Polymer-metal complexes as emerging catalysts for electrochemical reduction of carbon dioxide

Abstract

A class of metal-doped polyanilines (PANIs) was synthesized and investigated as electrocatalysts for the carbon dioxide reduction reaction (CO2RR). These materials show good affinity for the electrode substrate and allow to obtain stable binder-free electrodes, avoiding the utilization of expensive ionomer and additives. The emeraldine-base polyaniline (EB-PANI), in absence of metal dopant, shows negligible electrocatalytic activity and selectivity toward the CO2RR. Such behavior significantly improves once EB-PANI is doped with an appropriate cationic metal (Mn, Cu or Sn). In particular, the Sn-PANI outperforms other metal-doped samples, showing a good turnover frequency of 72.2 h−1 for the CO2RR at − 0.99 V vs the reversible hydrogen electrode and thus satisfactory activity of metal single atoms. Moreover, the Sn-PANI also displays impressive stability with a 100% retention of the CO2RR selectivity and an enhanced current density of 4.0 mA cm−2 in a 10-h test. PANI, a relatively low-cost substrate, demonstrates to be easily complexed with different metal cations and thus shows high tailorability. Complexing metal with conductive polymer represents an emerging strategy to realize active and stable metal single-atom catalysts, allowing efficient utilization of metals, especially the raw and precious ones. Graphic abstract

Structure-Performance Descriptors and the Role of the Axial Oxygen Atom on M–N4–C Single-Atom Catalysts for Electrochemical CO2 Reduction

“…Single-atom catalysts (SACs), possessing superior catalytic activity and maximum atom utilization, have been widely considered as efficient catalysts. − Especially, metal-coordinated four N-doped carbon two-dimensional SACs (M–N 4 –C, M = Fe, Co, Ni, Cu, etc.) have proven to be promising CO 2 RR electrocatalysts. − ,, Furthermore, the coordination environment and bonding characteristics of SACs play important roles in the regulation and optimization of catalytic performance, where heteroatomic doping is an effective strategy. , Compared with the introduction of impurity atoms in the carbon material plane, recent studies have reported that the introduction of an additional highly electronegative oxygen atom in the M–N 4 –C (M = Fe and Ni) axis can achieve efficient electrocatalytic CO 2 RR. − Wang et al revealed that CO 2 RR on an Fe–N 4 O-doped carbon material achieved a high Faradaic efficiency (FE) of CO of 96% with a low overpotential of 0.47 V. In addition, a Ni–N 4 –O/C catalytic site, by constructing a Ni site coordinated with four N atoms and one axial O atom, exhibited a FE CO of approximately 99.2% at −0.9 V . Chen et al reported an Fe 1 N 4 –O 1 electrocatalyst with an axial O atom coordinated at the Fe site by chelating an Fe precursor with an O-doped carbonaceous support, which showed outstanding catalytic performance of nearly 100% FE CO over a wide potential range.…”

“…have proven to be promising CO 2 RR electrocatalysts. [21][22][23][24][25][26][27]81,82 Furthermore, the coordination environment and bonding characteristics of SACs play important roles in the regulation and optimization of catalytic performance, where heteroatomic doping is an effective strategy. 28,29 Compared with the introduction of impurity atoms in the carbon material plane, recent studies have reported that the introduction of an additional highly electronegative oxygen atom in the M−N 4 −C (M = Fe and Ni) axis can achieve efficient electrocatalytic CO 2 RR.…”

Highly Dispersed Few-Nanometer Chlorine-Doped SnO2 Catalyst Embedded in a Polyaniline Matrix for Stable HCOO– Production in a Flow Cell

“…crystallography.net/cod/index.php). Since there are no contributions from the EB-PANI, 9 XRD data suggest that the SnO 2 /PANI catalyst consists of an amorphous polymeric matrix, decorated with nanometric SnO 2 -like structures (∼1.5 nm in size, based on Pawley refinement). Both morphology and structure were finally investigated with transmission electron microscopy (TEM).…”

“…Electrochemical Characterization. Due to the chemical nature as an ionomer of the polyaniline, 9 the catalyst was drop casted onto a gas diffusion electrode without the presence of a binder for the fabrication of the cathode electrode. The catalyst ink was composed of catalyst powder mixed with Vulcan carbon nanoparticles with a weight ratio of 9:1 for a total powder density of 1 mg cm −2 on each electrode; the introduction of carbon nanoparticles in the slurry was intended to increase the overall electrical conductivity across the electrode.…”

“…7,8 Since the presence of a metal active site is essential to perform a potential-accessible reduction of CO 2 , in order to keep the price down, the number of metal atoms required to form the active sites should be minimized. 9 The ideal design is the one of single-metal-atom materials 10,11 in which, similar to the metal−organic compounds, 12−14 the active site is a single metal atom or cation stabilized in a heterogeneous material or complexed with molecular ligands. The most common strategy is the design of defected catalysts that achieve the smallest possible cluster size, 15−17 thus maximizing the number of active sites per atoms involved in the cluster.…”