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Bifunctional 2D electrocatalysts of transition metal hydroxide nanosheet arrays for water splitting and urea electrolysis Pravin Babar, Abhishek C. Lokhande, Vijay Karade, Bharati Pawar, Myeng Gil Gang, Sambhaji Pawar, and Jin Hyeok Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01260 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Sustainable Chemistry & Engineering

Bifunctional 2D electrocatalysts of transition metal hydroxide nanosheet arrays for water splitting and urea electrolysis Pravin Babara, Abhishek Lokhandea, Vijay Karadea, Bharati Pawarb, Myeng Gil Ganga, Sambhaji Pawarb and Jin Hyeok Kima* a

Optoelectronic Convergence Research Center, Department of Materials Science and

Engineering, Chonnam National University, Gwangju 500-757, South Korea b

Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, South

Korea *Corresponding author: [email protected] (Jin Hyeok Kim) ABSTRACT The replacement of noble-metal-based electrocatalysts with earth-abundant, low-cost bifunctional electrocatalysts for efficient hydrogen generation is required. Herein, an amorphous and porous 2D NiFeCo hydroxide nanosheets grown on nickel foam (NF) (NiFeCo LDH/NF) by a cost-effective electrodeposition method was explored for efficient electrolytic water splitting and urea electrolysis. Experimental results show that porous confinement in 2D orientation, amorphous nature, and synergistic effect, leads to the excellent catalytical performance of the as-prepared 2D NiFeCo LDH/NF electrode for overall water splitting and urea electrolysis. The NiFeCo LDH/NF electrode presents promising behaviour for water electrolysis with a small overpotential of 210 1 ACS Paragon Plus Environment

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mV and 108 mV, respectively, is required for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) to gain 10 mA cm-2. More notably, the bifunctional NiFeCo LDH/NF catalyst, for water electrolysis, needs a lower potential of 1.57 V to gain 10 mA cm-2 in 1 KOH. Furthermore, the electrochemical urea oxidation results show that NiFeCo LDH/NF requires just 0.280 V (vs SCE) to drive 10 mA cm-2 in 1 M KOH with a 0.33 M urea. Whereas urea-mediated electrolysis cells require a very low potential of 1.49 V at 10 mA cm-2. The present results provide remarkable and notable insights into the preparation of non-noble and highly-efficient 2D transition metal hydroxide electrocatalysts with performances that allow them to compete for widespread use in various applications. KEYWORDS: 2D; nanosheets; NiFeCo LDH; water splitting; urea electrolysis.

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INTRODUCTION The dual concerns of energy storage and global environmental problems caused by increasing energy demands as well as fossil fuel depletion have accelerated the search for renewable energy resources

1,2.

Hydrogen is recognized as clean, renewable, a carbon-free and

ideal energy source and is considered to be a promising alternative to fossil fuels for the future 3– 5.

The hydrogen production from electrochemical water splitting is considered a sustainable

technology. Electrochemical water splitting devices require two electrodes: the anode and cathode, with their respective oxygen and hydrogen evolution reaction (OER and HER) activity 6. However, the high activation barriers and the kinetics of these two reactions make the process inefficient, and active electrocatalysts to lower the overpotential are needed 7,8. At present, the commercially available electrocatalysts are noble-metal-based materials such as Ir/Ru oxide and Pt group materials for OER and HER, respectively. However, the low abundance, high cost, and low bifunctionality remain a crucial impediment in practical applications 9–11. Over the past decade, a number of attempts have been made to fabricate noble-metal-free, efficient and cheap electrocatalysts that catalyse HER and OER simultaneously, such as transition metal hydroxides, oxides, nitrides, sulfides, and selenides 12–17. However, these catalysts have exhibited only singular excellent catalytic activity towards HER or OER, and thus to design bifunctional electrocatalysts in a single electrolytic solution is a great challenge 18,19. Among them, transition metal hydroxides have been endorsed as an appealing candidate for electrocatalysts due to their simple preparation and, low cost compared to metal phosphides and chalcogenides, but their poor conductivity and sparse catalytic edge sites limit their catalytic activity

20,21.

