Accelerative oxygen evolution by Cu-doping into Fe-Co oxides

Abstract

Development of cost-efficient, high-performance and robust earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is a continuous challenge. Herein, we develop a facile sol–gel strategy to fabricate a ternary Fe-Co-Cu oxide for efficient electrocatalysis of water oxidation. Low overpotentials of 256, 271, and 226 mV at 10 mA cm⁻² can be achieved over FeCoCuO_x supported on a GCE, a Pt electrode, and Ni foam, respectively, in an alkaline electrolyte. Due to the enhanced charge transfer ability and electrochemically active surface area, trimetallic FeCoCuO_x is more active than binary FeCoO_x and among the most active OER catalysts to date.

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Introduction

The oxygen evolution reaction (OER) is the core for some renewable energy technologies, such as electrocatalytic water-splitting, reversible fuel cells, and rechargeable metal–air batteries.^1–4 Due to the sluggish kinetics of the OER, high-efficiency electrocatalysts are demanded to overcome the high reaction barrier, facilitate the reaction rate, and reduce the overpotential to improve the energy efficiency.^5,6 Although iridium and ruthenium oxides possess good OER activity, their widespread application is impeded by the scarcity and high cost. Exploitation of cost-efficient, high-performance and robust noble-metal-free earth-abundant OER electrocatalysts is an ongoing challenge.⁷

Over the past few decades, various kinds of transition-metal-based catalysts have been reported to achieve electrochemical water oxidation in alkaline media, which include oxides or (oxy)hydroxides,^8–11 nitrides,^12–14 chalcogenides,^15–17 phosphides,^18,19 selenides,^20,21 and carbon-based hybrid materials.^22,23 It has been reported that the metal phosphides, sulfides and selenides would be partially or completely transformed into metal oxides and/or (oxy)hydroxides during the OER, especially when operated at a high overpotential.²⁴ Therefore, from the perspective of application, metal oxides or (oxy)hydroxides are preferred OER catalysts.^25,26 Compared with the crystalline counterparts, amorphous metal oxides show a higher surface area, better chemical stability, and stronger corrosion resistance.⁷ Although amorphous metal oxides lack long-term orderly arrangement of atoms, they maintain the short-range order within several atoms. The coordination number of the M–O polyhedron in amorphous metal oxides plays a crucial role in the OER. The configurations and electronic properties of the M–O polyhedron can be influenced by metal element doping.²⁵

Fe-Co-based oxides and (oxy)hydroxides showed excellent OER performance in alkaline media.^27,28 The catalytic OER activity can be improved by doping Cr,²⁹ Ni,³⁰ and W²⁵ into the Fe-Co-based system due to the production of defects or synergistic mechanisms. It has been reported that the introduction of copper into the Fe-Ni-based oxides increases the ECSA and intrinsic catalytic activity owing to a synergistic effect of Ni, Fe, and Cu.²⁴ Here, we adopt a sol–gel method to introduce copper into the Fe-Co-based oxide system in an atomically homogeneous dispersion, which is named FeCoCuO_x. The good dispersion of various metal ions favors the exposure of more active sites for the OER.

Results and discussion

The structure of the as-prepared FeCoCuO_x was analyzed by XRD. As shown in Fig. 1a, two broad peaks centered at ca. 35° and 61.5° can be observed, which is typically characteristic of amorphous oxide materials. Moreover, no definite diffraction peaks due to metal oxides/(oxy)hydroxides can be seen, further implying the amorphous state of FeCoCuO_x. The morphology of FeCoCuO_x was characterized by SEM. As illustrated in Fig. 1b, no regular morphology and crystals can be observed. The morphology of FeCoCuO_x was further investigated by TEM. As shown in Fig. 1c, irregular clumps are typically observed in agreement with the SEM result. From the HRTEM image of FeCoCuO_x (Fig. 1d), no lattice fringes can be seen, indicating that no crystals were formed. The dispersion of various metal elements in FeCoCuO_x was studied by a SEM-EDS mapping test. As exhibited in Fig. 2, Fe, Co, and Cu are homogeneously dispersed in the sample. The atomic ratio of Fe/Co/Cu is close to 1 : 1 : 1 as analyzed by the SEM-EDS, which is in agreement with the ICP results. The metal content of Fe, Co, and Cu in FeCoCuO_x is 20.1, 21.3, and 23.1 wt% determined by ICP-AES, respectively.

Fig. 1 (a) XRD patterns of FeCoCuO_x and referred samples (Fe(OH)₂, Co(OH)₂, and CuO), (b) HRSEM image of FeCoCuO_x, (c) TEM image of FeCoCuO_x, and (d) HRTEM image of FeCoCuO_x.

