Carbonaceous Hydrogen‐Evolution Catalyst Containing Cobalt Surrounded by a Tuned Local Structure

Abstract

The low activity and durability of noble-metal-free catalysts in acidic media are major problems that hinder their practical use. This study reports significantly enhanced activity of a cobalt-containing carbonaceous catalyst used for the hydrogen-evolution reaction in a water electrolyzer by using a proton-exchange membrane (PEM), which is an efficient energy-conversion device that is suitable for intermittent renewable energy sources. On the basis of X-ray absorption near-edge structure, the enhancement in activity was attributed to a change in the local structure surrounding the Co atom. The PEM water electrolyzer formed by using the catalyst with an optimized structure and Co surface concentration showed an enthalpy efficiency of 80 % at 1 A cm À2 .Water electrolysis is a process that converts electric energy into the chemical energy of H 2 and O 2 , and has recently attracted much attention as an energy-storage system accompanying renewable energy sources, such as photovoltaic cells and wind turbines. A suitable water electrolyzer for these intermittent and fluctuating power sources is one that uses a proton-exchange membrane (PEM) as the electrolyte owing to its quick response to changes in power input, its wide operating range, and its high efficiency. [1] However, the high cost and scarcity of noble-metal-based catalysts used in these electrodes is the main drawback of the PEM water electrolyzer for its widespread use.Various noble-metal-free catalysts for the hydrogen-evolution reaction (HER) have been reported, such as complexes of transition metals with organic ligands, molybdenum sulfides and oxides, tungsten oxides, tungstic acid salts, heteropolyanions, and nickel-based alloys, although there are very limited reports on the catalytic behavior evaluated in the PEM elec-trolyzer. [24][25][26] Quite recently, we developed a Co-containing carbonaceous nanoparticle aggregate by sublimation and pyrolysis of Co phthalocyanine (CoPc), which is a new type of HER catalyst functioning in the cathode of the PEM electrolyzer. [27] Highly stable catalytic activity was observed in a 100 h intermittent operation. However, the activity was much lower than that produced by conventional Pt-based catalysts, leading to a high overpotential and low efficiency of the PEM electrolyzer.The results of a previous study suggested that the active site in the carbonaceous catalyst was the Co atom coordinated by four N atoms and embedded in the carbon matrix (CoÀN 4 moiety), which was derived from the center of CoPc. In this study, we found that control of the local structure surrounding the Co atom was possible through double heat treatment, and the catalytic activity was enhanced on the basis of this structure change.The carbonaceous nanoparticle aggregate was formed by the first heat treatment of m:1 mixtures (m = 0.25, 0.5, 1, 2) of CoPc/Ketjenblack (KB) at 700 8C after increasing the temperature at a rate of 1 8C min À1

Synthesis of ordered carbonaceous frameworks from organic crystals

“…Ni 2 -CPD Py comprises two Ni-porphyrins linked by two butadiyne (diacetylene) moieties through phenyl groups; each porphyrin includes two meso -pyridyl groups. The M–N 4 (M = metal) unit in the porphyrin ring is thermally stable (~973 K) 23 , 24 . Diacetylene is thermally polymerized to poly(diacetylene) to form a rigid crosslinked network, allowing the precursor morphology to be maintained during carbonization.…”

Efficient Edge Plane Exposure on Graphitic Carbon Fiber for Enhanced Flow-Battery Reactions

“…The ratio of the CoPc and the Co–N 4 model components was 7:3. The Co foil component was attributed to Co metal aggregate nanoparticles formed by the decomposition of CoPc …”

A carbonaceous thin film containing N-coordinated Fe and Co with catalytic activity for oxygen reduction

“…1) showed cracks, which were caused by the shrinkage of the film during the heat treatment 27) . Wider cracks were observed in the CCoPc with lin- that the carbonaceous thin films formed from FePc and CoPc by the HPCVD method contained the metal-N 4 structure and the aggregated metal particles surrounded by carbon shells 27)- 30) . The spectra situated between the carbonaceous thin film formed from either of the metal and the metal foil, indicating that the extent of the metal aggregation was higher for CFeCo (3 : 1).…”