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Amorphous Multi-elements Electrocatalysts with Tunable Bifunctionality towards Overall Water Splitting Xiaomei Wang, Weiguang Ma, Chunmei Ding, Zhiqiang Xu, Hong Wang, Xu Zong, and Can Li ACS Catal., Just Accepted Manuscript • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Amorphous Multi-elements Electrocatalysts with Tunable Bifunctionality towards Overall Water Splitting Xiaomei Wang a, b, Weiguang Ma a, Chunmei Ding a, Zhiqiang Xu a, b, Hong Wang a, b, Xu Zong * a

a

, and Can Li * a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences; Dalian National Laboratory for Clean Energy, The Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Zhongshan Road 457, Dalian 116023, China b

University of Chinese Academy of Sciences, Beijing 100049, China.

ACS Paragon Plus Environment

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ABSTRACT: Economically producing hydrogen via electrocatalytic water splitting requires highly-efficient and low-cost catalysts and scalable synthetic strategies. Herein, we present the preparation of hierarchically structured multi-elements water splitting electrocatalysts consisted of Fe, Co, Ni, S, P and O with a one-step electrodeposition method. By tuning the non-metal compositions of the catalysts, the electrochemical performances of the catalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in 1 M KOH can be rationally modified, respectively. Under the optimum conditions, current densities of 100 and 1000 mA cm2

were obtained at overpotentials of only 135 and 264 mV on the HER catalyst, and 258 and 360

mV on the OER catalysts, respectively. When assembling the best-performed HER and OER catalyst in a two-electrode system for overall water splitting, a current density of 10 mA cm-2 can be obtained under a cell voltage of 1.46 V with long-term durability. As far as we know, this is among the lowest voltage ever reported for two-electrode electrolyzer based upon earth-abundant elements. Moreover, the catalysts can be facilely assembled on commercially available Ni mesh and demonstrate even higher performance, indicating its great potential for scale-up water electrolysis. We further demonstrate that S and P play different while pivot roles in modifying the apparent and intrinsic electrocatalytic activity of the as-prepared amorphous electrocatalysts, therefore pointing out a pathway towards the optimization of multi-elements catalysts.

KEYWORDS: Multi-elements electrocatalyst; Electrodeposition; Hydrogen evolution; Oxygen evolution; Overall water splitting


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1. INTRODUCTION Producing hydrogen via electrocatalytic water splitting has been supposed to be a viable approach to convert and store renewable energy resources such solar energy.1-2 Developing promising electrocatalysts towards cost-effective water splitting is important in this scenario and has received substantial attention. Although noble-metal-based materials like Pt, IrOx, and RuOx have been well-known as excellent catalysts for hydrogen evolution reaction (HER) or oxygen evolution reaction (OER), these materials are scarce and impractical for large scale applications.3-5 Therefore, exploring highly-efficient and low-cost catalysts for full water splitting is highly desirable. Recent research has identified a wide variety of earth-abundant metal-based materials such as oxide,6-9 sulfide,10-12 phosphide,13-16 carbides,17-19 and nitrides

20-21

for HER and OER, and great

efforts have been devoted to modify the structural 22-23 as well as electronic 24-25 properties of the catalysts. Among all the approaches under investigation, constructing multi-elements electrocatalysts has been found to be an effective approach due to the synergetic interplay of different elements on improving the electronic properties of catalysts. The modification of the electronic properties of the catalysts can regulate the adsorption energies of atomic hydrogen or OER intermediates such as OH*, O*, and OOH* to the electrocatalyst and thus the HER or OER activity of these catalysts.11, 24, 26-34 However, several challenging issues remain unresolved in this field. First, most of the reported work were dedicated to investigating the influence of multielements on tuning the electronic properties of the catalysts instead of the structural properties. Realizing simultaneous tunable modulation on the electronic and structural properties of multielements catalysts is attractive while remains a big challenge.35 Second, most of the multielements electrocatalysts reported up to now are generally confined to those consisted of unitary


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metal and multi-non-metal elements (CoPS,29 MoSSe,33 etc.) or unitary non-metal and multimetal elements (FeCoNiOx,24 NiFeCoP,31 and NiMo3S4,36 etc.). In principle, the participation of more elements in constructing electrocatalysts could afford more opportunities towards optimized electrocatalytic performance.32,

