Mass Transfer Analysis for Achieving High-Rate Lithium-Air Batteries.

Lithium-air batteries (LABs) have aroused worldwide interest due to their high energy density as a promising next-generation battery technology. From a practical standpoint, one of the most pressing issues currently in LABs is their poor rate performance. Accelerating the mass transfer rate within LABs is a crucial aspect for enhancing their rate capability. In this Perspective, we have meticulously analyzed the ion and oxygen transport processes to provide readers with a comprehensive understanding of the mass transfer within LABs. Following this, we have discussed potential misconceptions in the existing literature and propose our recommendations for improving the rate performance of LABs. This Perspective provides a deep insight into the mass transfer process in LABs and offers promising strategies for developing other high-rate metal-O2 batteries.

A Rotating Cathode with Periodical Changes in Electrolyte Layer Thickness for High-Rate Li‒O2 Batteries.

Li-O2 batteries (LOBs) possess the highest theoretical gravimetric energy density among all types of secondary batteries, but they are still far from practical applications. The poor rate performance resulting from the slow mass transfer is one of the primary obstacles in LOBs. To solve this issue, a rotating cathode with periodic changes in the electrolyte layer thickness is designed, decoupling the maximum transfer rate of Li+ and O2. During rotation, the thinner electrolyte layer on the cathode facilitates the O2 transfer, and the thicker electrolyte layer enhances the Li+ transfer. As a result, the rotating cathode enables the LOBs to undergo 58 cycles at 2.5 mA cm-2 and discharge stably even at a high current density of 7.5 mA cm-2. Besides, it also makes the batteries exhibit a large discharge capacity of 6.8 mAh cm-2, and the capacity decay is much slower with increasing current density. Notably, this rotating electrode holds great promise for utilization in other electrochemical cells involving gas-liquid-solid triple-phase interfaces, suggesting a viable approach to enhance the mass transfer in such systems.

Gram-level NH3 Electrosynthesis via NOx reduction on a Cu Activated Co Electrode.

Ambient electrochemical ammonia (NH3 ) synthesis is one promising alternative to the energy-intensive Haber-Bosch route. However, the industrial requirement for the electrochemical NH3 production with amperes current densities or gram-level NH3 yield remains a grand challenge. Herein, we report the high-rate NH3 production via NO2 - reduction using the Cu activated Co electrode in a bipolar membrane (BPM) assemble electrolyser, wherein BPM maintains the ion balance and the liquid level of electrolyte. Benefited from the abundant Co sites and optimal structure, the target modified Co foam electrode delivers a current density of 2.64 A cm-2 with the Faradaic efficiency of 96.45 % and the high NH3 yield rate of 279.44 mg h-1 cm-2 in H-type cell using alkaline electrolyte. Combined with in situ experiments and theoretical calculations, we found that Cu optimizes the adsorption behavior of NO2 - and facilitates the hydrogenation steps on Co sites toward a rapid NO2 - reduction process. Importantly, this activated Co electrode affords a large NH3 production up to 4.11 g h-1 in a homemade reactor, highlighting its large-scale practical feasibility.

A Bifunctional Catalyst for Green Ammonia Synthesis from Ubiquitous Air and Water.

Ammonia (NH3 ) is essential for modern agriculture and industry, and, due to its high hydrogen density and no carbon emission, it is also expected to be the next-generation of "clean" energy carrier. Herein, directly from air and water, a plasma-electrocatalytic reaction system for NH3 production, which combines two steps of plasma-air-to-NOx - and electrochemical NOx - reduction reaction (eNOx RR) with a bifunctional catalyst, is successfully established. Especially, the bifunctional catalyst of CuCo2 O4 /Ni can simultaneously promote plasma-air-to-NOx - and eNOx RR processes. The easy adsorption and activation of O2 by CuCo2 O4 /Ni greatly improve the NOx - production rate at the first step. Further, CuCo2 O4 /Ni can also resolve the overbonding of the key intermediate of * NO, and thus reduce the energy barrier of the second step of eNOx RR. Finally, the "green" NH3 production achieves excellent FENH3 (96.8%) and record-high NH3 yield rate of 145.8 mg h-1  cm-2 with large partial current density (1384.7 mA cm-2 ). Moreover, an enlarged self-made H-type electrolyzer improves the NH3 yield to 3.6 g h-1 , and the obtained NH3 is then rapidly converted to a solid of magnesium ammonium phosphate hexahydrate, which favors the easy storage and transportation of NH3 .

