Introduction

Proton exchange membrane water electrolysis (PEMWE) has sparked widely attention as a most promising technology for direct conversion of electrical energy into fuels by electrochemical water splitting1,2,3. Essential to the water electrolyzers is oxygen evolution reaction (OER) taking place at anode, where a complex multistep proton-coupled electron transfer process generates a sluggish kinetics3,4,5. In acid PEMWE, precious-metal oxides of IrO2 and RuO2 are currently regarded as the most active OER catalysts6. However, high costs and still poor intrinsic activities greatly limit their catalytic efficiencies. Therefore, develo** efficient catalysts with less content of precious-metal and higher intrinsic activity is quite appealing for acidic water oxidation.

Recently, the synthesis of complex oxides has been reported to be a promising approach for both reducing the precious-metal usage and promoting the intrinsic activity7,8,9,10,11,12,13. Various complex oxides with less precious-metal contents, including perovskite7,8,9,10 and pyrochlore-type11,12,13 compounds, were found to exhibit higher intrinsic OER activities relative to the simple binary oxides under acid conditions. For instance, the specific activity reached to about 2.8 mA cm−2oxide at 1.55 V for perovskite oxide Ba2NdIrO6, which is about 14 times larger than that of the commercial IrO2 (∼0.2 mA cm−2oxide)8. A more efficient IrOx/SrIrO3 catalyst with a high specific current density of 10 mA cm−2oxide at 1.50 V was revealed by strontium leaching from surface layers of SrIrO3 thin films during electrochemical testing9. Similar promotions were also observed in pyrochlore-type Y2Ir2O7 and Y2Ru2O7-δ catalysts12,13. Compared to iridium, ruthenium is much lower in price4,6,13. Moreover, ruthenium oxide typically exhibits higher activity than iridium oxide4,6. Therefore, the ruthenium oxides with complex structures represent promising catalysts for acidic water oxidation.

In this work, we report a complex perovskite oxide CaCu3Ru4O12 as a highly efficient catalyst for OER in acid. Unexpectedly, this ruthenate catalyst presents an ultralow overpotential of 171 mV at 10 mA cm−2geo in 0.5 M H2SO4 solution, surpassing the most reported robust OER catalysts in acid up to date. Moreover, it achieves large mass activity of 1942 A g−1Ru and specific activity of 22.1 mA cm−2oxide at 1.50 V, which are 170 and 96 times higher than those of the commercial RuO2, respectively. The long-term durability tests also reveal that CaCu3Ru4O12 shows much better stability than RuO2. The excellent OER performance of this ruthenate oxide would make it a promising catalyst in commercial PEMWE.

Results

Synthesis and characterization

The ruthenate oxide CaCu3Ru4O12 belongs to the typical A-site ordered quadruple perovskite compound AA'3B4O12, whose crystal structure can be considered as a 2 × 2 × 2 superstructure of simple cubic perovskite ABO3. As shown in Fig. 1a, one Ca2+ and three Cu2+ ions occupy A-sites in order, while the Ru4+ ions form corner-share RuO6 octahedra at B-sites14,15. The polycrystalline powders of this compound were synthesized by a conventional solid-state reaction. Scanning electron microscopy (SEM) image displays that the particle sizes are submicron (Supplementary Fig. 1). Figure 1b shows the powder X-ray diffraction (XRD) of as-prepared sample, where all the diffraction lines are well indexed to the cubic (Im-3) phase without detectable amount of impurities. On the base of Rietveld refinements on diffraction data, the lattice parameter a and Ru–O bond length are determined to be 7.4206(3) and 1.9808(7) Å (Supplementary Table 1), respectively, which well agrees with the reported values14. The good crystallinity and single-phase structure are further confirmed by high resolution transmission electron microscopy (HRTEM) image together with selected area electron diffraction (SAED) pattern (Fig. 1c). The observed lattice fringes with inter-plane spacing of 0.265 nm correspond to the \(\left( {02\bar 2} \right)\) or \(\left( {20\bar 2} \right)\) plane of cubic phase CaCu3Ru4O12. X-ray photoelectron spectra (XPS) measurements (Fig. 1d) reveal that CaCu3Ru4O12 exhibits two peaks at 464.2 and 486.3 eV, which are similar to those of RuO2 and assigned to 3p3/2 and 3p1/2 states of Ru4+ ions16, respectively. The energy dispersive X-ray spectrum (EDS, Supplementary Fig. 2) demonstrates that the molar ratio of Ca/Cu/Ru is 1:2.98:4.03, which suggests a good stoichiometry of the as-synthesized sample.

