Surface-Immobilized Conjugated Polymers Incorporating Rhenium

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Surface-Immobilized Conjugated Polymers Incorporating Rhenium Bipyridine Motifs for Electrocatalytic and Photocatalytic CO2 Reduction Nicholas M. Orchanian, Lorena E. Hong, John A. Skrainka, Jacques A. Esterhuizen, Damir A. Popov, and Smaranda C. Marinescu* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 188.68.1.70 on 12/25/18. For personal use only.

S Supporting Information *

ABSTRACT: The solar-driven conversion of CO2 to value-added products provides a promising route for solar energy storage and atmospheric CO2 remediation. In this report, a variety of supporting electrode materials were successfully modified with a [2,2′-bipyridine]5,5′-bis(diazonium) rhenium complex through a surface-localized electropolymerization method. Physical characterization of the resulting multilayer films confirms that the coordination environments of the rhenium bipyridine tricarbonyl sites are preserved upon immobilization and that the polymerized catalyst moieties exhibit long-range structural order with uniform film growth. UV−vis studies reveal additional absorption bands in the visible region for the polymeric films that are not present in the analogous rhenium bipyridine complexes. Electrochemical studies with modified graphite rod electrodes show that the electrocatalytic activity of these films increases with catalyst loading up to an optimal value, beyond which electron and mass transport through the material become rate-limiting. Electrocatalytic studies performed at −2.25 V vs Fc/Fc+ for 2 h reveal CO production with faradaic efficiencies and turnover numbers up to 99% and 3606, respectively. Photocatalytic studies of the modified TiO2 devices demonstrate enhanced activity at low catalyst loadings, with turnover numbers up to 70 during 5 h of irradiation. KEYWORDS: solar energy conversion, electrocatalysis, photocatalysis, rhenium bipyridine, surface modification, metallopolymers

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INTRODUCTION As the rapid growth in global population continues, the demands for sustainable energy sources and energy storage technologies are expected to increase.1 The annual availability of solar energy (23 000 TW per year) provides a potentially inexhaustible source of renewable, clean, and distributed power for the planet.2,3 However, the intermittency of solar energy requires the development of technologies to harness, store, and transport it efficiently. Storing this energy in the form of chemical bonds would serve as an ideal solution to global energy challenges.4 This transformation could be accomplished either by directly driving chemical reactions with solar photons (photocatalysis), by using solar-derived electricity to drive electrolysis (electrocatalysis), or a combination of both (photoelectrocatalysis).5 In particular, the catalytic conversion of carbon dioxide (CO2) to carbon monoxide (CO) presents an opportunity to transform an abundant small molecule to a value-added C1 chemical feedstock. However, this reaction suffers from a broad product distribution and competitive hydrogen evolution. As such, specialized catalysts are required for the selective conversion of CO2 to a single value-added product. Both homogeneous and heterogeneous catalysts have been developed for the CO2 reduction reaction (CO2RR), each with their respective benefits and drawbacks.6 The well-defined © XXXX American Chemical Society

active sites of homogeneous catalysts enable thorough mechanistic studies and subsequent synthetic optimization, though these systems typically suffer from limited stability, diffusion-dependent kinetics, and poor recyclability.7,8 In contrast, heterogeneous catalysts allow for increased lifetimes, diminished diffusion limitations, improved recyclability, and the elimination of catalyst separation cost, though their illdefined surface structures prohibit rational improvement and typically yield low selectivity for CO2 to CO conversion.9 An alternative to these pathways is the heterogenization of molecular catalysts by anchoring molecular species to a surface support.10 This has been shown to improve the activity of these molecular species and effectively combine the benefits of homogeneous and heterogeneous systems.65,66 Of the known molecular catalysts for CO2RR, the rhenium(I) bipyridine tricarbonyl chloride complex (Re(bpy)(CO)3Cl) is one of the most well-studied to date, offering high activity and selectivity for CO production under both Special Issue: New Chemistry to Advance the Quest for Sustainable Solar Fuels Received: October 11, 2018 Accepted: December 11, 2018

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DOI: 10.1021/acsaem.8b01745 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Scheme 1. (a) Illustration of the Molecular Structure Reported for Polymers Prepared through the Electropolymerization of Re(vbpy)(CO)3Cl (vbpy = 4-Vinyl-4′-methyl-2,2′-bipyridine);18−21 (b) Illustration of the Molecular Structure of the Polymers Studied Here, Prepared by Electropolymerization of a [2,2′-Bipyridine]-5,5′-bis(diazonium) Rhenium Complex

In contrast to the flexibility of methylene-spaced polymers generated by vinyl polymerization, polymers with conjugated backbones display structural rigidity.22 Several reports on rigid metallopolymers suggest that this class of materials displays promising properties for photocatalytic applications.23 Studies on the conjugated poly([2,2′-bipyridine]-5,5′-diyl) and related metallopolymers indicate that these structures present unique photophysical properties, including intraligand π−π* transitions and metal-to-ligand charge transfer (MLCT) bands, which facilitate photocatalytic H2-evolving activity.24,25 Rigid rhenium bipyridine metallopolymers with aryleneethynylene architectures were reported to exhibit π−π* transitions as well as dπ(Re)−πpolymer charge transfer bands.26 An analogous polymeric material generated from an alkyne-substituted rhenium bipyridine complex was shown to generate rigid polymers with electrocatalytic activity for CO2RR, but these materials performed with low Faradaic efficiency (FE for CO production = 33%) relative to the vinyl-polymerized materials and no studies under illumination were reported.27 Due to the promising photophysical properties of poly([2,2′-bipyridine]5,5′-diyl) materials and their underexplored catalytic applications, we sought to develop a method for the growth of surface-immobilized polymers with a poly(Re(CO)3Cl[2,2′bipyridine]-5,5′-diyl) structure. Diazonium chemistry offers a promising route for surface modification with a broad substrate scope, improved stability, and aryl-bond formation.28,29,67 While both chemical and electrochemical diazonium grafting have been previously applied to rhenium bipyridine species, the development of polymeric materials with extended conjugation and 5,5′ connectivity was not explored.52,70 It was shown that electropolymerization of the diazonium-substituted complex could facilitate continuous film growth to generate electrocatalytic films, though the resulting material was not structurally characterized nor were photocatalytic studies reported.70 A previous report on the electrochemical grafting of p-bis(diazonium)benzene has shown that bis(diazonium) salts offer several advantages over the typically employed mono(diazonium) analogues for the generation of fully conjugated films.30 The p-bis(diazonium)benzene unit was covalently anchored to the supporting electrode surface following the electrochemical reduction of a single diazonium group, while preserving the second diazonium group for subsequent chemical functionalization. The diazonium-termi-

