Enhanced OER Performance by Cu Substituted Spinel Cobalt Oxide

Cobaltite spinel oxides (CuCo2O4), which show electrocatalytic activity for oxygen evolution reaction (OER), are readily synthesized using a facile hydrothermal method, with a stationary ratio, uniform flower-like mesopores morphology, and high crystallinity. We have carried out first-principles calculations on the mechanism of the reaction pathway and the Gibbs free energy diagram of CuCo2O4 structures using density functional theory and purely confirmed by experimental results. This catalyst performed an outstanding OER performance with an overpotential 230 mV at 10 mA cm−2 in 1 M KOH, which was close to IrO2 with an overpotential 190 mV at 10 mA cm−2. This work provides a facile method for electrocatalytic oxygen production with enhanced conductivity and enhanced OER by replacing cobalt with copper.

In the past decades, Ternary transition metal oxides such as spinel containing different metal cations promise electrochemical active materials, which might be employed in lithium-ion batteries, supercapacitors and electrocatalysts [30]. Due to its richer redox reaction and lower electron transfer activation energy than single metal oxides, spinel demonstrates greater electrochemical performance. CuCo 2 O 4 has attracted wide attention among different spinels as a promising electrochemical electrode material. This is due to the low price, non-toxicity and the synergistic effect of cobalt and copper species as high conductivity species [31][32][33][34]. Up to now, various shapes of CuCo 2 O 4 materials such as nanowires, nanometer grass, nanometer flakes, nanometer cubes, hollow spheres, double-shell hollow spheres and nanotubes have been used to prepare electrodes [35][36][37][38][39][40][41].
As one of the well-known CuCo 2 O 4 has been investigated extensively as a corrosion-stable and highly efficient catalyst for OER in alkaline media. Compared with other spinel materials, MxCo 3−x O 4 material (M = Mn, Ni, Fe, Zn), the replacement of cobalt with Cu significantly improves the electrical conductivity and charge transfer ability, thereby increasing the alkalinity OER performance under sexual conditions [3,4,7]. The combination of the theoretical and experimental approaches towards the different active of CuCo 2 O 4 was conducted. In the experiment, the ratio, morphology and crystal structure of elements Cu and Co will all affect the oxygen production performance of CuCo 2 O 4 electrolytic water. Although DFT has succeeded in some simulation materials, it is not precise enough to calculate the electronic structure of the transition metal [8][9][10]. It is challenging for the experiment and theoretical calculations.
In the present work, we report that cobaltite spinel oxides CuCo 2 O 4 was readily synthesized using a facile hydrothermal method, with a stationary ratio (Cu/Co = 1/2), uniform flower-like mesopores morphology, and high crystallinity. Under theoretical prediction and experimental results, we found that CuCo 2 O 4 shows the best electrocatalytic activity.
The catalyst exhibits excellent OER performance in 1 M KOH with an overpotential of 230 mV at 10 mA cm −2 , which is close to IrO 2 with an overpotential of 190 mV at 10 mA cm −2 .

Experimental Section
Synthesis of MCo 2 -PTCDA MMOFs (mixed-metal-organic frameworks) is based on the method with some modification [42]. The detailed procedure is as follows.

Preparation of the CuCo 2 -PTCDA
Cu(OAc) 2 ·2H 2 O (0.133 mmol) and Co(OAc) 2 ·4H 2 O (0.267 mmol) were dissolved in 22.5 mL deionized water, and PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) (0.2 mmol) was dissolved in 12.5 mL NaOH solution (0.8 mmol NaOH). Under continuous stirring, PTCDA solution was dropped into the mixed solution of metal acetate. Immediate sediment formation was observed. The reaction mixture was agitated at room temperature for 30 min, and then transferred to a stainless steel vessel (45 mL) with Tefion-lined, and heated for 8 h at 100 °C. After cooling to room temperature, collect precipitation by centrifugation, wash and dry with water.

Preparation of the CuCo 2 O 4
Formation of CuCo 2 O 4 nanomaterials CuCo 2 -PTCDA MMOFs were thermally treated in air at 550 °C for 1 h with a ramp of 1 °C min −1 .

