Boosting the Heat Dissipation Performance of Graphene/Polyimide Flexible Carbon Film via Enhanced Through-Plane Conductivity of 3D Hybridized Structure.

The development of materials with efficient heat dissipation capability has become essential for next-generation integrated electronics and flexible smart devices. Here, a 3D hybridized carbon film with graphene nanowrinkles and microhinge structures by a simple solution dip-coating technique using graphene oxide (GO) on polyimide (PI) skeletons, followed by high-temperature annealing, is constructed. Such a design provides this graphitized GO/PI (g-GO/PI) film with superflexibility and ultrahigh thermal conductivity in the through-plane (150 ± 7 W m-1 K-1 ) and in-plane (1428 ± 64 W m-1 K-1 ) directions. Its superior thermal management capability compared with aluminum foil is also revealed by proving its benefit as a thermal interface material. More importantly, by coupling the hypermetallic thermal conductivity in two directions, a novel type of carbon film origami heat sink is proposed and demonstrated, outperforming copper foil in terms of heat extraction and heat transfer for high-power devices. The hypermetallic heat dissipation performance of g-GO/PI carbon film not only shows its promising application as an emerging thermal management material, but also provides a facile and feasible route for the design of next-generation heat dissipation components for high-power flexible smart devices.

Enhancing Oxygen Reduction Activity of Pt-based Electrocatalysts: From Theoretical Mechanisms to Practical Methods.

Pt-based electrocatalysts are considered as one of the most promising choices to facilitate the oxygen reduction reaction (ORR), and the key factor enabling their success is to reduce the required amount of platinum. Herein, we focus on illuminating both the theoretical mechanisms which enable enhanced and sustained ORR activity and the practical methods to achieve them in catalysts. The various multi-step pathways of ORR are firstly reviewed and the rate-determining steps based on the reaction intermediates and their binding energies are analyzed. We then explain the critical aspects of Pt-based electrocatalysts to tune oxygen reduction properties from the viewpoints of active sites exposure and altering the surface electronic structure, and further summarize representative research progress towards practically achieving these activity enhancements with a focus on platinum size reduction, shape control and core Pt elimination methods. We finally outline the remaining challenges and provide our perspectives with regard to further enhancing their activities.

A Single-Atom Iridium Heterogeneous Catalyst in Oxygen Reduction Reaction.

Combining the advantages of homogeneous and heterogeneous catalysts, single-atom catalysts (SACs) are bringing new opportunities to revolutionize ORR catalysis in terms of cost, activity and durability. However, the lack of high-performance SACs as well as the fundamental understanding of their unique catalytic mechanisms call for serious advances in this field. Herein, for the first time, we develop an Ir-N-C single-atom catalyst (Ir-SAC) which mimics homogeneous iridium porphyrins for high-efficiency ORR catalysis. In accordance with theoretical predictions, the as-developed Ir-SAC exhibits orders of magnitude higher ORR activity than iridium nanoparticles with a record-high turnover frequency (TOF) of 24.3 e- site-1 s-1 at 0.85 V vs. RHE) and an impressive mass activity of 12.2 A mg-1 Ir , which far outperforms the previously reported SACs and commercial Pt/C. Atomic structural characterizations and density functional theory calculations reveal that the high activity of Ir-SAC is attributed to the moderate adsorption energy of reaction intermediates on the mononuclear iridium ion coordinated with four nitrogen atom sites.

Longevous Cycling of Rechargeable Zn-Air Battery Enabled by "Raisin-Bread" Cobalt Oxynitride/Porous Carbon Hybrid Electrocatalysts.

Developing commercially viable electrocatalyst lies at the research hotspot of rechargeable Zn-air batteries, but it is still challenging to meet the requirements of energy efficiency and durability in realistic applications. Strategic material design is critical to addressing its drawbacks in terms of sluggish kinetics of oxygen reactions and limited battery lifespan. Herein, a "raisin-bread" architecture is designed for a hybrid catalyst constituting cobalt nitride as the core nanoparticle with thin oxidized coverings, which is further deposited within porous carbon aerogel. Based on synchrotron-based characterizations, this hybrid provides oxygen vacancies and Co-Nx -C sites as the active sites, resulting from a strong coupling between CoOx Ny nanoparticles and 3D conductive carbon scaffolds. Compared to the oxide reference, it performs enhanced stability in harsh electrocatalytic environments, highlighting the benefits of the oxynitride. Furthermore, the 3D conductive scaffolds improve charge/mass transportation and boost durability of these active sites. Density functional theory calculations reveal that the introduced N species into hybrid can synergistically tune the d-band center of cobalt and improve its bifunctional activity. As a result, the obtained air cathode exhibits bifunctional overpotential of 0.65 V and a battery lifetime exceeding 1350 h, which sets a new record for rechargeable Zn-air battery reported so far.