electrocatalytic and unsensitized photocatalytic conditions.11,12 The heterogenization of Re(bpy)(CO)3Cl has been successfully applied to a wide range of surfaces through both noncovalent and covalent interactions.13,14 In some cases, these architectures perform efficiently with high activity and orders of magnitude lower usage of expensive metals relative to their homogeneous counterparts.15 However, the functional groups selected for surface attachment limit the substrate scope of these methods. This limitation hinders catalyst development by preventing parallel studies of the heterogenized species across various supporting electrodes, which would allow for the translation of these devices between electrode architectures amenable to electrocatalysis and those amenable to photocatalysis. Electropolymerization presents a promising opportunity to generate nanostructured molecular films with broad substrate scope and tunable catalyst coverages.16 In particular, the electropolymerization of rhenium bipyridine complexes has been studied using a variety of polymerization methods, monomer units, and substrates.17,68,69 Following a report on the electropolymerization of a rhenium 4-vinyl-4′-methyl-2,2′bipyridine complex, Re(vbpy)(CO)3Cl, onto platinum disk electrodes, this procedure was subsequently applied to the modification of a variety of semiconductor materials (Scheme 1).18,19 These modified electrodes were shown to exhibit high turnover numbers (TONs) and faradaic efficiencies (FEs) for the electrocatalytic CO2RR to CO, with TON = 516 and FE = 90% after 80 min at −1.55 V vs SCE for a vinyl-substituted rhenium complex immobilized onto a platinum electrode.20 It was reported that diluting the rhenium complexes with a ruthenium tris(bipyridine) photosensitizer generated materials with improved stability and activity, with a TON = 3800 at −1.55 V vs SCE for a 30% Re and 70% Ru copolymer film (FE not reported).20 Although this technique was applied to semiconducting substrates for photoelectrocatalytic studies, unbiased photocatalytic studies were not reported.21 Despite promising activity, the flexibility imparted by the methylene spacers and the generation of vinyl radicals led to undesirable side reactions during grafting, including the formation of Re− Re and Re−C bonds via radical−radical coupling, as illustrated in Scheme 1. Consequently, these films were shown to exhibit limited stability, as anodic polarization led to film degradation, which was attributed to Re−C bond cleavage. B


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ACS Applied Energy Materials nated film could be directed toward either aryl- or azo-bond coupling reactions, though polymerization studies were not reported. We sought to modify this method of surface functionalization to generate surface-immobilized conjugated polymers incorporating rhenium bipyridine motifs. By targeting the synthesis of a [2,2′-bipyridine]-5,5′-bis(diazonium) rhenium complex (2), we sought to explore a 5′-directed electropolymerization mechanism for improved control over film structure and morphology. Grafting of this bis(diazonium) species is expected to generate a diazoniumterminated film, facilitating continued film growth. The structural rigidity and potential long-range order of the resulting films are expected to hinder dimer formation. The substrate scope of this method is investigated by modifying a variety of supporting electrode surfaces. Following grafting studies, the structure, morphology, and electrochemical behavior of these modified electrodes are investigated. Finally, the electrocatalytic and unbiased photocatalytic CO2RR activity of these devices are explored.

7.25 ppm, attributed to the aromatic protons, as well as a broad peak at δ 4.82 ppm which corresponds to the amine (−NH2) protons. Treatment of the recrystallized rhenium complex 1 with nitrosonium tetrafluoroborate (2.4 equiv) in anhydrous acetonitrile solutions at −40 °C leads to an immediate color change from pale-yellow to dark blue. Addition of diethyl ether results in the formation of a dark blue precipitate, which was collected by filtration, and stored in the dark at −27 °C. The dark blue precipitate was characterized by a variety of spectroscopic studies, including 1H and 19F NMR, ATRFTIR, UV−vis, and XPS. The 1H NMR spectrum of the dark blue precipitate displays peaks that are slightly shifted from that of the diamine complex 1; the amine peak is no longer present, and the aromatic peaks are shifted downfield (δ 10.10, 9.29, and 9.02 ppm), which indicates the clean formation of the desired rhenium bis(diazonium) complex 2 with strongly electron-withdrawing diazonium groups (Figure S2). The 19F spectrum exhibits a peak at −151.6 ppm, which is characteristic of the tetrafluoroborate anion (Figure S3). The ATR-FTIR spectrum of this complex displays characteristic carbonyl stretches for a fac-Re(CO)3 species at 2033 cm−1, 1957 cm−1, and 1929 cm−1, with an additional diazonium stretch appearing at 2310 cm−1 (Figure S4). These assignments are consistent with the gas-phase calculated frequencies using the M06 level of theory with the LANL2DZ basis set and ECP, as implemented in the Q-Chem software package (Figure S5, computational methods detailed in Supporting Information).32−34 The UV−vis spectrum of complex 2 maintains the MLCT band for the unsubstituted rhenium bipyridine species at 326 nm, with an additional broad feature at 612 nm, which we attribute to a π−π* transition of the bipyridine backbone upon incorporation of the diazonium substituents (Figure S6). XPS survey results confirm the presence of Re, C, O, N, Cl, B, and F in solid 2 (Figure S7). The high-resolution Re 4f region is consistent with that of complex 1, with features at binding energies of 43.4 and 40.9 eV, which correspond to the Re 4f5/2 and 4f7/2 levels, respectively.62 These binding energies suggest no change in the oxidation state of rhenium upon addition of NOBF4 (Figure S8). The Cl 2p region of complex 2 displays features at 198.9 and 197.1 eV, corresponding to the Cl 2p1/2 and Cl 2p3/2 levels (Figure S9), which are consistent with complex 1.62 The presence of the Cl 2p features confirms that the chloride ligand is not displaced upon diazotization. In contrast to complex 1, the N 1s region of complex 2 exhibits a peak at 404 eV, indicative of

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RESULTS AND DISCUSSION Synthesis of [2,2′-Bipyridine]-5,5′-bis(diazonium) Rhenium Complex (2). The rhenium [2,2′-bipyridine]-5,5′diamine complex 1 (Scheme 2) was synthesized according to Scheme 2. Immobilization of 2 via Electrochemical Grafting Studiesa

a (i) Treatment of complex 1 with 2.4 equiv of NOBF4 in anhydrous acetonitrile solution at −40 °C to generate complex 2 (ii) Electrochemical grafting of complex 2 by cyclic voltammetry in a 0.5 mM solution of 2 in acetonitrile with 0.1 M TBAPF6 supporting electrolyte. (iii) Electropolymerization of 2 by subsequent cyclic voltammetry scans.

our previous report by metalation of the [2,2′-bipyridine]-5,5′diamine ligand with rhenium(I) pentacarbonyl chloride in refluxed toluene solution.62 The 1H NMR spectrum of 1 in acetonitrile-d3 (Figure S1) displays peaks at δ 8.71, 7.64, and

Figure 1. Cyclic voltammograms for electropolymerization of 2 on a glassy carbon working electrode immersed in 0.5 mM 2 in acetonitrile solution with 0.1 M TBAPF6 electrolyte. All scans were performed at a scan rate (ν) of 100 mV/s and began at an initial potential (Pi) of −0.6 V with a switching potential (Ps) of (a) −1.60 V and (b) −2.60 V. C


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Figure 2. (a) High-resolution Re 4f XPS spectrum for a modified FTO electrode (n = 20). (b) SEM image of a modified FTO electrode (n = 20) at 25 000× magnification. (c) AFM topology of modified FTO electrode (n = 20).

