Molecularly defined interface created by porous polymeric networks

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Molecularly defined interface created by porous polymeric networks on gold surface for concerted and selective CO2 reduction Xin Cai, Haoyu Liu, Xing Wei, Zhenglei Yin, Jun Chu, Mingliang Tang, Lin Zhuang, and Hexiang Deng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04691 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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ACS Sustainable Chemistry & Engineering

Molecularly defined interface created by porous polymeric networks on gold surface for concerted and selective CO2 reduction Xin Cai,† Haoyu Liu,§ Xing Wei,† Zhenglei Yin,§ Jun Chu,† Mingliang Tang,# Lin Zhuang,†,§,* and Hexiang Deng†,§,* †Key

Laboratory of Biomedical Polymers-Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Luojiashan, Wuhan 430072, China. §UC Berkeley-Wuhan University Joint Innovative Center, The Institute for Advanced Studies, Wuhan University, Luojiashan, Wuhan 430072, China. #College of Life Sciences, Wuhan University, Luojiashan, Wuhan 430072, China. Corresponding Author: *[email protected] *[email protected]

Key Words: Electrochemical CO2 reduction, Concerted operation, In situ electropolymerization, Nanometer-thin films ABSTRACT: We report the design and synthesis of catalyst for electrochemical reduction of carbon dioxide, where the high geometric current of metal electrode and the excellent selectivity of molecular catalysts are combined. This was achieved by the construction of molecularly defined interface, here we term as MDI. Specifically, a porous polymer networks (PPNs) based on tetrakis-5,10,15,20-(4-aminophenyl) porphyrin (H2TAPP) were directly synthesized by electrochemical oxidation deposition on electrode. The thickness of this polymer layer was precisely controlled from 20 nm to 120 nm by the number of voltammetry cycles. The formation of MDI, composed of the amine linked polymer and Au electrode, was evidenced by scanning electron microscopy (SEM), atomic force microscopy (AFM) and attenuated total reflectance infrared spectroscopy (ATR-IR). This interface was endowed with abundant amino groups and Au atoms, providing excellent catalytic performance for electrochemical reduction of CO2 (CO2RR). Specifically, 95% of CO selectivity was achieved at the potential of -0.7 V (vs. RHE), while the same gold surface only gave a 25% selectivity under the same condition. The thickness of PPN layer was found to be critical for the catalytic performance, and in this construct the ideal thickness was achieved at 60 nm.

INTRODUCTION One promising approach to utilize and store intermittent energy was to covert CO2 to useful fuels by electrochemical CO2 reduction reaction (CO2RR)1-5. Among various critical considerations in the catalysts design for this technique, the conversion efficiency (geometric current density) and selectivity of the product were two primary aspects6-9. In general, there were two approaches to promote the efficiency and selectivity of CO2RR process: (1) surface modification of metal electrode by exposure of certain crystal lattice planes, (2) attachment of molecules and molecule-based materials including metal complex, proteins and polymers that carrying catalytic sites. These approaches have successfully demonstrated the possibility to achieve high conversion efficiency and/or selectivity in products, which provided insight to the mechanism of CO2RR8-23. Here, we reported a third approach to promote selectivity of CO2RR without sacrificing the efficiency, by creating molecularly defined interfaces (MDIs). Specifically, thin layers of porous polymer networks (PPNs) on the bases of tetrakis-5,10,15,20-(4-aminophenyl) porphyrin (H2TAPP) were deposited electrochemically on the surface of a flat gold electrode to construct PPN-on-Au MDIs through a cyclic voltammetry (CV) process (Figure 1A-C).

These PPNs contain amino functional group in their backbone, and we termed as TAPP-PPN. By gradual increase of the cycle numbers, a series of thin layers with progressively increased thickness, 20 nm, 40 nm, 60 nm, 80 nm and 120 nm, were synthesized leading to the creation of five PPN-on-Au MDIs, TAPP-PPN-20-on-Au, TAPP-PPN-40on-Au, TAPP-PPN-60-on-Au, TAPP-PPN-80-on-Au and TAPP-PPN-120-on-Au, respectively. The successful formation of these MDIs was confirmed by in situ attenuated total reflectance infrared spectroscopy (ATR-IR) and scanning electron microscopy (SEM). The thickness of PPN layers was measured by atomic force microscopy (AFM). With this design, a significant improvement of both efficiency and activity was brought to the CO2RR in comparison with bare gold electrode. As the thickness of PPNs layers increase from 20 nm to 60 nm, the conversion efficiency of CO2 to CO maximized at the thickness of 60 nm, 95%. Meanwhile, the geometric current density was also maximized at this thickness for all the potential tested (-0.5 V, -0.6 V and -0.7 V versus RHE). Further increase of the thickness from 60 nm to 120 nm did not result in better conversion efficiency, however, the product selectivity was well preserved. In addition, the best performing TAPP-PPN60-on-Au MDI exhibited excellent conversion efficiency and

