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Bi-metallic cobalt-nickel phosphide nanowires for electrocatalysis of the oxygen and hydrogen evolution reactions
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Bi-metallic cobalt-nickel phosphide nanowires for electrocatalysis of the oxygen and hydrogen evolution reactions Isilda Amorima, Junyuan Xua, Nan Zhanga, Dehua Xiongb, Sitaramanjaneya M. Thalluria, ⁎ Rajesh Thomasa, Juliana P.S. Sousaa, Ana Araújoa, Hong Lib, Lifeng Liua, a b
International Iberian Nanotechnology Laboratory (INL), Avenida Mestre José Veiga, 4715-330 Braga, Portugal State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxygen evolution reaction Hydrogen evolution reaction Transition metal phosphides Nanowires Water splitting
Transition metal phosphides (TMPs) have emerged as a new class of electrocatalysts capable of catalyzing the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) with high efficiency and good stability. Here, we report a facile fabrication of a series of cobalt-nickel phosphide nanowires (CoNiP NWs) containing different compositions of Co and Ni, which is accomplished by hydrothermal growth of cobalt-nickelcarbonate-hydroxide NWs and a subsequent post-phosphorization treatment. We investigate and compare the electrocatalytic performance of the CoNiP NWs for the OER and HER in alkaline solution. The results show that CoNiP NWs with equimol Co and Ni, i.e. CoNiP-1:1, show the best apparent and intrinsic activities for both OER and HER, compared to CoNiP NWs with other Co/Ni ratios as well as the mono-metallic CoP and Ni5P4 controls. The CoNiP-1:1 can deliver the benchmark apparent current density of 10 mA cm−2 at an overpotential of 301 and 252 mV for the OER and the HER, respectively, and show good stability over 24 h without obvious degradation, holding substantial promise for use as an effective and inexpensive alternative in alkaline water electrolysis.
1. Introduction The clean energy carrier hydrogen (H2) has been considered a promising alternative to replace fossil fuels since it does not produce greenhouse gas emissions when consumed. Currently, H2 is mainly produced by steam-reforming of natural gas [1], which is not an environmentally-friendly process and depletes fossil fuels. To decarbonize H2 production, electrochemical water splitting has been intensively investigated as it is a green and efficient process for H2 generation, especially if the electricity comes from renewable sources. Water splitting is composed of two half-reactions, i.e., the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), both of which need electrocatalysts to attain a practically good reaction rate and high energy efficiency [2–4]. Among different low-temperature water splitting technologies available now, alkaline water electrolysis (AWE) is a mature technology for H2 production and offers a big advantage over the proton-exchange-membrane water electrolysis (PEMWE) [5,6], namely, no need for using precious platinum group metal (PGM) electrocatalysts to catalyze the OER and HER. However, the electrocatalytic performance of alkaline water electrolyzers remains
⁎
unsatisfactory since comparatively large overpotentials are needed with the state-of-the-art HER (e.g., Ni) and OER (e.g., metal oxide) catalysts used in AWE [7]. In this context, many research efforts have recently been dedicated towards the development of new, efficient and low-cost electrocatalysts containing earth-abundant elements, which can enhance the catalytic performance of alkaline water electrolyzers and be potentially used in anion-exchange-membrane water electrolysis (AEMWE). For example, a variety of non-precious electrocatalysts active for OER and/or HER have been reported, such as transition metal oxides [8–12], hydroxides [13–15], chalcogenides [16–21], nitrides [22,23], carbides [24,25], and phosphides [26–41], among which transition metal phosphides (TMPs) are particularly interesting and have attracted considerable attention given that they exhibit intrinsically high catalytic activity and possess high electrical conductivity [42]. Early studies about TMPs were mainly concentrated on their applications in catalyzing the HER [35–38]. Later on, TMPs were found to be able to catalyze the OER under alkaline conditions [39,27–41,43,44] and be even more active than the corresponding metal oxide/hydroxide catalysts [32,45]. Comprehensive microstructure and composition investigations indicate that under OER
Corresponding author. E-mail address: [email protected] (L. Liu).
https://doi.org/10.1016/j.cattod.2019.05.037 Received 13 February 2019; Received in revised form 8 May 2019; Accepted 16 May 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Isilda Amorim, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.05.037
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electron microscope (FE-SEM, FEI Quanta 650 FEG) equipped with an INCA 350 spectrometer for energy dispersive X-ray spectroscopy (EDX). Microstructure and composition of samples were characterized by a transmission electron microscope (TEM, FEI Titan ChemiSTEM 80–200) operating at 200 keV. X-ray diffraction (XRD) measurements were carried out on a Panalytical X’Pert PRO diffractometer at 45 kV and 40 mA, using a PIXcel detector and Cu Kα radiation (λ = 1.540598 Å). The Bragg-Brentano configuration was applied to acquire patterns in the 2θ range of 10–80°. X-ray photoelectron spectroscopy (XPS) data were acquired on an ESCALAB 250 instrument with Al Kα X-rays (1489.6 eV). Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis was performed on an ICPE-9000 spectrometer (Shimadzu). Specifically, 5 mg of catalysts were dispersed in 1.8 mL of aqua regia. Subsequently, the acidic solution was diluted in a 100 mL volumetric flask. The analysis were performed three times using ca. 10 mL of solution each time to obtain an average value. The physically accessible surface area of the samples was measured using the nitrogen (N2) adsorption/desorption isotherms at 77 K, with a Quantachrome Autosorb IQ2 system. Approximately 100 mg of sample was introduced in the sample cell and outgassed for 4 h at 150 °C before the measurement. The specific surface area (SBET) of each catalyst was calculated based on the Brunauer–Emmett–Teller (BET) method.
conditions TMPs will be transformed to transition metal (oxy)hydroxide through in-situ oxidation, forming a heterojunction between the TMP core and the formed (oxy)hydroxide shell that may enhance the OER [43,46–48]. In this sense, TMPs serve as “pre-catalysts” for OER. Given the fact that TMPs are active for both OER and HER, they have recently been used as bifunctional (pre)catalysts for overall alkaline water splitting [28,30,34,46,49–51]. While various mono-metallic TMPs have been investigated to catalyze either OER or HER or both [26,27,29,34,50,52–54], recent studies have demonstrated that doping mono-metallic TMPs with a secondary transition metal can markedly improve the catalytic performance, due likely to the synergetic effect between different metal species [55,56]. In this work, we report the fabrication of cobalt-nickel phosphide nanowires (CoNiP NWs) with different ratios of Co:Ni and systematically investigate how the Co:Ni ratio influences the catalytic performance of CoNiP NWs for both OER and HER in alkaline media. We find that the bi-metallic CoNiP NWs with a Co:Ni ratio of 1:1 exhibit the best OER and HER activities among CoNiP NWs and the corresponding monometallic TMPs under investigation, requires an overpotential of 301 and 252 mV to deliver 10 mA cm−2 for OER and HER, respectively. They also show good long-term stability, catalyzing OER and HER for 24 h without noticeable degradation. 2. Materials and methods
2.3. Electrode preparation and electrochemical measurements
Cobalt (II) nitrate hexa-hydrate (Co(NO3)2.6H20, 98%), nickel (II) nitrate hexa-hydrate (Ni(NO3)2.6H2O, ≥97%), urea, ammonium fluoride (NH4F, ≥98%), and sodium hypophosphite (NaH2PO2) were purchased from Sigma-Aldrich. Potassium hydroxide (KOH ≥ 85%) was purchased from Alfa Aesar. All reagents were used without further purification in this work. Deionized water (DI), from a Millipore system (18.2 MΩ cm), was used for solutions preparation.
