Energy & Environmental Science

Activation of a water molecule by the electrochemical oxidation of a Mn-aquo complex accompanied by the loss of protons is reported. The sequential (2 × 1 electron/1 proton) and direct (2 electron/2 proton) proton-coupled electrochemical oxidation of a non-porphyrinic six-coordinated Mn(II)OH2 complex into a mononuclear Mn(O) complex is described. The intermediate Mn(III)OH2 and Mn(III)OH complexes are electrochemically prepared and analysed. Complete deprotonation of the coordinated water molecule in the Mn(O) complex is confirmed by electrochemical data while the analysis of EXAFS data reveals a gradual shortening of an Mn-O bond upon oxidation from Mn(II)OH2 to Mn(III)OH and Mn(O). Reactivity experiments, DFT calculations and XANES pre-edge features provide strong evidence that the bonding in Mn(O) is best characterized by a Mn(III)-oxyl description. Such oxyl species could play a crucial role in natural and artificial water splitting reactions. We provide here a synthetic example for such species, obtained by electrochemical activation of a water ligand.

The goal of this study was to measure spatially and temporally resolved effective diffusion coefficients (D(e)) in biofilms respiring on electrodes. Two model electrochemically active biofilms, Geobacter sulfurreducens PCA and Shewanella oneidensis MR-1, were investigated. A novel nuclear magnetic resonance microimaging perfusion probe capable of simultaneous electrochemical and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) techniques was used. PFG-NMR allowed noninvasive, nondestructive, high spatial resolution in situ D(e) measurements in living biofilms respiring on electrodes. The electrodes were polarized so that they would act as the sole terminal electron acceptor for microbial metabolism. We present our results as both two-dimensional D(e) heat maps and surface-averaged relative effective diffusion coefficient (D(rs)) depth profiles. We found that 1) D(rs) decreases with depth in G. sulfurreducens biofilms, following a sigmoid shape; 2) D(rs) at a given location decreases with G. sulfurreducens biofilm age; 3) average D(e) and D(rs) profiles in G. sulfurreducens biofilms are lower than those in S. oneidensis biofilms-the G. sulfurreducens biofilms studied here were on average 10 times denser than the S. oneidensis biofilms; and 4) halting the respiration of a G. sulfurreducens biofilm decreases the D(e) values. Density, reflected by D(e), plays a major role in the extracellular electron transfer strategies of electrochemically active biofilms.

In this study, we quantified electron transfer rates, depth profiles of electron donor, and biofilm structure of Geobacter sulfurreducens biofilms using an electrochemical-nuclear magnetic resonance microimaging biofilm reactor. Our goal was to determine whether electron donor limitations existed in electron transfer processes of electrode-respiring G. sulfurreducens biofilms. Cells near the top of the biofilms consumed acetate and were metabolically active; however, acetate concentration decreased to below detection within the top 100 microns of the biofilms. Additionally, porosity in the biofilms fell below 10% near the electrode surface, exacerbating exclusion of acetate from the lower regions. The dense biofilm matrix in the acetate-depleted zone acted as an electrical conduit passing electrons generated at the top of the biofilm to the electrode. To verify the distribution of cell metabolic activity, we used uranium as a redox-active probe for localizing electron transfer activity and X-ray absorption spectroscopy to determine the uranium oxidation state. Cells near the top reduced U(VI) more actively than the cells near the base. High-resolution transmission electron microscopy images showed intact, healthy cells near the top and plasmolyzed cells near the base. Contrary to models proposed in the literature, which hypothesize that cells nearest the electrode surface are the most metabolically active because of a lower electron transfer resistance, our results suggest that electrical resistance through the biofilm does not restrict long-range electron transfer. Cells far from the electrode can respire across metabolically inactive cells, taking advantage of their extracellular infrastructure produced during the initial biofilm formation.

The mechanisms and efficiency of charge transport in lithium peroxide (Li2O2) are key factors in understanding the performance of non-aqueous Li-air batteries. Towards revealing these mechanisms, here we use first-principles calculations to predict the concentrations and mobilities of charge carriers and intrinsic defects in Li2O2 as a function of cell voltage. Our calculations reveal that changes in the charge state of O2 dimers controls the defect chemistry and conductivity of Li2O2. Negative lithium vacancies (missing Li+) and small hole polarons are identified as the dominant charge carriers. The electronic conductivity associated with polaron hopping (5 x 10-20 S/cm) is comparable to the ionic conductivity arising from the migration of Li-ions (4 x 10-19 S/cm), suggesting that charge transport in Li2O2 occurs through a mixture of ionic and polaronic contributions. These data indicate that the bulk regions of crystalline Li2O2 are insulating, with appreciable charge transport occurring only at moderately high charging potentials that drive partial delithiation. The implications of limited charge transport on discharge and recharge mechanisms are discussed, and a two-stage charging process linking charge transport, discharge product morphology, and overpotentials is described. We conclude that achieving both high discharge capacities and efficient charging will depend upon access to alternative mechanisms that bypass bulk charge transport. More generally, we describe how the presence of a species that can change charge state - e.g., O2 dimers in alkaline metal-based peroxides - may impact rechargeability in metal-air batteries.