Fig. 1
figure 1

Structural characterization of CaCu3Ru4O12. a Crystal structure of CaCu3Ru4O12. Color code: Ca (gray), Cu (bronze), Ru (green), and O (red). b XRD pattern for CaCu3Ru4O12 together with the Rietveld refined results. c HRTEM image and SAED pattern (inset) for CaCu3Ru4O12. The scale bar is 2 nm. d Ru 3p XPS spectra

Full size image

OER performance

We evaluated the electrocatalytic OER activity of CaCu3Ru4O12 in O2-saturated 0.5 M H2SO4 solution using a standard three-electrode system. Figure 2a shows the polarization curve for CaCu3Ru4O12, along with the commercial RuO2 as a reference. For RuO2, a large overpotential (η) of 316 mV was required to achieve a current density of 10 mA cm−2geo, which is in line with those previous reports (Supplementary Table 2). Intriguingly, for CaCu3Ru4O12, the current density is remarkably enhanced and the overpotential is much reduced to 171 mV. Moreover, by normalizing the current density to catalyst mass or Ru metal mass, a significant increase in the mass activity is observed (Supplementary Fig. 3). For example, the mass activity of CaCu3Ru4O12 achieves 1942 A g−1Ru at 1.50 V, which is 170 times higher than that of RuO2 (11.4 A g−1Ru). As shown by Tafel plots in Fig. 2b, this enhanced performance is further verified to be intrinsic by the specific activity, which is obtained from normalizing the mass activity with Brunauer-Emmett-Teller (BET) surface area (Supplementary Fig. 4). Notably, the specific current density of CaCu3Ru4O12 at 1.50 V reaches up to the value of 22.1 mA cm−2oxide, which is 96 times larger than that of RuO2 (0.23 mA cm−2oxide). The similar feature is also found in the specific activities normalized by the electrochemically active surface areas (ECSA), which are derived from the electrochemical double-layer capacitance measurements (Supplementary Fig. 5, 6). Meanwhile, the Tafel plots illustrate that CaCu3Ru4O12 presents a smaller Tafel slope of 40 mV dec−1 than RuO2 of 67 mV dec−1, suggesting its significantly accelerated OER kinetic. The faster OER kinetic rate is further reflected by the electrochemical impedance spectroscopy (EIS) measurements, where a remarkable decrease of charge transfer resistance (Rct) is revealed for the ruthenate catalyst (Supplementary Fig. 7).

Fig. 2
figure 2

Electrochemical OER performance. a Polarization curves of CaCu3Ru4O12 and the commercial RuO2 measured in O2-saturated 0.5 M H2SO4 solution. b Tafel plots of specific OER activity. c Comparison of the overpotentials at 10 mA cm−2geo for CaCu3Ru4O12 and recent reported OER catalysts in acid media. d Chronopotentiometric measurements of CaCu3Ru4O12 and the commercial RuO2 at 10 mA cm−2geo

Full size image

Our electrochemical tests clearly demonstrate that CaCu3Ru4O12 exhibits a superior activity for acidic water oxidation. To the best of our knowledge, this complex oxide has the lowest overpotential at 10 mA cm−2geo among the excellent OER catalysts in acid electrolytes as reported up to date (Fig. 2c and Supplementary Table 3). Furthermore, its intrinsic activity also outperforms all of them. Particularly, the specific current density of the ruthenate at 1.50 V is even about two-fold higher than that of the recently reported best oxide catalyst IrOx/SrIrO3 (Supplementary Table 3).

To access the electrochemical stability of CaCu3Ru4O12, we performed a long-term chronopotentiometry at a constant current density of 10 mA cm−2geo, together with RuO2 for comparison. As shown in Fig. 2d, the commercial RuO2 nearly loses all its activity after 1 h. In constrast, the overpotential for CaCu3Ru4O12 just slightly increases from 171 to 192 mV after a 24 h continuous operation. The high stability of this complex oxide is also supported by the XRD, XPS, and HRTEM measurements after the OER test. No visible changes in the positions of XRD diffraction lines and XPS core level peaks (Supplementary Fig. 8) are found on the materials before and after the OER testing, while there is a marked decrease in the intensities due to the less catalyst mass used in the measurements after the OER testing. HRTEM image (Supplementary Fig. 9) further shows that the lattice fringes for the CaCu3Ru4O12 particle after the durability test still extend all the way to the surface, which suggests that the surface structure of CaCu3Ru4O12 is maintained during OER. In addition, inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements (Supplementary Table 4) reveal that less than 6.7% Ca, 2.9% Cu, and 2.7% Ru ions are found to be dissolved in the electrolyte, indicating just a slight cation leaching during the OER operation. This is also confirmed by TEM-EDS spectrum (Supplementary Fig. 10), which shows a small change of Ca/Cu/Ru ratio for ruthenate catalyst after the acidic OER testing. These results suggest that this ruthenate oxygen-evolving catalyst shows good chemical stability in acid, which will be beneficial for the commercial applications.

