A Universal Principle to Accurately Synthesize Atomically Dispersed Metal–N4 Sites for CO2 Electroreduction

Highlights A family of SAs–M–N–C consisted of carbon nanosheets supported atomic sites of isolated metal atom coordinated with four pyrrolic N atoms was fabricated. The SAs–Ni–N–C exhibited superior electrochemical CO2 electroreduction (CO2ER) activity and selectivity. Electronic supplementary material The online version of this article (10.1007/s40820-020-00443-z) contains supplementary material, which is available to authorized users.


Introduction
With the increasing concentration of atmospheric carbon dioxide (CO 2 ), how to effectively reduce CO 2 into available resources, such as carbon monoxide (CO) [1][2][3][4], formic acid (HCOOH) [5,6], hydrocarbons (C 2 , C 3 ) [7,8], and alcohols (CH 3 OH, CH 3 CH 2 OH) [9,10], by using electrochemical strategy has become a hot topic. Among all reported products from CO 2 electroreduction (CO 2 ER), CO gas is a relatively easy product to be yielded due to the CO 2 -to-CO conversion only involved two-step procedure of proton-coupled electron transfer. Moreover, the produced gaseous CO can be handily separated and be further used as resource in other industrial applications, like Fischer-Tropsch process [11]. Originally, noble metals (Au, Ag, Pd, etc.)-based materials are widely used to catalyze CO 2 into CO [12][13][14]; however, the application of these noble metal materials is highly hindered by their high cost and scarcity. Hence, significant efforts have been devoted to develop low-cost and highly effective alternative catalysts to replace the noble metals materials for CO 2 -to-CO conversion. Currently, atomically dispersed metal-nitrogen (M-N) sites-anchored carbon (M-N-C) materials is one of the most promising CO 2 ER electrocatalysts for CO production, owing to its simple synthetic procedure and excellent catalytic performance [15][16][17][18][19]. First, the M-N-C materials can be synthesized via a one-step pyrolysis of precursors contained carbon resources, nitrogen resources, and inorganic metal salt under optimized condition [20][21][22][23][24], and this strategy is a universal synthetic method that can be used to develop a series of M-N-C materials. Second, the electronic structure of central metal atom in M-N sites can be modified by the bonded N atoms, thus resulting in enhanced binding strength between the reaction intermediates and active M-N centers in their key step [25][26][27][28], promoting the catalytic activity and selectivity of M-N-C materials for CO 2 -to-CO conversion. Despite certain progress on developing M-N-C catalysts, it still suffers from imprecisely regulating the category and coordination number of ligating N atoms that bind to central metal atom. To be precise, several categories of N atoms such as pyridinic N, pyrrolic N, and graphitic N can provide coordinated possibility with metal atoms to form M-N structure during pyrolysis process; meanwhile, accurate ligand number between N and central metal atoms is difficult to be controlled.
Herein, we developed a universal approach to synthesize a series of single metal atom-N (SAs-M-N, M = Fe, Co, Ni, Cu) species immobilized on graphitized carbon supports (SAs-M-N-C) via an in situ pyrolysis of metalloporphyrin molecules and MCA polymer that was originated from the self-assemble of melamine (M) and cyanuric acid (CA). The SAs-M-N-C catalysts consisted of ultrathin carbon nanosheets supported accurate coordination structures of four pyrrole-type N atoms bonded with single metal atom (pyrroletype M-N 4 ). Benefitting from unique coordinated condition and discrepant intrinsic activity of pyrrole-type M-N 4 sites in as-prepared SAs-M-N-C catalysts, SAs-Ni-N-C exhibited an excellent activity, selectivity, and stability for CO 2 -to-CO conversion, in which the conversion started at low potential of − 0.3 V along with a small Tafel slope of 115 mV dec −1 ; meanwhile, a high Faradaic efficiency (F.E.) of 98.5% for CO production and durable catalytic stability of 50 h were achieved at − 0.7 V. Experimental measurements revealed that the CO 2 ER performance ranking of SAs-M-N-C was corresponding to the sequence of Ni > Fe > Cu > Co owing to the intrinsic nature of pyrrole-type M-N 4 structures, in which the high CO 2 ER performance catalyzed by SAs-Ni-N-C was appreciably superior to that of almost all previously reported M-N-C CO 2 ER electrocatalysts to date. The aberration-corrected high-angle annular dark field scanning transmission electron microscopy (AC HAADF-STEM) confirmed the atomic distribution of isolated metal atoms in SAs-M-N-C; X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) identified the accurate configuration of pyrrole-type M-N 4 centers with individual metal atom bonded by four pyrroletype N atoms. Furthermore, an integrated Zn-CO 2 battery equipped with the cathode of SAs-Ni-N-C delivered a peak power density of 1.4 mW cm −2 and the maximum CO F.E. of 93.3% during its discharge process, realizing the practical feasibility of CO 2 conversion and electric energy output.

