Atomically Dispersed Fe-Co Bimetallic Catalysts for the Promoted Electroreduction of Carbon Dioxide

Highlights X-ray photoelectron spectroscopy results confirmed the increased number of M–N sites in the bimetallic Fe–Co catalyst. Synchrotron-based X-ray absorption fine structure demonstrated that the interaction in the coordination environments of the different transition metal sites facilitated the CO production in electroreduction reaction of CO2 (ECO2RR). This bimetallic strategy has also been extended to fabricate other catalysts such as Cu–Co and Ni–Co, which also exhibited enhanced performance for ECO2RR. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00746-9.


S1.2 Synthesis of Co-ZIF
Typically, Co(NO3)2·6H2O (0.546 g) and Zn(NO3)2·6H2O (1.116 g) were dissolved in 40 mL of methanol. Then, 2-methylimidazole methanol solution (1.116 g in 40 mL) was slowly poured into the above solution under vigorously stirring, following with the subsequent stirring for 24 hours at room temperature. The as-obtained powders were collected by centrifugation at 7000 rpm for 5 minutes, washed three times with methanol, and finally dried in the oven at 90℃ overnight.

S1.3 Synthesis of Fe-Co-ZIF with Different Fe Loadings
The powder of as-prepared Co-ZIF (100 mg) was dispersed in methanol (10 ml) under ultrasound for 5 min at room temperature. After forming homogeneous solution, Fe(NO3)2·9H2O methanol solution (115 mg ml -1 , 50 μL for 0.8 wt% Fe, 100 μL for 1.6 wt% Fe, 200 μL for 3.2 wt% Fe, and 300 μL for 4.8 wt% Fe) was slowly injected into the mixed solution under ultrasound for 2 min at room temperature. Next, the mix solution was under vigorous stirring for 5 h at room temperature to make the salt solution be absorbed completely. Then the samples were centrifuged and dried in the oven at 90℃ overnight.

S1.4 Synthesis of C-Co-ZIF and C-Fe-Co-ZIFs
The powder of as-prepared Co-ZIF and Fe-Co-ZIFs were placed in the tube furnace, maintained at three temperature plateaus (800 ℃, 900 ℃, and 1000 ℃, 1 hour for each) with a heating rate of 25 ℃ min −1 under flowing argon gas, and then naturally cooled to room temperature. All the asprepared samples were directly used without any post-treatment.

S1.5 Synthesis of C-Cu-Co-ZIF and C-Ni-Co-ZIFs
The synthesis of C-Cu-Co-ZIF and C-Ni-Co-ZIFs follows the same procedure of C-Fe-Co-ZIF while replacing the Fe(NO3)2 methanol solution into the corresponding Cu(NO3)2 and Ni(NO3)2 methanol solution, respectively, during the ZIF preparation process.

S2 Supplementary Tables and Figures
solution at a scan rate of 20 mV s -1 (no iR-compensation was made during CV tests). f LSV curves of the samples in CO2-saturated 0.5 M KHCO3 solution at a scan rate of 5 mV s -1 (80% iRcompensation)   Double-layer capacitance tests for evaluating the electrochemical active surface area of the catalysts a CVs of C-Fe-Co-ZIF-1.6 wt%Fe sample with scan rate from 5 mV s -1 to 100 mV s -1 . The cathodic (dark line) and anodic (red line) currents were measured at 0.10 V vs. RHE as a function of the scan rate of b C-Co-ZIF. c C-Fe-Co-ZIF-0.8 wt%-Fe. d C-Fe-Co-ZIF-1.6 wt%-Fe. e C-Fe-Co-ZIF-3.2 wt%-Fe. f C-Fe-Co-ZIF-4.8 wt%-Fe. The average value of the cathodic and anodic slopes is taken as the double-layer capacitance of the catalyst electrode.

Fig. S9
SEM images of a Cu-Co-ZIF-1.6 wt%-Cu. b C-Cu-Co-ZIF-1.6 wt%-Cu. c Ni-Co-ZIF-1.6 wt%-Ni. d C-Ni-Co-ZIF-1.6 wt%-Ni All the original and carbonized Cu-Co-ZIFs and Ni-Co-ZIFs have the same morphology with a particle size of around 300 nm. It illustrates that the series of Cu and Ni modified Co-ZIF samples retain the morphology of the original Co-ZIFs independent of the metal sources during the synthesis and pyrolysis processes. Cu-Co-ZIF and Ni-Co-ZIF share the same crystalline structure as that of Fe-Co-ZIF, without any obvious characteristic peak assigned to Cu and Ni crystals. It indicates that Cu-Co-ZIF and Ni-Co-ZIF keep the same crystalline structure as that of Co-ZIF as well. After the pyrolysis, carbonized Cu-Co-ZIF (C-Cu-Co-ZIF) and carbonized Ni-Co-ZIF (C-Cu-Co-ZIF) also appear to be the amorphous carbon material, same as C-Fe-Co-ZIF. In Fig. S11a, the valances of Co atoms in C-Fe-Co-ZIF and C-Cu-Co-ZIF are close to that of C-Co-ZIF while the introduction of Ni reduces the valance of Co atoms in the catalyst, which is relatively greater than Fe and Cu. It implies The atomically dispersed state of the TM atoms and the interactions between Co and the introduced foreign TM atoms (Cu and Ni) are also confirmed by the analysis of EXAFS (Fig. S11b) where peaks at M-N and M-M regions appear for both C-Cu-Co-ZIF and C-Ni-Co-ZIF. Compared to C-Fe-Co-ZIF, the Co R space patterns in C-Cu-Co-ZIF and C-Ni-Co-ZIF are closer to that of C-Co-ZIF.

Fig. S12
The evaluation of the electrocatalytic performance of C-Cu-Co-ZIF with different Cu adding amounts. a LSV curves in CO2-saturated 0.5 M KHCO3 solution at a scan rate of 5 mV s -1 . b CO Faradaic efficiency of the catalysts at various applied potentials. c H2 Faradaic efficiency of the catalysts at various applied potentials. d Total Faradaic efficiency of CO and H2 of C-Cu-Co-ZIF-3.2 wt%-Cu at various applied potentials. e CO current density of the catalysts. f H2 current density of the catalysts.

Fig. S13
The evaluation of the electrocatalytic performance of C-Ni-Co-ZIF with different Ni adding amounts. a LSV curves in CO2-saturated 0.5 M KHCO3 solution at a scan rate of 5 mV s -1 . b CO Faradaic efficiency of the catalysts at various applied potentials. c) H2 Faradaic efficiency of the catalysts at various applied potentials. d Total Faradaic efficiency of CO and H2 of C-Ni-Co-ZIF-3.2wt%-Ni at various applied potentials. e CO current density of the catalysts. f) H2 current density of the catalysts. This work