Capturing Visible Light in Low-Band-Gap C4 N-Derived Responsive Bifunctional Air Electrodes for Solar Energy Conversion and Storage.

We report facile synthesis of low-band-gap mesoporous C4 N particles and their use as responsive bifunctional oxygen catalysts for visible-light-sensitive (VLS) rechargeable Zn-air battery (RZAB) and polymer-air battery (RPAB). Compared to widely studied g-C3 N4 , C4 N shows a smaller band gap of 1.99 eV, with a larger photocurrent response, and it can function as visible-light-harvesting antenna and bifunctional oxygen reduction/evolution (ORR/OER) catalysts, enabling effective photocoupling to tune oxygen catalysis. The C4 N-enabled VLS-RZAB displays a low charge voltage of 1.35 V under visible light, which is below the theoretical RZAB voltage of 1.65 V, corresponding to a high energy efficiency of 97.78 %. Pairing a C4 N cathode with a polymer anode also endows an VLS-RPAB with light-boosted charge performance. It is revealed that the ORR and OER active sites in C4 N are separate carbon sites near pyrazine-nitrogen atoms and photogenerated energetic holes can activate OER for improved reaction kinetics.

Redox Donor-Acceptor Conjugated Microporous Polymers as Ultralong-Lived Organic Anodes for Rechargeable Air Batteries.

Herein, we explore a new redox donor-acceptor conjugated microporous polymer (AQ-CMP) by utilizing anthraquinone and benzene as linkers via C-C linkages and demonstrate the first use of CMP as ultralong-lived anodes for rechargeable air batteries. AQ-CMP features an interconnected octupole network, which affords not only favorable electronic structure for enhanced electron transport and n-doping activity compared to linear counterpart, but also high density of active sites for maximizing the formula-weight-based redox capability. This coupled with highly cross-linked and porous structure endows AQ-CMP with a specific capacity of 202 mAh g-1 (96 % of theoretical capacity) at 2 Ag-1 and ≈100 % capacity retention over 60000 charge/discharge cycles. The assembled CMP-air full cell shows a stable and high capacity with full capacity recovery after only refreshing cathodes, while the decoupled electrolyte and cathode design boosts the discharge voltage and voltage efficiency to ≈1 V and 87.5 %.

Alkene-Linked Covalent Organic Frameworks Boosting Photocatalytic Hydrogen Evolution by Efficient Charge Separation and Transfer in the Presence of Sacrificial Electron Donors.

Covalent organic frameworks (COFs) are potential photocatalysts for artificial photosynthesis but they are much less explored for photocatalytic hydrogen evolution (PHE). COFs, while intriguing due to crystallinity, tunability, and porosity, tend to have low apparent quantum efficiency (AQE) and little is explored on atomistic structure-performance correlation. Here, adopting triphenylbenzene knots and phenyl linkers as a proof of concept, three structurally related COFs with different linkages are constructed to achieve a tunable COF platform and probe the effect of the linkage chemistry on PHE. Cyano-substituted alkene-linked COF (COF-alkene) yields a stable 2330 µmol h-1 g-1 PHE rate, much superior to imine- and imide-linked counterparts (<40 µmol h-1 g-1) under visible light irradiation. Impressively, COF-alkene achieves an AQE of 6.7% at 420 nm. Combined femtosecond transient absorption spectroscopy and theoretical calculation disclose the critical role of cyano-substituted alkene linkages toward high efficiency of charge separation and transfer in the presence of sacrificial electron donors-the decisive key to the superior PHE performance. Such alkene linkages can also be extended to design a series of high-performance polymeric photocatalysts, highlighting a general design idea for efficient PHE.

Erratum: In Situ Activating Strategy to Significantly Boost Oxygen Electrocatalysis of Commercial Carbon Cloth for Flexible and Rechargeable Zn-Air Batteries.

[This corrects the article DOI: 10.1002/advs.201800760.].

Humidity and Pressure Dual-Responsive Metal-Water Batteries Enabled by Three-In-One All-Polymer Cathodes for Smart Self-Powered Systems.