The lower electronic conductivity of

transition metal hydroxides can be improved through many strategies. Several reports have 3 ACS Paragon Plus Environment


demonstrated that the direct growth of the 2D nanosheet material onto conducting nickel foam (NF) substrate can take advantage of the porous 2D nanosheet which has a high surface to bulk ratio, providing an abundant density of active sites, and robust mechanical structure serves electrocatalytic purposes well

22,23.

Furthermore, the electronic confinement in 2D nanosheet

structure altered by the interaction between reacting species and active sites 24. Another convenient approach to enhance catalytical performance is to prepare amorphous material, as ample of research indicates the amorphous materials shows high catalytical activity than their crystalline materials 25,26. Accordingly, 2D nanosheet arrays provide the high electrochemical active surface area, good mechanical strength, and easy dissipation of gaseous products during the catalytical process while amorphous structure offers high active sites

26.

Inspired by this, the synthesis of

amorphous with 2D nanosheet arrays is a convenient approach for optimizing the overall water splitting for transition metal-based catalysts. Among transition metals, Ni, Fe, Co-based catalysts serve as active catalysts for water splitting. In comparison to monometallic or bimetallic tri metalbased catalysts show a significant increase in catalytic activity 27,28. Ample research indicates that NiFe-based electrocatalysts are active towards OER, and they have recently they gained interest as a plausible alternative to noble metal-based catalysts 29. Several studied report indicates that the doping of a third metal (e.g. Co) into NiFe hydroxide catalysts is advantageous, as it would lead to a greater increase in the electrocatalytic activity due to increased surface area and, electronic conductivity 28,30. So considering the above discussion, the preparation of amorphous and porous 2D NiFeCo hydroxide by simple and cost-effective electrodeposition method on NF (labelled as NiFeCo LDH/NF) is presented here. The 2D porous NiFeCo LDH/NF nanosheet can act as an efficient electrocatalyst for OER, HER and overall water splitting in alkaline electrolyte. Specifically, the 2D NiFeCo LDH/NF show low overpotential of 210 mV, 108 mV, and 1.57 V 4 ACS Paragon Plus Environment

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for OER, HER and overall water splitting respectively at 10 mA cm-2 which is excellent compared to recently reported transition metal-based catalysts. Another viable option for efficient hydrogen generation is replacing the anodic reaction, i.e., OER, with more easily oxidizable urea molecules, as urea oxidation requires a very low theoretical potential (0.37 V under standard conditions) 31. Being non-toxic, non-flammable and low-cost, urea enables the urea oxidation reaction (UOR) to be a promising candidate for hydrogen production 32,33. Additionally, eutrophication is a major problem facing the world, which includes urea. Therefore, urea electrolysis is not only an efficient method for hydrogen generation but also a promising way for eliminating harmful urea in urea-rich wastewater 34. The 2D NiFeCo LDH/NF electrode also shows remarkable UOR activity, with a low overpotential of 0.280 V vs SCE to afford 10 mA cm-2 current density in 1 M KOH with 0.33 M urea. Moreover, its bifunctional activity for urea electrolysis in the two-electrode system requires a relatively low potential of 1.49 V to reach 10 mA cm-2. The excellent performance of NiFeCo LDH/NF as bifunctional electrocatalysts for water splitting and urea electrolysis reveals that 2D nanosheets are leading candidate for electrochemical activity which exposes more catalytic active sites and increases the electrochemical kinetics. RESULTS AND DISCUSSION Morphological and structural analysis of 2D NiFeCo LDH/NF Herein, we begin our study with the synthesis of the NiFeCo LDH/NF catalyst via a onestep electrodeposition method. The amorphous, 2D NiFeCO LDH/NF synthesized at room temperature via a simple one-step electrodeposition method (see an experimental section for details). The morphology of the sample was investigated by a field emission scanning electron microscope (FE-SEM). The FE-SEM images of NiFeCo LDH/NF clearly show a densely 5 ACS Paragon Plus Environment


interconnected vertically-aligned porous 2D nanosheet morphology, whereas the surface of bare NF is smooth as shown in Figure 1(a-b) and Figure S1. Such porous 2D nanosheet arrays, built by NiFeCo LDH/NF, would afford a high number of electrochemically active sites, structural stability and empower the material with good electric conductivity, promoting electron transport efficiently 35. To deepen the understanding of the microstructure and composition of the NiFeCo LDH/NF electrocatalyst was studied by transmission electron microscope (TEM) images. The nanosheet like morphology was further confirmed by TEM images of NiFeCo LDH/NF (Figure 1c). The high-resolution TEM (HR-TEM) image in Figure 1d suggests that NiFeCo LDH/NF is an amorphous phase, as the typical lattice fringes are not observed. In the selected area diffraction pattern (SAED) a broad and diffuse halo ring further confirms the amorphous nature of the film (inset of Figure 1d)

36.