Nowadays, new technologies and breakthroughs in the fields of energy efficiency, alternative fuels and added-value electronics are leading to improved, more environmentally sustainable and green thinking applications. Due to the forecasted rapid increase of volume of air traffic, future aircraft generations have to face enhanced requirements concerning productivity, environmental compatibility and higher operational availability, thus effecting technical, operational and economical aspects of in-flight and on-ground power generation systems, even if air transport is responsible for only about 2% of all anthropogenic CO2 emissions. The trend in new aircraft development is toward ‘‘more electric’’ architectures which is characterized by a higher proportion of electrical systems substituting hydraulically or pneumatically driven components, and, as a result, increasing the amount of electrical power. Fuel cell systems in this context represent a promising solution regarding the enhancement of the energy efficiency for both cruise and ground operations. For several years the Institute of Technical Thermodynamics of the German Aerospace Center (Deutsches Zentrum f€ur Luft- und Raumfahrt, DLR) in Stuttgart and Hamburg has developed fuel cell systems for aircraft applications. The activities of DLR focus on: identification of fuel cell applications in aircraft in which the properties of fuel cell systems, namely high electric efficiency, low emissions and silent operation, are capitalized for the aircraft application; design and modeling of possible and advantageous system designs; theoretical and experimental investigations regarding specific aircraft relevant operating conditions; qualification of airworthy fuel cell systems; set up and full scale testing of fuel cell systems for application in research aircraft. In cooperation with Airbus, several fuel cell applications within the aircraft for both ground and cruise operation have been identified. As a consequence, fuel cell systems capable of supporting or even replacing existing systems have been derived. In this context, the provision of inert gas for the jet fuel (kerosene) tank and electrical cabin power supply, including water regeneration, represent the most promising application fields. This paper will present the state of development and the evolution discussing the following points: modeling of different system architectures and evaluation of promising fuel cell systems; experimental evaluation of fuel cell systems under relevant conditions (low pressure, vibrations, reformate operation, etc.); fuel cell test in DLR’s research aircraft ATRA (A320) including the test of an emergency system based on hydrogen and oxygen with 20 kW of electrical power. The fuel cell system was integrated into an A320 aircraft and tested up to a flight altitude of 25 000 feet under several acceleration and inclination conditions; fuel cell tests in Antares-H2—DLR’s new flying test bed.

Ultracapacitors are rapidly being adopted for use for a wide range of electrical energy storage applications. While ultracapacitors are able to deliver high rates of charge and discharge, they are limited in the amount of energy stored. The capacity of ultracapacitors is largely determined by the electrode material and as a result, research to improve the performance of electrode materials has dramatically increased. While test methods for packaged ultracapacitors are well developed, it is often not feasible for the materials scientist to assemble full sized, packaged cells to test electrode materials. Methodology to reliably measure a material's performance for ultracapacitor electrode use is not well standardized with the different techniques currently being used yielding widely varying results. In this manuscript, we review the best practice test methods that accurately predict a materials performance, yet are flexible and quick enough to accommodate a wide range of material sample types and amounts.

We formulate, solve computationally and study experimentally the problem of collecting solar energy in three dimensions(1-5). We demonstrate that absorbers and reflectors can be combined in the absence of sun tracking to build three-dimensional photovoltaic (3DPV) structures that can generate measured energy densities (energy per base area, kWh/m2) higher by a factor of 2-20 than stationary flat PV panels, versus an increase by a factor of 1.3-1.8 achieved with a flat panel using dual-axis sun tracking(6). The increased energy density is countered by a higher solar cell area per generated energy for 3DPV compared to flat panel design (by a factor of 1.5-4 in our conditions), but accompanied by a vast range of improvements. 3DPV structures are steadier sources of solar energy generation at all latitudes: they can double the number of peak power generation hours and dramatically reduce the seasonal, latitude and weather variations of solar energy generation compared to a flat panel design. Self-supporting 3D shapes can create new schemes for PV installation and the increased energy density can facilitate the use of cheaper thin film materials in area-limited applications. Our findings suggest that harnessing solar energy in three dimensions can open new avenues towards Terawatt-scale generation.

Solar radiation is the largest indigenous energy resource worldwide. It will gain a significantly more relevant role in covering the energy demand of many countries when national fuel reserves fall short and when demand increases as is expected within the next 10 years. If solar energy is transformed into heat by concentrating and absorbing the radiation, energy can be stored easily. Thermal energy from mirror fields that focus solar radiation not only is able to generate electricity but also can be used to generate storable heat, to desalinate salt water or to synthesise fuels from water and carbon dioxide to store, transport or use them on-site. The application of concentrated solar radiation as a primary energy source can help to decarbonise electricity generation and many other sectors to keep the chance of staying within the 2 C goal for limiting the effects of global warming. The aim of the present contribution is to give an overview on the state-of-the art of technologies for solar thermal power production and fuel production and to describe the status and outlook of commercial projects and perspectives of market development.

High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction.

The Comment by Nunes suggests a welcome refinement to an approximation made in the original paper. We show here that Nunes’ refinement is identical to a modified effective thermal conductivity, , where k is the thermal conductivity, ZT is the usual material figure of merit, and  is in the range 0.4 - 0.5. This form of keff was already identified in Section 3.3 of our original paper as an option to improve the accuracy of the calculations and is itself an approximation to the more sophisticated keff analysis of Baranowski, Snyder, and Toberer [J. Appl. Phys. 113, 204904 (2013)]. As noted by Nunes and ourselves, the main downside of such refinements is that they complicate the universality of the main result, the universal cost surface in Fig. 2 of the original paper. The simplified results in the original manuscript are justified and reasonable for ZT ~ 1 or less, for physical insight, scaling, and rapid screening. For the best accuracy in real systems, exact numerical solutions of the coupled cost and power equations are most appropriate, examples of which we have recently published for 30 bulk and thin film materials in Renewable and Sustainable Energy Reviews, 32, 313-327, 2014.

A recent paper by Yee, LeBlanc, Goodson, and Dames provides a powerful approach to the design of a low cost thermoelectric generation system, but makes an unjustified approximation. Avoiding that approximation is straightforward, and in no way undermines the validity of the original approach. It does, however, shift the optimal design of a thermoelectric generator and makes that design material dependent. The difference in cost between a generator designed using the results of the original paper, and one that uses the modification given here, could be as much as a factor of two.