A conceptually new class of humidity and pressure dual-responsive smart metal-water batteries (SMWBs) is presented, which displays self-tunable energy release and intriguing perceptibility of human respiration and environmental pressure. This battery is enabled by the direct contact of a metal (e.g., Mg or Zn) anode and a well-designed all-polymer dual-sensitive moisture electrode (DSME) made from semiconductive polymer (e.g., polypyrrole)-wrapped 3D macroporous polyurethane sponge, without additional electrolytes and separator. A DSME is cost-effective, easily scalable, compressible, and able to act as a moisture carrier, a hydrogen evolution catalyst, and a pressure and humidity dual-sensitive unit simultaneously. Unique three-in-one integration in the DSME enables favorable modulation of electron/mass transport or redox reactions in the SMWB upon different stimulations. Thus, the assembled SMWB not only delivers good discharge performance with smart energy management but also serves as a reliable self-powered bifunctional responsor for the real-time monitoring of respiration and the perceptibility of pressure. Based on various active metal-polymer pairs (Mg/Zn vs polypyrrole/polyaniline), we also developed a series of dual-responsive batteries, demonstrating a general design idea.

Integrated Photo-Responsive Batteries for Solar Energy Harnessing: Recent Advances, Challenges, and Opportunities.

Invited for this month's cover is the group of Prof. Dingshan Yu from Sun Yat-sen University, China. The Front Cover shows an integrated photo-responsive battery with the simplest two-electrode configuration powering a vehicle under light illumination. The Taiji diagram shows the working principle of this device, which combines photoexcited electrons (e- ) and/or holes (h+ ) with various redox species of the batteries during charging and/or discharging processes to realize the harnessing of solar energy. Read the full text of the Minireview at 10.1002/cplu.201900608.

Polyaniline/Pure Carbon Assemblies as Efficient Self-standing Metal-free Oxygen Electrodes in Alkaline Media for Zn-Air Batteries.

A facile design and fabrication of self-standing metal-free polyaniline (PANI)@carbon nanotubes (CNTs) composite membrane was initially proposed by straightforward noncovalent wrapping the polymer around pure CNTs. Without introduction of extra heteroatoms into CNTs, the optimized PANI@CNTs composite exhibits a much better electrocatalytic performance for oxygen evolution reaction (OER) than pure CNTs via favorable interfacial modification with PANI to largely expose the active sites of on the surface of pure CNTs. Besides, it displays good oxygen reduction reaction (ORR) performance. When directly utilized as bifunctional air electrode without extra additive agents, the composite membrane-enabled rechargeable Zn-air batteries not only deliver a high peak power density (201.9 W g-1 ) and a large energy density (850.3 Wh kgZn -1 ), but also present robust cycling performance for 216 cycles with a high energy efficiency of 57.8%.

Versatile, Aqueous Soluble C2N Quantum Dots with Enriched Active Edges and Oxygenated Groups.

C2N has emerged as a new family of promising two-dimensional (2D) layered frameworks in both fundamental studies and potential applications. Transforming bulk C2N into zero-dimensional quantum dots (QDs) could induce unique quantum confinement and edge effects that produce improved or new properties. Despite their appealing potential, C2NQDs remain unexplored, and their intriguing properties and a fundamental understanding of their prominent edge effects are still not well understood. Here, we report the first synthesis of water-soluble C2NQDs via a top-down approach without any foreign stabilizer and exploit their linear/nonlinear optical properties and unique edge-preferential electrocatalytic activity toward polysulfides for versatile applications. The resultant dispersant-free C2NQDs with an average size of less than 5 nm feature rich oxygen-carrying groups and active edges, not only enabling excellent dispersion in water but also creating interesting multifunctionality. They can emit not only blue one-photon luminescence (OPL) under ultraviolet (UV) excitation but also green two-photon luminescence (TPL) with a wide near-infrared (NIR) excitation range of 750-900 nm, enabling their use as a new fluorescent ink. Interestingly, when C2NQDs are introduced to modify commercial separators, they can function as new metal-free catalysts to boost polysulfide redox kinetics and endow Li-S batteries with excellent cycling stability, high rate capability, and large areal capacity (7.0 mA h cm-2) at a high sulfur loading of 8.0 mg cm-2. Detailed theoretical and experimental results indicate that the edge of C2N is more favorable for trapping and catalyzing the polysulfide conversion than the terrace and that the synergy between the active edges and oxygenated groups enriched in C2NQDs remarkably improves polysulfide immobilization and catalytic conversion.

