Oriented electron transmission in polyoxometalate-metalloporphyrin organic framework for highly selective electroreduction of CO2

The design of highly stable, selective and efficient electrocatalysts for CO2 reduction reaction is desirable while largely unmet. In this work, a series of precisely designed polyoxometalate-metalloporphyrin organic frameworks are developed. Noted that the integration of {ε-PMo8VMo4VIO40Zn4} cluster and metalloporphyrin endows these polyoxometalate-metalloporphyrin organic frameworks greatly advantages in terms of electron collecting and donating, electron migration and electrocatalytic active component in the CO2 reduction reaction. Thus-obtained catalysts finally present excellent performances and the mechanisms of catalysis processes are discussed and revealed by density functional theory calculations. Most importantly, Co-PMOF exhibits remarkable faradaic efficiency ( > 94%) over a wide potential range (−0.8 to −1.0 V). Its best faradaic efficiency can reach up to 99% (highest in reported metal-organic frameworks) and it exhibits a high turnover frequency of 1656 h−1 and excellent catalysis stability ( > 36 h).

1) There is a total lack of characterizations of the materials. I could not even find the detailed formulae. The authors only use the abbreviation M-PMOFs throughout the manuscript. There are no elemental analysis, no interpretation of the IR spectra and of the TGA measurements. 3) The stability of the M-PMOFs in a pH range from 5 to 11 and in the conditions of electrocatalysis cannot rely on a single experiment which is PXRD. Some peaks seem to be present at pH 5 for Co-PMOF and Ni-PMOF which are absent in the simulated powder pattern and could indicate the existence of an additional phase. The IR spectra of the solids must be recorded as well as an analysis of the material at least by EDX-chemical mapping and an analysis of the solution by ICP to check the absence of leaching. 4) TGA experiments are not sufficient and thermodiffraction experiments should be performed in order to assess that the structures are stable up to 200°C. 5) For the comparison with Co-TMCP, can the authors precise if the compound is studied as a solid in the carbon ink or in solution? I suspect this compound to be soluble in 0.5 M KHCO3. This must be clarified. A relevant comparison would be with a MOF containing Co-TMC such as MOF-525-Co.
6) The production of H2 and CO (TON) as a function of time must be given. 7) An isotopic experiment using 13CO2 as substrate should be performed in order to prove the origin of CO.
Reviewer #3 (Remarks to the Author): In this work, the authors reported several stable polyoxometalate-metalloporphyrin organic frameworks (i.e. Co-PMOF, Fe-PMOF, Ni-PMOF and Zn-PMOF) constructed from tetrakis [4carboxyphenyl]-porphyrin-M (M-TCPP) linkers and reductive Zn-ε-Keggin clusters by hydrothermal method. Incorporation of reductive POM units and metalloporphyrins in the hybrid materials results in oriented electronic transportation channels, and thus the obtained PMOFs exhibited excellent electrochemical CO2 reduction performances. In particular for Co-PMOF, it enables to converse selectively CO2 to CO with remarkable faradaic efficiency of 99%, high TOF of 1656 h-1 and excellent stability. The mechanisms of the catalysis are also clearly revealed by DFT calculations. Therefore, this work is very interesting and important. I therefore recommend to accept this manuscript for publication in Nature Communications after minor revisions.

Reviewer: 1
Comments to the Author: Lan and coworkers have synthesized a series of novel polyoxometalate-metalloporphyrin organic frameworks (PMOFs) crystals and explored their applications in CO 2 reduction. By integration of Zn-ε-Keggin cluster and metalloporphyrin the electric conductivity and electron donating capability of the fabricated MOFs are substantially strengthened. Among these PMOFs, Co-PMOF exhibits remarkable faradaic efficiency up to 99% for CO and long-term stability. Further, the mechanisms of catalysis processes are also discussed and revealed by DFT calculations. The results reported here are quite interesting and inspiring. Therefore, I recommend that this work for publication after minor revision before the following questions are fully addressed in order to further improve the quality of the nice work. 1. One of the major problem to hinder the MOFs as electrocatalysts is their stability. The authors claimed that the PMOFs, for example, Co-PMOF exhibited excellent elelctrocatalytic stability (> 36 h) and provided solid evidences of the Co-PMOFs in solutions with various pH values, but before electrocatalytic reaction. I am very curious to know whether the authors have characterized Co-PMOF after electrocatalysis. Therefore, I would suggest the authors, at least, should provide the XRD pattern of Co-PMOF after 36 h.