Highly Stable Low-Cost Electrochemical Gas Sensor with an Alcohol-Tolerant N,S-Codoped Non-Precious Metal Catalyst Air Cathode.

The emerging applications of electrochemical gas sensors (EGSs) in Internet of Things-enabled smart city and personal health electronics bring out a new challenge for common EGSs, such as alcohol fuel cell sensors (AFCSs) to reduce the dependence on a pricy Pt catalyst. Here, for the first time, we propose a low-cost novel N,S-codoped metal catalyst (FeNSC) to accelerate oxygen reduction reaction (ORR) and replace the Pt catalyst in the cathode of an AFCS. The optimal FeNSC shows high ORR activity, stability, and alcohol tolerance. Furthermore, the FeNSC-based AFCS not only demonstrates excellent linearity, low detection limit, high stability, and superior sensitivity to that of the commercial Pt/C-based AFCS but also outperforms commercial Pt/C-based AFCS in the exposed cell regarding great linearity, high sensitivity, and great stability. Such a promising sensor performance not just proves the concept of the FeNSC-based ACFS but enlightens the next-generation designs toward low-cost, highly sensitive, and durable EGSs.

Constructing multifunctional solid electrolyte interface via in-situ polymerization for dendrite-free and low N/P ratio lithium metal batteries.

Stable solid electrolyte interface (SEI) is highly sought after for lithium metal batteries (LMB) owing to its efficient electrolyte consumption suppression and Li dendrite growth inhibition. However, current design strategies can hardly endow a multifunctional SEI formation due to the non-uniform, low flexible film formation and limited capability to alter Li nucleation/growth orientation, which results in unconstrained dendrite growth and short cycling stability. Herein, we present a novel strategy to employ electrolyte additives containing catechol and acrylic groups to construct a stable multifunctional SEI by in-situ anionic polymerization. This self-smoothing and robust SEI offers multiple sites for Li adsorption and steric repulsion to constrain nucleation/growth process, leading to homogenized Li nanosphere formation. This isotropic nanosphere offers non-preferred Li growth orientation, rendering uniform Li deposition to achieve a dendrite-free anode. Attributed to these superiorities, a remarkable cycling performance can be obtained, i.e., high current density up to 10 mA cm-2, ultra-long cycle life over 8500 hrs operation, high cumulative capacity over 4.25 Ah cm-2 and stable cycling under 60 °C. A prolonged lifespan can also be achieved in Li-S and Li-LiFePO4 cells under lean electrolyte content, low N/P ratio or high temperature conditions. This facile strategy also promotes the practical application of LMB and enlightens the SEI design in related fields.

A Near-Isotropic Proton-Conducting Porous Graphene Oxide Membrane.

A graphene oxide (GO) membrane is an ideal separator for multiple applications due to its morphology, selectivity, controllable oxidation, and high aspect ratio of the 2D nanosheet. However, the anisotropic ion conducting nature caused by its morphology is not favorable toward through-plane conductivity, which is vital for solid-state electrolytes in electrochemical devices. Here, we present a strategy to selectively enhance the through-plane proton conductivity of a GO membrane by reducing its degree of anisotropy with pore formation on the nanosheets through the sonication-assisted Fenton reaction. The obtained porous GO (pGO) membrane is a near-isotropic, proton-conducting GO membrane, showing a degree of anisotropy as low as 2.77 and 47% enhancement of through-plane proton conductivity as opposed to the pristine GO membrane at 25 °C and 100% relative humidity. The anisotropic behavior shows an Arrhenius relationship with temperature, while the water interlayer formation between nanosheets plays a pivotal role in the anisotropic behavior under different values of relative humidity (RH); that is, as low RH increases, water molecules tend to orient in a bimodal distribution clinching the nanosheets and forming a subnanometer, high-aspect-ratio, water interlayer, resulting in its peak anisotropy. Further increase in RH fills the interlayer gap, resulting in behaviors akin to near-isotropic, bulk water. Lastly, implementation of the pGO membrane, as the solid proton-conductive electrolyte, in an alcohol fuel cell sensor has been demonstrated, showcasing the excellent selectivity and response, exceptional linearity, and ethanol detection limits as low as 25 ppm. The amalgamation of excellent performance, high customizability, facile scalability, low cost, and environmental friendliness in the present method holds considerable potential for transforming anisotropic GO membranes into near-isotropic ion conductors to further membrane development and sensing applications.

Deep-Breathing Honeycomb-like Co-Nx-C Nanopolyhedron Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries.

