Operando unraveling photothermal-promoted dynamic active-sites generation in NiFe2O4 for markedly enhanced oxygen evolution.

The ability to develop highly active and low-cost electrocatalysts represents an important endeavor toward accelerating sluggish water-oxidation kinetics. Herein, we report the implementation and unraveling of the photothermal effect of spinel nanoparticles (NPs) on promoting dynamic active-sites generation to markedly enhance their oxygen evolution reaction (OER) activity via an integrated operando Raman and density functional theory (DFT) study. Specifically, NiFe2O4 (NFO) NPs are first synthesized by capitalizing on amphiphilic star-like diblock copolymers as nanoreactors. Upon the near-infrared light irradiation, the photothermal heating of the NFO-based electrode progressively raises the temperature, accompanied by a marked decrease of overpotential. Accordingly, only an overpotential of 309 mV is required to yield a high current density of 100 mA cm-2, greatly lower than recently reported earth-abundant electrocatalysts. More importantly, the photothermal effect of NFO NPs facilitates surface reconstruction into high-active oxyhydroxides at lower potential (1.36 V) under OER conditions, as revealed by operando Raman spectroelectrochemistry. The DFT calculation corroborates that these reconstructed (Ni,Fe)oxyhydroxides are electrocatalytically active sites as the kinetics barrier is largely reduced over pure NFO without surface reconstruction. Given the diversity of materials (metal oxides, sulfides, phosphides, etc.) possessing the photo-to-thermal conversion, this effect may thus provide a unique and robust platform to boost highly active surface species in nanomaterials for a fundamental understanding of enhanced performance that may underpin future advances in electrocatalysis, photocatalysis, solar-energy conversion, and renewable-energy production.

Recent advances in activating surface reconstruction for the high-efficiency oxygen evolution reaction.

A climax in the development of cost-effective and high-efficiency transition metal-based electrocatalysts has been witnessed recently for sustainable energy and related conversion technologies. In this regard, structure-activity relationships based on several descriptors have already been proposed to rationally design electrocatalysts. However, the dynamic reconstruction of the surface structures and compositions of catalysts during electrocatalytic water oxidation, especially during the anodic oxygen evolution reaction (OER), complicate the streamlined prediction of the catalytic activity. With the achievements in operando and in situ techniques, it has been found that electrocatalysts undergo surface reconstruction to form the actual active species in situ accompanied with an increase in their oxidation state during OER in alkaline solution. Accordingly, a thorough understanding of the surface reconstruction process plays a critical role in establishing unambiguous structure-composition-property relationships in pursuit of high-efficiency electrocatalysts. However, several issues still need to be explored before high electrocatalytic activities can be realized, as follows: (1) the identification of initiators and pathways for surface reconstruction, (2) establishing the relationships between structure, composition, and electrocatalytic activity, and (3) the rational manipulation of in situ catalyst surface reconstruction. In this review, the recent progress in the surface reconstruction of transition metal-based OER catalysts including oxides, non-oxides, hydroxides and alloys is summarized, emphasizing the fundamental understanding of reconstruction behavior from the original precatalysts to the actual catalysts based on operando analysis and theoretical calculations. The state-of-the-art strategies to tailor the surface reconstruction such as substituting/doping with metals, introducing anions, incorporating oxygen vacancies, tuning morphologies and exploiting plasmonic/thermal/photothermal effects are then introduced. Notably, comprehensive operando/in situ characterization together with computational calculations are responsible for unveiling the improvement mechanism for OER. By delivering the progress, strategies, insights, techniques, and perspectives, this review will provide a comprehensive understanding of the surface reconstruction in transition metal-based OER catalysts and future guidelines for their rational development.

Lithium-Conducting Branched Polymers: New Paradigm of Solid-State Electrolytes for Batteries.

The past decades have witnessed rapid development of lithium-based batteries. Significant research efforts have been progressively diverted from electrodes to electrolytes, particularly polymer electrolytes (PEs), to tackle the safety concern and promote the energy storage capability of batteries. To further increase the ionic conductivity of PEs, various branched polymers (BPs) have been rationally designed and synthesized. Compared with linear polymers, branched architectures effectively increase polymer segmental mobility, restrain crystallization, and reduce chain entanglement, thereby rendering BPs with greatly enhanced lithium transport. In this Mini Review, a diversity of BPs for PEs is summarized by scrutinizing their unique topologies and properties. Subsequently, the design principles for enhancing the physical properties, mechanical properties, and electrochemical performance of BP-based PEs (BP-PEs) are provided in which the ionic conduction is particularly examined in light of the Li+ transport mechanism. Finally, the challenges and future prospects of BP-PEs in this rapidly evolving field are outlined.

Metal-Organic Frameworks for Ion Conduction.

Solid-state ionic conductors are compelling alternatives to liquid electrolytes in clean energy-harvesting and -storage technologies. The development of novel ionic conducting materials is one of the most critical challenges for next-generation energy technologies. Several advancements in design strategies, synthetic approaches, conducting properties, and underlying mechanisms for ionic conducting metal-organic frameworks (MOFs) have been made over the past five years; however, despite the recent, considerable expansion of related research fields, there remains a lack of systematic overviews. Here, an extensive introduction to ionic conducting performance for MOFs with different design strategies is provided, focusing primarily on ion mobility with the aid of hydrogen-bonding networks or solvated ionic charge. Furthermore, current theories on ion conducting mechanisms in different regimes are comprehensively summarized to provide an understanding of the underlying working principles in complex, realistic systems. Finally, challenges and future research directions at the forefront of ionic conducting MOF technologies are outlined.

Recent Advances in Silicon-Based Electrodes: From Fundamental Research toward Practical Applications.

The increasing demand for higher-energy-density batteries driven by advancements in electric vehicles, hybrid electric vehicles, and portable electronic devices necessitates the development of alternative anode materials with a specific capacity beyond that of traditional graphite anodes. Here, the state-of-the-art developments made in the rational design of Si-based electrodes and their progression toward practical application are presented. First, a comprehensive overview of fundamental electrochemistry and selected critical challenges is given, including their large volume expansion, unstable solid electrolyte interface (SEI) growth, low initial Coulombic efficiency, low areal capacity, and safety issues. Second, the principles of potential solutions including nanoarchitectured construction, surface/interface engineering, novel binder and electrolyte design, and designing the whole electrode for stability are discussed in detail. Third, applications for Si-based anodes beyond LIBs are highlighted, specifically noting their promise in configurations of Li-S batteries and all-solid-state batteries. Fourth, the electrochemical reaction process, structural evolution, and degradation mechanisms are systematically investigated by advanced in situ and operando characterizations. Finally, the future trends and perspectives with an emphasis on commercialization of Si-based electrodes are provided. Si-based anode materials will be key in helping keep up with the demands for higher energy density in the coming decades.