In situ Growth of Ni_xCu_1-x Alloy Nanocatalysts on Redox-reversible Rutile (Nb,Ti)O₄ Towards High-Temperature Carbon Dioxide Electrolysis

Abstract

In this paper, we report the in situ growth of Ni_xCu_1-x (x = 0, 0.25, 0.50, 0.75 and 1.0) alloy catalysts to anchor and decorate a redox-reversible Nb_1.33Ti_0.67O₄ ceramic substrate with the aim of tailoring the electrocatalytic activity of the composite materials through direct exsolution of metal particles from the crystal lattice of a ceramic oxide in a reducing atmosphere at high temperatures. Combined analysis using XRD, SEM, EDS, TGA, TEM and XPS confirmed the completely reversible exsolution/dissolution of the Ni_xCu_1-x alloy particles during the redox cycling treatments. TEM results revealed that the alloy particles were exsolved to anchor onto the surface of highly electronically conducting Nb_1.33Ti_0.67O₄ in the form of heterojunctions. The electrical properties of the nanosized Ni_xCu_1-x/Nb_1.33Ti_0.67O₄ were systematically investigated and correlated to the electrochemical performance of the composite electrodes. A strong dependence of the improved electrode activity on the alloy compositions was observed in reducing atmospheres at high temperatures. Direct electrolysis of CO₂ at the Ni_xCu_1-x/Nb_1.33Ti_0.67O₄ composite cathodes was investigated in solid-oxide electrolysers. The CO₂ splitting rates were observed to be positively correlated with the Ni composition; however, the Ni_0.75Cu_0.25 combined the advantages of metallic nickel and copper and therefore maximised the current efficiencies.

Introduction

Solid oxide electrolysers have demonstrated the tremendous advantages of electrochemical conversion of CO₂ into fuels with high efficiencies using renewable electrical energy^1,2,3. Oxide-ion-conducting solid oxide electrolysers can directly electrolyse CO₂ into CO and pure oxygen under external applied potentials. At the cathode, CO₂ molecules are electrochemically split into CO while the generated O²⁻ ions are transported through the electrolyte membrane to the anode compartment where pure O₂ gas is formed and released^4,5.

The conventional Ni/YSZ electrode has exhibited excellent steam-electrolysis performance under a reducing atmosphere; however, the Ni-cermets are not redox stable and require a significant concentration of reducing gas flowing over the Ni metal to avoid the oxidation of Ni to NiO^6,7. Similarly, the flowing of CO to the composite electrode is also necessary during electrolysis of CO₂ because the oxidation of Ni occurs at high temperatures⁸. Furthermore, the catalytic activity of Ni metal toward the splitting of CO₂ is relatively high; carbon deposition most likely occurs and results in the degradation of cell performance. Some researchers have demonstrated that the deposition of carbon is likely caused by reactions that occur over the catalyst and favour to occur only when CO is present in the chemical reaction system^8,9,10. Cu/YSZ and Cu/SDC have also been considered as potential electrodes because of their ability to adsorb CO₂, resistance to carbon fouling and low cost; however, the catalytic activity of Cu toward CO₂ splitting is inferior to that of Ni^11,12. Cu similarly oxidises during the direct electrolysis of CO₂ or H₂O in the absence of a reducing gas flowing over the composite cathode at high temperatures.

High-temperature electrolysis based on the redox-stable LSCM or LSTO cathode has been reported for the direct electrolysis of H₂O, CO₂ or H₂O/CO₂ and promising electrode performances have also been observed^{13,14,15,16,Fig. 1 (b) shows the XRD patterns of reduced NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (x = 1, 0.75, 0.5, 0.25, 0), which confirms that the reduced NbTi_0.5(Ni_xCu_1-x)_0.5O₄ are a mixture of two phases: Ni_xCu_1-x + Nb_1.33Ti_0.67O₄. As shown in Fig. 1 (b), the NbTi_0.5(Ni_xCu_1-x)_0.5O₄ changes into Nb_1.33Ti_0.67O₄ (PDF No. 053-0293) and metallic Ni_xCu_1-x alloy upon high-temperature reduction in 5% H₂/Ar (99.99%). This result confirms the phase change of NbTi_0.5(Ni_xCu_1-x)_0.5O₄ to highly electrically conducting Nb_1.33Ti_0.67O₄ and pure catalytically active metallic Ni_xCu_1-x alloy.}

Figure 1

XRD patterns of NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (a: patterns of the oxidised form; b: patterns of the reduced form; c: patterns of oxidised form after three redox cycles; d: patterns of the reduced form after three redox cycles).

