MoOx for Overall Water Splitting by

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Converting CoMoO4 into CoO/MoOx for Effective Overall Water Splitting by Hydrogenation Xiaodong Yan, Lihong Tian, Samuel Atkins, Yan Liu, James B. Murowchick, and Xiaobo Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00383 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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ACS Sustainable Chemistry & Engineering

Converting CoMoO4 into CoO/MoOx for overall water splitting by hydrogenation Xiaodong Yan,† Lihong Tian,†,ǁ Samuel Atkins,† Yan Liu,†,⊥ James Murowchick‡ and Xiaobo Chen†,* †

Department of Chemistry, University of Missouri − Kansas City, 5100 Rockhill Road, Kansas

City, Missouri 64110, USA ǁ

Hubei Collaborative Innovation Center for Advanced Organochemical Materials, Ministry-of-

Education Key Laboratory for the Synthesis and Applications of Organic Functional Molecules, Hubei University, No 3, Bayi Road, Wuchang, Wuhan, Hubei 430062, China ⊥

College of Environment, Sichuan Agricultural University, 211 Huimin Road, Wenjiang District,

Chengdu, Sichuan 611130, China. ‡

Department of Geosciences, University of Missouri – Kansas City, 5100 Rockhill Road, Kansas

City, Missouri 64110, USA

1 Environment ACS Paragon Plus


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ABSTRACT: Special structure of materials often bring in unprecedented catalytic activity which are critical in realizing large-scale hydrogen production by electrochemical water splitting. Herein, we report CoO/MoOx crystalline/amorphous structure as an effective bifunctional electrocatalyst for water splitting. Converted from CoMoO4 by hydrogenation, the CoO/MoOx, featured with crystalline CoO in amorphous MoOx matrix, displays superior catalytic activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). It shows small onset overpotentials of 40 and 230 mV for the HER and OER in 1.0 M KOH, respectively, and overall water splitting starting at 1.53 V with a robust stability. The high catalytic activity of the CoO/MoOx is benefited from the large defect-rich interface between CoO and MoOx, along with the amorphous nature of MoOx. Thus, this study demonstrates the effectiveness of structural manipulation in developing highly active electrocatalysts for overall electrochemical water splitting.

KEYWORDS: water electrolysis, hydrogen evolution reaction, cobalt monoxide, molybdenum oxide, composite


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INTRODUCTION Hydrogen generation has been intensively explored through photocatalytic and electrochemical water splitting for future clean energy applications.1−6 Compared to the lower solar-to-hydrogen efficiency of photocatalysis, electrolysis currently has a higher efficiency for large-scale hydrogen production.4−6 In water electrolysis, hydrogen is generated as a result of the cathodic hydrogen evolution reaction (HER), and oxygen is generated through anodic oxygen evolution reaction (OER). Electrocatalysts play key role in this process. To date, the state-of-the-art HER and OER electrocatalysts are platinum (Pt) and iridium oxide (IrO2), respectively.7,8 However, their scarcity and high cost limit large-scale applications. Earth-abundant and cost-effective electrocatalysts have thus been intensively and extensively explored over the past decade. For example, transition metal phosphides,9−12 sulfides,13−15 and double hydroxide,6,16−19 have been shown with great promise. FeP nanowires exhibited a small HER overpotential of 55 mV at 10 mA cm−2 in 0.5 M H2SO4;11 and NiFe layered double hydroxide films showed a small onset OER overpotential of ~230 mV in 0.1 M KOH.17 Besides new material discoveries, material modifications have been found as another effective approach to enhance the HER or OER performances of electrocatalysts. A series of oxide materials had markedly improved activity after electrochemical tuning.13,14,20−22 For example, not only the OER activity but also the HER activity of NiFeOx have been largely enhanced by electrochemical tuning,21 and remarkably improved HER activity has been achieved on Ni/NiO23−25, Co/Co3O426,27, MoO228, WO2,29 and WO2.9,30 by chemical reduction. CoMoO4 and MoO2/CoO have been reported to show decent OER activities.31−33 However, their HER activities have not been reported. In this work, we report that CoO/MoOx, converted from CoMoO4 through hydrogenation treatment, can act not only as an effective OER catalyst, 3 Environment ACS Paragon Plus


