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Replacing oxygen evolution with sodium sulfide electro-oxidation toward energy-efficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst
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Short Communication
Replacing oxygen evolution with sodium sulfide electro-oxidation toward energy-efficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst Shuai Hao a,b, Libin Yang b, Danni Liu b, Gu Du c, Yingchun Yang a,*, Abdullah M. Asiri d, Xuping Sun b,** a
College of Resources and Environment, Chengdu University of Information Technology, Chengdu 610225, Sichuan, China b College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China c Chengdu Institute of Geology and Mineral Resources, Chengdu 610081, Sichuan, China d Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
article info
abstract
Article history:
Water splitting is an important process to produce hydrogen fuels and oxygen evolution
Received 16 January 2017
reaction (OER), the anodic half reaction, is identified as the bottleneck for its kinetic and
Received in revised form
energetic complexity in improving the overall efficiency. In this communication, we report
17 May 2017
that cobalt phosphide nanoarray in situ grown on Ti mesh (CoP NA/TM) behaves as a du-
Accepted 31 May 2017
rable robust non-noble-metal electrocatalyst for sodium sulfide (Na2S) oxidation with the
Available online xxx
need of potential of 1.31 V to drive 20 mA cm2 in 1.0 M KOH with 50 mM Na2S. The high hydrogen-evolving activity for CoP NA/TM enables it as a bifunctional catalyst electrode for
Keywords:
energy-efficient electrochemical hydrogen generation by replacing OER with Na2S oxida-
Cobalt phosphide nanoarray
tion reaction. The CoP NA/TMjjCoP NA/TM couple needs a cell voltage of 1.49 V to drive
Sodium sulfide electro-oxidation
15 mA cm2, 210 mV less than of pure water splitting, with strong long-term electro-
Hydrogen production
chemical durability and 100% Faradic efficiency for hydrogen evolution.
Bifunctional catalyst
© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction In response to environmental pollution, but more because of the impending global energy crisis, increasing scientists dedicated to the exploration of a new energy carrier. Hydrogen
is regarded as an ideal candidate for replacing fossil fuels in the future [1e7]. Industrially, hydrogen is mainly produced from petroleum and coal with carbon dioxide emission [8e10]. Electrochemical water splitting provides an environmentally friendly way for large-scale production of pure hydrogen, but
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Yang), [email protected] (X. Sun). http://dx.doi.org/10.1016/j.ijhydene.2017.05.217 0360-3199/© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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the enormous electrical energy consumption becomes the major conditioned factor limiting its wide practical applications [11]. Thus, active electrocatalysts must be utilized for less energy-intensive process [12e14]. Compared to the cathodic hydrogen evolution reaction (HER), anodic oxygen evolution reaction (OER) involves multiproton-coupled electron-transfer steps and its sluggish kinetics limits the efficiency of hydrogen production [15,16]. Replacing oxygen evolution with the oxidation of more readily oxidizable species, such as methanol [17], ethanol [18], glycerol [19] and urea [20], offers an economical strategy to realize energy-saving hydrogen production. As an industrial raw material, Na2S is cheap with ultrahigh solubility in water and widely used in dyeing industry, electroplating and metal smelting [21,22]. The potential of Na2S oxidation is lower than that of water oxidation. It is expected to achieve energyefficient electrolytic hydrogen production by replacing OER with Na2S oxidation reaction (SSOR), which, however, has not been reported before. In this communication, we report that CoP nanoarray on Ti mesh (CoP NA/TM) behaves as a robust high-active electrocatalyst for SSOR in alkaline electrolytes with the need of potential of 1.31 V to reach 20 mA cm2 in 1.0 M KOH with 50 mM Na2S. The high HER activity enables this electrode as a bifunctional catalyst for energy-efficient electrolytic hydrogen production by replacing OER with SSOR. To drive 15 mA cm2, the two-electrode CoP NA/TMjjCoP NA/TM electrolyser demands a cell voltage of only 1.49 V, 210 mV less than for pure water splitting, with high long-term electrochemical durability and 100% Faradic efficiency for hydrogen evolution.
After the autoclave cooled down slowly at room temperature, the resulting Co(CO3)0.5(OH)$0.11H2O NA/TM (loading: 0.6 mg cm2) was washed with water several times and dried in air. Co(CO3)0.5(OH)$0.11H2O NA/TM and NaH2PO2 were put at two separate positions in a porcelain boat with NaH2PO2 at the upstream side of the furnace. The molar ratio for Co to P is 1:5. Subsequently, the sample was heated at 300 C for 2 h in Ar atmosphere, and then cooled to ambient temperature under Ar atmosphere to make CoP NA/TM.