Figure 2

Cell parameters of the oxidised NbTi_0.5Ni_xCu_1-xO₄ powder samples with x = 0, 0.25, 0.5, 0.75 and 1.0.

To investigate the reversibility of the exsolution of the metallic Ni_xCu_1-x, the composite powders were further treated for another three redox cycles at 1050°C under a reducing atmosphere (5% H₂/Ar) and a oxidising atmosphere (static air) for 20 h. Fig. 1 (c) shows the XRD patterns of the composite after oxidation, which again confirms the successful integration of Ni_xCu_1-x alloys into the Nb_1.33Ti_0.67O₄ substrate and the formation of a single-phase NbTi_0.5(Ni_xCu_1-x)_0.5O₄ solid solution. No phase impurities are observed in the case of NbTi_0.5(Ni_xCu_1-x)_0.5O₄, thereby firmly verifying the superior redox reversibility of the NbTi_0.5(Ni_xCu_1-x)_0.5O₄ ceramics. As shown in Fig. 1 (d), the corresponding XRD patterns confirm that the reduced NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (x = 1, 0.75, 0.5, 0.25, 0) are still composed of conducting ceramic Nb_1.33Ti_0.67O₄ and metallic alloy powders, which further demonstrates the excellent reversible exsolution of the metallic Ni_xCu_1-x from the lattice of NbTi_0.5(Ni_xCu_1-x)_0.5O₄ through a reversible phase transition. The three successive redox treatment cycles of NbTi_0.5(Ni_xCu_1-x)_0.5O₄ solid solutions suggest that the parent material NbTi_0.5(Ni_xCu_1-x)_0.5O₄ and the Ni_xCu_1-x + Nb_1.33Ti_0.67O₄ composite materials exhibit excellent redox reversibility. The metallic Ni_xCu_1-x alloys can be repeatedly exsolved from the NbTi_0.5(Ni_xCu_1-x)_0.5O₄ after the reduction and again integrated back into the Nb_1.33Ti_0.67O₄ substrate to form a homogeneous phase.

To further validate the elemental valence change in redox cycles of the NbTi_0.5(Ni_xCu_1-x)_0.5O₄ solid solution, we performed XPS on the oxidised and reduced NTN₃C₁O (NbTi_0.5(Ni_0.75Cu_0.25)_0.5O₄ and Ni_0.75Cu_0.25 + Nb_1.33Ti_0.67O₄) samples. All XPS patterns were fitted using a Shirley-type background subtraction method. The background functions for different the spectra of different elements were fitted using 80% Gaussian and 20% Lorentzian functions⁴⁶. As shown in Fig. 3 (a), only Nb⁵⁺ is observed in the oxidised NTN₃C₁O sample; however, the Nb⁵⁺ is reduced to Nb⁴⁺ in the Nb_1.33Ti_0.67O₄ substrate when the sample is treated under a reducing atmosphere at 1200°C for 20 h, as seen in Fig. 3 (b)^28,47. The low valence of the Nb⁴⁺ ions favours the formation of Nb-Nb metal bonds and the electronic conductivity is thus highly facilitated, resulting in enhanced electrical conductivity of Nb_1.33Ti_0.67O₄. By contrast, element Ti, as exhibited in Fig. 3 (c), is completely in the Ti⁴⁺ state in the oxidised NTN₃C₁O sample; however, the low-valence Ti³⁺ is also observed in the reduced sample, Nb_1.33Ti_0.67O₄ + Ni_0.5Cu_0.5, as shown in Fig. 3 (d), due to the chemical reduction of Ti⁴⁺ to Ti³⁺ after the heat treatment under a reducing atmosphere. We reasonably speculated that the Nb_1.33Ti_0.67O₄ is actually oxygen-deficient Nb_1.33Ti_0.67O_4-δ according to the charge balance²⁹. The Ti³⁺ (2p1/2) and Ti³⁺ (2p3/2) peaks are observed at 462.80 eV and 457.80 eV, respectively, whereas the Ti⁴⁺ (2p1/2) and Ti⁴⁺ (2p3/2) show peaks at 464.80 eV and 458.10 eV, respectively²⁹. Furthermore, the low-valence Ti³⁺ represents an approximately 0.174 mole ratio of all of the Ti according to this result, which indicates an oxygen deficiency of δ = 0.058 and a chemical formula of Nb_1.33Ti_0.67O_3.942. The Ni and Cu in the NTN₁C₁O are in the form of Ni²⁺ and Cu²⁺, as shown in Fig. 3 (e) and Fig. 3 (g), respectively. In Fig. 3 (f), the Ni⁰ (2p1/2) peaks are observed at 873.86 and 869.40 eV, whereas the Ni⁰ (2p3/2) has three peaks at 861.0, 869.40 and 852.0 eV^16,29,48. A similar change in the chemical state of elemental Cu is also observed in the NTN₁C₁O sample after reduction at high temperatures, as shown in Fig. 3 (h). The Cu⁰ (2p1/2) and Cu⁰ (2p3/2) peaks are observed at 951.88 and 932.20 eV, respectively. According to reports in the literature, the electronic potential energy of alloys of transition metals can be sensitively changed^49,50. In this work, some peaks of binding energy change by approximately 0.2 to 0.3 eV in the case of Ni⁰ and Cu⁰ in the Ni_0.5Cu_0.5 alloy. The Ni⁰ and Cu⁰ exist in the form of metallic alloy nanoparticles in the reduced sample, which further confirms the exsolution of metallic Ni_0.5Cu_0.5 alloy on the ceramic substrate. The XPS results further confirm the reversible transformation between the oxidised NbTi_0.5(Ni_xCu_1-x)_0.5O₄ solid solution and the reduced Nb_1.33Ti_0.67O₄ ceramic electronic conductor loaded with metallic Ni_xCu_1-x alloys.

