Photo‐Assisted Electrochemical CO2 Reduction to CH4 Using a Co‐Porphyrin‐Based Metal–Organic Framework

Metal–organic frameworks (MOFs) are a promising platform for assembling large concentrations of molecular catalysts on surfaces to drive the electroreduction of CO2. Yet until now, these MOF‐based systems were shown to produce only 2‐electron/proton products, i.e., CO or formic acid. Herein, it is demonstrated that a cobalt 5,10,15,20‐tetra(4‐carboxyphenyl) porphyrin (CoTCPP)‐based MOF can produce significant quantities of an 8‐electron/proton CH4, via a photo‐assisted electrocatalytic approach. Specifically, detailed electrochemical and spectro‐electrochemical analyses show that the addition of light illumination during electrocatalysis promotes the stabilization of a catalyst‐bound CO intermediate, allowing its further reduction to the final product, CH4. Using the photo‐assisted electrocatalysis method, maximum CH4 Faradaic efficiency of 14% was obtained at a low potential of −0.49 V NHE. Hence, the presented concept provides an additional step toward the design of more efficient MOF‐based electrocatalytic systems.


Introduction
Over the past two decades, metal-organic frameworks (MOFs) have evolved as a valuable class of porous functional materials. [1][2][3] They consist of metal nodes/clusters connected by organic linkers to form 3D porous, crystalline structures. [4] By virtue of their permanent porosity and structural modularity, MOFs satisfy some of the crucial requirements for a variety of important applications such as gas adsorption, [5] gas separation, [6] catalysis, [7] sensing, [8] and artificial photosynthesis. [9][10][11][12][13][14][15][16][17][18][19] Additionally, in recent years, the utilization of MOFs and MOF-based functional materials in electrochemical energy storage and electrocatalysis relevant to solar fuel production has evolved to be a fast-growing research field. [9,11,20,21] In this regard, Zr-oxo MOFs have emerged as a promising subclass because of their chemical robustness, structural modularity, and ability to incorporate functional molecular units. [22] The unique properties of MOFs enable one to perform heterogenization of high local concentration of catalytically active molecular species by their incorporation within the MOF's backbone, while maintaining their accessibility toward electrolyte-diffusing ions and reactants. Hence, these systems provide a great platform to drive energy-related electrocatalytic reactions, such as hydrogen evolution reaction (HER), [10,[23][24][25][26][27] oxygen reduction reaction (ORR), [28][29][30][31] and CO 2 reduction reaction (CO 2 RR). [32][33][34][35][36] Specifically, electrochemical CO 2 RR is a promising approach that can help lowering global CO 2 emissions. [37] Several types of molecular catalysts such as Fe, [32,[38][39][40] Ni, [41][42][43] Mn, [44,45] and Co-complexes [33,46,47] and metalloporphyrins [48][49][50] were previously reported to be active for CO 2 RR, while offering a suitable platform to achieve mechanistic insights into catalysis at the molecular level. Generally, these molecular catalysts follow a 2H þ /2e À proton coupled-electron transfer (PCET) reaction path, generating either carbon monoxide (CO), [51,52] formic acid (HCOOH), [53] or oxalate (C 2 O 4 2À ). [54] So far, however, reports of a molecular catalyst capable of producing multi-electron/proton reduction products (beyond 2e À /2H þ ) at reasonable rates are extremely rare. [47,55] Typically, for the successful generation of multi-electron products during CO 2 RR, catalyst-bound intermediates should be stable enough to undergo further proton-coupled electron transfer steps and release the end product. For instance, CO 2 -to-CH 4 conversion necessitates the stabilization of CO intermediates, to allow further 6e À /6H þ reduction to generate CH 4 . Indeed, Koper and coauthors have reported that Co-porphyrins immobilized on pyrolytic graphite electrodes can in principle form the 8-electron CH 4 , albeit with low Faradaic efficiency (FE) of 2.3% at À0.8 V RHE (pH = 1 and 10 atm). [47] Previous studies by Kadish and coauthors on the electrochemical behavior of Cobalt-5,10,15,20-tetraphenyl porphyrin (CoTPP) under a saturated CO environment revealed that CO binds preferably to the oxidized form of CoTPP i.e., [Co(III)-TPP] þ . [56] We hence postulated that by increasing the population of [Co(III)-TPP] þ , one could stabilize Co-TPP-bound CO and accelerate its electroreduction to CH 4 . Our recent study showed that a photoactive porphyrin-based MOF film can sustain large quantities of long-lived oxidized charged carriers during photocathodic operation. [10] Moreover, recent studies showed that added light DOI: 10.1002/solr.202201068 Metal-organic frameworks (MOFs) are a promising platform for assembling large concentrations of molecular catalysts on surfaces to drive the electroreduction of CO 2 . Yet until now, these MOF-based systems were shown to produce only 2-electron/proton products, i.e., CO or formic acid. Herein, it is demonstrated that a cobalt 5,10,15,20-tetra(4-carboxyphenyl) porphyrin (CoTCPP)-based MOF can produce significant quantities of an 8-electron/proton CH 4 , via a photoassisted electrocatalytic approach. Specifically, detailed electrochemical and spectro-electrochemical analyses show that the addition of light illumination during electrocatalysis promotes the stabilization of a catalyst-bound CO intermediate, allowing its further reduction to the final product, CH 4 . Using the photoassisted electrocatalysis method, maximum CH 4 Faradaic efficiency of 14% was obtained at a low potential of À0.49 V NHE . Hence, the presented concept provides an additional step toward the design of more efficient MOF-based electrocatalytic systems.