5 mg of the catalysts were ultrasonically dispersed in 1 mL of ethanol containing 50 μL of Nafion solution (5 wt %). To prepare electrodes for catalytic tests, 50 μL of the catalyst ink was drop-cast on a polished glassy carbon electrode (GCE, exposed area: 0.78 cm2), leading to a catalyst loading density of ca. 0.3 mg cm−2. The electrode was then air dried at room temperature. All electrochemical measurements were conducted in a typical three-electrode configuration for OER and HER tests using a Biologic VMP-3 potentiostat/galvanostat at room temperature in 1.0 M KOH. The GCE loaded with the catalyst, a graphite rod (for HER) or Pt wire (for OER) and a saturated calomel electrode (SCE) were utilized as working, counter, and reference electrodes, respectively. The SCE electrode was calibrated before each measurement in 0.5 M H2SO4 saturated with Ar gas containing 5% H2, employing a clean Pt wire as the working electrode. Unless otherwise stated, all potentials are reported versus the reversible hydrogen electrode (RHE), by converting the measured potentials vs SCE in accordance with the following formula:
2.1. Synthesis of CoNiP nanowires Cobalt-nickel-carbonate-hydroxide precursor NWs were synthesized through a simple hydrothermal approach according to a previous report [57]. First, 5 mmol urea, 10 mmol NH4F, and 2 mmol Co(NO3)2·6H2O were dissolved in 40 ml DI water, and 2, 1 or 0.2 mmol Ni(NO3)2·6H2O was added to the solution to tune the Co:Ni ratio to 1:1, 2:1 or 10:1, respectively. The mixture was then stirred vigorously for 30 min to form a transparent and homogeneous solution, and then was transferred into a Teflon-lined steel autoclave reactor. The reactor was sealed and heated up to 120 °C, and maintained at this temperature for 6 h. Subsequently, the reactor was naturally cooled down to room temperature. The obtained pink powders were rinsed three times using DI water and alcohol, respectively, and collected by centrifugation. Afterwards, the collected powders were dried under vacuum at 60 °C for further use. Also, precursors containing only Co or Ni were synthetized under similar conditions for comparison. CoNiP NWs were obtained by phosphorization of the precursor NWs powders using NaH2PO2 as the source of phosphorus. Specifically, the hydrothermally synthesized precursor NWs powders were loaded in a ceramic boat and 3.0 g of NaH2PO2 was placed 2.0–2.5 cm away from the precursor NWs. Afterwards, the ceramic boat was put into a tube furnace, with the NaH2PO2 placed on the upstream side. The furnace was then purged with nitrogen (N2, 99.999%) for 1 h to remove air, heated to 300 °C at a ramping rate of 5 °C min−1 and maintained at this temperature for 2 h. Finally, the furnace was naturally cooled down to room temperature. The N2 flow was maintained throughout the whole process. Likewise, CoP NW and Ni5P4 control samples were synthetized in the same way for comparison.
ERHE = ESCE + 0.059 × pH + 0.241
(1)
The current density (j) presented in this work is normalized with respect to the geometric surface area of the electrode. Cyclic voltammetry (CV) was performed at 5 mV s−1 in the potential window of 1.0–1.65 V vs RHE for OER and 0.24 to −0.45 V vs RHE for HER. An iRcorrection of 85% was applied to compensate the potential drop between the working and reference electrodes. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a d.c. voltage of 1.48 V vs RHE for OER and −0.25 V vs RHE for HER in the frequency range of 0.01 – 105 Hz with a 10 mV amplitude sinusoidal perturbation. The catalytic stability was assessed using chronopotentiometry (CP) at a constant current density of 10 mA cm-2 for OER and ‒10 mA cm-2 for HER. 2.4. Turnover frequency (TOF) calculation For OER and HER, the TOF values (s−1) were calculated assuming that every metal atom was involved in the catalysis, which represents the lower limit of the TOF [32,58]:
2.2. Materials characterization
OER:TOF =
The sample morphology was examined by a field-emission scanning 2
j 4nF
(2)
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Fig. 1. Morphology, microstructure, and composition of the representative CoNiP NWs (CoNiP-1:1). (a) SEM image. (b) Low-magnification TEM image. (c) HRTEM image. (d) TEM-EDX spectrum. (e) STEM-HAADF image and elemental maps of Co, Ni, P, O, and their overlay.
