Reduced Graphene Oxide Supported Cobalt-Calcium Phosphate Composite for Electrochemical Water Oxidation

: We report the oxygen evolution reaction (OER) catalyst composed of cobalt–calcium phosphate on reduced graphene oxide (CoCaP/rGO). Our catalyst is prepared by the anodic electrolysis of calcium phosphate/rGO mixture loaded on indium-tin-oxide (ITO) in Co 2+ aqueous solution. TEM, XPS and XRD experiments conﬁrm that the crystal phase of calcium phosphate (CaP) is trans-ferred into an amorphous phase of calcium oxide with phosphate (5.06 at%) after anodic electrolysis. Additionally, the main cation component of calcium is replaced by cobalt ion. The current–voltage characteristics of CoCaP/rGO showed a shoulder peak at 1.10 V vs. NHE, which originated from Co 2+ to higher oxidation states (Co 3+ or Co 4+ ) and a strong wave from water oxidation higher +1.16 V vs. NHE at neutral condition (pH 7). CoCaP and CoCaP/rGO showed 4.8 and 10 mA/cm 2 at 0.47 V of overpotential, respectively. The enhanced OER catalytic activity of CoCaP/rGO arises from the synergetic interaction between the amorphous phase of CoCaP and electric conducting graphene sheets.


Introduction
As the demand for sustainable energy increases, much research has been conducted on the development of renewable and environmentally benign energy [1]. In regard to solar energy conversion into chemical energy, water splitting has been extensively investigated for last three decades. Especially, electrochemical catalysts that enable the water oxidation have been considered as critical factor to improving the energy conversion efficiency. So far, precious metals or precious metals oxides (e.g., Ru/Ir oxides) are regarded as the prototype of oxygen evolution reaction (OER) catalysts [2][3][4][5][6]. However, these precious metals cannot be generally used because of their high cost [7,8]. Therefore, it is necessary to develop alternative catalysts that are made of relatively low-cost and earth-abundant elements with catalytic activity and durability. Hence, comparatively inexpensive transition metals (cobalt [9][10][11], copper [12], iron [13,14], etc.) deposited on carbon-based substrates graphene [15][16][17] and carbon nanotubes [18][19][20]) have been widely investigated. Generally, transition metal oxide/phosphates have insufficient electrical conductivity and a low surface area, which result in sluggish kinetics during the OER. Meanwhile, graphene oxide (GO) based materials have been widely used for oxygen/hydrogen evolution reaction and oxygen reduction reaction (ORR) because of their atomic thinness and high electrical conductivity after the reduction treatment [21][22][23][24]. For example, Co 3 O 4 nanoparticles reduced graphene oxide (rGO) [25], cobalt oxide on crumpled graphene with a high surface area [26], iron and nickel on GO [27], solvothermal synthesis of NiCo 2 S 4 nanoparticles on rGO [28] and GO-incorporated Cu-MOF composite [29] were utilized for HER, OER and ORR catalysts. The Nocera group demonstrated that an amorphous cobalt oxide containing phosphate group (so called Co-Pi) exhibited excellent OER catalytic properties under neutral pH conditions [30][31][32][33][34][35][36]. This Co-Pi has been successfully prepared on various substrates by in situ anodic electrolysis in phosphate buffer containing a small number of cobalt ions [31,37]. Previously, we developed a noble preparation method for the cobalt phosphate catalyst [38]. Particularly, we utilized the solid phase of hydroxyapatite (HAP) as a phosphate source instead of a solution phase of phosphate buffer. Our catalysts were prepared by the ion-exchange reaction of HAP (CoHAP) with aqueous cobalt ions solution [39]. Here, we developed the CoCaP/rGO OER catalyst through the anodic electrolysis of calcium phosphate and graphene mixture loaded on ITO under aqueous Co 2+ solution. After the anodic electrolysis, the calcium phosphate crystallites are changed into an amorphous cobalt oxide/phosphate. In addition, these cobalt oxide/phosphates are well dispersed in rGO without aggregation. Therefore, CoCaP/rGO exhibits enhanced OER catalytic activity compared to CoCaP (CoCaP/rGO requires only 457 mV to drive 10 mA). Furthermore, the Tafel slope was improved after the incorporation of Co 2+ ions (239 mV/dec to 132 mV/dec).

