3.1 Characterization results illustrating electrode morphology and structure
SEM and TEM have been applied to explore the morphology and microstructure of NiCu@NCNPs as well as corresponding control samples Cu@NCNPs and Ni@NCNPs electrodes. The SEM and TEM results of the prepared NiCu@NCNPs electrode are shown in Figs. 1a and 1b, which exhibit monodispersed spherical porous particles. According to literature, the surface micropores are formed by the evaporation of the zinc species during the high temperature pyrolysis step, thus leaving micropores in the carbon matrix, and are externally manifested as tiny micropores on the surface of the material (Yang et al. 2018a). The Ni@NCNPs and Cu@NCNPs obtained by the same preparation method yield similar results as shown in Figure S1, with an average particle size of 50 nm and clear spherical particles as NiCu@NCNPs. The corresponding energy dispersive X-ray spectroscopy (EDS) images demonstrate that the Ni and Cu elements are equally dispersed and precisely matched on the mapping.
The elemental composition and valence states of various elements were determined by XPS with surface sensitivity. Figure 2a shows the spectra of Cu 2p spectrum for Cu@NCNPs and NiCu@NCNPs. As illustrated in Fig. 2a, two deconvoluted peaks are observed at 932.7 eV and 934.9 eV for Cu@NCNPs which are attributed to Cu2+ and Cu+/Cu0 species, respectively. Similar peaks are observed on the electrode of NiCu@NCNPs in Fig. 2a demonstrating that doping another metal will not change the valence state of Cu species on the surface of the catalyst. Figure 2b indicates that both NiCu@NCNPs and Ni@NCNPs possess + 2 valence Ni species (856.1 eV). The N1s peaks of all three samples are depicted in Fig. 2c, with five deconvoluted peaks for N assigned to pyridinic-N (398.4 eV), M-N (399.3 eV), pyrrolic-N (400.2 eV), graphitic-N (401.2 eV), and oxidized-N (404.1 eV). The results observed in Fig. 2c are consistent with those in literature (Cheng et al. 2020; Hu et al. 2019), confirming the successful synthesis of heteroatom-embedded ZIF-derived N-doped carbon materials.
In addition, XRD was adopted to determine the composition of materials and the physical phases. As shown in Fig. 2d, two wide peaks of modest intensity are observed around 24° and 44° in the XRD patterns of all three samples, which are consistent with previous report (Li et al. 2021), confirming the M-N-C structure on all samples. However, no metal diffraction peak regarding Ni or Cu is found indicating the absence of metal particles. That means Ni and Cu coexist across the entire carbon matrix in the form of atoms.
3.2 Electrochemical measurements comparing electrode catalytic performance
Electrochemical experiments have been performed on various electrodes to compare the catalytic performance in CO2RR. Faradaic efficiency (FE) and particle current density (PCD) are used to describe the product selectivity and activity from CO2RR. As described in the experimental section above, catalysts with various Ni/Cu mole ratios were prepared and they were all tested for the electroreduction of CO2. To illustrate the results more evidently and explicitly, NiCu@NCNPs and control samples Cu@NCNPs and Ni@NCNPs were chosen as examples in Fig. 3a to display the profile of CO2 electroreduction selectivity under different potentials. As can be seen from Fig. 3a, the value of FECO on Cu@NCNPs at first grows with a continuing increase in the applied potential and reaches the highest value (55.9 ± 3.5%) at the optimal potential between − 0.78 V and − 0.90 V vs RHE. It then drops afterward because of the significant enhancement in the competing hydrogen evolution reaction (HER), consistent with the observed increasing FE of H2 when the potential applied is higher than the optimal potential, as shown in Fig. 4. A similar trend is observed about FE values from CO2RR on the Ni@NCNPs catalyst where the optimum FECO of 78.2 ± 2.4% is achieved at -0.9 V vs RHE. Surprisingly, different with the other two catalyst, the electrode with NiCu@NCNPs in this work displays a reversal outstanding performance. A high CO Faradaic efficiency of 93.1 ± 1.2% can be obtained at a low potential of -0.65 V vs RHE for NiCu@NCNPs, even higher than the optimal values of FECO on Cu@NCNPs and Ni@NCNPs, indicating that NiCu@NCNPs is more selective in CO production and consumes less energy in CO2RR. An optimal CO FE around 100% can be achieved on NiCu@NCNPs with trace production of H2 after the applied potential reaches at -0.9 V vs RHE. It seems that − 0.9 V vs RHE is the optimal potential for all three samples. Additionally, the CO FE remains almost unchanged in the voltage range of -0.9 V to -1.14V vs RHE, which signifies that at the appropriate voltage range, NiCu@NCNPs can significantly suppress HER and produce almost pure CO from CO2RR. This indicates the probability that on NiCu@NCNPs, CO2 can be completely converted to the required CO product in a proper voltage region without following separation procedures.
