Post-polymerization modification towards polymer-supported metalloporphyrins for heterogeneous electrocatalysis

Abstract

Metalloporphyrins and associated ligands of amino acids form the key cofactors in many redox enzymes. Mimicking such a core function of redox enzymes remains a compelling approach for catalyst design. While most designs focus on a single molecular metalloporphyrin and pendant ligand, it is attractive to integrate such a design into a polymer, which could form an enzyme-like microenvironment through its random-coil and globular conformations. In this study, we introduce a one-pot method for effectively conjugating metalloporphyrins and imidazole groups to a commercially available polymer. Multiple metalloporphyrins were incorporated onto a commercially available polymer to form catalyst P1, followed by a reaction of the same polymer with excess imidazole groups to afford catalyst P2. We examined this concept using Zn, Co, and Cu as metal centers for chemical characterization and electrochemical OER, HER, and CO₂RR. The results suggest that P1-Co exhibited a larger catalytic current and Tafel slope than P2-Co. While P1-Cu predominantly catalysed the HER, P2-Cu is the only catalyst in this study that showed significant activity in reducing CO₂ to CO. The carboxylic acid groups can act as distal groups and the imidazole groups as axial ligands, thereby enhancing the catalytic capability and selectivity compared with metalloporphyrins. Although there is room for further improvement in catalytic performance, this work demonstrates the advantages of incorporating intramolecular distal groups and axial ligands in a polymer chain to closely mimic enzyme-based catalysts. The synthetic strategies should inspire the synthesis of other polymer-supported heterogeneous catalysts that require ligands and/or distal groups to enhance their catalytic performance.

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1. Introduction

Catalysts are widely used in electrochemical redox reactions for modern chemical synthesis, energy conversion, and environmental remediation. These catalysts, including transition metals, metal oxides, and enzymes, not only lower the activation energy of reactions but also facilitate the electron transfer process between reactants. Metals like platinum, palladium, copper, and rhodium, along with metal oxides such as iron, vanadium, and molybdenum oxides, are common and effective catalysts.^1–6 However, these metal catalysts can be costly, and many are toxic, posing environmental risks. Alternatively, enzymes are natural catalysts characterized by their high catalytic efficiency and specificity. For example, redox enzymes are commonly used in water remediation and chemical synthesis. The high catalytic activity of redox-active enzymes, especially those with metal active centers as cofactors, stems from their metalloporphyrin centers and the unique microenvironment created by the three-dimensional structures of the surrounding protein. As a result, metalloporphyrins and their derivatives have been the most extensively studied organometallics for mimicking enzymes.^7–9

So far, metalloporphyrins have been widely used in chemical, electrochemical, and photochemical catalysis for alkane oxidation and alkene epoxidation,¹⁰ hydrogen evolution reactions (HER),¹¹ oxygen evolution reactions (OER), oxygen reduction reactions (ORR),¹² CO₂ reduction reactions (CO₂RR),^13,14 and water remediation. It has long been recognized that axial ligands and distal groups are crucial for tuning the catalytic activity of metalloporphyrin-containing enzymes.⁷ Therefore, when incorporating metalloporphyrins into materials, adding an axial ligand or distal group to the catalyst design is highly desired to mimic redox enzyme-based catalysts.⁷

In the early stages, thiolate or imidazole groups were attached to one of the phenyl rings on the metalloporphyrin.^10,15,16 Due to their proximity, this design enhanced ligand–metal center interactions in the metalloporphyrin, greatly improving its catalytic performance.^17,18 While highly efficient in catalysis, these single-metalloporphyrin catalysts require complex synthesis, tedious post-reaction separation, and are difficult to recycle.¹¹