Integrated Photo-Responsive Batteries for Solar Energy Harnessing: Recent Advances, Challenges, and Opportunities.

Photo-responsive batteries that enable the effective combination of solar harvesting and energy conversion/storage functionalities render a potential solution to achieve the large-scale utilization of unlimited and cost-effective solar energy and alleviate the limits of conventional energy storage devices. The internal integration of photo-responsive electrodes into rechargeable batteries with the simplest two-electrode configuration is regarded as a reliable and appealing strategy for highly-efficient and low-cost utilization of solar energy by simplifying the device architecture and improving the energy efficiency. This progress report provides a brief review on photo-responsive batteries with integrated two-electrode configuration that can achieve solar energy conversion/storage in one single device. The basic device architecture, operating principles and practical performance of various photo-responsive systems based on solar energy harvesting in various batteries including Li ion batteries, Li-S batteries, Li-I batteries, dual-liquid redox batteries, Li-O2 batteries, non-Li anode-O2 /air batteries are summarized and discussed. Finally, the future opportunities and challenges regarding the two-electrode photo-responsive batteries are proposed.

Donor-Acceptor Nanocarbon Ensembles to Boost Metal-Free All-pH Hydrogen Evolution Catalysis by Combined Surface and Dual Electronic Modulation.

A combined surface and dual electronic modulation strategy is used to realize metal-free all-pH catalysis towards the hydrogen evolution reaction (HER) by coupling a N-doped carbon framework (MHCF, electron acceptors) derived from MOFs with higher-Fermi-level pure carbon nanotubes (CNTs, electron donors), followed by surface modification with carboxyl-group-rich polymers. Although the three constituents are inactive, as-assembled ternary membranes yield superior HER performance with low overpotentials and high durability (≤5 % activity loss over 100 h) at all pH values. The C adjacent to pyrrolic N in MHCF is the most active site and the induced directional interfacial electron transfer from CNTs to MHCF coupled with N-driven intramolecular electron transfer in MHCF optimizes Gibbs free energy for hydrogen adsorption (ΔGH* ) near zero, while the polymer modulation enables local H+ enrichment in acidic media and enhanced water adsorption and activation in neutral and basic media.

Tactile UV- and Solar-Light Multi-Sensing Rechargeable Batteries with Smart Self-Conditioned Charge and Discharge.

A tactile, UV- and solar-light multi-sensing smart rechargeable Zn-air battery (SRZAB) with excellent cell performance, self-conditioned charge/discharge, and reliable environmental responsivity is made by using multi-scale conjugated block-copolymer-carbon nanotube-polyurethane foam assemblies as both a self-standing air electrode and a sensing unit. Multiscale engineering fully exploits the multi-synergy among components to endow the newly designed metal-free multi-sensing air electrode (MSAE) with bifunctional oxygen reduction and evolution activities, pressure sensitivity, and photothermal and photoelectric conversion functions in a single electrode, enabling effective regulation of interface properties, electronic/ionic transport, or redox reactions in SRZAB upon various stimulations and establishing multiple working principles. MSAE-driven SRZAB can be used as compressible power sources, self-powered pressure and optical sensors and light-to-electrochemical energy systems.

Ultrathin Black Phosphorus-on-Nitrogen Doped Graphene for Efficient Overall Water Splitting: Dual Modulation Roles of Directional Interfacial Charge Transfer.