illumination during electrochemical CO 2 RR improves product selectivity and accelerates reaction rates of porphyrin-based MOFs. [57][58][59] As a result, we hypothesized that for a Co-porphyrinbased MOF, added light illumination during electrochemical CO 2 RR (i.e., photo-assisted electrocatalysis) could generate a large concentration of oxidized, Co(III)-porphyrin bound CO intermediate, and thus, in turn, facilitate the formation of CH 4 . In this work, we demonstrate that under 1 sun illumination, within the potential range of À0.39 V NHE to À0.64 V NHE , a Cobalt 5,10,15,20-tetra(4-carboxyphenyl) porphyrin (CoTCPP)based Zr 6 -oxo MOF (MOF-525) can perform 8-electron 8-proton CO 2 RR, leading to the formation of CH 4 with maximum FE of 14% (measured at À0.49 V NHE ). Thus, the presented photoassisted electrocatalysis approach could facilitate the development of more efficient molecular assemblies that drive multielectron electroreduction of CO 2 .

Results and Discussion
The study was performed on MOF-525 (Scheme 1), which consists of Zr 6 -based nodes linked by tetrakis(4-carboxyphenyl) porphyrin (H 2 TCPP) ligands. The synthesis of MOF-525 was adapted from a previously reported method (see Supporting Information for detailed experimental procedure). [32] Thereafter, the as-synthesized MOF-525's H 2 TCPP ligands were postsynthetically metalated with cobalt to generate a CoTCPPcontaining MOF-525, termed Co-MOF-525 (for details see Supporting Information).
The as-synthesized Co-MOF-525 powder was characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) analysis ( Figure 1). The experimental XRD pattern ( Figure 1a) showed sharp diffraction peaks at 2θ angles of 4.6, 6.5, 7.9, and 9.2, in good agreement with the simulated XRD pattern of MOF-525. [22] Additionally, SEM analysis of Co-MOF-525 show a typical cubic-shaped morphology with an average particle size of %1 μm (Figure 1b).
Raman spectra of Co-MOF-525 was found to possess peaks at 1566 and 1367 cm À1 , which correspond to vibrational modes of the symmetric pyrrole stretching, ν 2, and ν 4 , respectively ( Figure S1, Supporting Information), [28,60] Thus confirming the presence of the porphyrin moiety in the Co-MOF-525.
The Co content within Co-MOF-525 was measured by inductively coupled plasma analysis, showing a Zr 6 :Co molar ratio of 1:2.9, which is equivalent to 97% metalation of    Figure S2, Supporting Information). [46] For the photo-assisted electrocatalytic CO 2 reduction studies, Co-MOF-525 films were prepared by drop-casting an ink containing the catalyst onto FTO electrodes (see Supporting Information for detailed procedure), yielding electrodes hereafter termed FTO-Co-MOF-525. The photo-assisted electrocatalytic CO 2 reduction by FTO-Co-MOF-525 was performed with 80/20 (% V/V) acetonitrile-H 2 O (H 2 O acting as a proton source) solution containing 0.1 M LiClO 4 electrolyte. All photo-electrochemical measurements were recorded under 1 sun illumination (100 mW cm 2 ), hereafter referred to as under-light measurements, whereas measurements were done in absence of illumination, hereafter termed as dark measurements. Electrochemical CO 2 RR measurements were carried out in CO 2 -saturated solutions and were compared to control experiments carried out in N 2 -saturated solutions.