Long-standing studies have explored how axial ligands and distal groups tune the catalytic activity of metalloporphyrin catalysts;⁷ therefore, incorporating these groups and metalloporphyrins into a single system offers simpler synthetic routes to mimic enzymes. Recent years have seen an increased focus on integrating metalloporphyrins, axial ligands, and distal groups into frameworks or polymer matrices, which facilitates easier synthesis and use in homogeneous and heterogeneous catalysis.^14,19,20 For example, Babu and co-workers²¹ reported the synthesis of 2D metalloporphyrin polymers with abundant hydroxyl groups. These hydroxyl groups, together with the porous framework, increased local proton concentration, thereby enhancing the HER. Later, Cao and co-workers²⁰ designed a methacrylate monomer bearing a single tetraphenylporphyrin with a cobalt center (Co-TPP). By copolymerizing with acidic, basic, or neutral monomers, the resulting polymer-supported metalloporphyrin catalysts exhibited a significant enhancement in electrochemical and photochemical performance for the HER. In particular, the catalyst with carboxylic acid groups exhibited the highest HER current. This is because the carboxylic acid acted as a distal group, facilitating proton transfer. Soon after, Cu-TPP-based covalent organic frameworks (COFs) containing 2,4-diamino-1,3,5-triazine were synthesized.¹⁴ These amine functionalities served as distal groups to stabilize the active intermediates, thereby enhancing the reduction of CO₂ to CO and CH₄. While these examples demonstrate the synthesis of metalloporphyrin catalysts with distal groups, incorporating axial ligands into metalloporphyrin-based heterogeneous catalysts remains a scarcely explored area.

In this work, we report a one-pot, two-step strategy for synthesizing polymer-supported metalloporphyrin catalysts through post-modification of a commercially available polymer, poly(methyl vinyl ether-alt-malic anhydride) (poly(MVE-alt-MA)). Three amino-functional metalloporphyrins, Zn-TPP-NH₂, Co-TPP-NH₂, and Cu-TPP-NH₂, were successively reacted with poly(MVE-alt-MA) followed by 1-(3-aminopropyl)imidazole (API), which is expected to act as an axial ligand (Scheme 1). The resulting metalloporphyrin-containing polymers bearing carboxylic acid groups, both with and without imidazole ligands, were then characterized and used for heterogeneous electrochemical OER, HER, and CO₂RR to test the effects of the presence of these axial ligands and distal groups. Structural characterization suggested the presence of an intramolecular ligand effect, and the catalytic results demonstrated that this all-in-one system could enhance various catalytic reactions.

Scheme 1 Synthesis and chemical composition of polymer-supported metalloporphyrin catalysts with or without imidazole as axial ligands.

2. Results and discussion

Polymer-supported metalloporphyrin catalysts with distal groups have previously been synthesized by first synthesizing a monomer. For example, metalloporphyrins are first modified with polymerizable groups such as methacrylate or methacrylamide.^20,22 However, this process can be time-consuming and tedious. In contrast, post-polymerization modification of a functional polymer with a reactive metalloporphyrin may be more straightforward; however, it requires highly reactive groups from the metalloporphyrin or parent polymer to achieve high conjugation efficiency. Here, we chose commercially available poly(MVE-alt-MA), hereafter denoted as P0, as the parent polymer. This polymer has been previously used to react with amino-functionalized nitroxide radicals and has proven effective in forming polymer-based cathodes for organic batteries.²³ More importantly, the modified polymers, while insoluble in aqueous electrolytes, are highly hydrophilic. Therefore, if we are to use a similar methodology, we need to synthesize amino-functionalized metalloporphyrins with various metal centres. Specifically, tetraphenylporphyrin (TPP) was selectively nitrated to produce 5-(4′-nitrophenyl)-10,15,20-triphenylporphyrin (TPP-NO₂) as described in previous reports.²⁴ The reduction of TPP-NO₂ to TPP-NH₂ was initially carried out using tin(II) chloride in an acidic medium, giving a yield of 53%. When switching to Na₂S as a reducing agent, the yield was slightly higher (60%) (SI). TPP-NH₂ was then treated with three metal salts to afford metalloporphyrins (M-TPP-NH₂), including Zn-TPP-NH₂, Co-TPP-NH₂, and Cu-TPP-NH₂. Since the latter two compounds, with Co²⁺ and Cu²⁺, are paramagnetic and could not be analyzed by NMR, metal insertion was monitored by UV-vis spectroscopy. Zn-TPP was included in the synthesis for indirect characterization of the post-modification reaction with P0.

Fig. 1 illustrates the changes in ¹H NMR spectra from TPP-NO₂ to TPP-NH₂ and Zn-TPP-NH₂. The reduction of an electron-withdrawing –NO₂ to an electron-donating –NH₂ resulted in a drastic upfield shift of resonances for protons a and b from 8.17 and 8.40 ppm to 7.0 and 7.9 ppm, respectively. Peak i of porphyrin NH proton resonance at −2.7 ppm remains intact. Insertion of a metal ion into the porphyrin decreases its electron density and thus should lead to a downfield shift of aromatic proton resonances, particularly for those adjacent to electron-donating groups. We observed these changes as peaks a and b from TPP-NH₂ shifted downfield to 7.7 and 8.0 ppm. Most importantly, peak i of the pyrrole NH protons fully disappeared, suggesting successful Zn²⁺ insertion. Insertion of Co²⁺ and Cu²⁺ ions was achieved using the same protocol. While the products could not be characterized by NMR, they could be characterized by other techniques, such as ATR-FTIR and UV-vis, as discussed below.