Soluble and Perfluorinated Polyelectrolyte for Safe and High-Performance Li-O2 Batteries.

The severe performance degradation of high-capacity Li-O2 batteries induced by Li dendrite growth and concentration polarization from the low Li+ transfer number of conventional electrolytes hinder their practical applications. Herein, lithiated Nafion (LN) with the sulfonic group immobilized on the perfluorinated backbone has been designed as a soluble lithium salt for preparing a less flammable polyelectrolyte solution, which not only simultaneously achieves a high Li+ transfer number (0.84) and conductivity (2.5 mS cm-1 ), but also the perfluorinated anion of LN produces a LiF-rich SEI for protecting the Li anode from dendrite growth. Thus, the Li-O2 battery with a LN-based electrolyte achieves an all-round performance improvement, like low charge overpotential (0.18 V), large discharge capacity (9508 mAh g-1 ), and excellent cycling performance (225 cycles). Besides, the fabricated pouch-type Li-air cells exhibit promising applications to power electronic equipment with satisfactory safety.

A Low-Volatile and Durable Deep Eutectic Electrolyte for High-Performance Lithium-Oxygen Battery.

The lithium-oxygen battery (LOB) with a high theoretical energy density (∼3500 Wh kg-1) has been regarded as a strong competitor for next-generation energy storage systems. However, its performance is still far from satisfactory due to the lack of stable electrolyte that can simultaneously withstand the strong oxidizing environment during battery operation, evaporation by the semiopen feature, and high reactivity of lithium metal anode. Here, we have developed a deep eutectic electrolyte (DEE) that can fulfill all the requirements to enable the long-term operation of LOBs by just simply mixing solid N-methylacetamide (NMA) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a certain ratio. The unique interaction of the polar groups in the NMA with the cations and anions in the LiTFSI enables DEE formation, and this NMA-based DEE possesses high ionic conductivity, good thermal, chemical, and electrochemical stability, and good compatibility with the lithium metal anode. As a result, the LOBs with the NMA-based DEE present a high discharge capacity (8647 mAh g-1), excellent rate performance, and superb cycling lifetime (280 cycles). The introduction of DEE into LOBs will inject new vitality into the design of electrolytes and promote the development of high-performance LOBs.

Three Birds with One Stone: An Integrated Cathode-Electrolyte Structure for High-Performance Solid-State Lithium-Oxygen Batteries.

Constructing solid-state lithium-oxygen batteries (SSLOBs) holds a great promise to solve the safety and stability bottlenecks faced by lithium-oxygen batteries (LOBs) with volatile and flammable organic liquid electrolytes. However, the realization of high-performance SSLOBs is full of challenges due to the poor ionic conductivity of solid electrolytes, large interfacial resistance, and limited reaction sites of cathodes. Here, a flexible integrated cathode-electrolyte structure (ICES) is designed to enable the tight connection between the cathode and electrolyte through supporting them on a 3D SiO2 nanofibers (NFs) framework. The intimate cathode-electrolyte structure and the porous SiO2 NFs scaffold combination are favorable for decreasing interfacial resistance and increasing reaction sites. Moreover, the 3D SiO2 NFs framework can also behave as an efficient inorganic filler to enhance the ionic conductivity of the solid polymer electrolyte and its ability to inhibit lithium dendrite growth. As a result, the elaborately designed ICES can simultaneously tackle the issues that limit the performance liberation of SSLOBs, making the batteries deliver a high discharge capacity and a long lifetime of 145 cycles with a cycling capacity of 1000 mAh g-1 at 60 °C, much superior to coventional SSLOBs (50 cycles).

Hybrid solid electrolyte enabled dendrite-free Li anodes for high-performance quasi-solid-state lithium-oxygen batteries.

The dendrite growth of Li anodes severely degrades the performance of lithium-oxygen (Li-O2) batteries. Recently, hybrid solid electrolyte (HSE) has been regarded as one of the most promising routes to tackle this problem. However, before this is realized, the HSE needs to simultaneously satisfy contradictory requirements of high modulus and even, flexible contact with Li anode, while ensuring uniform Li+ distribution. To tackle this complex dilemma, here, an HSE with rigid Li1.5Al0.5Ge1.5(PO4)3 (LAGP) core@ultrathin flexible poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) shell interface has been developed. The introduced large amount of nanometer-sized LAGP cores can not only act as structural enhancer to achieve high Young's modulus but can also construct Li+ diffusion network to homogenize Li+ distribution. The ultrathin flexible PVDF-HFP shell provides soft and stable contact between the rigid core and Li metal without affecting the Li+ distribution, meanwhile suppressing the reduction of LAGP induced by direct contact with Li metal. Thanks to these advantages, this ingenious HSE with ultra-high Young's modulus of 25 GPa endows dendrite-free Li deposition even at a deposition capacity of 23.6 mAh. Moreover, with the successful inhibition of Li dendrites, the HSE-based quasi-solid-state Li-O2 battery delivers a long cycling stability of 146 cycles, which is more than three times that of gel polymer electrolyte-based Li-O2 battery. This new insight may serve as a starting point for further designing of HSE in Li-O2 batteries, and can also be extended to various battery systems such as sodium-oxygen batteries.