Response:
Thanks for your kind suggestion. According to your suggestion, we have added the PXRD test of Co-PMOF after electrocatalysis. As shown in Fig. S27, the PXRD patterns of Co-PMOF agree well with the simulated one, indicating Co-PMOF can maintain the integrity of its structure after electrocatalysis. We have now included the discussion in the revised Supporting Information (Fig. S27, page 21, line 5). Figure 27. PXRD patterns of Co-PMOF after electrochemical experiment. Figure 4d, Co(I) species are involved during the electrocatalytic process. If the authors can provide direct evidences about the existence of the Co(I), such as EPR experiment, the proposed mechanism will be more solid.

Response:
Thanks for your suggestion. Co(I) is important in our proposed mechanism. CV curves and EPR tests are two promising characterization methods for the detection of Co(I) in experiment. As we mentioned in the first version of the manuscript, in  (Figure R1-2). In addition, Li et al. reported CoPPc/CNT as the CO 2 RR electrocatalyst with a similar broad cathodic wave for Co II /Co I (Chem, 2017, 3, 652-664). As for EPR tests, another powerful protocol for valance test, it is difficult to simulate the actual electrochemical measurement system in the EPR test process, such as under the condition of electrification, which leads to the rare use of EPR to characterize the catalyst under electrocatalysis conditions in reported works. Generally, most of works about electrochemical CO 2 RR applied CV to in-situ characterize the catalytic process. Thanks to the reviewer's kind suggestion, if possible, we will combine the electrochemical measurement system with EPR equipments in the future to explore the application of EPR in electrochemical CO 2 RR. Now we have properly cited relative references in the revised manuscript (ref 12, 29, 37 and 48, page 10, line 14).

The title "Oriented Electron Transmission in POM-Metalloporphyrin Organic
Framework for Highly Selective Electroreduction of CO 2 " makes me confused. Do the authors mean Oriented Electron Transportation? Response: Thanks for your suggestion. In this work, we applied polyoxometalates (POMs) and metalloporphyrins to construct polyoxometalate-metalloporphyrin organic frameworks. In the structure, reductive polyoxometalates mainly composed of low valent metal ions, such as Zn-ε-Keggin cluster ({ε-PMo 8 V Mo 4 VI O 40 Zn 4 }, including eight Mo V atoms), are usually electron-rich aggregates and can easily offer electrons when triggered by redox reaction or bias stimulus. Co-porphyrin, where inherent macrocycle conjugated π-electron system is very beneficial for electron mobility and Co(II) enables to be reduced to Co(I) during the process in many references (as mentioned in Response 1). The connection of POM and metalloporphyrin will presumably create an oriented electron transportation pathway under the motivation of electric field, abundant electrons flowing from POM cluster to metalloporphyrin motif can guarantee and facilitate the fulfillment of multiple electron migration process of CO 2 RR electrocatalysis. Now we have added relative references and discussion in the revised Supporting Information (page 4, line 10).
4. In the "Investigation of the catalytic mechanism" part, diverse contrast samples like NNU-12, Co-TMCP and TMCP were prepared and tested. The structure images of these contrast samples would be better provided.
Response: Per suggestion, we have provided the structure images of NNU-12, Co-TMCP and TMCP and added relative structure descriptions in the revised Supporting Information (Fig. S28 and S29, page 22, line 1). Figure 28. The structure images of NNU-12. (a) Secondary building block. (b) Basic construction unit. (c) 3D framework. (d) Six-fold interpenetrated structure with a dia topology. As presented in the image, each BCPT 2− ligand connects two Zn-ε-Keggin segments and each Zn-ε-Keggin connects four ligands, which generates a 3D framework with six-fold interpenetrated structure with a dia topology. NNU-12 contains the same Zn-ε-Keggin unit as Co-PMOF while the ligand is different. Figure 29. The structure images of TMCP and Co-TMCP. (a) TMCP. (b) Co-TMCP. TMCP is a kind of ester compound and Co-TMCP is a kind of Co-centered macrocycle.