3
The above chemicals were directly used as received without any further purification.

Preparation of SAs-M-N-C
The SAs-M-N-C samples were synthesized via a one-step in situ pyrolysis of metalloporphyrin molecules (Cu, Fe, Co, and Ni) on the surface of melamine (M) and cyanuric acid (CA)-polymerized polymer. First, 0.37 g (3.0 mmol) of M and 0.39 g (3.0 mmol) of CA were self-assembled in 40 mL of deionized water in an Erlenmeyer flask under ultrasonication condition to form a MCA polymer colloid. Then, the obtained MCA polymer colloid was separated by filtration and dried in vacuum at 60 °C for 10 h. Next, 1.0 g of solid MCA polymer was grinded with 0.2 g of metalloporphyrin to form a homogeneous mixed powder. Finally, the above mixture was placed in a tube furnace and heated at 700 °C for 2 h under N 2 atmosphere with a heating rate of 5 °C min −1 . After the calcination process, the corresponding black SAs-M-N-C product was obtained.

Preparation of N-C
The N-C was synthesized by direct pyrolysis of MCA polymer without adding metalloporphyrin molecules under the same pyrolysis condition for SAs-M-N-C synthesis.

Characterization
The field-emission scanning electron microscopy (FESEM) (SU-8010 Hitachi) and HRTEM (Tecnai G2 F20 S-TWIN) images were taken to identify the morphologies of samples. The X-ray diffraction (XRD) measurements were performed on ZETIUM DY 2186, 4 kW to display the crystal structures of samples. The metal content in samples was quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES) performed on Vista Axial. The XPS spectra of samples were collected on the Escalab 250Xi using an Al Kα radiation. The XAS results were obtained at the beamline 1W1B of the Q9 Beijing Synchrotron Radiation Facility (Beijing, China) using a transmission mode to detect the coordination environment of samples. Liquid-phase CO 2 ER products were identified by 1 H NMR (600 MHz, Bruker AVANCE AV III 500), in which 600 µL of 0.5 M KHCO 3 electrolyte after long-term CO 2 ER electrolysis was mixed with 70 µL of 10 mM dimethyl sulfoxide (DMSO) in D 2 O for 1 H NMR analysis. The DMSO was used as an internal standard, and the solvent suppression was used to decrease the area of H 2 O peak to make the CO2ER products peaks more clearly. Notably, all of the liquid-phase CO 2 ER products can be identified by 1 H NMR [29].

Electrochemical Measurements
Electrochemical measurements were tested on CHI 760E electrochemical workstation with a three-electrode cell (counter electrode: Pt wire; reference electrode: Ag/AgCl; working electrode: 1 × 1 cm 2 carbon paper loaded with catalyst). For the working electrode, homogeneous ink (10 mg mL −1 ) consisting of 10 mg of sample, 100 µL of 0.5% Nafion, and 900 µL of ethanol was prepared with sonication and stirring. Then, 100 µL of suspension solution was dropped onto the surface of carbon paper with loading amount of 1.0 mg cm −2 . The polarization curves were measured in 0.5 M KHCO 3 solution with a scan rate of 5 mV s −1 . The ECSA-referred cyclic voltammetry (CV) curves were performed at the potential of − 0.35 V ~ − 0.45 V (vs. Ag/ AgCl). The EIS spectra were measured with a frequency ranging from 100 kHz to 10 mHz and an AC voltage with 5 mV. The mentioned potentials versus reversible hydrogen electrode (RHE) were calculated by Eq. 1: The 0.197 V is V Ag/AgCl vs. NHE at 25 °C, and the pH value of CO 2 -saturated 0.5 M KHCO 3 solution is around 7.2.