Results
CoCaP/rGO is synthesized in two steps. First, CaP/rGO is prepared by the precipitation from the mixture of aqueous solution containing rGO, Ca 2+ and HPO 4 − . The ink of CaP/rGO is loaded onto the ITO substrate. Second, CaP/rGO is electrochemically converted into CoCaP/rGO by anodic electrolysis in the Co 2+ aqueous solution. Figure 1a shows the grayish color of CaP/rGO on ITO. Calcium phosphate and rGO are white and black colored, respectively. The mixture loaded on an ITO exhibits a gray color. After the anodic electrolysis of CaP/rGO in the Co 2+ aqueous solution for 60 min, the gray color of CaP/rGO is changed into a dark brown (Figure 1b, Supplementary Video S1). The dark brown color was previously reported in the OER catalyst based on Co-Pi [40][41][42]. Figure 1c represents the current density profile during anodic electrolysis. The current density is the highest at 5 min and gradually decreases, then reaches the baseline level after 20 min. These currents seem to be caused by the charge transfer involved in the CoCaP/rGO formation, not by the faradaic current of water oxidation. CoCaP/rGO is precipitated on ITO by the reaction between the phosphate group of calcium phosphate and the Co 2+ in aqueous solution, in which Co 2+ is changed into Co 3+ /Co 4+ given bias (+1.3 V vs. NHE). The total charge flow for 60 min is 150 mC/cm 2 . For the control experiment, the current density profile of the bare ITO electrode without CaP/rGO is carried out under the same electrolysis condition (Figure 1c). In this case, the current is barely flowed and the color of the ITO is light yellow (Supplementary Figure S1), which is obviously different from the dark brown color of CoCaP/rGO film. Figure 1d shows the change of XRD patterns before and after anodic electrolysis. The calcium phosphate from the reaction of calcium nitrate and sodium phosphate dibasic is well matched with monetite (CaHPO 4 , JCPDS No. 70-1425, marked with red dots) patterns. The average crystal size of CaP is 33.9 nm based on a calculation by the Scherrer equation (Supplementary Figure S2). After anodic electrolysis, the diffraction patterns of the monetite disappeared. In addition, no new diffraction patterns are observed. Therefore, we interpret that the monetite crystallite is changed into an amorphous form of cobalt/oxides in which the amorphous morphology of cobalt phosphate improves OER activity in the previous report [43,44]. TEM and EDS elemental mappings are performed to investigate the morphology and local/surface elemental distribution of CaP/rGO and CoCaP/rGO. CaP/rGO (Supplementary Figure S3) shows elongated shape of about 40.1 nm in width and 97.3 nm in length, on average. Previously, we investigated the impact of pH on the morphology of calcium phosphate in which monetite with an elongated shape is formed when the base is not added, as in this study [45]. CoCaP/rGO composite (Figure 2a,b), on the other hand, does not exhibit a distinct shape. These features are similarly observed in SEM images (Supplementary Figure S3). Therefore, we concluded that the anodic electrolysis is accompanied by the absence of diffraction patterns of monetite and morphology changes (Figure 1d, Figure 2 and Supplementary Figure S2). In addition, TEM-EDS mapping measurements (Figure 2c-i) show that all chemical species, especially cobalt ions, are evenly distributed in the entire sample of CoCaP/rGO without local aggregation. TEM-EDS analysis indicates that after the anodic electrolysis in the Co 2+ aqueous solution, the atomical composition of Ca 2+ decreased from 12.24 at.% to 0.48 at.%. Meanwhile, those of Co 2+ increased by 3.61 at.