Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B-O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co-O bond and the concentration of oxygen vacancies are controlled through Sr 2 þ substitution into La 1 À x Sr x CoO 3 À d . We attempt to rationalize the high activities of La 1 À x Sr x CoO 3 À d through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO 2.7 , with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.

Perovskite oxides have attracted significant attention as energy conversion materials for metal-air battery and solid-oxide fuel-cell electrodes owing to their unique physical and electronic properties. Amongst these unique properties is the structural stability of the cation array in perovskites that can accommodate mobile oxygen ions under electrical polarization. Despite oxygen ion mobility and vacancies having been shown to play an important role in catalysis, their role in charge storage has yet to be explored. Herein we investigate the mechanism of oxygen-vacancy-mediated redox pseudocapacitance for a nanostructured lanthanum-based perovskite, LaMnO3. This is the first example of anion-based intercalation pseudocapacitance as well as the first time oxygen intercalation has been exploited for fast energy storage. Whereas previous pseudocapacitor and rechargeable battery charge storage studies have focused on cation intercalation, the anion-based mechanism presented here offers a new paradigm for electrochemical energy storage.

Perovskites are of great interest as replacements for precious metals and oxides used in bifunctional air electrodes involving the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, we report the synthesis and activity of a phase-pure nanocrystal perovskite catalyst that is highly active for the OER and ORR. The OER mass activity of LaNiO3, synthesized by the calcination of a rapidly dried nanoparticle dispersion and supported on nitrogen-doped carbon, is demonstrated to be nearly 3-fold that of 6 nm IrO2 and exhibits no hysteresis during oxygen evolution. Moreover, strong OER/ORR bifunctionality is shown by the low total overpotential (1.02 V) between the reactions, on par or better than that of noble metal catalysts such as Pt (1.16 V) and Ir (0.92 V). These results are examined in the context of surface hydroxylation, and a new OER cycle is proposed that unifies theory and the unique surface properties of LaNiO3.

The ability to design and characterize uniform, bimetallic alloy nanoparticles, where the less active metal enhances the activity of the more active metal, would be of broad interest in catalysis. Herein, we demonstrate that simultaneous reduction of Ag and Pd precursors provides uniform, Ag-rich AgPd alloy nanoparticles (~5 nm) with high activities for the oxygen reduction reaction (ORR) in alkaline media. The particles are crystalline and uniformly alloyed, as shown by X-ray diffraction and probe corrected scanning transmission electron microscopy. The ORR mass activity per total metal was 60% higher for the AgPd2 alloy relative to pure Pd. The mass activities were 2.7 and 3.2 times higher for Ag9Pd (340 mA/mgmetal) and Ag4Pd (598 mA/mgmetal), respectively, than those expected for a linear combination of mass activities of Ag (60 mA/mgAg) and Pd (799 mA/mgPd) particles, based on rotating disk voltammetry. Moreover, these synergy factors reached 5-fold on a Pd mass basis. For silver-rich alloys (Ag≥4Pd), the particle surface is shown to contain single Pd atoms surrounded by Ag from cyclic voltammetry and CO stripping measurements. This morphology is favorable for the high activity through a combination of modified electronic structure, as shown by XPS, and ensemble effects, which facilitate the steps of oxygen bond breaking and desorption for the ORR. This concept of tuning the heteroatomic interactions on the surface of small nanoparticles with low concentrations of precious metals for high synergy in catalytic activity may be expected to be applicable to a wide variety of nanoalloys.

We present a series of perovskite electrocatalysts that are highly active for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in an aqueous alkaline electrolyte. Lanthanum-based perovskites containing different transition metal active sites (LaBO 3 , B = Ni, Ni 0.75 Fe 0.25 , Co, Mn) are synthesized by a general colloidal method, yielding phase pure catalysts of homogeneous morphology and surface area (8−14 m 2 /g). Each perovskite's ability to catalyze the OER and ORR is examined using thin film rotating disk electrochemistry (RDE). LaCoO 3 supported on nitrogen-doped carbon is shown to be ∼3 times more active for the OER than high-surface-area IrO 2 . Furthermore, LaCoO 3 is demonstrated to be highly bifunctional by having a lower total overpotential between the OER and ORR (ΔE = 1.00 V) than Pt (ΔE = 1.16) and Ru (ΔE = 1.01). The OER and ORR pathways are perturbed by the introduction of peroxide disproportionation functionality via support interactions and selective doping of the catalyst. LaNi 0.75 Fe 0.25 O 3 's ability to disproportionate peroxide is hypothesized to be responsible for the ∼50% improvement over LaNiO 3 in catalytic activity toward the ORR, despite similar electronic structure. These results allow us to examine the pathways for OER and ORR in context of support interactions, transition metal redox processes, and catalytic bifunctionality.