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selectivity throughout the entire long cycle process with a duration of 180 minutes.

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multiple choices of the function groups in the synthesis of PPN, this MDI approach provides a large variety for the design and modification of metal surface, in addition to the exposure of ideal crystal lattice planes of the metal electrode. Recently, we illustrated the advantage of MDI by simple deposition of covalent organic frameworks (COF) on silver surface31,32. In this study, the electrochemical deposition method provided better affinity between the PPN layer and the metal electrode in MDIs, without the use of any binding reagent. Only a small amount of PPN was necessary for their construction. In addition, an appropriate thickness was identified to balance both the catalytic surface area and the diffusion of CO2.

RESULTS AND DISCUSSION

Scheme 1. The illustration of molecularly defined interface. MDI combines the advantages of metallic electrode and molecular catalyst. In the catalysis using MDIs, both the metal surface and the functional groups close to the surface are involved. Electrons transfer directly from metallic electrode to the reactant without going through the backbone of molecules on the surface. The uniqueness of this MDI approach is that both the molecule based PPNs and metal surface are involved in catalysis in a concerted manner (Scheme 1), while in previous studies, these two components usually functioned separately3,6,16,24-29. In the molecule attachment approach, decrease of geometric current was commonly observed due to the coverage of catalytic sites on metal electrode19-21. In contrast, these MDIs exhibit porosity to provide direct access of guest molecules to metal electrode30, thus the geometric currents of the entire electrode remain unaltered throughout the CO2RR process. Furthermore, given the

Electropolymerization of the TAPP-PPN. The synthesis of TAPP-PPN and the fabrication of the corresponding MDIs were achieved by electrochemical deposition of H2TAPP on Au electrode through an oxidation process (Figure1B). The initial oxidation occurs primarily on the aminophenyl functional groups, reflected in the CV (Figure 1C). The synthetic potential of TAPP-PPN was below the oxidation decomposition potential of the porphyrin ring in the monomer33. Thus, the porphyrin structure remains intact throughout the entire electrochemical deposition process, which was confirmed by the unaltered characteristic Soret band at 375 nm and the Q band from 460 nm to 730 nm in the solid-state UV-vis (Figure S7). The oxidation initiated at 0.6 V versus Ag/AgCl, which indicated the start of deposition and the growth of TAPP-PPN on the electrode. In the first 5 cycles of the CV curves, a broad oxidation peak was observed between 0.8 V and 0.9 V, and a reduction peak appeared between 0.42 V and 0.45 V, respectively. The gradual increase in separation of these two peaks indicated that the electropolymerization process became irreversible (ΔEp > 350 mV)34-37. After 10 cycles of the CV scan, the thickness of the TAPP-PPN layer reached 60 nm, and the separation of the two peaks was 450 mV in the CV curve (Figure 1C). Similar phenomenon was observed in previous studies, where aniline and TAPP were polymerized on nonmetallic electrode37-40. Here, the growth of the PPN layer on Au electrode was monitored by in situ ATR-IR, a powerful technique to provide in-time and precise characterization of the chemical environment on metal surface (Figure 1D)41. In the ATR-IR spectra, the fingerprint peaks of phenazine at 1645 cm-1, 1538 cm-1, 1391 cm-1 started to emerge at 0.7 V in the oxidation branch of the CV process, marked the initiation of polymerization39,40,42. In the reduction branch, these peaks decrease gradually. This combined with the presence of dihydrophenazine fingerprint peaks (1305 cm-1 and 1240 cm-1) indicated that most of the phenazine linkages formed in the oxidation process were converted to dihydrophenazine linkages after reduction40,43,44 (Figure 1A, D). Characterization of TAPP-PPN. Unlike previously reported amorphous PPNs, these TAPP-PPNs exhibited sharp peaks at 5.2 and 6.0° in their powder X-ray diffraction (PXRD) patterns, indicating the ordered arrangement of TAPP units in the structure. Although we made efforts to