Discussion

To understand the enhanced OER activity in the complex oxide CaCu3Ru4O12, density functional theory (DFT) calculations were carried out. Figures 3a, b show the computed density of states (DOS) for RuO2 and CaCu3Ru4O12, respectively. The large DOS from Ru 4d and O 2p bands cross the Fermi level for both the oxides, which indicates that they present an intrinsically metallic behavior. However, compared to RuO2, CaCu3Ru4O12 exhibits a downshift in the projected DOS for both Ru 4d band and O 2p band relative to the Fermi level. For RuO2, the calculated Ru 4d band and O 2p band centers are 0.72 and 0.16 eV, while the values are reduced to −1.37 and −3.42 eV for CaCu3Ru4O12, respectively. To confirm these electronic features, we further conducted the electrical transport measurements. Figure 3c plots the temperature-dependent resistivity for RuO2 and CaCu3Ru4O12. Upon cooling, their electrical resistivities show a monotonous decrease. This temperature dependence reveals an intrinsic metallic state for both the oxides, agreeing well with the DFT calculations. The metallic conductivity could ensure fast charge transfer between catalyst-electrolyte and catalyst-support electrode interfaces, which is beneficial for the OER process17. Moreover, the resistivity of CaCu3Ru4O12 is apparently higher than that of RuO2 within the measured temperature range. For example, CaCu3Ru4O12 exhibits a more than threefold increase in the resistivity (1.5 mΩ cm) at room temperature compared to RuO2 (0.43 mΩ cm). The larger resistivity is supposed to be associated with the Ru 4d and O 2p band centers far away from the Fermi level owing to their downshifts to low energies.

Fig. 3
figure 3

Electronic structure studies. a, b Computed density of states (DOS) of a RuO2 and b CaCu3Ru4O12. c Temperature dependence of resistivity. d O K-edge XAS

Full size image

During the OER process, the conventional adsorbate mechanism demonstrates that the electrochemical activity of catalysts is determined by the binding strengths of adsorbed intermediates such as HO*, O*, and HOO* to active sites18,19,20,21,22. For transition metal oxide OER catalysts, the d-band center of active metal sites is closely related to the binding strength between metal sites and adsorbed oxygen species22,23,24. Usually, a lower d-band center generates a weaker metal-oxygen binding22,23,24. In our case, the refined Ru–O bond length of CaCu3Ru4O12 is 1.9808(7) Å, which is longer than the average Ru–O bond length of RuO2 (~1.97 Å)25,26. This implies that CaCu3Ru4O12 has a weaker Ru–O bonding strength compared to RuO2, well associated with that the former has a lower Ru 4d-band center. To further assess the strength of Ru–O bonds in our catalysts, we performed the O K-edge X-ray absorption spectroscopy (XAS). O 1 s spectra in K-edge XAS reflect the transition from O 1 s core level to unoccupied O 2p states hybridized with metal ions27,28. A higher absorption intensity of O 1 s spectra suggests a stronger hybridization between oxygen and metal ions. As shown in Fig. 3d, two pre-edge peaks are clearly observed at 528.8 and 531.7 eV for our catalysts, which are assigned to the unoccupied orbitals of O 2p hybridized with Ru 4d t2g and eg orbitals29,

$$E_{iR - {\mathrm{corrected}}} = E - iR,$$
(1)

where i is the current, and R is the uncompensated ohmic electrolyte resistance (~8 Ω) measured via high frequency ac impedance in O2-saturated 0.5 M H2SO4 solution. EIS measurements were carried out at different potential values with the frequency ranging from 100 kHz to 100 mHz under an AC voltage of 5 mV. The impedance spectra were presented in the form of Nyquist plot and fitted using ZView software with a representative equivalent electrical circuit. ECSA was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammetry (CV)10,17. The potential window of CV was set to be 1.21–1.31 V vs RHE and the scan rates were 10, 20, 40, 60, 80, and 100 mV s−1. The double-layer capacitance (Cdl) was estimated by plotting the Δj = (j+ − j−)/2 at 1.26 V vs RHE against the scan rate. The ECSA was calculated by the following equation:

$${\mathrm{ECSA = }}\left( {{\mathrm{C}}_{{\mathrm{dl}}}{\mathrm{/C}}_{\mathrm{s}}} \right){\mathrm{/M,}}$$
(2)

where Cs and M represented the specific capacitance and the loading mass for the catalyst, respectively. A Cs value of 60 μF cm−2 was used as the common estimate for oxide surfaces38,39. To conduct the long-term chronopotentiometric measurements, the catalysts were deposited on carbon paper with a mass loading of 0.25 mg cm−2 and followed by a heat-treating in ambient air at 300 °C. To carry out XRD and XPS measurements after the durability test for the catalysts, we followed the approaches frequently reported in recent literatures7,12,13, where the catalysts were loaded onto carbon paper with a high loading of 1 mg cm−2 for the durability test. After washing with ethanol and sonication, the specimens of ~2 mg were collected to use.

Computational methods

All the density functional theory (DFT) calculations were performed with the Vienna Ab-initio Simulation Package (VASP). The projector augmented wave (PAW) potentials and Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional were adopted. All calculations were conducted using a plane-wave kinetic energy cutoff of 520 eV. The energy convergence criterion was set to be 10−5 eV, and the force on each ion was converged to less than 0.05 eV/Å. The relevant details, models, and references are given in the Supplementary Methods section.