Fig. 2 (a) SEM image of FeCoCuO_x used in the EDS mapping test and (b) the corresponding EDS mapping of Fe, Co, Cu, and O.

The surface composition and metal valence state of FeCoCuO_x were analyzed by XPS. As displayed in Fig. S1,† Fe, Co, Cu, and O are mainly detected. The high-resolution XPS spectra of Fe 2p, Co 2p, Cu 2p, and O 1s are depicted in Fig. 3. The Fe 2p XPS spectrum is resolved into two pairs of 2p_1/2/2p_3/2 doublets for Fe²⁺ (710.8/724.0 eV) and Fe³⁺ (713.0/726.5 eV) with an energy separation of 13.1 eV.^31–34 The surface Fe²⁺/Fe³⁺ molar ratio is calculated to be around 1.3. Co 2p_3/2 and Co 2p_1/2 are located at 781.16 eV and 797.16 eV, respectively, with an energy separation of 16.0 eV, indicating the dominant existence of Co²⁺.^35–40 Two obvious satellite peaks centered at 786.4 and 802.7 eV further indicate the presence of Co²⁺. Cu 2p_3/2 and Cu 2p_1/2 can be fitted into Cu⁺ 2p_3/2 (932.8 eV), Cu²⁺ 2p_3/2 (934.8 eV), Cu⁺ 2p_1/2 (952.6 eV), and Cu²⁺ 2p_1/2 (954.7 eV).⁴¹ The Cu⁺/Cu²⁺ atomic ratio is calculated to be ca. 1 : 1. The O 1s XPS spectrum can be deconvoluted into the peaks for Fe–OH (530.2 eV), Co–OH (531.2 eV), Cu–OH (531.7 eV), and O from water (533.3 eV).^42,43

Fig. 3 High-resolution XPS spectra of Fe 2p (a), Co 2p (b), Cu 2p (c) and O 1s (d) of FeCoCuO_x.

The electrocatalytic activity of FeCoCuO_x toward the OER was first evaluated by supporting it on a glassy carbon electrode (GCE) and by linear sweep voltammetry (LSV) tests conducted in 1.0 M KOH electrolyte at a scan rate of 1.0 mV s⁻¹ (Fig. 4). The reference electrode was calibrated before the test (Fig. S2†). As expected, CuO_x and FeO_x show poor OER performance due to the lack of efficient OER active sites for the former and bad electrical conductivity for the latter (Fig. 4a). The introduction of Cu into FeO_x would improve the water oxidation performance, showing an overpotential of 438 mV at 10 mA cm⁻², still higher than that on CoO_x (348 mV) (Fig. 4b). The introduction of Cu into the CoO_x can also decrease the overpotential from 348 mV to 313 mV. It is interesting to find that the OER activity of FeCoO_x can be further increased by the addition of Cu (Fig. S3†). For FeCoCuO_x, the overpotential at 10 mA cm⁻² is only 256 mV, which is higher than or well comparable with that of previously reported electrocatalysts tested under similar conditions (Table S1†). The OER activity of FeCoCuO_x would exaggeratedly decrease after it was annealed at 500 °C in air for 2 h (Fig. 4a). The annealed FeCoCuO_x (A-FeCoCuO_x) shows distinct strong diffraction peaks in the XRD pattern (Fig. S4†), implying that amorphous FeCoCuO_x was transformed into a crystalline oxide. The control experiment indicates that the crystallized oxide shows worse OER activity than the non-crystallized one. The FeCoCuO_x catalyst shows a higher TOF of 0.034 s⁻¹ per total 3d metal atoms with 90% iR-correction at η = 300 mV than state-of-the-art IrO₂ (0.01 s⁻¹).⁴⁴ The excellent OER performance of FeCoCuO_x can be demonstrated by the low Tafel slope. As manifested in Fig. 4c, the Tafel slope of FeCoCuO_x-on-GCE is 42.9 mV dec⁻¹, which is lower than that of CuO_x (123.5 mV dec⁻¹), FeO_x (110.9 mV dec⁻¹), FeCuO_x (89.3 mV dec⁻¹), CoO_x (54.3 mV dec⁻¹), CoCuO_x (54.3 mV dec⁻¹), and FeCoO_x (43.0 mV dec⁻¹).

Fig. 4 (a) Polarization curves of FeO_x, CoO_x, CuO_x, FeCuO_x, CoCuO_x, FeCoO_x, IrO₂, and FeCoCuO_x supported on a GCE in 1.0 M KOH solution without iR-compensation. (b) Overpotential required for a current density of 10 mA cm⁻². (c) Tafel plots derived from (a). (d) Nyquist plots of the EIS test for FeCoO_x and FeCoCuO_x. The solid lines are the fits to the data using the simplified Randles circuit shown in the inset.