Characterization
The morphology, chemical composition, and structure of samples were characterized by high resolution field emission scanning electron microscopy (FESEM, Nova NanoSEM 450), transmission electron microscopy (TEM, 300 kV Titan Probe corrected TEM, Titan G2 60-300), and XPS (Thermo fisher-ESCALab 250). The Brunauer-Emmett-Teller (BET) surface area (SBET) and pore size distribution were determined using a Micromeritics ASAP 2000 nitrogen adsorption apparatus. All the samples were degassed at 180 °C prior to BET measurements. Fourier-transform infrared spectra (FT-IR) were measured by Nicolet 6700 (Thermo Fisher Scientific). Thermogravimetric analysis (TGA) was performed using a NETZSCH STA449F3 device, in an air flow with a heating rate of 5 °C min −1 .
The FE-SEM image reveals that the resulting CuCo 2 O 4 material is composed of nanoplate assemblies inherited from the CuCo 2 -PTCDA precursor with flower-like morphology (Fig. 1b). The nanoplates, which assemble to form the flower-like morphology, are porous under the observation of the TEM image (Fig. 1c). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analysis of an isolated nanoplate display polycrystal properties with periodic diffraction spots (Fig. 1e). The particle size of the nanoplate is about 1.6 µm. The six marked diffraction rings correspond to the (1 1 1) The thermal transformation process from MMOFs to spinel structured mixed-metal oxide was investigated by thermogravimetric (TG) analysis using CuCo 2 -PTCDA as a model (Fig. 3a). The sample has two consecutive weight loss  steps. The preliminary weight loss at 350 ℃ corresponds to the escape of perylene by decarboxylation. There is a welldefined weight loss due to the oxygen release of CuCo 2 O 4 thermal decomposition above 650 ℃. The spinel structure could not be formed at treatment temperatures below 450 ℃. Figure 3b is the FTIR spectra of the PTCDA, CuCo 2 -PTCDA and CuCo 2 O 4 electrode materials in the range of 1000-2000 cm −1 . The peaks at 1296, 1770 and 1400-1600 cm −1 are attributed to the cyclic anhydride C-O deformation of PTCDA, cyclic anhydride C=O stretching vibration and phenyl C=C vibration. The FTIR peaks of cyclic anhydride C=O stretching vibration (1776 cm −1 ) and C-O stretching vibration (1296 cm −1 ) disappeared after CuCo 2 O 4 was recombined on PTCDA by cyclic anhydride [43].
These oxides showed considerably improved specific surface area and apparent electrocatalytic activity. The surface area of porous structured CuCo 2 O 4 and Co 3 O 4 were characterized N 2 sorption measurements as shown in Fig. 3c. The samples for CuCo 2 O 4 and Co 3 O 4 had the specific Brunauer-Emmett-Teller (BET) surface area of 54.4 and 45.5 m 2 g −1 , respectively. uniform distribution of the mesopores with pore diameter of 14.8 and 12.3 nm, respectively. The large specific surface area and pore size are favorable for the reaction. The result of BET corresponded with the previous report [41,44].
Using cyclic voltammetry, C dl values were more straightforwardly estimated from the slope of the linear charging current i cp vs scan rate v plots obtained by recording cyclic voltammograms in a narrow potential region of 50 mV, i.e. of ± 25 mV around the same potentials as above. CV curves were performed without a redox process from 0 to 0.05 V (ranging from 10 to 120 mV s −1 (Fig. 3d)). According to the CV curves and the plots of the resultant current densities versus scan rates, the C dl of CuCo 2 O 4 is determined to be 1.0 mF cm −2 . C dl is related to the surface area. Large ECSA is conducive to the adsorption of water molecules and the rich active sites in the electrochemical reaction process. Supplementary Table 1 shows that CuCo 2 O 4 has a larger capacitance, which further illustrates that the introduction of Cu increases the electrochemically active surface area and thus the OER performance. In order to obtain the valence and the composition of the elements in the samples, we characterized the CuCo 2 O 4 and Co 3 O 4 by XPS. As shown the C 1s spectra in Fig. 4a, the peaks at 284.8 and 285.54 eV are attributed to C sp 2 and C sp 3 . The peak at 530.0 eV is attributed to M−O (where M represents metal), while the peak at 531.5 eV is attributed to O−C=O (Fig. 4b). The energy difference of 19.85 eV between the peaks at 932.51 and 952.36 eV assigned to Cu 2p 3/2 and Cu 2p 1/2 , is confirmed the existence of Cu 0 in the Cu 2p spectra (Fig. 4c) [45]. As shown Co 2p spectra in Fig. 4d, the two types of Co species (Co 2+ and Co 3+ ) are obtained. The two strong peaks at 780.32 and 796.21 eV are attributed to Co 2p 3/2 and Co 2p 1/2 , respectively. Moreover, the C 1s spectra and O 1s spectra of Co 3 O 4 , though similar to that of CuCo 2 O 4 , are not a precise match. The fitted peaks at 284.8, 285.54 and 289.00 eV are assigned to C sp 2 , C sp 3 and O-C=O in the C 1s spectra of Co 3 O 4 (Fig. S5a). The peaks at 531.5 and 529.6 eV are attributed to O−C=O and M−O in the O 1s spectra (Fig. S5b). We compared between the Co 2p spectra of CuCo 2 O 4 (Fig. S5c) and that of Co 3 O 4 (Fig. S5d). The three types of Co species (Co 0 , Co 2+ and Co 3+ ) are found in the Co 2p spectra of Co 3 O 4 (Fig. S5d). The energy difference of 15.89 eV between the two strong peaks at 780.32 and 796.21 eV are attributed to the two types of Co species (Co 2+ and Co 3+ ) for CuCo 2 O 4 . However, the energy difference of 15.03 eV between the two peaks are 780.09 and 795.12 eV is confirmed the existence of Co 0 for Co 3 O 4 [46]. The Cu 0 of CuCo 2 O 4 provides a higher conductivity than Co 0 of Co 3 O 4 to enable a good charge transfer for efficient electrocatalytic reaction [47,48].