The TEM elemental mapping images (Figure S2) show that NiFeCo

LDH/NF was fabricated with a stoichiometric atomic ratio of approximately 1:1:0.10 for Ni:Fe:Co. Moreover, the elemental mapping also illustrates that the Fe, Ni, and Co elements are distribute uniformly in NiFeCo LDH/NF nanosheets.


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Figure 1(a-b) FE-SEM images of 2D NiFeCo LDH/NF, (c) TEM image, and (d) HR-TEM images inset SAED pattern of 2D NiFeCo LDH/NF. Furthermore, the XRD pattern (Figure 2a) shows main diffraction peaks at 44.6º, 51.8º, and 76.4º corresponds to (111), (200), and (220) planes respectively, of NF (JCPDS 04-0850). No other peaks appeared, which shows that NiFeCo LDH/NF is in an amorphous phase, as explained earlier. The oxidation states and chemical states of 2D NiFeCo LDH/NF film was probed by Xray photoelectron spectroscopy (XPS) analysis. The Ni 2p spectrum (Figure 2c) shows two peaks 7 ACS Paragon Plus Environment


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at binding energies (BEs) of 873.1 and 855.9 eV corresponding to Ni 2p1/2 and Ni 2p3/2, respectively, which were ascribed to the nickel hydroxide phase with a Ni2+ oxidation state. Additionally, two peaks at BEs of 875.3 and 857.7 eV were observed, and these peaks suggest Ni3+ 37. The Fe 2p spectrum (Figure 2d) as deconvoluted into peaks at approximately 725 eV and 711.6 eV for Fe 2p1/2 and Fe 2p3/2 respectively, implying a Fe+2/Fe+3 oxidation state 38. In addition, the Co 2p binding energies at 781.3 eV and 797.2 eV were attributed to Co 2p3/2 and Co 2p1/2 along with their satellite peaks, indicating the presence of Co2+/Co3+ species (Figure 2e) 39. The O 1s a spectrum shows (Figure 2f) three deconvoluted peaks at 533.4, 531.4, and 529.4 eV, which are generally attributed to Co-O-H, Ni-O-H, and Fe-O-H bonds respectively

40.

Overall, the results

indicate the as-prepared NiFeCO LDH/NF is amorphous in nature with 2D nanosheet morphology.

Figure 2 (a) XRD pattern of 2D NiFeCo LDH/NF, (b) XPS survey scan, and (c) Ni 2p, (d) Fe 2p, (e) Co 2p and (f) O 1s core level XPS spectra of 2D NiFeCo LDH/NF.


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Electrocatalytical activity for OER and HER Next, we examined the OER properties of the 2D NiFeCo LDH/NF catalyst in a 1.0 M KOH electrolyte at room temperature using a standard three-electrode system. Figure 3a shows the polarization curve of the NiFeCo LDH/NF electrode along with electrodes comprised of bare NF, NiFe hydroxide on NF (NiFe LDH/NF), and RuO2/NF for comparison. The polarization curve was swept from higher to lower potential to neglect the effects of the current of the oxidation Ni2+ to Ni3+ 41. The redox peak observed at 1.35 V vs RHE, ascribed to one electron redox reaction involving in Ni