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Figure 1. Synthesis and structure characterization of TAPP-PPN. (A) The synthetic scheme of TAPP-PPN. (B) The illustration

of the set-up for the electropolymerization. (C) In situ cyclic voltammetry curves of TAPP-PPN along the entire electropolymerization process. The gradual shift of the peak around 0.8 V-0.9 V vs. Ag/AgCl in the oxidation branch demonstrate the increase in active sites of the electropolymerization process. (D) In situ ATR-IR spectra measured along the cyclic voltammetry curves in (C) reveals the formation of TAPP-PPN and the transformation of phenazine to dihydrophenazine in the electropolymerization process. simulate the possible crystal structure of these TAPP-PPNs, none of the models matched completely with PXRD patterns (Figure S3). N2 and CO2 adsorption measurements were also conducted on these PPNs and the corresponding monomers (Figure S5, Figure 3F). Much higher N2 and CO2 uptake were observed in PPNs, indicating the formation of pores after the polymerization process. In the CO2 isotherms of PPN, the large hysteresis loops between the desorption and adsorption curves indicated the strong interaction between PPN and CO2 molecules, due to the presence of amino functional groups in the framework. In order to further confirm the porosity of this TAPP-PPN, inclusion experiments were conducted to introduce small organic molecules into the pores of PPN (Figure 2M). Specifically, a fluorescent molecule, 6-carboxyfluorescein (FAM) with the largest dimension of 1 nm, was used to diffuse into the pores of PPN samples in aqueous solution. The inclusion of this fluorescent molecule was characterized by laser scanning confocal microscopy. Before the test, the FAM molecules on the surface of TAPP-PPN crystals were washed repeatedly by deionized water. Green fluorescence of FAM was observed across the entire PPN layer, indicating the

successful inclusion of FAM into the pores of TAPP-PPN (Figure S15, S16). Thickness control. In this electropolymerization system, the thickness of PPN layer can be controlled by simply adjusting the number of scans in the CV cycles. The precise control over the layer thickness is a unique feature of this electropolymerization method in comparison to other methods. In order to accurately measure the thickness of PPNs, AFM was used and PPNs was prepared on single crystal silicon wafer with [100] lattice plane. Both the PXRD patterns and Fourier transform infrared (FT-IR) spectra confirmed that the PPN layers obtained on silicon wafer were identical to those on Au electrode (Figure S4, S6). The PPN layers prepared by electropolymerization were continues and homogeneous (Figure 2B, E, H), where each PPN layer has a uniform thickness across a large area on the electrode to form MDIs, beyond 1 cm2 (Figure S14). The packing of TAPP units in these MDIs were illustrated in Figure 2A, D, G. As the cycle numbers increased, 3, 5, 10, 20 and 40, the thickness of TAPP-PPN layers increased gradually, 20, 40, 60, 80 and 120 nm, respectively (Figure 2C, F, I and Figure S12, S13). In addition, we prepared PPN



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Figure 2. MDIs with different thickness of PPN. (A), (D), (G) The illustration of MDIs formed by TAPP-PPNs with the thickness of 20, 60 and 120 nm on Au electrode, respectively. (B), (E), (H) The corresponding AFM images of PPN layers in (A), (D), (G). Scale bar, 0.5 μm. (C), (F), (I) The corresponding linear height collected from (B), (E), (H). (J), (K)SEM images of TAPP-PPN on silicon electrode taken from a side view of 45°(J) and 90°(K), respectively. Scale bar, J: 1 μm, K: 100 nm. (L) The thickness of PPN layer plotted against cycle numbers of cyclic voltammetry. (M) Confocal microscopy images of 8 continuous layers across the thickness of 2 μm for FAM included porous TAPP-PPN layer. Scale bar, 2 μm. sample by just 1 cycle of CV scan. Due to the detect limit, the thickness was not accurately collected by AFM. SEM was used to further reveal the morphology of TAPP-PPN-on-Au MDIs, where both the PPN layer and metal electrode were clearly identified (Figure 2J, K). We found that the thickness of PPN in MDIs was critical for their CO2RR performance. Figure 2L presents the plots of the thickness versus the cycle numbers in CV. The thickness of PPN layers exhibited a positive correlation to the cycle number. This tunability of the PPN thickness allows us to optimize the performance of CO2RR. Electrochemical Reduction of CO2 by MDIs. In the assessment of CO2RR performance for these PPN-on-Au MDIs, the selectivity and production rate of CO were two

