Redox mediators as charge agents for changing electrochemical reactions.

Redox mediators (RMs) play pivotal roles in enhancing the performance of electrochemical energy storage and conversion systems. Unlike the widely explored areas of electrode materials, electrolytes, separators, and electrolyte additives, RMs have received little attention. This review provides a comprehensive discussion toward understanding the effects of RMs on electrochemical systems, underlying redox mechanisms, and reaction kinetics both experimentally and theoretically. Our discussion focuses on the roles of RMs in various electrochemical systems such as lithium-ion batteries, Li-O2 batteries, Li-S batteries, decoupling electrolysis, supercapacitors, and microbial fuel cells. Depending on the reaction regions where the RMs become active, we can classify them into bulk, solid-solid interfacial, solid-liquid interfacial, and cell-unit RMs. The prospect of developing RMs with effective charge transfer properties along with minimal side-effects is an exciting research direction. Moreover, the introduction of an efficient RM into an electrochemical system can fundamentally change its chemistry; in particular, the electrode reaction polarization can be considerably decreased. In this context, we discuss the key properties of RMs applied for various purposes, and the main issues are addressed.

A Highly Reversible Long-Life Li-CO2 Battery with a RuP2 -Based Catalytic Cathode.

Rechargeable Li-CO2 batteries have attracted worldwide attention due to the capability of CO2 capture and superhigh energy density. However, they still suffer from poor cycling performance and huge overpotential. Thus, it is essential to explore highly efficient catalysts to improve the electrochemical performance of Li-CO2 batteries. Here, phytic acid (PA)-cross-linked ruthenium complexes and melamine are used as precursors to design and synthesize RuP2 nanoparticles highly dispersed on N, P dual-doped carbon films (RuP2 -NPCFs), and the obtained RuP2 -NPCF is further applied as the catalytic cathode for Li-CO2 batteries. RuP2 nanoparticles that are uniformly deposited on the surface of NPCF show enhanced catalytic activity to decompose Li2 CO3 at low charge overpotential. In addition, the NPCF its with porous structure in RuP2 -NPCF provides superior electrical conductivity, high electrochemical stability, and enough ion/electron and space for the reversible reaction in Li-CO2 batteries. Hence, the RuP2 -NPCF cathode delivers a superior reversible discharge capacity of 11951 mAh g-1 , and achieves excellent cyclability for more than 200 cycles with low overpotentials (<1.3 V) at the fixed capacity of 1000 mAh g-1 . This work paves a new way to design more effective catalysts for Li-CO2 batteries.

Ultrasmall TiO2-Coated Reduced Graphene Oxide Composite as a High-Rate and Long-Cycle-Life Anode Material for Sodium-Ion Batteries.

Because of the low cost and abundant nature of the sodium element, sodium-ion batteries (SIBs) are attracting extensive attention, and a variety of SIB cathode materials have been discovered. However, the lack of high-performance anode materials is a major challenge of SIBs. Herein, we have synthesized ultrasmall TiO2-nanoparticle-coated reduced graphene oxide (TiO2@RGO) composites by using a one-pot hydrolysis method, which are then investigated as anode materials for SIBs. The morphology of TiO2@RGO has been characterized using transmission electron microscopy, indicating that the TiO2 nanospheres uniformly grow on the surface of the RGO nanosheet. As-prepared TiO2@RGO composites exhibited a promising electrochemical performance in terms of cycling stability and rate capability, especially the initial cycle Coulombic efficiency of 60.7%, which is higher than that in previous reports. The kinetics of the electrode reaction has been investigated by cyclic voltammetry. The results indicate that the sodium-ion intercalation/extraction behavior is not controlled by the semiinfinite diffusion process, which gives rise to an outstanding rate performance. In addition, the electrochemical performance of TiO2@RGO composites in full cells, coupled with carbon-coated Na3V2(PO4)3 as the positive material, has been investigated. The discharge specific capacity was up to 117.2 mAh g-1, and it remained at 84.6 mAh g-1 after 500 cycles under a current density of 2 A g-1, which shows excellent cycling stability.

Crab-shell induced synthesis of ordered macroporous carbon nanofiber arrays coupled with MnCo2O4 nanoparticles as bifunctional oxygen catalysts for rechargeable Zn-air batteries.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are traditionally carried out using noble metals (such as Pt) and metal oxides (such as RuO2 and IrO2) as catalysts, respectively. Nevertheless, several key issues such as high cost, poor stability, and detrimental environmental effects limit the catalytic activity of these noble metal- and metal oxide-based catalysts. Herein, we have designed and synthesized macroporous carbon nanofiber arrays by using a natural crab shell template. Subsequently, spinel MnCo2O4 nanoparticles were embedded into the nitrogen-doped macroporous carbon nanofiber arrays (NMCNAs) by a hydrothermal method. Accompanied by the good conductivity, large surface area and doping of nitrogen, the as-prepared MnCo2O4/NMCNA exhibited remarkable catalytic performance and outstanding stability for both ORR and OER in alkaline media. The macroporous superstructures play vital role in reducing the ion transport resistance and facilitating the diffusion of gaseous products (O2). Finally, rechargeable Zn-air batteries using the MnCo2O4/NMCNA catalyst displayed appreciably lower overpotentials, higher power density and better stability than commercial Pt/C, thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries.

Sequentially surface modified hematite enables lower applied bias photoelectrochemical water splitting.

Hematite (α-Fe2O3) is a suitable candidate for photoelectrochemical water splitting due to its well-suited band structure, stability, and availability. However, water splitting using a low external potential is the major challenge that limits the practical application of hematite. Here, we achieve a very low onset potential using a sequential surface treatment approach to overcome two fundamental limiting factors, sluggish hole transfer, and interfacial recombination, independently. First, a heavily doped Fe2-xSnxO3 surface passivation layer was created by Sn4+ surface treatment which can robustly inhibit interfacial recombination. Then, an NiOOH catalyst layer was deposited that greatly enhances the charge transfer process across the passivated electrode/electrolyte interface. By exploiting this approach, the optimized sequentially treated photoanode (Fe2O3/Fe2-xSnxO3/NiOOH) exhibits a low photocurrent onset potential of 0.49 V vs. RHE and a saturated photocurrent density of 2.4 mA cm-2 V at 1.5 V vs. RHE. Transient photocurrent and impedance spectroscopy measurements further reveal that the combined Fe2-xSnxO3/NiOOH layers reduce interfacial recombination and enhance charge transfer across the electrode/electrolyte interface. The results provide convincing evidence that it is possible to address the problems of surface trap recombination and sluggish catalysis independently by employing surface passivation layers first and catalysts later sequentially.

Using hematite for photoelectrochemical water splitting: a review of current progress and challenges.

Photoelectrochemical (PEC) water splitting is a promising technology for solar hydrogen production to build a sustainable, renewable and clean energy economy. Hematite (α-Fe2O3) based photoanodes offer promise for such applications, due to their high chemical stability, great abundance and low cost. Despite these promising properties, progress towards the manufacture of practical water splitting devices has been limited. This review is intended to highlight recent advancements and the limitations that still hamper the full utilization of hematite electrodes in PEC water splitting systems. We review recent progress in manipulating hematite for PEC water splitting through various approaches, focused on e.g. enhancing light absorption, water oxidation kinetics, and charge carrier collection efficiency. As the morphology affects various properties, progress in morphological characterization from thicker planar films to recent ultrathin nanophotonic morphologies is also examined. Special emphasis has been given to various ultrathin films and nanophotonic structures which have not been given much attention in previous review articles.