2+/

Ni3+. The OER catalytic activity was assessed by comparing the main three

parameters: (i) the overpotential required, (ii) the long-term stability, and (iii) the Tafel slope of the catalysts 26. As shown in Figure 3a, the NiFe LDH/NF, RuO2/NF and bare NF reveal lower OER activities with overpotentials of 240 mV, 270 mV, and 350 mV respectively at 10 mA cm-2, indicating their poor catalytic activities. In contrast, the NiFeCo LDH/NF catalyst showed a remarkable increase in OER activity with a lower overpotential of 210 mV at the same 10 mA cm-2 current density. Furthermore, NiFeCo LDH/NF also displayed a high current density at a potential of 1.5 V (232 mA cm-2), which is almost 3.4 times that of NiFe LDH/NF (68 mA cm-2). With such high catalytic activity, the NiFeCo LDH/NF electrode shows excellent OER activity comparable with many recently reported transition-metal catalysts (Table S1). A similar trend was observed for the change of the Tafel slope, the NiFeCo LDH/NF had a smaller Tafel slope (39 mV dec-1) than those of NiFe LDH/NF (57.6 mV dec-1), RuO2/NF (72 mV dec-1), and bare NF (108 mV dec-1) (Figure 3b). These results indicate that the NiFeCo LDH/NF not only has the low overpotential needed to initialize OER but also has a great impact on the OER kinetics. The long-term stability is a crucial parameter to estimate the lifetime for the catalyst. The chronopotentiometry curve shows that at a constant current density of 10 mA cm-2 the static potential was retained for over 50 9 ACS Paragon Plus Environment


h of continuous operation (Figure 3c). It is worth pointing out that continuous operation over a period of 50 h, the NiFeCo LDH/NF shows negligible potential shift (inset Figure 3c). After the 50 h durability test, the catalyst was examined to check the structural stability. The FE-SEM images (Figure S3) show a slight change in the morphology after 50 h of electrolysis. Additionally, after a long-term durability test of NiFeCo LDH/NF, the survey spectra show the existence of Fe, Ni, Co and O elements. The XPS spectra of Ni, Fe, Co, and O show these elements are still in the same oxidation states. But the intensity of peak decreased and peaks are broad, it was due to continuous and vigorous evolution of gas bubble on the electrode surface. This slight change at the surface was observed after long-term stability of NiFeCo LDH/NF electrode (Figure S4). The above results confirm the stable structure of the 2D NiFeCo LDH/NF catalyst, which is an important parameter for practical application.

Figure 3 Electrochemical OER and HER performance: (a) OER polarization curve in 1 M KOH, (b) Tafel plots derived from the corresponding polarization curves, (c) chronopotentiometry stability test of 2D NiFeCo LDH/NF over 50 h at a constant current density of 10 mA cm-2, inset 10 ACS Paragon Plus Environment

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shows the polarization curve initially and after the 50 h stability test. (d) HER polarization curve in 1 M KOH, (e) Tafel plots derived from the corresponding polarization curves, and (f) chronopotentiometry stability test of 2D NiFeCo LDH/NF over 50 h at a constant current density of -10 mA cm-2, the inset shows the polarization curve initially and after the 50 h stability test. To check the feasibility for bifunctional water splitting, the catalytical performance of the NiFeCo LDH/NF electrode towards the HER was also investigated in a 1 M KOH solution. The polarization curve of the NiFeCo LDH/NF electrode showed enhanced HER activity in comparison to those of the NiFe LDH/NF, Pt/NF, and bare NF electrodes (Figure 3d). The Pt/NF shows the lower overpotential (40 mV). The NiFeCo LDH/NF electrode shows higher HER performance than that of the other two electrodes, indicating a rapid increase in current density with the applied potential. Specifically, the overpotential at 10 mA cm-2 for NiFeCo LDH/NF is 108 mV, lower than those the NiFe LDH/NF (145 mV), Pt/NF (64 mV dec-1), and bare NF (255 mV) electrodes. To acquire a relatively high current density of 100 and 200 mA cm-2 NiFeCo LDH/NF only 245 mV and 300 mV, respectively overpotential are required, whereas, for NiFe LDH/NF, 288 mV and 360 mV are required, respectively. Remarkably, it is noted that the overpotential of NiFeCo LDH/NF is better to that of previously published catalysts (Table S1). The enhanced HER performance of NiFeCo LDH/NF is further proved by the Tafel slope from the LSV curves. The Tafel slope of NiFeCo LDH/NF is 73 mV dec-1, lower compared to those of NiFe LDH/NF (90 mV dec-1) and bare NF (112 mV dec-1), indicating the fast HER kinetics of the NiFeCo LDH/NF catalysts (Figure 3e). Figure 3f shows that the NiFeCo LDH/NF catalyst retained a constant working potential during the 50 h continuous test, and after the 50 h long-term stability test, the NiFeCo LDH/NF maintained a nearly identical polarization curve (inset Figure 3f). 11 ACS Paragon Plus Environment