The positive role of Cu in improving the OER activity of the FeO_x, CoO_x, and FeCoO_x systems should be mainly ascribed to the modified charge transfer kinetics. The electrode reaction mechanism was controlled by the charge transfer step and ion diffusion process.⁴⁵ As depicted in Fig. 4d and S5,‡ the addition of copper to these systems significantly accelerates the charge transfer. For FeCoCuO_x-on-GCE, a rough semicircle at high frequency is ascribed to the faradaic charge-transfer resistance (R_ct) in parallel with the double-layer capacitance (CPE). The fitted result shows that the R_ct is as low as 0.53 Ω cm², indicating good conductivity and low internal resistance of the FeCoCuO_x electrode, which favors electrocatalysis.

To investigate the possible effect of the surface area on the OER, we determined the electrochemically active surface area (ECSA) by performing CV measurements at varying scan rates. The double-layer capacitance (C_dl) is usually used to represent the corresponding ECSA, which can be obtained from the linear slope by plotting Δj = (|j_charge − j_{off charge}|) in a faradaic silence potential range against the scan rates.²⁴ As shown in Fig. S6,† the C_dl values for CuO_x, FeO_x, CoO_x, FeCuO_x, CoCuO_x, FeCoO_x, and FeCoCuO_x are measured to be 0.043, 0.14, 0.35, 1.3, 6.5, 8.5, 28.6 mF cm⁻², respectively. The OER activity of FeCoCuO_x should be partially ascribed to its high ECSA with more electrochemical active sites. However, the ECSA is not linearly consistent with the specific surface area. As obtained by nitrogen sorption measurements (Fig. S7†), the BET specific surface areas of FeO_x, CoO_x, FeCoO_x, FeCuO_x, CoCuO_x, and FeCoCuO_x are 390, 33, 306, 252, 143, and 220 m² g⁻¹, respectively.

The electrocatalytic water oxidation activity of FeCoCuO_x was further evaluated by supporting it on a Pt electrode. As demonstrated in Fig. 5a, the bare Pt electrode shows low OER activity with an overpotential of 701 mV at 10 mA cm⁻². FeO_x and CuO_x supported on the Pt electrode show higher OER activity than those supported on the glassy carbon substrate, which should be attributed to a synergistic mechanism between Fe/Cu and Pt. Similar to the results obtained on the GCE, the introduction of copper into the FeO_x, CoO_x, and FeCoO_x systems can enhance the OER activity tested on the Pt electrode. The overpotential at 10 mA cm⁻² decreased from 304 mV for FeCoO_x to 271 mV for FeCoCuO_x (Fig. S8†). The FeCoCuO_x-on-Pt electrode shows a Tafel slope of 52.5 mV dec⁻¹ in 1.0 M KOH (Fig. 5b), slightly higher than that on the GCE.

Fig. 5 (a) Polarization curves of FeO_x, CoO_x, CuO_x, FeCuO_x, CoCuO_x, FeCoO_x, and FeCoCuO_x supported on Pt electrodes in 1.0 M KOH solution without iR-compensation. (b) Tafel plots derived from (a).

To obtain a high-quality electrode, we supported FeCoCuO_x on nickel foam (NF) with good conductivity and high surface area. The OER performance of the as-obtained FeCoCuO_x/NF was measured in 1.0 M KOH electrolyte. As exhibited in Fig. 6a, CuO_x-on-Ni foam shows worse OER activity than bare Ni foam, indicating the poor water oxidation ability of CuO_x. FeO_x-on-Ni foam shows better OER activity than CoO_x-on-Ni foam due to the synergistic effect between Fe and Ni, which has been reported.^46,47 Doping copper into the CoO_x system can improve the OER activity by lowering the overpotential at 10 mA cm⁻² from 333 mV for CoO_x-on-Ni to 307 mV for CoCuO_x-on-Ni (Fig. 6b). It is interesting to observe that the addition of copper to the FeCoO_x system can further improve the catalytic performance. The overpotential at 10 mA cm⁻² for FeCoO_x-on-Ni and FeCoCuO_x-on-Ni is 253 and 226 mV, respectively (Fig. S9†), outperforming some benchmark NiFe-based composite electrodes in alkaline media.^48,49 The Tafel slope of FeCoCuO_x-on-Ni is 49.4 mV dec⁻¹, which is close to that of FeCoCuO_x-on-GCE and FeCoCuO_x-on-Pt (Fig. 6c). The small Tafel slope of FeCoCuO_x implies the facile electron transfer for water oxidation on the surface of the composite.⁵⁰ The OER stability of FeCoCuO_x-on-Ni was investigated (Fig. 6d). FeCoCuO_x needs a small overpotential of about 250 mV to achieve 20 mA cm⁻² and can maintain relatively stable catalytic activity for more than 25 h.