%. The main cation of calcium ion is replaced by the cobalt ion after the anodic electrolysis. According to SEM images, the elongated crystallites of CaP/rGO cannot be distinguishable after the anodic electrolysis for an hour. The SEM-EDS elemental mapping measurements showed consistent results with the TEM-EDS experiments. The surface composition of the samples is analyzed by an X-ray photoelectron spectrum (XPS). The survey spectrum (Figure 3a) of the CaP/rGO and CoCaP/rGO confirms the presence of the carbon, calcium, oxygen, phosphorous and cobalt. While the Ca 2s and 2p peak of CaP/rGO almost disappeared after the anodic electrolysis, an additional new peak of Co 2p is observable. Based on the quantitative analysis by XPS, the atomic percentage of calcium is estimated as 6.54% for the CaP/rGO composite. After the anodic electrolysis, the atomic percentages of calcium decreased by 0.05%, while the cobalt species was 1.8%. The trend change of the elemental change is consistent with TEM-EDS (Figure 2c-2i). Therefore, we confirm that the main cation of composite changed from calcium to cobalt after the anodic electrolysis. High resolution Co 2p XPS spectra show 2p3/2 and 2p1/2 peaks for cobalt at 780.95 and 795.18 eV [46]. Co species is considered to be mainly in the 2+ oxidation state. Additionally, the Co 2p3/2 with a shoulder peak indicates that CoCaP/rGO may contain cobalt cations in the form of both Co 2+ and Co 3+ cations [37,47]. Supplementary Figure S4 represents that a phosphorous 2p peak at 132.78 eV indicates the existence of phosphate in CoCaP/rGO [48,49]. TEM and EDS elemental mappings are performed to investigate the morphology and local/surface elemental distribution of CaP/rGO and CoCaP/rGO. CaP/rGO (Supplementary Figure S3) shows elongated shape of about 40.1 nm in width and 97.3 nm in length, on average. Previously, we investigated the impact of pH on the morphology of calcium phosphate in which monetite with an elongated shape is formed when the base is not added, as in this study [45]. CoCaP/rGO composite (Figure 2a,b), on the other hand, does not exhibit a distinct shape. These features are similarly observed in SEM images (Supplementary Figure S3). Therefore, we concluded that the anodic electrolysis is accompanied by the absence of diffraction patterns of monetite and morphology changes (Figure 1d, Figure 2 and Supplementary Figure S2). In addition, TEM-EDS mapping measurements (Figure 2c-i) show that all chemical species, especially cobalt ions, are evenly distributed in the entire sample of CoCaP/rGO without local aggregation. TEM-EDS analysis indicates that after the anodic electrolysis in the Co 2+ aqueous solution, the atomical composition of Ca 2+ decreased from 12.24 at% to 0.48 at%. Meanwhile, those of Co 2+ increased by 3.61 at%. The main cation of calcium ion is replaced by the cobalt ion after the anodic electrolysis. According to SEM images, the elongated crystallites of CaP/rGO cannot be distinguishable after the anodic electrolysis for an hour. The SEM-EDS elemental mapping measurements showed consistent results with the TEM-EDS experiments. The surface composition of the samples is analyzed by an X-ray photoelectron spectrum (XPS). The survey spectrum (Figure 3a) of the CaP/rGO and CoCaP/rGO confirms the presence of the carbon, calcium, oxygen, phosphorous and cobalt. While the Ca 2s and 2p peak of CaP/rGO almost disappeared after the anodic electrolysis, an additional new peak of Co 2p is observable. Based on the quantitative analysis by XPS, the atomic percentage of calcium is estimated as 6.54% for the CaP/rGO composite. After the anodic electrolysis, the atomic percentages of calcium decreased by 0.05%, while the cobalt species was 1.8%. The trend change of the elemental change is consistent with TEM-EDS (Figure 2c-2i). Therefore, we confirm that the main cation of composite changed from calcium to cobalt after the anodic electrolysis. High resolution Co 2p XPS spectra show 2p 3/2 and 2p 1/2 peaks for cobalt at 780.95 and 795.18 eV [46]. Co species is considered to be mainly in the 2+ oxidation state. Additionally, the Co 2p 3/2 with a shoulder peak indicates that CoCaP/rGO may contain cobalt cations in the form of both Co 2+ and Co 3+ cations [37,47]. Supplementary Figure S4 represents that a phosphorous 2p peak at 132.78 eV indicates the existence of phosphate in CoCaP/rGO [48,49].   Figure 4a shows the cyclic voltammetry of CaP and CoCaP/rGO on a glassy carbon electrode (5 mm diameter). The oxidative wave at +1.1 V vs. NHE is observed from both the CV and LSV curves of Co-CaP/rGO (Figure 4a,b). This shoulder is commonly observed in Co-Pi catalysts and originated from the Co 2+ /Co 3+ or Co 4+ transitions [33,35,37,[50][51][52].   