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Origin of high catalytic activity To elucidate the origin of the excellent electrocatalytic activity, electrochemical active surface area (ESCA) analysis was conducted. ESCA is an important quantitative parameter that is applied to define the catalytical activity of a catalyst. An improvement in ESCA normally indicates more active sites and improvement of the catalytic activity 26. The ESCA of an electrode can be calculated by determining the electrochemical double layer capacitance (Cdl) from the cyclic voltammetry (CV) curves 42. By plotting the current density against scan rates, a linear slope is obtained, which has a positive correlation with Cdl. Figure S5 and Figure 4a show the CV curves of NiFeCo LDH/NF and NiFe LDH/NF and the corresponding Cdl of the catalysts, respectively. The Cdl values of the NiFeCo LDH/NF and NiFe LDH/NF electrodes were 11.35 mF cm-2 and 8.11 mF cm-2 respectively. NiFeCo LDH/NF possesses the highest Cdl value, indicating that NiFeCo LDH/NF has an improved ESCA and more active sites are obtained by the 2D porous nanosheet structure of NiFeCo LDH/NF 43. Moreover, electrical impedance spectra (EIS) analysis was conducted to analyse the charge transport mechanism of the electrodes

44.

The EIS results

(Figure 4b) indicate a smaller charge transfer resistance (Rct) of NiFCo LDH/NF compare to NiFe LDH/NF. The low Rct value of NiFeCo LDH/NF indicates that it has the fastest charge transfer process of the tested electrodes 45. To quantify the intrinsically high electrocatalytic activities for the OER, the turn over frequency (TOF), mass activity (MA) and specific activity were evaluated and compared among all of the catalysts 46. The TOF of NiFeCo LDH/NF was evaluated to be 0.037 s-1 at an overpotential of 270 mV, which is more than the corresponding value of NiFe LDH/NF (0.024 s-1), indicating a high intrinsic activity of NiFeCo LDH/NF 47,48. Figure 4c shows the TOF of NiFeCo/NF at various potentials. Moreover, the MA and specific activity results


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demonstrate that the NiFeCo LDH/NF catalyst provides higher catalytic activity than that of NiFe LDF/NF (Figure 4d).

Figure 4 (a) Linear fit of the capacitive current vs scan rates for 2D NiFeCo LDH/NF and NiFe LDH/NF, (b) Nyquist plot of 2D NiFeCo LDH/NF and NiFe LDH/NF. The inset shows the equivalent circuit. (c) TOF plot of OER, and (d) mass activity and specific activity of the catalysts at an overpotential of 270 mV. Bifunctional water splitting To explore the promising practical application of the as-prepared NiFeCo LDH/NF, for electrochemical water splitting, an electrolyser was assembled with NiFeCo LDH/NF electrodes in a two-electrode cell. As shown in Figure 5a, the NiFeCo LDH/NF(+) // NiFeCo LDH/NF(-) 13 ACS Paragon Plus Environment


couple exhibits a faster current enhancement with a cell voltage of 1.57 V to reach 10 mA cm-2. The NiFeCo LDH/NF exhibited, comparable and superior catalytic performance to that of other highly active transitional metal bifunctional electrocatalysts, as displayed in Table S2. To check the long-term durability for overall water splitting, the chronopotentiometry curve of the NiFeCo LDH/NF electrocatalyst was measured and is given in Figure 5b.

Figure 5 Electrocatalytic performance of 2D NiFeCo LDH/NF towards overall water splitting in 1 M KOH (a) polarization curve obtained in a two-electrode configuration and, (b) long-term stability over 50 h at 10 mA cm-2 for overall water splitting. The inset shows the polarization curves before and after the long-term stability test. As shown, the NiFeCo LDH/NF electrocatalyst exhibits impressive stability over 50 h with minimal degradation. The potential needed to deliver the same current density remained nearly unchanged after 50 h of continuous electrolysis. The above results demonstrate the superior electrocatalytic performance of NiFeCo LDH/NF as a bifunctional electrode in 1 M KOH that reach 10 mA cm-2 at a potential of 1.57 V, which is excellent compared to recently reported transitional metal-based catalysts. However, the sluggish kinetics and large potential of OER remain a barrier for efficient hydrogen generation. As