Metal organic framework (MOF) derivatives have been extensively used as bifunctional oxygen electrocatalysts. However, the utilization of active sites is still not satisfactory owing to the sluggish mass transport within their narrow pore channels. Herein, interconnected macroporous channels were constructed inside MOFs-derived Co-Nx-C electrocatalyst to unblock the mass transfer barrier. The as-synthesized electrocatalyst exhibits a honeycomb-like morphology with highly exposed Co-Nx-C active sites on carbon frame. Owing to the interconnected ordered macropores throughout the electrocatalyst, these active sites can smoothly "exhale/inhale" reactants and products, enhancing the accessibility of active sites and the reaction kinetics. As a result, the honeycomb-like Co-Nx-C displayed a potential difference of 0.773 V between the oxygen evolution reaction potential at 10 mA cm-2 and the oxygen reduction reaction half-wave potential, much lower than that of bulk-Co-Nx-C (0.842 V). The rational modification on porosity makes such honeycomb-like MOF derivative an excellent bifunctional oxygen electrocatalyst in rechargeable Zn-air batteries.

Nitrogen-doped graphene-TiO x N y nanocomposite electrode for highly efficient capacitive deionization.

In this work, the first-ever reported nanocomposite electrode of nitrogen-doped graphene-titanium oxynitride (NG-TiO x N y ) for capacitive deionization (CDI) was synthesized via hydrothermal reaction and a high-temperature nitridation process. The physiochemical characterizations revealed that the nitrogen was doped in the graphene structure mainly in the form of graphitic nitrogen and the TiO x N y was successfully formed via TiO2 nitridation process. The layered NG nanosheets facilitated the diffusion of ions in saline water and formed electrical double layer on the surface of the electrode material, while the presence of TiO x N y enhanced the electrochemical performance by increasing surface area and generating surface vacancies via nitridation. The CDI cell employed NG-TiO x N y electrode delivered a breakthrough salt adsorption capacity of 26.1 mg g-1 in 500 mg L-1 saline water, and retained over 90% of its initial salt removal efficacy after 12 regeneration cycles. Such high CDI performance exhibits the promising application of NG-TiO x N y as a novel CDI electrode candidate.

The Absence and Importance of Operando Techniques for Metal-Free Catalysts.

Operando characterization techniques have played a crucial role in modern technological developments. In contrast to the experimental uncertainties introduced by ex situ techniques, the simultaneous measurement of desired sample characteristics and near-realistic electrochemical testing provides a representative picture of the underlying physics. From Li-ion batteries to metal-based electrocatalysts, the insights offered by real-time characterization data have enabled more efficient research programs. As an emerging class of catalyst, much of the mechanistic understanding of metal-free electrocatalysts continues to be elusive in comparison to their metal-based counterparts. However, there is a clear absence of operando characterization performed on metal-free catalysts. Through the proper execution of operando techniques, it can be expected that metal-free catalysts can achieve exceptional technological progress. Here, the motivation of using operando characterization techniques for metal-free carbon-based catalyst system is considered, followed by a discussion of the possibilities, difficulties and benefits of their applications.

An Oxygen-Vacancy-Rich Semiconductor-Supported Bifunctional Catalyst for Efficient and Stable Zinc-Air Batteries.

The highly oxidative operating conditions of rechargeable zinc-air batteries causes significant carbon-support corrosion of bifunctional oxygen electrocatalysts. Here, a new strategy for the catalyst support design focusing on oxygen vacancy (OV)-rich, low-bandgap semiconductor is proposed. The OVs promote the electrical conductivity of the oxide support, and at the same time offer a strong metal-support interaction (SMSI), which enables the catalysts to have small metal size, high catalytic activity, and high stability. The strategy is demonstrated by successfully synthesizing ultrafine Co-metal-decorated 3D ordered macroporous titanium oxynitride (3DOM-Co@TiOx Ny ). The 3DOM-Co@TiOx Ny catalyst exhibits comparable activities for oxygen reduction and evolution reactions, but much higher cycling stability than noble metals in alkaline conditions. The zinc-air battery using this catalyst delivers an excellent stability with less than 1% energy efficiency loss over 900 charge-discharge cycles at 20 mA cm-2 . The high stability is attributed to the strong SMSI between Co and 3DOM-TiOx Ny which is verified by density functional theory calculations. This work sheds light on using OV-rich semiconductors as a promising support to design efficient and durable nonprecious electrocatalysts.

A Lithium-Sulfur Battery using a 2D Current Collector Architecture with a Large-Sized Sulfur Host Operated under High Areal Loading and Low E/S Ratio.

While backless freestanding 3D electrode architectures for batteries with high loading sulfur have flourished in the recent years, the more traditional and industrially turnkey 2D architecture has not received the same amount of attention. This work reports a spray-dried sulfur composite with large intrinsic internal pores, ensuring adequate local electrolyte availability. This material offers good performance with a electrolyte content of 7 µL mg-1 at high areal loadings (5-8 mg cm-2 ), while also offering the first reported 2.8 µL mg-1 (8 mg cm-2 ) to enter into the second plateau of discharge and continue to operate for 20 cycles. Moreover, evidence is provided that the high-frequency semicircle (i.e., interfacial resistance) is mainly responsible for the often observed bypassing of the second plateau in lean electrolyte discharges.