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but also as an excellent HER catalyst. The CoO/MoOx catalyst displays a much better performance than the original CoMoO4. The high catalytic activity of CoO/MoOx is benefited from the much faster charge-transfer kinetics resulting from the large interface between CoO and MoOx, along with the amorphous nature of MoOx. Thus, this study provides us a new approach in developing efficient catalysts for water splitting. EXPERIMENTAL SECTION Synthesis. In a typical synthesis, 0.2 mmol of Co(NO3)2·6H2O and 0.02857 mmol of (NH4)6Mo7O24·6H2O were dissolved in a solution of 7 mL deionized water and 8 mL ethanol in an autoclave. A piece of nickel foam was sonicated in 3 M HCl for 10 min to remove the possible surface oxide layer. After washed with deionized water, the nickel foam was transferred into the above solution and reacted at 160 °C for 6 h (hydrothermal process). After the reaction, the treated nickel foam was washed with deionized water and dried in air, followed by annealing at 500 °C for 2 h to obtain a CoMoO4 coated Ni foam. The nickel foam was then treated in a high-pressure hydrogen atmosphere at 400 °C for 3 h (hydrogenation), and labeled as CoO/MoOx. The mass loadings of the CoMoO4 and CoO/MoOx on nickel foam were about 1.1 mg cm−2. In order to investigate the structural evolution of CoMoO4 during hydrogenation, similar processes were also applied to the powders formed in the hydrothermal process. For comparison, NiO nanosheets on Ni foam was prepared according to our previous research.24 Typically, 3 mmol Ni(NO3)2·6H2O and 6 mmol hexamethylenetetramine were dissolved in a solution of 40 mL water and 20 mL ethanol in a glass bottle. A nickel foam (~20 cm2) was sonicated in 3 M HCl for 10 min to remove the possible surface oxide layer. After washed with deionized water, the nickel foam was transferred into the above solution and reacted at 90 °C for


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10 h. After the reaction, the nickel foam was washed with deionized water and dried in air, followed by annealing at 300 °C for 2 h to obtain a NiO coated Ni foam. Physical property characterization. Morphologies of the samples were examined using scanning and transmission electron microscopy (SEM and TEM). The SEM images were taken on a Hatachi 4700 field emission scanning electron microscope (FESEM). The foams were directly mounted on the sample stage for analysis. The TEM study was performed on a FEI Tecnai F20 STEM. The electron accelerating voltage was 200 kV. A small amount of powder sample dispersed in water was dropped onto a thin holey carbon film, and dried overnight before TEM measurement. Structural and chemical properties were studied with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD was performed using a Rigaku Miniflex X-ray diffractometer using Cu Kα radiation (wavelength = 1.5418 Å). XPS data were collected using a Kratos Axis 165 X-ray photoelectron spectrometer. Spectra were acquired using a photon beam of 1486.6 eV, selected from an Al/Mg dual-anode X-ray source. Fourier transform infrared (FTIR) spectra were recorded on a Thermo-Nicolet iS10 FT-IR spectrometer with an attenuated total reflectance unit. Electrochemical performance. Electrochemical measurements were carried out in a threeelectrode system at room temperature. The treated nickel foam, a Pt wire and an Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. 1.0 M KOH solution was used as the electrolyte. To prepare the IrO2 electrode, IrO2 (5 mg, 99%) were dispersed in mixed solvent of deionized water (1 mL) and 2-propanol (0.25 mL) via sonication for 0.5 h. Nafion solution (10 µL, 5 wt%) was added to increase the binding strength before sonication. 35 µL of the suspension was then drop-casted on Ni foam by micropipette, and the solvent was allowed to be evaporated at 70 °C for around 10 minutes. The