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stated above, UOR requires a lower thermodynamic voltage compared to that of OER, which gives an excellent alternative for electrocatalytic hydrogen generation. Electrochemical activity for UOR and HER The UOR for NiFeCo LDH/NF was studied using an LSV curve in 1 M KOH in the presence of 0.33 M urea and absence urea. The current density of the NiFeCo LDH/NF is very low when measured only in 0.33 M urea as shown in Figure 6a, which verifies that without KOH, the UOR process cannot occur. In contrast, in a 1 M KOH with 0.33 M urea electrolyte the electrode showed a significant enhancement in the current density. To reach 100 mA cm-2 in 1 m KOH with 0.33 M urea, only 0.35 V vs SCE was required, which surpasses the performance of noble metal-free UOR catalysts that have recently been recently reported (Table S3). Figure 6b shows the UOR activity of NiFeCo LDH/NF, NiFe LDH/NF, and bare NF electrodes in 1 M KOH with 0.33 M urea. Bare NF has a very low UOR activity, NiFe LDH/NF shows a UOR performance of 0.38 V vs SCE to reach 10 mA cm-2 whereas NiFeCo LDH/NF presents a high UOR activity with only 0.280 V vs SCE to reach the same 10 mA cm-2 current density. Furthermore, the Tafel slope of NiFeCo LDH/NF had a much lower value of 31 mv dec-1 than those of NiFe LDH/NF (42 mV dec-1) and bare NF (64 mV dec-1), suggesting favourable kinetics for UOR (Figure S6a). The NiFeCo LDH/NF was tested for long-term stability at a constant current of 10 mA cm-2 over 50 h. As observed, NiFeCo LDH/NF maintained high catalytic stability over 50 h, and there was a slight change in the overpotential after long-term stability testing (inset Figure 6c). After 50 h of the UOR stability test, the NiFeCo LDH/NF electrocatalyst was characterized by FE-SEM and XPS analysis. The interconnected nanosheet structure shows good retention of its original morphology, indicating its high structural stability (Figure S7). Additionally, the XPS results indicated the same



elemental peaks of Fe 2p, Ni 2p, Co 2p, and O 1s are at nearly the same BEs as before (Figure S8).

Figure 6 (a) Polarization curve in 1 M KOH with and without 0.33 M urea for 2D NiFeCo LDH/NF, (b) UOR polarization curve in 1 M KOH with 0.33 M urea, and (c) chronopotentiometry stability test of 2D NiFeCo LDH/NF over 50 h at a constant current density of 10 mA cm-2 in 1 M KOH with 0.33 M urea. The inset shows the polarization curve initially and after 50 h stability test. (d) Polarization curve in 1 M KOH with and without 0.33 M urea for 2D NiFeCo LDH/NF for HER, (e) HER polarization curve in 1 M KOH with 0.33 M urea, and (f) chronopotentiometry stability test of 2D NiFeCo LDH/NF over 50 h at a constant current density of -10 mA cm-2 in 1 M KOH with 0.33 M urea. The inset shows the polarization curve initially and after the 50 h stability test. Similar to UOR, the NiFeCo LDH/NF catalyst shows a similar trend in HER. Figure 6d shows the HER activity of NiFeCo LDH/NF in both 1 M KOH, 1 M KOH with 0.33 M urea and 0.33 M urea, and indicates a small shift in the potential in 1 M KOH with urea compared to that of 1 M 16 ACS Paragon Plus Environment