To verify the theory prediction, we tested electrochemical activity. The electrocatalytic activities of CuCo 2 O 4 for the OER were tested by loading the samples onto a glassy carbon electrode that served as the working electrode. The linear sweep voltammetry (LSV) was evaluated using a three-electrode electrochemical cell in a 1.0 M KOH solution. Under similar alkaline conditions, Fig. 5 shows the OER and Tafel slope. IrO 2 and CuCo 2 O 4 are for the OER with overpotentials at 190 and 230 mV, and with the Tafel slope of 79.7 and 80.7 mV dec −1 , respectively. It is consistent with the theory prediction. However, Fig. 5a and b shows that Co 3 O 4 has an overpotential of 270 mV and a Tafel slope of 83.7 mV dec −1 . It further illustrates that CuCo 2 O 4 has superior OER performance.
The above potentials were chosen to belong to the ideally polarized (i.e. flat) region (0-0.05 V) of the CV curves,   Fig. 5c as an example. The impedance further proved the charge transfer ability, and then the catalytic activity was evaluated (Fig. 5d). It can be seen that there are differences among different metal oxide electrodes, from Pt20%C (6.67 olm), CuCo 2 O 4 (16.67 olm) to the highest ZnCo 2 O 4 (133.33 olm), so CuCo 2 O 4 has good conductivity and charge transfer ability. This has been reported in previous articles and can be well confirmed [49,50]. In addition, The SEM image (Fig. S2) shows that the CuCo 2 O 4 nanoflowers still maintain the morphology of their nanoflowers after the stability test. Figure S1 shows that the current density of CuCo 2 O 4 is basically unchanged after continuous electrolysis at − 1.4 v for 18 h, indicating that CuCo 2 O 4 is an efficient and stable OER catalyst. We tested the Cu and Co contents of CuCo 2 O 4 before and after electrolysis by ICP-OES, which were 290.55 mg g −1 and 539.00 mg g −1 and 292.34 mg g −1 , 535.68 mg g −1 , respectively (Fig. S3). At the same time, Fig.  S4 shows the EDS elemental analysis of CuCo 2 O 4 catalyst, in which the ratio of Cu to Co is close to 1:2, which proved that the content did not change further before and after electrolysis. Combined with previous work, further understanding of the origin of OER activity [51]. CuCo 2 O 4 electrons can pass through Fermi levels and move freely to enhance conductivity (Fig. 6a). However, the Co 3 O 4 electrons failed to pass through the Fermi level, and the electron motion was blocked, resulting in poor conductivity (Fig. 6a). We begin with calculations to understand and identify active sites that favor high activity for hydrogen and oxygen evolution. In order to reveal the structural advantages of hydrogen and oxygen evolution of CuCo 2 O 4 , density functional theory (DFT) was used to analyze the mechanism of the reaction pathway and the Gibbs free energy diagram (see method for details) was obtained. Based on the experimental results and in order to provide supporting information about the reaction on pathway, different adsorption orientations of the intermediates at all possible active sites were determined. The first step in electrochemical reduction of H 2 O is adsorption of the target molecule. The likelihood estimation is performed for each reaction according to the energy difference of ∆G H values before and after each reaction. As shown in the Fig. 6b, in the alternating  mechanism, the adsorption energy of the *OOH intermediate of Cu on the surface of CuCo 2 O 4 is 2.3 eV, which is the energy required for the rate-decision step. However, the rate-decision step of Co 3 O 4 is 2.6 eV, which is more than the energy required of CuCo 2 O 4 for the rate-decision step (Fig. 6b). This also demonstrates that substitution, doping with highly conductive materials can further improve the OER activity of the material [52].

Conclusion
In short, this work introduces copper cobaltate nanoflowers for high-performance oxygen production. The newly designed CuCo 2 O 4 catalyst puts up an overpotential of 230 mV at 10 mA m −2 for OER electrocatalytic activity in 1 M KOH solution. Tafel slope 79.7 mV dec −1 lower than IrO 2 80.7 mV dec −1 . DFT calculation shows that the decisive step of electrocatalytic oxygen production is *O → *OOH. The required energy is 2.3 eV, which is consistent with experimental results. This is superior to the reported electrochemical activity. This research enhances its OER performance by replacing cobalt with copper to increase conductivity, which provides a direction for non-noble metal oxygen-producing catalysts.