Ir Nanoconfined and Doped MnO₂ Nanosheets for an Enhanced Electrocatalytic Oxygen Evolution Reaction in Acidic and Basic Medium

Abstract

One of the promising strategies to generate hydrogen for a future hydrogen economy is via electrocatalytic water splitting, where renewable electricity would be used to convert water into hydrogen and oxygen on an electrocatalyst within an electrolyzer. Oxygen evolution reaction (OER), the anodic half-reaction of water splitting is, however, kinetically sluggish and requires four electron-proton transfers making the overall reaction energy-intensive. For the economical production of hydrogen, improving the reaction kinetics in both low and high pH is of utmost necessity by designing an active and stable OER catalyst. Precious metals like Ir, Ru, and their oxides are known to be excellent OER catalysts but their large-scale application is hindered due to their low earth-abundancy. To address this issue, we have synthesized two types of OER catalysts with varying Ir wt.% to achieve a higher OER-activity per mass of Ir in the catalyst: Ir/MnO2, Ir nano-confined in the interlayer of layered MnO2 nanosheets and Ir-doped MnO2 nanosheets. In both acidic and basic environments, Ir/MnO2 exhibited better OER activity than Ir-doped MnO2. In 0.5 M H2SO4, 16.7 wt.% Ir/MnO2 and 17.5 wt.% Ir-doped MnO2 exhibited OER overpotentials (η) of 298 mV and 320 mV, respectively, at a current density of 10 mA cm-2. In 1M KOH electrolyte, 15.4 wt.% Ir/MnO2 and 17.5 wt.% of Ir-doped MnO2 exhibited OER η values of 213 mV and 245mV, respectively at 10 mA cm-2. These overpotentials are ~500 mV lower than the pristine MnO2 nanosheets and ~60-120 mV lower than commercial reference catalysts (20 wt.% Ir/C and IrO2) at both low and high pH conditions, respectively. These synthesized catalysts were able to outperform the commercial materials by exhibiting four times higher OER-activity per mass of Ir, in both acidic and basic media. Hence, this synthetic approach may be a method to achieve enhanced OER activity using a lower mass of Ir. The physical characterization of the materials was carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS).

Thermal‐Driven Orderly Assembly of Ir‐atomic Chains on α‐MnO2 with Enhanced Performance for Acidic Oxygen Evolution

“…The chemical valence states of Mn, Ir, Ce, and O on the surface of the synthesized α‐MnO 2 , α‐CeMnO 2 , IrMnO@Ir, and IrCeMnO@Ir NRs electrocatalysts were characterized using X‐ray photoelectron spectroscopy (XPS). The Mn 2p XPS spectrum of the prepared α‐MnO 2 catalyst (Figure 7a) showed two peaks at binding energies of about 642.1 and 653.9 eV, attributed to the Mn 2p 3/2 and Mn 2p 1/2 double peaks, respectively, with a spin‐energy spacing of 11.8 eV (similar to that of earlier reports) [18,27] . Meanwhile, as can be seen from Figure 7a, an additional peak at about 644 eV is observed in the XPS spectrum of Mn 2p, which can be regarded as an excited state or satellite peak corresponding to Mn 4+ .…”

“…As discussed in the XRD results, this lattice expansion can be attributed to the ionic radius difference between the doped solute ion (Ir 3+ 0.62 Å) and the parent element (Mn 4+ 0.54 Å) [26] . High‐angle annular dark‐field scanning transmission electron microscope (HAADF‐STEM) was used to identify the differences in their atomic distributions, as shown in Figure 6b, the atomic structure containing the element Ir is significantly brighter than the neighboring columns consisting of the elements Mn and Ce, with the Z‐contrast property of HAADF image (where Z denotes the atomic number), whereas the lighter O elements cannot be observed [8c,16,27] . According to these STEM descriptions, some irregularly spaced bright dots are Ir atomic chains and some smaller randomly distributed bright spots are iridium clusters deposited on the catalyst surface.…”