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catalyst loading was 0.7 mg cm−2. Cyclic voltammetry was performed 30 cycles between 0.4 and 0.55 V vs. Ag/AgCl at a scan rate of 10 mV s−1 until the oxygen evolution current of the IrO2 electrode showed negligible change. Linear sweep voltammetry was conducted at a scan rate of 5 mV s−1 to evaluate the HER and/or OER performances of all the working electrodes. Electrochemical impedance spectroscopy (EIS) analysis was performed using a 10 mV amplitude AC signal over a frequency range from 100 kHz to 10 mHz on a Biologic potentiostat/EIS electrochemical workstation (SP-150, Bio-Logic Science Instruments). The polarization curve was iR-corrected for an ohmic drop obtained from EIS Nyquist plot. The reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). The calibration was performed in a high purity hydrogen saturated 1 M KOH electrolyte with a Pt wire as the working electrode. The double layer capacitance (Cdl) was measured using a simple cyclic voltammetry method. The voltage window of cyclic voltammograms was 0.0 ‒ 0.1 V vs RHE. The scan rates were 20, 40, 60, 80 and 100 mV s−1. Cdl was estimated by plotting the ∆j = (ja−jc) at 0.05 V vs RHE against the scan rate, where ja and jc are charging and discharging current, respectively. RESULTS AND DISCUSSION Figure 1A showed the XRD patterns of CoMoO4 and CoO/MoOx. The XRD pattern of CoMoO4 matched well with the standard one of the monoclinic CoMoO4 (JCPDS card no. 21-0868). The diffraction peaks of the CoMoO4 disappeared in CoO/MoOx, indicating that the structure of CoMoO4 was completely converted in the hydrogenation. Strong XRD pattern corresponding to cubic CoO (JCPDS card no. 48-1719) was observed, but with no diffraction from the MoOx, suggesting an amorphous MoOx phase. The shift of the diffraction peaks of CoO at 42.4 and 61.5° were likely caused by slight structural changes due to the adjacent amorphous MoOx


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phase. XPS measurement was carried out to survey the valance states of ions in CoMoO4 and CoO/MoOx. Figure 1B displays the Co 2p XPS spectra of CoMoO4 and CoO/MoOx. At first glance, the two Co 2p spectra are similar with two core-level peaks. The two core-level peaks located around 796.7 and 780.5 eV were the Co2+ 2p1/2 and 2p3/2, respectively.34,35 Careful survey found that the binding energy of the Co 2p3/2 signal has a shift from 780.2 (CoMoO4) to 780.9 eV (CoO/MoOx). This was likely caused by the environmental change of Co2+.34,35 Also, we want to note that the trace amount of Co ions with higher valance states (Co3+ and Co4+) may exist on the surface of CoMoO4 and thus affected the location of the Co2+ 2p3/2 signal.36,37 The Mo 3d XPS spectrum of CoMoO4 (Figure 1C, curve a) showed two peaks located at 235.5 and 232.6 eV, corresponding to the 3d3/2 and 3d5/2 of Mo6+ ions.38 In CoO/MoOx, one extra peak was observed at 230.0 eV (Figure 1C, curve b), matched well to the Mo 3d5/2 of Mo4+ ions.39 This indicated that partial Mo6+ ions were reduced during the hydrogenation process. While only one peak located at 530.1 eV associated with lattice O2− was found in the O 1s XPS spectrum of CoMoO4 (Figure 1D), two types of oxygen associated with lattice O2− (530.1 eV) and oxygen vacancies or defects (531.0 eV) were observed, respectively.40 FTIR was further carried out to detect the surface chemical bonding. Usually, a broad band in the region of 3500−3400 cm−2 indicates the existence of OH groups or residual H2O that is molecularly adsorbed.41,42 The FTIR spectra (Figure S1) indicated no hydroxyl groups in both CoMoO4 and CoO/MoOx. Thus oxygen vacancies or defects were likely generated in CoO/MoOx from the hydrogenation treatment. The peaks of CoMoO4 centered at 942, 786 and 700 cm−1 were from the symmetric stretching mode of the Mo–O bond, the asymmetric stretching mode of oxygen in the O–Mo–O bond and the symmetric stretching mode of the Co–O–Mo bond,43 respectively. Those peaks disappeared in