Calculation of CO F.E
The Faradaic efficiency (F.E.) was calculated by Eq. 2: where x represents the concentration of CO (GC data); n corresponds to the amount of collected gas (the volume of the collected gas V 0 is 1.0 mL), calculated via n = PV 0 /RT (T = 299.15 K, P = 1.013 × 10 5 Pa, and R = 8.314 Nm K −1 ); Avogadro constant N A = 6.02 × 10 23 mol −1 ; the number of transfer electron is 2e; I (mA) represents the total current when collecting the pending tested gas; time (t) to collect 1.0 mL of gas is 3 s (CO 2 flow rate is 20 mL min −1 ); e = 1.602 × 10 −19 C e −1 .

Calculation of CO Turnover Frequency
In order to compare the catalytic activities of SAs-M-N-C with different metal concentrations, the turnover frequency (TOF) in SAs-M-N-C-catalyzed CO 2 ER was calculated according to Eq. 3 [30]: where I CO is the partial current for CO production, A; n is the transferred number of electron during CO 2 ER, which is 2 for CO production; F is the Faradaic constant, 96,485 C mol −1 ; m catalysts is the mass of catalysts loaded on the working electrode, which is 1.0 mg in our system; m is the metal concentration in SAs-M-N-C; M is the corresponding atomic mass of central metal.
The TOF values were calculated at the potential where the SAs-M-N-C delivered their maximum CO 2 ER performance for CO production. The TOF values of SAs-Fe-N-C, SAs-Co-N-C, SAs-Ni-N-C, and SAs-Cu-N-C for CO production were calculated to be 26.7, 11.3, 114.9, and 14.9 h −1 , respectively, suggesting that the catalytic activities of SAs-M-N-C followed the sequence of Ni > Fe > Cu > Co.

Results and Discussion
A family of SAs-M-N-C (M = Fe, Co, Ni, Cu) CO 2 ER catalysts were fabricated via a one-step in situ pyrolysis of metalloporphyrin molecules and MCA polymer derived from the polyreaction of M and CA. As shown in Fig. 1a, the M and CA precursors were firstly self-assembled to form MCA polymer, and then, the composite of metalloporphyrin loaded on MCA polymer was carbonized at 700 °C for 2 h under N 2 atmosphere. During the carbonization process, the MCA polymer was gradually evolved into graphitized carbon nanosheets, while the local chemical environment of pyrroletype M-N 4 structures in metalloporphyrin molecules was well preserved and in situ anchored into the graphitic carbon frameworks.  (Table S1). Further, the AC HAADF-STEM images of SAs-M-N-C displayed the homogenously distributed and isolated bright dots with single atomic diameter of ~ 0.25 nm on the surface of carbon nanosheets, which could be attributed to the isolated metal atoms due to its larger atomic number than C or N atoms ( Fig. 2a-d) [31][32][33]. Additionally, large-scale elemental distribution mapping images of SAs-M-N-C from the AC HAADF-STEM images displayed the uniform distribution of metal and N atoms on the carbon nanosheets, demonstrating the atomic dispersion of metal species and successful doping of N atoms into carbon frameworks ( Fig. 2e-h), which was further supported by the emerged characteristic peaks of C, N, and metal elements in XPS survey spectra (Fig.  S4). Based on the above results, it can be concluded that the isolated metal species in the SAs-M-N-C were atomically dispersed on the surface of graphitic carbon nanosheets with high density.