HER:TOF =
j 2nF
(CoNiP-1:1) were further examined by TEM. Fig. 1b is a typical TEM image showing that the NWs are composed of many crystal grains. A high-resolution TEM (HRTEM) image is illustrated in Fig. 1c, where the lattice fringes of crystallized domains can be clearly resolved. The measured interplanar distance of the crystallite is ca. 0.185 nm, which corresponds to the lattice spacing of (210) crystal planes of hexagonal CoNiP (ICDD No. 04-001-6153). Extensive SEM and TEM EDX analyses confirmed that CoNiP NWs consist of Co, Ni and P elements (Fig. 1d and Fig. S1e), and the Cu signal shown in Fig. 1d comes from the copper TEM grid used. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was carried out to investigate the elemental distribution along an individual NW. As displayed in Fig. 1e, the elements Co, Ni, and P are uniformly distributed along and across the NW. The oxygen signal should originate from surface oxidation upon the exposure of samples to air. Fig. 2a shows XRD patterns of all CoNiP NWs as well as CoP and Ni5P4 control samples, demonstrating the crystalline nature of all TMP nanostructures. CoNiP-1:1 can be indexed exclusively to the hexagonal CoNiP (ICDD No. 04-001-6153), while CoNiP-2:1 and CoNiP-10:1 are composed of a mixture of hexagonal CoNiP and orthorhombic CoP (ICDD No. 00-029-0497), perhaps because of the much higher feedstock of Co during the synthesis. The mono-metallic CoP and Ni5P4 controls comprise crystal phases that can be assigned to orthorhombic CoP and to hexagonal Ni5P4 (ICDD No. 00-018-0883), respectively. Furthermore, the surface composition and oxidation state of the CoNiP-1:1 catalyst were analyzed by XPS. Fig. 2b shows the high-resolution Co 2p3/2 XPS spectrum. The characteristic binding energy (BE) peak at 778.5 eV generally relates to the Co 2p contribution of metal phosphides as a consequence of the formation of Co−P [33,46,59]. The
(3)
Where j is the current (A) at a given overpotential, F is the Faraday constant (96,485 C mol−1) and n (mol) is the mole number of transition metal(s) loaded on the GC electrode which was determined by the ICPOES analysis. 3. Results and discussion CoNiP nanowires (NWs) catalysts with different Co:Ni ratios were fabricated by hydrothermal synthesis followed by a simple and reproducible low-temperature phosphorization process during which the precursor NWs were converted in situ into CoNiP. Hereafter, CoNiP NWs with Co:Ni ratios of 1:1, 2:1 and 10:1 are denoted as CoNiP-1:1, CoNiP2:1 and CoNiP-10:1, respectively, and the mono-metallic control catalysts are named CoP and Ni5P4. ICP-OES analyses confirmed that the Co:Ni ratios in CoNiP-1:1, CoNiP-2:1 and CoNiP-10:1 are close to the desired stoichiometry of 1:1, 2:1 and 10:1 (Table S1, Supporting Information). SEM examination revealed that all CoNiP and CoP samples exhibit a wire-like morphology (Fig. 1a and Fig. S1a–c) with diameters in the range of 60–270 nm. CoP NWs have an average length of about 4 μm. With the increase in the Ni content, the NWs length becomes shorter and shorter, being 1–3 μm, 550–630 nm, and 240–440 nm for CoNiP10:1, CoNiP-2:1 and CoNiP-1:1, respectively. In contrast to CoNiP and CoP NWs, the mono-metallic Ni5P4 shows a sheet-like morphology (Ni5P4 NSs, Fig. S1d), likely because of the crystallographic growth habit of Ni5P4. The microstructure and composition of representative CoNiP NWs 3
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Fig. 2. (a) XRD patterns of CoP, Ni5P4 and CoNiP with different Co:Ni ratios, namely 1:1, 2:1 and 10:1. The standard ICDD powder diffraction patterns of CoP, Ni5P4 and CoNiP are given for reference. (b) Co 2p, (c) Ni 2p and (d) P 2p XPS spectra of the CoNiP-1:1 NWs sample.
samples, the CoNiP-1:1 still shows the best specific activity, indicating that it is an intrinsically more active “pre-catalyst”. It is worth noting that not all physically accessible surface areas are electrocatalytically active, so the BET-based specific activities represent the lower limit of the intrinsic catalytic performance. The OER kinetics of all TMP “pre-catalysts” was analyzed by Tafel analysis. The Tafel slope decreases with an order similar to that of the OER apparent activity, namely, CoNiP-1:1 > CoNiP-2:1 > CoNiP10:1 > CoP > Ni5P4 (Fig. 3c). The CoNiP-1:1 NWs show the lowest Tafel slope of 54.0 mV dec−1, suggesting a faster kinetics for the electron transfer; while the Tafel slope of the other catalysts falls in the range of 60–120 mV dec−1, indicating that the chemical adsorption of OH− might be the rate-determining step (RDS) [63]. The fast OER kinetics of the CoNiP-1:1 was also confirmed by EIS measurements (Fig. S2, Supporting Information). The charge transfer resistance (Rct) of CoNiP-1:1 (10.63 Ω) is substantially lower than that of the mono-metallic CoP (60.59 Ω) and Ni5P4 (141.6 Ω), as shown in Fig. 3c. The Rct values of all TMP “pre-catalysts” follow the same trend as that of the Tafel slopes, implying that Ni doping enhances the charge transfer ability during the OER. The electrochemical double-layer capacitance (Cdl) of each catalyst was measured using CV in the non-faradaic region (Fig. S4, Supporting Information), to estimate the electrochemically accessible surface area (ECSA) which correlates to the active sites number. As shown in Fig. 3d the introduction of Ni helps improve the Cdl value. For instance, the Cdl value of CoNiP-1:1 NWs (4.65 m F cm−2) is significantly higher than that of CoP NWs (0.82 m F cm−2). The intrinsic OER activity of the TMPs was further evaluated based on TOF, assuming all transition metal species in the TMPs are catalytically active (i.e., the lower limit). Fig. 3e shows the TOF values of all TMP “pre-catalysts” calculated at η = 300, 350 and 400 mV. The trend in TOF is found to be the same as that observed for the apparent activity, indicating that CoNiP-1:1 NWs are intrinsically more active than other TMPs for the OER. In particular, the TOF value of CoNiP-1:1 is as large as 0.022 s-1 at η = 350 mV, superior to that of many nonprecious OER (pre)catalysts including OCoP/GO nanoparticle (TOF350 = 0.018 s-1) [27], Ni2P/Ni/NF urchinlike (TOF350 = 0.015 s-1) [29], and CFP/NiCo2O4 (TOF440 = 0.0023 s1 ) [64].