main parameters for comparison, where the production rate was quantified by the corresponding current. The impact of metal electrode types, porosity of MDIs, functional groups and thickness of the PPN layers were discussed. In both TAPP-PPN-on-Au and pure Au electrodes, currents corresponding to CO generation were clearly observed beyond the onset potential of -0.3 V versus RHE, which is characteristic for Au electrode (Figure S22). This indicated that the electrons were directly transferred from Au to CO2 molecules, instead of going through PPN framework. A much higher Faraday efficiency of CO was observed in TAPP-PPN-on-Au electrode (95%) under the potential of -0.7 V, in comparison to that of Au electrode (25%) under the same condition (Figure 3C). Concurrently,


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Figure 3. Performance of CO2RR on different MDIs. (A) The illustration of the concerted CO2RR taking place in TAPP-PPN-

on-Au MDI. (B), (C) Geometric current density and selectivity of CO for TAPP-PPN-60-on-Au, TTP-PPN-on-Au, H2TAPP-onAu and Au foil at different potentials, respectively. Geometric current density and selectivity of CO are displayed in the Y axis on the left and right, respectively. (D) Selectivity of CO and total current density for TAPP-PPN-on-Au related to the thickness at -0.7 V versus RHE, displayed in the Y axis on the left and right, respectively. (E) Pb-UPD curves of the TAPP-PPN-on-Au and bare Au disc electrodes, demonstrating the preservation of the majority of the active metal surface in MDI. (F) CO2 adsorption isotherm for H2TAPP and TAPP-PPN.

the hydrogen evolution reaction (HER) was suppressed from more than 70% in Au electrode to nearly 0% in TAPPPPN-on-Au electrode under -0.5 V and -0.6 V (Figure S19). To investigate the mechanism for the high performance of MDI in CO2RR, several control experiments was performed. Another type of PPN, constructed by tetrakis-5,10,15,20thiophene porphyrin (H2TTP), here we termed as TTP-PPN, was used to demonstrated the impact of functional groups (Figure S2). Although HER suppression was observed for TTP-PPN-on-Au, 40% at -0.7 V, less than that of Au electrode, 60% (Figure S19), there was no obvious improvement in the absolute current corresponding to the CO generation (Figure 3B). Similar phenomenon was observed in electrode prepared by deposition of organic linker, H2TAPP, on Au, where HER was further suppressed (20%) at -0.7 V, but CO current remained the same (Figure 3B). These two control experiments above clearly revealed the importance of amino functional groups in the suppression of HER. The formation of PPN further promoted the generation and selectivity of CO, thus the porosity of PPN in MDI was also critical for CO2RR. The Faraday efficiency of the CO and the total current were measured to assess the performance of CO2RR for TAPP-PPN-on-Au MDIs with different PPN thickness, from 20 nm to 120 nm, at -0.5 V, -0.6 V and -0.7 V, respectively (Figure 3D and Figure S20, S21). Both the total current and

Faraday efficiency increased as the thickness of PPN layers grew from 20 nm to 60 nm. The further increase of the thickness resulted in slight decrease of the total currents, while the Faraday efficiency still remained at a relatively high level. This could be attributed to the increase in the length of the diffusion pathway for CO2 to reach the metal surface. The TAPP-PPNs were also deposited on glassy carbon (GC) electrode and titanium electrode using the same method as control. Neither of these electrodes exhibited improvement in CO2RR, while only the titanium one suppressed HER at -0.8 and -1.0 V (Figure S23). This clearly demonstrated that TAPP-PPN alone had no activity in CO2RR, thus, further outlined the importance of the synergy between TAPP-PPN and Au electrode. Mechanism of CO2RR at MDI. The mechanism of electrochemical reduction of CO2 at TAPP-PPN-on-Au MDI was investigated by underpotential deposition (UPD) of Pb and in situ ATR-IR. Pb-UPD was performed on TAPP-PPN60-on-Au and bare Au electrode as control, where both curves displayed two significant peaks corresponding to the (111) and (110) facets of Au (Figure 3E). Integration of the UPD curves showed that TAPP-PPN-60-on-Au MDI preserved 83% of the active electrochemical surface area on the basis of bare Au electrode, indicating that a large portion of the active sites on Au electrode remained unaffected in MDI construct. In the in situ ATR-IR study of



