The development of high-energy and high-power density supercapacitors (SCs) is critical for enabling next-generation energy storage applications. Nanocarbons are excellent SC electrode materials due to their economic viability, high-surface area, and high stability.Although nanocarbons have high theoretical surface area and hence high double layer capacitance, the net amount of energy stored in nanocarbon-SCs is much below theoretical limits due to two inherent bottlenecks: i) their low quantum capacitance and ii) limited ionaccessible surface area. Here, we demonstrate that defects in graphene could be effectively used to mitigate these bottlenecks by drastically increasing the quantum capacitance and opening new channels to facilitate ion diffusion in otherwise closed interlayer spaces. Our results support the emergence of a new energy paradigm in SCs with 250% enhancement in double layer capacitance beyond the theoretical limit.Furthermore, we demonstrate prototype defect engineered bulk SC devices with energy densities 500% higher than state-of-the-art commercial SCs without compromising the power density. IntroductionSupercapacitors (SCs) are novel electrochemical devices that store energy through reversible adsorption of ionic species from an electrolyte on highly porous electrode surfaces. SCs are highly durable (lifetime >10,000 cycles) with power densities (10 kW/kg) that are an order of magnitude larger than batteries. But the low energy density (10 Wh/kg) of SCs 1 relative to batteries precludes their use in practical applications despite their ability to withstand >10,000 cycles. Graphene-based nanocarbons are ideal electrode materials for SCs due to their low cost, high stability, and high specific surface area. Indeed, an outstanding characteristic of single-layer graphene is its high specific surface area ~2675 m 2 /g, which sets an upper limit for electrical double layer capacitance (C dl ) ~21 µF/cm 2 (~550 F/g). 1-4 Notwithstanding this theoretical limit, there are two intrinsic bottlenecks that are impeding the emergence of high energy density SC devices:i) typically only 50-70% of the theoretical surface area is accessible to ionic species from the electrolyte, which limits the overall capacitance (10-15 µF/cm 2 ) and leads to low energy density, and ii) although the total energy that can be harnessed from a SC device depends predominantly on ion-accessible surface area, it is not the only factor. The presence of the so-called small quantum capacitance (C Q ) in series for nanocarbon electrodes, arising from their low electronic density of states at the Fermi level (DOS(E F )), overwhelms the high C dl further reducing the already limited capacitance and low energy density. 5-7While the efforts to increase energy density have been focused either on increasing the active surface area or the addition of pseudo-capacitance through redox active materials, there is a clear lack of methodologies to simultaneously address the inherent challenges described above. Here, we experimentally show that eng...

It is of increasing importance to explore new low‐cost and high‐activity electrocatalysts for oxygen reduction reaction (ORR), which have had a substantial impact across a diverse range of energy conversion system, including various fuel cell and metal–air batteries. Although engineering carbon nanostructures have been widely explored as a candidate class of Pt‐based ORR electrocatalysts owing to their proved high activity, outstanding stability, and ease of use, there still remains a daunting challenge to develop high activity metal‐free electrocatalysts in pH‐universal electrolyte system. Here, a reliable and controllable route amenable to prepare nitrogen‐doped porous carbon (NPC) with high yields and exceptional quality is described. The as‐prepared NPC shows advantages of high activity, high durability, and methanol‐tolerant as an efficient pH‐universal electrocatalyst for ORR, showing comparable or even better activity as compared with the commercial Pt/C catalysts not only in alkaline media but also in acidic and neutral electrolyte. Systematic electrochemical studies, combining with density functional theory calculation, demonstrate the unique nitrogen‐doping species and favorable pores in the as‐designed NPC synergistically contribute to the significantly improved catalytic activity in pH‐universal medium. The present work potentially presents an important breakthrough in developing ORR electrocatalysts for various fuel cells.

Development of highly active and stable bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts from earth-abundant elements remains a grand challenge for highly demanded reversible fuel cells and metal-air batteries. Carbon catalysts have many advantages over others due to their low cost, excellent electrical conductivity, high surface area, and easy functionalization. However, they typically cannot withstand the highly oxidative OER environment. We report here a new class of ultra-large sized nitrogen doped graphene tubes (N-GTs) (>500 nm) decorated with FeCoNi alloy particles as a bifunctional electrocatalyst. These tubes are prepared from an inexpensive dicyandiamide via a template-free graphitization process. The ORR/OER activity and the stability of these graphene tube catalysts depends strongly on the transition metal precursors. The best performing FeCoNi-derived N-GT catalyst exhibits excellent ORR and OER activity along with sufficient electrochemical durability over a wide potential window (0 to 1.9 V) in alkaline media. The measured OER current is solely due to desirable O2 evolution, rather than carbon oxidation. Extensive electrochemical and physical characterization indicated that high graphitization degree, thicker tube walls, proper nitrogen doping, and presence of FeCoNi alloy particles are vital for high bifunctional activity and electrochemical durability of tubular carbon catalysts.

