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3D graphene foam-supported cobalt phosphate and borate electrocatalysts for high-efficiency water oxidation
Sci. Bull. (2015) 60(16):1426–1433 DOI 10.1007/s11434-015-0861-5
www.scibull.com www.springer.com/scp
Article
Materials Science
3D graphene foam-supported cobalt phosphate and borate electrocatalysts for high-efficiency water oxidation Min Zeng • Hao Wang • Chong Zhao • Jiake Wei • Wenlong Wang • Xuedong Bai
Received: 18 April 2015 / Accepted: 22 July 2015 / Published online: 11 August 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract The cobalt phosphate-/cobalt borate-based oxygen-evolving catalysts (OECs) are the important class of earth-abundant electrocatalysts that can operate with high activity for water splitting under benign conditions. This article reports the integration of cobalt phosphate (CoPi) and cobalt borate (Co-Bi) OECs with three-dimensional (3D) graphene foam (GF) for the electrocatalytic water oxidation reaction. The GF showed a unique advantage to serve as a highly conductive 3D support with large capacity for anchoring and loading Co-OECs, thereby facilitating mass and charge transfer due to the large amount of active sites provided by the 3D graphene scaffold. As a result, this integrated system of GF and Co-OECs exhibits synergistically enhanced catalytic activity. The overpotential (g) of Co-Pi and Co-Bi/graphene catalysts is about 0.390 and 0.315 V in neutral solutions, respectively. Besides, the integrated Co-OECs/graphene catalysts have also exhibited improved and stable oxygen evolution catalytic ability in alkaline solution. Keywords Water splitting Oxygen evolution catalyst Cobalt phosphate and borate Graphene foam
M. Zeng H. Wang C. Zhao J. Wei W. Wang (&) X. Bai (&) Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China e-mail: [email protected] X. Bai e-mail: [email protected]
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1 Introduction The use of earth-abundant materials for electrocatalytic and/or photocatalytic splitting of water for hydrogen production is one of the most prevailing ways for meeting the future need for environmentally friendly and renewable energy sources [1–3]. Water-splitting process consists of the two H2O/O2 and H2O/H2 half-cell reactions. The water oxidation half reaction is considerably more complex because it requires the removal of four protons and four electrons to form a relatively weak oxygen–oxygen bond [4–6]. Therefore, a key determinant of high-efficiency water splitting is the searching of robust and high-activity oxygen evolution catalysts (OECs) that can oxidize water with low kinetic overpotential (g) [7, 8]. In recent years, the cobalt phosphate/cobalt borate (Co-Pi/Co-Bi)-based OECs have emerged as an important class of earth-abundant electrocatalysts that can operate with high activity for water splitting under benign conditions [9, 10]. The Co-Pi- and Co-Bi-based OECs can be typically formed as thin films on conducting surfaces when aqueous solutions of Co2? salts are electrolyzed in the presence of phosphate or borate [9, 10]. During the deposition process, Co2? is a high-spin ion and soluble, whereas Co3? and higher oxidation states are low spin and substitutionally inert in an oxygen-atom ligand field [11]. The mechanisms for the high oxygen evolution reaction (OER) efficiency suggest the involvement of Co2?, Co3?, and likely Co4? oxidation states in the redox cycles of O2/H2O at cobalt centers [12]. Besides multivalent states of cobalt ions, Pi and Bi electrolytes are the efficient proton-accepting electrolytes which enable the formation of the catalyst with sustained activity and functional stability [10]. The high activity of Co-based molecular catalysts on the inert substrates (indium tin oxide (ITO)-coated glass, or
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fluorine-doped tin oxide (FTO)-coated glass) at neutral pH has been demonstrated. However, the practical utilization of these molecular catalysts requires their grafting with the retention of the catalytic activity onto an ideal electrode material. To optimize the performance, an electrode with high surface area, resistance to corrosion, and high conductivity is an alternative approach to improving the activity. Graphene is such a good electrode material for its extraordinary electronic properties, unusual mechanical strength, and ultra-large specific surface area [13, 14]. Compared with other materials, graphene represents an excellent geometrical support for catalysts with a large open surface area. In addition, the synergistic effects between graphene and other functional nanomaterials lead to vast unprecedented possibilities [15, 16]. Moreover, graphene also possesses a rich surface chemistry and has the potential to further promote the catalytic activity and stability of the supported catalyst systems through cation–p interactions or p–p stacking [17]. Those unique properties make graphene a promising material for energy conversion and storage applications. However, these excellent properties are relevant at the nanoscale to harness these properties for the applications at the macro-size. So the macroscopic three-dimensional (3D) architectures integrated with two-dimensional (2D) nanoscale graphene components will be an ideal electrode substrate for the further applications. Here, the 3D graphene foams (GFs) grown through chemical vapor deposition (CVD) are seamlessly continuous and have the highly conductive graphene network [18, 19]. This 3D porous structure is ideal to serve as the scaffold for the fabrication of monolithic composite electrodes. The porous, interconnected network is beneficial to ion diffusion and transfer kinetics and provides a special reaction microenvironment and multiplexed conductive pathway for rapid charge transfer and conduction. Furthermore, the GF is a mechanically robust and flexible macroscale network and easy to contact [17]. This article uses this macro GF-like 3D network as the substrate to load Co-OEC catalyst. Compared with conventional FTO substrate, Co-Pi/GF and Co-Bi/GF composite electrode has worked well for OER with a smaller g and higher catalytic ability.
2 Materials and methods 2.1 Materials Co(NO3)2 (99.999 %), KH2PO4 (99 %), K2HPO4 (98 %), H3BO3 (99.5 %), and KOH (85 %) were purchased from Aldrich. All electrolyte solutions were prepared with
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deionized water (resistivity, 18 MX cm). FTO-coated glass was purchased from Aldrich, and all experiments used FTO with 15 X/sq surface resistivity.
