The need/desire to lower the consumption of fossil fuels and its environmental consequences has reached unprecedented levels in recent years. A global effort has been undertaken to develop advanced renewable energy generation and especially energy storage technologies, as they would enable a dramatic increase in the effective and efficient use of renewable (and often intermittent) energy sources. The development of electrical energy storage (EES) technologies with high energy and power densities, long life, low cost, and safe use represents a challenge from both the fundamental science and technological application points of view. While the advent and broad deployment of lithium-ion batteries (LIBs) has dramatically changed the EES landscape, their performance metrics need to be greatly enhanced to keep pace with the ever-increasing demands imposed by modern consumer electronics and especially the emerging automotive markets. Current battery technologies are mostly based on the use of a transition metal oxide cathode (e.g., LiCoO, LiFePO, or LiNiMnCoO) and a graphite anode, both of which depend on intercalation/insertion of lithium ions for operation. While the cathode material currently limits the battery capacity and overall energy density, there is a great deal of interest in the development of high-capacity cathode materials as well as anode materials. Conversion reaction materials have been identified/proposed as potentially high-energy-density alternatives to intercalation-based materials. However, conversion reaction materials react during lithiation to form entirely new products, often with dramatically changed structure and chemistry, by reaction mechanisms that are still not completely understood. This makes it difficult to clearly distinguish the limitations imposed by the mechanism and practical losses from initial particle morphology, synthetic approaches, and electrode preparations. Transition metal compounds such as transition metal oxides, sulfides, fluorides, phosphides, and nitrides can undergo conversion reactions yielding materials with high theoretical capacity (generally from 500 to 1500 mA h g). In particular, a number of transition metal oxides and sulfides have shown excellent electrochemical properties as high-capacity anode materials. In addition, some transition metal fluorides have shown great potential as cathode materials for Li rechargeable batteries. In this Account we present mechanistic studies, with emphasis on the use of operando methods, of selected examples of conversion-type materials as both potentially high-energy-density anodes and cathodes in EES applications. We also include examples of the conceptually similar conversion-type reactions involving chalcogens and halogens, with emphasis on the Li-S system. In this case we focus on the problems arising from the low electrical conductivities of elemental sulfur and LiS and the "redox shuttle" phenomena of polysulfides. In addition to mechanistic insights from the use of operando methods, we also cover several key strategie...

Alkaline fuel cells have drawn increasing attention as next-generation energy-conversion devices for electrical vehicles, since high pH enables the use of non-precious-metal catalysts. Herein, we report on a family of rationally designed Mn-doped cobalt ferrite (MCF) spinel nanocrystals, with an optimal composition Mn0.8(CoFe2)0.73O4 (MCF-0.8), that are effective electrocatalysts for the oxygen reduction reaction. MCF-0.8 exhibits a half-wave potential (E 1/2) of 0.89 V vs RHE in 1 M NaOH, only 0.02 V less than that of commercial Pt/C under identical testing conditions and, to the best of our knowledge, one of the highest recorded values in the literature. Moreover, MCF-0.8 exhibits remarkable durability (ΔE 1/2 = 0.014 V) after 10 000 electrochemical cycles. In situ X-ray absorption spectroscopy (XAS) reveals that the superior performance of the trimetallic MCF-0.8 originates from the synergistic catalytic effect of Mn and Co, while Fe helps preserve the spinel structure during cycling. We employed in situ XAS to track the evolution of the oxidation states and the metal–oxygen distances not only under constant applied potentials (steady state) but also during dynamic cyclic voltammetry (CV) (nonsteady state). The periodic conversion between Mn­(III, IV)/Co­(III) and Mn­(II, III)/Co­(II) as well as the essentially constant oxidation state of Fe during the CV suggests collaboration efforts among Mn, Co, and Fe. Mn and Co serve as the synergistic coactive sites to catalyze the oxygen reduction, apparently resulting in the observed high activity, while Fe works to maintain the integrity of the spinel structure, likely contributing to the remarkable durability of the catalyst. These findings provide a mechanistic understanding of the electrocatalytic processes of trimetallic oxides under real-time fuel cell operating conditions. This approach provides a new strategy to design high-performance non-precious-metal electrocatalysts for alkaline fuel cells.

Hydrogen fuel cells have attracted growing attention for high-performance automotive power but are hindered by the scarcity of platinum (and other precious metals) used to catalyze the sluggish oxygen reduction reaction (ORR). We report on a family of nonprecious transition metal nitrides (TMNs) as ORR electrocatalysts in alkaline medium. The air-exposed nitrides spontaneously form a several-nanometer-thick oxide shell on the conductive nitride core, serving as a highly active catalyst architecture. The most active catalyst, carbon-supported cobalt nitride (Co 3 N/C), exhibited a half-wave potential of 0.862 V and achieved a record-high peak power density among reported nitride cathode catalysts of 700 mW cm −2 in alkaline membrane electrode assemblies. Operando x-ray absorption spectroscopy studies revealed that Co 3 N/C remains stable below 1.0 V but experiences irreversible oxidation at higher potentials. This work provides a comprehensive analysis of nonprecious TMNs as ORR electrocatalysts and will help inform future design of TMNs for alkaline fuel cells and other energy applications.

Exploring Pt-free electrocatalysts with high activity and long durability for the oxygen reduction reaction (ORR) has been long pursued by the renewable energy materials community. In this work, we have designed an ordered intermetallic PdZn/C (O-PdZn) with a several atomic-layer Pd shell, which achieved a 3-fold enhancement in ORR mass activities (MA) in alkaline media, relative to Pd/C and Pt/C. Further Au incorporation in O-PdZn/C (Au-O-PdZn/C) yielded a catalyst with superior durability with less than 10% loss in MA after 30000 potential cycles. These effects have attributed to the rationally designed ordered structure and stabilizing effect of Au atoms. Aberration-corrected scanning transmission electron microscopy and synchrotron-based X-ray fluorescence spectroscopy were employed to show that Au not only galvanically replaced Pd and Zn on the surface but also penetrated through the PdZn lattice and distributed uniformly within the particles. Au-O-PdZn/C was also tested as an effective oxygen cathode in broad applications in rechargeable Li–air and Zn–air batteries.

Designing high-performance nonprecious electrocatalysts to replace Pt for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report a systematic study of 15 different AB2O4/C spinel nanoparticles with well-controlled octahedral morphology. The 3 most active ORR electrocatalysts were MnCo2O4/C, CoMn2O4/C, and CoFe2O4/C. CoMn2O4/C exhibited a half-wave potential of 0.89 V in 1 M KOH, equal to the benchmark activity of Pt/C, which was ascribed to charge transfer between Co and Mn, as evidenced by X-ray absorption spectroscopy. Scanning transmission electron microscopy (STEM) provided atomic-scale, spatially resolved images, and high-energy-resolution electron-loss near-edge structure (ELNES) enabled fingerprinting the local chemical environment around the active sites. The most active MnCo2O4/C was shown to have a unique Co-Mn core–shell structure. ELNES spectra indicate that the Co in the core is predominantly Co2.7+ while in the shell, it is mainly Co2+. Broader Mn ELNES spectra indicate less-ordered nearest oxygen neighbors. Co in the shell occupies mainly tetrahedral sites, which are likely candidates as the active sites for the ORR. Such microscopic-level investigation probes the heterogeneous electronic structure at the single-nanoparticle level, and may provide a more rational basis for the design of electrocatalysts for alkaline fuel cells.