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It has been found that the multi-metal catalysts

consisted of Fe, Co, and Ni generally exhibit higher performance than the unary and binary ones for both HER and OER due to the interplay of the metals in fine-tuning the electronic properties and therefore the adsorption energies of the reaction intermediates.31,

37-40

Moreover, the

incorporation of non-metals like P and S into metal compounds has been reported to lead to enhanced electrochemical performance.27,

29

We envision that the simultaneous and tunable

assembly of milt-metals of Fe, Co, Ni and milt-non-metals of S and P will create new opportunities towards more efficient electrocatalysis. Third, although most multi-elements catalysts were reported to show improved activity for HER or OER, respectively, few of them showed both remarkable HER and OER performances in the same electrolyte. Developing ideal multi-elements water splitting electrocatalysts that have exceptional activity towards both the HER and OER will simplify the electrocatalytic system and reduce the energy consumption and the manufacturing cost. And last, most of the reported strategies for synthesizing multi-elements electrocatalyst involved complicated and time-consuming synthesis steps and are essentially incompatible for large scale deployment,27, 41 which defines the imperative to develop low-cost and scalable synthetic strategies. Electrodeposition provides a versatile, time-saving and scalable platform to realize precise control on the compositions, structures and morphologies of the as-deposited materials through tuning the deposition electrolytes and parameters.42-43 Moreover, electrodeposition can enable the straightforward assembly of electrocatalysts on conductive substrate to prepare binder-free


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electrodes, therefore reducing the internal resistance and also improving the stability of the catalysts when compared with those using organic binders such as Nafion.32 However, although this technique has been well applied to the synthesis of water splitting electrocatalysts with simple compositions,44-46 its capability for synthesizing more complex electrocatalyst system consisted of both multiple metals and non-metals for water splitting reaction is still unclear. Herein, we demonstrate the power of this strategy for the one-pot assembly of Fe, Co, Ni, S, P and O elements towards the synthesis of multi-elements electrocatalysts. Through systematically tailoring the non-metal modifiers of P and S in the catalysts, we realized tunable modulation on the structural as well as electronic properties and therefore the electrocatalytic performance of the FeCoNiPxSy samples. Under optimum conditions, the as-prepared HER catalyst yields current densities of 10, 100, and 1000 mA cm-2 at low overpotentials of 43 mV, 135 mV, and 264 mV in 1 M KOH. Moreover, overpotentials of only 258 and 360 mV are required to yield current densities of 100 and 1000 mA cm-2 for OER. When assembling the best-performed HER and OER catalyst for overall water splitting, a current density of 10 mA cm-2 can be achieved at 1.46 V. As far as we know, this is among the lowest cell voltages ever reported for two-electrode water splitting devices. More importantly, we showed the pivot and different roles of S and P in modifying the apparent and intrinsic electrocatalytic activity of the amorphous electrocatalysts, which as far as we know has not been well illustrated. 2. MATERIAL AND METHODS FeSO4·7H2O, NiSO4·6H2O, H3BO3, NH4Cl, NaH2PO2 and KOH were purchased from Sinopharm Chemical Reagent Co., Ltd. CoSO4·7H2O and Thiourea (H2NCSNH2) were obtained from Tianjin Kemiou Chemical Reagent and Tianjin Bodi Co., Ltd, respectively. All the reagents are analytical grade and used directly.