2.2 Synthesis of 3D GF Nickel forms (*320 g/m2 in areal density, 10 mm 9 12 mm in size and *1.6 mm in thickness, Alantum Advanced Technology Materials) were used as 3D scaffold templates for the CVD growth of GF. They were placed in a quartz tube furnace, heated to 1,050 °C at a heating rate of 20 °C/min, and then annealed for 30 min under H2 (1,000 sccm, sccm: standard-state cubic centimeter per minute) and Ar (1,000 sccm) to clean the surfaces and eliminate the thin surface oxide layer. After annealing, a small amount of CH4 (15 sccm) was then introduced into the reaction tube at ambient pressure for the growth process. After 30 min, the sample was slowly cooled to 500 °C at a cooling rate of 5 °C/min under H2/Ar flow and then cooled to the room temperature naturally. Finally, the nickel substrate was etched away with FeCl3 (3 mol/L) solution to leave the freestanding 3D GFs [18]. The specific surface area of 3D GF is about 800 cm2/g. 2.3 Preparation of GF and Co-Pi/Bi composite electrodes Bulk electrolyses were performed in a two-compartment electrochemical cell with a glass frit junction of fine porosity. For catalyst electrodeposition, the auxiliary side held 40 mL of 0.1 mol/L potassium phosphate (KPi, pH 7.0) or 0.1 mol/L potassium borate (KBi, pH 9.2) electrolyte and the working side held 40 mL of the same electrolyte containing 0.5 mmol/L Co2?. Cobalt solutions were prepared fresh for each experiment. Subsequently, the 1 cm 9 1.2 cm 3D GFs fixed on a glass slide. Typically, 1 cm 9 1 cm was immersed into the solution. 1 cm 9 2 cm Pt sheet was used as the auxiliary electrode. Electrolysis was carried out at 1.3 V for 8 h without stirring and internal resistance (iR) compensation, and the reference electrode placed a few millimeters from the GF surface. Before the deposition, the 3D GF was treated with oxygen or H2O plasma to realize the hydrophilic surface. A 1.0 cm2 catalyst was prepared in an electrodeposition with the deposition time of 5.6 h (Co-Pi/GF) and 8 h (Co-Bi/ GF) and then transferred to 0.1 mol/L KPi or 0.1 mol/L KBi electrolyte without Co2? to carry on other electrochemical experiments. Co-Pi and Co-Bi were deposited on FTO in the same way as GF for substrate. We estimate the loading of the active materials through a rough weigh, and it is in the range of 1–2 mg/cm2 for Co-Pi/GF (5.6 h), CoBi/GF (8 h), and Co-Pi/FTO (8 h).
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2.4 Characterization
2.6 Tafel plot data collection
Raman spectra were recorded at ambient temperature on a Horiba Jobin–Yvon LabRAM HR-800 Raman microscope (k = 532 nm, power = 1 mW). The microstructure and compositional analysis were studied by the Hitachi S4800 scanning electron microscope (SEM) instrument with energy-dispersive X-ray spectrum (EDX) facilities. After the electrodeposition of OECs, catalyst samples were rinsed gently with deionized water and dried in air before loading into the instrument. Images were obtained with an acceleration voltage of 5 or 10 kV, and EDX spectra were obtained with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) imaging measurements were taken at 200 kV in a JEOL-2010F fieldemission-type high-resolution TEM.
The current–voltage behavior of the catalyst in the region of water oxidation was measured in 10–20 mV increments between 1.25 and 1.1 V for Co-Pi/graphene foam and 1.05–0.90 V for Co-Bi/GF. Electrolysis was conducted at each potential in Co-free Pi or Bi electrolyte until the current density (ip) reached a steady-state value in 3–5 min. The measurements were taken twice and averaged, and the variation in steady-state current between two runs at a particular potential was \5 %. The solution resistance measured prior to the data collection was used to correct the Tafel plot for iR drop. The potential values were converted to g by correcting for ohmic potential losses and subtracting the thermodynamic potential for water oxidation under the experimental conditions (g = Vappl - iR - EpH, where Vappl is the applied potential and EpH = 1.229 - 0.059 9 pH).
2.5 Electrochemical methods All electrochemical experiments were performed with Autolab PGSTAT 128N Instrument. The 3D GF and Co-Pi composite serves as the working electrode, while Pt foil and Ag/AgCl electrodes (with saturated KCl as electrolyte) were used as the counter electrode and the reference electrode, respectively. Unless otherwise stated, the electrolyte was 0.1 mol/L KPi electrolyte (pH 7.0) for Co-Pi electrode, while 0.1 mol/L KBi electrolyte (pH 9.2) for CoBi. All potentials reported in this manuscript were converted to the normal hydrogen electrode (NHE) reference scale using ENHE = EAg/AgCl ? 0.197 V. Electrochemical impedance spectroscopy (EIS) measurements were taken in the frequency range from 0.01 to 100 kHz at open-circuit potential with an alternating current (AC) perturbation of 5.0 mV.
