Proton is Essential or Not: A Fresh Look on Pseudocapacitive Energy Storage of PANI Composites.

Protonation has been considered essential for the pseudocapacitive energy storage of polyaniline (PANI) for years, as proton doping in PANI chains not only activates electron transport pathways, but also promotes the proceeding of redox reactions. Rarely has the ability for PANI of storing energy without protonation been investigated, and it remains uncertain whether PANI has pseudocapacitive charge storage properties in an alkaline electrolyte. Here, this work first demonstrates the pseudocapacitive energy storage for PANI without protonation using a PANI/graphene composite as a model material in an alkaline electrolyte. Using in situ Raman spectroscopy coupled with electrochemical quartz crystal microbalance (EQCM) measurements, this work determines the formation of -N= group over potential on a PANI chain and demonstrates the direct contribution of OH- in the nonprotonation type of oxidation reactions. This work finds that the PANI/graphene composite in an alkaline electrolyte has excellent cycling stability with a wider operation voltage of 1 V as well as a slightly higher specific capacitance than that in an acidic electrolyte. The findings provide a new perspective on pseudocapacitive energy storage of PANI-based composites, which will influence the selection of electrolytes for PANI materials and expand their application in energy storage fields.

A Review of Compact Carbon Design for Supercapacitors with High Volumetric Performance.

Volumetric performance is of great importance in today's energy storage devices, and is used to evaluate their competitiveness in the markets of miniaturized electronic devices and space-constrained electric vehicles. Supercapacitors suffer from a low volumetric energy density in spite of their high power and long cycle life because of their use of porous but low-density carbons. This review considers compact carbon design strategies for high volumetric performance supercapacitors based on four key electrode parameters: density, thickness, gravimetric capacitance, and nonactive components. A guide is provided for constructing a conductive additive-/binder-free self-supported ultrathick, dense electrode to maximize the volumetric energy density. The research status of emerging micro-supercapacitors and hybrid supercapacitors is then briefly discussed, emphasizing the importance of their volumetric performance and the opportunities as well as challenges they face in the trendy Internet of things applications or larger device systems.

Building Carbon-Based Versatile Scaffolds on the Electrode Surface to Boost Capacitive Performance for Fiber Pseudocapacitors.

In order to fabricate high performance fiber pseudocapacitors, the trade-off between high mass loading and high utilization efficiency of pseudocapacitive materials should be carefully addressed. Here, a solution that is to construct a carbon-based versatile scaffold is reported for loading pseudocapacitive materials on carbonaceous fibers. The scaffold can be easily built by conformally coating commercial pen ink on the fibers without any destruction to the fiber skeleton. Due to the high electrical conductivity and abundant macropore structure, it can provide sufficient loading room and a high ion/electron conductive network for pseudocapacitive materials. Therefore, their loading mass and utilization efficiency can be increased simultaneously, and thus the as-designed fibrous electrode displays a high areal capacitance of 649 mF cm-2 (or 122 mF cm-1 based on length), which is higher than most of the reported fiber pseudocapacitors. The simple and low-cost strategy opens up a new way to prepare high performance portable/wearable energy storage devices.

Interlayer Affected Diamond Electrochemistry.

Diamond electrochemistry is primarily influenced by quantities of sp3-carbon, surface terminations, and crystalline structure. In this work, a new dimension is introduced by investigating the effect of using substrate-interlayers for diamond growth. Boron and nitrogen co-doped nanocrystalline diamond (BNDD) films are grown on Si substrate without and with Ti and Ta as interlayers, named BNDD/Si, BNDD/Ti/Si, and BNDD/Ta/Ti/Si, respectively. After detailed characterization using microscopies, spectroscopies, electrochemical techniques, and density functional theory simulations, the relationship of composition, interfacial structure, charge transport, and electrochemical properties of the interface between diamond and metal is investigated. The BNDD/Ta/Ti/Si electrodes exhibit faster electron transfer processes than the other two diamond electrodes. The interlayer thus determines the intrinsic activity and reaction kinetics. The reduction in their barrier widths can be attributed to the formation of TaC, which facilitates carrier tunneling, and simultaneously increases the concentration of electrically active defects. As a case study, the BNDD/Ta/Ti/Si electrode is further employed to assemble a redox-electrolyte-based supercapacitor device with enhanced performance. In summary, the study not only sheds light on the intricate relationship between interlayer composition, charge transfer, and electrochemical performance but also demonstrates the potential of tailored interlayer design to unlock new capabilities in diamond-based electrochemical devices.

Reconfiguring Hard Carbons with Emerging Sodium-Ion Batteries: A Perspective.

Hard carbons, an important category of amorphous carbons, are non-graphitizable and are widely accepted as the most promising anode materials for emerging sodium-ion batteries (SIBs)， because of their changeable low-potential charge/discharge plateaus. However, their microstructures are not fixed and are difficult to accurately demonstrate as graphites do. The successful use of hard carbons in SIBs revives the interest to clearly picture their complicated microstructures that are in close relevance to sodium storage. In this review, the past definitions and structural models of hard carbons are revisited first, and a renewed understanding of their sodium storage is presented. Three critical structural features are highlighted for hard carbons, namely crystallites, defects, and nanopores, which are directly responsible for the presence of the low-potential plateaus and their reversible extension. The impact of these structural features upon the sodium storage is then deeply discussed and sieving carbons is finally proposed as an ideal configuration of carbon anode for superhigh sodium storage. This review is expected to offer a clear picture of hard carbons, and help realize a truly rational design of high-capacity carbon anodes, driving the industrialization of SIBs, and more promisingly open up a window for exploring their possible new uses.