As shown by the cyclic voltammogram (CV) in Figure S3, Supporting Information, sweeping the potential toward the cathodic direction, a reduction wave is observed, reaching its peak current at À0.50 V NHE . In the reverse scan, an anodic peak is observed at À0.26 V NHE . Those peaks correspond to a quasireversible redox process of the porphyritic metal center Co(II/I) TCPP (E 1/2 = À0.38 V NHE ). [61,62] Figure 2 plots CVs of electrocatalytic operation of FTO-Co-MOF-525. Under N 2 saturated condition, FTO-Co-MOF-525 shows catalytic current due to HER, when scanning the applied potential negatively to Co(II/I) TCPP redox peak. [62] In saturated CO 2 solution, higher catalytic current and lower onset potential were observed, compared to saturated N 2 solution (HER catalytic currents), suggesting that FTO-Co-MOF-525 is active toward CO 2 reduction. The current further increases upon the addition of 1 sun illumination. Interestingly, under 1 sun illumination, the catalytic current density corresponding to HER also increases. This might indicate that some MOF-based photo-electrochemical behavior is taking place during catalytic operation. In other words, under illumination MOF-residing CoTCPP's are photoexcited, generating free charge carriers that in turn can participate and ideally can promote both the HER and CO 2 RR.
Moreover, to reveal the component responsible for the observed photo-current in the FTO-Co-MOF-525 system, we have performed an incident photon to current efficiency (IPCE) measurement. The measurement was done at À0.49 V NHE using a set of band-pass filters, covering a spectral range of 380-600 nm. As shown in Figure 3, the IPCE response of the electrode fits the electrode's UV-vis absorption spectra (recorded after 40 min of 1 sun illumination), showing the Soret band of CoTCPP. This result indicates that CoTCPP is the photoactive species responsible for the photoelectrochemical response of the system.
To analyze the CO 2 RR products, chronoamperometric measurements were performed within the potential range of -0.39 to -0.84 V NHE , at intervals of 0.05 V, at dark and under 1 sun illumination in a CO 2 -saturated electrolyte solution ( Figure S4, Supporting Information). Figure 4 shows the measured Faradaic efficiencies (FEs) of the produced CH 4 , CO, and H 2 at each potential. At dark conditions, FE for the 2e À /2H þ reduction product CO and the 8e À /8H þ CH 4 remain almost unchanged (both %1%) throughout the whole measured potential range. Notably, under 1 sun illumination a significant increase in the FE of CH 4 is clearly seen within the potential range of À0.39 to À0.64 V NHE , reaching a maximum FE value of 14% at À0.49 V NHE (see Figure 2b). However, the FE of CO increases only to %2% and remains almost unchanged through the whole potential range. Thus, CH 4 is the major CO 2 reduction product formed for FTO-Co-MOF-525 under light-assisted electrocatalytic conditions. H 2 , the product of the competitive reaction, HER, first appeared at À0.64 V NHE with an FE of 3.5%. Its FE continues to grow at higher cathodic potentials in both dark and light, reaching up to 87% at À0.89 V NHE . The increase in H 2 evolution is accompanied by a significant drop in the formation of CH 4 .  The formation of CH 4 was initially unexpected since CoTCPPs are mainly reported for reducing CO 2 to CO. [63] As reported earlier, the CO 2 RR by CoTCPP begins with activation of the CoTCPP by electron transfer from the electrode, reducing the Co(II)TCPP center to Co(I)TCPP, which can further interact with CO 2 through the metal center and form an intermediate complex, i.e., CO 2 -Co(I)TCPP. This complex was reported to further undergo multiple reductions and protonation steps that yield a catalyst-bound CO intermediate, CO-Co(II)TCPP. Eventually, the CO is released, and the catalyst is regenerated. [63] Hence, to understand better the mechanisms involved in the generation of CH 4 with the FTO-Co-MOF-525 catalyst, we performed a series of thin-layer spectro-electrochemical measurements. The technique enables us to follow the changes in the CoTCPP oxidation state by means of change in the UV-vis absorption spectral features under applied potential. The large extinction coefficient of porphyrins and the clear spectral difference among the species with different oxidation states allowed us to identify the probable active species involved in various steps of the catalysis and elucidate mechanistic insights.