Figure 3

XPS results for Nb(a), Ti(c), Ni(e) and Cu(g) in the oxidised NbTi_0.5(Ni_0.75Cu_0.25)_0.5O₄ and for Nb(b), Ti(d), Ni(f) and Cu(h) in the reduced NbTi_0.5(Ni_0.75Cu_0.25)_0.5O₄.

TGA of the oxidised NTN₁C₁O and NTNO samples was conducted from room temperature to 1000°C at a heating rate of 4°C·min⁻¹ in a reducing atmosphere of 5% H₂/Ar. To ensure sufficient reduction of the powder sample, the predetermined temperature at 1000°C is stabilised for as long as 3 h and then drifts to room temperature at the same rate, as previously stated. Fig. 4 (a) presents the percentage weight change of the oxidised NTNO powder as a function of temperature when heated under a 5% H₂/Ar atmosphere. The weight loss reaches 7.35% for the NTNO sample because of the loss of oxygen under such reducing conditions, which is in a good agreement with the chemical change of the NbTi_0.5Ni_0.5O₄ solid solution to the reduced Ni + Nb_1.33Ti_0.67O₄ composite material. In comparison, the weight loss of the studied NTN₁C₁O powder reached approximately 7.59%, as seen in Fig. 4 (b), which is reasonably consistent with the theoretical value of 7.61% corresponding to the oxygen loss according to the chemical reaction presented in equation (1). Notably, the onset temperature of weight loss for the NTNO sample is approximately 750°C, as determined from the results in Fig. 4 (a), where the in situ exsolution of Ni metal is anticipated. Nevertheless, the weight loss of NTN₁C₁O starts at temperatures as low as approximately 600°C to grow the metallic alloy. This growth may occur because the incorporation of copper reduces the chemical reaction barrier of metal exsolution and thus facilitates the reversible phase change to grow the metallic NiCu alloys. As evident in both Fig. 4 (a) and Fig. 4 (b), a small weight loss always occurred during the process of cooling. This weight loss is likely to exceed 7.40 and 7.61% for the NTNO and NTN₁C₁O, respectively, if the samples experience longer reduction times at higher temperatures. Ultimately, the obtained reduced sample may be oxygen-deficient Nb_1.33Ti_0.67O_4-δ + Ni or oxygen-deficient Nb_1.33Ti_0.67O_4-δ + Ni_0.5Cu_0.5. Despite the different proportions, our conclusion through reasoning is that these results coincide with the aforementioned results, as shown in Fig 3.

