Electrocatalysts Formed by Pyrolysis: Temperature-Dependent Progression of Active Sites in Co-Phthalocyanine/Carbon-Derived Electrocatalysts for Hydrogen Evolution Reaction and Oxygen Reduction Reaction

Herein, the governing role of pyrolysis in the development of active sites in cobalt phthalocyanine (CoPc)-functionalized carbon for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) has been investigated. CoPc was mixed with two different carbon substrates, i.e, Vulcan XC72R and Ketjenblack EC-600JD (KJB), and then subjected to pyrolysis. The influence of pyrolysis temperature (25–800 °C) on the progression of morphological features, textural properties, and surface chemistries in both sets of samples was thoroughly analyzed using state-of-the-art in situ and ex situ characterization techniques. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray absorption spectroscopy (XAS) confirmed the preservation of original CoPc-based species dispersed in carbonaceous frameworks until 600 °C, and afterward, the evolution of CoO and metallic Co nanoparticles was observed. X-ray photoelectron spectroscopy (XPS) indicated a transformation in the superficial active moieties over the course of pyrolysis. Using a rotating disk electrode (RDE) and a rotating ring disk electrode (RRDE), the electrocatalytic performances of the derived electrocatalysts were analyzed toward HER and ORR, respectively, in alkaline media. The samples pyrolyzed at 600–700 °C showed peak electrochemical performances. Overall, KJB-based samples demonstrated slightly superior ORR activities, whereas Vulcan-based samples showed enhanced HER kinetics. The influence of the pyrolysis temperature and the nature of the carbon support on the evolution and transformation of active structures have been studied by developing a relationship among processing parameters, evolved active moieties, and electrochemical activities. Such insights can help in the optimization of structural attributes and performance of the electrocatalysts for diverse electrocatalytic applications.