The electrochemical characterizations (cyclic voltammetry (CV), linear sweep voltammetry (LSV) and bulk electrolysis) of CoCaP and CoCaP/rGO are performed by commercial potentiostat with three electrode system under neutral pH conditions (0.1 M KPi, pH 7). Details are described in the experimental method. Figure 4a shows the cyclic voltammetry of CaP and CoCaP/rGO on a glassy carbon electrode (5 mm diameter). The oxidative wave at +1.1 V vs. NHE is observed from both the CV and LSV curves of Co-CaP/rGO (Figure 4a,b). This shoulder is commonly observed in Co-Pi catalysts and originated from the Co 2+ /Co 3+ or Co 4+ transitions [33,35,37,[50][51][52].   Figure 4a shows the cyclic voltammetry of CaP and CoCaP/rGO on a glassy carbon electrode (5 mm diameter). The oxidative wave at +1.1 V vs. NHE is observed from both the CV and LSV curves of CoCaP/rGO (Figure 4a,b). This shoulder is commonly observed in Co-Pi catalysts and originated from the Co 2+ /Co 3+ or Co 4+ transitions [33,35,37,[50][51][52]. Figure 4b presents LSV curves with a sweep rate of 5 mV/s in 0.1 M KPi (pH 7). The current densities of CoCaP and CoCaP/rGO exhibit 3.5 mA/cm 2 and 15 mA/cm 2 at 1.3 V vs. NHE, respectively. Due to their low electric conductivity, transition metal oxide/phosphates combined with rGO and CoCaP significantly induces the improvement of OER catalytic activity. The improvements of ORR and HER catalytic activity are additionally reported by the hybrid of the catalyst and GO [53][54][55]. Previously, we reported that cobalt incorporated hydroxyapatite (Co-HAP) OER catalyst. CoCaP/rGO showed approximately ten-fold higher OER activity than CoHAP (1.2 mA/cm 2 at 1.3 V vs. NHE). CoHAP is prepared by the ion exchange reaction of HAP by the simple immersion of HAP in aqueous Co 2+ solution, in which cobalt ions are coated on the surface of HAP. Therefore, the number of cobalt species in CoHAP is much less than that of cobalt species in CoCaP/rGO prepared by anodic electrolysis. The electrochemical active surface area of both samples is determined by measuring double-layer capacitance using CV at different scan rates (20,30,50, 100 and 200 mV/s) in the potential region having no Faradaic current (Figure 4c and Figure S6). The capacitance of CoCaP/rGO is approximately eleven-fold over that of CoCaP. Therefore, the increased catalytic active area by the dispersion of CoCaP on rGO surface enhances OER catalytic activity. Figure 4d and Supplementary Movie S2 illustrate a stability of CoCaP and CoCaP/rGO loaded on ITO as a working electrode at the overpotential of 680 mV through the bulk electrolysis over 24 h. Both CoCaP/rGO and CoCaP present durability with 90% and 95% after the potential is applied. The electrocatalytic properties of both CaP/rGO and CoCaP/rGO composite (Figure 4e) are estimated by Tafel plots and showed linearity in the range of ca. 100 mV of overpotentials. The Tafel slope for CoCaP/rGO shows an improved activity with a slope of 132 mV/dec compared to the that of CoCaP (239 mV/dec). This lower value is comparable to other Tafel slopes of cobalt based OER catalysts performed under pH 7 conditions (166 mV/dec for Co-Bi nanoarray [56], 121 mV/dec for CoFe(CN) 5 -PVP [57], 89~95 mV/dec for Co/Fe PBAs [58] and 137 mV/dec for CoO/CoSe 2 hybrid [59] electrocatalysts). The improved OER activity and Tafel slope of CoCaP/rGO seem to originate from the efficient charge transfer from rGO to active catalytic species due to the well dispersion of CoCaP on rGO. Generally, transition metal oxide or phosphates exhibit poor electric conductivity. Therefore, combining active catalytic species with carbon supports has been widely applied in catalytic system of OER or ORR. For example, NiCo double hydroxide on N-doped graphene [60], CoFe 2 O 4 on graphene [61] and graphene-Co 3 O 4 nanocomposite [62] were demonstrated for OER catalysts with higher OER/ORR activities and material durability. In addition to the charge transfer issue, the dispersion of CoCaP on rGO may provide more exposed catalytic active sites and a higher rate of mass transfer, which has been proven in carbon supports catalytic systems in previous reports [54,[63][64][65].
