Helmholtz Institute Erlangen-Nürnberg

Abstract Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis.

ConspectusThe need to drastically reduce CO2 emissions will lead to the transformation of our current, carbon-based energy system to a more sustainable, renewable-based one. In this process, hydrogen will gain increasing importance as secondary energy vector. Energy storage requirements on the TWh scale (to bridge extended times of low wind and sun harvest) and global logistics of renewable energy equivalents will create additional driving forces toward a future hydrogen economy. However, the nature of hydrogen requires dedicated infrastructures, and this has prevented so far the introduction of elemental hydrogen into the energy sector to a large extent. Recent scientific and technological progress in handling hydrogen in chemically bound form as liquid organic hydrogen carrier (LOHC) supports the technological vision that a future hydrogen economy may work without handling large amounts of elemental hydrogen. LOHC systems are composed of pairs of hydrogen-lean and hydrogen-rich organic compounds that store hydrogen by repeated catalytic hydrogenation and dehydrogenation cycles. While hydrogen handling in the form of LOHCs allows for using the existing infrastructure for fuels, it also builds on the existing public confidence in dealing with liquid energy carriers. In contrast to hydrogen storage by hydrogenation of gases, such as CO2 or N2, hydrogen release from LOHC systems produces pure hydrogen after condensation of the high-boiling carrier compounds.This Account highlights the current state-of-the-art in hydrogen storage using LOHC systems. It first introduces fundamental aspects of a future hydrogen economy and derives therefrom requirements for suitable LOHC compounds. Molecular structures that have been successfully applied in the literature are presented, and their property profiles are discussed. Fundamental and applied aspects of the involved hydrogenation and dehydrogenation catalysis are discussed, characteristic differences for the catalytic conversion of pure hydrocarbon and nitrogen-containing LOHC compounds are derived from the literature, and attractive future research directions are highlighted.Finally, applications of the LOHC technology are presented. This part covers stationary energy storage (on-grid and off-grid), hydrogen logistics, and on-board hydrogen production for mobile applications. Technology readiness of these fields is very different. For stationary energy storage systems, the feasibility of the LOHC technology has been recently proven in commercial demonstrators, and cost aspects will decide on their further commercial success. For other highly attractive options, such as, hydrogen delivery to hydrogen filling stations or direct-LOHC-fuel cell applications, significant efforts in fundamental and applied research are still needed and, hopefully, encouraged by this Account.

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A–D–A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (Eloss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4′,5′]thieno-[2′,3′:4,5]pyrrolo-[3,2-g]thieno-[2′,3′:4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A–DA′ D–A configuration has further propelled the PCEs to go beyond 15% due to smaller Eloss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure–property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

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Maximum atom efficiency as well as distinct chemoselectivity is expected for electrocatalysis on atomically dispersed (or single site) metal centres, but its realization remains challenging so far, because carbon, as the most widely used electrocatalyst support, cannot effectively stabilize them. Here we report that a sulfur-doped zeolite-templated carbon, simultaneously exhibiting large sulfur content (17 wt% S), as well as a unique carbon structure (that is, highly curved three-dimensional networks of graphene nanoribbons), can stabilize a relatively high loading of platinum (5 wt%) in the form of highly dispersed species including site isolated atoms. In the oxygen reduction reaction, this catalyst does not follow a conventional four-electron pathway producing H2O, but selectively produces H2O2 even over extended times without significant degradation of the activity. Thus, this approach constitutes a potentially promising route for producing important fine chemical H2O2, and also offers opportunities for tuning the selectivity of other electrochemical reactions on various metal catalysts.

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π-π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.

A viable hydrogen infrastructure is one of the main challenges for fuel cells in mobile applications. Several studies have investigated the most cost-efficient hydrogen supply chain structure, with a focus on hydrogen transportation. However, supply chain models based on hydrogen produced by electrolysis require additional seasonal hydrogen storage capacity to close the gap between fluctuation in renewable generation from surplus electricity and fuelling station demand. To address this issue, we developed a model that draws on and extends approaches in the literature with respect to long-term storage. Thus, we analyse Liquid Organic Hydrogen Carriers (LOHC) and show their potential impact on future hydrogen mobility. We demonstrate that LOHC-based pathways are highly promising especially for smaller-scale hydrogen demand and if storage in salt caverns remains uncompetitive, but emit more greenhouse gases (GHG) than other gaseous or hydrogen ones. Liquid hydrogen as a seasonal storage medium offers no advantage compared to LOHC or cavern storage since lower electricity prices for flexible operation cannot balance the investment costs of liquefaction plants. A well-to-wheel analysis indicates that all investigated pathways have less than 30% GHG-emissions compared to conventional fossil fuel pathways within a European framework.