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The FeCoNiPxSy catalysts were prepared by electrodeposition on a planar Ti foil at -1 A cm-2 without stirring. For the deposition of FeCoNiPxSy, NiSO4, FeSO4, and CoSO4 were used as the metal salts and NaH2PO2 and H2NCSNH2 were used as the salts for introducing P and S dopants. Moreover, H3BO3 and NH4Cl were added in the deposition solution as the addition agents to improve the quality of the deposits. The electrolyte bath contained 0.09 M NiSO4, 0.03 M FeSO4, 0.053 M CoSO4, 0.4 M H3BO3, 0.28 M NH4Cl and certain amount of NaH2PO2 and H2NCSNH2. The metal ratio of Fe: Co: Ni was maintained at 17:31:52 because this ratio has been reported to afford catalyst with the best electrochemical performance.37-40, 42 When tuning the P and S compositions of FeCoNiPxSy catalysts, the total amount of NaH2PO2 and H2NCSNH2 is maintained at 1 M while those of NaH2PO2 (x M) and H2NCSNH2 (y M) were systematically varied and the resulting samples were denoted as FeCoNiPxSy. There were FeCoNiP1S0, FeCoNiP0.8S0.2, FeCoNiP0.5S0.5, FeCoNiP0.2S0.8 and FeCoNiP0S1 that were prepared with the two-electrode electrodeposition. In order to optimize the electrodeposition time of catalysts towards HER and OER, the FeCoNiP1S0 catalysts with deposition time of 30, 60, 75, 90, 105, 120 and 150 s were prepared and evaluated. The optimized deposition time of FeCoNiP1S0 was determined to be 120 s for HER and 105 s for OER, respectively. As a result, all the FeCoNiPxSy catalysts were prepared at 120 s for HER and 105 s for OER. In order to investigate the roles of Fe, Co and Ni played for FeCoNiPxSy catalysts towards HER and OER, MP0S1 (M: Fe, Co, Ni, FeCo, FeNi, and CoNi) samples with a deposition duration of 120 s for HER and MP0.5S0.5 with a deposition duration of 105 s for OER were prepared at their deposition solution, respectively, in which the total content of metal salts were maintained at 0.173 M while the compositions of the metals were systematically varied.


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The physicochemical properties of the FeCoNiPxSy samples were investigated with elemental analysis, X-ray analysis, and electron microscopical analysis, etc. (see Supporting Information for details). The electrocatalytic performance of the FeCoNiPxSy catalysts were measured by linear sweep voltammetry (LSV), chronopotentiometry, electrochemical impedance spectroscopy (EIS), and faradaic efficiency (FE), etc. (see Supporting Information for details). On the measurement, FeCoNiPxSy/Ti and saturated calomel electrode (saturated KCl, SCE) were used as the working and reference electrode, respectively. As for the counter electrode, carbon plate was employed to avoid the interference of Pt for HER measurement. For the OER measurements, we are also aware of the possible interference of Pt counter electrode. However, Pt counter electrode is employed because we find it is more suitable than graphite counter electrode in the measurements (Figure S6-S7). All the LSV curves were recorded with iR compensation (90%). 3. RESULTS AND DISCUSSION Preparation and characterizations of the catalysts. Multi-elements catalysts containing Fe, Co, Ni, S, P and O were prepared via electrodeposition in solution containing 0.03 M FeSO4, 0.053 M CoSO4, 0.09 M NiSO4. Different amounts of sodium hypophosphite (x M) and thiourea (y M) were added to the deposition solution as the P and S modifiers. For simplicity, the as-prepared electrocatalysts were denoted as FeCoNiPxSy although the actual ratios of the elements are different from those in the deposition electrolyte. X-ray diffraction (XRD) patterns of FeCoNiPxSy catalysts only exhibited weak diffraction peaks at ca. 23o besides those indexed to the Ti substrate, indicating that the as-prepared FeCoNiPxSy catalysts are amorphous (Figure S1). Scanning electron microscopy (SEM) analysis (Figure S2) indicates that all the FeCoNiPxSy