Preparation of CoP NW/GCE Pure CoP nanowires were synthesized by the same way without the presence of TM. In a typical procedure, the glassy carbon electrode (GCE, diameter 3 mm) was respectively polished with 0.3 and 0.05 mm alumina slurry and cleaned by brief ultrasonication. Then cleaned electrode was dried under nitrogen flow. 2 mg CoP NW were dispersed in 20 mL 5 wt% Nafion and 980 mL of aqueous ethanol solution (1:1). The CoP NW modified GCE (CoP NW/GCE) was prepared by casting 17.5 mL of CoP NW suspension (2 mg mL1) on a GCE surface and dried in air as working electrode.
Characterization
Experimental
Powder X-ray diffraction (XRD) data were collected on a RigakuD/MAX 2550 diffractometer with Cu Ka radiation (l ¼ 1.5418 A). Scanning electron microscopy (SEM) measurements were carried out on a Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 25 kV. X-ray photoelectron spectrometer (XPS) measurements were carried out on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the excitation source.
Reagents and materials
Electrochemical measurements
Cobalt nitrate hexahydrate (Co(NO3)2$6H2O), ammonium fluoride (NH4F), and urea were purchased from Beijing Chemical Corp. Sodium hypophosphite (NaH2PO2) was purchased from Aladdin Ltd. (Shanghai, China). Potassium hydroxide (KOH), ethanol and hydrochloric acid (HCl) were purchased from Tianjin Chemical Corporation. Ti mesh is provided by Hongshan District, Wuhan Instrument Surgical Instruments business. All the chemicals in the experiment were analytical grade and used without further purification. The water used throughout all experiments was purified through a Millipore system.
Electrochemical measurements were performed with a CHI 660E electrochemical analyzer (CH Instruments, Inc., Shanghai) using CoP NA/TM as working electrode, a graphite rod as the counter electrode, and a Ag/AgCl electrode used as the reference electrode in a H-type electrolyzer. Working and reference electrodes are in the one cell which contains KOH and Na2S solution, and counter electrode is in the other with KOH only. All potentials measured were calibrated to RHE using the following equation: E (RHE) ¼ E (Ag/AgCl) þ 1.024 V. All experiments were carried out at room temperature (~25 C).
Preparation of CoP NA/TM
Results and discussion CoP NA/TM was converted from Co(CO3)0.5(OH)$0.11H2O NA/ TM [23] according to reported method [24]. To prepare Co(CO3)0.5(OH)$0.11H2O NA/TM, Co(NO3)2$6H2O (0.002 mol), NH4F (0.004 mol) and urea (0.01 mol) were dissolved in 20 mL water under vigorous stirring for 30 min. Ti mesh was washed with diluted HCl and water several times to remove the surface impurities. Then the solution was transferred into a Teflon-lined stainless autoclave (25 mL) and a piece of Ti mesh (3 cm 2 cm) was immersed into the solution. The autoclave was sealed and maintained at 95 C for 24 h in an electric oven.
Fig. 1a shows the XRD pattern of the precursor and its phosphided product scratched down from Ti substrate. The precursor shows diffraction peaks characteristic of Co(CO3)0.5(OH)$0.11H2O (JCPDS No. 48-0083) while only peaks corresponding to CoP phase (JCPDS No. 29-0497) are observed for phosphided product, confirming successful conversion of Co(CO3)0.5(OH)$0.11H2O into CoP after the low-temperature phosphidation reaction. SEM analysis shows the full coverage of bare TM (Fig. 1b) with Co(CO3)0.5(OH)$0.11H2O
Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Fig. 1 e (a) Powder XRD pattern for Co(CO3)0.5(OH)·0.11H2O (red) and CoP (black). SEM images for (b) bare Ti mesh, (c) CO(CO3)0.5(OH)·0.11H2O NA/TM and (d) CoP NA/TM. XPS spectra of CoP in the (e) Co 2p and (f) P 2p regions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
nanoarray (Fig. 1c) after hydrothermal treatment. The resulting CoP still maintains nanoarray feature (Fig. 1d). The intact CoP NA/TM chemical property was further evaluated by XPS analysis (Fig. 1e and f). The sample demonstrates apparent Co 2p peaks for both CoP at 779.1 (2p3/2) and 794.0 eV (2p1/2) and oxidized cobalt at 781.3(2p3/2) and 797.3 eV (2p1/2), respectively. The BE at 129.8 eV is typical of metal-phosphide bonds. The additional peaks at 133.5 eV can be matched to oxidized P species. These results are consistent with previous reports [24e27]. We examined the electrocatalytic SSOR activity of CoP NA/ TM (CoP loading: 0.5 mg cm2) in a typical three-electrode setup and all potentials were reported on a reversible hydrogen electrode (RHE) scale. Because as-measured currents cannot
directly reflect the intrinsic behavior of electrocatalysts for the effect of ohmic resistance, an iR correction was applied to all original data [28,29]. As shown in Fig. 2a, the linear sweep voltammetry (LSV) curve for CoP NA/TM in the absence of Na2S exhibits an anodic catalytic current beyond 1.6 V accompanied by vigorous bubble evolution upon further positive scan. With the presence of different concentration of Na2S, the catalytic current onset shifts cathodically, implying CoP NA/TM is able to preferably catalyze SSOR at potentials less positive than those required for water oxidation in alkaline media. The SSOR activity increases with the increasing concentration of Na2S, and further addition (60 mM) leads to slight decrease, due to the excess adsorption of sulfion on electrode, leading to sluggish diffusion of gas and electrolyte [30]. Fig. 2b compares the
Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Fig. 2 e (a) LSV curves of CoP NA/TM in 1.0 M KOH with different concentration of Na2S. (b) LSV curves for CoP NA/TM (curve 1), Co(CO3)0.5(OH)·0.11H2O NA/TM (curve 2) and bare TM (curve 3) toward 50 mM Na2S with a scan rate of 5 mV s¡1 for SSOR. (c) Chronopotentiometric curve of CoP NA/TM with constant current density of 20 mA cm¡2. (d) Multi-current process of CoP NA/TM. The current density started at 20 mA cm¡2 and ended at 40 mA cm¡2, with an increment of 2.5 mA cm¡2 per 200 s without iR correction. All experiments were carried out in 1.0 M KOH.