diazonium moieties (Figure S10).29,62 The high-resolution B 1s and F 1s regions of 2 display features with binding energies at 192.1 and 682.9 eV, respectively, which correspond to the tetrafluoroborate anion (BF4−) (Figures S11−S12). Electropolymerization Studies. Electrochemical Behavior of 2. The electrochemical behavior of 2 was assessed by cyclic voltammetry (CV) experiments. Glassy carbon and FTO substrates were employed for these studies as models for carbon-based and metal oxide substrates, respectively. Slow scan rates (ν = 100 mV/s) were employed for these analyses to minimize the background current measured due to capacitive charging currents. Figure 1a presents consecutive CV scans for a glassy carbon (GC) working electrode immersed in a 0.5 mM solution of 2 with 0.1 M TBAPF6 supporting electrolyte in acetonitrile. On the initial scan, a broad irreversible reduction feature is observed at −0.71 V vs Fc/Fc+ (all subsequent potentials are referenced to the ferrocene/ferrocenium reversible couple). This broad reduction feature appears at potentials much more positive than those of complex 1 (−1.74 V and −2.12 V vs Fc/Fc+), and it is assigned to the oneelectron reduction of a diazonium group.62 This reduction leads to liberation of dinitrogen and subsequent formation of an aryl radical in close proximity to the electrode surface, as is established in the literature for electrochemical diazonium reduction.28 Additional grafting scans cause this feature to sharpen and shift to more negative potentials. This behavior is consistent with the literature precedent for the electropolymerization of diazonium species and indicates that charge transport from the substrate to the solution becomes increasingly impeded as polymerization proceeds.35 Following this initial reduction feature, a quasi-reversible feature at −2.01 V and an irreversible feature at −2.24 V appear (Figure 1b). These features are characteristic of Re(bpy)(CO)3Cl reduction events, previously described as predominantly bipyridinebased.36 The increase in current for these features with consecutive grafting scans provides evidence for electropolymerization, as more redox-active rhenium bipyridine species are available at the electrode surface with continued film growth.37 As subsequent scans are performed, the reduction features for complex 2 begin to diminish and a decrease in current density across the CV window is observed, which we attribute to hindered electrode kinetics as a result of the electrodeposited film (Figure S13).38 This decrease in current density

for the reduction features was observed to be dependent on the potential window of the grafting scans. As seen above in Figure 1, if the CV switching potential (Ps) is limited to −1.6 V the diazonium reduction feature appears at −1.2 V on the fifth scan, whereas if grafting is conducted with Ps = −2.6 V, the diazonium feature appears at −1.8 V on the fifth scan. This behavior illustrates that the potential window selected for grafting can influence the deposition of complex 2, as has previously been reported for diazonium electropolymerization.39 Similar characteristics were observed in analogous experiments with FTO working electrodes, which suggests that grafting proceeds in an analogous fashion (Figures S14−S15). Studies on FTO, however, indicate relatively lower current densities for grafting, which indicates subtle differences in grafting mechanism or efficiency across different supporting electrodes. Additionally, grafting studies performed with varying CV parameters indicate subtle changes in the rate of film deposition, as determined by estimation of catalyst loading through CV experiments (detailed further below). Film deposition is shown to change as a function of both potential window and scan rate (Figure S16), consistent with the changing rate for the shift in the diazonium feature observed during electropolymerization studies. Standardized Film Growth Methodology. To prepare films on various electrode materials (FTO, TiO2, gold, glassy carbon, graphite rod, or carbon nanotubes) for structural characterization and catalytic studies, a standardized grafting methodology was employed. Cyclic voltammetry scans were performed between an initial potential of −0.6 V (Pi = −0.6 V) and a switching potential of −1.6 V (Ps = −1.6 V). Cyclic voltammetry, rather than controlled potential electrolysis or linear sweep voltammetry, was selected to minimize the trapping of charged species in the film as the return sweep serves to repel these species from the electrode surface between successive grafting scans.39 Fast scan rates (ν = 1 V/s) were employed to minimize the Helmholtz layer during electropolymerization and subsequently minimize the trapping of electrolyte in the film, facilitating uniform film growth. An increasing number of scans, n, were run across a series of electrode samples to produce modified electrodes with varying catalyst loadings. Each of these samples exhibits a distinct orange film with increasing coloration across the series (Figure S36). These modified substrates were rinsed with acetonitrile and acetone to remove potentially physisorbed diazonium D


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redox features at −2.01 V and −2.24 V as the characteristic Re(bpy)(CO)3Cl reduction events. Analysis of the P 2p region reveals minor features at 135.8 and 134.9 eV, corresponding to the 2p1/2 and 2p3/2 levels, respectively, for films with n = 10 and n = 20. These features correspond to the PF6− anion, introduced during grafting as the electrolyte, which may be trapped within the film during electropolymerization or present on the electrode surface from residual electrolyte solution. The absence of these features for n = 1 and 5 is likely due to subdetection-limit concentrations of this species. The F 1s region for all films studied display a prominent feature at 684.4 eV, corresponding to trapped BF4− anions, suggesting that this is the dominant ion trapped within the film following polymerization (Figures S21−S22). Only the film modified with n = 5 expresses a feature at 687.9 eV, corresponding to the PF6− anion, suggesting that this is a minor species and that the rinsing procedure is successful in removing residual electrolyte. Additionally, the high-resolution C 1s spectra exhibit a π−π* shakeup feature at 292 eV, a characteristic feature in carbonbased materials with conjugation (Figure S23).41 This provides an indication for the growth of conjugated polymers during electropolymerization. Scanning Electron Microscopy and Atomic Force Microscopy. A scanning electron microscopy (SEM) image acquired at 25 000× magnification for a modified FTO electrode prepared with 20 grafting scans (n = 20) is presented in Figure 2b. This image illustrates that film growth is uniform across the substrate. Images acquired for films of varying catalyst loadings show that the observed morphology evolves from that of the underlying FTO substrate to a uniform and smooth surface (Figure S24). Analysis of the interface between the bare region and the modified region of the FTO electrodes highlights the applicability of this deposition method to masking and patterned growth, as well-defined boundaries were formed even with the simple Teflon-tape masks employed here (Figure S25). In an effort to estimate the absolute thickness of these films, side-on SEM imaging was attempted. Due to poor contrast between the glass, FTO, and film layers as well as difficulties observed for elemental mapping due to the near-detection-limit concentrations of relevant species, only the highest catalyst loading studied (n = 20) resulted in images appropriate for thickness estimation. A side-on SEM image for a modified FTO substrate (n = 20) indicates an ∼420 nm thick layer above the glass substrate, which can be attributed to the combined thickness of the film and the underlying FTO (Figure S55). Based on the thickness of FTO provided by the manufacturer (250 nm, MTI Corp.), we roughly estimate the film thickness of this sample as ∼170 nm, which represents an upper-bound estimate for the films studied in this report. This thickness is consistent with the passivation of the Sn 3d signal observed by XPS, as the film has grown well beyond the depth penetration of XPS. For quantitative data regarding the morphology of the modified electrode materials, atomic force microscopy (AFM) was applied to the analysis of FTO samples. Figure 2c presents the results from an AFM tapping mode experiment on a modified FTO electrode (n = 20), which was performed on a series of electrodes with increasing catalyst loading (Figures S26−S27). As illustrated above, these films grow as dense, uniform films. The root-mean-square surface roughness (Sq) was measured for four modified FTO films (n = 1, 5, 10, 20), with results tabulated in Table S1. These measurements reveal a decreasing trend from Sq = 11.25 ± 0.6 (n = 1) to Sq = 9.88