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CoO/MoOx. This suggested that the CoMoO4 phase was completely converted during the hydrogenation process, consistent with the XRD result. Both CoMoO4 and CoO/MoOx existed in the form of nanorods on the nanosheets. The growth of the nonorods seemed to be triggered at some special points on the nanosheets and the exact reasons were still under investigation. The nanorods were approximately had a diameter of ~1 µm and a length of ~ 12 µm (Figure 2A and S2). After hydrogenation, the CoMoO4 was converted into CoO/MoOx, with similar morphology and size (Figure 2B and S3). As seen in Figure 2C, the CoMoO4 was highly crystalized with clear lattice fringes (Figure 2C) having dspacing of 0.47 nm, corresponding to the (−201) plane of CoMoO4 (JCPDS card no. 21-0868). In contrast, the CoO/MoOx had a crystalline phase within an amorphous phase (Figure 2D). The crystalline phase had lattice fringes with the d-spacing of 0.21 nm of the (200) plane of CoO (JCPDS card no. 48-1719). So it seemed that CoO/MoOx had a crystalline CoO phase embedded in an amorphous MoOx phase. The HER activities of the CoMoO4 and CoO/MoOx were evaluated in 1.0 M KOH by linear sweep voltammetry in a standard three-electrode system. For comparison, the commercial Pt and a bare Ni foam were compared under the same condition, along with NiO nanosheets. The polarization curve of NiO was given as NiO nanosheets could also be formed on the Ni foam.24 CoMoO4 showed the worst catalytic activity among all the electrodes, even worse than bare Ni foam. This evidenced collaterally that the nanosheets and nanorods were pure CoMoO4. The apparent difference in the polarization curves of NiO, CoMoO4 and CoO/MoOx suggested that the contribution of NiO was negligible. The HER activity of the CoO/MoOx was much higher than the pristine CoMoO4 (Figure 3A). The CoO/MoOx reached a benchmark current density of 10 mA cm−2 under a small overpotential of 163 mV. In contrast, a large overpotential (353 mV)


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was needed for the CoMoO4. Meanwhile, the CoO/MoOx showed a very small onset potential (~40 mV), comparable to the state-of-the-art Pt catalyst. Figure 3B showed the Tafel plots of the CoMoO4 and CoO/MoOx derived from Figure 3A. The Tafel slope (44 mV dec−1) of the CoO/MoOx was smaller than that (61 mV dec−1) of the CoMoO4, displaying the higher catalytic activity of the CoO/MoOx. In alkaline media, two mechanisms were developed to explain the HER pathway: Volmer-Heyrovesky (2) or Volmer-Tafel (4) processes.23,44,45 H2O + e− → Had + OH− (Volmer)

(1)

H2O + Had + e− → H2 + OH− (Heyrovesky)

(2)

H2O + e− → Had + OH− (Volmer)

(3)

Had + Had → H2 (Tafel)

(4)

Tafel slope of 120, 40, or 30 mV dec−1 was expected if the Volmer, Heyrovsky, or Tafel step was the rate-determining step, respectively.44,45 Thus, the experimentally observed Tafel slope of 44 mV dec−1 indicated that the Heyrovsky process was the rate-determining step for the CoO/MoOx. The stability of the CoO/MoOx was evaluated using chronopotentiometry at a constant voltage of −0.2 V versus RHE without iR-compensation. Negligible current degradation was observed during a long time period of 6000s. This indicated that the CoO/MoOx were stable when operated in 1.0 M KOH electrolyte. To reveal the HER kinetics on the surface of the catalysts, EIS analyses were performed at a potential of −0.2 V versus RHE. Only one semicircle corresponding to the charge-transfer kinetics was observed in the Nyquist plots of both CoMoO4 and CoO/MoOx (Figure 3D). The charge-transfer resistance (~10 Ω) of the CoO/MoOx was much smaller than that (~80 Ω) of the CoMoO4. This suggested a much faster charge-transfer kinetics on the surface of the CoO/MoOx electrode. The faster charge-transfer kinetics may be partially