electrocatalytic SSOR performance of CoP NA/TM, CO(CO3)0.5(OH) 0.11H2O NA/TM and bare TM. Bare TM has poor SSOR activity with a negligible current density and Co(CO3)0.5(OH)$0.11H2O NA/TM also has very limited activity. Remarkably, CoP NA/TM electrode shows remarkably high SSOR activity and it only demands a low potential of 1.31 V to drive 20 mA cm2, suggesting its superior SSOR performance. Because durability of catalysts is also critical for practical applications, we carried out Na2S oxidation at a constant current density of 20 mA cm2 for 8 h (Fig. 2c). The potential for CoP NA/ TM only increases by 20 mV after electrolysis, due to the consumption of Na2S which is inevitable. The stable reaction process can be obtained from adding 2.5 mmol Na2S in 50 mL electrolyze every 6 h (Fig. S1). XRD pattern (Fig. S2a) and SEM image (Fig. S2b) of the post-SSOR CoP NA/TM (after 8 h electrolysis) indicate that the anodic catalyst maintain its structure and nanoarray feature after electrolysis. Fig. S3 shows the highsolution Co 2p and P 2p XPS spectra of the as-prepared and postSSOR CoP sample. In sharp contrast, two weakened peaks belong to the CoP and two broad peaks corresponding to the oxidized cobalt can be observed for the post-SSOR sample, respectively. The characteristic signals of P almost completely disappeared in the similar situation, implying the occurrence of oxidation during the SSOR process. Furthermore, XPS 10 nm depth profile collected for the post-SSOR sample shows two typical peaks corresponding to CoP in the Co 2p region. These findings reveal that the Na2S oxidation process only occurs on the surface of CoP sample and the CoP core remains intact after
SSOR [31]. Fig. 2d shows the mutil-step chronopotentiometric curve for CoP NA/TM with anodic current density increasing from 20 to 40 mA cm2. The potential immediately levels off at 1.31 V at the start current value and remains unchanged for the rest 200 s and the other steps also gives similar results, demonstrating the excellent mass transportation, conductivity and mechanical robustness of this catalyst electrode [32,33]. We further estimated their electrochemically active surface area for CoP NA/TM electrode using cyclic voltammetry measurement by extracting the double-layer capacitance [34]. Fig. 3a and b shows the cyclic voltammograms (CVs) collected in the region of 0.87e0.97 V, where the current response should be only due to the charging of the double layer. The Capacitances of CoP NA/TM and Co(CO3)0.5(OH)$0.11H2O NA/ TM are 73.8 and 7.6 mF cm2 (Fig. 3c), respectively, indicating CoP NA/TM has a much higher surface roughness than Co(CO3)0.5(OH)$0.11H2O NA/TM and thus exposes more active sites for SSOR [34]. As shown in Fig. 3d, CoP NA/TM possesses an extremely smaller radius of semicircle than that of Co(CO3)0.5(OH)$0.11H2O NA/TM, demonstrating a much lower Rp and thus a better charge transport capability and more rapid catalytic kinetics [35]. The CoP NA/TM electrocatalyst has to maintain excellent HER performance in the presence of Na2S due to the potential permeation of Na2S across the proton exchange membrane from the anode compartment to the cathode site. Therefore, the HER performance for CoP NA/TM was also studied in alkaline electrolyte with Na2S. As expected, the inherent excellent HER
Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Fig. 3 e CVs of (a) CoP NA/TM and (b) CO(CO3)0.5(OH) 0.11H2O NA/TM in the non-faradaic capacitance current range at scan rates of 5, 10, 20, 30, 40, 50, 60, 80 and 100 mV s¡1. (c) The capacitive currents at 0.92 V as a function of scan rate for Co(CO3)0.5(OH)·0.11H2O NA/TM (point 1) and CoP NA/TM (point 2). (d) Nyquist plots for Co(CO3)0.5(OH)·0.11H2O NA/TM (curve 1) and CoP NA/TM (curve 2) with a fitted equivalent circuit (inset).