Single-walled carbon nanotube (SWNT) / polyaniline (PANI) hybrid films were prepared by casting the suspension containing well-dispersed SWNTs and CSA-doped PANI. The electrical conductivity of SWNT/PANI film at first increased with the increasing SWNT content and then decreased at high SWNT content, whereas the Seebeck coefficient increased monotonically in the present SWNT content range. Moreover, the electrical conductivity values of the SWNT/PANI composites are much higher than the values calculated based on the series-connected two-component mixture model, whereas the dependence of Seebeck coefficient on the SWNT content fits well with the mixture model. The thermal conductivities increased with the SWNT content, but the increasing rate is much lower than the estimated values on the mixture model. The maximum values of electrical conductivity and Seebeck coefficient of hybrid films are up to 769S/cm and 65V/K. Consequently, the maximum thermoelectric power factor and ZT value at room temperature reach 176μW/mK2 and 0.12, respectively. The optimal TE property of the SWNT/PANI hybrid film is remarkably higher than those of either individual component of the composite, and among the highest values in inorganic-organic composite materials reported so far. The XRD and Raman analyses revealed that the PANI molecules in the composite film have more expanded conformation, and are more orderly arrangement compared with both pure PANI bulk and pure PANI film. The abnormally enhanced thermoelectric performance is attributed to the highly ordered PANI interface layer on the SWNT surface, which formed by the synergetic effect of the chain-expanding by the chemical interactions between PANI and solvent and the chain-ordering of the π-π conjugation between PANI and CNT.

High temperature looping cycles can be used to produce hydrogen or capture CO2 from power stations, though sintering of absorbents is frequently a problem, reducing reactivity. In this work we develop materials, in which the crystal structure and volume of polymorphic materials change with temperature, as active spacers to reduce sintering.

Hybrid organo-metal halide perovskites are an exciting new class of solar absorber materials and have exhibited a rapid increase in solar cell efficiencies throughout the past two years to over 17% in both meso-structured and thin-film device architectures. We observe slow transient effects causing hysteresis in the current-voltage characterization of these devices that can lead to an over- or underestimation of the solar cell device efficiency. We find that the current-voltage (IV) measurement scan direction, measurement delay time, and light and voltage bias conditions prior to measurement can all have a significant impact upon the shape of the measured IV light curves and the apparent device efficiency. We observe that hysteresis-free light IV curves can be obtained at both extremely fast and slow voltage scan rates but only in the latter case are quasi-steady-state conditions achieved for a valid power conversion efficiency measurement. Hysteretic effects are also observed in devices utilizing alternative selective contacts but differ in magnitude and time scale, suggesting that the contact interfaces have a big effect on transients in perovskite-absorber devices. The transient processes giving rise to hysteresis are consistent with a polarization response of the perovskite absorber that results in changes in the photocurrent extraction efficiency of the device. The strong dependence of the hysteresis on light and voltage biasing conditions in thin film devices for a period of time prior to the measurement suggests that photo-induced ion migration may additionally play an important role in device hysteresis. Based on these observations, we provide recommendations for correct measurement and reporting of IV curves for perovskite solar cell devices.

An overview on processes that are relevant in light induced fuel generation, such as water photoelectrolysis or carbon dioxide reduction, is given. Considered processes encompass the photophysics of light absorption, excitation energy transfer to catalytically active sites and interfacial reactions at the catalyst solution phase boundary. The two major routes envisaged for realization of photoelectrocatalytic systems, e.g. bio inspired single photon catalysis and multiple photon inorganic or hybrid tandem cells, are outlined. For development of efficient tandem cell structures that are based on non oxidic semiconductors, stabilization strategies are presented. Physical surface passivation is described using the recently introduced nanoemitter concept which is also applicable in photovoltaic solid state or electrochemical solar cells and first results with p Si and p InP thin films are presented. Solar to hydrogen efficiencies reach 12.1 for homoepitaxial InP thin films covered with Rh nanoislands. In the pursuit to develop biologically inspired systems, enzyme adsorption onto electrochemically nanostructured silicon surfaces is presented and tapping mode atomic force microscopy images of heterodimeric enzymes are shown. An outlook towards future envisaged systems is given

Electrocatalysis plays a key role in the energy conversion processes central to several renewable energy technologies that have been developed to lessen our reliance on fossil fuels. However, the best electrocatalysts for these processes—which include the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), and the redox reactions that enable regenerative liquid-junction photoelectrochemical solar cells—often contain scarce and expensive noble metals, substantially limiting the potential for these technologies to compete with fossil fuels. The considerable challenge is to develop robust electrocatalysts composed exclusively of low-cost, earth-abundant elements that exhibit activity comparable to that of the noble metals. In this review, we summarize recent progress in the development of such high-performance earth-abundant inorganic electrocatalysts (and nanostructures thereof), classifying these materials based on their elemental constituents. We then detail the most critical obstacles facing earth-abundant inorganic electrocatalysts and discuss various strategies for further improving their performance. Lastly, we offer our perspectives on the current directions of earth-abundant inorganic electrocatalyst development and suggest pathways toward achieving performance competitive with their noble metal-containing counterparts.

Lithium–sulfur (Li–S) batteries are highly attractive for future generations of portable electronics and electric vehicles due to their high energy density and potentially low cost. In the past decades, various novel electrodes and electrolytes have been tested to improve Li–S battery performance. However, these designs on electrodes and electrolytes have not fully addressed the problem of low cycling stability of Li–S batteries. Here, we show the role of the separator in the capacity decay of the Li–S battery, namely that it can accommodate a large amount of polysulfides inside which then precipitates as a thick layer of inactive S-related species. Using a thin conductive coating on the separator to prevent the formation of the inactive S-related species layer, we show that the specific capacity and cycling stability of the Li–S battery are both improved significantly compared to the battery with a pristine separator. Combining this separator design with a monodisperse sulfur nanoparticle cathode, we show Li–S batteries with a life of over 500 cycles with an initial specific capacity of 1350 mA h g−1 at C/2 and a cycle decay as low as 0.09% per cycle.

This research introduces an alternative mixed culture fermentation technology for anaerobic digestion to recover valuable products from low grade biomass. In this mixed culture fermentation, organic waste streams are converted to caproate and caprylate as precursors for biodiesel or chemicals. It was found that acetate, as the main intermediate of anaerobic digestion, can be elongated to medium chain fatty acids with six and eight carbon atoms. Mixed microbial communities were able to produce 8.17 g l−1 caproate and 0.32 g l−1 caprylate under methanogenesis-suppressed conditions in a stable batch reactor run. The highest production rate was 25.6 mM C caproate per day with a product yield of 0.6 mol C per mol C. This elongation process occurred with both ethanol and hydrogen as electron donors, demonstrating the flexibility of the process. Microbial characterization revealed that the microbial populations were stable and dominated by relatives of Clostridium kluyveri

Vinyl ethylene sulfite (VES) is studied as a new additive in propylene carbonate (PC)-based electrolyte for lithium ion batteries. The electrochemical results show that the artificial graphite material exhibits excellent electrochemical performance in a PC-based electrolyte with the addition of the proper amount of VES. According to our spectroscopic results, VES is reduced to ROSO2Li (R=C4H6), Li2SO3 and butadiene (C4H6) through an electrochemical process which precedes the decomposition of PC. Furthermore, some of the Li2SO3 could be further reduced to Li2S and Li2O. All of these products are proven to be components of the solid electrolyte interface (SEI ) layer. National Natural Science Foundation of China (NNSFC) [29925310, 20433060, 20473068]; Ministry of Science and Technology, China [2007CB209702]

Carbon dioxide (CO2) capture using solid sorbents has been recognized as a very promising technology that has attracted intense attention from both academic and industrial fields in the last decade. It is astonishing that around 2000 papers have been published from 2011 to 2014 alone, which is less than three years after our first review paper in this journal on solid CO2 sorbents was published. In this short period, much progress has been made and the major research focuses have more or less changed. Therefore, we feel that it is necessary to give a timely update on solid CO2 capture materials, although we still have to keep some important literature results published in earlier years so as to keep the good continuity. We believe this work will benefit researchers working in both academic and industrial areas. In this paper, we still organize the CO2 sorbents according to their working temperatures by classifying them as such: (1) low-temperature (<200 °C), (2) intermediate-temperature (>400 °C), and (3) high-temperature (> 400 °C). Since the sorption capacity, kinetics, recycling stability and cost are important parameters when evaluating a sorbent, these features will be carefully considered and discussed. In addition, due to the huge amounts of cost-effective CO2 sorbents demanded and the importance of waste resources, solid CO2 sorbents prepared from waste resources and their performance are reviewed. Finally, the techno-economic assessments of various CO2 sorbents and technologies in real applications are briefly discussed.

Macroporous structure has attracted great interest for a wide range of applications due to the unique structure-determined physical and chemical properties. In regard to the inherent low surface area, decoration of macroporous structure with small building blocks (including nanoparticles, nanorods, nanowires, nanosheets, etc.) to form macroporous composite materials therefore have been highly pursued very recently. In this review, we summarize the recent advances in synthesis of hierarchical macroporous composite materials and their potential applications as photoelectrode materials for dye-sensitized solar cells (DSSCs), quantum dot-sensitized solar cells (QDSSCs) and photoelectrochemical (PEC) cells.

Photovoltaic devices based on organic semiconductors (OPVs) hold great promise as a cost-effective renewable energy platform because they can be processed from solution and deposited on flexible plastics using roll-to-roll processing. Despite important progress and reported power conversion efficiencies of more than 10% the rather limited stability of this type of devices raises concerns towards future commercialization. The tandem concept allows for both absorbing a broader range of the solar spectrum and reducing thermalization losses. We designed an organic tandem solar cell with an inverted device geometry comprising environmentally stable active and charge-selecting layers. Under continuous white light irradiation, we demonstrate an extrapolated, operating lifetime in excess of one decade. We elucidate that for the current generation of organic tandem cells one critical requirement for long operating lifetimes consists of periodic UV light treatment. These results suggest that new material approaches towards UV-resilient active and interfacial layers may enable efficient organic tandem solar cells with lifetimes competitive with traditional inorganic photovoltaics.

Rechargeable metal-air batteries are widely considered as the next generation high energy density electrochemical storage devices. The performance and rechargeability of these metal-air cells is highly dependent on the positive electrode material, where oxygen reduction and evolution reactions take place. Here, for the first time, we provide a detailed account of the kinetics and rechargeability of sodium-air batteries through a series of carefully designed tests on treated commercial carbon material. Surface area and porous structure of the positive electrode material was modified in order to gain detailed information surrounding the reaction kinetics of sodium-air batteries. The results indicate that discharge capacity is linearly correlated with surface area while morphology of the solid discharge product is strongly dependent on specific surface area and pore size. Furthermore, it was found that the chemical composition of discharge products as well as charging overpotential is affected by discharge reaction rate.

There has been growing concern about the way cultivating biomass for the production of agro-biofuels competes with food production. To avoid this competition biomass production for biofuels will, in the long term, have to be completely decoupled from food production. This is where microalgae have enormous potential. Here we propose a novel process based on microalgae cultivation using dilute fossil CO2 emissions and the conversion of the algal biomass through a catalytic hydrothermal process. The resulting products are methane as a clean fuel and concentrated CO2 for sequestration. The proposed gasification process mineralizes nutrient-bearing organics completely. Here we show that complete gasification of microalgae (Spirulina platensis) to a methane-rich gas is now possible in supercritical water using ruthenium catalysts. 60-70% of the heating value contained in the algal biomass would be recovered as methane. Such an efficient algae-to-methane process opens up an elegant way to tackle both climate change and dependence on fossil natural gas without competing with food production.

Novel mixed-matrix membranes prepared by blending sodium alginate (NaAlg) with polyvinyl alcohol (PVA) and certain heteropolyacids (HPAs), such as phosphomolybdic acid (PMoA), phosphotungstic acid (PWA) and silicotungstic acid (SWA), followed by ex-situ cross-linking with glutaraldehyde (GA) to achieve the desired mechanical and chemical stability, are reported for use as electrolytes in direct methanol fuel cells (DMFCs). NaAlg-PVA-HPA mixed matrices possess a polymeric network with micro-domains that restrict methanol cross-over. The mixed-matrix membranes are characterised for their mechanical and thermal properties. Methanol cross-over rates across NaAlg-PVA and NaAlg-PVA-HPA mixed-matrix membranes are studied by measuring the mass balance of methanol using a density meter. The DMFC using NaAlg-PVA-SWA exhibits a peak power-density of 68 mW cm(-2) at a load current-density of 225 mA cm(-2), while operating at 343 K. The rheological properties of NaAlg and NaAlg-PVA-SWA viscous solutions are studied and their behaviour validated by a non-Newtonian power-law.

The interface stoichiometry of cuprous oxide (Cu2O) was controlled by adjusting the O2 and Zn partial pressures during ZnO sputter deposition and measured by high-resolution X-ray photoelectron spectroscopy of ultrathin (<3 nm) ZnO films on Cu2O wafers. Open circuit voltage measurements for ZnO/Cu2O heterojunctions under AM1.5 illumination were measured and it was found that a stoichiometric interface can achieve the voltage entitlement dictated by the ZnO/Cu2O band alignment, whereas non-stoichiometric interfaces showed large VOC deficits. These results highlight not only the need for stoichiometric interfaces in Cu2O devices, but also a reproducible experimental method for achieving stoichiometric interfaces that could be applied to any potential heterojunction partner. Additionally, valence-band offset measurements indicated changing the interface stoichiometry shifted the band alignment between Cu2O and ZnO, which accounts for the variation in previously reported band offset values.

A current challenge to alkaline polymer electrolyte fuel cells (APEFCs) is the unexpectedly sluggish kinetics of the hydrogen oxidation reaction (HOR). A recently proposed resolution is to enhance the oxophilicity of the catalyst, so as to remove the Had intermediate through reacting with OHad, but this approach is questioned by other researchers. Here we report a clear and convincing test on this problem. By using PtRu/C as the HOR catalyst for APEFC, the peak power density is boosted to 1.0 W/cm2, in comparison to 0.6 W/cm2 when using Pt/C in the anode. Such a remarkable improvement, however, can hardly be explained as an oxophilic effect, because, as monitored by CO stripping, reactive hydroxyl species can generate on certain sites of the Pt surface at more negative potentials than on the PtRu surface in KOH solution. Rather, the incorporation of Ru has posed an electronic effect on weakening the Pt-Had interaction, as revealed by the voltammetric behavior and from density-functional calculations, which thus benefits the oxidative desorption of Had, the rate determining step of HOR in alkaline media. These findings further our fundamental understanding of the HOR catalysis, and cast a new light on the exploration of better catalysts for APEFC.

Although II-VI semiconductors such as CdS, CdTe, CdSe, ZnTe, and alloys thereof, can have nearly ideal band gaps and band-edge positions for the production of solar fuels, II-VI photoanodes are well-known to be unstable towards photocorrosion or photopassivation when in contact with aqueous electrolytes. Atomic-layer deposition (ALD) of amorphous, “leaky” TiO2 films coated with thin films or islands of Ni oxide has been shown to robustly protect Si, GaAs, and other III-V materials from photocorrosion and therefore to facilitate the robust, solar-driven photoelectrochemical oxidation of H2O to O2(g). We demonstrate herein that ALD-deposited 140-nm thick amorphous TiO2 films also effectively protect single crystalline n-CdTe photoanodes from corrosion or passivation. An n-CdTe/TiO2 electrode with a thin overlayer of a Ni-oxide based oxygen-evolution electrocatalyst produced 435 ± 15 mV of photovoltage with a light-limited current density of 21 ± 1 mA cm-2 under 100 mW cm-2 of simulated Air Mass 1.5 illumination. The ALD-deposited TiO2 films are highly optically transparent and electrically conductive. We show that an n-CdTe/TiO2/Ni oxide electrode enables the stable solar-driven oxidation of H2O to O2(g) in strongly alkaline aqueous solutions, where passive, intrinsically safe, efficient systems for solar-driven water-splitting can be operated.

High yields of liquid straight-chain alkanes were obtained directly from cellulosic feedstock in a one-pot biphasic catalytic system. The catalytic reaction proceeds at elevated temperatures under hydrogen pressure in the presence of tungstosilicic acid, dissolved in the aqueous phase, and modified Ru/C, suspended in the organic phase. Tungstosilicic acid is primarily responsible for cellulose hydrolysis and dehydration steps, while the modified Ru/C selectively hydrogenates intermediates en route to the liquid alkanes. Under optimal conditions, microcrystalline cellulose is converted to 82% n-decane-soluble products, mainly n-hexane, within a few hours, with a minimum formation of gaseous and char products. The dominant route to the liquid alkanes proceeds via 5-hydroxymethylfurfural (HMF), whereas the more common pathway via sorbitol appears to be less efficient. High liquid alkane yields were possible through i) selective conversion of cellulose to glucose and further to HMF by gradually heating the reactor, ii) a proper hydrothermal modification of commercial Ru/C to tune its chemoselectivity to furan hydrogenation rather than glucose hydrogenation, and iii) the use of a biphasic reaction system with optimal partitioning of the intermediates and catalytic reactions. The catalytic system is capable of converting subsequent batches of fresh cellulose, enabling accumulation of the liquid alkanes in the organic phase during subsequent runs. Its robustness is illustrated in the conversion of the holocellulosic part of wood sawdust.

Nanostructured photovoltaics has attracted an enormous amount of attention in recent years owing to its potency for significant device performance enhancement over the conventional technologies. Nonetheless, conventional fabrication approaches for nanostructured scaffolds rely on glass or silicon substrates which are costly, brittle and have limited scalability. Meanwhile, rational design guidelines for optical and electrical performance optimization of solar cells are of urgent need for their practical applications. In this work, flexible and quasi-ordered three-dimensional (3-D) nanospike (NSP) arrays are fabricated on a reasonable large scale with well controlled geometry. Systematic investigations by experiments discovered that photovoltaic devices based on NSPs with optimal geometry can accommodate the trade-off between optical absorption and electrical performance, demonstrating a power conversion efficiency of 7.92%, which is among the highest efficiency reported for single junction a-Si:H solar cells on a flexible substrate. Furthermore, we have demonstrated the superior omnidirectional device performance by utilizing such a 3-D NSP. This unique feature is of paramount importance for practical photovoltaic applications.

Ambient-unstable hole transporters and expensive and complicated noble metal electrode deposition are incompatible with the large scale and low-cost production of perovskite solar cells and thus would hamper their commercialization. Herein we report a new modality of perovskite solar cells that do away with the use of conventional hole transporters by directly clamping a selective hole extraction electrode made of candle soot and a deliberately engineered perovskite photoanode. The key soot/perovskite interface, which promotes hole extraction and electron blocking by forming a Schottky junction, was established seamlessly by pre-wetting and reaction embedding the carbon particles. Femtosecond time-resolved photoluminescence revealed a high hole extraction rate at 1.92 ns-1. We have now achieved 11.02 % efficiency, making an important step towards roll-to-roll production of perovskite solar cells.

Two Ru hydride complexes, Cp*Ru(PPh2NBn2)H (1-H) and Cp*Ru(PtBu2NBn2)H (2-H) supported by cyclic PR2NR'2 ligands (Cp* = η5-C5Me5; 1,5-dibenzyl,-3,7-R-1,5-diaza-3,7-diphosphacyclooctane, where R = Ph or tBu) have been developed as electrocatalysts for oxidation of H2 (1.0 atm, 22 °C). The turnover frequency of 2-H is 1.2 s-1 at 22 °C (1.0 atm H2) with an overpotential at Ecat/2 of 0.5 V in the presence of exogenous base, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), while catalysis by 1-H has a turnover frequency of 0.6 s-1 and an overpotential of 0.6 V at Ecat/2. Addition of H2O facilitates oxidation of H2 by 2-H and increases its turnover frequency to 1.9 s-1, while H2O slows down the catalysis by 1-H. In addition, studies of Cp*Ru(dmpm)H (where dmpm = bis(dimethylphosphino)methane), a control complex lacking pendent amines in its diphosphine ligand, confirms the critical roles of the pendant amines of the P2N2 ligands as proton relays in the oxidation of H2.

The first examples of di-branched di-anchoring organic sensitizers were synthesized and used in dye-sensitized solar cells leading to red-shifted IPCE maxima and increased photocurrent when compared to the corresponding mono-branched mono-anchoring dye, yielding power conversion efficiency of 5.7% (4.9% with ionic liquid electrolyte) with enhanced stability under 1 sun conditions from the dianchoring groups.

Given the intermittent nature of solar radiation, large-scale use of solar energy requires an efficient energy storage solution. So far, the only practical way to store such large amounts of energy is in the form of a chemical energy carrier, i.e., a fuel. Photoelectrochemical (PEC) cells offer the ability to convert solar energy directly into chemical energy in the form of hydrogen. Cuprous oxide (Cu2O) is being investigated for photoelectrochemical solar water splitting since it has a band gap of 2.0 eV with favorable energy band positions for water cleavage, it is abundant and environmentally friendly. A major challenge with Cu2O is the limited chemical stability in aqueous environments. We present a simple and low-cost treatment to create a highly stable photocathode configuration for H2 production, consisting of a steam treatment of the multilayer structures. The role of this treatment was investigated and the optimized electrodes showed photocurrents over –5 mA cm–2 with 90% stability over more than 50 h of light chopping (biased at 0 VRHE in pH 5 electrolyte).

We report an aqueous lithium–sulfur battery based on aqueous dissolved Li2S4/Li2S redox couple as the cathode, metallic lithium as the anode and Li1.35Ti1.75Al0.25P2.7Si0.3O12 (LATP) as the separator. A reversible specific capacity of up to 1030 mA h g−1 is attained. Moreover, aqueous lithium–polysulfide batteries have a discharge potential of 2.53 V versus Li+/Li.

The application of inverse opal structured materials is extended to the ceria–zirconia (Ce_(0.5)Zr_(0.5)O_2) system and the significance of material architecture on heterogeneous catalysis, specifically, chemical oxidation, is examined.

Si microwire-array solar cells with Air Mass 1.5 Global conversion efficiencies of up to 7.9% have been fabricated using an active volume of Si equivalent to a 4 μm thick Si wafer. These solar cells exhibited open-circuit voltages of 500 mV, short-circuit current densities (J_(sc)) of up to 24 mA cm^(-2), and fill factors >65% and employed Al_2O_3 dielectric particles that scattered light incident in the space between the wires, a Ag back reflector that prevented the escape of incident illumination from the back surface of the solar cell, and an a-SiN_x:H passivation/anti-reflection layer. Wire-array solar cells without some or all of these design features were also fabricated to demonstrate the importance of the light-trapping elements in achieving a high J_(sc). Scanning photocurrent microscopy images of the microwire-array solar cells revealed that the higher J_(sc) of the most advanced cell design resulted from an increased absorption of light incident in the space between the wires. Spectral response measurements further revealed that solar cells with light-trapping elements exhibited improved red and infrared response, as compared to solar cells without light-trapping elements.

Herein, we introduce an ultra-thin self-powered artificial skin (SPAS) based on a piezoelectric nanogenerator, which harvests stored elastic deformation energy produced by the bending and stretching actions of the skin. This finding is an important step toward building a self-powered “smart skin.”

Paper-based electronics have been considered as one of the most exciting technologies in the near future due to sustainability, low cost and mechanical flexibility etc. Even though there have been numerous studies regarding this technology, there isn’t any available quantitative study on how paper electronics would minimize the impact to the environment. This work aims to give the first detailed analysis regarding this important question. To this end, we for the first time designed and prototyped the paper-based multi-layer printed circuit boards (P-PCB), which show comparable functions to the currently available organic printed circuit boards (O-PCB); yet the P-PCB adopt a “green” preparation process. A life cycle assessment study was performed to quantify the P-PCB’s environmental impacts, e.g. Acidification Potential Global Warming Potential, Human Toxic Potential, and Ozone Layer Depletion Potential etc. Our current research reveals that the P-PCB have about two magnitude lower impact to the environment than O-PCB based on the results of the life cycle assessment, which suggests that the P-PCB technique is beneficial for the environment at the regional or global production level. The current study gives useful information and sheds light on the future technological directions for the various paper based electronics studies.

The conversion of carbon dioxide and water into fuels in a solar refinery presents a potential solution for reducing greenhouse gas emissions, while providing a sustainable source of fuels and chemicals. Towards realizing such a solar refinery, there are many technological advances that must be met in terms of capturing and sourcing the feedstocks (namely CO2, H2O, and solar energy) and in catalytically converting CO2 and H2O. In the first part of this paper, we review the state-of-the-art in solar energy collection and conversion to solar utilities (heat, electricity, and as a photon source for photo-chemical reactions), CO2 capture and separation technology, and non-biological methods for converting CO2 and H2O to fuels. The two principal methods for CO2 conversion include (1) catalytic conversion using solar-derived hydrogen and (2) direct reduction of CO2 using H2O and solar energy. Both hydrogen production and direct CO2 reduction can be performed electro-catalytically, photo-electrochemically, photo-catalytically, and thermochemically. All four of these methods are discussed. In the second part of this paper, we utilize process modeling to assess the energy efficiency and economic feasibility of a generic solar refinery. The analysis demonstrates that the realization of a solar refinery is contingent upon significant technological improvements in all areas described (solar energy capture and conversion, CO2 capture, and catalytic conversion processes).

Conjugated donor (D)-π-acceptor (A) copolymers, PBDT-TPD, PBDT-ttTPD, PBDTT-TPD, and PBDTT-ttTPD, based on a benzodithiophene (BDT) donor unit and thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) acceptor unit were designed and synthesized with different π bridges via Pd-catalyzed Stille-coupling. The π bridges between BDT and TPD were thiophene in PBDT-TPD and PBDTT-TPD, and 6-alkylthieno[3,2-b]thiophene in PBDT-ttTPD and PBDTT-ttTPD. The effects of the π bridges on the optical, electrochemical, and photovoltaic properties of the polymers were investigated, in addition to the film crystallinities and carrier mobilities. Copolymers with the 6-alkylthieno[3,2-b]thiophene π-bridge exhibited high crystallinity and hole mobility. Improved Jsc and FF were obtained to increase the overall power conversion efficiencies (PCE) in inverted single organic photovoltaic cells. A PCE of 6.81% was achieved from the inverted single device fabricated using the PBDTT-ttTPD:PC71BM blend film with 3 vol% 1,8-diiodooctane. A tandem photovoltaic device comprising the inverted PBDTT-ttTPD cell and a PTB7-based cell as the bottom and top cell components, respectively, showed a maximum PCE of 9.35% with a Voc of 1.58 V, Jsc of 8.00 mA/cm2, and FF of 74% under AM 1.5 G illumination at 100 mW/cm2. The obtained PCE of the bottom cell and FF of the tandem cell are, to the best of our knowledge, the highest reported to date for a tandem OPV device. This work demonstrates that PBDTT-ttTPD may be very promising in applications in tandem solar cells. Furthermore, 6-alkylthieno[3,2-b]thiophene π-bridge systems in medium bandgap polymers can improve the performance of tandem organic photovoltaic cells.

Reduced graphene oxide-based films were prepared to assess their effects as gas barriers on the stability of organic photovoltaic (OPV) devices. The direct spin-casting of a graphene oxide suspension onto an aluminum electrode was performed to encapsulate the associated OPV device with a reduced graphene oxide film. The lifetime of the OPV device after the reduction process was found to be increased by a factor of 50. The gas barrier properties of a graphene oxide layer are closely related to its surface roughness and dispersibility. Furthermore, these gas barrier properties can be enhanced by controlling the thermal reduction conditions. The thermal reduction of a graphene oxide film at a low heating rate results in a low water vapor permeability, only 0.1% of that of an as-prepared polyethylene naphthalate film. These results indicate that the dispersibility, surface roughness, and reduction conditions of a graphene oxide film significantly influence its gas barrier performance. Further investigations of the reduction of graphene oxide films are expected to enable further improvements in performance.

In this paper, the recent developments and their characteristics of membrane separators for lithium-ion batteries are reviewed. In recent years, there have been intensive efforts to develop advanced battery separators for rechargeable lithium-ion batteries for different applications such as portable electronics, electric vehicles, and energy storage for power grids. Separator is a critical component of lithium-ion batteries since it provides a physical barrier between the positive and negative electrodes in order to prevent electrical short circuits. The separator also serves as the electrolyte reservoir for the transport of ions during the charging and discharging cycles of a battery. The performance of lithium-ion batteries is greatly affected by the materials and structure of the separators. This paper introduces the requirements of battery separators and the structure and properties of four important types of membrane separators which are microporous membranes, modified microporous membranes, non-woven mats, and composite membranes. Each separator type has inherent advantages and disadvantages which influence the performance of lithium-ion batteries. The structures, characteristics, manufacturing, modification, and performance of separators are described in this review paper. The outlooks and future directions in this research field are also given.

Here, we have successfully developed a novel contact mode between MoS2 and graphene, where graphene rolls up into a hollow nanotube and thin MoS2 nanosheets are uniformly standing on the inner surface of graphitic nanotubes, thus forming mechanically robust, free-standing, interwoven MoS2@graphene nanocable webs (MoS2@G). Such a hybrid structure can maximize the MoS2 loading in the electrode in which over 90% of MoS2 nanosheets with stacked layer number of less than 5 can be installed. Remarkably, when calculated on the basis of the whole electrode, this binder free electrode not only shows high specific capacity (ca. 1150 mA h g−1) and excellent cycling performance (almost 100% capacity retention even after 160 cycles at a current density of 0.5 A g−1) but exhibits a surprisingly high-rate capability of 700 mA h g−1 at the rate of 10 A g−1 despite such a high MoS2 loading content, which is one of the best results of MoS2-based electrode materials ever reported thus far.