High-resolution transmission electron microscopy (HR-TEM) analysis of the oxidised NTN₁C₁O reveals lattice spacings of 2.546 Å (101) and 3.322 Å (110), as shown in Figs. 5 (a) and (b), consistent with the separation spacing determined by the XRD analysis. As presented in Figs. 5 (c) and 5 (d), the corresponding lattice spacing of the parent material, Nb_1.33Ti_0.67O₄, can be obtained as 2.537 Å (101) and 3.363 Å (110). Moreover, the interplanar spacing is 2.238 Å (111). These spacings can be demonstrated by the results of the XRD analysis, shown in Figs. 1 and 2. Furthermore, the reduction of the NTN₁C₁O leads to the growth of Ni_0.5Cu_0.5 alloy nanoparticles on the Nb_1.33Ti_0.67O₄ surfaces in Fig. 5 (d). The analysis results for the nickel copper alloy indicates a lattice spacing of 1.781 Å (200), consistent with the standard data for the alloy Ni₁Cu₁ data⁵¹. These TEM results further demonstrate and validate the reversible exsolution of metallic particles on the ceramic substrate through control of the phase transformation between the NbTi_0.5(Ni_xCu_1-x)O₄ solid solution and Ni_xCu_1-x/Nb_1.33Ti_0.67O₄, as confirmed by the aforementioned XRD, XPS and TGA analyses. More importantly, the TEM results reveal that the alloy particles are exsolved to in situ grow and anchor to the surface of the highly electronically conducting Nb_1.33Ti_0.67O₄ in the form of heterojunctions, which avoids any possible agglomeration of exsolved metallic nanoparticles on the substrate surface at high temperatures. The metal alloy nanoparticles can be re-incorporated or re-exsolved into/from the host material, yielding a catalyst that can be regenerated through periodic exposure of the material to oxidising/reducing conditions.

Figure 4

TGA thermograms of the oxidised form of NbTi_0.5Ni_0.5O₄ (a) and NbTi_0.5Ni_0.25Cu_0.25O₄ (b) in 5% H₂/Ar; the heating rate was 5°C min⁻¹.

Figure 5

TEM results for the oxidised form of NbTi_0.5Ni_0.25Cu_0.25O₄ (a, b) and for the reduced form of NbTi_0.5Ni_0.25Cu_0.25O₄ (c, d).

Fig. 6 shows the SEM and energy-dispersive X-ray spectroscopy (EDS) maps taken from the oxidised and reduced NbTi_0.5(Ni_0.5Cu_0.5)_0.5O₄ pellets. The sintered samples were reduced in 5% H₂/Ar at 900°C for 17 h. As shown in Fig. 6 (a), the Ni and Cu are homogeneously dispersed in the oxidised NTN₁C₁O sample and the other elements are also well distributed in the bulk. Although the oxidised sintered NTN₁C₁O pellet is not highly dense, clear images are still observed for the sample with low sinterability and high porosity. In comparison, uniform exsolved nanoparticles anchor on the surface of the reduced NbTi_0.5(Ni_0.5Cu_0.5)_0.5O₄ pellet, as shown in Fig. 6 (b), which is further confirmed by the EDS maps of the Ni and Cu. In addition, the Nb, Ti and O, which are components of the new phase of Nb_1.33Ti_0.67O₄, as previously discussed, are distributed evenly in the sample. The SEM and EDS results presented in Figure 6 (b) of the reduced NTN₁C₁O suggest the exsolution of Ni_0.5Cu_0.5 alloy nanoparticles to anchor on the substrate surfaces. The presence of Ni_0.5Cu_0.5 alloy nanoparticles is further confirmed by the SEM and EDS results in addition to the XRD, XPS and TEM results, as previously mentioned. These results indicate that the reversible exsolution of NiCu alloy nanoparticles from the parent NbTi_0.5(Ni_xCu_1-x)_0.5O₄ material to grow in situ and anchor the metal alloy nanocatalyst on the Nb_1.33Ti_0.67O₄ surface is successful. The dispersed Ni_xCu_1-x nanoparticles anchoring on the Nb_1.33Ti_0.67O₄ surface are expected to prohibit the agglomeration and improve the electrocatalytic activity of the composite cathode for high-temperature CO₂ electrolysis.