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caused by the possibly improved conductivity of the CoO/MoOx due to the oxygen vacancies generated during the hydrogenation process. The catalytic activity is normally influenced by the electrochemical surface area (ECSA) of the catalysts. The larger ECSA, the higher the activity. ECSA was estimated from Cdl (Figure S4A and B), as Cdl was linearly proportional to the effective active surface area.24,46 The linear slope of the capacitive current against scan rate was used to represent the ECSA (Figure S4C).24,46 The ECSA of CoO/MoOx was 60% smaller than that of CoMoO4. A smaller ECSA meant less exposed active sites, contradictory to the higher catalytic activity of CoO/MoOx. Thus, the high catalytic activity of CoO/MoOx was likely benefited from the synergistic effect of crystalline CoO and amorphous MoOx. According to the literature, CoO itself has extremely low catalytic activity toward HER.27 However, dramatically enhanced catalytic activity has been observed when CoO was composited with metal cobalt/nitrogen-rich carbon and MnO.27,47,48 Herein, we propose that the interface between CoO and the deliberately introduced materials is expected to be defect-rich. It has been widely reported that amorphous or defect-rich materials had higher catalytic activity than their counterparts.15,28,30 Therefore, it can be concluded that the abundant defect-rich interface between CoO and MoOx, along with the amorphous nature of MoOx, synergistically rendered CoO/MoOx high catalytic activity toward HER. The OER activities of the CoMoO4 and CoO/MoOx were also evaluated. For comparison, the polarization curves of IrO2 and bare Ni foam were collected under similar conditions. Compared to the pristine CoMoO4, CoO/MoOx exhibited highly enhanced OER activity with an onset potential of 1.46 V vs RHE and a high current density of 20 mA cm−2 at a small overpotential of 310 mV (Figure 4A). In contrast, the overpotential for CoMoO4 and NiO at 20 mA cm−2 was much larger (370 and 410 mV, respectively). This implied that the high catalytic activity was


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originated from Co/MoOx rather than CoMoO4 or NiO. The highly enhanced OER activity was beneﬁted from the synergistic effect between MoOx and CoO which allowed the faster ion diffusion and electron transport at the electrode/electrolyte interface.33 Also, The OER activity of CoO/MoOx is higher than those of the CoMoO4 nanosheets, MoO2-CoO-carbon composite and CoP/C composite.32,33,49 The higher OER activity of the CoO/MoOx than the MoO2-CoO-carbon composite can be attributed to the amorphous nature of MoOx, as amorphous metal oxides have been reported to have higher OER activity than their counterparts.50−52 Herein, we also want to point out that the Fe impurity from KOH may positively affect the catalytic activity of the catalysts toward OER.53−55 The stability of the CoO/MoOx towards OER was evaluated using chronopotentiometry at a constant voltage of 1.58 V versus RHE without iR-compensation. Stable performance was observed during a long time period of 6000 s without current degradation (Figure 4B). An electrolyzer with CoO/MoOx as the anode and cathode electrodes was further assembled to test the activity for overall water splitting in 1 M KOH. The liner sweep voltammetry of the CoO/MoOx || CoO/MoOx system showed that the complete electrolysis proceeded at an applied potential of 1.53 V (Figure 5A). This is one of the smallest values of all the catalysts. A current density of 20 mA cm−2 was achieved at a voltage of ~1.82 V (Figure 5A). This performance was better than the NiCo2S4 nanowire system (20 mA cm−2 at 1.86 V),56 and was comparable to the NiFe LDH system (20 mA cm−2 at 1.78 V).6 In sharp contrast, the CoMoO4 || CoMoO4 system required a high voltage of > 2.0 V to achieve 20 mA cm−2. The durability of the electrolyzer was tested at 1.70 V in 1 M KOH at room temperature. Both CoO/MoOx || CoO/MoOx and CoMoO4 || CoMoO4 systems showed stable performance with negligible current degradation during the 10 h test (Figure 5B). The robust long-term stability might be derived from the MoOx matrix which