Au nanorods are optically tunable anisotropic nanoparticles with built-in catalytic activities. The state-of-the-art seed-mediated nanorod synthesis offers excellent control over the aspect ratios of cylindrical Au nanorods, which enables fine-tuning of plasmon resonances over a broad spectral range. However, facet control of Au nanorods with atomic-level precision remains significantly more challenging. The coexistence of various types of low-index and high-index facets on the highly curved nanorod surfaces makes it extremely challenging to quantitatively elucidate the atomic-level structure-property relationships that underpin the catalytic competence of Au nanorods. Here we demonstrate that cylindrical Au nanorods undergo controlled facet evolution during their overgrowth in the presence of Cu(2+) and cationic surfactants, resulting in the formation of anisotropic nanostructures enclosed by well-defined facets, such as low-index faceting nanocuboids and high-index faceting convex nanocuboids and concave nanocuboids. These faceted Au nanorods exhibit enriched optical extinction spectral features, broader plasmonic tuning range, and enhanced catalytic tunability in comparison to the conventional cylindrical Au nanorods. The capabilities to both fine-tailor the facets and fine-tune the plasmon resonances of anisotropic Au nanoparticles open up unique opportunities for us to study, in great detail, the facet-dependent interfacial molecular transformations on Au nanocatalysts using surface-enhanced Raman scattering as a time-resolved spectroscopic tool.

While great success has been achieved in fine-tuning the aspect ratios and thereby the plasmon resonances of cylindrical Au nanorods, facet control with atomic level precision on the highly curved nanorod surfaces has long been a significantly more challenging task. The intrinsic structural complexity and lack of precise facet control of the nanorod surfaces remain the major obstacles for the atomic-level elucidation of the structure-property relationships that underpin the intriguing catalytic performance of Au nanorods. Here we demonstrate that the facets of single-crystalline Au nanorods can be precisely tailored using cuprous ions and cetyltrimethylammonium bromide as a unique pair of surface capping competitors to guide the particle geometry evolution during nanorod overgrowth. By deliberately maneuvering the competition between cuprous ions and cetyltrimethylammonium bromide, we have been able to create, in a highly controllable and selective manner, an entire family of nanorod-derived anisotropic multifaceted geometries whose surfaces are enclosed by specific types of well-defined high-index and low-index facets. This facet-controlled nanorod overgrowth approach also allows us to fine-tune the particle aspect ratios while well-preserving all the characteristic facets and geometric features of the faceted Au nanorods. Taking full advantage of the combined structural and plasmonic tunability, we have further studied the facet-dependent heterogeneous catalysis on well-faceted Au nanorods using surface-enhanced Raman spectroscopy as an ultrasensitive spectroscopic tool with unique time-resolving and molecular finger-printing capabilities.

Structures and morphologies of Fe-N-C catalysts are believed to be crucial because of the number of active sites and local bonding structures governing the overall catalyst performance for the oxygen reduction reaction (ORR). However, the knowledge how to rationally design catalysts is still lacking. By combining different nitrogen/carbon precursors, including polyaniline (PANI), dicyandiamide (DCDA), and melamine (MLMN), we aim to tune catalyst morphology and structure to facilitate the ORR. Instead of the commonly studied single precursors, multiple precursors were used during the synthesis; this provides a new opportunity to promote catalyst activity and stability through a likely synergistic effect. The best-performing Fe-N-C catalyst derived from PANI+DCDA is superior to the individual PANI or DCDA-derived ones. In particular, when compared to the extensively explored PANI-derived catalysts, the binary precursors have an increased half-wave potential of 0.83 V and an enhanced electrochemical stability in challenging acidic media, indicating a significantly increased number of active sites and strengthened local bonding structures. Multiple key factors associated with the observed promotion are elucidated, including the optimal pore size distribution, highest electrochemically active surface area, presence of dominant amorphous carbon, and thick graphitic carbon layers with more pyridinic nitrogen edge sites likely bonded with active atomic iron.

) surface areas and, more importantly, the highest concentration of nitrogen incorporated into the carbon planes.Thus, in addition to the intrinsic high activity of Fe-derived catalysts, the high surface area and nitrogen doping contribute to high ORR activity. This work, for the first time, demonstrates size-controlled synthesis of large-diameter N-doped carbon tube electrocatalysts by varying the metal used in N-CNT generation. Electrocatalytic activity of the Fe-derived catalyst is already the best among studied metals, due to the high intrinsic activity of possible Fe-N coordination. This work further provides a promising route to advanced Fe-N-C nonprecious metal catalysts by generating favorable morphology with more active sites and improved mass transfer.

A highly efficient process for producing bulk chemical diethyl maleate is achieved with polyoxometalate ionic liquids from a cleavage lignin aromatic unit with high yield and selectivity, which is ascribed to the intensive synergistic effect between the acidic depolymerization, oxidative aromatic ring cleavage, and in situ esterification. This work offers new insight into the versatile petroleum-based chemical production from renewable resources.