Boosting Production of HCOOH from CO2 Electroreduction via Bi/CeOx.

Formic acid (HCOOH) is one of the most promising chemical fuels that can be produced through CO2 electroreduction. However, most of the catalysts for CO2 electroreduction to HCOOH in aqueous solution often suffer from low current density and limited production rate. Herein, we provide a bismuth/cerium oxide (Bi/CeOx ) catalyst, which exhibits not only high current density (149 mA cm-2 ), but also unprecedented production rate (2600 µmol h-1 cm-2 ) with high Faradaic efficiency (FE, 92 %) for HCOOH generation in aqueous media. Furthermore, Bi/CeOx also shows favorable stability over 34 h. We hope this work could offer an attractive and promising strategy to develop efficient catalysts for CO2 electroreduction with superior activity and desirable stability.

An Illumination-Assisted Flexible Self-Powered Energy System Based on a Li-O2 Battery.

The flexible Li-O2 battery is suitable to satisfy the requirements of a self-powered energy system, thanks to environmental friendliness, low cost, and high theoretical energy density. Herein, a flexible porous bifunctional electrode with both electrocatalytic and photocatalytic activity was synthesized and introduced as a cathode to assemble a high-performance Li-O2 battery that achieved an overpotential of 0.19 V by charging with the aid of solar energy. As a proof-of-concept application, a flexible Li-O2 battery was constructed and integrated with a solar cell via a scalable encapsulate method to fabricate a flexible self-powered energy system with excellent flexibility and mechanical stability. Moreover, by exploring the evolution of the electrode morphology and discharge products (Li2 O2 ), the charging process of the Li-O2 battery powered by solar energy and solar cell was demonstrated.

Generating Defect-Rich Bismuth for Enhancing the Rate of Nitrogen Electroreduction to Ammonia.

The electrochemical N2 fixation, which is far from practical application in aqueous solution under ambient conditions, is extremely challenging and requires a rational design of electrocatalytic centers. We observed that bismuth (Bi) might be a promising candidate for this task because of its weak binding with H adatoms, which increases the selectivity and production rate. Furthermore, we successfully synthesized defect-rich Bi nanoplates as an efficient noble-metal-free N2 reduction electrocatalyst via a low-temperature plasma bombardment approach. When exclusively using 1 H NMR measurements with N2 gas as a quantitative testing method, the defect-rich Bi(110) nanoplates achieved a 15 NH3 production rate of 5.453 µg mgBi -1 h-1 and a Faradaic efficiency of 11.68 % at -0.6 V vs. RHE in aqueous solution at ambient conditions.

A Simple and Effective Principle for a Rational Design of Heterogeneous Catalysts for Dehydrogenation of Formic Acid.

Efficient and selective dehydrogenation of formic acid is a key challenge for a fuel-cell-based hydrogen economy. Though the development of heterogeneous catalysts has received much progress, their catalytic activity remains insufficient. Moreover, the design principle of such catalysts are still unclear. Here, experimental and theoretical studies on a series of mono-/bi-metallic nanoparticles supported on a NH2 -N-rGO substrate are combined for formic acid dehydrogenation where the surface energy of a metal is taken as a relevant indicator for the adsorption ability of the catalyst for guiding catalyst design. The AuPd/NH2 -N-rGO catalyst shows record catalytic activity by reducing the energy barrier of rate controlling steps of formate adsorption and hydrogen desorption. The obtained excellent results both in experiments and simulations could be extended to other important systems, providing a general guideline to design more efficient catalysts.

Prevention of dendrite growth and volume expansion to give high-performance aprotic bimetallic Li-Na alloy-O2 batteries.