Supplementary
5. What is the main concern of choosing Zn-ε-Keggin cluster as the desired POM to construct various PMOFs? It should be discussed from the viewpoint of structure design or target properties.

Response:
Thanks for your insightful suggestion. The Zn-ε-Keggin cluster {ε-PMo 8 V Mo 4 VI O 40 Zn 4 } is embedded by four Zn 2+ located in a regular tetrahedral arrangement, which offers a 4-connected mode to form interesting structures with outstanding stability. As a kind of reductive POMs composed of low valent metal ions (including eight Mo V atoms), Zn-ε-Keggin cluster is electron-rich aggregate and can easily offer electrons when triggered by redox reaction or bias stimulus. The connection of Zn-ε-Keggin cluster and metalloporphyrin will presumably create an oriented electron transportation pathway under the motivation of electric field, abundant electrons flowing from Zn-ε-Keggin cluster to metalloporphyrin motif can guarantee and facilitate the fulfillment of multiple electron migration process of CO 2 RR electrocatalysis. Hence in the structure design, we deduce that reductive polyoxometalate-metalloporphyrin organic frameworks will probably be promising candidates to enhance the efficiency and selectivity of CO 2 RR. As a proof-of-concept, Co-PMOF exhibits remarkable faradaic efficiency (FE CO > 94%) over a wide potential range (-0.8 to -1.0 V) in this work. Its best FE CO can reach up to 99% (highest in reported MOFs) and it exhibits a high TOF of 1656 h -1 and excellent catalysis stability (> 36 h).
6. Two kinds of additives (i.e. acetylene black (AB) and Nafion solution) were used to prepare the CCE working electrode. The authors should explain the reasons.

Response:
Thanks for your kind suggestion. Given the poor intrinsic electrical conductivity of MOFs, acetylene black (AB) was introduced to mix with the as-synthesized MOFs to improve the conductivity. Nafion solution was introduced as a kind of MOF dispersion solution generally applied in many reported works, which can form a homogeneous ink with MOF and further help to attach onto the surface of carbon cloth. Now we have added relative discussion in the revised manuscript (page 12, line 23).
Response: Per suggestion, we have deleted the unnecessary blank spaces and checked through the manuscript (page 3, line 17). Table 2, the compound name like Co-TCPOM listed in the table should be Co-PMOF, correct it.

In Supplementary
Response: Per suggestion, we have corrected the compound name "Co-TCPOM" to "Co-PMOF" and checked through the manuscript (Table S2, page 28). 9. Line 102: converse CO 2 to CO should be convert CO 2 to CO.
Response: Per suggestion, we have corrected the word "converse" to "convert" and checked through the manuscript (page 3, line 14).

Reviewer: 2
Comments to the Author: This manuscript reports the synthesis and characterizations of a series of polyoxometalate-metalloporpyrin organic frameworks (M-PMOFs). The Co-PMOF is an efficient catalyst for the reduction of CO 2 into CO. This material represents a rare example of a POM-based material which can reduce CO 2 . The originality is however lowered by the fact that the catalytic species is not the POM but the metalloporphyrin. Furthermore there are several problems which are listed below. Therefore I cannot recommend the publication of this manuscript in Nature Communications.
1. There is a total lack of characterizations of the materials. I could not even find the detailed formulae. The authors only use the abbreviation M-PMOFs throughout the manuscript. There are no elemental analysis, no interpretation of the IR spectra and of the TGA measurements.