In Situ Polymer Graphenization Ingrained with Nanoporosity in a Nitrogenous Electrocatalyst Boosting the Performance of Polymer-Electrolyte-Membrane Fuel Cells.

Rich, porous graphene frameworks decorated with uniformly dispersed active sites are prepared by using polyaniline as a graphene precursor and introducing phenanthroline as a pore-forming agent. The unprecedented fuel-cell performance of this electrocatalyst is linked to the graphene frameworks with vast distribution of pore sizes, which maximizes the active-sites accessibility, facilitates mass-transport properties, and improves the carbon corrosion resistance.

Co-N Decorated Hierarchically Porous Graphene Aerogel for Efficient Oxygen Reduction Reaction in Acid.

Nitrogen-functionalized graphene materials have been demonstrated as promising electrocatalyst for the oxygen reduction reaction (ORR), owning to their respectable activity and excellent stability in alkaline electrolyte. However, they exhibit unacceptable catalytic activity in acid medium. Here, a hierarchically porous Co-N functionalized graphene aerogel is prepared as an efficient catalyst for the ORR in acid electrolyte. In the preparation process, polyaniline (PANI) is introduced as a pore-forming agent to aid in the self-assembly of graphene species into a porous aerogel networks, and a nitrogen precursor to induce in situ nitrogen doping. Therefore, a Co-N decorated graphene aerogel framework with a large surface area (485 m(2) g(-1)) and an abundance of meso/macropores is effectively formed after heat treatment. Such highly desired structures can not only expose sufficient active sites for the ORR but also guarantee the fast mass transfer in the catalytic process, which provides significant catalytic activity with positive onset and half wave potentials, low hydrogen peroxide yield, high resistance to methanol crossover, and remarkable stability that is comparable to commercial Pt/C in acid medium.

Molecular Functionalization of Graphene Oxide for Next-Generation Wearable Electronics.

Acquiring reliable and efficient wearable electronics requires the development of flexible electrolyte membranes (EMs) for energy storage systems with high performance and minimum dependency on the operating conditions. Herein, a freestanding graphene oxide (GO) EM is functionalized with 1-hexyl-3-methylimidazolium chloride (HMIM) molecules via both covalent and noncovalent bonds induced by esterification reactions and electrostatic πcation-π stacking, respectively. Compared to the commercial polymeric membrane, the thin HMIM/GO membrane demonstrates not only slightest performance sensitivity to the operating conditions but also a superior hydroxide conductivity of 0.064 ± 0.0021 S cm(-1) at 30% RH and room temperature, which was 3.8 times higher than that of the commercial membrane at the same conditions. To study the practical application of the HMIM/GO membranes in wearable electronics, a fully solid-state, thin, flexible zinc-air battery and supercapacitor are made exhibiting high battery performance and capacitance at low humidified and room temperature environment, respectively, favored by the bonded HMIM molecules on the surface of GO nanosheets. The results of this study disclose the strong potential of manipulating the chemical structure of GO to work as a lightweight membrane in wearable energy storage devices, possessing highly stable performance at different operating conditions, especially at low relative humidity and room temperature.

Quaternized graphene oxide nanocomposites as fast hydroxide conductors.

Nanocomposites play a key role in performance improvements of hydroxide conductors employed in a wide range of alkaline-electrochemical systems such as fuel cells and metal-air batteries. Graphene oxide (GO) nanosheets are considered to be outstanding nanofillers for polymeric nanocomposites on account of their excellent physicochemical strength and electrochemical properties. In this work, a fast hydroxide conductor was developed on the basis of a chemically modified GO nanocomposite membrane. The high surface area of GO was functionalized with highly stable hydroxide-conductive groups using a dimethyloctadecyl [3-(trimethoxysilyl)propyl]ammonium chloride (DMAOP) precursor, named QAFGO, and then composed with porous polybenzimidazole PBI (pPBI) as a well-suited polymeric backbone. The nanocomposite exhibited outstanding hydroxide conductivity of 0.085 S cm(-1), high physicochemical strength, and electrochemical stability for 21 days. An alkaline fuel cell (AFC) setup was fabricated to determine the functionality of QAFGO/pPBI nanocomposite in an alkaline-based system. The high AFC performance with peak power density of 86.68 mW cm(-2) demonstrated that QAFGO/pPBI nanocomposite membrane has promising potential to be employed as a reliable hydroxide conductor for electrochemical systems working in alkaline conditions.