Rechargeable aprotic alkali metal (Li or Na)-O2 batteries are the subject of great interest because of their high theoretical specific energy. However, the growth of dendrites and cracks at the Li or Na anode, as well as their corrosive oxidation lead to poor cycling stability and safety issues. Understanding the mechanism and improving Li/Na-ion plating and stripping electrochemistry are therefore essential to realizing their technological potential. Here, we report how the use of a Li-Na alloy anode and an electrolyte additive realizes an aprotic bimetal Li-Na alloy-O2 battery with improved cycling stability. Electrochemical investigations show that stripping and plating of Li and Na and the robust and flexible passivation film formed in situ (by 1,3-dioxolane additive reacting with the Li-Na alloy) suppress dendrite and buffer alloy anode volume expansion and thus prevent cracking, avoiding electrolyte consumption and ensuring high electron transport efficiency and continued electrochemical reactions.

Recent Advances toward the Rational Design of Efficient Bifunctional Air Electrodes for Rechargeable Zn-Air Batteries.

Large-scale application of renewable energy and rapid development of electric vehicles have brought unprecedented demand for advanced energy-storage/conversion technologies and equipment. Rechargeable zinc (Zn)-air batteries represent one of the most promising candidates because of their high energy density, safety, environmental friendliness, and low cost. The air electrode plays a key role in managing the many complex physical and chemical processes occurring on it to achieve high performance of Zn-air batteries. Herein, recent advances of air electrodes from bifunctional catalysts to architectures are summarized, and their advantages and disadvantages are discussed to underline the importance of progress in the evolution of bifunctional air electrodes. Finally, some challenges and the direction of future research are provided for the optimized design of bifunctional air electrodes to achieve high performance of rechargeable Zn-air batteries.

In Situ CVD Derived Co-N-C Composite as Highly Efficient Cathode for Flexible Li-O2 Batteries.

To promote the development of high energy Li-O2 batteries, it is important to design and construct a suitable and effective oxygen-breathing cathode. Herein, activated cobalt-nitrogen-doped carbon nanotube/carbon nanofiber composites (Co-N-CNT/CNF) as the effective cathodes for Li-O2 batteries are prepared by in situ chemical vapor deposition (CVD). The unique architecture of these electrodes facilitates the rapid oxygen diffusion and electrolyte penetration. Meanwhile, the nitrogen-doped carbon nanotube/carbon nanofiber (N-CNT/CNF) and Co/CoNx serve as reaction sites to promote the formation/decomposition of discharge product. Li-O2 batteries with Co-N-CNT/CNF cathodes exhibit superior electrochemical performance in terms of a positive discharge plateau (2.81 V) and a low charge overpotential (0.61 V). Besides, Li-O2 batteries also present a high discharge capacity (11512.4 mAh g-1 at 100 mA g-1 ), and a long cycle life (130 cycles). Meanwhile, the Co-N-CNT/CNF cathode also has an excellent flexibility, thus the assembled flexible battery with Co-N-CNT/CNF can work normally and hold a wonderful capacity rate under various bending conditions.

Amorphizing of Cu Nanoparticles toward Highly Efficient and Robust Electrocatalyst for CO2 Reduction to Liquid Fuels with High Faradaic Efficiencies.

Conversion of carbon dioxide (CO2 ) into valuable chemicals, especially liquid fuels, through electrochemical reduction driven by sustainable energy sources, is a promising way to get rid of dependence on fossil fuels, wherein developing of highly efficient catalyst is still of paramount importance. In this study, as a proof-of-concept experiment, first a facile while very effective protocol is proposed to synthesize amorphous Cu NPs. Unexpectedly, superior electrochemical performances, including high catalytic activity and selectivity of CO2 reduction to liquid fuels are achieved, that is, a total Faradaic efficiency of liquid fuels can sum up to the maximum value of 59% at -1.4 V, with formic acid (HCOOH) and ethanol (C2 H6 O) account for 37% and 22%, respectively, as well as a desirable long-term stability even up to 12 h. More importantly, this work opens a new avenue for improved electroreduction of CO2 based on amorphous metal catalysts.

Anchoring and Upgrading Ultrafine NiPd on Room-Temperature-Synthesized Bifunctional NH2 -N-rGO toward Low-Cost and Highly Efficient Catalysts for Selective Formic Acid Dehydrogenation.