In order to find out the ideal Ni/Cu mole ratio in carbon material for CO2RR, the electrocatalytic performance of electrodes with various Ni/Cu mole ratios was examined under the optimal potential at -0.9V vs. RHE. As shown in Fig. 3b, the CO Faradaic efficiencies of Ni@NCNPs, Ni3Cu1@NCNPs, NiCu@NCNPs, Ni1Cu3@NCNPs and Cu@NCNPs samples were compared to illustrate the optimal Ni/Cu mole ratio for CO2RR at -0.9 V vs RHE. It is obvious that dual-metal-doped catalysts possess higher selectivity than monometallic ones, and among the various dual-metal-doped catalysts, NiCu@NCNPs exhibit the highest CO Faraday efficiency (nearly 100%), indicating the optimal Ni/Cu mole ratio is 1.
Figure 3c compares the PCD values of CO on all three materials with the change in applied potential. As shown in Fig. 3c, the current density of CO on NiCu@NCNPs is 8 times the value on Cu@NCNPs and 4 times that on Ni@NCNPs at the optimized potential at -0.9 V vs. RHE. The results in Fig. 3c indicate that compared with the other two electrodes, NiCu@NCNPs has exhibited enhanced activity in CO2RR towards CO.
Besides of activity and product selectivity, the catalytic stability is also an important criterion in evaluating its catalytic performance in CO2RR. The stability of the NiCu@NCNPs electrode has also been evaluated using five successive CO2RR experiments at the optimized applied potential. It is demonstrated in Fig. 3d that the CO selectivity of the material keeps almost the same during the five cycles of CO2RR experiments, verifying that the NiCu@NCNPs electrode exhibits excellent stability during the electrochemical reduction of CO2.
In order to illustrate this point more clearly, XPS tests were performed for the NiCu@NCNPs sample before and after CO2RR. Figure S2 shows the spectra of N 1s spectrum for NiCu@NCNPs before and after the reaction separately. The N spectrum after the reaction exhibited 5 deconvoluted peaks assigned to pyridinic-N (398.4 eV), M-N (399.3 eV), pyrrolic-N (400.2 eV), graphitic-N (401.2 eV), and oxidized-N (404.1 eV) which stay same with the N spectrum of NiCu@NCNPs before reaction. Which indicates that the structure of the NiCu@NCNPs sample was not altered throughout the process of CO2RR.
3.3 The correlation between the structure of electrodes and corresponding catalytic performance
The excellent electrocatalytic effect of NiCu@NCNPs is directly related to its structure. Combined with various characterization results, conclusions can be easily drawn that NiCu@NCNPs is a crystalline material with an organized pore structure, allowing the active center of the material to be exposed at a high density and therefore being well suited for catalytic applications. Furthermore, the low dimensionality of NiCu@NCNPs enhances its external specific surface area, further optimizing the electrocatalytic efficiency. In order to illustrate this point more clearly, cyclic voltammetry experiments have been performed to compare the electrochemically active surface areas (ECSA) on all three materials. Figure S3 shows cyclic voltammograms for three catalysts at various scan rates. To compare the ESCA of three materials, double layer capacitance (Cdl) measurements were carried out. As clearly illustrated in Fig. 5, NiCu@NCNPs shows the highest Cdl value among all three catalysts. Specifically, the Cdl value of NiCu@NCNPs is at 2.858 mF cm− 2, which is roughly 3.3 times that of Ni@NCNPs (0.878 mF cm− 2) and 8.2 times that of Cu@NCNPs (0.349 mF cm− 2). The ESCA value for each electrode can be calculated from the formula: \(ESCA={C}_{dl} / {C}_{s}\) and the Cs in the work is 20µF cm−2. Thus, the calculated ECSA values on the three electrodes follow this trend: NiCu@NCNPs (142.5 cm− 2) > Ni@NCNPs (43.9 cm−2) > Cu@NCNPs (17.5 cm−2). Based on this result, the introduction of NiCu diatomic metals contributes to more surface-active sites, thus leading to the enhanced catalytic performance in CO2RR.