Fig. 1 ¹H NMR spectra of (a) TPP-NO₂, (b) TPP-NH₂, and (c) Zn-TPP-NH₂ in CDCl₃ at 600 MHz.

Next, we tried to modify P0 with the amino-functional M-TPPs and 1-(3-aminopropyl)imidazole (API) (Scheme 1). The latter is introduced to the polymer to serve as an axial ligand to the metalloporphyrin metal center. Modification of P0 with the various M-TPP-NH₂ molecules was achieved by a nucleophilic acyl substitution reaction through a one-pot strategy, as illustrated in Scheme 1. The reaction forms a carboxylic acid and an amide linkage between M-TPP and the polymer. Three sets of parallel reactions were set up using P0 and Zn-TPP-NH₂, Co-TPP-NH₂, and Cu-TPP-NH₂ (two identical reactions for each M-TPP-NH₂) at a 1 : 10 ratio of –NH₂/maleic anhydride (MA) and the same concentration at 60 °C in DMF. After 24 hours, one set of reactions was terminated and the mixture purified to give P1-Zn, P1-Co, and P1-Cu. The other set was charged with excess API to further react with the remaining unreacted MA moieties within the polymer. Ultimately, the second group of polymers conjugated with imidazoles was obtained, designated P2-Zn, P2-Co, and P2-Cu. All P1 polymers exhibited good solubility in chloroform and DMF, while P2 polymers showed little to no solubility in many common solvents. This suggests strong intermolecular interactions between the imidazole ligands and the metal centers of the metalloporphyrins. All six polymers were analyzed by elemental analysis to determine the grafting efficiency of M-TPP-NH₂ and API by measuring the increase in nitrogen content. The content of M-TPP ranged from 7.4 to 11.3 mol%, and API ranged from 58% to 90%, resulting in API/TPP ratios of 5 to 12. Since P0 has a number-average molecular weight (M_n) of approximately 80 kDa, which corresponds to a degree of polymerization (DP) of 512, it is expected that this post-modification method can attach 38–60 metalloporphyrin groups to a single polymer chain (Table 1). It should be noted that the pendant imidazole group can also induce charge interactions with carboxylic acid groups and form an interlocked polymer network, making P2 water-insoluble but hydrophilic, an important feature for heterogeneous electrocatalysis in water.

Table 1 Characterization of the chemical composition of each polymer-supported M-TPP catalyst

Entry^a	% N	% C	M-TPP (mol)%	M-TPP/chain^b	Im (mol)%	Im/M-TPP	MW^c (g mol^−1)
a The molar percentage of M-TPP and imidazole on polymers was calculated based on elemental analysis. b The number of M-TPPs per polymer chain was calculated from their molar percentage and the repeating units of maleic anhydride (DP = 512), with the latter determined from the Mn of P0, i.e., 80 kDa. c The molar mass per mole of M-TPP.
P1-Zn	3.37	51.30	11.30	60			2066
P2-Zn	10.70	51.65	11.30	60	58.0	5	2660
P1-Co	2.51	53.16	7.40	38			2794
P2-Co	13.78	57.50	7.40	38	90.0	12	4201
P1-Cu	3.08	53.46	9.80	51			2278
P2-Cu	12.11	52.51	9.80	51	72.3	7	3131

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This approach is distinct from previous copolymerization strategies, which only allow one metalloporphyrin unit to be incorporated into each polymer chain.²⁰ Moreover, an excess of pendant imidazole groups should increase the likelihood of binding with the metal centre of a TPP moiety and, hence, their ability to act as axial ligands within the polymer chain. We successfully obtained the ¹H NMR spectrum of P1-Zn and the parent polymer, P0 (Fig. 2). Compared with P0, a set of new peaks appeared between 7 and 9 ppm, corresponding to the aromatic and pyrrolic proton resonances from Zn-TPP. This conjugation also led to a partial shift of peak d to ∼2.8 ppm. These results indicate the successful conjugation of Zn-TPP-NH₂ to P0.