species or polymerization products. No visible change in the coloration of these films was observed after extensive rinsing or sonication in acetone, suggesting that the coloration was due to a robustly immobilized material. Grafting studies performed with ν = 1 V/s on glassy carbon, FTO, and gold electrodes show similar reduction features to those performed with ν = 100 mV/s with higher measured current densities, as expected (Figure S48). Larger current densities were measured for glassy carbon substrates relative to FTO substrates in these studies, suggesting that grafting may proceed more efficiently on carbon-based supports. This grafting methodology was also applied to graphite rod, TiO2, and gold substrates (Figure S48). The graphite rod electrode shows comparable current densities to the glassy carbon substrate, while the gold electrodes exhibit similar behavior to that of FTO. TiO2 electrodes generate the highest current densities, which we attribute to the high surface roughness and surface area of these substrates. The high passive charging currents and poorly resolved electrochemical features observed during electropolymerization studies on graphite rod, carbon nanotube, and TiO2 substrates precluded independent electropolymerization optimization for these electrodes. This behavior is consistent with the literature for diazonium electrografting to nanotube bucky paper.35 Characterization of Electropolymerized Films. XPS. FTO substrates were employed as the primary samples for Xray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), infrared reflection absorption spectroscopy (IRRAS), and UV−vis spectroscopy, as FTO substrates are flat and easily characterized by a variety of methods. After grafting and rinsing, modified FTO slides were analyzed by high-resolution XPS (Figure S17). The rhenium 4f region is illustrated in Figure 2a. The bare FTO substrate displays no peaks in the rhenium 4f region (Figure S49), as expected. Following electrodeposition of 2 (n = 20), the modified substrate displays two features at 43.4 and 41.0 eV, corresponding to the Re 4f5/2 and 4f7/2, respectively. Similar values were observed for the Re 4f signals of complex 1 as well as complex 2.62 These features are present and unchanged following additional sonication cycles. This provides evidence that the rhenium species is robustly immobilized to the substrate, pointing to successful reduction of the diazonium salts and subsequent covalent attachment to the surface. These Re 4f features increase in intensity for films deposited with a greater number of grafting scans, providing a strong initial indication for CV-controlled film thickness (Figure S18). This can also be inferred from the decrease in intensity for the high-resolution Sn 2p features. The bare FTO substrate exhibits features at 493.9 and 485.4 eV, corresponding to the Sn 3d1/2 and 3d5/2, respectively, which are clearly present for a film modified with n = 1 (Figure S19). These features are completely passivated as the catalyst loading is increased (up to n = 20), demonstrating that film thickness has exceeded the XPS sampling depth (∼5−10 nm). For films grafted with Ps = −1.60 V, the high-resolution Cl 2p spectra show two characteristic features for the Cl 2p1/2 (198.3 eV) and 2p3/2 (196.7 eV) levels for all catalyst loadings studied, consistent with Re(bpy)(CO)3Cl moieties in the film (Figure S20). For films grafted with Ps = −2.60 V, no chlorine was detected by XPS, consistent with the literature precedent for chloride dissociation associated with a one-electron reduction of the bipyridine ligand with subsequent ligand-to-metal charge transfer.40,31 This further corroborates our assignment of the E


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Figure 3. (a) Polarized IRRAS results measured for a modified Au substrate (n = 10) under both p- (blue trace) and s- (red trace) polarization. The a1′ (pink), a′′ (blue), and a2′ (green) are illustrated with their corresponding carbonyl stretching vectors and identified by colored asterisks. (b) UV−vis absorption spectrum measured for a modified FTO substrate (n = 10).

± 0.5 (n = 20) as the catalyst loading is increased. This trend demonstrates that film morphology is uniform across the electrode surface and that smooth electrode coverage is facilitated during electropolymerization. Film thicknesses could not be determined by AFM due to the gradual, rather than sharp, growth of these films at the film−FTO interface. Infrared Reflection Absorption and UV−vis Spectroscopy. Infrared reflection absorption spectroscopy (IRRAS) was employed as a surface-sensitive method to confirm the integrity of the fac-tricarbonyl coordination environment of the rhenium centers after electropolymerization. Gold substrates were selected for this study, as their high reflectivity allowed for spectra with increased signal-to-noise ratio. It has been welldocumented that rhenium fac-tricarbonyl complexes exhibit three distinct carbonyl stretching modes in the 1900−2100 cm−1 frequency range (though two of these modes often coalesce to one broad feature), corresponding to one highenergy, fully symmetric mode (a1′) and two nearly degenerate lower-energy modes (a′′ and a2′), illustrated in Figure 3a.42 Due to π-backbonding between the rhenium center and its carbonyl ligands, these stretching modes exhibit shifts in frequency related to the electron density of the metal cation and therefore function as convenient reporter ligands.43 The observed spectra for the electropolymerized films are consistent with fac-tricarbonyl rhenium, with one sharp highenergy band at 2030 cm−1 and a broad lower-energy band at 1929 cm−1. The small red shift in the stretching frequency (Δν = 3 cm−1 for the high-energy mode) indicates increased electron density at rhenium compared to the bis(diazonium) complex 2, as is expected for the loss of the electronwithdrawing diazonium substituents. To determine whether these films display long-range order, polarized IRRAS studies were conducted by introducing a ZnSe polarizing lens. Spectra collected for a modified gold electrode with n = 10, as an intermediate catalyst loading for comparison, are presented in Figure 3a. The spectrum obtained under s-polarized irradiation (perpendicular to surface normal) displays increased absorption for the carbonyl stretching modes relative to the corresponding spectrum obtained under p-polarization (parallel to surface normal). This can be quantified as the ratio of the peak height (ΔR) for the carbonyl stretching mode at 2030 cm−1 under s- and ppolarization (ΔRs/ΔRp). For a modified Au electrode (n = 10), this corresponds to a value of 74. The large polarizationdependence of these stretching modes demonstrates that the