peak at 782.2 eV can be ascribed to a Co oxidation state associated with Co−POx, which is likely resulting from the surface oxidation [46,59,60]. Similarly, in the Ni 2p spectrum (Fig. 2c), the Ni 2p3/2 peaks observed at 853.2 and 857.1 eV should arise from Ni−P and Ni−POx, respectively [33,47,49]. As far as the P 2p XPS spectrum is concerned, two peaks appearing at 129.4 and 130.1 eV can be assigned to P 2p3/2 and P 2p1/2 components, respectively (Fig. 2d), which is a characteristic of metal − P bonds in metal phosphides [61,62]. Additionally, a strong peak at 134.2 eV is observed, arising from the PeO bond due to the air exposure of CoNiP NWs [35]. The electrocatalytic performance of CoNiP NWs (pre)catalysts towards the OER and HER was assessed on GC substrates in 1.0 M KOH using CV, EIS, and CP. The CoP and Ni5P4 references were also tested under the same conditions for comparison. Prior to each catalytic test, pre-activation was carried out by repetitive CV scans at 5 mV s−1 for OER in the potential range of 1.0–1.65 V vs RHE and at 50 mV s−1 for HER in the potential range of −0.26–0.24 V vs RHE, respectively. Fig. 3a shows the reduction branches of iR-corrected CV curves of all samples after pre-activation for OER. The overpotential (η) needed at a specific current density has been widely used as an extrinsic parameter to evaluate the performance of electrocatalysts. The η values needed to deliver 10 (η10), 20 (η20) and 50 (η50) mA cm-2 are compared in the inset of Fig. 3a, where it’s seen that all CoNiP NWs show lower η values than both CoP NWs and Ni5P4 NSs. To reach the current density of 10 mA cm-2, Ni5P4 NSs need a η value of 373 mV, while the best-performing CoNiP-1:1 only demands 301 mV, showing a remarkable enhancement in the OER activity. In the low η region (η ≤ 350 mV) the OER activity follows the order of CoNiP-1:1 > CoNiP-2:1 ≈ CoNiP-10:1 > CoP > Ni5P4; while in the high η region (η > 400 mV), the same order basically remains but the activity of CoNiP-2:1 becomes higher, close to that of CoNiP-1:1, i.e., CoNiP-1:1 ≈ CoNiP-2:1 > CoNiP-10:1 > CoP > Ni5P4. For all “pre-catalysts” under investigation, CoNiP-1:1 exhibits the best OER activity at 10 mA cm-2, outperforming many other non-noble OER (pre)catalysts reported in recent years (Table S2, Supporting Information). Furthermore, the specific activity of all catalysts was calculated upon normalizing the catalytic current by their BET surface area derived from N2 adsorption/desorption isotherms (Fig. S3). Compared to CoNiP-10:1, CoNiP-2:1, and CoP and Ni5P4 control 4
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Fig. 3. Electrocatalytic performance of the CoNiP, CoP and Ni5P4 “pre-catalysts” for the OER in 1.0 M KOH with a catalyst loading density of 0.3 mg cm−2. (a) iRcorrected polarization curves recorded at 5 mV s-1. The inset shows the potentials needed for the “pre-catalysts” to deliver current densities of 10, 20, 50 and 100 mA cm−2. (b) BET-normalized specific activity. (c) Tafel slopes and charge-transfer resistance of the different “pre-catalysts” extracted from the fitting of the Nyquist plots. (d) Double-layer capacitance calculated from the CVs in non-faradaic region at different scan rates. (e) TOF values calculated at η = 300, 350 and 400 mV. (f) Chronopotentiostatic curve of the “pre-catalysts” recorded at a constant current density of 10 mA cm−2.
Supporting Information). The η10 value of CoNiP-10:1 (397 mV) is similar to that of CoP NWs (403 mV), indicating that doping a little Ni into CoP does not improve the HER activity markedly. The HER apparent activity was found to follow an order different from that attained for the OER, namely, CoNiP-1:1 > CoNiP-2:1 > Ni5P4 > CoNiP10:1 > CoP. The Tafel slopes obtained follow the same trend, as shown in Fig. 4b. The HER kinetics was also probed by EIS measurements (Fig. S5, Supporting Information). The obtained Nyquist plots were fitted with an equivalent circuit and the Rct at the electrode/electrolyte interface was extracted. The trend in Rct values is in agreement with that in the Tafel analyses, confirming the faster HER kinetics of CoNiP-1:1 NWs. The intrinsic HER activity of all TMP catalysts was further evaluated with BET-based specific activity and catalyst mass-based TOF assuming all transition metal species in the TMPs are catalytically active (i.e., the lower limit). Like for the OER, the specific activity for CoNiP-1:1 is
The stability of all TMP “pre-catalysts” was evaluated using chronopotentiometry at 10 mA cm−2. As shown in Fig. 3f, the overpotential needed to maintain 10 mA cm−2 decreases gradually in the first 2–3 h, indicative of an activation process, and then stabilizes up to 24 h, suggesting good long-term durability for all “pre-catalysts” except for the CoNiP-10:1 that shows slight degradation (i.e. increase in overpotential) after 9 h. The activation process may be associated with the electrochemical in-situ dephosphorization and oxidation process of the TMP, which has been reported recently in many TMP-based “pre-catalysts” and is believed to help enhance the OER performance due to the formation of catalytically active (oxy)hydroxide [30,43,46,65,66]. Likewise, the HER performance of all TMP catalysts was assessed in 1.0 M KOH. All CoNiP NWs are electrocatalytically active towards the HER, with the best activity achieved by CoNiP-1:1 which exhibits an overpotential of 252 mV at 10 mA cm−2 (Fig. 4a), outperforming a number of HER nano-catalysts reported in the literature (Table S3, 5
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Fig. 4. Electrocatalytic performance of the CoNiP, CoP and Ni5P4 catalysts for the HER in 1.0 M KOH with a catalyst loading density of 0.3 mg cm−2. (a) iR-corrected polarization curves recorded at 5 mV s-1. (b) BET-normalized specific activity. (c) Tafel plots and charge-transfer resistance of the different catalysts extracted from the fitting of the Nyquist plots. (d) TOF values calculated at η = 350 and 400 mV. (e) Chronopotentiostatic curve of the catalysts recorded at a constant current density of −10 mA cm−2.
good electrocatalytic performance, the low-cost nature and the simplicity of synthetic procedures, the bi-metallic transition metal phosphide nanowires hold substantial promise for use in alkaline and anionexchange-membrane water electrolysis.
higher than that of other TMPs (Fig. 4c). The TOF values of different catalysts calculated at η = 350 mV are compared in Fig. 4d, where it’s seen that the TOF value is greatly improved for CoNiP NWs with high Ni contents (i.e., Co:Ni = 1:1 and 2:1). Similarly, the TOF values of TMP catalysts follow an order the same as that of the Tafel slopes and Rct. In particular, the TOF value of CoNiP-1:1 is 0.071 s−1 at η = 350 mV. Furthermore, the stability of the best-performing catalyst, i.e. CoNiP1:1, was studied by chronopotentiometry at −10 mA cm-2. CoNiP-1:1 can sustain constant HER electrolysis for 24 h with little degradation, as shown in Fig. 4e.
Acknowledgments The authors acknowledge the financial support from the Portuguese Foundation of Science and Technology (FCT) under the project “PTDC/ CTM-ENE/2349/2014” (grant agreement No. 016660). This work was also partially funded by the Horizon 2020 project CritCat (grant agreement No. 686053).
4. Conclusion
Appendix A. Supplementary data
In summary, we have synthetized bi-metallic CoNiP nanowires with different compositions of Co and Ni using a simple hydrothermal method followed by a post-phosphorization treatment. We comprehensively investigate the electrocatalytic performance of CoNiP NWs toward the OER and HER in alkaline solution and compare them to the mono-metallic CoP and Ni5P4 control (pre)catalysts. We found that the CoNiP NWs with equimol Co and Ni exhibit the best apparent and intrinsic activities as well as the reaction kinetics both OER and HER, requiring 301 mV to afford a benchmark current density of 10 mA cm−2 for OER and 252 mV at −10 mA cm−2 for HER. Moreover, CoNiP-1:1 shows good catalytic stability for both OER and HER as well. Given the
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.05.037. References [1] T.S. Veras, T.S. Mozer, D.C.S.R. Santos, A.S. César, Int. J. Hydrogen Energy 42 (2017) 2018–2033. [2] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I. Chorkendorff, J.K. Nørskov, T.F. Jaramillo, Science 355 (2017) eaad4998. [3] B. You, Y. Sun, Acc. Chem. Res. 51 (2018) 1571–1580.