For the analysis of coordinated environment of metal atoms in SAs-M-N-C, taking the SAs-Ni-N-C as an example, high-resolution Ni 2p, N 1s XPS spectra, and XAS spectra were conducted. Figure 3a displays high-resolution Ni 2p XPS spectrum of SAs-Ni-N-C, in which the valence state of atomically dispersed Ni species was fitted to be Ni 2+ and Ni 3+ according to the peaks located at binding energies of 855.2/872.3 and 855.7/872.9 eV, respectively, whereas the Ni 0 with the peak located at 852.6 eV was not observed [34], suggesting that the Ni species in SAs-Ni-N-C exist in an oxidized state instead of metallic Ni. This result was inconsistent well with XPS spectra of controlled Ni-porphyrin sample, in which the valence state of Ni species was also fitted to be Ni 2+ and Ni 3+ based on corresponding characteristic peaks, thus excluding the existence of metallic Ni 0 . Based on above results, it can be found that the valence state of Ni atoms was not transformed during the pyrolysis process, and the Ni species was well preserved in the atomic level and not aggregated into the Ni NPs. Additionally, high-resolution N 1s XPS spectra of SAs-Ni-N-C and pure N-doped carbon nanosheets (denoted as N-C, Fig. S5) that were synthesized under the same pyrolysis condition as SAs-Ni-N-C but free of adding Ni-porphyrin molecules are shown in Fig. 3b. As compared with N-C, the characteristic peak of pyrrolic N in SAs-Ni-N-C was chemically shifted with 0.5 eV, whereas no changes on the characteristic peaks of other types of N dopants were observed on . Such unique coordination category in SAs-Ni-N-C could be attributed to the reserved pyrrole-type Ni-N 4 architectures originated from Ni-porphyrin molecules during pyrolysis, since high thermal stability of Ni-porphyrin preserved its local chemical structures [36], as confirmed by thermogravimetric analysis (TGA) results (Fig. S6). Further, the XAS spectra were used to accurately identify the local geometric structures of SAs-Ni-N-C at atomic level. Figure 3c displays the Ni k-edge X-ray absorption near edge structure (XANES) spectra of SAs-Ni-N-C with NiO and Ni foil as references, in which the adsorption edge energy of SAs-Ni-N-C was higher than that of Ni foil and NiO, indicating that the valence state of Ni species in SAs-Ni-N-C was a little bit higher than +2 [37,38], inconsistent well with the fitting results of Ni 2+ and Ni 3+ from Ni 2p XPS spectra. The Fourier-transformed Ni k-edge k 3 -weighted extended X-ray absorption fine structure (EXAFS) in R space confirmed that the characteristic peak of Ni-Ni bonds from the Ni foil was located at 2.17 Å (Fig. 3d), which was absent in the spectrum of SAs-Ni-N-C, demonstrating the inexistence of Ni-based clusters/particles in the SAs-Ni-N-C. Meanwhile, the peaks ranged from 1.24 to 1.7 Å can be attributed to the first coordination shell of Ni-N bonds, which was inconsistent with the peaks in standard Ni-porphyrin [27,30,39,40], suggesting the successful formation of Ni-N structures in the SAs-Ni-N-C. Since the characteristic peak of Ni-N bonds in Fig. 3d was located at the similar position with the Ni-O bonds in NiO, the wavelet transform (WT) of Ni k-edge EXAFS oscillations was further analyzed, and the results confirmed that the backscattering atoms bonded with Ni atom were indeed N atoms instead of O atoms because the WT-EXAFS analysis can provide the resolutions in both R and k spaces [21,41]. In particular, the maximum intensity at 4.0 Å that associated with Ni-N bonds from SAs-Ni-N-C was quite different to that of Ni-O bonds at 7.5 Å in NiO (Fig. 3e). The fitting results of FT-EXAFS from SAs-Ni-N-C further revealed that the coordination number of Ni-N bonds was quantified to be four (Fig. 3f, g and Table S2). Based on the XPS and XAS fitting results, one can conclude that the local geometric structure of SAs-Ni-N-C was accurately confirmed to be the chemical configuration of single Ni atom coordinated with four pyrrolic N atoms (Fig. 3h). Likewise, the metal species in