majority of the rhenium tricarbonyl moieties are oriented with respect to one another, confirming the presence of long-range order in the deposited films. While quantitative orientation analysis is beyond the scope of this study, these results are consistent with the vertical growth mechanism proposed. Similar results are observed for modified FTO electrodes, although the polarization-dependence is somewhat less pronounced (ΔRs/ΔRp = 13 for n = 10) which may be a result of the rougher FTO surface or differences in substrate− polymer interactions (Figure S28). Further, plotting the peak height for the carbonyl stretching modes as a function of the number of grafting scans reveals a linear trend, as is consistent with increasing surface concentration of Re(bpy)(CO)3Cl moieties with sequential grafting scans (Figure S29). This trend is also seen across a series of modified gold substrates (Figure S53). Modified m-TiO2 devices were also characterized by IRRAS, confirming the presence of carbonyl stretches and the absence of diazonium features which provide evidence for electrodeposition with retention of the rhenium coordination environment (Figure S50). These films also exhibit an increase in peak height for the carbonyl stretches with increasing grafting scans, further suggesting that electropolymerization proceeds successfully on these substrates. Negligible polarization dependence (ΔRs/ΔRp = 1.5 for n = 10) was measured for modified TiO2 substrates, which is attributed to their high degree of surface roughness which is expected to prohibit the growth of highly oriented films. Similarly, modified graphite rod electrodes also express these characteristic carbonyl stretching modes (Figure S54) and display a low degree of polarization dependence (ΔRs/ΔRp = 3.5 for n = 10). The UV−vis spectrum of a modified FTO electrode (n = 10) is shown in Figure 3b. Unlike the unsubstituted rhenium bipyridine complex, Re(bpy)(CO)3Cl, the films grown in this study exhibit features in the visible region with transitions at 398, 496, and 644 nm, in addition to the characteristic MLCT band at 336 nm. These new transitions are consistent with those reported previously for conjugated rhenium bipyridine polymers and have been attributed to metal−ligandπ‑backbone charge transfer and intraligand π−π* transitions.26 The red shift of these bands relative to the MLCT transition of complex 2 demonstrate that electrons are promoted to an optically excited state with extensive delocalization.43,44 As the photocatalytic behavior of the unsubstituted rhenium bipyridine complex is hindered by its limited absorption cross section, F


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Figure 4. (a) Cyclic voltammetry of glassy carbon electrodes modified with varying numbers of grafting scans in acetonitrile solutions with 0.1 M TBAPF6 electrolyte (no diazonium salt in solution) (ν = 100 mV/s). (b) Catalyst loading (normalized to electrode surface area) as a function of the number of grafting scans applied (Pi = −0.6 V, Ps = −1.6 V, ν = 1 V/s) as determined by cyclic voltammetry (red) and ICP-OES (blue). (c) Double-layer capacitance (Cdl) measurement for a modified electrode (n = 5); traces indicate CV experiments at the open-circuit potential (0.120 V) with varying scan rates (ν = 1, 5, 10, and 20 mV/s). (d) Cdl of deposited films as a function of the number of grafting scans, illustrating the linear increase in electrochemically active surface coverage.

nondiffusing, surface-immobilized species (Figure S31).48 As the catalyst loading increases (n = 10), the peak separation increases as well (ΔE = 63.3 mV), which suggests that the diffusion-like transport of electrons is more substantial for longer polymeric chains. Randles−Sevcik analyses were performed on the cyclic voltammetry data experiments conducted with variable scan rates, and in all cases, the slopes of the logarithmic plots are near to the ideal value of 1 for an immobilized species, with the exception of the thinnest film (n = 1), which is dominated by passive charging current (Figures S31−S34). After the initial cathodic scan, subsequent scans exhibit lower current densities due to inefficient reoxidation of the reduced film. If an anodic scan (up to Ps = +0.4 V) is performed between cathodic scanning, the cathodic current density remains stable for repeated experiments (Figure S35). Similarly, if the modified electrode is exposed to air overnight (12 h), the redox feature at −1.95 V returns to the current density measured for the first scan after grafting. This behavior coincides with a dramatic color change in these films. After cathodic sweeping through the reduction feature at −1.95 V, a color change from orange to blue is observed for deposited films (Figures S35 and S36). After resting in air for 5 min, the blue film returns to its original orange color. This color change is not observed if the cathodic scan is followed by an anodic sweep (Ps = +0.4 V). This behavior is consistent with the reported behavior for poly([2,2′-bipyridine]-5,5′-diyl), which undergoes electrochemical n-doping.24,49 While further characterization of the blue films was precluded by their air

broadening this into the visible range is greatly beneficial for applications in artificial photosynthesis. Similar UV−vis spectra were recorded for a series of films with varying catalyst loadings, and all exhibit similar transitions (Figure S30), suggesting that oligomeric species are formed even for electrodes modified with n = 1.45 Electrochemistry of Films. After grafting, modified electrodes were washed vigorously with acetone for 1 min, dried under nitrogen, and immersed in an acetonitrile solution for electrochemical studies. Cyclic voltammetry experiments for a series of modified GC electrodes (n = 1, 5, 10, and 20) in acetonitrile with 0.1 M TBAPF6 supporting electrolyte are presented in Figure 4a. An irreversible cathodic peak appears at −1.62 V (for n = 20), which has previously been attributed to the buildup of excess electrolyte in the polymer matrix and subsequent discharge of these species.37,46,47 As the catalyst loading is increased, this feature shifts to more cathodic potentials and exhibits higher current densities, indicating hindered charge transport and increased electrolyte buildup for thicker films. This feature diminishes with subsequent CV scans, further indicating that a change occurs in the film upon cathodic scanning. The two rhenium bipyridine-based cathodic waves of 2 have converged into a single broad feature at −1.95 V, now corresponding to a two-electron reduction of the deposited films with retained quasi-reversibility. The broadening of this feature illustrates the hindered electron transfer kinetics resulting from growth of the polymer film.48 For thin films (n = 5), there is a 13.2 mV peak separation between the cathodic and anodic peak potentials, as expected for a G


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Figure 5. Cyclic voltammograms of modified glassy carbon electrodes in acetonitrile solution with 0.1 M TBAPF6 electrolyte under an inert atmosphere (blue), 1 atm of CO2 (red), and 1 atm of CO2 with 0.5 M trifluoroethanol added (green). Results are shown for (a) n = 1, (b) n = 5, (c) n = 10, and (d) n = 20. Control experiments with bare glassy carbon electrodes are also shown under 1 atm of CO2 (solid black) and with 0.5 M trifluoroethanol added (dashed black).

nitric acid to determine a bulk loading, this technique provides a more accurate representation of the amount of catalyst deposited onto the supporting electrode. The results of these quantification experiments suggest that similar catalyst loadings are obtained with similar grafting parameters for glassy carbon (Figure 4b) and graphite rod electrodes (Figure S42), both reaching loadings of about 40 nmol/cm2, while FTO electrodes exhibit lower per-area catalyst loadings of up to 26.8 ± 2.7 nmol/cm2 (Figure S51). A departure from this behavior is observed for the m-TiO2 substrates. These substrates display much higher surface areas than the previously mentioned electrodes due to their porosity and nanostructured surfaces. As a result, catalyst loadings about twice that of the other substrates, up to 79.1 ± 8.0 nmol/cm2, are observed for m-TiO2 (Figure S43). Despite these discrepancies, all electrodes exhibit a linear increase in catalyst loading as a function of the number of grafting scans performed, as measured by both CV and ICP, indicating that this electropolymerization technique is widely applicable. The double layer capacitance, Cdl, of each film was measured to provide a proxy for electroactive surface coverage (Figure 4c).51 Cyclic voltammetry scans were performed with variable scan rates in a 100 mV window around the open-circuit potential for each electrode (Figures S37−S40). A linear correlation was observed between the Cdl value and the number of grafting scans applied, shown in Figure 4d, which agrees with the linear trend observed from our estimation of surface coverage by eq S1. As a surface coverage of ∼0.1 nM/ cm2 corresponds to a dense monolayer for analogous metal complexes, the catalyst loadings measured in this study suggest that multilayer film growth is achieved during the initial grafting scan (∼10 equiv layers for n = 1).47