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protected the CoO from oxidation. Also, we compared the electrocatalytic performances of various materials applied in two-electrode systems (Table 1). All the date is original without considering the capacitance current. Although the CoO/MoOx is not among the best electrocatalysts for overall water splitting, its performance is comparable to those of NiFe LDH and NiCo2S4. More importantly, this sheds light on the rational design of bifunctional electrocatalysts for overall water splitting. Conclusions CoO/MoOx electrocatalyst, featured with crystalline CoO embedded in amorphous MoOx, displays highly enhanced HER and OER performances with small overpotentials of 40 and 230 mV in 1.0 M KOH, respectively, and with a stable performance for overall water splitting. The high performance is likely benefited from fast charge transfer kinetics on the interface between CoO and the amorphous MoOx in the CoO/MoOx. This study enriches our knowledge and material choice in catalysts towards affordable water electrolysis. ASSOCIATED CONTENT Supporting Information Figures S1 to S4 are included. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT X. C. acknowledges the support from College of Arts and Sciences, University of Missouri Kansas City. X. Y. thanks the funds provided by the University of Missouri-Kansas City, School of Graduate Studies. L. T. and Y. L. appreciates the National Natural Science Foundation of China (No. 51302072) and China Scholarship Council for their financial supports. REFERENCES [1] Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 331 (2011) 746–750. [2] Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 110 (2010) 6503−6570. [3] Chen, X.; Liu, L.; Huang, F. Black titanium dioxide (TiO2) nanomaterials. Chem. Soc. Rev. 44 (2015) 1861−1885. [4] Wang, H.; Tsai, C.; Kong, D.; Chan, K.; Abild-Pedersen, F.; Nørskov, J. K.; Cui, Y. Transition-metal doped edge sites in vertically aligned MoS2 catalysts for enhanced hydrogen evolution. Nano Research 8 (2015) 566−575. [5] Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 nanoparticles grown on carbon fiber paper: an efficient and stable electrocatalyst for hydrogen evolution reaction. J. Am. Chem. Soc. 136 (2014) 4897−4900.



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[57] Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. NiSe nanowire film supported on nickel foam: an efficient and stable 3d bifunctional electrode for full water splitting. Angew. Chem. Int. Edit. 54 (2015) 9351−9355. [58] Peng, Z.; Jia, D.; Al-Enizi, A. M.; Elzatahry, A. A.; Zheng, G. From water oxidation to reduction:

homologous

Ni–Co

based

nanowires

as

complementary

water

splitting

electrocatalysts. Adv. Energy Mater. 5 (2015) 1402031. [59] Jiang, N.; You, B.; Sheng, M.; Sun, Y. electrodeposited cobalt-phosphorous-derived films as competent bifunctional catalysts for overall water splitting. Angew. Chem. Int. Ed. 54 (2015) 6251–6254. [60] You, B.; Sun, Y. Hierarchically porous nickel sulfide multifunctional superstructures. Adv. Energy Mater. 6 (2016) 1502333.



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Figure Captions Figure 1. (A) XRD patterns, (B) Co 2p, (C) Mo 3d, and (D) O 1s core-level XPS spectra of (a) CoMoO4 and (b) CoO/MoOx. Figure 2 SEM of CoMoO4 (A) and CoO/MoOx (B), TEM images of CoMoO4 (C) and CoO/MoOx (D). Figure 3 (A) HER polarization curves of various electrodes at a scan rate of 5 mV s−1 in 1.0 M KOH. (B) Tafel plots derived from Figure 3A. (C) Chronopotentiometry curve of CoO/MoOx without iR-compensation. (D) Nyquist plots of CoMoO4 and CoO/MoOx obtained at a potential of −0.2 V versus RHE. Figure 4 (A) OER polarization curves of various electrodes at a scan rate of 5 mV s−1 in 1.0 M KOH. (B) Current-time characteristics of CoO/MoOx at overpotential of 350 mV without iRcompensation. Figure 5 Overall water-splitting characteristics of CoO/MoOx || CoO/MoOx and CoMoO4 || CoMoO4 in two-electrode configurations in 1 M KOH. All the data are without iR compensation. (A) Polarization curves at a scan rate of 5 mV s−1. (B) Current-time characteristics at a voltage of 1.70 V.


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(b) CoO PDF#48-1719 (a)

Intensity / a.u.

Intensity / a.u.

B

Co 2p

A

781.0 796.7

(b) Sat.

Sat.

(a)

CoMoO4 PDF#21-0868

20

30

40

50

60

805

70

800

Mo 3d

790

785

O 1s Intensity / a.u.