Fig. 4 e (a) Polarization curves for CoP NA/TM at a scan rate of 5 mV s¡1 with and without 50 mM Na2S. (b) Polarization curves of water electrolysis for CoP NA/TMjjCoP NA/TM couple with a scan rate of 5 mV s¡1 with and without 50 mM Na2S. (c) Time-dependent current density curve for CoP NA/TMjjCoP NA/TM couple under a cell potential of 1.2 V for 10 h. The inset shows the narrow chronopotentiometric curve. (d) The measured H2 quantity compared with theoretically calculated H2 quantity vs. time for CoP NA/TM. All experiments were carried out in 1.0 M KOH. Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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performance was not impacted after the addition of trace amount of Na2S (Fig. 4a). We also made an electrochemical cell in two-electrode configuration using CoP NA/TM as cathode for HER and anode for SSOR. As shown in Fig. 4b, This CoP NA/ TMjjCoP NA/TM couple shows superior performance with a cell voltage of only 1.49 V to afford 15 mA cm2 in the presence of Na2S. In contrast, in the absence of Na2S, the CoP NA/TMjjCoP NA/TM couple was able to catalyze overall water splitting to produce H2 and O2 but needs a 210 mV higher potential (1.70 V) to approach 15 mA cm2 and 1.67 V for 10 mA cm2. It should be noted that these voltages is lower than those of many reported bifunctional electrocatalysts to afford a 10 mA cm2 current density for overall water splitting, including NiFe LDH/NF (1.70 V) [36], NiCo2S4 NA/CC (1.68 V) [37] and commercial electrolyzers (1.8 Ve2.0 V) [13]. When the cell potential was fixed at 1.2 V, CoP NA/TMjjCoP NA/TM couple for SSOR and HER shows very good stability and only a slight change of potential is observed over about 10 h (Fig. 4c). The fluctuant current densitytime curve in the narrow time period also suggested the growth and released of uniform hydrogen on the electrocatalyst surface. To quantify the produced H2 in the cathode compartment of the H-type electrolytic cell, a long-term electrolysis at a constant current density of 20 mA cm2 was performed. As shown in Fig. 4d, the Faradic efficiency for the hydrogen generation quantified by gas chromatography analysis matches the calculated amount based on passed charge, suggesting a 100% Faradic efficiency [38]. CoP nanowires was also immobilized on glassy carbon electrode (CoP NW/GCE) using 5 wt% nafion as binder (CoP Loading: 0.5 mg cm2) for electrochemical tests (Figs. S4aed). Note that CoP NA/TM is a superior SSOR catalyst than CoP NW/GCE (1.46 V for 20 mA cm2). This electrode exhibits HER activity requiring an overpotential of 0.43 V to attain 100 mA cm2, much larger than the value of CoP NA/TM. The two-electrode system using CoP NW/GCE as both cathode and anode drives 15 mA cm2 at a cell potential of 1.65 V, 0.16 V larger than that for CoP NA/TM. The superior catalytic activity of CoP NA/TM can be attributed to the following three reasons: (1) The direct growth of CoP on Ti mesh substrate leads to strong mechanical adhesion and reliable electrical connection [39]. (2) The reaction proceeds along with vectorial electro transport and facilitated electrolyte diffusion, due to the 1D nanoarray feature and meshy substrate structure, respectively [39,40]. (3) The binder-free electrode not only benefits from its high conductivity [41], but effectively reduces the blocking of active sites [42]. Indeed, the Nyquist plots show that CoP NA/TM has much lower electrochemical impedance and thus more rapid catalytic kinetics than CoP NW/GCE.
Conclusions In summary, CoP nanoarray has been proven as a high-active and durable catalyst for Na2S electro-oxidation in alkaline media. As a bifunctional catalyst for HER and SSOR, its corresponding two-electrode configuration demands a cell voltage of only 1.49 to afford 15 mA cm2 in 1.0 M KOH with 50 mM Na2S, which is 210 mV less that for pure water splitting. This work not only provides us a low-cost catalyst electrode
for Na2S electro-oxidation, but would open the new opportunity toward the development of Na2S-assisted energy-saving electrochemical hydrogen-producing system.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.05.217.
