Dynamic active-site generation via deprotonation process on iridium atom for oxygen evolution

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Electrocatalytic water splitting for hydrogen and oxygen production provides an attractive path to obtain sustainable energy via the conversion and storage of intermittent solar and wind energies. However, the bottleneck in improving water electrolysis is mainly caused by sluggish reaction kinetics at the anode, where water is oxidized and the oxygen evolution reaction occurs. Currently, precious metal oxides, iridium, and ruthenium oxides are the highly efficient oxygen evolution reaction electrocatalysts but suffer from relative scarcity and high-cost.

Before that, we focused on nanoporous metals and metal compounds, as low-cost catalysts for hydrogen evolution reaction and oxygen evolution reaction1, 2. Even if the nanoporous metal compounds have excellent oxygen evolution reaction performance, their catalytic performance remains far from satisfactory. Fortunately, the emergence of single-atom catalysts has brought dawn to this field, which could maximize the catalytic activity while significantly reducing the amount of noble metals. So we began to try to combine the single-atom catalyst with the nanoporous metal phosphide.

Inspired by the self-reconstruction of catalysts under oxygen evolution reaction conditions, we successfully synthesized the single-atom iridium catalyst via a self-reconstruction strategy. The as-synthesized catalyst exhibited an outstanding oxygen evolution reaction performance in alkaline aqueous solutions, showing an oxygen evolution reaction overpotential of 197 mV versus reversible hydrogen electrode to achieve a current density of 10 mA cm-2, an ultralow Tafel slope of 29.6 mV dec-1, a high mass activity of 39.3 A mg-1 (131 times higher than that of the commercial IrO2). Further insights into the origin of the outstanding oxygen evolution reaction catalytic activity of Ir atoms incorporation by controlled operando XAS measurement confirm that the isolated iridium sites undergo a deprotonation process to form the multiple active sites during oxygen evolution reaction, promoting the O-O coupling (see figure).

Schematic illustration of the deprotonation process determined by the operando XAS analysis. Ir, Ni, Fe, O, and H atoms are presented as blue, dark blue, purple, grey, and pink balls, respectively.

Our work suggests that the self-reconstruction on the surface of metal phosphide could stabilize isolated iridium atom, which might open up new opportunities for the preparation of stable single-atom catalyst. More importantly, with single-atom catalysts widely used in the field of electrocatalytic oxygen evolution, our operando mechanism can provide a reasonable explanation for the origin of high catalytic activity of single-atom catalysts.

If you are interested in our work, you may find the full paper here.

Reference:

  • Tan, Y. et al. 3D nanoporous metal phosphides toward high-efficiency electrochemical hydrogen production. Adv. Mater. 28, 2951-2955 (2016).

2. Tan, Y. et al. Versatile nanoporous bimetallic phosphides towards electrochemical water splitting. Energy Environ. Sci. 9, 2257-2261 (2016).

More relevant research can be found here: http://www.tanresearchgroup.com/.

Go to the profile of Yongwen Tan

Yongwen Tan

Teacher, Hunan university

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