Biofunctionalized conductive polymers enable efficient CO2 electroreduction

Abstract

Selective electrocatalysts are urgently needed for carbon dioxide (CO₂) reduction to replace fossil fuels with renewable fuels, thereby closing the carbon cycle. To date, noble metals have achieved the best performance in energy yield and faradaic efficiency and have recently reached impressive electrical-to-chemical power conversion efficiencies. However, the scarcity of precious metals makes the search for scalable, metal-free, CO₂ reduction reaction (CO₂RR) catalysts all the more important. We report an all-organic, that is, metal-free, electrocatalyst that achieves impressive performance comparable to that of best-in-class Ag electrocatalysts. We hypothesized that polydopamine-a conjugated polymer whose structure incorporates hydrogen-bonded motifs found in enzymes-could offer the combination of efficient electrical conduction, together with rendered active catalytic sites, and potentially thereby enable CO₂RR. Only by developing a vapor-phase polymerization of polydopamine were we able to combine the needed excellent conductivity with thin film-based processing. We achieve catalytic performance with geometric current densities of 18 mA cm^-2 at 0.21 V overpotential (-0.86 V versus normal hydrogen electrode) for the electrosynthesis of C₁ species (carbon monoxide and formate) with continuous 16-hour operation at >80% faradaic efficiency. Our catalyst exhibits lower overpotentials than state-of-the-art formate-selective metal electrocatalysts (for example, 0.5 V for Ag at 18 mA cm^-1). The results confirm the value of exploiting hydrogen-bonded sequences as effective catalytic centers for renewable and cost-efficient industrial CO₂RR applications.

Fig. 1. Hydrogen bonds as catalytic motifs for CO₂ reduction.

(A) PDA electrocatalytic wires consisting of a conjugated-conductive body with functional groups on a carbon-based carrier electrode. (B to D) The functional groups drive the selective reduction of CO₂, (B) BEs at this site (DFT calculation) for CO₂, (C) two-spot delocalized amine-carbonyl hydrogen–bonded catalytic center on PDA, and (D) BEs shown as functions of the distances between nitrogen and CO₂ (1.99 Å) and oxygen and CO₂ (2.11 Å), indicating the most favorable energy-steric conformation.

Fig. 2. Synthesis of conductive-catalytic PDA.

Reaction of dopamine to conductive PDA by contact with sulfuric acid as oxidant during vapor-phase polymerization involves building blocks such as (i) condensed and oxidized diketoindole, (ii) dopamine, and (iii) oxidized dopamine, yielding the final product with periodically repeating units of conductive PDA.

Fig. 3. Fingerprints of free charge carriers in conductive PDA.

(A) Mid-infrared spectra of nonconductive PDA and conductive PDA, showing fingerprint spectral features for free-carrier generation in the conjugated system, confirmed in particular by signature infrared-activated vibrations (IRAVs). Note the strong carbonyl oscillation, which indicates dominant oxidation of the original hydroxyl groups. (B) The dc electrical conductivity (versus T) was as high as 0.43 S cm⁻¹ at 300 K in conductive PDA.

Fig. 4. PDA on CF for the electrocatalysis of CO₂.

(A) Cyclic voltammogram of conductive PDA on a CF electrode (area, 1 cm²) including control scans. The catalytic activity shows a reductive current in acetonitrile-water purged with CO₂. Fc/Fc⁺, ferrocene/ferrocenium; QRE, quasi-reference electrode. (B) SEMs of conductive PDA on CF before and after 8-hour electrocatalysis. (C) Faradaic efficiencies (F.E.) of CO, H₂, and formate as functions of time. (D) Sixteen-hour chronoamperometric scans (potential, −860 mV versus NHE) showing current stability.

Fig. 5. Electrochemical activation of amine-carbonyl hydrogen bridge to a nucleophilic center.

(A) Initial steps driving CO₂RR in conductive PDA: (i) electrochemical activation of the hydrogen-stabilized carbonyl group, (ii) subsequent formation of a nucleophilic center via the adjacent amine, and (iii) attachment to CO₂ creating an amide. (B) The in situ Fourier transform infrared (FTIR) differential spectra plot the individual initial steps shown above. The negative signs reflect the depletion of PDA-carbonyl, whereas positive signs correspond to the emergence of new absorptions, that is, CO₂-related vibrations (2350 cm⁻¹) and amide-carbonyl vibrations (1650 and 1635 cm⁻¹). SHE, standard hydrogen electrode.