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Bi-metallic cobalt-nickel phosphide nanowires for electrocatalysis of the oxygen and hydrogen evolution reactions Isilda Amorima, Junyuan Xua, Nan Zhanga, Dehua Xiongb, Sitaramanjaneya M. Thalluria, ⁎ Rajesh Thomasa, Juliana P.S. Sousaa, Ana Araújoa, Hong Lib, Lifeng Liua, a b
International Iberian Nanotechnology Laboratory (INL), Avenida Mestre José Veiga, 4715-330 Braga, Portugal State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxygen evolution reaction Hydrogen evolution reaction Transition metal phosphides Nanowires Water splitting
Transition metal phosphides (TMPs) have emerged as a new class of electrocatalysts capable of catalyzing the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) with high efficiency and good stability. Here, we report a facile fabrication of a series of cobalt-nickel phosphide nanowires (CoNiP NWs) containing different compositions of Co and Ni, which is accomplished by hydrothermal growth of cobalt-nickelcarbonate-hydroxide NWs and a subsequent post-phosphorization treatment. We investigate and compare the electrocatalytic performance of the CoNiP NWs for the OER and HER in alkaline solution. The results show that CoNiP NWs with equimol Co and Ni, i.e. CoNiP-1:1, show the best apparent and intrinsic activities for both OER and HER, compared to CoNiP NWs with other Co/Ni ratios as well as the mono-metallic CoP and Ni5P4 controls. The CoNiP-1:1 can deliver the benchmark apparent current density of 10 mA cm−2 at an overpotential of 301 and 252 mV for the OER and the HER, respectively, and show good stability over 24 h without obvious degradation, holding substantial promise for use as an effective and inexpensive alternative in alkaline water electrolysis.
1. Introduction The clean energy carrier hydrogen (H2) has been considered a promising alternative to replace fossil fuels since it does not produce greenhouse gas emissions when consumed. Currently, H2 is mainly produced by steam-reforming of natural gas [1], which is not an environmentally-friendly process and depletes fossil fuels. To decarbonize H2 production, electrochemical water splitting has been intensively investigated as it is a green and efficient process for H2 generation, especially if the electricity comes from renewable sources. Water splitting is composed of two half-reactions, i.e., the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), both of which need electrocatalysts to attain a practically good reaction rate and high energy efficiency [2–4]. Among different low-temperature water splitting technologies available now, alkaline water electrolysis (AWE) is a mature technology for H2 production and offers a big advantage over the proton-exchange-membrane water electrolysis (PEMWE) [5,6], namely, no need for using precious platinum group metal (PGM) electrocatalysts to catalyze the OER and HER. However, the electrocatalytic performance of alkaline water electrolyzers remains
⁎
unsatisfactory since comparatively large overpotentials are needed with the state-of-the-art HER (e.g., Ni) and OER (e.g., metal oxide) catalysts used in AWE [7]. In this context, many research efforts have recently been dedicated towards the development of new, efficient and low-cost electrocatalysts containing earth-abundant elements, which can enhance the catalytic performance of alkaline water electrolyzers and be potentially used in anion-exchange-membrane water electrolysis (AEMWE). For example, a variety of non-precious electrocatalysts active for OER and/or HER have been reported, such as transition metal oxides [8–12], hydroxides [13–15], chalcogenides [16–21], nitrides [22,23], carbides [24,25], and phosphides [26–41], among which transition metal phosphides (TMPs) are particularly interesting and have attracted considerable attention given that they exhibit intrinsically high catalytic activity and possess high electrical conductivity [42]. Early studies about TMPs were mainly concentrated on their applications in catalyzing the HER [35–38]. Later on, TMPs were found to be able to catalyze the OER under alkaline conditions [39,27–41,43,44] and be even more active than the corresponding metal oxide/hydroxide catalysts [32,45]. Comprehensive microstructure and composition investigations indicate that under OER
Corresponding author. E-mail address: [email protected] (L. Liu).
https://doi.org/10.1016/j.cattod.2019.05.037 Received 13 February 2019; Received in revised form 8 May 2019; Accepted 16 May 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Isilda Amorim, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.05.037
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electron microscope (FE-SEM, FEI Quanta 650 FEG) equipped with an INCA 350 spectrometer for energy dispersive X-ray spectroscopy (EDX). Microstructure and composition of samples were characterized by a transmission electron microscope (TEM, FEI Titan ChemiSTEM 80–200) operating at 200 keV. X-ray diffraction (XRD) measurements were carried out on a Panalytical X’Pert PRO diffractometer at 45 kV and 40 mA, using a PIXcel detector and Cu Kα radiation (λ = 1.540598 Å). The Bragg-Brentano configuration was applied to acquire patterns in the 2θ range of 10–80°. X-ray photoelectron spectroscopy (XPS) data were acquired on an ESCALAB 250 instrument with Al Kα X-rays (1489.6 eV). Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis was performed on an ICPE-9000 spectrometer (Shimadzu). Specifically, 5 mg of catalysts were dispersed in 1.8 mL of aqua regia. Subsequently, the acidic solution was diluted in a 100 mL volumetric flask. The analysis were performed three times using ca. 10 mL of solution each time to obtain an average value. The physically accessible surface area of the samples was measured using the nitrogen (N2) adsorption/desorption isotherms at 77 K, with a Quantachrome Autosorb IQ2 system. Approximately 100 mg of sample was introduced in the sample cell and outgassed for 4 h at 150 °C before the measurement. The specific surface area (SBET) of each catalyst was calculated based on the Brunauer–Emmett–Teller (BET) method.
conditions TMPs will be transformed to transition metal (oxy)hydroxide through in-situ oxidation, forming a heterojunction between the TMP core and the formed (oxy)hydroxide shell that may enhance the OER [43,46–48]. In this sense, TMPs serve as “pre-catalysts” for OER. Given the fact that TMPs are active for both OER and HER, they have recently been used as bifunctional (pre)catalysts for overall alkaline water splitting [28,30,34,46,49–51]. While various mono-metallic TMPs have been investigated to catalyze either OER or HER or both [26,27,29,34,50,52–54], recent studies have demonstrated that doping mono-metallic TMPs with a secondary transition metal can markedly improve the catalytic performance, due likely to the synergetic effect between different metal species [55,56]. In this work, we report the fabrication of cobalt-nickel phosphide nanowires (CoNiP NWs) with different ratios of Co:Ni and systematically investigate how the Co:Ni ratio influences the catalytic performance of CoNiP NWs for both OER and HER in alkaline media. We find that the bi-metallic CoNiP NWs with a Co:Ni ratio of 1:1 exhibit the best OER and HER activities among CoNiP NWs and the corresponding monometallic TMPs under investigation, requires an overpotential of 301 and 252 mV to deliver 10 mA cm−2 for OER and HER, respectively. They also show good long-term stability, catalyzing OER and HER for 24 h without noticeable degradation. 2. Materials and methods
2.3. Electrode preparation and electrochemical measurements
Cobalt (II) nitrate hexa-hydrate (Co(NO3)2.6H20, 98%), nickel (II) nitrate hexa-hydrate (Ni(NO3)2.6H2O, ≥97%), urea, ammonium fluoride (NH4F, ≥98%), and sodium hypophosphite (NaH2PO2) were purchased from Sigma-Aldrich. Potassium hydroxide (KOH ≥ 85%) was purchased from Alfa Aesar. All reagents were used without further purification in this work. Deionized water (DI), from a Millipore system (18.2 MΩ cm), was used for solutions preparation.
