Pulsed Light Synthesis of High Entropy Nanocatalysts with Enhanced Catalytic Activity and Prolonged Stability for Oxygen Evolution Reaction

Abstract The ability to synthesize compositionally complex nanostructures rapidly is a key to high‐throughput functional materials discovery. In addition to being time‐consuming, a majority of conventional materials synthesis processes closely follow thermodynamics equilibria, which limit the discovery of new classes of metastable phases such as high entropy oxides (HEO). Herein, a photonic flash synthesis of HEO nanoparticles at timescales of milliseconds is demonstrated. By leveraging the abrupt heating and cooling cycles induced by a high‐power‐density xenon pulsed light, mixed transition metal salt precursors undergo rapid chemical transformations. Hence, nanoparticles form within milliseconds with a strong affinity to bind to the carbon substrate. Oxygen evolution reaction (OER) activity measurements of the synthesized nanoparticles demonstrate two orders of magnitude prolonged stability at high current densities, without noticeable decay in performance, compared to commercial IrO2 catalyst. This superior catalytic activity originates from the synergistic effect of different alloying elements mixed at a high entropic state. It is found that Cr addition influences surface activity the most by promoting higher oxidation states, favoring optimal interaction with OER intermediates. The proposed high‐throughput method opens new pathways toward developing next‐generation functional materials for various electronics, sensing, and environmental applications, in addition to renewable energy conversion.


Equipment
Pulsed light processing of nanoparticles uses Xenon 2100S flash lamp machine. This machine can generate high-power-density pulses (see schematic Fig. 1) of light covering the whole visible light spectrum. The xenon flash lamp works at very high voltages with a minimum of 1.9 kV and a maximum of 3.1 kV. The pulses of light can range from 100 microseconds to 3 milliseconds. The delay in Fig.1a is a function of the pulse voltage, pulse duration, and pulse number. The pulses occur in a closed box having control over the environment of reduction occurs. The sealed box contains a platform to hold the substrate distanced 2.5 cm from the lamp. The Xenon lamp produces a 3 inch x 3 inch of light footprint over the platform.
Scanning electron microscopy was performed in Notre Dame using a Helios G4 Ux Dual Beam Microscope. XPS was performed on Thermo K-Alpha+. STEM imaging was performed on Titan Themis Z G3 Cs-Corrected S/TEM. The size measurement is performed using ImageJ software where brighter nanoparticles could be automatically separated from the darker background. The statistical analysis of the nanoparticle size is conducted with Origin software.

Method
In a typical process, separate 10 mM solutions of single metal salt in DI water are produced in glass vials with 15 minutes of stirring and 15 minutes of sonication. The solutions are then mixed in equal ratios to formulate a mixture of ternary, quaternary, and quinary precursor solution and sonicated again for 15 more minutes for uniform mixing. The prepared mixture is drop cast (200 μL) on a washed and oxygen plasma-treated substrate. A hot plateassisted in-situ heating system is used to vaporize the water solvent and form a deposition of the precursor material on the substrate surface. Nafion is added on top of the precursor as binder to avoid detaching of the precursor materials from the substrate. The substrate is then transferred to the Xenon flash lamp machine for nanoparticle processing. Several light pulses generated by 3kV voltage with a 3ms duration were used for the high entropy alloy nanoparticle processing. The delay between two adjacent pulses was kept at 858ms. The whole procedure occurred in a closed conduit in less than 6 seconds of total time with a flowing nitrogen (N 2 ) atmosphere. The N 2 is used to avoid extensive oxidation of the nanoparticles at high temperature.

Flash Synthesis Parameter Selection
Flash synthesis parameters for all the samples were selected based on a separate study using a mixed chloride precursor of iron, nickel, and cobalt at equal ratio. The drop cast sample is irradiated with different pulse conditions from the Xenon flash lamp. The precursor's incomplete decomposition could be observed in the low-energy pulses ( Figure S1 a-b). Uniform dispersion with sizes smaller than 100 nm could be found using voltage 3 kV, duration 3 ms, and 7 pulses with a delay between pulses of 858ms ( Figure S1c). The theoretical maximum energy deliverable at this state by the total seven pulses is estimated as 0.32 kJ/cm 2 Increasing pulse number beyond this parameter causes the nanoparticles to be welded together and become more prominent in size.

Electrochemical testing
All experiments were conducted in 1M KOH electrolyte. N 2 bubbling was performed for 45-60 min prior to the experiment to deaerate the solution from any oxygenated species. Hg/HgO was used as the reference electrode, enclosed in a plastic housing, to provide adequate robustness against strong alkaline environments. Catalyst activation was done by performing Cyclic Voltammetry (CV) at 20 and 50 mV/s scan rate. Linear sweep voltammetry (LSV) was conducted at 5 mV/s scan rate. Stability analysis was performed by applying constant current (i.e., Chronopotentiometry (CP)). CP was performed at different current densities (10, 50 and 200 mA/cm2), successively.