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catalysts are uniformly deposited on the Ti foil substrate. A close examination with high magnification SEM reveals that FeCoNiP1S0 exhibits a cauliflower-like morphology (Figure 1a), which is similar with that of FeCoNiP0S0 (Figure S3). Therefore, P was supposed to have little effect on the morphology of the resulting catalysts. On the contrary, upon the introduction of S, FeCoNiP0.8S0.2 shows smoother surface with no obvious cauliflower-like structure (Figure 1b). With further increase in S and decrease in P, the morphologies of FeCoNiPxSy catalysts gradually convert to thinner, rippled and porous nanosheet (Figure 1c-e), indicating the significant effect of S on tuning the morphology of the FeCoNiPxSy catalysts. We suppose that the introduction of thiourea that has good complexation ability with metal ions will significantly influence the deposition environment of the metal ions and therefore the morphologies of the resulting FeCoNiPxSy catalysts.47-52 Similar morphological changes were observed using parallel transmission electron microscopy (TEM) characterizations (Figure 1f-j). Therefore, tailoring the relative ratios of thiourea and sodium hypophosphite can significantly modulate the morphological structures of the resulting FeCoNiPxSy catalysts. Inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis (Table S1) indicates that all the catalysts have a metal composition of Fe17Co31Ni52. This is similar with that in the deposition solution, highlighting the advantage of the present approach on fine-controlling the metal stoichiometry in the catalysts. Moreover, the S content in FeCoNiPxSy catalysts gradually increases while P content decreases with the increased use of thiourea and decreased use of sodium hypophosphite in the deposition solution (Table S2). Energy-dispersive X-ray (EDX) results show that the amount of O (see Table S2 for details) is much higher than those of S and P in all the catalysts, indicating that FeCoNiPxSy actually exists mainly as oxides and S and P are concomitant as the dopants that will potentially influence the morphological and


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electronic properties of the catalysts. EDX elemental mapping (Figure S4) analysis indicates that all the measured elements (Fe, Co, Ni, P, S, and O) are evenly distributed over the examined area for all the FeCoNiPxSy catalysts, again demonstrating the advantage of the present approach on fine-controlling the synthesis of the catalysts.

Figure 1. SEM images of (a) FeCoNiP1S0, (b) FeCoNiP0.8S0.2, (c) FeCoNiP0.5S0.5, (d) FeCoNiP0.2S0.8, and (e) FeCoNiP0S1, and TEM images of (f) FeCoNiP1S0, (g) FeCoNiP0.8S0.2, (h) FeCoNiP0.5S0.5, (i) FeCoNiP0.2S0.8, and (j) FeCoNiP0S1. X-ray photoelectron spectroscopy (XPS) was then employed to investigate the chemical features of the metals and non-metals in the FeCoNiPxSy multi-elements catalysts. As a typical example, FeCoNiP0.5S0.5 was first investigated. Figure 2a shows that the surface of FeCoNiP0.5S0.5 is composed of Fe, Co, Ni, P, S and O. There are four peaks for the Fe 2p spectrum (Figure 2b). Peaks at 716.45 and 707.13 eV, which belongs to Fe 2p1/2 and Fe 2p3/2, can be indexed to Fe-P or Fe-S, and peaks of Fe 2p1/2 at 723.31 eV and Fe 2p3/2 at 712.26 eV can be attributed to the oxidized Fe species.53-57 The Co 2p spectrum shown in Figure 2c are fitted with two spin-orbit doublets: one at 798.04 eV (Co 2p1/2) and 782.16 eV (Co 2p3/2) and the other at 796.39 eV (Co 2p1/2) and 781.11 eV (Co 2p3/2), which correspond to Co2+ and Co3+, respectively.


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In addition, the rest of two peaks located at 786.50 eV and 803.09 eV are ascribed to satellite peaks.10, 27, 29, 58 As for Ni 2p spectrum (Figure 2d), the binding energy (BE) at 874.76 eV for Ni 2p1/2 and at 856.24 eV for Ni 2p3/2 can be indexed to Ni3+, and the BE at 873.36 eV for Ni 2p1/2 can be indexed to Ni2+. The peaks located at 861.82 and 880.19 eV belong to the satellite peaks.45, 57-59 The P 2p spectrum (Figure 2e) exhibits two peaks at 133.49 eV and 132.92 eV, which can be ascribed to the P 2p1/2 and P 2p3/2 of the phosphate-like P, respectively. The peak at 130.02 eV is attributed to the phosphorus anions, which are bonded to the metals in the catalyst.27, 60 The S 2p spectrum exhibits four peaks (Figure 2f). The peaks at 164.05 (S 2p1/2) and 162.67 eV (S 2p3/2) can be indexed to sulfide species of M-S (M: Fe, Co and Ni), respectively, and the BE of 169.74 and 168.21 eV is attributed to the S 2p1/2 and 2p3/2 of sulfate species.9-10, 36, 58 The above mentioned results indicate that the Fe, Co, and Ni elements are all in mixed 2+ and 3+ states and the FeCoNiP0.5S0.5 catalyst contains ca. 6.9% of Fe2+, 10.1% of Fe3+, 17.4% of Co2+, 13.6% of Co3+, 23.4% of Ni2+ and 28.6% of Ni3+, respectively. (The approximate percentage of variable oxidation states was calculated from their area of deconvoluted spectra). Moreover, the S and P elements are also in a complex states corresponding to sulfide, sulfate, phosphide, or phosphate. Similar with FeCoNiP0.5S0.5, other FeCoNiPxSy catalysts exhibit parallel XPS spectra for all the elements (Figure 3). However, slight shifts of the peak positions were observed for the metals’ spectra with the variation of the S and P contents (Figure 3a for Fe 2p, 3b for Co 2p and 3c for Ni 2p, and Table S3), indicating the role of the non-metal elements on the modification of the microelectronic environments of the metals.22, 27, 35, 61 Moreover, in the P 2p and S 2p spectra, the peak intensities for P decrease and those for S increase with the increase of thiourea and decrease of sodium hypophosphite in the deposition solution (Figure 3d-e, Figure S5 and Table