C 232.4 230.0

235.5

(b)

(a)

238

795

780

775

Binding Energy / eV

2 Theta / degree

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D 531.0 530.1

(b)

(a)

236

234

232

230

228

Binding Energy / eV

226

534

532

530

528

Binding Energy / eV

Figure 1. (A) XRD patterns, (B) Co 2p, (C) Mo 3d, and (D) O 1s core-level XPS spectra of (a) CoMoO4 and (b) CoO/MoOx.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. SEM of CoMoO4 (A) and CoO/MoOx (B), TEM images of CoMoO4 (C) and CoO/MoOx (D).


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0

0.4

A

B

-10

c /de mV 61

CoMoO4 CoO/MoOx Pt

-20

η/V

j / mA cm

-2

0.3

-30 CoMoO4 CoO/MoOx Pt NiO Ni

-40 -50 -0.4

-0.3

-0.2

-0.1

0.0

0.2 c /de mV 44

0.1 V 28 m

0.0 0.0

0.1

/dec

0.5

1.0

1.5

-2

E / V vs. RHE

Log j / mA cm

-20

40

C

D −Z'' / Ω

-2

30

j / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-10

20

CoMoO4 CoO/MoOx

10

0

0

1000

2000

3000

4000

5000

6000

0

0

20

Time / s

40

60

80

Z' / Ω

Figure 3 (A) HER polarization curves of various electrodes at a scan rate of 5 mV s−1 in 1.0 M KOH. (B) Tafel plots derived from Figure 3A. (C) Chronopotentiometry curve of CoO/MoOx without iR-compensation. (D) Nyquist plots of CoMoO4 and CoO/MoOx obtained at a potential of −0.2 V versus RHE.



40

B

80

A

30

-2

j / mA cm

-2

60

j / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ni CoMoO4 CoO/MoOx IrO2 NiO

40

20

0 1.1

20

10

0 1.2

1.3

1.4

1.5

1.6

1.7

0

1000

2000

3000

4000

5000

6000

Time / s

E / V vs. RHE

Figure 4. (A) OER polarization curves of various electrodes at a scan rate of 5 mV s−1 in 1.0 M KOH. (B) Current-time characteristics of CoO/MoOx at overpotential of 350 mV without iRcompensation.


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60

10

B

8

CoO/MoOx

20

O Mo o C

0 1.2

j / mA cm

-2

40 Co O/ Mo Ox

-2

A j / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.4

1.6

1.8

4

6 4 CoMoO4

2

2.0

0

0

2

Voltage / V

4

6

8

10

Time / h

Figure 5. Overall water-splitting characteristics of CoO/MoOx || CoO/MoOx and CoMoO4 || CoMoO4 in two-electrode configurations in 1 M KOH. All the data are without iR compensation. (A) Polarization curves at a scan rate of 5 mV s−1. (B) Current-time characteristics at a voltage of 1.70 V.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1 Comparison of electrocatalytic performances of various materials in two-electrode systems. Sample

Electrolyte

V10 (V)

Ref.

NiFe LDH || NiFe LDH

1 M NaOH

~1.70

[6]

Ni2P || Ni2P

1 M KOH

~1.63

[9]

NiFeOx || NiFeOx

1 M KOH

1.51

[21]

CoOx/N-C || CoOx/N-C

1 M KOH

~1.30

[27]

NiCo2S4 || NiCo2S4

1 M KOH

~1.68

[56]

NiSe/Ni || NiSe/Ni

1 M KOH

~1.63

[57]

NiCo2O4 || Ni0.33Co0.67S2

1 M KOH

~1.65

[58]

Co-P || Co-P

1 M KOH

~1.64

[59]

NiSx || NiSx

1 M KOH

~1.45

[60]

CoMoO4 || CoMoO4

1 M KOH

1.92

This work

CoO/MoOx || CoO/MoOx

1 M KOH

1.72

This work

V20 reprrsents the potential at an HER current density of 10 mA cm−2.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Converting CoMoO4 into CoO/MoOx for overall water splitting by hydrogenation Xiaodong Yan, Lihong Tian, Samuel Atkins, Yan Liu, James Murowchick and Xiaobo Chen

TOC

Efficient overall water splitting has been achieved with nanorods-on-nanosheets CoO/MoOx catalyst in alkaline conditions starting at 1.58 V.


MoOx for Overall Water Splitting by

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