Reducing CO2 emissions is an urgent global priority. The enforcement of a CO2 tax, stringent regulations, and investment in renewables are some of the mitigation strategies currently in place. For a smooth transition to renewable energy, the energy storage issue must be addressed decisively. Hydrogen is regarded as a clean energy carrier; however, its low density at ambient conditions makes its storage challenging. The storage of hydrogen in liquid organic hydrogen carriers (LOHC) systems has numerous advantages over conventional storage systems. Most importantly, hydrogen storage and transport in the form of LOHC systems enables the use of the existing infrastructure for fuel. From a thermodynamic point of view, hydrogen storage in LOHC systems requires an exothermic hydrogenation step and an endothermic dehydrogenation step. Interestingly, hydrogenation and dehydrogenation can be carried out at the same temperature level. Under high hydrogen pressures (typically above 20 bar as provided from electrolysis or methane reforming), LOHC charging occurs and catalytic hydrogenation takes place. Under low hydrogen pressures (typically below 5 bar), hydrogen release from the LOHC system takes place. Hydrogen release from charged LOHC systems is always in conflict between highly power-dense hydrogen production and LOHC stability over many charging/discharging cycles. We therefore discuss the role of different catalyst materials on hydrogen productivity and LOHC stability. The use of density functional theory techniques to determine adsorption energies and to identify rate-determining steps in the LOHC conversion processes is also described. Furthermore, the performance of a LOHC dehydrogenation unit is strongly dependent on the applied reactor configuration. Industrial implementation of the LOHC technology has started but is still in an early stage. Related to this, we have identified promising application scenarios for the South African energy market.

Abstract Understanding the pathways of catalyst degradation during the oxygen evolution reaction is a cornerstone in the development of efficient and stable electrolyzers, since even for the most promising Ir based anodes the harsh reaction conditions are detrimental. The dissolution mechanism is complex and the correlation to the oxygen evolution reaction itself is still poorly understood. Here, by coupling a scanning flow cell with inductively coupled plasma and online electrochemical mass spectrometers, we monitor the oxygen evolution and degradation products of Ir and Ir oxides in situ. It is shown that at high anodic potentials several dissolution routes become possible, including formation of gaseous IrO 3 . On the basis of experimental data, possible pathways are proposed for the oxygen‐evolution‐triggered dissolution of Ir and the role of common intermediates for these reactions is discussed.

emissions will become a relevant economic factor. Solar- and wind-powered electricity will become significantly cheaper, such that hydrogen produced from electrolysis will be competitively priced against hydrogen manufactured from natural gas. However, to handle the unsteadiness of system input from fluctuating energy sources, energy storage technologies that cover the full scale of power (in megawatts) and energy storage amounts (in megawatt hours) are required. Hydrogen, in particular, is a promising secondary energy vector for storing, transporting, and distributing large and very large amounts of energy at the gigawatt-hour and terawatt-hour scales. However, we also discuss energy storage at the 120-200-kWh scale, for example, for onboard hydrogen storage in fuel cell vehicles using compressed hydrogen storage. This article focuses on the characteristics and development potential of hydrogen storage technologies in light of such a changing energy system and its related challenges. Technological factors that influence the dynamics, flexibility, and operating costs of unsteady operation are therefore highlighted in particular. Moreover, the potential for using renewable hydrogen in the mobility sector, industrial production, and the heat market is discussed, as this potential may determine to a significant extent the future economic value of hydrogen storage technology as it applies to other industries. This evaluation elucidates known and well-established options for hydrogen storage and may guide the development and direction of newer, less developed technologies.

Over the last two decades, lattice Boltzmann methods have become an increasingly popular tool to compute the flow in complex geometries such as porous media. In addition to single phase simulations allowing, for example, a precise quantification of the permeability of a porous sample, a number of extensions to the lattice Boltzmann method are available which allow to study multiphase and multicomponent flows on a pore scale level. In this article, we give an extensive overview on a number of these diffuse interface models and discuss their advantages and disadvantages. Furthermore, we shortly report on multiphase flows containing solid particles, as well as implementation details and optimization issues.

Exposing Fe–N–C catalysts to H ₂ O ₂ -byproduct leaves their catalytic sites untouched but decreases the turnover frequency <italic>via</italic> oxidation of the carbon surface.