Hydrogen is widely considered to be a sustainable and clean energy alternative to the use of fossil fuels in the future. Its high hydrogen content, nontoxicity, and liquid state at room temperature make formic acid a promising hydrogen carrier. Designing highly efficient and low-cost heterogeneous catalysts is a major challenge for realizing the practical application of formic acid in the fuel-cell-based hydrogen economy. Herein, a simple but effective and rapid strategy is proposed, which demonstrates the synthesis of NiPd bimetallic ultrafine particles (UPs) supported on NH2 -functionalized and N-doped reduced graphene oxide (NH2 -N-rGO) at room temperature. The introduction of the NH2 N group to rGO is the key reason for the formation of the ultrafine and well-dispersed Ni0.4 Pd0.6 UPs (1.8 nm) with relatively large surface area and more active sites. Surprisingly, the as-prepared low-cost NiPd/NH2 -N-rGO dsiplays excellent hydrophilicity, 100% H2 selectivity, 100% conversion, and remarkable catalytic activity (up to 954.3 mol H2 (mol catalyst)-1 h-1 ) for FA decomposition at room temperature even with no additive, which is much higher than that of the best catalysts so far reported.

A Water-/Fireproof Flexible Lithium-Oxygen Battery Achieved by Synergy of Novel Architecture and Multifunctional Separator.

To meet the increasing demands for portable and flexible devices in a rapidly developing society, it is urgently required to develop highly safe and flexible electrochemical energy-storage systems. Flexible lithium-oxygen batteries with high theoretical specific energy density are promising candidates; however, the conventional half-open structure design prevents it from working properly under water or fire conditions. Herein, as a proof-of-concept experiment, a highly safe flexible lithium-oxygen battery achieved by the synergy of a vital multifunctional structure design and a unique composite separator is proposed and fabricated. The structure can effectively prevent the invasion of water from the environment and combustion, which is further significantly consolidated with the help of a polyimide and poly(vinylidene fluoride-co-hexafluoropropylene) composite separator, which holds good water resistance, thermal stability, and ionic conductivity. Unexpectedly, the obtained lithium-oxygen battery exhibits superior flexibility, water resistance, thermal resistance, and cycling stability (up to 218 cycles; at a high current of 1 mA and capacity of 4 mA h). This novel water/fireproof, flexible lithium-oxygen battery is a promising candidate to power underwater flexible electronics.

Amorphizing of Au Nanoparticles by CeOx -RGO Hybrid Support towards Highly Efficient Electrocatalyst for N2 Reduction under Ambient Conditions.

Ammonia synthesis is one of the most kinetically complex and energetically challenging chemical processes in industry and has used the Harber-Bosch catalyst for over a century, which is processed under both harsh pressure (150-350 atm) and hightemperature (623-823 K), wherein the energy and capital intensive Harber-Bosch process has a huge energy cost accounting for about 1%-3% of human's energy consumption. Therefore, there has been a rough and vigorous exploration to find an environmentally benign alternative process. As the amorphous material is in a metastable state and has many "dangling bonds", it is more active than the crystallized one. In this paper, CeOx -induced amorphization of Au nanoparticles anchored on reduced graphite oxide (a-Au/CeOx -RGO) has been achieved by a facile coreduction method under ambient atmosphere. As a proof-of-concept experiment, a-Au/CeOx -RGO hybrid catalyst containing the low noble metal (Au loading is 1.31 wt%) achieves a high Faradaic efficiency (10.10%) and ammonia yield (8.3 µg h-1 mg-1cat. ) at -0.2 V versus RHE, which is significantly higher than that of the crystalline counterpart (c-Au/RGO), and even comparable to the yields and efficiencies under harsh temperatures and/or pressures.

Materials Design and System Construction for Conventional and New-Concept Supercapacitors.

With the development of renewable energy and electrified transportation, electrochemical energy storage will be more urgent in the future. Supercapacitors have received extensive attention due to their high power density, fast charge and discharge rates, and long-term cycling stability. During past five years, supercapacitors have been boomed benefited from the development of nanostructured materials synthesis and the promoted innovation of devices construction. In this review, we have summarized the current state-of-the-art development on the fabrication of high-performance supercapacitors. From the electrode material perspective, a variety of materials have been explored for advanced electrode materials with smart material-design strategies such as carbonaceous materials, metal compounds and conducting polymers. Proper nanostructures are engineered to provide sufficient electroactive sites and enhance the kinetics of ion and electron transport. Besides, new-concept supercapacitors have been developed for practical application. Microsupercapacitors and fiber supercapacitors have been explored for portable and compact electronic devices. Subsequently, we have introduced Li-/Na-ion supercapacitors composed of battery-type electrodes and capacitor-type electrode. Integrated energy devices are also explored by incorporating supercapacitors with energy conversion systems for sustainable energy storage. In brief, this review provides a comprehensive summary of recent progress on electrode materials design and burgeoning devices constructions for high-performance supercapacitors.