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Figure 2. (a) The survey XPS spectrum of FeCoNiP0.5S0.5 catalyst, and high resolution XPS spectra for the (b) Fe 2p, (c) Co 2p, (d) Ni 2p, (e) P 2p, and (f) S 2p regions of FeCoNiP0.5S0.5. S4). Moreover, the P intensity is weakened substantially once thiourea is introduced to the deposition solution (Figure 3d). This should be ascribed to more preferable deposition of S species compared with that for P deposition. This is well consistent with the trends observed in the ICP-AES and EDX analysis. At present, the accurate identification of coordination environments of the metal elements of Fe, Co, and Ni in FeCoNiPxSy catalysts is unavailable due to the quite complex compositions. However, based upon the above analysis, it is reasonable to conclude that a series of amorphous multi-elements FeCoNiPxSy catalysts with tunable morphological and electronic properties can be obtained in a scalable manner with the present approach.


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Figure 3. The high resolution XPS spectra for the (a) Fe 2p, (b) Co 2p, (c) Ni 2p, (d) P 2p, and (e) S 2p regions of FeCoNiPxSy catalysts. Electrocatalytic HER and OER performances. The electrocatalytic performances of FeCoNiPxSy for HER and OER were evaluated. In this work, the electrocatalytic performances of the catalysts are normalized according to the geometric surface area of the electrode unless otherwise stated. To achieve rational control on the catalytic activity of the FeCoNiPxSy catalysts, the deposition time was first optimized. We found that FeCoNiP1S0 catalyst demonstrated the optimum performance for HER and OER at a deposition time of 120 and 105 s,


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respectively (Figure S8), which should be due to the balance between the decreased conductivity and the increased number of the active sites for the electrocatalysts with increased deposition time.46 Therefore, these deposition durations were used to prepare other FeCoNiPxSy catalysts in the following studies. Figure 4a shows the HER activities of FeCoNiPxSy catalysts with different ratios of P and S prepared at a deposition time of 120 s. FeCoNiP1S0 obtains current densities of 10 and 100 mA cm-2 at relatively high overpotentials of 111 and 203 mV, respectively. With increasing the S and decreasing the P contents, a drastic and monotonic increase in the current densities was observed and FeCoNiP0S1 with thickness of ca. 21.6 µm (Figure S9) shows the highest HER activity. Current densities of 10, 100, and 1000 mA cm-2 are obtained at the lowest overpotentials of 43, 135, and 264 mV, respectively. As far as we know, these are among the best values ever reported for noble-metal-free catalysts towards HER (Table S5). Moreover, all the FeCoNiPxSy catalysts demonstrate much higher activity than the catalysts without P and S dopants, indicating the beneficial roles of P and S on improving the electrocatalytic performance (Figure S10). Besides, Figure S11 indicates that the ternary metal catalyst of FeCoNiP0S1 exhibits higher HER activity than the other binary and unitary ones, demonstrating the simultaneous presence of Fe, Co, and Ni contributes to the improved HER performance for the catalyst with non-metal dopants. Figure 4b shows that the FeCoNiP0S1 exhibits long-term durability during continuous HER test for 40 h. The faradaic efficiency (FE) of HER on FeCoNiP0S1 was calculated to be ca. 100% (Figure S12).