For the measurement, 0.3 mg of Co-MOF-525 was loaded on a 1 mm thick ITO glass and placed in a quartz cuvette together with Pt and Ag wires used as counter and reference electrodes, respectively. The measurement was performed under a CO 2 environment under two conditions: 1) dark ( Figure 5a) and 2) light (Figure 5b), in which the electrode was subject to 1 sun illumination for 40 min prior to spectro-electrochemical measurement.
A series of chronoamperometric measurements were performed with the ITO-Co-MOF-525 within the potential window of À0.39 V NHE to À0.79 V NHE , while recording its UV-vis absorption spectra. For the measurement performed at dark (Figure 5a), two peaks at 408 and 430 nm were observed at open circuit potential. According to previous reports, the peak at 408 nm corresponds to the neutral Co(II)TCPP while the 430 nm peak corresponds to the oxidized form [Co(III) TCPP] þ . [56,64] The relative intensity of the two peaks changes with the application of potential. As the applied potential becomes more cathodic, the intensity of the peak corresponding to [Co(III)TCPP] þ (430 nm) decreases while the peak of Co(II) TCPP (408 nm) increases. Additionally, at À0.59 V NHE a new peak appears at 370 nm, which according to the literature corresponds to CO 2 reduction active species, [Co(I)TCPP] À . [46] Application of more negative potentials results in an increase of [Co(I)TCPP] À peak area, followed by the diminishing of the [Co(III)TCPP] þ and Co(II)TCPP peaks. The trend observed in the dark spectro-electrochemical measurement is logical since higher reduction potentials should result in a higher population of the reduced form of CoTCPP, [Co(I)TCPP] À .
On the contrary, for the light spectro-electrochemical measurements (Figure 5b), the peaks at 430 and 408 nm were observed at the open-circuit potential but with the addition of a broad absorption feature at longer wavelengths (%460 nm). According to the literature, it suggests the existence of a higher oxidation form of the catalyst, namely [Co(III)TCCP] 2þ , which is the product of porphyrin ring oxidation. [64,65] This broad absorption feature decreases significantly at higher reduction potentials, which strengthens our hypothesis. No significant difference was observed in the relative intensity of the absorption peaks corresponding to [Co(III)TCPP] þ and Co(II)TCPP throughout the whole reduction potential range. Meaning, the relative amount of [Co(III)TCPP] þ and Co(II)TCPP remained similar throughout the entire measurement. Additionally, it is noteworthy that under light, the absorption peak of [Co(I)TCPP] À was not detected at all applied potentials.
As mentioned earlier, it is well-known that CO binds preferably to the oxidized form of Co-TPP, i.e., [Co(III)-TPP] þ . [56] The spectro-electrochemical analysis shows that under 1 sun illumination, CO-MOF-525 maintains a higher relative concentration of [Co(III)TCPP] þ . Thus, we have hypothesized that the reason behind the formation of CH 4 under light lies in the stabilization of CO through the formation of a CO-Co(III)TCPP intermediate.
The qualitative spectro-electrochemical measurements reveal that upon 1 sun illumination the relative amount of [Co(III) TCPP] þ remains similar throughout the chronoamperometric measurements at different increasing reduction potentials. As can be seen in Figure 6a, upon switching on the light, an immediate positive shift in the VOC is observed, reaching a Plato at VOC of þ 600 mV (starting from À400 mV up to þ200 mV) after %25 min. In other words, the VOC measurement under light indicates slow positive charge accumulation on Co-MOF-525, namely generation of the positively-charged [Co(III)TCPP] þ species. When the light is switched off, the VOC remains almost unchanged (decays only by %50 mV over the course of half an hour), which is indicative of slow charge recombination in the absence of light illumination, thus explaining how the relative amount of [Co(III)TCPP] þ remained practically constant during light-assisted chronoamperometric measurements at various reduction potentials (Figure 5b).