5 mg of the catalysts were ultrasonically dispersed in 1 mL of ethanol containing 50 μL of Nafion solution (5 wt %). To prepare electrodes for catalytic tests, 50 μL of the catalyst ink was drop-cast on a polished glassy carbon electrode (GCE, exposed area: 0.78 cm2), leading to a catalyst loading density of ca. 0.3 mg cm−2. The electrode was then air dried at room temperature. All electrochemical measurements were conducted in a typical three-electrode configuration for OER and HER tests using a Biologic VMP-3 potentiostat/galvanostat at room temperature in 1.0 M KOH. The GCE loaded with the catalyst, a graphite rod (for HER) or Pt wire (for OER) and a saturated calomel electrode (SCE) were utilized as working, counter, and reference electrodes, respectively. The SCE electrode was calibrated before each measurement in 0.5 M H2SO4 saturated with Ar gas containing 5% H2, employing a clean Pt wire as the working electrode. Unless otherwise stated, all potentials are reported versus the reversible hydrogen electrode (RHE), by converting the measured potentials vs SCE in accordance with the following formula:
2.1. Synthesis of CoNiP nanowires Cobalt-nickel-carbonate-hydroxide precursor NWs were synthesized through a simple hydrothermal approach according to a previous report [57]. First, 5 mmol urea, 10 mmol NH4F, and 2 mmol Co(NO3)2·6H2O were dissolved in 40 ml DI water, and 2, 1 or 0.2 mmol Ni(NO3)2·6H2O was added to the solution to tune the Co:Ni ratio to 1:1, 2:1 or 10:1, respectively. The mixture was then stirred vigorously for 30 min to form a transparent and homogeneous solution, and then was transferred into a Teflon-lined steel autoclave reactor. The reactor was sealed and heated up to 120 °C, and maintained at this temperature for 6 h. Subsequently, the reactor was naturally cooled down to room temperature. The obtained pink powders were rinsed three times using DI water and alcohol, respectively, and collected by centrifugation. Afterwards, the collected powders were dried under vacuum at 60 °C for further use. Also, precursors containing only Co or Ni were synthetized under similar conditions for comparison. CoNiP NWs were obtained by phosphorization of the precursor NWs powders using NaH2PO2 as the source of phosphorus. Specifically, the hydrothermally synthesized precursor NWs powders were loaded in a ceramic boat and 3.0 g of NaH2PO2 was placed 2.0–2.5 cm away from the precursor NWs. Afterwards, the ceramic boat was put into a tube furnace, with the NaH2PO2 placed on the upstream side. The furnace was then purged with nitrogen (N2, 99.999%) for 1 h to remove air, heated to 300 °C at a ramping rate of 5 °C min−1 and maintained at this temperature for 2 h. Finally, the furnace was naturally cooled down to room temperature. The N2 flow was maintained throughout the whole process. Likewise, CoP NW and Ni5P4 control samples were synthetized in the same way for comparison.
ERHE = ESCE + 0.059 × pH + 0.241
(1)
The current density (j) presented in this work is normalized with respect to the geometric surface area of the electrode. Cyclic voltammetry (CV) was performed at 5 mV s−1 in the potential window of 1.0–1.65 V vs RHE for OER and 0.24 to −0.45 V vs RHE for HER. An iRcorrection of 85% was applied to compensate the potential drop between the working and reference electrodes. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a d.c. voltage of 1.48 V vs RHE for OER and −0.25 V vs RHE for HER in the frequency range of 0.01 – 105 Hz with a 10 mV amplitude sinusoidal perturbation. The catalytic stability was assessed using chronopotentiometry (CP) at a constant current density of 10 mA cm-2 for OER and ‒10 mA cm-2 for HER. 2.4. Turnover frequency (TOF) calculation For OER and HER, the TOF values (s−1) were calculated assuming that every metal atom was involved in the catalysis, which represents the lower limit of the TOF [32,58]:
2.2. Materials characterization
OER:TOF =
The sample morphology was examined by a field-emission scanning 2
j 4nF
(2)
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Fig. 1. Morphology, microstructure, and composition of the representative CoNiP NWs (CoNiP-1:1). (a) SEM image. (b) Low-magnification TEM image. (c) HRTEM image. (d) TEM-EDX spectrum. (e) STEM-HAADF image and elemental maps of Co, Ni, P, O, and their overlay.