Fig. 6 (a) Polarization curves of FeO_x, CoO_x, CuO_x, FeCuO_x, CoCuO_x, FeCoO_x, and FeCoCuO_x supported on nickel foam in 1.0 M KOH solution with 90% iR-compensation. (b) Overpotential required for a current density of 10 mA cm⁻². (c) Tafel plots derived from (a). (d) Plot of current density vs. time for the FeCoCuO_x catalyst supported on nickel foam at a constant anode voltage of 1.48 V vs. RHE in 1.0 M KOH.

From the above results, it can be inferred that Cu is not the efficient active site for the OER since CuO_x shows poor electrocatalytic water oxidation activity by supporting it on a GCE, a Pt electrode, and Ni foam. The OER activity of FeCoO_x is much higher than that of monometallic FeO_x and CoO_x on different substrates, suggesting a cooperative mechanism between Fe and Co during the OER in agreement with previous reports.^28,51–54 However, it is still controversial which one is the actual active site for the OER. Very recently, Hu et al. reported that a dimeric Co-Fe moiety might be the active site for the OER on a single-atom Co precatalyst in the presence of Fe³⁺.⁵² The main role of Cu in accelerating the OER of FeCoO_x should be ascribed to the promoted charge transfer kinetics as manifested by EIS.

The structure of FeCoCuO_x after the OER stability test was characterized by XRD. Since it is very difficult to obtain sufficient sample for the XRD analysis by supporting it on the GCE, we supported it on a FTO (fluorine doped tin oxide) coated glass electrode for the OER and XRD tests. As manifested in Fig. S10,† besides the characteristic peaks for FTO in the XRD patterns, no additional peaks can be found, suggesting that no crystalline oxides were formed during the OER. The TEM image further verifies that FeCoCuO_x after the OER test still maintains the amorphous state (Fig. S11†). The change of surface metal valence states in FeCoCuO_x was characterized by XPS of the used catalyst. Since it was very difficult to reclaim the used catalysts from GC electrodes to perform XPS analysis, we used FTO coated glass with a large area as the substrate to support the electrocatalyst for the OER. After reaction, the used sample was washed with water several times, dried, and then scraped off from FTO to carry out XPS. The position of the Fe 2p_3/2 XPS peaks shifts from 711.0 eV before the OER to 713.2 eV after OER electrocatalysis (Fig. 7a), suggesting that a large proportion of Fe²⁺ was oxidized to Fe³⁺. The fitted result shows that the surface Fe³⁺/Fe²⁺ ratio increases from 0.77 : 1 in the fresh sample to 6.6 : 1 in the used sample. There is a slight shift of Co 2p to higher binding energy (from 781.1 to 781.8 eV) after the OER (Fig. 7b), indicating that a part of Co²⁺ was oxidized to Co³⁺ during water oxidation. The Cu 2p_3/2 XPS spectrum shows that the Cu species remains after the OER (Fig. 7c), but the binding energy shifts to a higher value of 934.8 eV, revealing that Cu⁺ was completely oxidized to Cu²⁺ during the OER measurement. From the fitted O 1s XPS spectrum (Fig. 7d), the relative content of Fe–OH, Co–OH, and Cu–OH remains unchanged, suggesting the chemical stability of FeCoCuO_x. The electrolyte after the OER was analyzed by ICP-AES, which did not detect obvious Fe/Co/Cu ions (<1 ppm) in the solution, further demonstrating the good stability of FeCoCuO_x for the OER.

Fig. 7 High-resolution XPS spectra of Fe 2p (a), Co 2p (b), Cu 2p (c) and O 1s (d) of FeCoCuO_x after the OER test.

Conclusions

In summary, trimetallic FeCoCuO_x has been displayed as a high-performance and robust electrocatalyst for the OER. The obtained FeCoCuO_x showed good OER activity with low overpotentials of 256, 271, and 226 mV at 10 mA cm⁻² on a GCE, a Pt electrode, and Ni foam, respectively. FeCoCuO_x is more active than bimetallic FeCoO_x, due to the introduction of Cu to promote the charge transfer. In addition, the addition of Cu to the FeCoO_x system significantly enhances the ECSA, which can provide more electrochemically active sites for OER catalysis. This work paves a facile way for developing highly active OER electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of Jilin Province (20180101291JC).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9se00928k

‡ These authors contributed equally to this work.

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