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Preparation of Reduced Graphene Oxide (rGO)
The slightly modified Hummer's method was used to prepare graphene oxide (GO) [66]. 750 mg of graphite flakes were mixed with a 9:1 mixture of H 2 SO 4 /H 3 PO 4 (90:10 mL) and stirred in an ice bath. Then, 4.5 g of potassium permanganate was added very slowly to the reaction mixture by maintaining the temperature at 35-40 • C. The reaction mixture was stirred at 50-55 • C for 12 h and then cooled to room temperature, followed by the addition of 100 mL of iced water containing 30% H 2 O 2 [67][68][69]. The resulting solution was centrifuged at 8000 rpm for 30 min and the supernatant was removed. The gel-like product was washed 5-6 times with 50 mL of distilled water. GO powder was obtained by dialysis with the membrane for 3 days to remove acid in the product completely, followed by lyophilization for 3 days. A total of 250 mg of as-obtained GO powder were dispersed in 250 mL of DI water; then, the pH of the solution was adjusted to 10.0-11.0 by adding a trace amount of ammonium hydroxide solution (28%, w/w). A total of 1.25 mL of hydrazine hydrate were added to the solution and stirred at 95 • C for 1 h. The solution color became slightly darker as the reaction progressed [67][68][69]. The reaction mixture was centrifuged at 12,000 rpm for 30 min, then washed with DI water. Finally, the rGO dispersion was lyophilized to obtain rGO powder.

Preparation of Catalyst
Calcium phosphate [70] reduced graphene oxide (CaP/rGO) composite was prepared by the precipitation method. 20 mg of rGO powder were dispersed in 10 mL of DI water by ultrasonication for 1 h. 750 µL of calcium nitrate tetrahydrate (0.656 M) were added into 10 mL of the rGO suspension, followed by addition of 750 µL of sodium phosphate dibasic (0.394 M) solution. The reaction mixture was aged at 75 • C for 24 h. Finally, the CaP/rGO composite was obtained by washing using DI water for three times with centrifugation. The composite was lyophilized for 24 h. CaP was obtained by the same steps as preparing CoCaP/rGO without adding any rGO powder in the first step.

Preparation of Working Electrode
The glassy carbon electrode (5 mm in diameter) was cleaned with polishing using 1 µm and 0.05 µm alumina, followed by the sonication 3 × 30 s in DI water. The Indium Tin Oxide (ITO) electrode was cleaned by the sonication 3 × 30 s in DI water and acetone for two times, respectively. A total of 5 mg of as-obtained CaP/rGO composite were added into 150 µL of ethanol and 80 µL of 5% Nafion 117 solution [71]. This ink was ultrasonicated for 30 min. A total of 2.0 µL and 10 µL of ink were loaded onto glassy the carbon electrode (0.197 cm 2 ) and ITO electrode (area: 2 cm × 0.5 cm) individually, then dried at ambient conditions.