HER:TOF =
j 2nF
(CoNiP-1:1) were further examined by TEM. Fig. 1b is a typical TEM image showing that the NWs are composed of many crystal grains. A high-resolution TEM (HRTEM) image is illustrated in Fig. 1c, where the lattice fringes of crystallized domains can be clearly resolved. The measured interplanar distance of the crystallite is ca. 0.185 nm, which corresponds to the lattice spacing of (210) crystal planes of hexagonal CoNiP (ICDD No. 04-001-6153). Extensive SEM and TEM EDX analyses confirmed that CoNiP NWs consist of Co, Ni and P elements (Fig. 1d and Fig. S1e), and the Cu signal shown in Fig. 1d comes from the copper TEM grid used. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was carried out to investigate the elemental distribution along an individual NW. As displayed in Fig. 1e, the elements Co, Ni, and P are uniformly distributed along and across the NW. The oxygen signal should originate from surface oxidation upon the exposure of samples to air. Fig. 2a shows XRD patterns of all CoNiP NWs as well as CoP and Ni5P4 control samples, demonstrating the crystalline nature of all TMP nanostructures. CoNiP-1:1 can be indexed exclusively to the hexagonal CoNiP (ICDD No. 04-001-6153), while CoNiP-2:1 and CoNiP-10:1 are composed of a mixture of hexagonal CoNiP and orthorhombic CoP (ICDD No. 00-029-0497), perhaps because of the much higher feedstock of Co during the synthesis. The mono-metallic CoP and Ni5P4 controls comprise crystal phases that can be assigned to orthorhombic CoP and to hexagonal Ni5P4 (ICDD No. 00-018-0883), respectively. Furthermore, the surface composition and oxidation state of the CoNiP-1:1 catalyst were analyzed by XPS. Fig. 2b shows the high-resolution Co 2p3/2 XPS spectrum. The characteristic binding energy (BE) peak at 778.5 eV generally relates to the Co 2p contribution of metal phosphides as a consequence of the formation of Co−P [33,46,59]. The
(3)
Where j is the current (A) at a given overpotential, F is the Faraday constant (96,485 C mol−1) and n (mol) is the mole number of transition metal(s) loaded on the GC electrode which was determined by the ICPOES analysis. 3. Results and discussion CoNiP nanowires (NWs) catalysts with different Co:Ni ratios were fabricated by hydrothermal synthesis followed by a simple and reproducible low-temperature phosphorization process during which the precursor NWs were converted in situ into CoNiP. Hereafter, CoNiP NWs with Co:Ni ratios of 1:1, 2:1 and 10:1 are denoted as CoNiP-1:1, CoNiP2:1 and CoNiP-10:1, respectively, and the mono-metallic control catalysts are named CoP and Ni5P4. ICP-OES analyses confirmed that the Co:Ni ratios in CoNiP-1:1, CoNiP-2:1 and CoNiP-10:1 are close to the desired stoichiometry of 1:1, 2:1 and 10:1 (Table S1, Supporting Information). SEM examination revealed that all CoNiP and CoP samples exhibit a wire-like morphology (Fig. 1a and Fig. S1a–c) with diameters in the range of 60–270 nm. CoP NWs have an average length of about 4 μm. With the increase in the Ni content, the NWs length becomes shorter and shorter, being 1–3 μm, 550–630 nm, and 240–440 nm for CoNiP10:1, CoNiP-2:1 and CoNiP-1:1, respectively. In contrast to CoNiP and CoP NWs, the mono-metallic Ni5P4 shows a sheet-like morphology (Ni5P4 NSs, Fig. S1d), likely because of the crystallographic growth habit of Ni5P4. The microstructure and composition of representative CoNiP NWs 3
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Fig. 2. (a) XRD patterns of CoP, Ni5P4 and CoNiP with different Co:Ni ratios, namely 1:1, 2:1 and 10:1. The standard ICDD powder diffraction patterns of CoP, Ni5P4 and CoNiP are given for reference. (b) Co 2p, (c) Ni 2p and (d) P 2p XPS spectra of the CoNiP-1:1 NWs sample.
samples, the CoNiP-1:1 still shows the best specific activity, indicating that it is an intrinsically more active “pre-catalyst”. It is worth noting that not all physically accessible surface areas are electrocatalytically active, so the BET-based specific activities represent the lower limit of the intrinsic catalytic performance. The OER kinetics of all TMP “pre-catalysts” was analyzed by Tafel analysis. The Tafel slope decreases with an order similar to that of the OER apparent activity, namely, CoNiP-1:1 > CoNiP-2:1 > CoNiP10:1 > CoP > Ni5P4 (Fig. 3c). The CoNiP-1:1 NWs show the lowest Tafel slope of 54.0 mV dec−1, suggesting a faster kinetics for the electron transfer; while the Tafel slope of the other catalysts falls in the range of 60–120 mV dec−1, indicating that the chemical adsorption of OH− might be the rate-determining step (RDS) [63]. The fast OER kinetics of the CoNiP-1:1 was also confirmed by EIS measurements (Fig. S2, Supporting Information). The charge transfer resistance (Rct) of CoNiP-1:1 (10.63 Ω) is substantially lower than that of the mono-metallic CoP (60.59 Ω) and Ni5P4 (141.6 Ω), as shown in Fig. 3c. The Rct values of all TMP “pre-catalysts” follow the same trend as that of the Tafel slopes, implying that Ni doping enhances the charge transfer ability during the OER. The electrochemical double-layer capacitance (Cdl) of each catalyst was measured using CV in the non-faradaic region (Fig. S4, Supporting Information), to estimate the electrochemically accessible surface area (ECSA) which correlates to the active sites number. As shown in Fig. 3d the introduction of Ni helps improve the Cdl value. For instance, the Cdl value of CoNiP-1:1 NWs (4.65 m F cm−2) is significantly higher than that of CoP NWs (0.82 m F cm−2). The intrinsic OER activity of the TMPs was further evaluated based on TOF, assuming all transition metal species in the TMPs are catalytically active (i.e., the lower limit). Fig. 3e shows the TOF values of all TMP “pre-catalysts” calculated at η = 300, 350 and 400 mV. The trend in TOF is found to be the same as that observed for the apparent activity, indicating that CoNiP-1:1 NWs are intrinsically more active than other TMPs for the OER. In particular, the TOF value of CoNiP-1:1 is as large as 0.022 s-1 at η = 350 mV, superior to that of many nonprecious OER (pre)catalysts including OCoP/GO nanoparticle (TOF350 = 0.018 s-1) [27], Ni2P/Ni/NF urchinlike (TOF350 = 0.015 s-1) [29], and CFP/NiCo2O4 (TOF440 = 0.0023 s1 ) [64].