In order to further explore the potential mechanisms leading to the superiority of the electrode with NiCu@NCNPs over the other two materials, electrochemical impedance spectroscopy (EIS) has been employed to comprehensively understand the reaction kinetics of electrochemical CO2RR over the three catalysts. As shown in Fig. 6, the impedance arc in the Nyquist curve of NiCu@NCNPs is much smaller than that of the control samples, implying that NiCu@NCNPs presents the lowest electron transfer resistance during CO2 reduction. The results in Fig. 6 lead to the conclusion that the NiCu@NCNPs electrode features the swiftest interfacial charge transfer process followed by Ni@NCNPs and Cu@NCNPs, which is consistent with their experimental results shown in Fig. 3.
As discussed above, on one hand, the introduction of NiCu diatomic metals contributes to the formation of more surface-active sites, which will facilitate the adsorption of CO2 and corresponding reaction intermediates on the surface of the electrode. On the other hand, a lower impedance on the NiCu@NCNPs electrode leads to a faster electron transfer during the electrochemical reduction of CO2. Both points will result in a higher CO2 reduction efficiency.
3.4 Mechanism of enhanced catalytic performance on NiCu@NCNPs
As shown in previous literature (Liu et al. 2019; Liu et al. 2020a; Rosen et al. 2015), the electrocatalytic reduction of CO2 to CO involves three steps: CO2 gas molecules are first adsorbed and protonated to *COOH on active sites (*) of the catalyst surface. Afterwards *COOH is converted to *CO in the presence of electron, which will ultimately desorb from the surface as gaseous CO. This process can be described as follows:
$${CO}_{2}\text{(g) }+\text{ }{e}^{-}\text{ }+ {H}^{+}+ * \text{→ }\text{*COOH}$$
3
$$\text{*COOH}\text{ }+\text{ }{e}^{-}\text{ +} {H}^{+} \to \text{*}CO + {H}_{2}O$$
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$$\text{*}CO \to CO\left(g\right) + *$$
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Tafel analysis and DFT calculations have been performed to further obtain a profound understanding of the reaction kinetics of electrochemical CO2RR over the three catalysts towards CO. It has been concluded from previous studies that a Tafel slope of 118 mV dec− 1 means that the rate-determining step in CO2 reduction is the first electron transfer step (Yang et al. 2018b). As shown in Fig. 7, the Tafel slope of NiCu@NCNPs is very close to 118 mV dec− 1, so the *COOH generation step with the first proton-coupled electron transfer is the rate-determining step. In contrast, the Tafel slopes for Ni@NCNPs and Cu@NCNPs are 142 mV dec− 1 and 163 mV dec− 1, respectively, which indicates that the reaction rates are influenced not only by the first proton-coupled electron transfer to generate the *COOH intermediate, but also by some other factors such as the mass transfer(Liu et al. 2017; Shi et al. 2020; Yang et al. 2018a).
On this basis, DFT calculations were carried out to theoretically explore the reaction mechanism of CO2 electroreduction on these three electrodes in this work. Figures 8a-8c show the reaction pathways of CO2RR on Cu@NCNPs, Ni@NCNPs and NiCu@NCNPs, respectively. The optimized configurations of important intermediates *COOH and *CO are also shown on each catalytic surface.
Figure 8d is the Gibbs free energy diagrams for CO2 reduction to CO. As shown in Fig. 8d, the first reaction step about CO2 activation for *COOH production presents an uphill potential for each electrode, while the *CO production step and CO desorption step are thermodynamically downhill and can proceed spontaneously. Among all three steps, the first step displays the highest Gibbs free energy difference (∆G). Therefore, the step involving intermediate *COOH formation is the rate-determining step which agrees well with the results from Tafel analysis. Figure 8d also demonstrates that for the rate-determining step about the formation of *COOH, NiCu@NCNPs presents the lowest ∆G of 1.19 eV, followed by Ni@NCNPs (2.0 eV) and Cu@NCNPs (2.04 eV), which will lead to the highest CO2RR activity and CO selectivity on NiCu@NCNPs. According to the results of Tafel analysis and DFT calculations, the lowest ΔG for the rate-determining step on NiCu@NCNPs among all three catalysts leads to its superior catalytic performance in CO2RR.