sensitivity, this behavior points to the successful generation of conjugated polymers with the desired poly(Re(CO)3Cl[2,2′bipyridine]-5,5′-diyl) structure. To assess the degree of control over catalyst loading provided by electrochemical deposition, we performed a series of studies to determine the electrochemically active surface coverage of rhenium bipyridine species. After washing the working electrode, CV scans in clean electrolyte solution (no diazonium salt in solution) were used to quantify the electroactive rhenium bipyridine concentration. The current− time plots were integrated for the range of the two-electron reduction feature at −1.95 V (as a convenient electrochemical signature of electroactive rhenium sites). The charge passed in this region was then converted to [Re] using eq S1, though this provides only an estimate of coverage due to the peak broadness of these reduction features and possible side phenomena.50 To corroborate these estimates, bulk catalyst loadings were determined by ICP-OES measurements for films digested in nitric acid. The results of these quantification studies are plotted in Figure 4b (details tabulated in Table S2). The linear correlation between electroactive rhenium coverage and the number of grafting scans performed demonstrates that nanomolar control over the surface coverage is feasible through this electropolymerization technique. In all cases, the catalyst loading as estimated by CV appears larger than that estimated by ICP. We attribute this discrepancy to the poor estimation of loading provided by CV measurements. The large capacitive charging currents coupled with the poorly resolved reduction features for the electrodes studied (particularly graphite rod and m-TiO2 substrates) yield overestimates for charge passed which subsequently result in overestimates of catalyst loading. As ICP measurements are conducted by digesting the film in H


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Figure 6. Cyclic voltammograms in acetonitrile solutions with 0.1 M TBAPF6 supporting electrolyte under either N2 (blue) or CO2 (red) atmosphere with (a) modified graphite rod electrode (n = 10) and (b) modified Nafion-MWCNT (n = 10). Respective bare electrodes (n = 0) shown in black under nitrogen (dashed) and CO2 (solid) atmospheres.

Catalytic Studies. Cyclic Voltammetry. Voltammograms with modified glassy carbon electrodes under a saturated CO2 atmosphere (shown in Figure 5) exhibit a loss of reversibility and an increase in current relative to experiments under a nitrogen atmosphere. For films of varying catalyst loadings, CVs under CO2 show increasing current densities with increasing surface coverage of catalyst, up to n = 10, before beginning to decrease again as n is increased to 20 (Figure S57). We attribute this behavior to hindered charge transport and substrate diffusion through the thicker films. As CO2RR represents a two-electron and two-proton reaction, the addition of an external proton source has been shown to increase the rate of catalysis by facilitating protonation of the bound CO2-adduct.31 To test the impact of an external proton source on the modified films, experiments were performed with the addition of a 0.5 M trifluoroethanol (Figure 5), which serves as an appropriate acid for CO2RR as hydrogen evolution is minimized.31,63 These studies again result in increasing current densities with increasing surface coverage of catalyst, up to n = 10, before beginning to decrease again as n is increased to 20, verifying that an optimal film thickness is reached before n = 20 (Figure S57). This saturation in current density for films with a higher catalyst loading in the presence of an external proton source indicates hindered mass transport as a result of the increased film thickness. As glassy carbon and FTO represent flat substrates, surface coverage of the catalytic film is limited in these cases. In an effort to improve the electrochemically active surface coverage of this material, alternative carbon-based electrodes were explored for CO2 electrolysis studies. We chose to explore graphite rod substrates and a composite electrode material composed of multiwalled carbon nanotubes (MWCNTs) with a proton-conducting binder (Nafion 117) on FTO as highsurface-area electrodes. For the MWCNT electrodes, an MWCNT-Nafion ink was prepared and drop-cast onto FTO substrates for subsequent electropolymerization of 2. The results of cyclic voltammetry experiments conducted on modified electrodes are presented in Figure 6. Upon switching from a nitrogen atmosphere to a CO2-saturated atmosphere, both electrodes show a large, irreversible increase in current density with a corresponding change in waveform to that of the kinetic zone, indicating fast catalysis with minimal substrate depletion near the electrode surface. Both substrates result in trace-crossing under CO2, which is present even after 2 h of controlled potential electrolysis under catalytic conditions (Figure S56). The trace-crossing observed here indicates the

formation of a new species upon reduction which exhibits an oxidation potential more positive than that of the as-prepared film, as has previously been established in the literature.64 This may indicate the formation of a long-lived CO2 adduct species which remains present for the return scan. Due to the lower background current densities measured for the graphite rod electrodes, these substrates were selected for preparative-scale electrolysis experiments. Controlled Potential Electrolysis. Controlled potential electrolysis (CPE) experiments were performed with modified graphite rod electrodes to determine the current efficiencies, product distributions, and lifetimes of these catalysts. Electrodes were prepared with various catalyst loadings (Figure S42) and assessed under electrolytic conditions for 2 h at −2.25 V under a CO2 atmosphere. This potential was selected for CPE studies, as it corresponds to the potential at which the modified electrode reaches current densities of 10 mA/cm2 under a saturated CO2 atmosphere by cyclic voltammetry. The results of these studies are summarized below in Figure 7, with details tabulated in Table S3. In contrast to the per-area normalized catalyst loadings presented elsewhere in this work, CPE results are discussed in terms of absolute catalyst loadings, as this provides a straightforward estimation of

Figure 7. Results of controlled potential electrolysis experiments with modified graphite rod electrodes (n = 1, 5, 10, and 20). All experiments were performed at −2.25 V for 2 h in acetonitrile solutions with 0.1 M TBAPF6 supporting electrolyte. Left axis corresponds to the Faradaic efficiency (red), and the right axis corresponds to TON based on ICP-OES bulk loading (blue). I


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Figure 8. Device architecture (a) and energy diagram (b) for photocatalytic CO2 reduction.

min of electrolysis. The low TOF measured for the film with n = 20 (0.06 s−1) is attributed to slow catalysis due to the inefficient reoxidation of films observed during cyclic voltammetry experiments. This inefficient reoxidation suggests that electrons may occupy trap states within the polymer which are subsequently not competent for CO2RR. Despite this, these conjugated polymers exhibit enhanced TON and FE for CO production (up to TON = 3583 with a FE of 99 ± 7%) in comparison to the previously reported vinyl-polymerized films (TON = 516 and FE = 90% for a 100% rhenium bipyridine film).20 Photocatalysis. The photocatalytic activities of modified TiO2 substrates were measured to assess the applicability of this material toward device fabrication for artificial photosynthesis. As catalyst immobilization has previously been shown to improve excited state lifetimes, the selection of supporting substrate, anchoring method, and device architecture has a profound impact on photocatalytic activity.29,53,54 The electrode architecture explored in this study is depicted below in Figure 8. Mesoporous TiO2 (m-TiO2) was selected as a wide-band-gap semiconducting support with a rough morphology for increased surface area. The roughness of these substrates allows for increased catalyst coverage relative to the flat FTO and glassy carbon electrodes (Figure S43). Further, TiO2 has been shown to mediate electron transfer through its conduction band and to stabilize surface-bound catalysts and chromophores.56,68 Triethanolamine (TEOA) was selected as a sacrificial electron donor, and all photocatalytic experiments were performed under sacrificial conditions in DMF (5:1 DMF/TEOA) to study the reductive half-reaction.57,58 Electrodes were prepared with various catalyst loadings, which was estimated by both cyclic voltammetry and ICPOES (Figure S43). As TiO2 has been shown to perform unassisted CO2RR, albeit with typically low selectivity for CO production, the activity of the bare substrates under photocatalytic conditions was measured to benchmark this background activity.55 The bare TiO2 substrate generate 0.11 μmol of CO during 5 h of irradiation. By introducing a 399 nm cut-on filter between the light source and substrate, excitation of TiO2 was inhibited and no CO was detected after 5 h of irradiation. Following these control studies, the photocatalytic activities of modified devices were tested with various catalyst loadings. The results of these studies are summarized in Figure 9, with details tabulated in Table S4. For the lowest coverage studied (n = 1) a TON of 70 and an overall TOF of 14.0 h−1 was measured after 5 h of irradiation. For n = 5, a TON of 28 and an overall TOF of 5.6