Fig. 2 ¹H NMR spectra of (a) P0 in CDCl₃ and (b) P1-Zn in DMSO-d₆, at 600 MHz.

Although obtaining ¹H NMR spectra for P2-Zn and other polymers with Co and Cu centers was challenging, we gathered more structural details through UV-vis and ATR-FTIR characterization. Fig. 3 shows the ATR-FTIR spectra (a–c) and UV-vis spectra (d–f) of all three sets of polymers. Compared with P0, all P1 polymers exhibited a decrease in C=O adsorption at 1772 cm⁻¹ from MA. Notably, this reduction was more significant than expected, as the elemental analysis showed only 7.4–11.3% conjugation of M-TPP-NH₂, likely due to the hydrolysis of the anhydride during the purification steps. This is not a concern, as carboxylic acids are distal groups that can facilitate some catalytic reactions in metalloporphyrin systems.²⁰ Further reaction with the API nearly converted the remaining anhydrides to pendant imidazole and carboxylic acid groups, as shown by the increased C=O peaks at 1700 and 1640 cm⁻¹, corresponding to the amide and carboxylic acid, respectively. Additionally, a strong, broad peak around 1550 cm⁻¹ in all P2 polymers indicated the stretching and bending of the CN and CH bonds in the imidazole groups.

Fig. 3 ATR-FTIR spectra (a–c) and UV-vis spectra in DMF (d–f) of polymers conjugated with (a and d) Zn-TPP, (b and e) Co-TPP, and (c and f) Cu-TPP, along with imidazole.

UV-vis spectra of all polymers were measured in DMF. In the absence of a metal, the UV-vis spectrum of TPP-NH₂ shows one Soret band at 417 nm and four Q-bands at 516, 557, 592 and 652 nm. The addition of a metal ion increases the symmetry of a porphyrinic molecule, reducing the number of Q-bands from four to two, with a stronger peak at a lower wavelength and a weaker peak at a higher wavelength (Fig. S1 and Table S1).²⁵ The metal ions also induce redshifts of both Soret band and Q-band adsorptions due to the change in electronic structure of the porphyrin (Table S1).²⁶ When M-TPP was incorporated into polymers, P1-Co and P2-Co exhibited the most pronounced redshift, corresponding to a Soret band shift from 415 nm for Co-TPP to 429 nm for P1-Co and 434 nm for P2-Co, and a similar trend for Q1 and Q2. This is due to the Co-TPP being able to create a distorted octahedral coordination site with two axial ligands, presumably imidazole and DMF in this case. A weaker redshift from Zn-TPP to its polymers could be due to its coordination with one ligand to form a square-pyramidal metal coordination site, whereas for Cu-TTP and its polymers, no obvious shift was observed. Previous work suggests that Cu has weak binding to the ligand;²⁶ still, we deemed that DMF may coordinate with Cu-TPP and its polymers (note: DMF is the only suitable solvent for both P1 and P2 polymers), as DMF is a good ligand for copper, even stronger than water.²⁷ At a very low concentration (20 µg mL⁻¹), intermolecular interactions could be minimized. Therefore, these spectroscopic characterizations could provide important insights into intramolecular ligand–metal coordination.

Next, P1 and P2 were mixed with multiwalled carbon nanotubes (MWCNTs) in DMF to form catalyst composites. The morphologies of these composites were analysed using SEM. The polymers formed uniform morphologies with polymers coated on the MWCNT surface. The slight aggregation observed in P2 polymers is likely due to their insolubility in solvents because of strong intermolecular ionic interactions between the carboxylic acid and imidazole groups (Fig. S2).

The OER performance of cobalt-containing polymers P1-Co and P2-Co was then tested in a pH = 10 buffer. Linear-scan voltammograms (LSVs) were recorded at 0, 600, and 1200 rpm over a potential range of 0.8 to 1.6 V vs. Ag/AgCl (Fig. 4a). The current density is normalised based on the loading of metalloporphyrin catalysts (per µmol cm⁻²) and the potential versus RHE. It was found that the normalized OER currents for P1-Co were higher than those for P2-Co at various rotation rates (Fig. S3), presumably due to greater resistance, as indicated by EIS (Fig. S4 and Table S2). As a result, it required a slightly higher overpotential to drive the reaction. Notably, once the OER occurred, the Tafel slope for P2-Co was 320 mV dec⁻¹, significantly lower than that for P1-Co (510 mV dec⁻¹) (Fig. 4b). This difference is also reflected in the stable catalytic current density for P2-Co (i.e., 1 mA µmol⁻¹ cm⁻²) (Fig. 4c). Although P1-Co initially exhibited a higher current density, it rapidly decreased to a level lower than that of P2-Co after two hours of electrolysis. The CVs were also recorded before and after controlled-current electrolysis (Fig. S5), where P1-Co showed a more pronounced decrease in current. We also found that the buffer became slightly coloured. For this, we deemed that under basic conditions, the carboxylic acid groups in P1-Co are deprotonated, making the polymer soluble in water. The OER follows a proton-coupled electron-transfer mechanism,²⁸ in which the proximal carboxylic acid groups act as proton shuttles (Fig. 4d). P2-Co has faster OER kinetics due to the coordination of axial imidazole ligands with Co-TPP, which is also seen in a small M-TPP system.²⁹