SAs-Fe-N-C, SAs-Co-N-C, and SAs-Cu-N-C were confirmed to be an oxidized state instead of metallic phase (Fig. S7); combined with the AC HAADF-STEM images of the above three catalysts, it can be deduced that the corresponding metal atoms were distributed with atomic level in SAs-Fe-N-C, SAs-Co-N-C, and SAs-Cu-N-C. Further, high-resolution N 1s spectra of SAs-Fe-N-C, SAs-Co-N-C, and SAs-Cu-N-C all exhibited a chemical shift of pyrrolic N characteristic peak with respect to that of the N-C (Fig. S8), demonstrating the formation of pyrrole-type M-N structures in the above three samples. Besides, the fitting results of FT-EXAFS from SAs-Fe-N-C, SAs-Co-N-C, and SAs-Cu-N-C all confirmed that the coordination number of M-N bonds in corresponding sample was quantified to be four (Figs. S9, S10 and Table S2), revealing that the local geometric structures in SAs-Fe-N-C, SAs-Co-N-C, and SAs-Cu-N-C were similar to that in SAs-Ni-N-C in the form of pyrrole-type M-N 4 structures. These results demonstrated that this in situ pyrolysis of metalloporphyrin molecules loaded on surface of MCA polymer was a universal method to accurately synthesize atomically dispersed pyrrole-type M-N 4 structures. The electrochemical CO 2 ER activity and selectivity of as-prepared SAs-M-N-C catalysts were performed in H-cell reactor with a typical three-electrode system. The linear sweep voltammetry (LSV) curves of SAs-M-N-C catalysts measured in CO 2 -and Ar-saturated 0.5 M KHCO 3 solutions are shown in Fig. S11. Considering that the difference of delivered current densities between the CO 2 -saturated one and the Ar-saturated counterpart was originated from the CO 2 ER catalysis, the SAs-Ni-N-C displayed the highest catalytic activity for CO 2 ER among all investigated SAs-M-N-C samples. To further evaluate the selectivity of SAs-M-N-C for CO 2 ER by calculating Faradaic efficiency (F.E.), gaseous and liquid-phase products from CO 2 ER were quantified by gas chromatography (GC) and 1 H nuclear magnetic resonance spectroscopy ( 1 H NMR). Notably, the gaseous products produced from SAs-M-N-C-catalyzed CO 2 ER were identified to be CO and H 2 gases, and the total F.E. of CO and H 2 was calculated to be 100%, thus excluding the formation of liquid-phase products, as supported by 1 H NMR results (Fig. S12). The corresponding F.E.s for CO and H 2 products are shown in Fig. 4a, b, in which the SAs-Ni-N-C and SAs-Fe-N-C delivered a much higher CO F.E. than the SAs-Cu-N-C and SAs-Co-N-C under all the applied potentials, demonstrating high selectivity of SAs-Ni-N-C and SAs-Fe-N-C for CO 2 ER catalysis. Although the SAs-Ni-N-C exhibited a slightly lower CO 2 ER selectivity for CO generation than the SAs-Fe-N-C under the relatively positive potentials of − 0.3 ~ − 0.6 V, the former delivered a higher current density within this interval than the latter. Besides, with the applied potentials increased negatively, the CO F.E. of SAs-Ni-N-C was obviously superior to that of SAs-Fe-N-C and reached the maximum of 98.5% at − 0.7 V. The superior CO 2 ER performance of SAs-Ni-N-C with respect to SAs-Fe-N-C was further revealed by their partial current densities for CO production (Fig. 4c). These results suggested that, among the family of SAs-M-N-C catalysts, SAs-Ni-N-C was more suitable for the practical application in CO 2 ER because it delivered large current density and high selectivity during CO 2 ER process. From the results of electrochemical impedance spectroscopy (EIS) of SAs-M-N-C samples, the SAs-Ni-N-C possessed the lowest charge-transfer resistance among all the investigated catalysts for CO 2 ER, which was supported by the smallest radius in the Nyquist plot (Fig. 4d) [42]. Additionally, SAs-Ni-N-C-catalyzed CO 2 ER delivered a much lower Tafel slope of 115 mV dec −1 than that from the SAs-Fe-N-C (124 mV dec −1 )-, SAs-Co-N-C (221 mV dec −1 )-, and SAs-Cu-N-C (216 mV dec −1 )catalyzed counterparts (Fig. 4e), demonstrating the fastest reaction kinetics in SAs-Ni-N-C-catalyzed CO 2 ER process [22].
