Researchers at S. Korea’s DGIST (Daegu Gyeongbuk Institute of Science and Technology), with colleagues at Pacific Northwest National Laboratory (PNNL), have developed a low-cost, highly efficient and ultra-durable core-shell nanostructured electrocatalyst that exhibits an improved oxygen evolution activity and stability compared to that of the commercial noble metal electrodes.
They developed a Prussian blue analogue-derived nitrogen-doped nanocarbon (NC) layer-trapped, cobalt-rich, core–shell nanostructured electrocatalysts (core–shell Co@NC). The core–shell Co@NC-loaded nickel foam exhibits a lower overpotential of 330 mV than that of IrO2 on nickel foam at 10 mA cm−2 and has a durability of more than 400 h. A commercial Pt/C cathode-assisted, core–shell Co@NC–anode water electrolyzer delivers 10 mA cm−2 at a cell voltage of 1.59 V—70 mV lower than that of the IrO2–anode water electrolyzer.
Long-term chronopotentiometry durability testing showed that while the IrO2–anode water electrolyzer shows a cell voltage loss of 230 mV (14%) at 95 h, the loss of the core–shell Co@NC–anode electrolyzer is only 60 mV (4%) even after 350 h cell-operation. A paper on the work is published in the journal Advanced Energy Materials.
In electrocatalytic water splitting, oxygen gas generates in the anode due to the oxygen evolution reaction (OER). OER is a slow electrochemical reaction as compared with the hydrogen evolution reaction (HER); thus, a suitable electrocatalyst is needed. Ruthenium and iridium oxides are considered as state-of-the-art electrocatalysts in OER, but the lack of stability limits their use in large-scale water splitting and hinders widespread commercialization.
The DGIST and PNNL team focused on developing an alternative, low-cost, non-noble metal electrocatalyst to replace the noble metal anode electrode in efficient water splitting. Carbon-supported metal has been considered as an efficient electrocatalytic material for the enhanced OER in water splitting.
So far, most of the developed electrocatalysts have featured higher carbon content and less metal active specious content. The higher carbon amount mired the real metal active sites and thus resulted in a faster carbon corrosion conditions. This further led to lower electrocatalytic activity, stability and large-scale water splitting processes (scalability).
In the study, the researchers found that a large amount of inorganic cobalt metal ions bridged by organic ligands in the Prussian blue analog were a suitable precursor for developing efficient and ultra-stable, metal-rich, nitrogen-doped graphitic nanocarbon-encapsulated core-shell electrocatalysts for the sluggish OER (anode) in water splitting.
When heated (600-900°C) in an inert atmosphere, the cobalt metal ions and organic ligands in the salt are transformed into cobalt metal and nitrogen-doped graphitic thin carbon layers, respectively, which form the thin carbon layer, encapsulated metallic, cobalt core-shell nanostructures (Core-Shell Co@NC).
The thin carbon layers have a strong interaction with cobalt metal, which can promote less carbon corrosion, excellent electron movement, and more cobalt metal exposure to the reaction medium, including the formation of nanosized morphology without particle aggregation.
The combined effect of carbon and cobalt metal in the electrodes achieves the more efficient electrocatalytic activity of the OER than that of the precious metal electrodes to allow efficient water splitting. The researches concluded that the non-noble metal-rich electrode is an alternative, active, stable, and less expensive OER anode for cost-effective H2 gas production in commercial-scale water electrolysis.
Anticipate this to be a unique approach to developing metal-rich, reduced-carbon composite nanostructures that have enhanced metal active sites, which feature thin carbon layer protection and ultra-fast electron movement in the catalyst surface, that will enhance the electrochemical activity and stability of electrocatalysts. We will carry out the follow-up studies that can be used to understand the real OER mechanism on the active species in the presence of nanocarbon coating.—Professor Sangaraju Shanmugam, DGIST, corresponding author
Arumugam Sivanantham, Pandian Ganesan, Luis Estevez, B. Peter McGrail, Radha Kishan Motkuri, and Sangaraju Shanmugam (2018) “A Stable Graphitic, Nanocarbon-Encapsulated, Cobalt-Rich Core-Shell Electrocatalyst as an Oxygen Electrode in a Water Electrolyzer”, Advanced Energy Materials doi: 10.1002/aenm.201702838