Fig. 4 Electrochemical OER using P1-Co and P2-Co in 0.1 M carbonate–bicarbonate buffer (pH = 10). (a) LSV, (b) Tafel slopes, and (c) controlled-potential electrolysis at 1.5 V vs. Ag/AgCl. Current densities were normalized to 1 µmol cm⁻² of Co-TPP. (d) Proposed OER mechanism using polymer-supported catalysts.

To better demonstrate the intramolecular axial ligand effect within the polymer-supported metalloporphyrin catalysts, we then tested the Cu-TPP-NH₂ modified polymers P1-Cu and P2-Cu for the HER and CO₂RR at pH 8.8 and 7.2. Fig. 5a and b show the CV at 100 mV s⁻¹ under N₂ and CO₂ atmospheres, respectively. Cu-TPP and its polymer catalysts, P1-Cu and P2-Cu, exhibited higher reduction currents in the presence of CO₂. With the same amount of CNTs, this current difference is caused by the strong capacitive effect of ionic polymers, which facilitate the diffusion of electrolyte ions through the electrode layer. The resistance from EIS showed that P2-Cu exhibited a much smaller charge transfer resistance (R_ct) of 2.4 Ω vs. 99 Ω for P1-Cu, and a smaller ion transport resistance reflected by the Warburg coefficient (σ = 7.22) was also observed (Fig. S4 and Table S2). Additionally, the pair of redox peaks at −0.7 V was attributed to Cu-TPP binding with CO₂. Comparing their LSV with normalized Cu-TPP content, P2-Cu showed the highest current density of 2 mAh cm⁻² under a CO₂ atmosphere, at −1.0 V vs. Ag/AgCl. When the potential approached −1.2 V, the current densities for P1-Cu and P2-Cu were similar. Specifically, for P2-Cu at scan rates of 50, 100, and 150 mV s⁻¹, the reduction currents under CO₂ were consistently higher than those under N₂. The voltammetry data suggest that P1-Cu without an axial imidazole ligand exclusively catalyzed the HER. Unlike Co-TPP, the CVs of polymer-supported Cu-TPP catalysts before and after electrolysis were very similar (Fig. S5c and d) due to the CO₂RR/HER at near-neutral pH.

Fig. 5 Electrochemical behaviour of Cu-TPP and its polymer catalysts: CV under (a) N₂ and (b) CO₂ atmospheres, and a comparison of LSV (c) for different catalysts under a CO₂ atmosphere and (d) for P2-Cu at various scan rates under a N₂ or CO₂ atmosphere (N₂: pH = 8.8 and CO₂: pH = 7.2) in an electrolyte with Cu-TPP loading of 1 µmol cm⁻².

To verify the products, the Cu-TPP catalysts were then applied to carbon felt for heterogeneous HER and CO₂RR in either a N₂ or CO₂ atmosphere, and the produced gases were detected by GC. It was found that at −1.25 V vs. Ag/AgCl, the polymer-supported Cu-TPP catalysts P1-Cu and P2-Cu produced significantly more H₂ than Cu-TPP under both N₂ and CO₂. Likely, for P1-Cu, the remaining anhydride can be hydrolyzed into carboxylic acid groups, and the further conjugation of API to form P2-Cu also produced abundant carboxylic acid groups. Carboxylic acid groups act as proton relays, facilitating the HER and reinforcing previous findings in the polymer-supported Co-TPP system.²⁰ Such a relay effect could be slightly diminished when imidazole groups were introduced into P2-Cu via the anhydride moieties, leading to lower H₂ production (Fig. 6a and b). Notably, the higher HER efficiency under a CO₂ atmosphere could be attributed to a slight decrease in pH from 8.8 to 7.2 when the atmosphere was switched from N₂ to CO₂. Both the HER and CO₂RR are potential-dependent. Lowering the potential from −1.1 to −1.4 V vs. Ag/AgCl significantly increased H₂ and CO production (Fig. 6c and e). Among those limited examples of polymer-supported metalloporphyrin catalysts, the HER current of our P2 polymers is comparable to previous research under neutral to slightly basic conditions, even at higher pH (Table S3).