In order to clarify the intrinsic property of isolated M-N centers in SAs-M-N-C, the CO 2 ER performance of control N-C sample was evaluated (Fig. S13). Although the content of doped N species in N-C was much higher than that in SAs-M-N-C, the N-C exhibited a finite CO 2 ER performance in terms of catalytic activity, CO selectivity, and reaction kinetics with respect to SAs-M-N-C. This result indicated that the M-N centers played the key role in enhancing CO 2 ER performance. Furthermore, for precisely evaluating the intrinsic nature of SAs-M-N-C, electrochemical active surface area (ECSA) of SAs-M-N-C samples was calculated via measuring the corresponding double-layer capacitance (Figs. 4f and S14) [39,43]. The ECSA of SAs-Fe-N-C, SAs-Co-N-C, SAs-Cu-N-C, and SAs-Ni-N-C was calculated to be 102, 98, 32, and 35 cm 2 , respectively. Despite the more exposed numbers of Fe-N and Co-N centers caused by the larger ECSAs in SAs-Fe-N-C and SAs-Co-N-C than that of Ni-N centers in SAs-Ni-N-C, the latter displayed the largest ECSA-normalized partial current density for CO production among the above investigated samples (Fig. 4g), suggesting that the superior CO 2 ER activity and selectivity of SAs-Ni-N-C are mainly attributed to high intrinsic property of the Ni-N sites, instead of other extrinsic factors, like ECSA. Besides, the ECSA-normalized partial current  Fig. 4 a-g F.E.s for CO and H 2 productions, geometric surface area-normalized partial current density for CO production, Nyquist plot, Tafel slope, double-layer capacitance, and ECSA-normalized partial current density for CO production of SAs-Fe-N-C, SAs-Co-N-C, SAs-Ni-N-C, and SAs-Cu-N-C. h Long-term stability tests of 50 h, CO F.E., and current density from SAs-Ni-N-C-catalyzed CO 2 ER at − 0.7 V 1 3 density for CO production by SAs-M-N-C samples also revealed the intrinsic property of M-N sites following the sequence of Ni-N > Fe-N > Cu-N > Co-N, inconsistent well with the sequence for CO selectivity. These results demonstrated that the CO 2 ER performance of SAs-M-N-C was strongly depend on the catalytic nature of central transition metal in M-N sites, giving the catalytic activity sequence of Ni > Fe > Cu > Co. Additionally, SAs-Ni-N-C also displayed a favorable CO 2 ER stability over 50 h of continuous reaction under a constant potential of -0.7 V (Fig. 4h), during which a slight attenuation was observed in both total geometric current density and CO F.E. after operated for 50 h, whereas the CO F.E. was still maintained at above 80%, illustrating a superior performance of SAs-Ni-N-C for catalyzing CO 2 ER with high selectivity and stability. Such excellent activity and stability of SAs-Ni-N-C for CO 2 ER catalysis were superior to that of the most previously reported M-N-C CO 2 ER electrocatalysts (Table S3).
