced accessible surface area, which facilitate mass transport and gas diffusion. For both OER and HER, FeCoNi-LTH/NiCo2O4/CC exhibites first-class catalytic performances, and they are comparable to the state-of-the-art commercial noble-metal catalysts. Moreover, FeCoNi-LTH/NiCo2O4/CC showed superior stability in terms of structure and performance. The superb bifunctional catalysis performance and stability make FeCoNi-LTH/NiCo2O4/CC suitable candidate for practical water splitting system. This work not only provides an advanced material, but also provides a variety of reference ideas to improve the performance of overall water splitting catalyst, e.g., the use of self-supported structures to avoid using binder, composition of a variety of materials to cause synergies and open structure assembly to promote mass transfer. In general, this work provides a new approach for us to develop high performance catalysts.

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ASSOCIATED CONTENT Supporting Information Experiment section. XRD patterns and SEM images of precursor of the NiCo2O4. SEM

images

of

FeCoNi-LTH/NiCo2O4/CC,

NiCo-LDH/NiCo2O4/CC

and

FeCoNi-LTH/CC. The full XPS spectrum of FeCoNi-LTH/NiCo2O4/CC. Charging currents measured in the non-Faradaic potential from 0.87 V to 1.07 V at different scan rates. SEM images and LSV curves of FeCoNi-LTH/NiCo2O4/CC with different electrodeposition time. SEM images and LSV curves of FeCoNi-LTH/NiCo2O4/CC electrodeposited under different metal concerntration ratios. Comparison of the OER electrocatalytic performance of different catalysts. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Bai) *E-mail: [email protected] (K.N. Sun) ORCID Yu Bai: 0000-0003-2617-7536 Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This project is supported by the National Natural Science Foundation of China (Grant No. 51203036), the Postdoctoral Science Special Foundation of China 18


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(Grant No. 2013T60380), the Postdoctoral Science Foundation of China (Grant No. 2012M520748), and the Scientific Research Foundation of Beijing Institute of Technology. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http:// REFRENCES

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Scheme 1 Schematic illustration of the preparing procedure for FeCoNi-LTH/NiCo2O4/CC.

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Fig. 1 SEM images of (a) NiCo2O4/CC and (b) FeCoNi-LTH/NiCo2O4/CC, insets are SEM images at higher magnifications. (c) TEM image of an individual NiCo2O4 nanoneedle decorated with FeCoNi-LTH nanosheets. (d) HRTEM image of the typical FeCoNi-LTH/NiCo2O4/CC, and the inset shows the corresponding SAED pattern.

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Fig. 2 (a) EDS elemental mapping images of Cl, Co, Ni, and Fe elements and (b) EDS spectrum for FeCoNi-LTH/NiCo2O4/CC. (c) XRD patterns of FeCoNi-LTH/NiCo2O4/CC, NiCo2O4/CC and FeCoNi-LTH/CC.

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Fig. 3 XPS spectra of the (a) Co 2p, (b) Ni 2p, (c) Fe 2p and (d) O 1s of FeCoNi-LTH/NiCo2O4/CC.

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Fig. 4 (a) LSV curves for FeCoNi-LTH/NiCo2O4/CC, NiCo-LDH/NiCo2O4/CC, FeCoNi-LTH/CC, NiCo2O4/CC, CC and RuO2/CC with a scan rate of 2 mV s-1 for OER. (b) Tafel plots of FeCoNi-LTH/NiCo2O4/CC, NiCo-LDH/NiCo2O4/CC, FeCoNi-LTH/CC, NiCo2O4/CC, CC and RuO2/CC. (c) Polarization curves of FeCoNi-LTH/NiCo2O4/CC before and after 500 CV cycles at a scan rate of 2 mV s-1. (d) Time-dependent current density curves of FeCoNi-LTH/NiCo2O4/CC. Inset in (d) shows the SEM image of FeCoNi-LTH/NiCo2O4/CC after 24 hours of electrolysis. All the measurements are performed in 1.0 M KOH solution.

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Fig.5 Charging current density differences (∆j) plotted against scan rates.

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Fig. 6 (a) LSV curves for FeCoNi-LTH/NiCo2O4/CC, NiCo-LDH/NiCo2O4/CC, FeCoNi-LTH/CC, NiCo2O4/CC, CC and Pt/C on CC with a scan rate of 2 mV s-1 for HER. (b) Tafel plots of FeCoNi-LTH/NiCo2O4/CC, NiCo-LDH/NiCo2O4/CC, FeCoNi-LTH/CC, NiCo2O4/CC, CC and Pt/C on CC. (c) Polarization curves of FeCoNi-LTH/NiCo2O4/CC before and after 500 CV cycles at a scan rate of 2 mV s−1. (d) Time-dependent current density curve of FeCoNi-LTH/NiCo2O4/CC. Inset in (d) shows the SEM image of FeCoNi-LTH/NiCo2O4/CC after 24 hours of electrolysis. All the measurements are in electrolyte of 1.0 M KOH.

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Fig 7 (a) Photograph of a homemade two-electrode device for overall water splitting, with FeCoNi-LTH/NiCo2O4/CC acting as both cathode and anode. (b) Polarization curve and (c) Chronoamperometry curves for overall water splitting. All the measurements are carried out in the electrolyte of 1.0 M KOH.

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Graphic for Manuscript

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110x80mm (300 x 300 DPI)


NiCo2O4 ... - ACS Publications

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