A cobalt(Ⅱ)porphyrin with a tethered imidazole for efficient oxygen reduction and evolution electrocatalysis

Electrocatalytic oxygen reduction and evolution reactions are involved in new energy conversion and storage technologies,such as various fuel cells and metal-air batteries and also water splitting devices[1,2].However,both reactions are very slow in kinetics,and thus catalysts are required[3,4].

Cobalt porphyrins supported on carbon nanotubes as model catalysts of metal-N_(4)/C sites for oxygen electrocatalysis

Transition-metal based M-N_4/C catalysts are appealing for electrocatalytic oxygen reduction reaction(ORR) and oxygen evolution reaction(OER). Employing model catalysts, which have well-defined molecular structures and coordination environments, to investigate electrocatalytic performance of M-N_4/C sites for ORR and OER is of fundamental significance. Herein, we reported the use of Co tetra(phenyl)porphyrin 1 and Co tetra(pentafluorophenyl)porphyrin 2 as models to probe the role of Co-N_4/C sites for oxygen electrocatalysis. We showed that Co porphyrin 1 is more efficient than its structural analogue 2 for oxygen electrocatalysis in alkaline aqueous solutions, indicating that the electronrich Co-N_4/C site is more favored when noncovalently adsorbed on carbon supports. This work inspires rational design of reaction-oriented catalysts for sustainable energy storage and conversion technologies.

Comparing electrocatalytic hydrogen and oxygen evolution activities of first-row transition metal complexes with similar coordination environments

Developing cheap and efficient electrocatalysts for water splitting is required for energy conversion techniques.Many first-row transition metal complexes have been shown to be active for the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).Metal ions play crucial roles in these catalytic processes,but the activity dependence on the nature of metal ions has been rarely studied due to the difficulty to compare metal complexes with different coordination environments.We herein reported the synthesis of a series of metal complexes of azido-substituted porphyrin(1),in which metal ions have very similar coordination environments.By grafting 1-M(M=Mn,Fe,Co,Ni,and Cu)onto alkynefunctionalized carbon nanotubes(CNTs)through the same covalent connection,the resulted hybrids 1-M@CNT were all active and robust for both electrocatalytic HER and OER in alkaline aqueous solutions.Among these hybrids,1-Fe@CNT displayed the highest electrocatalytic activity for HER,while 1-Co@CNT was the most active one for OER.Moreover,a two-electrode water electrolysis cell assembled with 1-Fe@CNT as the cathode and 1-Co@CNT as the anode required smaller applied bias potential by210 mV to get 10 mA/cm^(2)current density as compared to that assembled with Pt/C and Ir/C with the same amount of metal loading.This work is significant to correlate HER and OER activity with the nature of first-row transition metal ions and to highlight promising potential applications of molecular electrocatalysis in water splitting.

A Cobalt(Ⅲ)Corrole with a Tethered Imidazole for Boosted Electrocatalytic Oxygen Reduction Reaction

Developing electrocatalysts with high activity and selectivity for the oxygen reduction reaction(ORR)is vital to promote the performance of the next-generation energy technologies,which depend on the efficiency of the catalytic reduction of dioxygen.In the structure of cytochrome c oxidases(CcOs),a histidine imidazole residue binding to the axial position of Fe plays a crucial role in facilitating the selective reduction of O_(2)to water.Inspired by nature,we herein report on the synthesis of CoIII corrole 1 tethered with an imidazole ligand as well as its electrocatalytic ORR and O_(2)binding features.As compared to the imidazolium-free analogue,complex 1 displayed remarkably boosted activity for the selective four-electron/four-proton(4e-/4H+)ORR with a half-wave potential of E1/2=0.82 V versus reversible hydrogen electrode(RHE)in 0.1 mol/L KOH solutions.Importantly,we demonstrate that the tethered axial imidazole ligand improves the O_(2)binding ability of 1 thermodynamically and dynamically,which is crucial to boost electrocatalytic ORR performance.This work presents an example to improve electrocatalytic ORR activity and selectivity of Co corroles by introducing an axial imidazole ligand to enhance the O_(2)binding and activation.

Electropolymerization of cobalt porphyrins and corroles for the oxygen evolution reaction

Developing large-scale electrocatalysts using molecular complexes for the oxygen evolution reaction(OER)is of great importance. Herein, four cobalt porphyrins and corroles are deposited on electrode substrates using a simple and fast electropolymerization method. Our results showed that Co-1-P@CC, formed by electropolymerizing Co tetrakis(p-N-pyrrolylphenyl)porphyrin(Co-1-P) on carbon cloth(CC), is the most active OER catalyst in the examined Co porphyrins and corroles in alkaline aqueous solutions by displaying an onset overpotential of 380 m V. Long-term electrolysis tests confirmed the stability of these electropolymerized films by functioning as OER electrocatalysts.

Tuning Electronic Structures of Covalent Co Porphyrin Polymers for Electrocatalytic CO_(2) Reduction in Aqueous Solutions

Improving the selectivity of the electrocatalytic CO_(2) reduction reaction(CO_(2)RR)over hydrogen evolution in aqueous solutions is required but challenging because the two reactions occur at close thermodynamic potentials and compete with each other.Herein,we report on the selective CO_(2)RR in aqueous solutions utilizing covalent Co porphyrin polymers with fine-tuned electronic structures.

Bioinspired iron porphyrins with appended poly-pyridine/amine units for boosted electrocatalytic CO_(2) reduction reaction

Developing highly efficient electrocatalysts for the CO_(2) reduction reaction(CO_(2)RR)has attracted increasing interest in the past decade.Herein,we report on the design and synthesis of Fe porphyrin 1 with an appended N,N-di(2-picolyl)ethylenediamine(DPEN)unit that boosts electrocatalytic activity for CO_(2)-to-CO conversion in acetonitrile with water as the proton source.By mimicking carbon monoxide dehydrogenase(CODH),1 has poly-pyridine/amine units located at the active site to form hydrogen-bonded water-containing networks that enable fast proton transfer.The protonated and positively charged DPEN unit can also stabilize CO_(2) reduction intermediates through electrostatic and hydrogen-bonding interactions.These factors make 1 a highly active electrocatalyst for the CO_(2)RR by achieving a TOFmax of 5.0​×​10^(4)​s^(−1) with water providing the protons.These critical roles of the DPEN unit in the CO_(2)RR are further supported by theoretical studies.This work is significant to highlight the benefits of using molecular catalysts to elucidate structural effects.