Figure 6

SEM and EDS results for the oxidised form of NbTi_0.5Ni_0.25Cu_0.25O₄ (a) and for the reduced form of NbTi_0.5Ni_0.25Cu_0.25O₄ (b).

The dependence of the conductivity of the samples on the temperature and oxygen partial pressure was systematically investigated, as shown in Fig. 7. According to the literature, Nb_1.33Ti_0.67O₄ is a mixed conductor because of the edge sharing of NbO₆ octahedra along the c-axis, which results in Nb-Nb metal bond overlap with an inter-metallic distance of 3 Ų⁹. The reduced NbTi_0.5(Ni_xCu_1-x)_0.5O₄ ceramic samples (Nb_1.33Ti_0.67O₄ + Ni_xCu_1-x) exhibit greater conductivity, as shown in Fig. 7 (a), because the metallic Ni_xCu_1-x nanoparticles dispersed on the substrate improve the electrical conductivity under a reducing atmosphere (5% H₂/Ar, 99.99% purity). The reduced NbTi_0.5Cu_0.5O₄ (Nb_1.33Ti_0.67O₄ + Cu) sample exhibits the highest conductivity, which reaches approximately 500 S·cm⁻¹ at 730°C. With increasing nickel composition, the conductivity of the reduced samples gradually decreases to 204 S·cm⁻¹ with the maximum Ni content in the reduced NbTi_0.5Ni_0.5O₄ (Nb_1.33Ti_0.67O₄ + Cu) at the same temperature. This phenomenon is possibly due to the Cu metal exhibiting conductivity superior to that of Ni metal at high temperatures. The conductivities measured under a reducing atmosphere is mainly attributed to the electronically conducting ceramic Nb_1.33Ti_0.67O₄, in agreement with results reported in the literature^26,27; however, the percolating metal network is not formed due to the presence of dispersed Ni_xCu_1-x alloys. As shown in Fig. 7 (b), the conductivities of Nb_1.33Ti_0.67O4 + Ni_xCu_1-x are generally independent of the oxygen partial pressure at partial pressures less than 10⁻⁴ atm; however, the conductivity significantly decreases when the samples are oxidised at high oxygen partial pressure due to the oxidation of Nb_1.33Ti_0.67O₄ + Ni_xCu_1-x to the NbTi_0.5(Ni_xCu_1-x)_0.5O₄ solid solution.

Figure 7

(a) The dependence of conductivity on temperature of the reduced form of NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (x = 0, 0.25, 0.5 and 1.0) samples; (b) the dependence of the conductivity on the oxygen partial pressure of the reduced form of NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (x = 0, 0.25, 0.5 and 1.0) samples at 800°C.

Fig. 8 shows the AC impedance plots of the symmetric cells tested under a series of different hydrogen partial pressures (0, 2, 5, 20, 40, 50, 60, 80 and 100% H₂) with nitrogen to adjust the hydrogen concentrations at 800°C. The impedance plots show two intercepts with the real axis, where the series resistant of the cell (R_s) corresponds to the first intercept and the difference between the two intercepts is a measure of the electrode polarisation resistance (R_p). The Zview software (IM6, Zahner, Germany) was employed to calculate R_s and R_p, as reported in our previous work^46,48. The ionic resistance of the 1 mm YSZ electrolyte is the primary contributor to the R_s, which is approximately 2 Ω and is generally stable over a wide range of hydrogen partial pressures. In our previous work, we reported that a symmetric solid-oxide cell with a composite electrode based on Nb_1.33Ti_0.67O₄ exhibits poor performance, where the electrode polarisation resistance is approximately 300 Ω cm²²⁹. As shown in Figs. 8 (a), (b) and (c), the R_p of the symmetric cell based on NTNO-SDC decreases from 18.65 to 3.02 Ω cm² over the hydrogen partial pressure ranging of 0 to 100%. Similarly, the R_p values of the symmetric cells based on NTN₃C₁O, NTN₁C₁O, NTN₁C₃O and NTCO are shown in Figs. 8 (d, e, f), Figs. 8 (g, h, i), Figs. 8 (g, k, l) and Figs. 8 (m, n, o), respectively; the R_p decreases from 34.72 to 6.08 Ω cm², 45.61 to 14.26 Ω cm², 57.04 to 7.64 Ω cm² and 39.85 Ω cm² to 10.81 Ω cm², respectively. All of the R_p values change as functions of the hydrogen partial pressure, which suggests that the stronger reducing atmosphere improves the electrode polarisations. A stronger reducing atmosphere favours the growth of metallic Ni_xCu_1-x alloy particles on the surface of the electrode skeleton and thus enhances the electrocatalytic activity of the composite electrodes. However, the polarisation resistance gradually decreases with increasing nickel content when compared with all of the proportions, which is attributed to the catalytic activity of nickel metal being greater than that of copper. The electrocatalytic activity of the composite electrode can be continuously tuned according to the strong dependence of the electrode activity on the alloy compositions under reducing atmospheres at high temperatures.