3 Results and discussion A two-step process was used to prepare the Co-Pi/GF hybrid structure (Fig. 1). First, the 3D GF was synthesized by CVD. In this process, nickel foam was used as an interconnected 3D template to growth of GF. In brief, CH4 was used as the carbon source at 1,000 °C under atmospheric pressure, and GF was then precipitated on the surface of the nickel foam [18]. To obtain the free-standing GF, the nickel skeleton was etched by 3 mol/L FeCl3 solution and washed by abundant deionized water and alcohol. Then, Co-Pi was deposited on the GF in situ through electrochemical method. Here, 1.3 V versus the NHE was applied for 8 h in 0.1 mol/L KPi electrolyte
Fig. 1 (Color online) Schematic of the preparation process and the working model of Co-Pi/GF electrode
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containing 0.5 mmol/L Co2? during the electrolysis. Finally, this Co-Pi/graphene composite electrode was obtained for the further experiments. The morphology of GF was examined by SEM, which was the continuous and interconnected graphene network (Fig. 2a). The free-standing GF picture is shown in the inset of Fig. 2a. Ripples and wrinkles were formed on the GFs for the different thermal expansion coefficients between nickel and graphene (Fig. 2b). TEM images presented the separated graphene sheets from 3D network (Fig. 2c). It is clear that the graphene sheet exhibits different layers (up to 5 layers). To identify whether we get the high-quality GF or not, the GF was further investigated by Raman spectra (Fig. 2d). Two characteristic peaks at *1,580 cm-1 (G-band) and *2,700 cm-1 (2D-band) are observed. The intensity ratio of 2D-band to G-band (I2D/IG) is related to the number of layers of graphene (\5 layers) [20, 21], which is consistent with the result of TEM. Extremely weak D-band (*1,350 cm-1) in Fig. 2d indicated that the GF obtained here is of high quality with few defects. The high-quality GF here provides a good platform for the further experiments. During the electrochemical deposition of Co-Pi, the deposition current of GF is about five times as high as FTO at the same deposition potential (E = 1.3 V), which demonstrates the advantages of high surface area and electrical conductivity of GF as the substrate (Fig. 2e). To study the growth progress of Co-Pi, the SEM images of the electrode surface taken at indicated time I (2.8 h) and II (5.6 h) are shown in Fig. 2f, g, respectively. During the deposition of Co-Pi, there are two main stages: nucleation and germination. In the stage of nucleation, Co-Pi was nucleated at single dot and then grown along the wrinkles or edges of graphene (Fig. 2f), while in the stage of germination, enough Co-Pi catalyst particles coalesced into a thin film to cover the surface of GF (Fig. 2g), and the graphene substrate could be seen from the crack of Co-Pi. The thickness of the Co-Pi on graphene increased with the deposition time. We could estimate that the thickness of Co-Pi deposited on graphene was in the range of several hundred nanometers (\1 lm) at deposition time of 5.6 h. The elemental compositions of Co-Pi/GF were examined by using EDX spectra. Co, P, K, and O were identified as the principal elemental components of Co-Pi, and C is also detected for the graphene substrate (Fig. 2g). Here, the TEM image showed that the amorphous structure of Co-Pi was grown on GF, and the corresponding electron diffraction pattern exhibited only diffuse Debye rings (Fig. 2h). Together, all the results in Fig. 2 demonstrate that we have made a high-quality GF and Co-Pi has been successfully deposited on this 3D substrate. After the deposition of Co-Pi, the oxygen evolution ability was evaluated through polarization curve in 0.1 mol/L KPi electrolyte (pH 7.0) (Fig. 3a). The ip of CoPi/GF at E = 1.5 V (vs NHE) is about five times as high as
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that of Co-Pi/FTO, which indicates that the oxygen evolution ability of Co-Pi/GF is higher than of Co-Pi/FTO. To further examine the OER ability between these two substrates, the current densities of Co-Pi/GF and Co-Pi/FTO were measured as a function of the g (Fig. 3b). From this Tafel plot, a ip of 1 mA/cm2 requires g = 0.390 V for CoPi/GF electrode, while g = 0.415 V for Co-Pi/FTO electrode. So the g of Co-Pi/GF electrode shifts 25 mV lower than that of Co-Pi/FTO electrode. Besides, the slope of Tafel plot for Co-Pi/GF electrode is 68 mV/dec (dec: decade) which is smaller than 74 mV/dec for Co-Pi/FTO. Here, smaller Tafel slope resulted from faster electron transport for Co-Pi/GF. All the results indicate that the OER activity of Co-Pi/GF is higher. Moreover, EIS was employed to investigate fundamental electrochemical processes for both Co-Pi/GF and Co-Pi/ FTO electrodes (Fig. 3c). The equivalent series resistance (ESR) of the Co-Pi/GF composite electrode is about 12.0 X, which is smaller than that of Co-Pi/FTO electrode (118.0 X). Smaller ESR indicated faster charge transfer at interface of Co-Pi/GF. This further suggests that the Co-Pi/ GF electrode is more advantageous for OER. To optimize the performance of Co-Pi/GF, we investigated the catalytic efficiency dependence on catalyst loading. Figure 3d shows that the go2 versus deposition time exhibits an initial decrease then rise. And the ip (the current at E = 1.5 V in cyclic voltammetry (CV) curves) increases with the time firstly and then decreases. All these results indicated that the oxygen evolution activity will be highest at a moderate deposition time. During the early stage for the growth process of Co-Pi, the catalyst particles are nucleated and grow as the activated sites for OER, so the catalytic ability will increase with time. When the CoPi catalyst particles coalesce into a thin film to cover the whole surface of GF, the catalytic ability will increase to the maximum. If the electrochemical deposition time is longer, the Co-Pi catalyst particles will also grow to the limiting thickness and the activity will decrease with time for the poor conductivity of catalyst film. That is the reason why the OER activity first increases and then declines with the deposition time. Besides Co-Pi, Co-Bi was also deposited on GF through an electrolysis at 1.3 V (vs NHE) in KBi electrolyte (pH 9.2) containing 0.5 mmol/L Co2?. It exhibits a rising ip that reaches a high value [20 mA/cm2 after 8 h during the deposition (Fig. 4a). The SEM image (the inset in Fig. 4a) shows that the Co-Bi catalyst film covered on the GF is consisted of smaller particles compared with Co-Pi. As seen in the Talfel plot, the g at 1 mA/cm2 is 315 mV and the Tafel slope is 59 mV/dec which corresponds to 2.3RT/ F (Fig. 4b). This Tafel slope is characteristic of an O2 evolution mechanism involving a reversible one-electron transfer prior to a chemical turnover-limiting step [22]. The
