Non‐Fermi Liquids as Highly Active Oxygen Evolution Reaction Catalysts

Abstract The oxygen evolution reaction (OER) plays a key role in emerging energy conversion technologies such as rechargeable metal‐air batteries, and direct solar water splitting. Herein, a remarkably low overpotential of ≈150 mV at 10 mA cm−2 disk in alkaline solutions using one of the non‐Fermi liquids, Hg2Ru2O7, is reported. Hg2Ru2O7 displays a rapid increase in current density and excellent durability as an OER catalyst. This outstanding catalytic performance is realized through the coexistence of localized d‐bands with the metallic state that is unique to non‐Fermi liquids. The findings indicate that non‐Fermi liquids could greatly improve the design of highly active OER catalysts.

initial 5 wt.% proton-type Nafion ® suspension from 1-2 to 11. The catalyst inks were prepared from a mixture of the as-prepared samples (25 mg), acetylene black (AB, Strem Chemicals Inc., 5 mg), and 3.33 wt.% K + ion-exchanged Nafion ® suspension (0.15 mL). The total ink volumes were adjusted to 5 mL by the addition of tetrahydrofuran (THF, Sigma-Aldrich), to give final concentrations of 5 mg sample/mL, 1mg AB/mL, and 1 mg Nafion/mL in the ink. A sample of the ink (3.6 μL) was then drop-casted on a rotating-disk electrode composed of a glassy carbon (GC) disk (0.15 × 0.15 × cm 2 ) (BAS Inc., Japan), which was used as the working electrode after mirror polishing with 0.05 μg alumina slurry (BAS Inc., Japan). The catalyst layer on the GC disk was dried overnight in vacuum at room temperature, and was composed of 0.25 mg oxide /cm 2 disk , 0.05 mg AB /cm 2 disk , and ~0.05 mg Nafion /cm 2 disk .

Electrochemical measurements
Electrochemical measurements were conducted using a rotating ring disk electrode rotator (RRDE-3A, BAS Inc., Japan) at 1600 rpm, in combination with a bipotentiostat (BAS Inc., Japan). In addition, a Pt wire counter electrode, and an  Figure 14), and capacitance-corrected by averaging the anodic and cathodic scans [5] to remove the non-faradaic current contribution. The current densities (mA/cm 2 disk ) were obtained by dividing the OER current with the glassy carbon disk electrode area.

Chemical analysis
X-ray photoemission spectra (XPS) were obtained by a XPS-7000 spectrometer (Rigaku Co., Japan) using Al-K radiation at room temperature. XPS data were collected with an energy step of 0.1 eV. Hg 2 Ru 2 O 7 and Ca 2 Ru 2 O 7 samples (as-synthesized, as-cast, and after 100 or 50 cycles) were mixed with a Au powder, which was also used as a Au 4f 7/2 (84.0 eV) standard. XPS data of a RuO 2 powder was collected as a reference for the Ru 3d 5/2 peak. The Ru 3d 5/2 peak positions of Hg 2 Ru 2 O 7 (281.5-281.7 eV) are 0.6-0.8 eV above that of RuO 2 (280.9 eV) for all the samples. This confirms that the Ru of Hg 2 Ru 2 O 7 sustains its Ru 5+ valence during the OER measurement. The compositional information was obtained by the integrated Hg 4f and Ru 3p 3/2 peak areas. Hg 2 Ru 2 O 7 reported by Yamamoto et al. [20] . Therefore, our XPS data confirms that the Ru of the catalysts sustains its Ru 5+ valence during the OER measurement. The Ru 3d peaks were not used for obtaining the compositional information due to the severe overlap of Ru 3d 3/2 peaks with C 1s peaks. The severe overlap of Ru 3d 3/2 peaks with C 1s peaks (the intensity of which greatly depends on the amount of acetylene black attached to the surface of the catalyst) causes the significant change in the intensity ratio between the Ru 3d 3/2 and 3d 5/2 peaks for the as synthesized catalyst, the cast catalyst and the catalyst after 50/100 cycles (already at the stage of as synthesized, a small amount of carbon species from the air are attached to the surface).   shows the overall view of the impedance spectra between the frequency of 40 kHz and 100 mHz. Impedance spectra were measured using a potentiostat/galvanostat with frequency response analyzer (Bio-Logic SAS, SP-150).

Supplementary
Supplementary Figure 15. The OER mechanism of cubic pyrochlore ruthenates in this study based on the electron transfer between Ru ion and the OHadsorbate. The rate-determining step is either the formation of O-O bond (reaction 2) or the deprotonation (reaction 3) along with the redox reaction of Ru ions in analogy with the perovskite oxides in Suntivich et al. [5] and Yagi et al. [6] , and the high OER activity of Hg 2 Ru 2 O 7 (especially the low overpotential) can be explained by the smooth electron transfer between Ru ion and the OHadsorbate.   [S2] , f i = 1-exp(-  /4), where f i and  denote bond ionicity and Pauling's electronegativity difference between the cation-anion pair, respectively.

Supplementary
This  Figure 9(a)). The Ca/Ru ratio was obtained by the integrated Ca 2p 3/2 and Ru 3p 3/2 peak areas in comparison with the as-synthesized Ca 2 Ru 2 O 7 (Supplementary Figure 9( [S3] . Hg 2 Ru 2 O 7 (non-Fermi liquid) exhibits a comparable overpotential with the highly active chalcogenide OER catalysts.

Material
Overpotential  (V)   S4] , SrRuO 3 [S5] , and RuO 2 -NiO [S6] were taken from the previous literature. The turnover frequency (TOF) was calculated by TOF = j/4Fn, where j is the OER current at 1.5 V vs. RHE, F is the Faraday constant, and n is the number of moles of the Ru atom on the working electrode. Hg 2 Ru 2 O 7 (non-Fermi liquid) exhibits the highest OER activity in Ru-based catalysts.