Figure 8

AC impendence plots for the symmetrical cells based on NbTi_0.5(Ni_xCu_1-x)_0.5O₄ electrodes with x = 1 (a, b, c), x = 0.75 (d, e, f), x = 0.5, (g, h, i), x = 0.25 (j, k, l) and x = 0 (m, n, o) tested at 800°C under different hydrogen partial pressures.

To investigate the sealing of the single solid-oxide electrolyser based on the NTN_xC_1-xO composite cathode, we recorded the open-circuit voltage (OCV) with the cathode and anode exposed to 5% H₂/Ar and static air, respectively and examined the separation between the anodic and cathodic gases. The OCVs of the cells reach approximately 1.0 V, which indicates good separation between the anodic and cathodic gases. Fig. 9 shows the I–V curve at 800°C with 100% CO₂ introduced to the cathode and with the anode exposed to static air. The relationship between the current and the applied voltage is far from linear, with the maximum current densities reaching 169.71, 113.17, 80.46, 69.14 and 56.57 mA cm⁻² at 1.6 V for the electrolysers based on NTNO, NTN₃C₁O, NTN₁C₁O, NTN₁C₃O and NTCO, respectively. Numerous investigators have demonstrated that the electrolysis current depends on the availability of the three-phase interfaces. However, the electrolysis current also increases rapidly with increasing nickel content because of the high catalytic activity of nickel. Thus, a high content of nickel catalyst improves the cathode performance.

Figure 9

I–V curves of the solid oxide electrolysers based on NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) cathodes for CO₂ electrolysis at 800°C.

In situ AC impedance spectroscopy was carried out under different applied voltages to investigate the changes to R_s and R_p, where R_s and R_p correspond to the series resistance and polarisation resistance of the electrolyser, respectively. As shown in Fig. 10, all of the series resistances, R_s, are stable; however, the R_p values sharply decrease as a function of the applied voltage in the range of 1.2 to 1.4 V. The R_p is significantly enhanced by the application of higher external potentials because the passing current density not only activates the electrode, but also electrochemically reduces the cathode to enhance the mixed conductivity and electrocatalytic activity. We observed that the R_p is only 2.17 Ω·cm² at 1.4 V in CO₂ in the case of the NTNO cathode, whereas the R_p is approximately 2.85 Ω·cm² at low voltage of 1.1 V as shown in Fig. 10 (a). However, as seen in Figs. 10 (b), (c) and (d), the R_p values are 6.21, 9.94 and 6.22 Ω·cm² for the electrolysers with NTN₃C₁O, NTN₁C₃O and NTCO cathodes at 1.4 V, respectively. Here, the exsolution of metallic nanoparticles may significantly improve the electrocatalytic activity of the composite cathodes and the passing current may further activate the composite electrodes and improve the electrode polarisation. To a certain extent, we conclude that the nickel content in NiCu alloys and the crystal structure of NiCu alloys may have a coordinated action for CO₂ electrolysis; if this is not the case, the R_p should increase with decreasing proportion of nickel because nickel exhibits superior catalytic performance for CO₂ decomposition.

Figure 10

AC impendence plots of the solid oxide electrolysers based on NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (a: x = 1; b: x = 0.75; c: x = 0.25; d: x = 0) composite electrodes for CO₂ electrolysis with the oxygen electrode exposed in static air at 800°C.