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Fig. 2 (Color online) a SEM image of GF; the inset is the picture of GF; b higher magnification SEM image of GF; c TEM image of graphene sheet separated from the GF; d Raman spectra of GF; e i-t curves of Co-Pi/GF and Co-Pi/FTO electrode at E = 1.3 V (vs NHE); f, g SEM image of Co-Pi/GF at different deposition time for 2.8 h and 5.6 h which is shown in (e) as I and II, respectively; the illustration in (g) is the typical EDX histogram; h TEM image of Co-Pi separated from Co-Pi/GF; the inset is its corresponding electron diffraction pattern, showing the Co-Pi is amorphous
CV curve of Co-Bi/GF in 0.1 mol/L KBi showed a higher OER catalytic ability (the inset in Fig. 4b). The ip at E = 1.5 V is about 40 mA/cm2 which much higher than
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10 mA/cm2 for Co-Pi/GF in 0.1 mol/L KPi (Fig. 3a). Together, all the results in Fig. 4 demonstrate that Co-Bi/ GF has excellent OER catalytic ability. Besides, the Co-Pi/
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Fig. 3 (Color online) a Polarization curve of Co-Pi/GF and Co-Pi/FTO in 0.1 mol/L KPi from 0 to 1.5 V with the scan rate 5 mV/s; b Tafel plot for Co-Pi/GF and Co-Pi/FTO in 0.1 mol/L KPi electrolyte (pH 7.0), corrected for the iR drop of the solution; c Nyquist plots of Co-Pi/GF and Co-Pi/FTO electrodes at open-circuit potential; d ip at E = 1.5 V (vs NHE) in cyclic voltammetry curves and anodic overpotential go2 dependence on deposition time in 0.1 mol/L KPi electrolyte for Co-Pi/GF
GF and Co-Bi/GF have excellent OER catalytic ability in 0.1 mol/L KOH. The high specific surface area, high conductivity, and mechanically robust and flexible properties make GF as an advantageous platform for OER. Co-Pi/GF displayed better OER performance for both GF and Co-Pi/FTO, which demonstrated the synergistic interaction between Co-Pi and GF (Fig. 5a). Besides, Co-Pi/GF electrode kept the high stability for 8 h at 0.1 mol/L KOH electrolyte (Fig. 5b), and the activity of Co-Bi/GF decreased a little. However,
both Co-Pi/GF and Co-Bi/GF showed better performance than Co-Pi/FTO. 3D GF exhibits large specific surface area, extremely high electrical conductivity and low inter-sheet junction contact resistance, which is an ideal substrate for loading catalyst. A working model of the Co-Pi/GF electrode is shown in the bottom left corner of Fig. 1. The Co2? is oxidized to Co3? and then is deposited on the electrode in the presence of HPO42-. The Co4?-oxo is produced from the latter proton-coupled electron transfer (PCET) process,
Fig. 4 (Color online) a i-t curve of Co-Bi/GF electrode at E = 1.3 V (vs NHE); the inset is SEM of Co-Bi/GF; b Tafel plot for Co-Bi/GF in 0.1 mol/L KBi electrolyte (pH 9.2), corrected for the iR drop of the solution; the inset is CV curve of Co-Bi/GF in 0.1 mol/L KBi
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Fig. 5 (Color online) a CV curves of GF, Co-Pi/FTO, Co-Pi/GF in 0.1 mol/L KOH from 0.2 to 1.4 V; b stability test for Co-Pi/FTO, Co-Pi/GF, and Co-Bi/GF in 0.1 mol/L KOH at E = 1.0 V for 8 h
from which O2 is produced and cobalt is returned to the 2? oxidation state [23]. The mechanism for oxygen evolution at molecular scale is not very clear for this special amorphous catalyst with multivalent states, which may cycle through heterogeneous and homogeneous phase. The special network structure and high conductivity of 3D graphene are benefit for fast electron and mass transport, which is favor of the PCET process and the complex equilibria processes between different valent states for cobalt. Besides, the cation–p interactions between Co2? and graphene improve the stability of Co-Pi during turnover. Co-Pi, Co-Bi and graphene contact well at the interface and the contact resistance cut down. The synergistic integration between the Co-Pi, Co-Bi and GF substrate has improved the catalytic ability for OER.
4 Conclusions In this study, we demonstrate a facile two-step synthesis (CVD growth of GF and in situ electrodepositions of Co-Pi and Co-Bi) to prepare Co-Pi/GF and Co-Bi/GF. As demonstrated here, GF is uniquely advantageous to serve as a 3D support of large capacity to anchor Co-OEC with well-defined size and shapes. Co-OECs/GF has shown improved performance for splitting water into oxygen for the synergistic integration between these two novel nanomaterials. The overpotential of Co-Pi/GF and Co-Bi/GF is about 0.390 and 0.315 V in benign conditions, respectively, which is lower than that of Co-OECs deposited on the conventional FTO substrate. Besides, Co-Pi/GF and Co-Bi/GF electrodes also have excellent and stable OER catalytic ability in alkaline solution. The successful integration of Co-OECs with 3D GF for the high-efficiency water oxidation described in our present work may afford a pathway toward the practical implementation of low-cost and technologically competitive water-splitting systems for sustainable hydrogen production.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21322304, 11290161) and the National Basic Research Program of China (2012CB933003, 2013CB932603). Conflict of interest of interest.