Response:
Per suggestion, in this version, we have added detailed structure and stability characterizations (IR, TGA, EA and ICP) of M-PMOF and provided relative discussions in both revised manuscript and Supporting Information.
For the formulae, in the first version of the manuscript, we have provided the empirical formulae of Co-PMOF ( The calculated formulae were further supported by TGA test (Fig. S8). Taking Fe-PMOF for example, in the test, about 6.8% mass loss at temperature range from 0 to 200 o C is attributed to the loss of guest molecules, which matches well with the content of guest molecules in Fe-PMOF. Now we have added relative discussion in the revised manuscript (page 11, line 31) and Supporting Information (page 9, line 2).
For IR tests, relative IR peaks for Co-PMOF, Fe-PMOF, Ni-PMOF and Zn-PMOF are added in the syntheses part in the revised manuscript (page 11, line 24). The peak attribution in the IR spectra is discussed in Fig. S7 (Supporting Information, page 8,  line 3).
The thermal stability of M-PMOF samples is studied by thermogravimetric analyses (TGA) in O 2 atmosphere. Under O 2 atmosphere, all these M-PMOF samples can be stable at temperatures higher than 200 o C. Taking Co-PMOF for example, about 4% mass loss at temperature range from 0 to 100 o C is attributed to the loss of guest molecules. After 200 o C, the framework of Co-PMOF starts to collapse and ends at about 500 o C (Fig. S8). To further certify the thermal stability, PXRD patterns of M-PMOF samples treated at 200 ºC in the presence of ultrapure O 2 were tested. As shown in Fig. S9, the PXRD patterns of M-PMOF samples still agree well with the simulated ones. Now we have added relative discussion in the revised manuscript (page 4, line 20) and Supporting Information (page 10, line 3). Figure 3 raises several problems. First it is really hard to see if the experimental PXRD patterns correspond to the simulated ones because of the very small sizes of the Figures. The Figures must be enlarged and shown on a single column and a zoom should be made for 2 Theta between 0 and 20 degrees. In the absence of elemental analysis, it is thus hard to conclude about the purity of the phase.

Supplementary
Response: Thanks for your suggestion. We have enlarged the figures and a zoom have been made for 2 Theta between 3 and 20 degrees (catalyst has no peeks before 3 degrees) for all of the M-PMOF (Co-PMOF, Fe-PMOF, Ni-PMOF and Zn-PMOF) (Fig. S3-6). As showed in the image, all of the PXRD patterns of the as-synthesized samples match well with the simulated ones, which indicates the high purity and crystalline of these M-PMOF. Besides, the elemental analysis tests mentioned in Response 1 further prove the purity of the phase (Supporting Information, page 6-7). 3. The stability of the M-PMOFs in a pH range from 5 to 11 and in the conditions of electrocatalysis cannot rely on a single experiment which is PXRD. Some peaks seem to be present at pH 5 for Co-PMOF and Ni-PMOF which are absent in the simulated powder pattern and could indicate the existence of an additional phase. The IR spectra of the solids must be recorded as well as an analysis of the material at least by EDX-chemical mapping and an analysis of the solution by ICP to check the absence of leaching.