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Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Short Communication
Replacing oxygen evolution with sodium sulfide electro-oxidation toward energy-efficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst Shuai Hao a,b, Libin Yang b, Danni Liu b, Gu Du c, Yingchun Yang a,*, Abdullah M. Asiri d, Xuping Sun b,** a
College of Resources and Environment, Chengdu University of Information Technology, Chengdu 610225, Sichuan, China b College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China c Chengdu Institute of Geology and Mineral Resources, Chengdu 610081, Sichuan, China d Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
article info
abstract
Article history:
Water splitting is an important process to produce hydrogen fuels and oxygen evolution
Received 16 January 2017
reaction (OER), the anodic half reaction, is identified as the bottleneck for its kinetic and
Received in revised form
energetic complexity in improving the overall efficiency. In this communication, we report
17 May 2017
that cobalt phosphide nanoarray in situ grown on Ti mesh (CoP NA/TM) behaves as a du-
Accepted 31 May 2017
rable robust non-noble-metal electrocatalyst for sodium sulfide (Na2S) oxidation with the
Available online xxx
need of potential of 1.31 V to drive 20 mA cm2 in 1.0 M KOH with 50 mM Na2S. The high hydrogen-evolving activity for CoP NA/TM enables it as a bifunctional catalyst electrode for
Keywords:
energy-efficient electrochemical hydrogen generation by replacing OER with Na2S oxida-
Cobalt phosphide nanoarray
tion reaction. The CoP NA/TMjjCoP NA/TM couple needs a cell voltage of 1.49 V to drive
Sodium sulfide electro-oxidation
15 mA cm2, 210 mV less than of pure water splitting, with strong long-term electro-
Hydrogen production
chemical durability and 100% Faradic efficiency for hydrogen evolution.
Bifunctional catalyst
© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction In response to environmental pollution, but more because of the impending global energy crisis, increasing scientists dedicated to the exploration of a new energy carrier. Hydrogen
is regarded as an ideal candidate for replacing fossil fuels in the future [1e7]. Industrially, hydrogen is mainly produced from petroleum and coal with carbon dioxide emission [8e10]. Electrochemical water splitting provides an environmentally friendly way for large-scale production of pure hydrogen, but
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Yang), [email protected] (X. Sun). http://dx.doi.org/10.1016/j.ijhydene.2017.05.217 0360-3199/© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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the enormous electrical energy consumption becomes the major conditioned factor limiting its wide practical applications [11]. Thus, active electrocatalysts must be utilized for less energy-intensive process [12e14]. Compared to the cathodic hydrogen evolution reaction (HER), anodic oxygen evolution reaction (OER) involves multiproton-coupled electron-transfer steps and its sluggish kinetics limits the efficiency of hydrogen production [15,16]. Replacing oxygen evolution with the oxidation of more readily oxidizable species, such as methanol [17], ethanol [18], glycerol [19] and urea [20], offers an economical strategy to realize energy-saving hydrogen production. As an industrial raw material, Na2S is cheap with ultrahigh solubility in water and widely used in dyeing industry, electroplating and metal smelting [21,22]. The potential of Na2S oxidation is lower than that of water oxidation. It is expected to achieve energyefficient electrolytic hydrogen production by replacing OER with Na2S oxidation reaction (SSOR), which, however, has not been reported before. In this communication, we report that CoP nanoarray on Ti mesh (CoP NA/TM) behaves as a robust high-active electrocatalyst for SSOR in alkaline electrolytes with the need of potential of 1.31 V to reach 20 mA cm2 in 1.0 M KOH with 50 mM Na2S. The high HER activity enables this electrode as a bifunctional catalyst for energy-efficient electrolytic hydrogen production by replacing OER with SSOR. To drive 15 mA cm2, the two-electrode CoP NA/TMjjCoP NA/TM electrolyser demands a cell voltage of only 1.49 V, 210 mV less than for pure water splitting, with high long-term electrochemical durability and 100% Faradic efficiency for hydrogen evolution.
After the autoclave cooled down slowly at room temperature, the resulting Co(CO3)0.5(OH)$0.11H2O NA/TM (loading: 0.6 mg cm2) was washed with water several times and dried in air. Co(CO3)0.5(OH)$0.11H2O NA/TM and NaH2PO2 were put at two separate positions in a porcelain boat with NaH2PO2 at the upstream side of the furnace. The molar ratio for Co to P is 1:5. Subsequently, the sample was heated at 300 C for 2 h in Ar atmosphere, and then cooled to ambient temperature under Ar atmosphere to make CoP NA/TM.