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Figure 4. (a) Linear sweep voltammetry curves of the as-prepared FeCoNiPxSy catalysts. (b) Chronopotentiometric curve of FeCoNiP0S1 for HER at 10 mA cm−2. The potentials are given without iR-correction. (c) Current densities at 200 mV and the electrochemical surface areas (ECSA), (d) normalization of the current densities with ECSA at 200 mV, (e) the Tafel plots, (f) Nyquist plots of the FeCoNiPxSy samples for HER in 1 M KOH. Capacitance measurements show that the electrochemical surface areas (ECSAs) of the FeCoNiPxSy catalysts increase monotonically with the increase of the S content and the decrease of P content (Figure S13). By correlating with the ECSA, it is interesting to find that the catalytic current densities of FeCoNiPxSy catalysts increase almost linearly with the ECSA (Figure 4c). Moreover, at an overpotential of 200 mV, normalization of the geometric current density with active sites was shown in Figure 4d. Due to the unknown capacitive behaviour (Cs) of the FeCoNiPxSy catalysts, active site activity*Cs (ASA*Cs) was used as the standard for comparing the intrinsic activity, which has been well applied in literature.62-63 All the FeCoNiPxSy catalysts


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exhibit similar ASA*Cs, indicating their similar intrinsic activity for HER. Tafel slope was then employed as an indicator to investigate the effect of S and P on the rate-determining step and reaction pathway of the HER catalyzed by FeCoNiPxSy catalysts. As shown in Figure 4e, similar Tafel slopes were observed for all the FeCoNiPxSy catalysts, indicating that the catalysts exhibit similar catalytic reaction mechanism. The minor differences in the Tafel slopes are mainly caused by the different morphologies and porosities of the catalysts, which affect the mass transport and thus the rate of the rate-limiting step instead of the reaction pathway.64-65 Therefore, it is reasonable to suppose that the promotion in the apparent activity of the catalysts rather than the improvement in their intrinsic activity for HER is the main reason for the drastically enhanced activities of FeCoNiPxSy with higher S content.66-69 Electrochemical impedance spectroscopy (EIS) analysis indicates that the charge-transfer resistance increased substantially with the decrease in the S content (Figure 4f), which is well consistent with the increased trend observed for the HER activity.


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Figure 5. (a) Linear sweep voltammetry curves of the as-prepared FeCoNiPxSy catalysts. (b) chronopotentiometric curve of FeCoNiP0.5S0.5 for OER at 10 mA cm−2. The potentials are given without iR-correction. (c) Current densities at 300 mV and the electrochemical surface areas (ECSA), (d) normalization of the current density with ECSA at 300 mV, (e) the Tafel plots, and (f) Nyquist plots of the FeCoNiPxSy for OER in 1 M KOH. Likewise, we also tested and analysed the OER performance of the FeCoNiPxSy samples with different ratios of P and S prepared at a deposition time of 105 s. As shown in Figure 5a, FeCoNiP1S0 exhibits relatively low OER activity with overpotential of 280 mV to yield current density of 100 mA cm-2. With the increase of the S dopant, a significant improvement in the OER activity was observed initially. However, further increase in the S content and decrease in the P content led to decreased OER activity. Among all the FeCoNiPxSy catalysts, FeCoNiP0.5S0.5 with thickness of ca. 19.8 µm (Figure S14) exhibits the best performance. Current densities of 100 and 1000 mA cm-2 are obtained at the overpotentials of only 258 and 360 mV. As far as we know, the performance obtained on FeCoNiP0.5S0.5 catalyst is among the best values obtained on noble-metal free OER electrocatalysts (Table S6). Moreover, the charge-transfer and synergistic effects in Fe, Co and Ni for FeCoNiP0.5S0.5 towards OER were also observed (Figure S15) as has been well reported.24, 70 Figure 5b shows that the FeCoNiP0.5S0.5 exhibits long-term durability during continuous OER test for 40 h. The FE for OER on the FeCoNiP0.5S0.5 catalyst was calculated to be ca. 100% (Figure S16). Capacitance measurements show that the ECSAs for OER increased initially and then levelled off with the increase of S and the decrease of P contents (Figure 5c and Figure S17), which is different from the trend observed for the OER activity. Therefore, the increase in ECSAs cannot be simply translated to the increase in the OER activity, which should also be influenced by