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KOH. The HER activities of all electrodes shown in Figure 6e clearly demonstrate that NiFeCo LDH/NF has the lowest overpotential of 1.1 V vs SCE to reach 10 mA cm-2 compared to that of NiFe LDH/NF (1.2 V vs SCE) and bare NF (1.25 V vs SCE). The Tafel slopes of NiFeCo LDH/NF, NiFe LDH/NF, and bare NF were 55 mV dec-1, 79 mV dec-1, and 224 mV dec-1 respectively as shown in Figure S6b, demonstrating the fast kinetics of NiFeCo LDH/NF for HER. The long-term stability of NiFeCo LDH/NF was further examined using the chronopotentiometry curve. As observed in Figure 6f, the NiFeCo LDH/NF exhibits excellent catalytic stability over 50 h under a constant current density of -10 mA cm-2. Urea electrolysis Taking into account the excellent experimental results of UOR and HER for NiFeCo LDH/NF in 1 M KOH with urea (0.33 M), we constructed a urea electrocatalytic cell using NiFeCo LDH/NF as both the cathode and anode. For the urea electrolyser, the NiFeCo LDH/NF delivered the excellent performance with 10 mA cm-2 at the low cell potential of 1.49 V, and a high current density of 100 mA cm-2 can be achieved at 1.72 V (Figure 7a). This performance for NiFeCo LDH/NF is excellent compared to that of MnO2/MnCo2O4/NF (1.55 V), NiMoS/Carbon cloth (1.59 V), and CoS2/Ti mesh (1.59 V)

49–51.

At the same time, the long-term stability of NiFeCo

LDH/NF catalysts was examined over 50 h in a 1 M KOH with 0.33 M urea solution (Figure 7b). Over 50 h of continuous operation, NiFeCo LDH/NF showed a slight change in potential (inset in Figure 7b).



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Figure 7 (a) Overall electrolysis in a two-electrode system in 1 M KOH with 0.33 M urea and, (b) Long-term stability over 50 h. The inset shows the polarization curves before and after the longterm stability test. The stable and efficient electrochemical performance towards alkaline water splitting and urea electrolysis of the 2D NiFeCo LDH/NF nanosheets mainly benefits from the following facts: (i) the amorphous 2D NiFeCo LDH/NF nanosheet arrays was grown directly on conductive NF to form an electrode with high electron transportation and electrolyte diffusion, which improves the catalytic activity 52. (ii) The improvement of catalytical activity by incorporation of Co in the NiFe catalyst, which is due to the simple intercalation of OH- by enhancing the porosity/disorder and increases the amount of edge sites or defects in NiFeCo LDH structure

26,53.

(iii) The

interconnected 2D nanosheet arrays maintain good structural stability and allow electrolyte ions access to the surface for fast water splitting 54,55. (iv) The ternary compound shows better structural stability and catalytic activity compared to those of the binary compound owing to a synergistic effect between the components 56,57. The facile preparation, low-cost, high electrocatalytic activity,


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and decent bifunctionality of the NiFeCo LDH/NF enable it to serve as competitive electrocatalysts for large-scale urea electrolysis and alkaline water splitting applications. CONCLUSION In summary, we demonstrated that amorphous and porous 2D NiFeCo hydroxide nanosheets grown on NF via a simple and cost-effective electrodeposition method are highlyefficient bifunctional electrocatalyst for alkaline water splitting and urea electrolysis. The high electrocatalytic performance enhancement could be attributed to the amorphous structure, 2D nanosheet morphology, and the synergistic effect. The as-prepared 2D NiFeCo LDH/NF electrode exhibited an excellent OER and HER activity with overpotentials of 210 and 108 mV to achieve 10 mA cm-2 with excellent long-term durability. The 2D NiFeCo LDH/NF-based two-electrode water electrolyser only requires a cell voltage of 1.57 V to achieve a current density of 10 mA cm-2. Furthermore, this electrode also acts as an UOR with a low potential of 0.280 V (vs SCE) at 10 mA cm-2 in KOH (1 M) with urea (0.33 M) and a urea-based water electrolysis cell requires only 1.49 V to achieve 10 mA cm-2 with high stability. The present work provides a new approach for designing and utilizing earth-abundant element-based materials as an efficient and robust electrocatalyst for water splitting applications.

ACKNOWLEDGMENTS This work was supported by the Human Resources Development program (No.20164030201310) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry, and Energy and supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2018R1A6A1A03024334). 19 ACS Paragon Plus Environment


Electronic Supplementary material: Supplementary material (Experimental procedure, FE-SEM, XPS analysis) is available in the online version of this article at


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TOC

The amorphous 2D hydroxide nanosheet arrays on Ni foam (NiFeCo LDH/NF) prepared by simple and cost-effective electrodeposition method exhibited remarkable performance for water splitting and urea electrolysis.


Bifunctional 2D electrocatalysts of transition metal ... - ACS Publications

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