Deep Eutectic Solvent Synthesis of Perovskite Electrocatalysts for Water Oxidation

Oxide perovskites have attracted great interest as materials for energy conversion due to their stability and structural tunability. La-based perovskites of 3d-transition metals have demonstrated excellent activities as electrocatalysts in water oxidation. Herein, we report the synthesis route to La-based perovskites using an environmentally friendly deep eutectic solvent (DES) consisting of choline chloride and malonic acid. The DES route affords phase-pure crystalline materials on a gram scale and results in perovskites with high electrocatalytic activity for oxygen evolution reaction. A convenient, fast, and scalable synthesis proceeds via assisted metathesis at a lower temperature as compared to traditional solid-state methods. Among LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 perovskites prepared via the DES route, LaCoO3 was established to be the best-performing electrocatalyst for water oxidation in alkaline medium at 0.25 mg cm–2 mass loading. LaCoO3 exhibits current densities of 10, 50, and 100 mA cm–2 at respective overpotentials of approximately 390, 430, and 470 mV, respectively, and features a Tafel slope of 55.8 mV dec–1. The high activity of LaCoO3 as compared to the other prepared perovskites is attributed to the high concentration of oxygen vacancies in the LaCoO3 lattice, as observed by high-resolution transmission electron microscopy. An intrinsically high concentration of O vacancies in the LaCoO3 synthesized via the DES route is ascribed to the reducing atmosphere attained upon thermal decomposition of the DES components. These findings will contribute to the preparation of highly active perovskites for various energy applications.


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homogenized in a bath sonicator USC-TH (VWR) for 30 min, and then using an ultrasonic probe Vibra-cell 75185 (Thermo Fisher Scientific) for 1 min.
Electrocatalytic anodes containing perovskite materials and the reference IrO2 electrocatalyst (99.99%, Alfa Aesar) were prepared by loading the ink on a Ni foam supporting material (Heze Jiaotong, 110 pores per inch, 0.3 mm thick). Prior ink deposition, Ni foam was cleaned by sequential 30-min ultrasonication in acetone, ethanol, and Milli-Q water. The ink was loaded in 20 μL batches on the surface of the Ni foam current collector, while letting ethanol to evaporate between the batches. The exposed geometrical surface area of the anode was fixed to be a 1 cm 2 and the total mass of electrocatalyst loaded on the anode was varied from 0.25 to 3 mg cm -2 . Finally, the obtained anode was air-dried and subjected to electrochemical testing.

Electrochemical Measurements
Electrochemical studies were conducted at room temperature using Autolab PGSTAT302N potentiostat/galvanostat (Methrohm), equipped with a FRA32M frequency response analyzer. The performance of the electrocatalysts in oxygen evolution reaction (OER) was evaluated under moderate Ar bubbling (1 bubble s -1 ) while stirring at 150 rpm in a three-electrode system filled with purified 1 M NaOH aqueous electrolyte solution. The electrocatalytic anode, a calibrated saturated calomel electrode (SCE), and a Pt wire served as the working, reference, and counter electrodes, respectively. All potentials reported in the work were converted to a reversible hydrogen electrode (RHE) reference scale according to the following equation: ERHE = ESCE + 0.059pH + 0.241. An iRcorrection of 85% was applied in the polarization experiments to compensate for the voltage drop between the reference and working electrodes, which was estimated by a single-point highfrequency impedance measurement.
OER anodic polarization curves were recorded using cyclic voltammetry (CV) with a scan rate of 5 mV s -1 . In the case of electrocatalyst activation, the scan rate was augmented to 50 mV s -1 .
Electrochemical impedance spectroscopy (EIS) measurements were carried out at the overpotential (10) that provide the current density of 10 mA cm -2 in the frequency range from 105 to 0.01 Hz with a 10 mV sinusoidal perturbation. The EIS measurements and the interpretation of results were conducted in accordance with aqueous electrochemical assembly, the so-called supported system.
The electrocatalytic stability of the anodes was evaluated as a function of time by means of chronopotentiometry at constant current density of 10 mA cm -2 .
The relative electrochemically active surface area (ECSA) of the solid-electrolyte interface of the anodes was assessed on the basis of geometric double-layer capacitance, Cdl, measurements. CV S4 cycles were recorded from 1.17 VRHE to 1.27 VRHE for all perovskite electrodes under the standard conditions of alkaline OER experiment using increasing scan rates of 5, 10, 25, 50 and 100 mV s −1 .
A linear trend is obtained via plotting half the difference in current density, j, between the anodic and cathodic sweeps, ½ (janodic − jcathodic), in the non-Faradaic region of the cyclic voltammograms (ca. 1.2 V) as a function of the scan rate. The slope of the linear fitting of these data is the geometric Cdl (mF cm −2 ), which is linearly proportional to the ECSA for a given surface.   a Optimal mass loading of perovskites was estimated experimentally measuring alkaline OER activity as a function of mass loading of the electrocatalyst ( Figure S5). b Estimated experimentally using CV cycling with scan rate of 50 mV s -1 in potential range of 1.06-1.80 V ( Figure S6). c Estimated by Tafel fit (Figure 2b) of anodic polarization curves (Figure 2a) of the respective electrocatalysts. d Estimated by fitting the Nyquist plots (Figure 2c), measured at 10, to the equivalent circuit model (Figure 2c, inset). e Estimated by linear fitting of the data representing anodic and cathodic difference in current density as a function of cyclic voltammetry scan rate ( Figure S7).  Figure S1. Rietveld refinement plots of powder X-ray diffraction patterns of DES-derived perovskites collected at room temperature; experimental powder patterns are in blue, calculated patterns are in green, differences are in black.