Fig. 11 shows the rate of CO production and the current efficiency of the electrolysers versus the nickel composition based on NTN_xC_1-xO composite cathodes for direct CO₂ electrolysis under applied voltages of 1.2, 1.3 and 1.4 V, respectively. As shown in Fig. 11 (a), the CO production rates gradually improve with increasing applied voltage for all of the electrolysers and exhibit the highest rates at a final applied voltage of 1.4 V because high voltages favour CO₂ splitting at the cathode. The CO production rate of the electrolyser with the NTN₃C₁O (x = 0.75) composite cathode is 0.1629 ml·min⁻¹·cm⁻² at 1.4 V, which is approximately three times greater than the rate of 0.04113 ml·min⁻¹·cm⁻² for the cell at the same applied voltage of 1.2 V. However, we note that the productionrate of CO is strongly dependent on the nickel composition in the parent material, i.e., the NbTi_0.5(Ni_xCu_1-x)_0.5O₄ solid solution, which definitely determines the nickel concentration of the metallic Ni_xCu_1-x alloy nanocatalysts. The electrolyser produces CO at a rate of 0.07663 ml·min⁻¹·cm⁻² in the case of the NTCO composite cathode and was significantly enhanced to as high as 0.3866 ml·min⁻¹·cm⁻² in the case of the NTNO cathode at a voltage of 1.4 V. These results again verify the superior catalytic activity of the nickel catalyst toward electrochemical CO₂ splitting. Similar behaviour of the CO production rate versus the nickel composition was also observed at applied voltages of 1.2 and 1.3 V. Clearly, the CO production improves slowly when x < 0.5 and then increases rapidly in the interval of 0.5 ≤ x ≤ 1. This behaviour can be attributed to the excellent catalytic activity of the Ni_xCu_1-x alloy when the nickel concentration is high.

Figure 11

(a) CO production rate for the cells based on NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) composite cathode versus x of NbTi_0.5(Ni_xCu_1-x)_0.5O₄; (b) the relationship between the current efficiency of the CO₂ electrolysis and x in the NbTi_0.5(Ni_xCu_1-x)_0.5O₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) composite cathodes.

Fig. 11 (b) shows the current efficiencies of the electrolysers versus the nickel composition for the case of NTN_xC_1-xO composite cathodes during direct CO₂ electrolysis under different applied voltages. The current efficiencies gradually improve with increasing copper content in the composition range of 0 ≤ x ≤ 0.75 because of the adsorption of CO₂ onto copper. However, the current efficiencies begin to decrease when the copper content is in the rage of 0.75 ≤ x ≤ 1, although they are still greater than the current efficiencies of the cathode with x = 0. Copper with excellent CO₂/CO adsorption properties is expected to facilitate the electrochemical conversion of CO₂ to CO and to therefore improve the current efficiencies. The optimum composition of Ni_0.75Cu_0.25 alloy catalyst combines the advantages of both metallic nickel and metallic copper to maximise current efficiencies for the direct CO₂ electrolysis. The highest current efficiency of 74.29% are obtained in the case of the NTN₃C₁O cathode at 1.4 V. Similarly, the current efficiencies are accordingly improved versus the applied voltage because the applied voltages electrochemically reduce/activate the composite cathodes to enhance the electrocatalytic activity⁵⁶. The in situ growth and anchoring of Ni_xCu_1-x alloy nanocatalysts on the Nb_1.33Ti_0.67O₄ surface combines the advantages of metallic nickel and copper and produces a synergistic effect to maximise the current efficiencies for the direct electrolysis of CO₂. The coupling of metal catalysts with the electronically conducting ceramic substrate offers possibilities for the direct CO₂ electrolysis at high temperatures.

Conclusion

In this work, the in situ growth of Ni_xCu_1-x (x = 0, 0.25, 0.50, 0.75 and 1.0) alloy catalysts was achieved to anchor on the redox-reversible Nb_1.33Ti_0.67O₄ ceramic substrate in the form of heterojunctions. The exsolution/dissolution of the metal particles is completely reversible during redox cycling. The electrocatalytic activity of the composite electrode based on Ni_xCu_1-x/Nb_1.33Ti_0.67O₄ can be continuously tuned by tailoring the alloy composition. The coupling of metal catalyst with the electronically conducting ceramic substrate achieves direct CO₂ electrolysis at high temperatures, whereas the combined advantages of metallic nickel and copper produces a synergistic effect to maximise the current efficiencies. Electrocatalytic activity of the composite materials was achieved through direct exsolution of metal particles from the crystal lattice of the ceramic oxides under a reducing atmosphere at higher temperatures. The current work expands our understanding of advanced ceramic cathodes with excellent electrocatalytic activity in the field of high-temperature electrolysis.