Unraveling Thermodynamics, Stability, and Oxygen Evolution Activity of Strontium Ruthenium Perovskite Oxide

Abstract

Extensive investigations in understanding the functional mechanisms of metal oxides behind oxygen evolution have been carried out since an electrolyzer has demonstrated promising possibilities as a device to produce hydrogen for electrochemical energy conversion systems. In particular, perovskite oxides are reputable for high activity toward the oxygen evolution reaction (OER). Here, we revisited the list of active perovskite oxides constructed based on theoretical oxygen binding energies of reaction intermediates to the catalyst surface. From this list, Ru-based perovskites, i.e. SrRuO 3 and LaRuO 3 , have been predicted as active perovskites to exhibit a particularly high OER activity. We report on the stability of nanoscaled SrRuO 3 perovskite prepared by a simple and scalable flame synthesis method. Attempts to obtain LaRuO 3 were made; however, its DFT calculated phase diagram suggests that its perovskite phase is not thermodynamically stable, which supports our experimental results such that only a mixture of different La−Ru−O phases has been obtained. Nanoscaled SrRuO 3 is evaluated for its electrochemical activity with a particular emphasis pointed toward stability in both alkaline and acidic media. Through conjoining electrochemical methods, operando X-ray absorption spectroscopy (XAS), and theoretical calculations, we show that SrRuO 3 exhibits trivial activity toward OER that decreases promptly. The loss in activity is rationalized through DFT based computations, which corroboratively suggest the poor chemical stability of both selected perovskites. Regardless of the predicted theoretical OER activity, the intrinsic instability strongly suggests that Sr-and La-based ruthenium oxides are not viable catalysts for OER in aqueous media. This further suggests that their activities are independent of their binding energies between intermediates and catalyst surface but rather closely associated with material dissolution. We highlight that understanding the origin of stability under a real operating environment is absolutely essential for the design of a sustainable electrocatalyst with optimal balance between activity and stability.

Recent Progress in Electrocatalysts for Acidic Water Oxidation

“…From 1970s, the electrocatalysts in acidic environment are mainly thin films based on metal oxides, such as MoO 2 , WO 2 , ReO 2 , OsO 2 , RuO 2 , and IrO 2 . [ 36–38 ] At present, the catalytic performance and stability of catalysts in acid have made great progress, including precious metal, [ 39–42 ] oxides, [ 43,44 ] perovskite, [ 45–47 ] nonprecious metal, [ 27,48 ] and metal‐free compounds. [ 49 ]…”

In Situ X-ray Absorption Spectroscopy Studies of Nanoscale Electrocatalysts

“…Many mechanistic studies of heterogeneous catalysts are complimented by density functional theory (DFT) calculation of reaction mechanisms, active sites, and adsorbate interactions. Many of these studies have used an indirect comparison between the XAS analysis and DFT calculations (e.g., activation energies at various reaction coordinates) to confirm active sites and favorable reaction paths, such as role of different active sites in HER activity of different CoP-based catalysts [41,42], modified IrO 2 and nanoscale SrRuO 3 catalyst for OER [43,44], zeolite chemistry under reactive SCR conditions [45,46], and many more [47][48][49][50]. Coupling of DFT calculations with XAS simulations [51] and implementation of different spectroscopy modeling modules into common computational tools for materials modeling have made direct theoretical predictions and analysis of XAS spectra more accessible and provided molecular insights into catalytic reaction mechanism and active sites.…”

Recent Advances in Novel Nanostructuring Methods of Perovskite Electrocatalysts for Energy‐Related Applications

“…However, this process requires large power consumption, and more often only produces perovskite nanomaterials with irregular shape, contaminants, or amorphous components. Additionally, researchers also succeeded in fabricating perovskite nanostructures using other approaches, such as combustion synthesis, flame spray synthesis, microwave‐assisted method, and others . For example, Liu et al used a one‐pot combustion process to synthesize a layered perovskite‐metal oxide nanohybrid, NiO–(La 0.613 Ca 0.387 ) 2 NiO 3.562 .…”