Oxygen Evolution Reaction (OER) of Precious-Metal-Free Perovskite Nanocatalysts Made by Spray-Flame Synthesis—Effect of A- and B-Site Substitutions and B-Site Doping

The sustainable large-scale generation of hydrogen – <i>via</i> electrochemical water splitting – demands a large amount – and thus necessarily precious metal-free – active catalyst materials which allow the evolution of H₂ (HER) and O₂ (OER) at high rates while requiring low overpotentials.¹ Especially for the sluggish OER, cost-effective and abundant materials are sought as alternatives to RuO₂ and IrO₂ compounds which are scarce and poorly stable in alkaline electrolytes.² Perovskite-based materials (ABO₃) incorporating lanthanides (A-site) and 3<i>d</i> transition metal ions (B-site) have been identified as such promising alternatives as their electronic structure and surface defects can be tuned by substituting/doping the A-and B-sites. The synthesis of such compounds is commonly done in time-consuming wet-chemical batch processes followed by a calcination step.^{3, 4} Alternatively, the scalable spray-flame synthesis (SFS) allows the formation of functional high-surface-area doped/substituted perovskite nanoparticles in a single step.^{5, 6}<br/>Using the SFS method, we investigated the A-site (La_1-<i>x</i>Sr<i>_x</i>CoO₃, <i>x</i> = 0 – 0.4) and B-site (LaCo_1-<i>y</i>Fe<i>_y</i>O₃, <i>y</i> = 0 – 1) substitution, and the B-site doping (LaCo_0.94M_0.06O₃ (M = Cr, Mn, Fe, Ni, Cu) of the LaCoO₃ perovskite. XRD measurements indicated the formation of oxygen-deficient (e.g. LaCoO_3-_d) trigonal R-3cR structures in the La_1-<i>x</i>Sr<i>_x</i>CoO₃ series, a transition from the rhombohedrally- to the orthorhombically-distorted perovskite structure at <i>y</i> &gt; 0.4 in the LaCo_1-<i>y</i>Fe<i>_y</i>O₃ series, and the same rhombohedral LaCoO₃ structure in all samples from the LaCo_0.94M_0.06O₃ series. Investigations using XPS measurements revealed a surface enrichment of La³⁺ and Sr²⁺ ions and the presence of Sr-oxides in the La_1-<i>x</i>Sr<i>_x</i>CoO₃ series, an enrichment of La³⁺ at <i>y</i> &gt; 0.4 and the presence of iron in the oxidation state +3 regardless of the concentration (<i>y</i>) in the LaCo_1-<i>y</i>Fe<i>_y</i>O₃ series, and variable Co²⁺/Co³⁺ ratios (e.g. <b>~</b>0.57 and <b>~</b>0.75 for the Cr- and Ni-doped samples, respectively) depending on the doping element in the LaCo_0.94M_0.06O₃ series. The nanoparticle products were further characterized using TEM/EDX, FTIR, and BET measurements. Selected samples were characterized <i>via</i> EXAFS, XANES, and XPS using synchrotron radiation.<br/>For the oxygen-evolution reaction (OER) catalytic performance, the lowest overpotentials – 325, 345, and 397 mV vs. RHE at 10 mA cm^-2 – were obtained using the La_0.7Sr_0.3CoO₃, LaCo_0.6Fe_0.4O₃, and LaCo_0.94Ni_0.06O₃samples, respectively. These results point to the relevance of increasing the B–O covalency⁷ and the electrical conductivity⁸ of the perovskite structures and are comparable to the performance of commercial IrO₂ reference⁹ materials (1.59 V vs. RHE at 10 mA cm^-2). Relationships between chemical composition, surface properties, and OER activity of the spray-flame synthesized catalysts will be discussed in detail.<br/><b>References</b><br/>1. <i>Electrochemical Science Advances </i><b>2022</b>, 00 e2100206.<br/>2. <i>Journal of Materials Chemistry A </i><b>2022,</b> 10, (5), 2271-2279.<br/>3. <i>Electrochimica Acta </i><b>2017,</b> 241, 433-439.<br/>4. <i>International Journal of Hydrogen Energy </i><b>2018,</b> 43, (9), 4682-4690.<br/>5. <i>Proceedings of the Combustion Institute </i><b>2019,</b> 37, (1), 83-108.<br/>6. <i>AIChE Journal </i><b>2019,</b> 66, (1), e16748.<br/>7. <i>ACS Applied Materials & Interfaces </i><b>2022,</b> 14, (12), 14129-14136.<br/>8. <i>Science and Technology of Advanced Materials </i><b>2015,</b> 16, (2), 026001.<br/>9. <i>ChemCatChem </i><b>2017,</b> 9, (15), 2988-2995.