Next, to verify if CO binds strongly to the positively charged [Co(III)TCPP] þ state, CV measurements were performed to compare the electrochemical behavior of Co-MOF-525 in CO and N 2 -saturated solutions (Figure 6b). While scanning the potential anodically, a quasi-reversible redox peak is observed, corresponding to the Co(II/III)TCPP redox couple. Notably, under CO-saturated condition, the Co(II/III)TCPP peak potential shifts cathodically by %50 mV, thus confirming that CO favorably binds with the oxidized form of CoTCPP i.e., [Co(III)TCPP] þ . [56] This result indicates that during light-assisted electrocatalysis, the CO formed during CO 2 electroreduction may bind with the [Co(III)TCPP] þ present in the Co-MOF-525, allowing its further reduction to CH 4 . Hence, if indeed this is the case, we should also observe a cathodic shift in the Co(II/III)TCPP redox potential when measured under CO 2 -saturated conditions with added light irradiation. As can be seen in Figure 6c, compared to dark conditions, the Co(II/III)TCPP redox peak shows a cathodic shift of %50 mV under light illumination. Consequently, this cathodic shift infers that CO is the most probable intermediate that forms during the CO 2 reduction performed by Co-MOF-525, which interacts with the oxidized form of CoTCPP, i.e., [Co(III)TCPP] þ present in abundance during the electrochemical measurements under 1 sun illumination and result in the cathodic shift of the redox peak position of Co(II/III)TCPP compared to the measurements performed in dark.
At this point, we have realized that by adding light illumination during electrocatalytic operation, one can stabilize catalystbound CO intermediates and thus further reduce them to CH 4 . Nevertheless, a question remains whether similar results could be obtained only by a photochemical route (photocatalysis). Hence, to decouple dark electrocatalytic contributions from photo-assisted ones, a photocatalytic CO 2 reduction reaction was performed over the FTO-Co-MOF-525 electrode. After 5 h of photocatalysis under 1 sun illumination, 0.037 μmol of CH 4 and 0.047 μmol of CO were generated. As can be clearly seen in Figure S5, Supporting Information, under photo-assisted electrocatalytic conditions (measured at À0.49 V NHE ) the amount of www.advancedsciencenews.com www.solar-rrl.com produced CH 4 is 2.35 times higher (while the produced amount of CO is lowered by a factor of 1.34) compared to the photocatalytic condition. Moreover, the CO 2 reduction product selectivity is profoundly affected by the mode of catalytic operation. For the photo-assisted electrocatalytic measurement, the obtained CH 4 selectivity is 71.4% (CO is the remaining 28.6%), while in the photocatalytic reaction CH 4 selectivity reaches only 44.1% (CO is the remaining 55.9%). Meaning, although the reduction of CO 2 can be performed photocatalytically, the introduction of electrical potential in our setup not only increases the total amount of products but also increases the selectivity toward CH 4 . Finally, the stability of the FTO-Co-MOF-525 electrode was tested by conducting a 5 h bulk-electrolysis measurement at À0.49 V NHE , which according to FE calculations is the optimal potential for CH 4 evolution in our setup. The measurement was performed in a CO 2 -saturated solution both at dark and light. As seen in Figure S6, Supporting Information, a stable, enhanced catalytic current was obtained under light-assisted condition. At the end of the measurement 0.087 μmol of CH 4 , and 0.035 μmol of CO were generated, corresponding to FE of 11.53% and 1.15%, respectively. Under dark, however, only trace amounts of CO and no CH 4 were detected after 5 h of bulkelectrolysis. The observed result emphasizes the contribution of photo-assisted electrocatalysis to the multi-electron reduction process of CO 2 . PXRD and Raman spectra measured before and after bulk-electrolysis under light show the retainment of the MOF's crystallinity. In addition, SEM images confirmed that the MOF's morphology was also preserved ( Figure S7, Supporting Information).
To conclude, in this work, we have presented a photo-assisted electrocatalytic CO 2 reduction approach to enable the generation of multielectron products by heterogenized molecular catalysts. Specifically, we show that a CoTCPP-based MOF, Co-MOF-525, can produce CH 4 , an 8e À /8H þ CO 2 reduction product when the light illumination is added during electrochemical operation, reaching an FE of %14% at À0.49 V NHE . Detailed electrochemical and spectroelectrochemical analysis revealed that under illumination, a large concentration of holes is accumulated, in the form of oxidized [Co(III)TCPP] þ species. In turn, this [Co(III)TCPP] þ species can bind CO favorably, thus stabilizing it for further electroreduction to CH 4 . As such, these results provide a step forward in our understanding of MOF-based solar energy conversion schemes, toward the future design of efficient catalytic systems that sustain multiple proton-coupled electron transfer reactions.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.