TON per rhenium site. In all cases, CO was the only product determined by GC and no formic acid was detected by 1H NMR, illustrating high selectivity for CO production. For the lowest catalyst coverage studied (n = 1), 3.3 μmol of CO were produced during a 2 h electrolysis experiment with FE = 48 ± 5% (TON = 1149, TOF = 0.32). Increasing the catalyst loading (n = 5) led to an improvement in activity, producing 18.8 μmol of CO with FE = 99 ± 7%. This corresponds to a TON of 3583, based on the bulk catalyst loading by ICP-OES, and an overall TOF of 0.50 s−1. For n = 10, a modified electrode produced 27.2 μmol of CO with FE = 88 ± 8%, corresponding to a TON of 3606 and TOF of 0.50 s−1. For the highest coverage studied (n = 20), 13.8 μmol of CO was produced with FE = 64 ± 6% (TON = 623, TOF = 0.06 s−1). Based on these results, it is apparent that both FE and TON increase up to an optimal value for catalyst loading, beyond which both parameters decrease. The low FE and TON for low catalyst loadings (n = 1) are consistent with the behavior reported for a chemically grafted diazonium-substituted rhenium bipyridine species on graphene substrates.52 Chemical grafting led to much lower catalyst coverages (0.1 nmol/cm2) and lower FEs for CO production (≤44%, comparable to the n = 1 electrode). As the catalyst loading of modified graphite rod electrodes is increased, side processes are eliminated by passivating the electrode surface and the FE for CO production increases. Based on the trend observed in Figure 7, the optimal catalyst loading is reached before n = 20 and appears to lie between n = 5 and n = 10. Using an average of the loadings measured through ICP-OES for n = 5 and n = 10, an optimal loading of ∼6 nmol is estimated. As previously discussed, increased catalyst loading corresponds to both reduced electron transfer and mass transport kinetics, which are expected to hinder catalytic activity for thick films. These results demonstrate that high catalytic performance can be reached with relatively low concentrations of catalyst. In all cases, the current densities drop off rapidly within the first 5 min of electrolysis (Figure S52), before stabilizing to lower current densities (2 h) exhibit substantially decreased current densities relative to the as-deposited films (>80% decrease, Figure S56). ICP studies were conducted on the postcatalysis acetonitrile solutions for graphite rod experiments, indicating that no rhenium had leached into solution for all catalyst loadings studies. This provides evidence that deactivation is not due to catalyst desorption from the graphite rod electrodes and suggests strong graphite−polymer interactions. IRRAS studies confirm this, as the high-energy carbonyl stretching mode decreases in intensity by only 5% following electrocatalytic studies (Figure S47). These studies do indicate a sharpening of the lower-energy carbonyl stretches and an increase in their peak separation, which may hint at catalyst restructuring as a deactivation pathway. In contrast to the graphite rod electrodes used for electrocatalysis, the m-TiO2 electrodes employed for photocatalysis display substantial loss of material from the electrode surface. IRRAS studies on the postphotocatalysis substrates indicate a 67% decrease in the high-energy carbonyl stretch (Figure S47). ICP was not conducted on the photocatalysis solution, due to the incompatibility of TEOA with nitric acid. For more thorough characterization of these polymer films after electrolysis, FTO electrodes were prepared and subjected to 2 h of controlled potential electrolysis at −2.25 V in acetonitrile solution under 1 atm of CO2. XPS on the postcatalysis FTO electrodes indicates a decrease in intensity for the high-resolution rhenium 4f peaks, implying a loss of material from the electrode surface (Figure S44). No features corresponding to Re0 or oxidized rhenium species were detected, confirming that the formation of nanoparticles did not occur during catalytic studies. A similar decrease in intensity was observed for the carbonyl stretching modes of the postcatalysis films in IRRAS studies (Figure S45). These peaks were also 10 cm−1 red-shifted relative to the precatalysis film, suggesting a more nucleophilic rhenium center, which is consistent with the reduced rhenium bipyridine molecular complex. The appearance of two new features at 1604 and 1660 cm−1 is observed and have previously been attributed to the presence of Re-CO2− speciesan intermediate in the proposed catalytic cycle for the homogeneous complex.60,61 Further, no evidence for Re−C bond formation was observed by FTIR, in contrast to previous reports for vinyl polymerization, suggesting that these side reactions have been successfully mitigated through this diazonium electropolymerization methodology. UV−vis studies on the postcatalysis FTO electrodes show a similar decrease in intensity for the electronic transitions of the film (Figure S46). No shifts in these features were observed, nor were any new features present after catalytic studies. This provides a strong indication that dimers are not generated under catalytic conditions, as the characteristic electronic transitions of the dimer species observed after vinyl electropolymerization remain absent,

Figure 9. TON as a function of catalyst loading for photocatalytic studies with modified TiO2 electrodes.

h−1 was determined. After introducing a 399 nm cut-on filter, similar activity was measured for a film modified under the same conditions (n = 5, TON = 31, TOF = 6.1 h−1). This confirms that the CO generated for modified electrodes is predominantly generated by the catalyst moieties rather than the mesoporous TiO2 electrode. For n = 10, the photocatalytic activity decreases to a TON of 26 and TOF of 5.2 h−1. The highest coverage studied (n = 20) displayed the lowest catalytic activity, with a TON of 22 and TOF of 4.4 h−1. After 5 h of irradiation, the solvent mixture had changed from colorless to pale orange and the color intensity of the postcatalysis devices had diminished. These results suggest promising unbiased photocatalytic activity for thin films of poly(Re(CO)3Cl[2,2′-bipyridine]-5,5′diyl). Prior systems incorporating rhenium bipyridine complexes immobilized on TiO2 were reported to exhibit a photocatalytic TON up to 62 during 24 h for a phosphonated complex, and a photoelectrocatalytic TON of 70 for a carboxylate-substituted complex on Cu2O-TiO2 photocathodes (during 1.5 h at −2.05 V).59,14 The measured TON of 70 during 5 h of irradiation for the n = 1 electrode compares favorably to these previous systems as a higher loading is attainable with comparable TON per rhenium site (yielding a larger CO output per device), and no external potential was applied to reach these TON values. The high activity for low catalyst loadings demonstrates that only a small quantity of the deposited material remains active through the 5 h irradiation experiment. Beyond this loading, photocatalytic activity drops to TON ≤ 28. The homogeneous, unsubstituted rhenium bipyridine species was reported to generate CO with a TON of 27 during 4 h of irradiation in a mixture of 5:1 DMF/TEOA. Furthermore, the color change of both the electrodes and DMF solution reveal that catalyst desorption occurs for TiO2 substrates and that these polymers are partially soluble in DMF following surface desorption. As rhenium bipyridine complexes are known to exhibit absorption features with large molar extinction coefficients, it is likely that the activity of films with higher catalyst loadings are dominated by the desorbed material, which absorbs incident photons before they are able to reach the electrode−catalyst assembly. Although these solvated polymeric species may also be competent for photocatalytic CO2RR, it has previously been shown that the solvated molecular complex performs with slower rates of catalysis than the analogous surface-immobilized species.18 For photons that do reach the device, a thin layer of photocatalytic K