Preparation of CoP NW/GCE Pure CoP nanowires were synthesized by the same way without the presence of TM. In a typical procedure, the glassy carbon electrode (GCE, diameter 3 mm) was respectively polished with 0.3 and 0.05 mm alumina slurry and cleaned by brief ultrasonication. Then cleaned electrode was dried under nitrogen flow. 2 mg CoP NW were dispersed in 20 mL 5 wt% Nafion and 980 mL of aqueous ethanol solution (1:1). The CoP NW modified GCE (CoP NW/GCE) was prepared by casting 17.5 mL of CoP NW suspension (2 mg mL1) on a GCE surface and dried in air as working electrode.
Characterization
Experimental
Powder X-ray diffraction (XRD) data were collected on a RigakuD/MAX 2550 diffractometer with Cu Ka radiation (l ¼ 1.5418 A). Scanning electron microscopy (SEM) measurements were carried out on a Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 25 kV. X-ray photoelectron spectrometer (XPS) measurements were carried out on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the excitation source.
Reagents and materials
Electrochemical measurements
Cobalt nitrate hexahydrate (Co(NO3)2$6H2O), ammonium fluoride (NH4F), and urea were purchased from Beijing Chemical Corp. Sodium hypophosphite (NaH2PO2) was purchased from Aladdin Ltd. (Shanghai, China). Potassium hydroxide (KOH), ethanol and hydrochloric acid (HCl) were purchased from Tianjin Chemical Corporation. Ti mesh is provided by Hongshan District, Wuhan Instrument Surgical Instruments business. All the chemicals in the experiment were analytical grade and used without further purification. The water used throughout all experiments was purified through a Millipore system.
Electrochemical measurements were performed with a CHI 660E electrochemical analyzer (CH Instruments, Inc., Shanghai) using CoP NA/TM as working electrode, a graphite rod as the counter electrode, and a Ag/AgCl electrode used as the reference electrode in a H-type electrolyzer. Working and reference electrodes are in the one cell which contains KOH and Na2S solution, and counter electrode is in the other with KOH only. All potentials measured were calibrated to RHE using the following equation: E (RHE) ¼ E (Ag/AgCl) þ 1.024 V. All experiments were carried out at room temperature (~25 C).
Preparation of CoP NA/TM
Results and discussion CoP NA/TM was converted from Co(CO3)0.5(OH)$0.11H2O NA/ TM [23] according to reported method [24]. To prepare Co(CO3)0.5(OH)$0.11H2O NA/TM, Co(NO3)2$6H2O (0.002 mol), NH4F (0.004 mol) and urea (0.01 mol) were dissolved in 20 mL water under vigorous stirring for 30 min. Ti mesh was washed with diluted HCl and water several times to remove the surface impurities. Then the solution was transferred into a Teflon-lined stainless autoclave (25 mL) and a piece of Ti mesh (3 cm 2 cm) was immersed into the solution. The autoclave was sealed and maintained at 95 C for 24 h in an electric oven.
Fig. 1a shows the XRD pattern of the precursor and its phosphided product scratched down from Ti substrate. The precursor shows diffraction peaks characteristic of Co(CO3)0.5(OH)$0.11H2O (JCPDS No. 48-0083) while only peaks corresponding to CoP phase (JCPDS No. 29-0497) are observed for phosphided product, confirming successful conversion of Co(CO3)0.5(OH)$0.11H2O into CoP after the low-temperature phosphidation reaction. SEM analysis shows the full coverage of bare TM (Fig. 1b) with Co(CO3)0.5(OH)$0.11H2O
Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Fig. 1 e (a) Powder XRD pattern for Co(CO3)0.5(OH)·0.11H2O (red) and CoP (black). SEM images for (b) bare Ti mesh, (c) CO(CO3)0.5(OH)·0.11H2O NA/TM and (d) CoP NA/TM. XPS spectra of CoP in the (e) Co 2p and (f) P 2p regions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
nanoarray (Fig. 1c) after hydrothermal treatment. The resulting CoP still maintains nanoarray feature (Fig. 1d). The intact CoP NA/TM chemical property was further evaluated by XPS analysis (Fig. 1e and f). The sample demonstrates apparent Co 2p peaks for both CoP at 779.1 (2p3/2) and 794.0 eV (2p1/2) and oxidized cobalt at 781.3(2p3/2) and 797.3 eV (2p1/2), respectively. The BE at 129.8 eV is typical of metal-phosphide bonds. The additional peaks at 133.5 eV can be matched to oxidized P species. These results are consistent with previous reports [24e27]. We examined the electrocatalytic SSOR activity of CoP NA/ TM (CoP loading: 0.5 mg cm2) in a typical three-electrode setup and all potentials were reported on a reversible hydrogen electrode (RHE) scale. Because as-measured currents cannot
directly reflect the intrinsic behavior of electrocatalysts for the effect of ohmic resistance, an iR correction was applied to all original data [28,29]. As shown in Fig. 2a, the linear sweep voltammetry (LSV) curve for CoP NA/TM in the absence of Na2S exhibits an anodic catalytic current beyond 1.6 V accompanied by vigorous bubble evolution upon further positive scan. With the presence of different concentration of Na2S, the catalytic current onset shifts cathodically, implying CoP NA/TM is able to preferably catalyze SSOR at potentials less positive than those required for water oxidation in alkaline media. The SSOR activity increases with the increasing concentration of Na2S, and further addition (60 mM) leads to slight decrease, due to the excess adsorption of sulfion on electrode, leading to sluggish diffusion of gas and electrolyte [30]. Fig. 2b compares the
Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Fig. 2 e (a) LSV curves of CoP NA/TM in 1.0 M KOH with different concentration of Na2S. (b) LSV curves for CoP NA/TM (curve 1), Co(CO3)0.5(OH)·0.11H2O NA/TM (curve 2) and bare TM (curve 3) toward 50 mM Na2S with a scan rate of 5 mV s¡1 for SSOR. (c) Chronopotentiometric curve of CoP NA/TM with constant current density of 20 mA cm¡2. (d) Multi-current process of CoP NA/TM. The current density started at 20 mA cm¡2 and ended at 40 mA cm¡2, with an increment of 2.5 mA cm¡2 per 200 s without iR correction. All experiments were carried out in 1.0 M KOH.