The authors declare that they have no conflict
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www.scibull.com www.springer.com/scp
Article
Materials Science
3D graphene foam-supported cobalt phosphate and borate electrocatalysts for high-efficiency water oxidation Min Zeng • Hao Wang • Chong Zhao • Jiake Wei • Wenlong Wang • Xuedong Bai
Received: 18 April 2015 / Accepted: 22 July 2015 / Published online: 11 August 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract The cobalt phosphate-/cobalt borate-based oxygen-evolving catalysts (OECs) are the important class of earth-abundant electrocatalysts that can operate with high activity for water splitting under benign conditions. This article reports the integration of cobalt phosphate (CoPi) and cobalt borate (Co-Bi) OECs with three-dimensional (3D) graphene foam (GF) for the electrocatalytic water oxidation reaction. The GF showed a unique advantage to serve as a highly conductive 3D support with large capacity for anchoring and loading Co-OECs, thereby facilitating mass and charge transfer due to the large amount of active sites provided by the 3D graphene scaffold. As a result, this integrated system of GF and Co-OECs exhibits synergistically enhanced catalytic activity. The overpotential (g) of Co-Pi and Co-Bi/graphene catalysts is about 0.390 and 0.315 V in neutral solutions, respectively. Besides, the integrated Co-OECs/graphene catalysts have also exhibited improved and stable oxygen evolution catalytic ability in alkaline solution. Keywords Water splitting Oxygen evolution catalyst Cobalt phosphate and borate Graphene foam
M. Zeng H. Wang C. Zhao J. Wei W. Wang (&) X. Bai (&) Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China e-mail: [email protected] X. Bai e-mail: [email protected]
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1 Introduction The use of earth-abundant materials for electrocatalytic and/or photocatalytic splitting of water for hydrogen production is one of the most prevailing ways for meeting the future need for environmentally friendly and renewable energy sources [1–3]. Water-splitting process consists of the two H2O/O2 and H2O/H2 half-cell reactions. The water oxidation half reaction is considerably more complex because it requires the removal of four protons and four electrons to form a relatively weak oxygen–oxygen bond [4–6]. Therefore, a key determinant of high-efficiency water splitting is the searching of robust and high-activity oxygen evolution catalysts (OECs) that can oxidize water with low kinetic overpotential (g) [7, 8]. In recent years, the cobalt phosphate/cobalt borate (Co-Pi/Co-Bi)-based OECs have emerged as an important class of earth-abundant electrocatalysts that can operate with high activity for water splitting under benign conditions [9, 10]. The Co-Pi- and Co-Bi-based OECs can be typically formed as thin films on conducting surfaces when aqueous solutions of Co2? salts are electrolyzed in the presence of phosphate or borate [9, 10]. During the deposition process, Co2? is a high-spin ion and soluble, whereas Co3? and higher oxidation states are low spin and substitutionally inert in an oxygen-atom ligand field [11]. The mechanisms for the high oxygen evolution reaction (OER) efficiency suggest the involvement of Co2?, Co3?, and likely Co4? oxidation states in the redox cycles of O2/H2O at cobalt centers [12]. Besides multivalent states of cobalt ions, Pi and Bi electrolytes are the efficient proton-accepting electrolytes which enable the formation of the catalyst with sustained activity and functional stability [10]. The high activity of Co-based molecular catalysts on the inert substrates (indium tin oxide (ITO)-coated glass, or
Sci. Bull. (2015) 60(16):1426–1433
fluorine-doped tin oxide (FTO)-coated glass) at neutral pH has been demonstrated. However, the practical utilization of these molecular catalysts requires their grafting with the retention of the catalytic activity onto an ideal electrode material. To optimize the performance, an electrode with high surface area, resistance to corrosion, and high conductivity is an alternative approach to improving the activity. Graphene is such a good electrode material for its extraordinary electronic properties, unusual mechanical strength, and ultra-large specific surface area [13, 14]. Compared with other materials, graphene represents an excellent geometrical support for catalysts with a large open surface area. In addition, the synergistic effects between graphene and other functional nanomaterials lead to vast unprecedented possibilities [15, 16]. Moreover, graphene also possesses a rich surface chemistry and has the potential to further promote the catalytic activity and stability of the supported catalyst systems through cation–p interactions or p–p stacking [17]. Those unique properties make graphene a promising material for energy conversion and storage applications. However, these excellent properties are relevant at the nanoscale to harness these properties for the applications at the macro-size. So the macroscopic three-dimensional (3D) architectures integrated with two-dimensional (2D) nanoscale graphene components will be an ideal electrode substrate for the further applications. Here, the 3D graphene foams (GFs) grown through chemical vapor deposition (CVD) are seamlessly continuous and have the highly conductive graphene network [18, 19]. This 3D porous structure is ideal to serve as the scaffold for the fabrication of monolithic composite electrodes. The porous, interconnected network is beneficial to ion diffusion and transfer kinetics and provides a special reaction microenvironment and multiplexed conductive pathway for rapid charge transfer and conduction. Furthermore, the GF is a mechanically robust and flexible macroscale network and easy to contact [17]. This article uses this macro GF-like 3D network as the substrate to load Co-OEC catalyst. Compared with conventional FTO substrate, Co-Pi/GF and Co-Bi/GF composite electrode has worked well for OER with a smaller g and higher catalytic ability.
2 Materials and methods 2.1 Materials Co(NO3)2 (99.999 %), KH2PO4 (99 %), K2HPO4 (98 %), H3BO3 (99.5 %), and KOH (85 %) were purchased from Aldrich. All electrolyte solutions were prepared with
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deionized water (resistivity, 18 MX cm). FTO-coated glass was purchased from Aldrich, and all experiments used FTO with 15 X/sq surface resistivity.