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Family for undergraduate fellowships to L.E.H. We thank USC for the Undergraduate Provost Fellowship to J.A.S. and J.A.E. We thank Prof. Jahan M. Dawlaty for discussion and for providing access to the IRRAS accessory, and to Prof. Mark E. Thompson for providing access to AFM instrumentation. XPS and SEM data were collected with instrumentation provided by the USC Center of Excellence in Nano Imaging. We thank Andrew J. Clough for assistance with XPS and SEM and Courtney A. Downes for discussion. FTIR and UV−vis data were collected with instrumentation provided by USC. NMR data were collected with instrumentation provided by the USC Center of Excellence for Molecular Characterization. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant Number ACI-1548562. All calculations were performed on the COMET cluster operated by the San Diego Supercomputer Center at UC San Diego.

which further highlights the benefits of these rigid, fully conjugated films.

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CONCLUSIONS We report here the directed electropolymerization of molecular films for photocatalytic and electrocatalytic CO2 reduction. A diamine-functionalized rhenium bipyridine system was converted to a bis(diazonium) species, 2, which was immobilized to a broad range of substrates. The catalyst loading was controlled by cyclic voltammetry, and the resulting films exhibit extended conjugation. The conjugation in these films enables a high degree of long-range order, supported by polarized IRRAS studies, as well as new intraligand π−π* and metal−πpolymer transitions which are not present for the unsubstituted rhenium bipyridine species. In contrast to previous reports for vinyl electropolymerization, no evidence for Re−Re dimer formation or Re−C bond formation was observed for these conjugated films. Cyclic voltammetry of these films shows a large, irreversible increase in current under a CO2 atmosphere at a potential that coincides with the rhenium bipyridine reduction features. CPE studies performed on a modified graphite rod electrode confirm the catalytic reduction of CO2 to CO with turnover numbers up to 3606 in 2 h, a TOF of 0.50 s−1, and an FE of 99% for electrolysis at −2.25 V vs Fc/Fc+. Photocatalytic studies on modified TiO2 electrodes confirm promising activity with TONs up to 70 after 5 h of irradiation, corresponding to an overall TOF of 14 h−1. This work demonstrates that (1) rigid, conjugated, and highly oriented polymers can be produced on a broad range of substrates by the electropolymerization of aryl p-bis(diazonium) salts and that (2) conjugated polymers of rhenium bipyridine moieties produced in this manner exhibit promising electrocatalytic and photocatalytic performance.

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(1) Energy International Administration (EIA). International Energy Outlook Executive Summary. 2018, 1−14. (2) Gray, H. B. Powering the Planet with Solar Fuel. Nat. Chem. 2009, 1, 7. (3) Lewis, N. S. Research Opportunities to Advance Solar Energy Utilization. Science 2016, 351, aad1920. (4) House, R. L.; Iha, N. Y. M.; Coppo, R. L.; Alibabaei, L.; Sherman, B. D.; Kang, P.; Brennaman, M. K.; Hoertz, P. G.; Meyer, T. J. Artificial Photosynthesis: Where Are We Now? Where Can We Go? J. Photochem. Photobiol., C 2015, 25, 32−45. (5) Kim, D.; Sakimoto, K. K.; Hong, D.; Yang, P. Artificial Photosynthesis for Sustainable Fuel and Chemical Production. Angew. Chem., Int. Ed. 2015, 54, 3259−3266. (6) Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. A Review of Catalysts for the Electroreduction of Carbon Dioxide to Produce Low-Carbon Fuels. Chem. Soc. Rev. 2014, 43, 631−675. (7) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and Homogeneous Approaches to Conversion of CO2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38, 89−99. (8) Morris, A. J.; Meyer, G. J.; Fujita, E. Molecular Approaches to the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels. Acc. Chem. Res. 2009, 42, 1983−1994. (9) Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, M. T. M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073−4082. (10) Bullock, R. M.; Das, A. K.; Appel, A. M. Surface Immobilization of Molecular Electrocatalysts for Energy Conversion. Chem. - Eur. J. 2017, 23, 7626−7641. (11) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Electrocatalytic Reduction of Carbon Dioxide Mediated by Re(Bipy)(CO)3Cl (Bipy = 2,2′Bipyridine). J. Chem. Soc., Chem. Commun. 1984, 984, 328−330. (12) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Efficient Photochemical Reduction of CO2 to CO by Visible Light Irradiation of Systems Containing Re(Bipy)(CO)3X or Ru(Bipy)32+-Co2+ Combinations as Homogeneous Catalysts. J. Chem. Soc., Chem. Commun. 1983, 9, 536− 538. (13) Blakemore, J. D.; Gupta, A.; Warren, J. J.; Brunschwig, B. S.; Gray, H. B. Noncovalent Immobilization of Electrocatalysts on Carbon Electrodes for Fuel Production. J. Am. Chem. Soc. 2013, 135, 18288−18291. (14) Schreier, M.; Luo, J.; Gao, P.; Moehl, T.; Mayer, M. T.; Grätzel, M. Covalent Immobilization of a Molecular Catalyst on Cu2O Photocathodes for CO2 Reduction. J. Am. Chem. Soc. 2016, 138, 1938−1946. (15) Oh, S.; Gallagher, J. R.; Miller, J. T.; Surendranath, Y. GraphiteConjugated Rhenium Catalysts for Carbon Dioxide Reduction. J. Am. Chem. Soc. 2016, 138, 1820−1823.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01745. Synthetic methods, electrochemical methods, computational methods, NMR data, ATR-FTIR data, IRRAS data, cyclic voltammetry data, XPS data, double-layer capacitance data, SEM and AFM images, electrolysis and photocatalysis results, ICP-OES data, and photographs of modified electrodes (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nicholas M. Orchanian: 0000-0001-5752-6845 Smaranda C. Marinescu: 0000-0003-2106-8971 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Southern California (USC) and the National Science Foundation (NSF) through the NSF CAREER award (CHE-1555387). We are grateful to the USC Wrigley Institute for Norma and Jerol Sonosky summer fellowships to N.M.O. and D.A.P. We are grateful to the Rose Hills Foundation and the Morrisroe L


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