In order to further extend the practical application of CO 2 ER catalyzed by SAs-M-N-C, taking the SAs-Ni-N-C as an example, an aqueous Zn-CO 2 battery composed of anode with Zn foil and cathode with SAs-Ni-N-C was designed to achieve the reaction of CO 2 ER along with the electricity output during its discharge process (Fig. 5a) [44][45][46][47], in which 6.0 M KOH with 0.2 M Zn(Ac) 2 solution was used as anolyte and 0.5 M KHCO 3 solution was used as catholyte; bipolar membranes were set to maintain the pH value of two chambers. The charge-discharge polarization  Fig. 5b demonstrated the rechargeable characterizations of this Zn-CO 2 battery equipped with the cathode of SAs-Ni-N-C. Notably, the Zn-CO 2 battery delivered a maximum power density of 1.4 mW cm −2 along with the current density of 5.3 mA cm −2 during the discharge process (Fig. 5c), which was much higher than the previously reported Zn-CO 2 batteries equipped with the cathodes of atomically dispersed metal-anchored carbon materials (such as 0.21 mW cm −2 for NiPG [44] and 0.62 mW cm −2 for Cu-N 2 /GN nanosheets [47].) Meanwhile, the integrated Zn-CO 2 battery with SAs-Ni-N-C cathode displayed a superior durability under cyclic charging and discharging processes with a constant current density of 1.0 mA cm −2 (Fig. 5d). Additionally, the CO 2 ER performance of SAs-Ni-N-C during discharge process of Zn-CO 2 battery was identified, where the CO F.E. was achieved to the maximum of 93.3% under the power density of 0.7 mW cm −2 (Fig. 5e). Based on the above results, we conclude that the Zn-CO 2 battery equipped with the SAs-Ni-N-C cathode can effectively realize the energy conversion of chemical energy into electric energy during its discharge process, that is, the occurring of the redox reactions achieved the reduction of CO 2 into CO, and the electricity output was synchronously realized in the circuit. Consequently, both the bulbs and homemade LED array were lighted by the integrated Zn-CO 2 battery (Fig. 5f). Furthermore, the emerged intersection at a voltage of 1.37 V between the charge polarization curves of this Zn-CO 2 battery and the current-voltage (J-V) curve of solar cell (irradiated with Xe lamp, AM 1.5G, 100 mW cm −2 ) suggested the practical feasibility of using the solar cell to charge the Zn-CO 2 battery (Fig. 5g). In this respect, solar cell irradiated under natural solar energy was used to effectually charge the Zn-CO 2 battery, as featured by the occurring of anodic oxygen evolution reaction (Fig. 5h) [44,46,48], achieving the promising process of energy storage.

Conclusions
In summary, we developed a universal principle for in situ pyrolysis of the MCA polymer-supported metalloporphyrin molecules to synthesize a family of atomically dispersed SAs-M-N-C catalysts. The experimental results revealed that the isolated metal species was bonded by pyrrolic N atoms and atomically distributed on the ultrathin carbon nanosheets with accurate pyrrole-type M-N 4 structures. Owing to the specific nature of pyrroletype M-N 4 structure in SAs-M-N-C catalysts, the CO 2 ER performances catalyzed by SAs-M-N-C followed the sequence of Ni > Fe > Cu > Co, in which the SAs-Ni-N-C catalyst exhibited an excellent performance in CO 2 ER with the measured onset potential and Tafel slope of − 0.3 V and 115 mV dec −1 , along with the detected maximum CO selectivity and long-term stability of 98.5% and 50 h at the optimized − 0.7 V, respectively. Such superior CO 2 ER performance achieved by SAs-Ni-N-C was outperforming almost all of the previously reported M-N-C electrocatalysts. Additionally, an integrated Zn-CO 2 battery equipped the SAs-Ni-N-C cathode achieved the energy conversion and output with the maximum CO F.E. of 93.3% and peak power density of 1.4 mW cm −2 . The developed strategy for synthesizing accurate pyrrole-type M-N 4 sites in SAs-M-N-C catalysts as introduced in this work may give an alternative approach to construct the structurally controllable M-N 4 centers supported on carbon materials for other promising electrochemical reactions, like hydrogen evolution reaction, nitrogen reduction reaction, and oxygen evolution reaction. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.

Electronic supplementary material
The online version of this article (https ://doi.org/10.1007/s4082 0-020-00443 -z) contains supplementary material, which is available to authorized users.