TAPP-PPN-60-on-Au, three peaks were observed between 1727 cm-1, and 1867 cm-1, representing the stretch vibration of CO molecules detected at -0.6 V, -0.7 V and -0.8 V versus RHE, respectively (Figure S17). The attachment of CO molecules on Au surface showed that the reduction of CO2 took place on Au electrode, where electrons transferred directly to CO2 molecules without going through PPN backbone. This also demonstrated facile diffusion of CO2 in this MDI (Scheme 1, Figure 3A). The mechanism of electrochemical reduction of CO2 at MDI is different from previous studies, where molecules carrying catalytic sites were deposited on inactive electrode, in that the Au electrode here also plays a critical role. Indeed, when the electrode was switched to glassy carbon and titanium, the corresponding performances of CO2RR were poor at both TAPP-PPN-on-GC and TAPP-PPNon-Ti. This MDI approach is also different from lattice plane modification at surface of metal electrode, in that the amino functional groups in TAPP-PPN layer of MDI here is critical to activated CO2 molecules for selective reduction. According to the previous work, the amino functional groups have strong interaction with CO2 molecules by forming carbamate intermediates31,45, thus the amine framework could concentrate and activate CO2 molecules to help Au electrode reduce CO2 in a concerted manner. This was also evidenced in the limited improvement of CO current in CO2RR of TTP-PPN-on-Au in comparison to that of bare Au electrode. The thiophene functional groups in TTP-PPN were not as active as amino functional groups in TAPP-PPN in catalyzing CO2 electrochemical reduction. The critical role of both PPN and Au electrode in MDI was also confirmed by the inferior performance of H2TAPP deposited Au electrode. The formation of porous PPN in these MDIs obviated the compact coverage of small molecules on metal electrode, thus providing sufficient pathways for CO2 diffusion, and leading to drastic improvement in the current for CO generation. In comparison to our previous study, where MDI was formed by simple particle deposition of COF-300-AR31, the electropolymerization method here allowed us to precisely control the thickness of PPN layer on metal electrode. As we discussed above, the thickness is indeed a critical factor for the CO2RR performance. When the thickness of TAPP-PPN was below 20 nm, the number of active sites between TAPPPPN and Au was limited, while the CO2 diffusion pathways might be blocked as the thickness increased beyond 80 nm. Both resulted in decline of CO2RR performance (Figure 2A, G). The TAPP-PPN with 60 nm thickness exhibit the best selectivity of CO and highest total current. In this case, the optimized porosity favored CO2 diffusion kinetics for their access to the abundant active sites created by TAPP-PPN and Au electrode, and the sequential selective conversion to CO in a concerted manner (Figure 2D).

CONCLUSIONS PPN layers were directly synthesized on electrode by electrochemical deposition to form a special MDI. Au electrode decorated with TAPP-PPN was subjected to CO2

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electrochemical reduction in a concerted manner to achieved high selectivity and activity. The homogeneous fabrication method by electropolymerization led to a precise control of the thickness. The porosity and amino functional groups in TAPP-PPN-on-Au MDI are the two key elements for the excellent CO2RR performance. This MDI approach provided a new way in achieving synergy between distinct components with complementary properties to access high efficiency and selectivity in electrochemical catalytic reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information includes Supplemental Experimental Procedures and 26 figures and 2 table, it is available free of charge on the ACS Publications website. at DOI: ××× ×××.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (21471118, 91545205, 91622103), National Key Basic Research Program of China (2014CB239203), Key Program of Hubei Provence (2015CFA126) and Innovation Team of Wuhan University (2042017kf0232). We thank the test center and Core Research Facilities of Wuhan University, and we would like to acknowledge the invaluable assistance and discussion with Zhiyue Dong at Huazhong University of Science and Technology, Chao Wang, Yi Zhou and all the group members’ support and help in Deng’s lab and Zhuang’s group.

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Table of Contents Graphic

Electrochemical reduction of CO2 to useful fuels stands as a promising direction to provide CO2 balance and energy sustainability in modern society.


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Molecularly defined interface created by porous polymeric networks

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