William G. Hardin

Highly Active, Nonprecious Metal Perovskite Electrocatalysts for Bifunctional Metal-Air Battery Electrodes.

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.

Anion charge storage through oxygen intercalation in LaMnO3 perovskite pseudocapacitor electrodes

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.

Atomic Ensemble and Electronic Effects in Ag-Rich AgPd Nanoalloy Catalysts for Oxygen Reduction in Alkaline Media

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 AgPd(2) alloy relative to pure Pd. The mass activities were 2.7 and 3.2 times higher for Ag(9)Pd (340 mA/mg(metal)) and Ag(4)Pd (598 mA/mg(metal)), respectively, than those expected for a linear combination of mass activities of Ag (60 mA/mg(Ag)) and Pd (799 mA/mg(Pd)) particles, based on rotating disk voltammetry. Moreover, these synergy factors reached 5-fold on a Pd mass basis. For silver-rich alloys (Ag(≥4)Pd), 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.

Water electrolysis on La1-xSrxCoO3-δ perovskite electrocatalysts

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 Sr2+ substitution into La1−xSrxCoO3−δ. We attempt to rationalize the high activities of La1−xSrxCoO3−δ 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, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.

Exceptional electrocatalytic oxygen evolution via tunable charge transfer interactions in La 0.5 Sr 1.5 Ni 1−x Fe x O 4±δ Ruddlesden-Popper oxides

The electrolysis of water is of global importance to store renewable energy and the methodical design of next-generation oxygen evolution catalysts requires a greater understanding of the structural and electronic contributions that give rise to increased activities. Herein, we report a series of Ruddlesden–Popper La0.5Sr1.5Ni1−xFexO4±δ oxides that promote charge transfer via cross-gap hybridization to enhance electrocatalytic water splitting. Using selective substitution of lanthanum with strontium and nickel with iron to tune the extent to which transition metal and oxygen valence bands hybridize, we demonstrate remarkable catalytic activity of 10 mA cm−2 at a 360 mV overpotential and mass activity of 1930 mA mg−1ox at 1.63 V via a mechanism that utilizes lattice oxygen. This work demonstrates that Ruddlesden–Popper materials can be utilized as active catalysts for oxygen evolution through rational design of structural and electronic configurations that are unattainable in many other crystalline metal oxide phases.Water electrolysis provides a potential means to large-scale renewable fuel generation, although sluggish oxygen evolution kinetics challenges progress. Here, authors report on Ruddlesden–Popper oxides as active oxygen evolution electrocatalysts that provide impetus for overcoming kinetic barriers.