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other factors besides ECSA.66-67, 71-72 When normalizing the current density with ECSA at 300 mV (Figure 5d), the specific activity (ASA*Cs) of the catalyst increases with the increase in P content and the decrease in S content, indicating the variation in the intrinsic activity of the catalyst. Figure 5e exhibits that the Tafel slope obtained on the FeCoNiP0S1 catalyst is ca. 80 mV dec-1, while much lower values of 60, 62, 49 and 51 mV dec-1 are obtained on FeCoNiP1S0, FeCoNiP0.8S0.2, FeCoNiP0.5S0.5 and FeCoNiP0.2S0.8, respectively. This can be ascribed to the modulated microelectronic environment of the catalysts incorporated with S and P, which will regulate the adsorption energies of OER intermediates and thus influence the OER kinetics. More importantly, P species were found to play a significantly role in shifting the reaction mechanism. The presence of P has been reported to modify the microelectronic environment of the catalysts and therefore their intrinsic activity.31, 61, 73-74 Moreover, phosphate is known to play an important role in mediating OER performance and the decrease of phosphate will influence the proton-coupled electron transfer and thus reducing the OER activity and shifting the catalytic mechanism.72, 75-77 In our case, the initial modification of FeCoNiP1S0 with S can significantly increase the ECSA of the resulting catalysts and therefore promote the catalyst activity. However, excess use of S will simultaneously lead to the sharp reduction of the P content in the catalyst, which will in turn deteriorate the OER activity. Therefore, fine-controlling the P and S elements in FeCoNiPxSy will not only modulate the apparent activity by changing the ECSA but also the intrinsic activity by influencing the microelectronic environment of the catalyst. It is reasonable to suppose that a best compromise between the two factors was achieved on FeCoNiP0.5S0.5, which led to the optimum activity. EIS analysis (Figure 5f) shows that for all the catalysts the decrease of charge-transfer resistance is not completely consistent with the increase


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of OER activity, again indicating that the OER activities of FeCoNiPxSy catalysts were influenced by both the charge transfer kinetics and the intrinsic activity.78 Based upon the above results, we can conclude that tailoring the amount of S and P in FeCoNiPxSy will modulate the morphological and electronic structures of the resulting FeCoNiPxSy catalysts. This will consequently mediate the mass transport and adsorption energies of the HER and OER intermediates and thus influence their electrocatalytic performance. As for the HER, the monotonic increase in the activity with increased S content is mainly ascribed to the increase in ECSA due to the evolution of the ripped and porous nanostructure. Moreover, the modified nanostructure will promote mass transport by allowing more facile diffusion of gaseous products and electrolyte. As for the OER, the electrocatalytic performances of the FeCoNiPxSy catalysts are affected not only by the change in the surface area due to S incorporation but also by the variation in the microelectronic environments of the catalysts due to P and S co-doping. As a consequence, we obtained the highest HER and OER catalytic activities on FeCoNiP0S1 and FeCoNiP0.5S0.5, respectively. These best-performed catalysts also show advantages over many other HER and OER catalysts in terms of other evaluation parameters (Table S5 and S6). We expect that theoretical calculations on FeCoNiPxSy could provide additional insight on the interplay of the P and S elements on modifying the electrocatalytic kinetics and activity of the catalysts, while this effort is baffled at present due to the amorphous and complex nature of the FeCoNiPxSy catalyst that prevents us from establishing a reasonable model.