Fig. 6 Controlled-potential electrolysis for the HER and CO₂RR using Cu-TPP and its polymer catalysts. A comparison of H₂ production under (a) N₂ (pH = 8.8) and (b) CO₂ (pH = 7.2) atmospheres. (c) Potential-dependent H₂ production catalysed by P2-Cu and (d) a comparison of CO production by P1-Cu and P2-Cu under a CO₂ atmosphere at −1.25 V vs. Ag/AgCl, (e) a comparison of CO production by P2-Cu under a CO₂ atmosphere at various voltages. Voltage vs. Ag/AgCl, at pH = 7.2 with Cu-TPP loading of 1 µmol cm⁻². (f) A proposed CO₂RR catalyzed by P2-Cu with imidazole as an axial ligand and carboxylic acid as a distal group.

As discussed earlier, a much lower potential (−1.4 V vs. Ag/AgCl) could greatly promote the HER; therefore, we examined the CO₂RR at −1.25 V vs. Ag/AgCl. In this case, neither carbon felt nor Cu-TPP produced any detectable CO (Fig. 6d). In contrast, P1-Cu produced only a detectable amount of CO after 2000 seconds (i.e., 0.01 µmol). Conversely, P2-Cu was the only catalyst capable of producing CO, reaching up to 1 µmol after 2000 seconds. Introducing an axial imidazole ligand exerts a ‘push effect’ and increases electron density of the Cu centre, thereby facilitating the adsorption of CO₂ to the catalytic center.^30,31 Simultaneously, a carboxylic acid group acting as a distal group stabilizes the CO₂-bound intermediate via hydrogen bonding, thereby lowering the activation energy.^32,33 The proximity of a carboxylic acid and a Cu-TPP center brought about by the random coil conformation of the polymer chain enables a proton relay for proton-coupled electron transfer, a key step in converting CO₂ to CO (Fig. 6f).³⁴ Our design should also allow for well-separated TPP units along the polymer chain, preventing their stacking and aggregation, which has been found to significantly inhibit electrocatalytic activity in the CO₂RR.³⁵ Taken together, all results suggested that poly(MVE-alt-MA) is a useful parent polymer and, through a one-pot two-step strategy, both axial ligands and distal groups can be easily introduced to a polymer-supported metalloporphyrin catalyst, and readily applied in OER, HER, or CO₂RR reactions.

3. Conclusions

In conclusion, we have introduced a universal post-modification strategy to conjugate amino-functional M-TPP and amino-functional ligands to a commercial polymer, yielding heterogeneous, polymer-supported metalloporphyrin catalysts. This one-pot and two-step method avoids the need for complex monomer synthesis, and structural characterization has shown that dozens of M-TPP molecules can be easily incorporated onto each polymer chain. Proof-of-concept catalytic experiments on the OER, HER, and CO₂RR demonstrated the proton relay effect of P1 polymers in the HER and the axial ligand effect of P2 polymers in the OER and CO₂RR. In particular, P2-Co showed a smaller Tafel slope for the OER and a more stable catalytic current compared to P1-Co. P1-Cu and P2-Cu were capable of enhancing the HER compared with Cu-TPP; only the latter can exclusively produce CO. Notably, we observed that CO was not the dominant product. This is likely due to the strong intramolecular ionic interaction between imidazole and carboxylic acids, which makes P2 polymers difficult to dissolve in organic solvents during catalyst slurry preparation. A more precise synthetic strategy is currently being developed in our laboratories to ensure that axial ligands are in proximity to the metalloporphyrin center, rather than relying on the random-coil conformation of the polymer chain to facilitate these ligand effects.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available on request.

Supplementary information: synthesis and characterizations of functional metalloporphyrins, and electrochemical properties of polymer-supported catalysts. See DOI: https://doi.org/10.1039/d5ta07789c.

Acknowledgements

The authors thank the Australian Research Council (DP230100642) and Flinders University High Impact Collaborative Research Development Fund for financial support.

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