peak at 782.2 eV can be ascribed to a Co oxidation state associated with Co−POx, which is likely resulting from the surface oxidation [46,59,60]. Similarly, in the Ni 2p spectrum (Fig. 2c), the Ni 2p3/2 peaks observed at 853.2 and 857.1 eV should arise from Ni−P and Ni−POx, respectively [33,47,49]. As far as the P 2p XPS spectrum is concerned, two peaks appearing at 129.4 and 130.1 eV can be assigned to P 2p3/2 and P 2p1/2 components, respectively (Fig. 2d), which is a characteristic of metal − P bonds in metal phosphides [61,62]. Additionally, a strong peak at 134.2 eV is observed, arising from the PeO bond due to the air exposure of CoNiP NWs [35]. The electrocatalytic performance of CoNiP NWs (pre)catalysts towards the OER and HER was assessed on GC substrates in 1.0 M KOH using CV, EIS, and CP. The CoP and Ni5P4 references were also tested under the same conditions for comparison. Prior to each catalytic test, pre-activation was carried out by repetitive CV scans at 5 mV s−1 for OER in the potential range of 1.0–1.65 V vs RHE and at 50 mV s−1 for HER in the potential range of −0.26–0.24 V vs RHE, respectively. Fig. 3a shows the reduction branches of iR-corrected CV curves of all samples after pre-activation for OER. The overpotential (η) needed at a specific current density has been widely used as an extrinsic parameter to evaluate the performance of electrocatalysts. The η values needed to deliver 10 (η10), 20 (η20) and 50 (η50) mA cm-2 are compared in the inset of Fig. 3a, where it’s seen that all CoNiP NWs show lower η values than both CoP NWs and Ni5P4 NSs. To reach the current density of 10 mA cm-2, Ni5P4 NSs need a η value of 373 mV, while the best-performing CoNiP-1:1 only demands 301 mV, showing a remarkable enhancement in the OER activity. In the low η region (η ≤ 350 mV) the OER activity follows the order of CoNiP-1:1 > CoNiP-2:1 ≈ CoNiP-10:1 > CoP > Ni5P4; while in the high η region (η > 400 mV), the same order basically remains but the activity of CoNiP-2:1 becomes higher, close to that of CoNiP-1:1, i.e., CoNiP-1:1 ≈ CoNiP-2:1 > CoNiP-10:1 > CoP > Ni5P4. For all “pre-catalysts” under investigation, CoNiP-1:1 exhibits the best OER activity at 10 mA cm-2, outperforming many other non-noble OER (pre)catalysts reported in recent years (Table S2, Supporting Information). Furthermore, the specific activity of all catalysts was calculated upon normalizing the catalytic current by their BET surface area derived from N2 adsorption/desorption isotherms (Fig. S3). Compared to CoNiP-10:1, CoNiP-2:1, and CoP and Ni5P4 control 4
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Fig. 3. Electrocatalytic performance of the CoNiP, CoP and Ni5P4 “pre-catalysts” for the OER in 1.0 M KOH with a catalyst loading density of 0.3 mg cm−2. (a) iRcorrected polarization curves recorded at 5 mV s-1. The inset shows the potentials needed for the “pre-catalysts” to deliver current densities of 10, 20, 50 and 100 mA cm−2. (b) BET-normalized specific activity. (c) Tafel slopes and charge-transfer resistance of the different “pre-catalysts” extracted from the fitting of the Nyquist plots. (d) Double-layer capacitance calculated from the CVs in non-faradaic region at different scan rates. (e) TOF values calculated at η = 300, 350 and 400 mV. (f) Chronopotentiostatic curve of the “pre-catalysts” recorded at a constant current density of 10 mA cm−2.
Supporting Information). The η10 value of CoNiP-10:1 (397 mV) is similar to that of CoP NWs (403 mV), indicating that doping a little Ni into CoP does not improve the HER activity markedly. The HER apparent activity was found to follow an order different from that attained for the OER, namely, CoNiP-1:1 > CoNiP-2:1 > Ni5P4 > CoNiP10:1 > CoP. The Tafel slopes obtained follow the same trend, as shown in Fig. 4b. The HER kinetics was also probed by EIS measurements (Fig. S5, Supporting Information). The obtained Nyquist plots were fitted with an equivalent circuit and the Rct at the electrode/electrolyte interface was extracted. The trend in Rct values is in agreement with that in the Tafel analyses, confirming the faster HER kinetics of CoNiP-1:1 NWs. The intrinsic HER activity of all TMP catalysts was further evaluated with BET-based specific activity and catalyst mass-based TOF assuming all transition metal species in the TMPs are catalytically active (i.e., the lower limit). Like for the OER, the specific activity for CoNiP-1:1 is
The stability of all TMP “pre-catalysts” was evaluated using chronopotentiometry at 10 mA cm−2. As shown in Fig. 3f, the overpotential needed to maintain 10 mA cm−2 decreases gradually in the first 2–3 h, indicative of an activation process, and then stabilizes up to 24 h, suggesting good long-term durability for all “pre-catalysts” except for the CoNiP-10:1 that shows slight degradation (i.e. increase in overpotential) after 9 h. The activation process may be associated with the electrochemical in-situ dephosphorization and oxidation process of the TMP, which has been reported recently in many TMP-based “pre-catalysts” and is believed to help enhance the OER performance due to the formation of catalytically active (oxy)hydroxide [30,43,46,65,66]. Likewise, the HER performance of all TMP catalysts was assessed in 1.0 M KOH. All CoNiP NWs are electrocatalytically active towards the HER, with the best activity achieved by CoNiP-1:1 which exhibits an overpotential of 252 mV at 10 mA cm−2 (Fig. 4a), outperforming a number of HER nano-catalysts reported in the literature (Table S3, 5
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Fig. 4. Electrocatalytic performance of the CoNiP, CoP and Ni5P4 catalysts for the HER in 1.0 M KOH with a catalyst loading density of 0.3 mg cm−2. (a) iR-corrected polarization curves recorded at 5 mV s-1. (b) BET-normalized specific activity. (c) Tafel plots and charge-transfer resistance of the different catalysts extracted from the fitting of the Nyquist plots. (d) TOF values calculated at η = 350 and 400 mV. (e) Chronopotentiostatic curve of the catalysts recorded at a constant current density of −10 mA cm−2.
good electrocatalytic performance, the low-cost nature and the simplicity of synthetic procedures, the bi-metallic transition metal phosphide nanowires hold substantial promise for use in alkaline and anionexchange-membrane water electrolysis.
higher than that of other TMPs (Fig. 4c). The TOF values of different catalysts calculated at η = 350 mV are compared in Fig. 4d, where it’s seen that the TOF value is greatly improved for CoNiP NWs with high Ni contents (i.e., Co:Ni = 1:1 and 2:1). Similarly, the TOF values of TMP catalysts follow an order the same as that of the Tafel slopes and Rct. In particular, the TOF value of CoNiP-1:1 is 0.071 s−1 at η = 350 mV. Furthermore, the stability of the best-performing catalyst, i.e. CoNiP1:1, was studied by chronopotentiometry at −10 mA cm-2. CoNiP-1:1 can sustain constant HER electrolysis for 24 h with little degradation, as shown in Fig. 4e.
Acknowledgments The authors acknowledge the financial support from the Portuguese Foundation of Science and Technology (FCT) under the project “PTDC/ CTM-ENE/2349/2014” (grant agreement No. 016660). This work was also partially funded by the Horizon 2020 project CritCat (grant agreement No. 686053).
4. Conclusion
Appendix A. Supplementary data
In summary, we have synthetized bi-metallic CoNiP nanowires with different compositions of Co and Ni using a simple hydrothermal method followed by a post-phosphorization treatment. We comprehensively investigate the electrocatalytic performance of CoNiP NWs toward the OER and HER in alkaline solution and compare them to the mono-metallic CoP and Ni5P4 control (pre)catalysts. We found that the CoNiP NWs with equimol Co and Ni exhibit the best apparent and intrinsic activities as well as the reaction kinetics both OER and HER, requiring 301 mV to afford a benchmark current density of 10 mA cm−2 for OER and 252 mV at −10 mA cm−2 for HER. Moreover, CoNiP-1:1 shows good catalytic stability for both OER and HER as well. Given the
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