electrocatalytic SSOR performance of CoP NA/TM, CO(CO3)0.5(OH) 0.11H2O NA/TM and bare TM. Bare TM has poor SSOR activity with a negligible current density and Co(CO3)0.5(OH)$0.11H2O NA/TM also has very limited activity. Remarkably, CoP NA/TM electrode shows remarkably high SSOR activity and it only demands a low potential of 1.31 V to drive 20 mA cm2, suggesting its superior SSOR performance. Because durability of catalysts is also critical for practical applications, we carried out Na2S oxidation at a constant current density of 20 mA cm2 for 8 h (Fig. 2c). The potential for CoP NA/ TM only increases by 20 mV after electrolysis, due to the consumption of Na2S which is inevitable. The stable reaction process can be obtained from adding 2.5 mmol Na2S in 50 mL electrolyze every 6 h (Fig. S1). XRD pattern (Fig. S2a) and SEM image (Fig. S2b) of the post-SSOR CoP NA/TM (after 8 h electrolysis) indicate that the anodic catalyst maintain its structure and nanoarray feature after electrolysis. Fig. S3 shows the highsolution Co 2p and P 2p XPS spectra of the as-prepared and postSSOR CoP sample. In sharp contrast, two weakened peaks belong to the CoP and two broad peaks corresponding to the oxidized cobalt can be observed for the post-SSOR sample, respectively. The characteristic signals of P almost completely disappeared in the similar situation, implying the occurrence of oxidation during the SSOR process. Furthermore, XPS 10 nm depth profile collected for the post-SSOR sample shows two typical peaks corresponding to CoP in the Co 2p region. These findings reveal that the Na2S oxidation process only occurs on the surface of CoP sample and the CoP core remains intact after
SSOR [31]. Fig. 2d shows the mutil-step chronopotentiometric curve for CoP NA/TM with anodic current density increasing from 20 to 40 mA cm2. The potential immediately levels off at 1.31 V at the start current value and remains unchanged for the rest 200 s and the other steps also gives similar results, demonstrating the excellent mass transportation, conductivity and mechanical robustness of this catalyst electrode [32,33]. We further estimated their electrochemically active surface area for CoP NA/TM electrode using cyclic voltammetry measurement by extracting the double-layer capacitance [34]. Fig. 3a and b shows the cyclic voltammograms (CVs) collected in the region of 0.87e0.97 V, where the current response should be only due to the charging of the double layer. The Capacitances of CoP NA/TM and Co(CO3)0.5(OH)$0.11H2O NA/ TM are 73.8 and 7.6 mF cm2 (Fig. 3c), respectively, indicating CoP NA/TM has a much higher surface roughness than Co(CO3)0.5(OH)$0.11H2O NA/TM and thus exposes more active sites for SSOR [34]. As shown in Fig. 3d, CoP NA/TM possesses an extremely smaller radius of semicircle than that of Co(CO3)0.5(OH)$0.11H2O NA/TM, demonstrating a much lower Rp and thus a better charge transport capability and more rapid catalytic kinetics [35]. The CoP NA/TM electrocatalyst has to maintain excellent HER performance in the presence of Na2S due to the potential permeation of Na2S across the proton exchange membrane from the anode compartment to the cathode site. Therefore, the HER performance for CoP NA/TM was also studied in alkaline electrolyte with Na2S. As expected, the inherent excellent HER
Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Fig. 3 e CVs of (a) CoP NA/TM and (b) CO(CO3)0.5(OH) 0.11H2O NA/TM in the non-faradaic capacitance current range at scan rates of 5, 10, 20, 30, 40, 50, 60, 80 and 100 mV s¡1. (c) The capacitive currents at 0.92 V as a function of scan rate for Co(CO3)0.5(OH)·0.11H2O NA/TM (point 1) and CoP NA/TM (point 2). (d) Nyquist plots for Co(CO3)0.5(OH)·0.11H2O NA/TM (curve 1) and CoP NA/TM (curve 2) with a fitted equivalent circuit (inset).