Response:
Thanks for your kind suggestion. In the new version, stability tests of M-PMOF are carefully characterized with PXRD, IR, EDX-chemical mapping and ICP tests and the obtained data are well-organized and discussed.
For PXRD tests, the images are enlarged and showed on a single column ( Fig.  S10-13). In the enlarged images, the PXRD patterns of M-PMOF (i.e. Co-PMOF, Fe-PMOF, Ni-PMOF and Zn-PMOF) after treating in acid, base and 0.5 M KHCO 3 solutions for 24 h match well with the simulated ones, which verify the high chemical stability of M-PMOF (Supporting Information, pages 10-12).
As for the claim "Some peaks seem to be present at pH 5 for Co-PMOF and Ni-PMOF which are absent in the simulated powder pattern and could indicate the existence of an additional phase", some peaks of M-PMOF for simulated patterns are weak in grouped figures in the first version and they can be visible in the enlarged figures in this one. After carefully comparison, the simulated PXRD pattern is consistent well with patterns obtained at pH = 5 (Figs. S10-13). Besides, we have added the IR spectra studies of M-PMOF after stability test in acid, base and 0.5 M KHCO 3 solutions. As shown in Fig. S14, the IR spectra of M-PMOF in acid, base and 0.5 M KHCO 3 solutions have negligible change compared with the as-synthesized ones, indicating M-PMOF can maintain the integrity of their structures after chemical stability tests. We have added relative discussion in the revised Supporting Information (page 12, line 7).
Moreover, Fig. S15 presents SEM and energy dispersive X-ray (EDX) elemental mapping images for Co-PMOF, Fe-PMOF, Ni-PMOF and Zn-PMOF after stability test in acid solution. In SEM tests, M-PMOF are all in regular cubic shapes and EDX tests show that metal elements distribute uniformly on the M-PMOF crystals after stability tests. Further proved by the ICP leaching test, negligible leaching metal ions were detected in the solution after chemical stability tests. Now we have added relative discussion in the revised Supporting Information (page 13, line 4). 5. For the comparison with Co-TMCP, can the authors precise if the compound is studied as a solid in the carbon ink or in solution? I suspect this compound to be soluble in 0.5 M KHCO 3 . This must be clarified. A relevant comparison would be with a MOF containing Co-TMC such as MOF-525-Co.

Supplementary
Response: Thanks for your insightful suggestion. Co-TMCP is a kind of ester compound, which is insoluble in water or 0.5 M KHCO 3 . Co-TMCP is fuchsia. If it is dissolved in the solution, the color might have some change. After electrocatalysis test, the solution has no color change (Fig. S38). Recycle test was further preformed to verify the insolubility of Co-TMCP and an efficiency of 43.4% at -0.8 V vs. RHE similar to the original one (43.6% at -0.8 V vs. RHE) was achieved (Supporting Information, page 27, line 5).
Moreover, MOF-525(Co) was prepared as relevant comparison to further support the catalytic mechanism of M-PMOF. MOF-525(Co) has similar ligand as Co-PMOF and is constructed with Zr 6 O 4 (OH) 4 unit without POM. In the structure, each TCPP 4− ligand connects with four Zr 6 O 4 (OH) 4 units to generate a 3D framework with a ftw topology. The comparison between MOF-525(Co) and Co-PMOF can further reveal the function of POM in the electrochemical CO 2 RR. MOF-525(Co) is successfully prepared and certified by the PXRD tests (Fig. S32). The electrochemical CO 2 RR of MOF-525(Co) is measured adopting the same testing methods. As shown in Fig. S33, MOF-525(Co) shows lower FE CO (47.9% at -0.8 V) than Co-PMOF (98.7% at -0.8 V), which might be attributed to the poorer proton and electron transfer efficiency. This indicates POMs actually act as electron-rich aggregates in the catalytic mechanism of M-PMOF. In Co-PMOF, the connection of POM and metalloporphyrin will create an oriented electron transportation pathway under the motivation of electric field, abundant electrons flowing from POM cluster to metalloporphyrin motif can guarantee and facilitate the fulfillment of multiple electron migration process of CO 2 RR to achieve higher performance than MOF-525(Co). Now we have added relative discussion in the revised manuscript (page 8, line 30) and Supporting Information (page 24, line 3). Figure 38. The photo images of Co-TMCP before and after CO 2 RR test. (a) Co-TMCP powder. (b) Co-TMCP loaded carbon cloth electrode before CO 2 RR test. (c) Co-TMCP loaded carbon cloth electrode after CO 2 RR test. (d) The electrolyte before CO 2 RR test. (e) The electrolyte after CO 2 RR test. 6. The production of H 2 and CO (TON) as a function of time must be given.

Supplementary
Response: Thanks for your suggestion. According to your suggestion, we have added the calculation in the Fig. S24 and provided a discussion in the revised manuscript (page 6, line 32) and supporting information (page 2, line 29; page 19, line 3).
Turnover number (TON) is defined as the mole of reduction product generated per electrocatalytic active site over a given period of time.
The TON for CO was calculated as follows: