Showing papers in "Energy and Environmental Science in 2018"

Carbon capture and storage (CCS): the way forward

Abstract: Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

Rational design of electrocatalysts and photo(electro)catalysts for nitrogen reduction to ammonia (NH3) under ambient conditions

Abstract: As one of the most important chemicals and carbon-free energy carriers, ammonia (NH3) has a worldwide annual production of ∼150 million tons, and is mainly produced by the traditional high-temperature and high-pressure Haber–Bosch process which consumes massive amounts of energy. Very recently, electrocatalytic and photo(electro)catalytic reduction of N2 to NH3, which can be performed at ambient conditions using renewable energy, have received tremendous attention. The overall performance of these electrocatalytic and photo(electro)catalytic systems is largely dictated by their core components, catalysts. This perspective for the first time highlights the rational design of electrocatalysts and photo(electro)catalysts for N2 reduction to NH3 under ambient conditions. Fundamental theory of catalytic reaction pathways for the N2 reduction reaction and the corresponding material design principles are introduced first. Then, recently developed electrocatalysts and photo(electro)catalysts are summarized, with a special emphasis on the relationship between their physicochemical properties and NH3 production performance. Finally, the opportunities in this emerging research field, in particular, the strategy of combining experimental and theoretical techniques to design efficient and stable catalysts for NH3 production, are outlined.

CdS-Based photocatalysts

Abstract: To solve the problem of the global energy shortage and the pollution of the environment, in recent years, semiconductor photocatalytic technology that converts solar energy into chemical fuel has been widely studied. Regarding semiconductor-based photocatalysts, CdS has attracted extensive attention due to its relatively narrow bandgap for visible-light response and sufficiently negative potential of the conduction band edge for the reduction of protons. Studies have shown that CdS-based photocatalysts possess excellent photocatalytic performance in terms of solar-fuel generation and environmental purification. This critical review presents the recent advances and progress in the design and synthesis of various CdS and CdS-based photocatalysts. The basic physical and chemical properties of CdS and the related growth mechanism have been briefly summarized. Moreover, the applications of CdS-based photocatalysts have been discussed such as in photocatalytic hydrogen production, reduction of CO2 to hydrocarbon fuels and degradation of pollutants. Finally, a brief perspective on the challenges and future directions for the development of CdS and CdS-based photocatalysts are also presented.

Precision and correctness in the evaluation of electrocatalytic water splitting: revisiting activity parameters with a critical assessment

Abstract: The number of research reports published in recent years on electrochemical water splitting for hydrogen generation is higher than for many other fields of energy research. In fact, electrochemical water splitting, which is conventionally known as water electrolysis, has the potential to meet primary energy requirements in the near future when coal and hydrocarbons are completely consumed. Due to the sudden and exponentially increasing attention on this field, many researchers across the world, including our group, have been exerting immense efforts to improve the electrocatalytic properties of the materials that catalyze the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode, aided by the recent revolutionary discovery of nanomaterials. However, the pressure on the researchers to publish their findings rapidly has caused them to make many unnoticed and unintentional errors, which is mainly due to lack of clear insight on the activity parameters. In this perspective, we have discussed the use and validity of ten important parameters, namely overpotential at a defined current density, iR-corrected overpotential at a defined current density, Tafel slope, exchange current density (j0), mass activity, specific activity, faradaic efficiency (FE), turnover frequency (TOF), electrochemically active surface area (ECSA) and measurement of double layer capacitance (Cdl) for different electrocatalytic materials that are frequently employed in both OER and HER. Experimental results have also been provided in support of our discussions wherever required. Using our critical assessments of the activity parameters of water splitting electrocatalysis, researchers can ensure precision and correctness when presenting their data regarding the activity of an electrocatalyst.

New horizons for inorganic solid state ion conductors

Abstract: Among the contenders in the new generation energy storage arena, all-solid-state batteries (ASSBs) have emerged as particularly promising, owing to their potential to exhibit high safety, high energy density and long cycle life. The relatively low conductivity of most solid electrolytes and the often sluggish charge transfer kinetics at the interface between solid electrolyte and electrode layers are considered to be amongst the major challenges facing ASSBs. This review presents an overview of the state of the art in solid lithium and sodium ion conductors, with an emphasis on inorganic materials. The correlations between the composition, structure and conductivity of these solid electrolytes are illustrated and strategies to boost ion conductivity are proposed. In particular, the high grain boundary resistance of solid oxide electrolytes is identified as a challenge. Critical issues of solid electrolytes beyond ion conductivity are also discussed with respect to their potential problems for practical applications. The chemical and electrochemical stabilities of solid electrolytes are discussed, as are chemo-mechanical effects which have been overlooked to some extent. Furthermore, strategies to improve the practical performance of ASSBs, including optimizing the interface between solid electrolytes and electrode materials to improve stability and lower charge transfer resistance are also suggested.

Isolated Ni single atoms in graphene nanosheets for high-performance CO2 reduction

Abstract: Single-atom catalysts have emerged as an exciting paradigm with intriguing properties different from their nanocrystal counterparts Here we report Ni single atoms dispersed into graphene nanosheets, without Ni nanoparticles involved, as active sites for the electrocatalytic CO2 reduction reaction (CO2RR) to CO While Ni metal catalyzes the hydrogen evolution reaction (HER) exclusively under CO2RR conditions, Ni single atomic sites present a high CO selectivity of 95% under an overpotential of 550 mV in water, and an excellent stability over 20 hours’ continuous electrolysis The current density can be scaled up to more than 50 mA cm−2 with a CO evolution turnover frequency of 21 × 105 h−1 while maintaining 97% CO selectivity using an anion membrane electrode assembly Different Ni sites in graphene vacancies, with or without neighboring N coordination, were identified by in situ X-ray absorption spectroscopy and density functional theory calculations Theoretical analysis of Ni and Co sites suggests completely different reaction pathways towards the CO2RR or HER, in agreement with experimental observations

Li+ intercalated V2O5·nH2O with enlarged layer spacing and fast ion diffusion as an aqueous zinc-ion battery cathode

Abstract: Aqueous zinc-ion batteries (ZIBs) show conspicuous potential in large-scale energy storage systems due to their cost-effectiveness and environmentally friendliness. Yet developmental cathodes in aqueous ZIBs suffer from sluggish Zn2+ diffusion kinetics. Herein, we introduce an effective strategy by the chemical intercalation of Li+ into the interlayer of V2O5·nH2O, i.e. LixV2O5·nH2O (LVO), with enlarged layer spacing and fast Zn2+ diffusion. As a cathode in aqueous ZIBs with a 2 M ZnSO4 electrolyte, the cotton-like LVO-250 demonstrates high rate capacities and excellent cycling performance (232 mA h g−1 after 500 cycles at 5 A g−1, and 192 mA h g−1 after 1000 cycles at 10 A g−1). The electrochemical reaction kinetics and zinc storage mechanism are investigated in detail.

Construction of hierarchical Ni–Co–P hollow nanobricks with oriented nanosheets for efficient overall water splitting

Abstract: Complex nano-architectures with ordered two-dimensional (2D) building blocks are a class of promising electrocatalysts for different electrochemical technologies. In this work, a novel template-engaged strategy followed by sequential etching and phosphorization treatments is demonstrated to fabricate open and hierarchical Ni–Co–P hollow nanobricks (HNBs) via the assembly of oriented 2D nanosheets. Benefiting from the unique nano-architectures with large electrolyte-accessible surface and abundant mass diffusion pathways, the as-prepared Ni–Co–P HNBs exhibit high electrocatalytic activity, which affords the current density of 10 mA cm−2 at low overpotentials of 270 mV and 107 mV for oxygen and hydrogen evolution reactions respectively, and excellent stability in an alkaline medium. Remarkably, when used as both the anode and cathode, a low cell voltage of 1.62 V is required to reach the current density of 10 mA cm−2, making the Ni–Co–P HNBs an efficient bifunctional electrocatalyst for overall water splitting.

Membrane distillation at the water-energy nexus: limits, opportunities, and challenges

Abstract: Energy-efficient desalination and water treatment technologies play a critical role in augmenting freshwater resources without placing an excessive strain on limited energy supplies. By desalinating high-salinity waters using low-grade or waste heat, membrane distillation (MD) has the potential to increase sustainable water production, a key facet of the water-energy nexus. However, despite advances in membrane technology and the development of novel process configurations, the viability of MD as an energy-efficient desalination process remains uncertain. In this review, we examine the key challenges facing MD and explore the opportunities for improving MD membranes and system design. We begin by exploring how the energy efficiency of MD is limited by the thermal separation of water and dissolved solutes. We then assess the performance of MD relative to other desalination processes, including reverse osmosis and multi-effect distillation, comparing various metrics including energy efficiency, energy quality, and susceptibility to fouling. By analyzing the impact of membrane properties on the energy efficiency of an MD desalination system, we demonstrate the importance of maximizing porosity and optimizing thickness to minimize energy consumption. We also show how ineffective heat recovery and temperature polarization can limit the energetic performance of MD and how novel process variants seek to reduce these inefficiencies. Fouling, scaling, and wetting can have a significant detrimental impact on MD performance. We outline how novel membrane designs with special surface wettability and process-based fouling control strategies may bolster membrane and process robustness. Finally, we explore applications where MD may be able to outperform established desalination technologies, increasing water production without consuming large amounts of electrical or high-grade thermal energy. We conclude by discussing the outlook for MD desalination, highlighting challenges and key areas for future research and development.

Correction: Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage

Abstract: Correction for ‘Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage’ by Turgut M. Gur, Energy Environ. Sci., 2018, DOI: 10.1039/c8ee01419a.

An extremely safe and wearable solid-state zinc ion battery based on a hierarchical structured polymer electrolyte

Abstract: Flexible and safe batteries, coupled with high performance and low cost, constitute a radical advance in portable and wearable electronics, especially considering the fact that these flexible devices are likely to experience more mechanical impacts and potential damage than well-protected rigid batteries. However, flexible lithium ion batteries (LIBs) are vastly limited by their intrinsic safety and cost issues. Here we introduce an extremely safe and wearable solid-state zinc ion battery (ZIB) comprising a novel gelatin and PAM based hierarchical polymer electrolyte (HPE) and an α-MnO2 nanorod/carbon nanotube (CNT) cathode. Benefiting from the well-designed electrolyte and electrodes, the flexible solid-state ZIB delivers a high areal energy density and power density (6.18 mW h cm−2 and 148.2 mW cm−2, respectively), high specific capacity (306 mA h g−1) and excellent cycling stability (97% capacity retention after 1000 cycles at 2772 mA g−1). More importantly, the solid-state ZIB offers a high wearability and an extreme safety performance over conventional flexible LIBs, and performs very well under various severe conditions, such as being greatly cut, bent, hammered, punctured, sewed, washed in water or even put on fire. In addition, flexible ZIBs were integrated in series to power a commercial smart watch, a wearable pulse sensor, and a smart insole, which has been achieved to the best of our knowledge for the first time. These results demonstrate the promising potential of ZIBs in many practical wearable applications and offer a new platform for flexible and wearable energy storage technologies.

A salt-rejecting floating solar still for low-cost desalination

Abstract: Although desalination technologies have been widely adopted as a means to produce freshwater, many of them require large installations and access to advanced infrastructure. Recently, floating structures for solar evaporation have been proposed, employing the concept of interfacial solar heat localization as a high-efficiency approach to desalination. However, the challenge remains to prevent salt accumulation while simultaneously maintaining heat localization. This paper presents an experimental demonstration of a salt-rejecting evaporation structure that can operate continuously under sunlight to generate clean vapor while floating in a saline body of water such as an ocean. The evaporation structure is coupled with a low-cost polymer film condensation cover to produce freshwater at a rate of 2.5 L m−2 day−1, enough to satisfy individual drinking needs. The entire system's material cost is $3 m−2 – over an order of magnitude lower than conventional solar stills, does not require energy infrastructure, and can provide cheap drinking water to water-stressed and disaster-stricken communities.

A hydrogel-based antifouling solar evaporator for highly efficient water desalination

Abstract: Solar desalination is a promising method for large-scale water purification by utilizing sustainable energy. However, current high-rate solar evaporation often relies on optical concentration due to the diffusion of natural sunlight, which leads to inadequate energy supply. Here we demonstrate a hydrogel-based solar evaporator that is capable of generating vapor at a high rate of ∼2.5 kg m−2 h−1 under one sun irradiation (1 kW m−2), among the best values reported in the literature. Such highly efficient solar evaporation is achieved by a hybrid hydrogel composed of a hydrophilic polymer framework (polyvinyl alcohol, PVA) and solar absorber (reduced graphene oxide, rGO), which has internal capillary channels. The PVA can greatly facilitate the water evaporation owing to the reduced water evaporation enthalpy in the hydrogel network. The rGO penetrating into the polymeric network enables efficient energy utilization. The capillary channels sustain an adequate water supply for continuous solar vapor generation at a high rate. This hydrogel-based solar evaporator also exhibits promising antifouling properties, enabling long-term water desalination without recycling. The high-efficiency hydrogel-based solar vapor generators open significant opportunities to enhance solar water evaporation performance and reduce the cost of solar desalination systems.

Interpretation and evolution of open-circuit voltage, recombination, ideality factor and subgap defect states during reversible light-soaking and irreversible degradation of perovskite solar cells

Abstract: Metal halide perovskite absorber materials are about to emerge as a high-efficiency photovoltaic technology. At the same time, they are suitable for high-throughput manufacturing characterized by a low energy input and abundant low-cost materials. However, a further optimization of their efficiency, stability and reliability demands a more detailed optoelectronic characterization and understanding of losses including their evolution with time. In this work, we analyze perovskite solar cells with different architectures (planar, mesoporous, HTL-free), employing temperature dependent measurements (current–voltage, light intensity, electroluminescence) of the ideality factor to identify dominating recombination processes that limit the open-circuit voltage (Voc). We find that in thoroughly-optimized, high-Voc (≈1.2 V) devices recombination prevails through defects in the perovskite. On the other hand, irreversible degradation at elevated temperature is caused by the introduction of broad tail states originating from an external source (e.g. metal electrode). Light-soaking is another effect decreasing performance, though reversibly. Based on FTPS measurements, this degradation is attributed to the generation of surface defects becoming a new source of non-radiative recombination. We conclude that improving long-term stability needs to focus on adjacent layers, whereas a further optimization of efficiency of top-performing devices requires understanding of the defect physics of the nanocrystalline perovskite absorber. Finally, our work provides guidelines for the design of further dedicated studies to correctly interpret the diode ideality factor and decrease recombination losses.

Coordinatively unsaturated nickel–nitrogen sites towards selective and high-rate CO2 electroreduction

Abstract: High Faradaic efficiency and appreciable current density are essential for future applications of the electrochemical CO2 reduction reaction (CO2RR). However, these goals are difficult to achieve simultaneously due to the severe side reaction – the hydrogen evolution reaction (HER). Herein, we successfully synthesized coordinatively unsaturated nickel–nitrogen (Ni–N) sites doped within porous carbon with a nickel loading as high as 5.44 wt% by pyrolysis of Zn/Ni bimetallic zeolitic imidazolate framework-8. Over the Ni–N composite catalysts, the CO current density increases with the overpotential and reaches 71.5 ± 2.9 mA cm−2 at −1.03 V (vs. a reversible hydrogen electrode, RHE), while maintaining a high CO Faradaic efficiency of 92.0–98.0% over a wide potential range of −0.53 to −1.03 V (vs. the RHE). Density functional theory calculations suggest that the CO2RR occurs more easily than the HER over the coordinatively unsaturated Ni–N site. Therefore, highly doped and coordinatively unsaturated Ni–N sites achieve high current density and Faradaic efficiency of the CO2RR simultaneously, breaking current limits in metal–nitrogen composite catalysts.

Recent progress on sodium ion batteries: potential high-performance anodes

Abstract: Due to massively growing demand arising from energy storage systems, sodium ion batteries (SIBs) have been recognized as the most attractive alternative to the current commercialized lithium ion batteries (LIBs) owing to the wide availability and accessibility of sodium. Unfortunately, the low energy density, inferior power density and poor cycle life are still the main issues for SIBs in the current drive to push the entire technology forward to meet the benchmark requirements for commercialization. Over the past few years, tremendous efforts have been devoted to improving the performance of SIBs, in terms of higher energy density and longer cycling lifespans, by optimizing the electrode structure or the electrolyte composition. In particular, among the established anode systems, those materials, such as metals/alloys, phosphorus/phosphides, and metal oxides/sulfides/selenides, that typically deliver high theoretical sodium-storage capacities have received growing interest and achieved significant progress. Although some review articles on electrodes for SIBs have been published already, many new reports on these anode materials are constantly emerging, with more promising electrochemical performance achieved via novel structural design, surface modification, electrochemical performance testing techniques, etc. So, we herein summarize the most recent developments on these high-performance anode materials for SIBs in this review. Furthermore, the different reaction mechanisms, the challenges associated with these materials, and effective approaches to enhance performance are discussed. The prospects for future high-energy anodes in SIBs are also discussed.

Ni–Mo–O nanorod-derived composite catalysts for efficient alkaline water-to-hydrogen conversion via urea electrolysis

Abstract: Photo/electrochemical splitting of water to hydrogen (H2) fuel is a sustainable way of meeting our energy demands at no environmental cost, but significant challenges remain: for example, the sluggish anodic reaction imposes a considerable overpotential requirement. By contrast, urea electrolysis offers the prospect of energy-saving H2 production together with urea-rich wastewater purification, whereas the lack of inexpensive and efficient urea oxidation reaction (UOR) catalysts places constraints on the development of this technique. Here we report a porous rod-like NiMoO4 with high oxidation states of the metal elements enabling highly efficient UOR electrocatalysis, which can be readily produced through annealing solid NiMoO4·xH2O as a starting precursor in Ar. This precursor gives the derived Ni/NiO/MoOx nanocomposite when switching the shielding gas from Ar to H2/Ar, exhibiting platinum-like activity for the hydrogen evolution reaction (HER) in alkaline electrolytes. Assembling an electrolytic cell using our developed UOR and HER catalysts as the anode and cathode can provide a current density of 10 milliamperes per square centimeter at a cell voltage of mere 1.38 volts, as well as remarkable operational stability, representing the best yet reported noble-metal-free urea electrolyser. Our results demonstrate the potential of nickel–molybdenum-based materials as efficient electrode catalysts for urea electrolysers that promises cost-effective and energy-saving H2 production.

Aqueous vs. nonaqueous Zn-ion batteries: consequences of the desolvation penalty at the interface

Abstract: Zinc ion batteries using metallic zinc as the negative electrode have gained considerable interest for electrochemical energy storage, whose development is crucial for the adoption of renewable energy technologies, as zinc has a very high volumetric capacity (5845 mA h cm−3), is inexpensive and compatible with aqueous electrolytes. However, the divalent charge of zinc ions, which restricts the choice of host material due to hindered solid-state diffusion, can also pose a problem for interfacial charge transfer. Here, we report our findings on reversible intercalation of up to two Zn2+ ions in layered V3O7·H2O. This material exhibits very high capacity and power (375 mA h g−1 at a 1C rate, and 275 mA h g−1 at an 8C rate) in an aqueous electrolyte compared to a very low capacity and slow rate capabilities in a nonaqueous medium. Operando XRD studies, together with impedance analysis, reveal solid solution behavior associated with Zn2+-ion diffusion within a water monolayer in the interlayer gap in both systems, but very sluggish interfacial charge transfer in the nonaqueous electrolyte. This points to desolvation at the interface as a major factor in dictating the kinetics. Temperature dependent impedance studies show high activation energies associated with the nonaqueous charge transfer process, identifying the origin of poor electrochemical performance.

Dissolution, migration, and deposition of transition metal ions in Li-ion batteries exemplified by Mn-based cathodes – a critical review

Abstract: Unlike the revolutionary advances in the anodes of lithium-ion batteries from Li intercalation materials to Li alloy and/or conversion reaction materials, the development of the cathode is still dominated by the Li intercalation compounds. Transition metal ions are essential in these cathodes as the rapid redox reaction centers, and one of the biggest challenges for the TM-based cathodes is the capacity and power fading especially at an elevated temperature, which is directly associated with the dissolution–migration–deposition (DMD) process of TMs from the cathode materials. This process not only alters the surface structure of the cathode materials, but more importantly, changes the SEI composition at the anode side. There is no doubt that the TM-DMD issue should be addressed thoroughly to unlock the potential of these compounds to enable a prolonged battery lifetime. This review article mainly focuses on research activities with regard to the DMD process in TM-based cathode materials. In the first four sections, we choose Mn-based cathodes as an example to discuss how Mn DMD relates to the capacity fade of the cell, and what possible approaches might suppress the DMD process by modification of the electrode or electrolyte. In the fifth section, we discuss the TM DMD process in Ni-, Co-, Fe- and V-containing cathode materials. This article reviews the frontier electrochemical research on TM-based cathodes and summarizes the progress and challenges, thereby helping to advance future R&D of LIBs.

Vertical Co9S8 hollow nanowall arrays grown on a Celgard separator as a multifunctional polysulfide barrier for high-performance Li–S batteries

Abstract: Lithium–sulfur (Li–S) batteries have been regarded as one of the most promising next-generation energy-storage devices, due to their low cost and high theoretical energy density (2600 W h kg−1). However, the severe dissolution of lithium polysulfides (LiPSs) and the fatal shuttle effect of the sulfur cathode seriously hinder the practical applications of Li–S batteries. To address such issues, we present here, for the first time, a novel metal organic framework (MOF)-derived Co9S8 nanowall array with vertical hollow nanoarchitecture and high electrical conductivity, which is grown in situ on a Celgard separator (Co9S8–Celgard) via a feasible and scalable liquid-reaction approach, as an efficient barrier for LiPSs in Li–S batteries. Benefiting from the direct in situ growth of vertical Co9S8 hollow nanowall arrays as a multifunctional polar barrier, the Co9S8–Celgard separator possesses large surface area, excellent mechanical stability, and particularly strong LiPS-trapping ability via chemical and physical interactions. With these advantages, even with a pure sulfur cathode with a high sulfur loading of 5.6 mg cm−2, the Li–S cells with the Co9S8–Celgard separator exhibit outstanding electrochemical performance: the initial specific capacity is as high as 1385 mA h g−1 with a retention of 1190 mA h g−1 after 200 cycles. The cells deliver a high capacity of 530 mA h g−1 at a 1C rate (1675 mA g−1) even after an impressive number of 1000 cycles with an average capacity fade of only 0.039% per cycle, which is promising for long-term cycling application at high charge/discharge current densities, and pouch-type Li–S cells with the Co9S8–Celgard separator display excellent cycling performance. When the optimized cathode with the sulfur loading in well-designed yolk–shelled carbon@Fe3O4 (YSC@Fe3O4) nanoboxes is employed, the cell with Co9S8–Celgard delivers a high initial capacity of 986 mA h g−1 at a 1C rate with a capacity retention as high as 83.2% even after a remarkable number of 1500 cycles. This work presents a strategy to grow on the separator a multifunctional polar interlayer with unique nanoarchitecture and high conductivity to chemically and physically trap the LiPSs, thus significantly enhancing the performance of Li–S batteries.

Iodine chemistry determines the defect tolerance of lead-halide perovskites

Abstract: Metal-halide perovskites are outstanding materials for photovoltaics Their long carrier lifetimes and diffusion lengths favor efficient charge collection, leading to efficiencies competing with established photovoltaics These observations suggest an apparently low density of traps in the prototype methylammonium lead iodide (MAPbI3) contrary to the expected high defect density of a low-temperature, solution-processed material Combining first-principles calculations and spectroscopic measurements we identify less abundant iodine defects as the source of photochemically active deep electron and hole traps in MAPbI3 The peculiar iodine redox chemistry leads, however, to kinetic deactivation of filled electron traps, leaving only short-living hole traps as potentially harmful defects Under mild oxidizing conditions the amphoteric hole traps can be converted into kinetically inactive electron traps, providing a rationale for the defect tolerance of metal-halide perovskites Bromine and chlorine doping of MAPbI3 also inactivate hole traps, possibly explaining the superior optoelectronic properties of mixed-halide perovskites

Chemo-mechanical expansion of lithium electrode materials – on the route to mechanically optimized all-solid-state batteries

Abstract: Charge and discharge of lithium ion battery electrodes is accompanied by severe volume changes. In a confined space, the volume cannot expand, leading to significant pressures induced by local microstructural changes within the battery. While volume changes appear to be less critical in batteries with liquid electrolytes, they will be more critical in the case of lithium ion batteries with solid electrolytes and they will be even more critical and detrimental in the case of all-solid-state batteries with a lithium metal electrode. In this work we first summarize, compare, and analyze the volume changes occurring in state of the art electrode materials, based on crystallographic studies. A quantitative analysis follows that is based on the evaluation of the partial molar volume of lithium as a function of the degree of lithiation for different electrode materials. Second, the reaction volumes of operating full cells (“charge/discharge volumes”) are experimentally determined from pressure-dependent open-circuit voltage measurements. The resulting changes in the open-circuit voltage are in the order of 1 mV/100 MPa, are well measurable, and agree with changes observed in the crystallographic data. Third, the pressure changes within solid-state batteries are approximated under the assumption of incompressibility, i.e. for constant volume of the cell casing, and are compared to experimental data obtained from model-type full cells. In addition to the understanding of the occurring volume changes of electrode materials and resulting pressure changes in solid-state batteries, we propose “mechanical” blending of electrode materials to achieve better cycling performance when aiming at “zero-strain” electrodes.

Electrode–electrolyte interfaces in lithium-based batteries

Abstract: The electrode–electrolyte interface has been a critical concern since the birth of lithium(Li)-based batteries (lithium or Li+-ion batteries) that are operated with liquid electrolytes and in recent years to increase the operating voltages. The electrode–electrolyte interfacial behavior has also been in sharp focus with respect to intensively pursued solid-electrolyte-based (polymer- or ceramic-electrolyte) lithium and Li+-ion batteries. Understanding the relevant chemical/electrochemical reactions, structural/compositional characteristics, and thermodynamic/kinetic behaviors at the electrode–electrolyte interface is of paramount importance for the development of strategies to enhance overall battery performances. Although the relevant research has been emphasized for many years, both a fundamental understanding of the interfacial phenomena and the practical strategies for enhancing the interfacial properties are still limited or vague. Due to the complexity involved in the interfacial behavior, the future research and development require collaborative efforts involving the disciplines of chemistry, physics, materials science, nanoscience/nanotechnology, as well as computational modeling/simulation. This review presents the key findings, recent progress, current status, and a bold perspective/vision for further understanding and manipulating the electrode–electrolyte interfaces in lithium and Li+-ion batteries. The information provided in this review is expected to benefit the current Li+-ion technologies and future-generation solid-state lithium and Li+-ion batteries that are based on polymer electrolytes or ceramic solid electrolytes.

Synchronous immobilization and conversion of polysulfides on a VO2–VN binary host targeting high sulfur load Li–S batteries

Abstract: Lithium–sulfur (Li–S) batteries are deemed as one of the most promising next-generation energy storage systems. However, their practical application is hindered by existing drawbacks such as poor cycling life and low Coulombic efficiency due to the shuttle effect of lithium polysulfides (LiPSs). We herein present an in situ constructed VO2–VN binary host which combines the merits of ultrafast anchoring (VO2) with electronic conducting (VN) to accomplish smooth immobilization–diffusion–conversion of LiPSs. Such synchronous advantages have effectively alleviated the polysulfide shuttling, promoted the redox kinetics, and hence improved the electrochemical performance of Li–S batteries. As a result, the sulfur cathode based on the VO2–VN/graphene host exhibited an impressive rate capability with ∼1105 and 935 mA h g−1 at 1C and 2C, respectively, and maintained long-term cyclability with a low capacity decay of 0.06% per cycle within 800 cycles at 2C. More remarkably, favorable cyclic stability can be attained with a high sulfur loading (13.2 mg cm−2). Even at an elevated temperature (50 °C), the cathodes still delivered superior rate capacity. Our work emphasizes the importance of immobilization–diffusion–conversion of LiPSs toward the rational design of high-load and long-life Li–S batteries.

A search for selectivity to enable CO2 capture with porous adsorbents

Abstract: Fundamental aspects and actual developments of selective CO2 capture from relevant sources (flue gas or air) by reversible physisorption are critically reviewed. Thermodynamic as well as kinetic principles of CO2 adsorption in the presence of other gases are linked to current approaches of materials development. Whilst hundreds or even thousands of porous materials have been evaluated for CO2 capture, research in this field is still full of challenges, as for instance a feasible physical adsorbent for CO2 capture for direct capture from air has still not been found. Current attempts towards the optimization of materials in terms of CO2 uptake/selectivity, regenerability, tolerance against water, and cost most often exclude each other. The aim of this article is not to summarize all recent attempts towards tailoring of materials for selective CO2 capture but to discuss the most fundamental aspects of adsorptive CO2 separation in order to illuminate the “sweet spot” to be explored when electronic structure, polarity, and pore size/geometry are rationally balanced and optimized – just like nature does when exerting selective binding of gases.

Nanoconfined phase change materials for thermal energy applications

Abstract: Phase change materials (PCMs) have been extensively characterized as constant temperature latent heat thermal energy storage (TES) materials. Nevertheless, the widespread utilization of PCMs is limited due to the flow of liquid PCMs during melting, phase separation, supercooling and low heat transfer rate. In order to overcome these inherent problems and to improve thermo-physical properties, the confinement of PCMs at the nanoscale has been identified as a versatile strategy, which ensures the encapsulation of PCMs in much smaller nano-containers. Such strategies including core–shell, longitudinal, interfacial and porous confinement have been widely presented in recent years to efficiently encapsulate PCMs in nanospaces and are presenting attractive ways to enhance thermal performance. This review summarizes the recent advancement and critical issues of nanoconfinement technologies of PCMs from the point of view of material design. In addition, the potential applications of nanoconfined PCMs in diverse fields, including energy conversion and storage, thermal rectification and temperature controlled drug delivery systems, are presented in detail. Finally, the major drawbacks associated with nanoconfined PCMs and their prospective solutions are also provided.

Synergistic effect of well-defined dual sites boosting the oxygen reduction reaction

Abstract: Herein, we construct a novel electrocatalyst with Fe–Co dual sites embedded in N-doped carbon nanotubes ((Fe,Co)/CNT), which exhibits inimitable advantages towards the oxygen reduction reaction. The electrocatalyst shows state-of-the-art ORR performance with an admirable onset potential (Eonset, 1.15 V vs. 1.05 V) and half-wave potential (E1/2, 0.954 V vs. 0.842 V), outperforming those of the commercial Pt/C. The ORR test reveals that the performance of the (Fe,Co)/CNT is superior to most of the reported non-precious catalysts in alkaline electrolytes. Furthermore, when employed as a cathode catalyst in a Zn–air battery, the (Fe,Co)/CNT exhibits high voltages of 1.31 V and 1.23 V at discharge current densities of 20 mA cm−2 and 50 mA cm−2, respectively. In addition, the power density and the specific energy density reach 260 mW cm−2 and 870 W h kgZn−1. We discover that the Fe–Co dual sites embedded in N-doped porous carbon are beneficial for the activation of oxygen by weakening the OO bonds.

Efficient visible-light-driven selective oxygen reduction to hydrogen peroxide by oxygen-enriched graphitic carbon nitride polymers

Abstract: H2O2 is a green, environmentally friendly potential energy source. The photocatalytic reduction of molecular oxygen to synthesise H2O2 is an eco-friendly strategy compared with the anthraquinone method and H2/O2 direct synthesis. We proposed oxygen-enriched carbon nitride polymer (OCN) models, which were proven to more easily produce 1,4-endoperoxide species and have a high selectivity for molecular oxygen reduction to H2O2, rather than superoxide radicals, through theoretical calculations and experiments. The apparent quantum yield for H2O2 production by OCNs reached 10.2% at 420 nm under an O2 atmosphere, which was 3.5 times higher than that of g-C3N4 and the activity did not decay over 20 h. OCN has a better oxygen reducibility and electron–hole separation efficiency than g-C3N4 and is more prone to 2-electron reduction in the ORR. This work promotes understanding of the mechanism of photocatalytic oxygen reduction and provides a new idea for the design and synthesis of new materials for the preparation of H2O2.

Initiating a mild aqueous electrolyte Co3O4/Zn battery with 2.2 V-high voltage and 5000-cycle lifespan by a Co(III) rich-electrode

Abstract: The Zn/Co3O4 battery is one of the few aqueous electrolyte batteries with a potential >2 V voltage. Unfortunately, so far, all reported Zn/Co3O4 batteries are using an alkaline electrolyte, resulting in poor cycling stability and environmental problems. Here, we report a Co(III) rich-Co3O4 nanorod material with vastly improved electrochemical kinetics. Zn/Co(III) rich-Co3O4 batteries can work well in ZnSO4 with a CoSO4 additive aqueous solution as a mild electrolyte, delivering a high voltage of 2.2 V, a capacity of 205 mA h g−1 (Co3O4) and an extreme cycling stability of 92% capacity retention even after 5000 cycles. Further mechanistic study reveals a conversion reaction between in situ formed CoO and Co3O4, which has never been observed in an alkaline Zn/Co3O4 battery. Subsequently, a flexible solid-state battery is constructed and reveals high flexibility and a high energy density of 360.8 W h kg−1 at a current density of 0.5 A g−1. Our research initiates the first Zn/Co3O4 battery working in a mild electrolyte, resulting in excellent electrochemical performance. It also indicates that the electrochemical kinetics can be effectively enhanced by fine tuning the atomic structure of electrode materials, opening a new door to improve the performance of aqueous electrolyte batteries.

Water splitting by electrolysis at high current densities under 1.6 volts

Abstract: Splitting water into hydrogen and oxygen by electrolysis using electricity from intermittent waste heat, wind, or solar energies is one of the easiest and cleanest methods for high-purity hydrogen production and an effective way to store the excess electrical power. The key dilemma for efficient large-scale production of hydrogen by splitting of water via the hydrogen and oxygen evolution reactions (HER and OER, respectively) is the high overpotential required, especially for the OER. We report an exceptionally active and durable OER catalyst yielding current densities of 500 and 1000 mA cm−2 at overpotentials of only 259 mV and 289 mV in alkaline electrolyte, respectively, fulfilling the commercial criteria of the OER process. Together with a good HER catalyst, we have achieved the commercially required current densities of 500 and 1000 mA cm−2 at 1.586 and 1.657 V, respectively, with very good stability, dramatically lower than any previously reported voltage. This discovery sets the stage for large-scale hydrogen production by water splitting using excess electrical power whenever and wherever available.