2.2 Synthesis of 3D GF Nickel forms (*320 g/m2 in areal density, 10 mm 9 12 mm in size and *1.6 mm in thickness, Alantum Advanced Technology Materials) were used as 3D scaffold templates for the CVD growth of GF. They were placed in a quartz tube furnace, heated to 1,050 °C at a heating rate of 20 °C/min, and then annealed for 30 min under H2 (1,000 sccm, sccm: standard-state cubic centimeter per minute) and Ar (1,000 sccm) to clean the surfaces and eliminate the thin surface oxide layer. After annealing, a small amount of CH4 (15 sccm) was then introduced into the reaction tube at ambient pressure for the growth process. After 30 min, the sample was slowly cooled to 500 °C at a cooling rate of 5 °C/min under H2/Ar flow and then cooled to the room temperature naturally. Finally, the nickel substrate was etched away with FeCl3 (3 mol/L) solution to leave the freestanding 3D GFs [18]. The specific surface area of 3D GF is about 800 cm2/g. 2.3 Preparation of GF and Co-Pi/Bi composite electrodes Bulk electrolyses were performed in a two-compartment electrochemical cell with a glass frit junction of fine porosity. For catalyst electrodeposition, the auxiliary side held 40 mL of 0.1 mol/L potassium phosphate (KPi, pH 7.0) or 0.1 mol/L potassium borate (KBi, pH 9.2) electrolyte and the working side held 40 mL of the same electrolyte containing 0.5 mmol/L Co2?. Cobalt solutions were prepared fresh for each experiment. Subsequently, the 1 cm 9 1.2 cm 3D GFs fixed on a glass slide. Typically, 1 cm 9 1 cm was immersed into the solution. 1 cm 9 2 cm Pt sheet was used as the auxiliary electrode. Electrolysis was carried out at 1.3 V for 8 h without stirring and internal resistance (iR) compensation, and the reference electrode placed a few millimeters from the GF surface. Before the deposition, the 3D GF was treated with oxygen or H2O plasma to realize the hydrophilic surface. A 1.0 cm2 catalyst was prepared in an electrodeposition with the deposition time of 5.6 h (Co-Pi/GF) and 8 h (Co-Bi/ GF) and then transferred to 0.1 mol/L KPi or 0.1 mol/L KBi electrolyte without Co2? to carry on other electrochemical experiments. Co-Pi and Co-Bi were deposited on FTO in the same way as GF for substrate. We estimate the loading of the active materials through a rough weigh, and it is in the range of 1–2 mg/cm2 for Co-Pi/GF (5.6 h), CoBi/GF (8 h), and Co-Pi/FTO (8 h).
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2.4 Characterization
2.6 Tafel plot data collection
Raman spectra were recorded at ambient temperature on a Horiba Jobin–Yvon LabRAM HR-800 Raman microscope (k = 532 nm, power = 1 mW). The microstructure and compositional analysis were studied by the Hitachi S4800 scanning electron microscope (SEM) instrument with energy-dispersive X-ray spectrum (EDX) facilities. After the electrodeposition of OECs, catalyst samples were rinsed gently with deionized water and dried in air before loading into the instrument. Images were obtained with an acceleration voltage of 5 or 10 kV, and EDX spectra were obtained with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) imaging measurements were taken at 200 kV in a JEOL-2010F fieldemission-type high-resolution TEM.
The current–voltage behavior of the catalyst in the region of water oxidation was measured in 10–20 mV increments between 1.25 and 1.1 V for Co-Pi/graphene foam and 1.05–0.90 V for Co-Bi/GF. Electrolysis was conducted at each potential in Co-free Pi or Bi electrolyte until the current density (ip) reached a steady-state value in 3–5 min. The measurements were taken twice and averaged, and the variation in steady-state current between two runs at a particular potential was \5 %. The solution resistance measured prior to the data collection was used to correct the Tafel plot for iR drop. The potential values were converted to g by correcting for ohmic potential losses and subtracting the thermodynamic potential for water oxidation under the experimental conditions (g = Vappl - iR - EpH, where Vappl is the applied potential and EpH = 1.229 - 0.059 9 pH).
2.5 Electrochemical methods All electrochemical experiments were performed with Autolab PGSTAT 128N Instrument. The 3D GF and Co-Pi composite serves as the working electrode, while Pt foil and Ag/AgCl electrodes (with saturated KCl as electrolyte) were used as the counter electrode and the reference electrode, respectively. Unless otherwise stated, the electrolyte was 0.1 mol/L KPi electrolyte (pH 7.0) for Co-Pi electrode, while 0.1 mol/L KBi electrolyte (pH 9.2) for CoBi. All potentials reported in this manuscript were converted to the normal hydrogen electrode (NHE) reference scale using ENHE = EAg/AgCl ? 0.197 V. Electrochemical impedance spectroscopy (EIS) measurements were taken in the frequency range from 0.01 to 100 kHz at open-circuit potential with an alternating current (AC) perturbation of 5.0 mV.