The Li-air (or Li-O2) battery has attracted wide attention, since it has the highest theoretical specific gravimetric energy density. In spite of the rapid progress made on improving its cyclic performance and reducing its voltage polarization, many key issues on thermodynamics and kinetics in nonaqueous Li-O2 batteries are still unresolved. In this study, by using the galvanostatic intermittent titration technique, several novel phenomena have been observed, such as zero voltage gap for the open circuit voltage (OCV) between charging and discharging, asymmetrical polarization behaviours at different current densities and temperatures, a continuous increase of overpotential during charging, and a negative temperature coefficient of the cell's thermodynamic equilibrium voltage. These results could inspire other researchers to comprehensively investigate the complicated reaction mechanisms, thermodynamics, and kinetic properties of the Li-air battery, as well as other advanced batteries.

Here we present a multiple ion-exchange membrane (IEM) cell design for redox flow batteries (RFBs) that can generally accommodate redox pair combinations with any mixed ion charges. This design also allows flexible choice of electrolytes such as an acid electrolyte at one electrode while a base in the other. More specifically, the double-IEM cell with one anion-exchange membrane (AEM), one cation-exchange membrane (CEM), and a middle electrolyte in between can work with all redox pairs except for the case of two hybrid redox pairs (i.e., an anion–cation pair vs. an anion–cation pair). For the combination of two hybrid pairs, a triple-IEM cell with three membranes (CEM/AEM/CEM or AEM/CEM/AEM) and two middle electrolytes is needed. The double- and triple-IEM cells bring unprecedented freedom in choosing redox pairs and supporting electrolytes. Of particular importance, two featured aqueous RFBs are demonstrated here: (1) ultra-high voltage zinc–cerium RFB with 3.08 V standard cell voltage, the highest among all known aqueous RFBs; and (2) ultra-low cost sulfur–iron RFB with 1.22 V standard cell voltage, with two highly available elements (iron and sulfur are the 1st and 5th most produced elements worldwide, respectively).

At first glance, the combination of the lightest, most electropositive metal (lithium) with a safe, abundant (and reasonably light) non-metal (sulfur) makes good sense as a prospective battery. However, while the lithium-sulfur battery offers a very high theoretical specific energy (~2600 Wh kg-1) the actual performance delivered is proving to be severely limited — in many cases, this is directly related to the role of the electrolyte. The fundamental issue is that the reduction of sulfur proceeds through a series of polysulfide species, which are for the most part soluble in common organic solvents, including those employed in battery electrolyte solutions. So, despite the fact that the ultimate product (Li2S) is essentially insoluble, the intermediate stages of discharge see a migration of redox-active species out of the cathode, from where they can react with the lithium anode, which sets in train a series of equilibria that cause both a loss of charging efficiency and a gradual loss of discharge capacity. In the last decade, a major stream of the research to overcome this complex situation has focused on minimizing the solubility of polysulfides. From this we now have a range of media in which the lithium-sulfur system can operate with much improved charge–discharge characteristics: ionic liquids (and blends with organic media); super-saturated salt-solvent mixtures; polymer-gelled organic media; solid polymers; solid inorganic glasses. Underlining the multi-faceted nature of interactions within the lithium-sulfur cell, though, none of these improved electrolytes has been able to bring the performance of this system up to the levels of reliability and capacity maintenance (without sacrificing high specific energy) that are benchmarks in energy storage applications. Our survey indicates that only by combining particular electrolytes with cathode materials that are designed to actively retain sulfur and its reduction products, have a relatively few studies been able to obtain the desired levels of performance. Ultimately the successful development of the lithium-sulfur battery requires careful coordination of the choice of modified electrolyte with the specific nature of the cathode material, underpinned by the assumption that the resulting electrolyte composition will meet established criteria for compatibility with the lithium anode.

Molecular electrocatalysts for H2 evolution are usually studied under various conditions (solvent, proton sources) that prevent direct comparison of their performances. We provide here a rational method for such a benchmark based on (i) the recent analysis of the current-potential response for two-electron-two-step mechanisms and (ii) the derivation of catalytic Tafel plots reflecting the interdependency of turnover frequency and overpotential based on the intrinsic properties of the catalyst, independently of contingent factors such as the cell characteristics. Such a methodology is exemplified on a series of molecular catalysts among the most efficient in recent literature

A critical step in the conversion of pyrolysis and liquefaction bio-oils to transportation fuels is deoxygenative upgrading. With this perspective we introduce a two-step thermochemical cycle which harnesses concentrated solar radiation to drive bio-oil deoxygenation. In this cycle, which we have termed “looped-oxide catalysis” (LOC), a metal oxide is reduced in a high-temperature solar thermal reactor and subsequently reacted with bio-oil, reforming the original metal oxide and yielding a deoxygenated biofuel product. By augmenting the chemical energy stored in bio-oil with solar thermal energy, LOC may increase fuel yields for a given quantity of biomass beyond what is currently achievable with standalone biomass-to-fuel pathways. We identify five promising candidate LOC materials based on equilibrium thermodynamics, diffusion kinetics and catalytic performance in hydrodeoxygenation (HDO) reactions. Additionally, we present new experimental results and mechanistic implications from an investigation of the LOC upgrading of acetic acid. The formation of acetaldehyde with comparable selectivity in LOC with zinc metal as well as HDO with zinc oxide suggests that surface oxygen vacancies are the catalytically relevant sites in both processes.

The growing use of bioenergy goes hand in hand with a heated public debate, in which conflicting claims are made regarding the amount of biomass that can be sustainably used for this purpose. This article assesses the current knowledge on biomass resource potentials and interrelated factors such as water availability, biodiversity, food demand, energy demand and agricultural commodity markets. A sensitivity analysis of the available information narrows the range of biomass potentials from 0–1500 EJ/yr to approximately 200–500 EJ/yr in 2050. In determining the latter range, water limitations, biodiversity protection and food demand are taken into consideration. Key factors are agricultural efficiency and crop choice. In principle, global biomass potentials could meet up to one third of the projected global energy demand in 2050.