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Figure 6. (a) Linear sweep voltammetry polarization curve of two-electrode electrolyzer with FeCoNiP0.5S0.5 and FeCoNiP0S1 as the anode and cathode at a scan rate of 1 mV s−1. Insert is the multi-current process of overall water splitting. (b) Chronopotentiometric curve of the twoelectrode device at 500 mA cm−2 for water splitting. The potentials are collected without iRcorrection. Two-electrode system for overall water splitting. After systematically investigating the OER and HER catalytic performances of FeCoNiPxSy catalysts, we assembled an overall water splitting device by using FeCoNiP0.5S0.5 and FeCoNiP0S1 as the anode and cathode. The polarization curve (Figure 6a) of the two-electrode electrolyzer displays a cell voltage of 1.46 V to afford a current density of 10 mA cm−2, which is as far as we know among the best value ever reported for water splitting devices (Table S7). The insert in Figure 6a shows a multi-current process for the two-electrode electrolyzer, which has been generally used to detect the mass transport properties, mechanical robustness and conductivity of electrodes by measuring the potential response upon abrupt change in current density.46 In the experiment, current densities at 10, 50, and 100 to 200 mA cm-2 were used, and the corresponding potentials were recorded simultaneously. When performed at a current density of 10 mA cm-2, the recorded potential levels off at 1.53 V and maintains unchanged for 0.5 h. Similar phenomena are observed for


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other measurement performed at different current densities, demonstrating the excellent mass transport properties, mechanical robustness and conductivity of the FeCoNiP0S1 and FeCoNiP0.5S0.5 electrodes. Due to the fact that commercial electrolyzers typically operate at current densities of from 100 to 500 mA cm-2, 79 we further tested the two-electrode system at 500 mA cm-2. Figure 6b exhibits that the applied potential of the system remains almost unchanged during operation that lasts for 60 h, indicating the outstanding stability of the device for water splitting (Table S7). Moreover, the FeCoNiPxSy catalysts can be well assembled with a scalable manner on Ni mesh (Figure S18) that is used in commercial water splitting device and demonstrate even higher performance than those prepared on planer Ti substrate for overall water splitting (Figure 7). These results suggest that the present synthetic strategy is compatible with industry, enabling the hierarchically structured multi-elements FeCoNiPxSy catalysts as promising electocatalysts towards economic and practical production of hydrogen by electrocatalytic water splitting.

Figure 7. Linear sweep voltammetry polarization curve of two-electrode electrolyzer assembled on Ni mesh with FeCoNiP0.5S0.5 (anode) and FeCoNiP0S1 (cathode) for water splitting.


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4. CONCLUSIONS In summary, we assembled multi-elements with a scalable one-step electrodeposition method to obtain a series of HER and OER electrocatalysts solely based upon earth-abundant elements. We found that rationally tailoring the non-metal elements of S and P in the catalyst can realize tunable modulation on the structural as well as the electronic properties of the as-prepared catalysts and therefore the electrocatalytic performance. Under optimum conditions, FeCoNiP0S1 delivers tiny overpotentials of 43, 135 and 264 mV to yield current densities of 10, 100 and 1000 mA cm-2 for HER. Moreover, current densities of 100 and 1000 mA cm-2 are achieved at low overpotentials of only 258 and 360 mV towards OER for FeCoNiP0.5S0.5 catalyst. When assembling the best-performed electrocatalysts in a two-electrode device, a current density of 10 mA cm-2 can be obtained under a really low cell voltage of 1.46 V with long-term durability, which is as far as we know among the best performance reported for water splitting devices. This work not only presents a series of highly efficient and stable electrocatalysts and their scalable and low-cost synthetic strategies toward electrocatalytic water splitting but also highlights the importance of dual electronic and morphological modulation through multi-elements assembly strategy towards individual HER and OER optimization.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]; [email protected] Notes


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The authors declare no competing financial interest. SUPPORTING INFORMATION Details about additional characterization for FeCoNiPxSy catalysts via XRD, SEM, EDX, ICPAES, XPS, LSV curves, faradaic efficiency, CV curves, chronopotentiometric curves, and the comparision of electrocatalytic performance for FeCoNiPxSy catalysts with other electrocatalysts in alkaline. This information is available free of charge on the ACS Publications website. (PDF) ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21573219), National Key R&D Program of China (No. 2017YFA0204804), the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB17000000), and CNPC-DICP Joint Research Center. X. Z. acknowledges the support from Young Thousand Talents Program of China. REFERENCES (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (2) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Graetzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-abundant Catalysts. Science 2014, 345, 1593-1596. (3) Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Norskov, J. K. Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23-J26.


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