3 Results and discussion A two-step process was used to prepare the Co-Pi/GF hybrid structure (Fig. 1). First, the 3D GF was synthesized by CVD. In this process, nickel foam was used as an interconnected 3D template to growth of GF. In brief, CH4 was used as the carbon source at 1,000 °C under atmospheric pressure, and GF was then precipitated on the surface of the nickel foam [18]. To obtain the free-standing GF, the nickel skeleton was etched by 3 mol/L FeCl3 solution and washed by abundant deionized water and alcohol. Then, Co-Pi was deposited on the GF in situ through electrochemical method. Here, 1.3 V versus the NHE was applied for 8 h in 0.1 mol/L KPi electrolyte
Fig. 1 (Color online) Schematic of the preparation process and the working model of Co-Pi/GF electrode
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containing 0.5 mmol/L Co2? during the electrolysis. Finally, this Co-Pi/graphene composite electrode was obtained for the further experiments. The morphology of GF was examined by SEM, which was the continuous and interconnected graphene network (Fig. 2a). The free-standing GF picture is shown in the inset of Fig. 2a. Ripples and wrinkles were formed on the GFs for the different thermal expansion coefficients between nickel and graphene (Fig. 2b). TEM images presented the separated graphene sheets from 3D network (Fig. 2c). It is clear that the graphene sheet exhibits different layers (up to 5 layers). To identify whether we get the high-quality GF or not, the GF was further investigated by Raman spectra (Fig. 2d). Two characteristic peaks at *1,580 cm-1 (G-band) and *2,700 cm-1 (2D-band) are observed. The intensity ratio of 2D-band to G-band (I2D/IG) is related to the number of layers of graphene (\5 layers) [20, 21], which is consistent with the result of TEM. Extremely weak D-band (*1,350 cm-1) in Fig. 2d indicated that the GF obtained here is of high quality with few defects. The high-quality GF here provides a good platform for the further experiments. During the electrochemical deposition of Co-Pi, the deposition current of GF is about five times as high as FTO at the same deposition potential (E = 1.3 V), which demonstrates the advantages of high surface area and electrical conductivity of GF as the substrate (Fig. 2e). To study the growth progress of Co-Pi, the SEM images of the electrode surface taken at indicated time I (2.8 h) and II (5.6 h) are shown in Fig. 2f, g, respectively. During the deposition of Co-Pi, there are two main stages: nucleation and germination. In the stage of nucleation, Co-Pi was nucleated at single dot and then grown along the wrinkles or edges of graphene (Fig. 2f), while in the stage of germination, enough Co-Pi catalyst particles coalesced into a thin film to cover the surface of GF (Fig. 2g), and the graphene substrate could be seen from the crack of Co-Pi. The thickness of the Co-Pi on graphene increased with the deposition time. We could estimate that the thickness of Co-Pi deposited on graphene was in the range of several hundred nanometers (\1 lm) at deposition time of 5.6 h. The elemental compositions of Co-Pi/GF were examined by using EDX spectra. Co, P, K, and O were identified as the principal elemental components of Co-Pi, and C is also detected for the graphene substrate (Fig. 2g). Here, the TEM image showed that the amorphous structure of Co-Pi was grown on GF, and the corresponding electron diffraction pattern exhibited only diffuse Debye rings (Fig. 2h). Together, all the results in Fig. 2 demonstrate that we have made a high-quality GF and Co-Pi has been successfully deposited on this 3D substrate. After the deposition of Co-Pi, the oxygen evolution ability was evaluated through polarization curve in 0.1 mol/L KPi electrolyte (pH 7.0) (Fig. 3a). The ip of CoPi/GF at E = 1.5 V (vs NHE) is about five times as high as
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that of Co-Pi/FTO, which indicates that the oxygen evolution ability of Co-Pi/GF is higher than of Co-Pi/FTO. To further examine the OER ability between these two substrates, the current densities of Co-Pi/GF and Co-Pi/FTO were measured as a function of the g (Fig. 3b). From this Tafel plot, a ip of 1 mA/cm2 requires g = 0.390 V for CoPi/GF electrode, while g = 0.415 V for Co-Pi/FTO electrode. So the g of Co-Pi/GF electrode shifts 25 mV lower than that of Co-Pi/FTO electrode. Besides, the slope of Tafel plot for Co-Pi/GF electrode is 68 mV/dec (dec: decade) which is smaller than 74 mV/dec for Co-Pi/FTO. Here, smaller Tafel slope resulted from faster electron transport for Co-Pi/GF. All the results indicate that the OER activity of Co-Pi/GF is higher. Moreover, EIS was employed to investigate fundamental electrochemical processes for both Co-Pi/GF and Co-Pi/ FTO electrodes (Fig. 3c). The equivalent series resistance (ESR) of the Co-Pi/GF composite electrode is about 12.0 X, which is smaller than that of Co-Pi/FTO electrode (118.0 X). Smaller ESR indicated faster charge transfer at interface of Co-Pi/GF. This further suggests that the Co-Pi/ GF electrode is more advantageous for OER. To optimize the performance of Co-Pi/GF, we investigated the catalytic efficiency dependence on catalyst loading. Figure 3d shows that the go2 versus deposition time exhibits an initial decrease then rise. And the ip (the current at E = 1.5 V in cyclic voltammetry (CV) curves) increases with the time firstly and then decreases. All these results indicated that the oxygen evolution activity will be highest at a moderate deposition time. During the early stage for the growth process of Co-Pi, the catalyst particles are nucleated and grow as the activated sites for OER, so the catalytic ability will increase with time. When the CoPi catalyst particles coalesce into a thin film to cover the whole surface of GF, the catalytic ability will increase to the maximum. If the electrochemical deposition time is longer, the Co-Pi catalyst particles will also grow to the limiting thickness and the activity will decrease with time for the poor conductivity of catalyst film. That is the reason why the OER activity first increases and then declines with the deposition time. Besides Co-Pi, Co-Bi was also deposited on GF through an electrolysis at 1.3 V (vs NHE) in KBi electrolyte (pH 9.2) containing 0.5 mmol/L Co2?. It exhibits a rising ip that reaches a high value [20 mA/cm2 after 8 h during the deposition (Fig. 4a). The SEM image (the inset in Fig. 4a) shows that the Co-Bi catalyst film covered on the GF is consisted of smaller particles compared with Co-Pi. As seen in the Talfel plot, the g at 1 mA/cm2 is 315 mV and the Tafel slope is 59 mV/dec which corresponds to 2.3RT/ F (Fig. 4b). This Tafel slope is characteristic of an O2 evolution mechanism involving a reversible one-electron transfer prior to a chemical turnover-limiting step [22]. The