Fig. 4 e (a) Polarization curves for CoP NA/TM at a scan rate of 5 mV s¡1 with and without 50 mM Na2S. (b) Polarization curves of water electrolysis for CoP NA/TMjjCoP NA/TM couple with a scan rate of 5 mV s¡1 with and without 50 mM Na2S. (c) Time-dependent current density curve for CoP NA/TMjjCoP NA/TM couple under a cell potential of 1.2 V for 10 h. The inset shows the narrow chronopotentiometric curve. (d) The measured H2 quantity compared with theoretically calculated H2 quantity vs. time for CoP NA/TM. All experiments were carried out in 1.0 M KOH. Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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performance was not impacted after the addition of trace amount of Na2S (Fig. 4a). We also made an electrochemical cell in two-electrode configuration using CoP NA/TM as cathode for HER and anode for SSOR. As shown in Fig. 4b, This CoP NA/ TMjjCoP NA/TM couple shows superior performance with a cell voltage of only 1.49 V to afford 15 mA cm2 in the presence of Na2S. In contrast, in the absence of Na2S, the CoP NA/TMjjCoP NA/TM couple was able to catalyze overall water splitting to produce H2 and O2 but needs a 210 mV higher potential (1.70 V) to approach 15 mA cm2 and 1.67 V for 10 mA cm2. It should be noted that these voltages is lower than those of many reported bifunctional electrocatalysts to afford a 10 mA cm2 current density for overall water splitting, including NiFe LDH/NF (1.70 V) [36], NiCo2S4 NA/CC (1.68 V) [37] and commercial electrolyzers (1.8 Ve2.0 V) [13]. When the cell potential was fixed at 1.2 V, CoP NA/TMjjCoP NA/TM couple for SSOR and HER shows very good stability and only a slight change of potential is observed over about 10 h (Fig. 4c). The fluctuant current densitytime curve in the narrow time period also suggested the growth and released of uniform hydrogen on the electrocatalyst surface. To quantify the produced H2 in the cathode compartment of the H-type electrolytic cell, a long-term electrolysis at a constant current density of 20 mA cm2 was performed. As shown in Fig. 4d, the Faradic efficiency for the hydrogen generation quantified by gas chromatography analysis matches the calculated amount based on passed charge, suggesting a 100% Faradic efficiency [38]. CoP nanowires was also immobilized on glassy carbon electrode (CoP NW/GCE) using 5 wt% nafion as binder (CoP Loading: 0.5 mg cm2) for electrochemical tests (Figs. S4aed). Note that CoP NA/TM is a superior SSOR catalyst than CoP NW/GCE (1.46 V for 20 mA cm2). This electrode exhibits HER activity requiring an overpotential of 0.43 V to attain 100 mA cm2, much larger than the value of CoP NA/TM. The two-electrode system using CoP NW/GCE as both cathode and anode drives 15 mA cm2 at a cell potential of 1.65 V, 0.16 V larger than that for CoP NA/TM. The superior catalytic activity of CoP NA/TM can be attributed to the following three reasons: (1) The direct growth of CoP on Ti mesh substrate leads to strong mechanical adhesion and reliable electrical connection [39]. (2) The reaction proceeds along with vectorial electro transport and facilitated electrolyte diffusion, due to the 1D nanoarray feature and meshy substrate structure, respectively [39,40]. (3) The binder-free electrode not only benefits from its high conductivity [41], but effectively reduces the blocking of active sites [42]. Indeed, the Nyquist plots show that CoP NA/TM has much lower electrochemical impedance and thus more rapid catalytic kinetics than CoP NW/GCE.
Conclusions In summary, CoP nanoarray has been proven as a high-active and durable catalyst for Na2S electro-oxidation in alkaline media. As a bifunctional catalyst for HER and SSOR, its corresponding two-electrode configuration demands a cell voltage of only 1.49 to afford 15 mA cm2 in 1.0 M KOH with 50 mM Na2S, which is 210 mV less that for pure water splitting. This work not only provides us a low-cost catalyst electrode
for Na2S electro-oxidation, but would open the new opportunity toward the development of Na2S-assisted energy-saving electrochemical hydrogen-producing system.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.05.217.
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Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217
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Please cite this article in press as: Hao S, et al., Replacing oxygen evolution with sodium sulfide electro-oxidation toward energyefficient electrochemical hydrogen production: Using cobalt phosphide nanoarray as a bifunctional catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.217