Enhancing the electrocatalytic oxygen reduction reaction via pyridine in cobalt porphyrins

Abstract

The expensive and scarce precious metal catalysts required for the cathodic oxygen reduction reaction (ORR) in H₂-O₂ proton exchange membrane fuel cells severely limit their large-scale application. Inspired from nature, design and developing metal porphyrins is a very attractive solution. However, the activity and selectivity of metal porphyrins still need to be improved. During the ORR process of metal porphyrins, the breaking of OO bond to form M-oxo species, is crucial. Hence, the ease of forming M-oxo species determines the selectivity of ORR. Early transition metals, such as Mn and Fe, readily form M-oxo species, favoring the 4e⁻ pathway. In contrast, late transition metals have difficulty forming M-oxo species, making the catalytic cleavage of OO bond less likely and thus favoring the 2e⁻ pathway. Given that Co-oxo species possess suitable stability and reactivity, mononuclear Co centers can theoretically catalyze both 2e⁻ and 4e⁻ ORR, with the selectivity largely dependent on the electronic structure of Co and its surrounding environment. In this work, we designed Co porphyrin 2 with a pyridyl group, Co porphyrin 1 with a chlorine atom connected to the single adjacent nitrogen atom of pyridyl group, and Co porphyrin 3 without a pyridyl group to study the influence of pyridyl group on electrocatalytic ORR under acidic conditions. Experimental results demonstrate that pyridyl significantly enhances ORR activity and selectivity. Upon the addition of trifluoroacetic acid at the same concentration in homogeneous catalysis, the catalytic current of 2 was 1.8 times of Co porphyrin 1 and 4.5 times of Co porphyrin 3, with an earlier onset potential and higher turnover frequency (TOF) value. When loaded onto multiwalled carbon nanotubes (CNTs) under acidic conditions, the half-wave potentials (E_1/2) of 1@CNT, 2@CNT, and 3@CNT were 0.53 V, 0.57 V, and 0.46 V (versus reversible hydrogen electrode, vs RHE), respectively, with electron transfer numbers of 3.37, 3.80, and 3.16. This work reveals the critical importance of pyridyl group in improving the electrocatalytic ORR performance.