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Fig. 2 (Color online) a SEM image of GF; the inset is the picture of GF; b higher magnification SEM image of GF; c TEM image of graphene sheet separated from the GF; d Raman spectra of GF; e i-t curves of Co-Pi/GF and Co-Pi/FTO electrode at E = 1.3 V (vs NHE); f, g SEM image of Co-Pi/GF at different deposition time for 2.8 h and 5.6 h which is shown in (e) as I and II, respectively; the illustration in (g) is the typical EDX histogram; h TEM image of Co-Pi separated from Co-Pi/GF; the inset is its corresponding electron diffraction pattern, showing the Co-Pi is amorphous
CV curve of Co-Bi/GF in 0.1 mol/L KBi showed a higher OER catalytic ability (the inset in Fig. 4b). The ip at E = 1.5 V is about 40 mA/cm2 which much higher than
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10 mA/cm2 for Co-Pi/GF in 0.1 mol/L KPi (Fig. 3a). Together, all the results in Fig. 4 demonstrate that Co-Bi/ GF has excellent OER catalytic ability. Besides, the Co-Pi/
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Fig. 3 (Color online) a Polarization curve of Co-Pi/GF and Co-Pi/FTO in 0.1 mol/L KPi from 0 to 1.5 V with the scan rate 5 mV/s; b Tafel plot for Co-Pi/GF and Co-Pi/FTO in 0.1 mol/L KPi electrolyte (pH 7.0), corrected for the iR drop of the solution; c Nyquist plots of Co-Pi/GF and Co-Pi/FTO electrodes at open-circuit potential; d ip at E = 1.5 V (vs NHE) in cyclic voltammetry curves and anodic overpotential go2 dependence on deposition time in 0.1 mol/L KPi electrolyte for Co-Pi/GF
GF and Co-Bi/GF have excellent OER catalytic ability in 0.1 mol/L KOH. The high specific surface area, high conductivity, and mechanically robust and flexible properties make GF as an advantageous platform for OER. Co-Pi/GF displayed better OER performance for both GF and Co-Pi/FTO, which demonstrated the synergistic interaction between Co-Pi and GF (Fig. 5a). Besides, Co-Pi/GF electrode kept the high stability for 8 h at 0.1 mol/L KOH electrolyte (Fig. 5b), and the activity of Co-Bi/GF decreased a little. However,
both Co-Pi/GF and Co-Bi/GF showed better performance than Co-Pi/FTO. 3D GF exhibits large specific surface area, extremely high electrical conductivity and low inter-sheet junction contact resistance, which is an ideal substrate for loading catalyst. A working model of the Co-Pi/GF electrode is shown in the bottom left corner of Fig. 1. The Co2? is oxidized to Co3? and then is deposited on the electrode in the presence of HPO42-. The Co4?-oxo is produced from the latter proton-coupled electron transfer (PCET) process,
Fig. 4 (Color online) a i-t curve of Co-Bi/GF electrode at E = 1.3 V (vs NHE); the inset is SEM of Co-Bi/GF; b Tafel plot for Co-Bi/GF in 0.1 mol/L KBi electrolyte (pH 9.2), corrected for the iR drop of the solution; the inset is CV curve of Co-Bi/GF in 0.1 mol/L KBi
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Fig. 5 (Color online) a CV curves of GF, Co-Pi/FTO, Co-Pi/GF in 0.1 mol/L KOH from 0.2 to 1.4 V; b stability test for Co-Pi/FTO, Co-Pi/GF, and Co-Bi/GF in 0.1 mol/L KOH at E = 1.0 V for 8 h
from which O2 is produced and cobalt is returned to the 2? oxidation state [23]. The mechanism for oxygen evolution at molecular scale is not very clear for this special amorphous catalyst with multivalent states, which may cycle through heterogeneous and homogeneous phase. The special network structure and high conductivity of 3D graphene are benefit for fast electron and mass transport, which is favor of the PCET process and the complex equilibria processes between different valent states for cobalt. Besides, the cation–p interactions between Co2? and graphene improve the stability of Co-Pi during turnover. Co-Pi, Co-Bi and graphene contact well at the interface and the contact resistance cut down. The synergistic integration between the Co-Pi, Co-Bi and GF substrate has improved the catalytic ability for OER.
4 Conclusions In this study, we demonstrate a facile two-step synthesis (CVD growth of GF and in situ electrodepositions of Co-Pi and Co-Bi) to prepare Co-Pi/GF and Co-Bi/GF. As demonstrated here, GF is uniquely advantageous to serve as a 3D support of large capacity to anchor Co-OEC with well-defined size and shapes. Co-OECs/GF has shown improved performance for splitting water into oxygen for the synergistic integration between these two novel nanomaterials. The overpotential of Co-Pi/GF and Co-Bi/GF is about 0.390 and 0.315 V in benign conditions, respectively, which is lower than that of Co-OECs deposited on the conventional FTO substrate. Besides, Co-Pi/GF and Co-Bi/GF electrodes also have excellent and stable OER catalytic ability in alkaline solution. The successful integration of Co-OECs with 3D GF for the high-efficiency water oxidation described in our present work may afford a pathway toward the practical implementation of low-cost and technologically competitive water-splitting systems for sustainable hydrogen production.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21322304, 11290161) and the National Basic Research Program of China (2012CB933003, 2013CB932603). Conflict of interest of interest.
The authors declare that they have no conflict
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