Few-layered exfoliated black phosphorus (EBP) has attracted surging interest for electronics, optoelectronics, and catalysis. As compared to excellent progress in electronic and optoelectronic applications, very few reports are available for electrocatalysis by metal-free EBPs. Herein, we couple solution-processable ultrathin EBP nanosheets with higher Fermi level of N-doped graphene (NG) into a new metal-free 2D/2D heterostructure (EBP@NG) with well-designed interfaces and unique electronic configuration, as efficient and durable bifunctional catalysts toward hydrogen evolution and oxygen evolution reactions (HER/OER) for overall water splitting in alkaline media. By rational interface engineering, the synergy of EBP and NG is fully exploited, which not only improves the stability of EBP, but also effectively modulates electronic structures of each component to boost their intrinsic activities. Specifically, due to the lower Fermi level of EBP relative to NG, their electronic interaction induces directional interfacial electron transfer, which not only enriches the electron density over EBP and optimizes H adsorption/desorption to promote HER, but also introduces abundant positively charged carbon sites on NG and provides favorable formation of key OER intermediates (OOH*) to improve OER energetics. Thus, despite that pure EBP or NG alone has poor or negligible activity, EBP@NG achieves remarkably enhanced bifunctional HER/OER activities, along with an excellent durability. This endows an optimized electrolyzer using EBP@NG as anode and cathode with a low cell voltage of 1.54 V at 10 mA cm-2, which is smaller than that of the costly integrated Pt/C@RuO2 couple (1.60 V).

In Situ Activating Strategy to Significantly Boost Oxygen Electrocatalysis of Commercial Carbon Cloth for Flexible and Rechargeable Zn-Air Batteries.

An in situ strategy to simultaneously boost oxygen reduction and oxygen evolution (ORR/OER) activities of commercial carbon textiles is reported and the direct use of such ubiquitous raw material as low-cost, efficient, robust, self-supporting, and bifunctional air electrodes in rechargeable Zn-air batteries is demonstrated. This strategy not only furnishes carbon textiles with a large surface area and hierarchical meso-microporosity, but also enables efficient dual-doping of N and S into carbon skeletons while retaining high conductivity and stable monolithic structures. Thus, although original carbon textile has rather poor catalytic activity, the activated textiles without loading other active materials yield effective ORR/OER bifunctionality and stability with a much lower reversible overpotential (0.87 V) than those of Pt/C (1.10 V) and RuO2 (1.02 V) and many reported metal-free bifunctional catalysts. Importantly, they can concurrently function as current collectors and as ORR/OER catalysts for rechargeable aqueous and flexible solid-state Zn-air batteries, showing excellent cell performance, long lifetime, and high flexibility.

A General Electrode Design Strategy for Flexible Fiber Micro-Pseudocapacitors Combining Ultrahigh Energy and Power Delivery.

Herein, a general strategy is proposed to boost the energy storage capability of pseudocapacitive materials (i.e., MnO2) to their theoretical limits in unconventional 1D fiber configuration by rationally designing bicontinuous porous Ni skeleton@metal wire "sheath-core" metallic scaffold as a versatile host. As a proof of concept, the 1D metallic scaffold supported-MnO2 fiber electrode is demonstrated. The proposed "sheath" design not only affords large electrode surface area with ordered macropores for large electrolyte-ion accessibility and high electroactive material loading, but also renders interconnected porous metallic skeleton for efficient electronic and ionic transport, while the metallic "core" functions as an extra current collector to promote long-distance electron transport and electron collection. Benefiting from all these merits, the optimized fiber electrode yields unprecedented specific areal capacitance of 1303.6 mF cm-2 (1278 F g-1 based on MnO2, approaching the theoretical value of 1370 F g-1) in liquid KOH and 847.22 mF cm-2 in polyvinyl alcohol (PVA)/KOH gel electrolyte, 2-350 times of previously reported fiber electrodes. The solid-state fiber micro-pseudocapacitors simultaneously achieve remarkable areal energy and power densities of 18.83 µWh cm-2 and 16.33 mW cm-2, greatly exceeding the existing symmetric fiber supercapacitors, together with long cycle life and high rate capability.