Significantly boosted oxygen electrocatalysis with cooperation between cobalt and iron porphyrins

Covalent Tethering of Cobalt Porphyrins on Phenolic Resins for Electrocatalytic Oxygen Reduction and Evolution Reactions

Using functionalized supporting materials for the immobilization of molecular catalysts is an appealing strategy to improve the efficiency of molecular electrocatalysis. Herein, we report the covalent tethering of cobalt porphyrins on phenolic resins (PR) for improved electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A cobalt porphyrin bearing an alkyl bromide substituent was covalently tethered on phenolic resins, through the substitution reaction of alkyl bromides with phenolic hydroxyl groups, to afford molecule-engineered phenolic resins (Co-PR). The resulted Co-PR was efficient for electrocatalytic ORR and OER by displaying an ORR half-wave potential of E1/2=0.78 V versus RHE and an OER overpotential of 420 mV to get 10 mA/cm2 current density. We propose that the many residual phenolic hydroxyl groups on PR will surround the tethered Co porphyrin and play critical roles in facilitating proton and electron transfers. Importantly, Co-PR outperformed unmodified PR and PR loaded with Co porphyrins through simple physical adsorption (termed Co@PR). The zinc-air battery assembled using Co-PR displayed a performance comparable to that using Pt/C+Ir/C. This work is significant to present phenolic resins as a functionalized material to support molecular electrocatalysts and demonstrate the strategy to improve molecular electrocatalysis with the use of phenolic resin residues.

The pH Influence on the Water-Splitting Electrocatalytic Activity of Graphite Electrodes Modified with Symmetrically Substituted Metalloporphyrins

Hydrogen, considered to be an alternative fuel to traditional fossil fuels, can be generated by splitting water molecules into hydrogen and oxygen via the use of electrical energy, in a process whose efficiency depends directly on the employed catalytic material. The current study takes part in the relentless search for suitable and low-cost catalysts relevant to the water-splitting field by investigating the electrocatalytic properties of the O₂ and H₂ evolution reactions (OER and HER) of two metalloporphyrins: Zn(II) 5,10,15,20-tetrakis(4-pyridyl)-porphyrin and Co(II) 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin. The TEM/STEM characterisation of the porphyrin samples obtained using different organic solvents revealed several types of self-assembled aggregates. The HER and OER experiments performed on porphyrin-modified graphite electrodes in media with different pH values revealed the most electrocatalytically active specimens. For the OER, this specimen was the electrode manufactured with one layer of Co-porphyrin applied from dimethylsulfoxide, exhibiting an overpotential of 0.51 V at i = 10 mA/cm² and a Tafel slope of 0.27 V/dec. For the HER, it was the sample obtained by drop casting one layer of Zn-porphyrin from N,N-dimethylformamide that displayed a HER overpotential of 0.52 V at i = -10 mA/cm² and a Tafel slope of 0.15 V/dec.

Recent Advances in Porphyrin-Based Systems for Electrochemical Oxygen Evolution Reaction

Oxygen evolution reaction (OER) plays a pivotal role in the development of renewable energy methods, such as water-splitting devices and the use of Zn–air batteries. First-row transition metal complexes are promising catalyst candidates due to their excellent electrocatalytic performance, rich abundance, and cheap price. Metalloporphyrins are a class of representative high-efficiency complex catalysts owing to their structural and functional characteristics. However, OER based on porphyrin systems previously have been paid little attention in comparison to the well-described oxygen reduction reaction (ORR), hydrogen evolution reaction, and CO2 reduction reaction. Recently, porphyrin-based systems, including both small molecules and porous polymers for electrochemical OER, are emerging. Accordingly, this review summarizes the recent advances of porphyrin-based systems for electrochemical OER. Firstly, the electrochemical OER for water oxidation is discussed, which shows various methodologies to achieve catalysis from homogeneous to heterogeneous processes. Subsequently, the porphyrin-based catalytic systems for bifunctional oxygen electrocatalysis including both OER and ORR are demonstrated. Finally, the future development of porphyrin-based catalytic systems for electrochemical OER is briefly prospected.

One-dimensional cobalt oxide nanotubes with rich defect for oxygen evolution reaction

For the electrochemcial hydrogen production, the oxygen evolution reaction (OER) is a pivotal half-reaction in water splitting. However, OER suffers sluggish kinetics and high overpotential, leading to the increase of overall energy consumption and decrease of the energy efficiency. In this work, high-quality cobalt oxide porous nanotubes (Co₃O₄-PNTs) are easily obtained by simple self-template approach. One-dimensional (1D) porous structure provides the large specific surface area, enough abundant active atoms and effective mass transfer. In addition, Co₃O₄-PNTs also own self-stability of 1D architecture, benefitting the their durability for electrocatalytic reaction. Thus, Co₃O₄-PNTs with optimal annealing temperature and time reveal the attractive alkaline OER performance (Tafel slope of 56 mV dec^-1and 323 mV overpotential at 10 mA cm^-2), which outperform the Co₃O₄nanoparticles and benchmark commercial RuO₂nanoparticles. Furthermore, Co₃O₄-PNTs also exhibit excellent OER durability for least 10 h at the 10 mA cm^-2. Overall, Co₃O₄-PNTs with low cost can be serve as a highly reactive and economical catalyst for OER.