In Situ Preparation of Cobalt Calcium Phosphate/rGO Composite (CoCaP/rGO)
The CoCaP/rGO was prepared by in situ anodic electrolysis. The electrolysis was done with a three-electrode electrochemical cell system using commercial potentiostat (EC Epsilon, BASi and ZIVE SP1, Wonatech). CaP/rGO on GCE (or ITO), Ag/AgCl and Pt-wire (or Pt plate) was used for a working, reference and counter electrode, respectively. The electrolysis was carried out under 50 mL of 20 mM cobalt nitrate solution by applying +1.3 V vs. NHE for 1 h without stirring. The CoCaP was prepared by the same way in the absence of rGO.

Characterization
The crystallinity of CoCaP/rGO film was determined by the X-ray diffraction (XRD, 3 kW Cu X-ray Diffractometer, D8 Advance, Bruker AXS, Hamburg, Germany). The shape of analysis of the CaP/rGO and CoCaP/rGO was performed using a Field Emission Transmission Electron Microscope (FE-TEM, JEM-F200, JEOL, Tokyo, Japan) and Scanning Electron Microscopy (SEM, JSM-6700F, JEOL, Tokyo, Japan). The morphology of the sample was analyzed using a holey carbon grid to map the carbon elements for TEM-EDS measurements. The surface elemental analysis was characterized by X-ray Photoelectron Spectroscopy (XPS, ULVAC-PHI Inc. PHI 5000 Versaprobe, Chigasaki, Japan). Electrochemical measurements were performed by a three-electrode electrochemical cell system with commercial potentiostats (EC Epsilon, BASi and ZIVE SP1, Wonatech). Linear sweep voltammetry (LSV) was carried out at 5 mV/s, and Tafel data were obtained from LSV. The electrochemical behavior of cyclic voltammetry was studied in 0.1 M potassium phosphate buffer solution (pH 7) with a sweep rate of 10 mV/s without iR compensation. Both CV and LSV were performed using a glassy carbon electrode (GCE) of 5 mm in diameter as the working electrode. The electrochemical double-layer capacitance (C dl ) was investigated using the CV method [72] in a potential range not affected by the faradaic current. All potentials were converted to the normal hydrogen electrode (NHE) using the following formula: E(NHE) = E(Ag/AgCl) + 0.197 V The overpotential for oxygen evolution was calculated using the following formula: η = E(NHE) + 0.059 × pH − 1.23

Conclusions
In summary, we prepared cobalt-calcium phosphate on reduced graphene oxide (CoCaP/rGO) by applying the anodic electrolysis in cobalt aqueous solution. During the anodic electrolysis process, the CaP/rGO was electrochemically converted into an amorphous phase of CoCaP/rGO and identified by XRD experiment. The TEM-EDS and XPS experiments of CoCaP/rGO suggest that amorphous cobalt oxide/phosphates are well dispersed on rGO. Furthermore, bulk electrolysis of chronoamperometric measurements demonstrated the stability of catalysts under the neutral conditions. Comparing the catalytic activity between CoCaP and CoCaP/rGO, we conclude that the dispersion of CoCaP on rGO with high conductivity enhanced catalytic activities in neutral conditions. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/catal11080960/s1: Figure S1: photographs taken in each step of the experiment; Figure S2: XRD patterns for CaHPO 4 (monetite), CaP/rGO and CoCaP/rGO and Miller facets of the nine peaks of CaHPO 4 ; Figure S3: TEM images and EDS elements mapping images of CaP/rGO; Figure S4: SEM images of CaP/rGO, CoCaP/rGO and EDS elemental mapping images of CoCaP/rGO; Figure S5: XPS spectrum comparing before and after anodic electrolysis of CaP/rGO; Figure S6: cyclic voltammetry experiments of the current density of CoCaP and CoCaP/rGO composite; Video S1: the anodic electrolysis of CaP/rGO for 1 h in 20 mM Co 2+ aqueous solution; Video S2: the surface of CoCaP/rGOcoated ITO electrode at 1.5 V vs. NHE in 0.1 M KPi (pH 7); Table S1: the crystal size of CaHPO 4 derived from XRD peaks using the Scherrer equation.