Self-driving laboratories (SDLs) promise an accelerated application of the scientific method. Through the automation of experimental workflows, along with autonomous experimental planning, SDLs hold the potential to greatly accelerate research in chemistry and materials discovery. This review provides an in-depth analysis of the state-of-the-art in SDL technology, its applications across various scientific disciplines, and the potential implications for research and industry. This review additionally provides an overview of the enabling technologies for SDLs, including their hardware, software, and integration with laboratory infrastructure. Most importantly, this review explores the diverse range of scientific domains where SDLs have made significant contributions, from drug discovery and materials science to genomics and chemistry. We provide a comprehensive review of existing real-world examples of SDLs, their different levels of automation, and the challenges and limitations associated with each domain.

Abstract All‐inorganic perovskite semiconductors have recently drawn increasing attention owing to their outstanding thermal stability. Although all‐inorganic perovskite solar cells (PSCs) have achieved significant progress in recent years, they still fall behind their prototype organic–inorganic counterparts owing to severe energy losses. Therefore, there is considerable interest in further improving the performance of all‐inorganic PSCs by synergic optimization of perovskite films and device interfaces. This review article provides an overview of recent progress in inorganic PSCs in terms of lead‐based and lead‐free composition. The physical properties of all‐inorganic perovskite semiconductors as well as the hole/electron transporting materials are discussed to unveil the important role of composition engineering and interface modification. Finally, a discussion of the prospects and challenges for all‐inorganic PSCs in the near future is presented.

Abstract The “volcano”‐relationship for the electrocatalytic hydrogen evolution reaction seems to be a generally accepted phenomenology in electrochemistry. Based on the Sabatier principle, which suggests a not too strong, nor too weak binding of reaction intermediates as prerequisite for high reaction rates, it provides a straightforward and intuitive explanation for a plethora of experimental results. However, while the Sabatier principle as a main paradigm of heterogeneous catalysis was never really disputed in the case of gas‐phase reactions, it remains questionable if it can be the main driving principle that governs activity trends of electrocatalytic reactions. This work provides an overview on this topic for the model hydrogen evolution reaction (HER), pointing out certain inconsistencies and contradictions found in literature. The critical assessment provides a viewpoint which could have important practical consequences and could provide different perspective on future catalyst design.

Water electrolysis is considered as an important technology for an increased renewable energy penetration. This perspective on low-temperature water electrolysis joins the dots between the interdisciplinary fields of fundamental science describing physicochemical processes, engineering for the targeted design of cell components and the development of operation strategies. Within this aim, the mechanisms of ion conduction, gas diffusion, corrosion and electrocatalysis are reviewed and their influence on the optimum design of separators, electrocatalysts, electrodes and other cell components are discussed. Electrocatalysts for the water splitting reactions and metals for system components are critically accessed towards their stability and functionality. On the basis of the broad scientific analysis provided, challenges for the design of water electrolyzers are elucidated with special regard to the alkaline or acidic media of the electrolyte.

Abstract Proton‐exchange membrane water electrolyzers (PEMWEs) will play a key role in future sustainable hydrogen production for mobility, households or chemical industry. Yet, the anode in PEMWEs, where the pivotal oxygen evolution reaction takes place, needs further improvement in terms of performance and cost. Both catalyst materials and electrode structure have to be optimized in order to inhibit degradation and reduce noble metal loadings. This review focuses on a holistic approach, covering all catalyst material, electrode structure, and transport layers within the framework of an overall electrode design, which not only optimizes the catalyst but also, all components of the electrode in conjunction. This review defines the goals for performance metrics of future PEMWEs in terms of power density and durability of the anode. Moreover, it summarizes manufacturing techniques and approaches that have a chance to be upscaled to meet the megawatt deployment of PEMWEs. The different aspects described jointly in this review such as novel catalyst system with higher intrinsic and structural performance or graded porous transport layers shall help to advance a next generation of electrodes.

Combination of atom probe tomography, isotope-labelling and online electrochemical mass spectrometry provides direct correlation of atomic scale structure of Ir oxide catalysts with the mechanism of oxygen formation from the lattice atoms.

The foreseeable worldwide energy and environmental challenges demand renewable alternative sources, energy conversion, and storage technologies. Therefore, electrochemical energy conversion devices like fuel cells, electrolyzes, and supercapacitors along with photoelectrochemical devices and batteries have high potential to become increasingly important in the near future. Catalytic performance in electrochemical energy conversion results from the tailored properties of complex nanometer-sized metal and metal oxide particles, as well as support nanostructures. Exposed facets, surface defects, and other structural and compositional features of the catalyst nanoparticles affect the electrocatalytic performance to varying degrees. The characterization of the nanometer-size and atomic regime of electrocatalysts and its evolution over time are therefore paramount for an improved understanding and significant optimization of such important technologies like electrolyzers or fuel cells. Transmission electron microscopy (TEM) and scanning transmission electron microscope (STEM) are to a great extent nondestructive characterization tools that provide structural, morphological, and compositional information with nanoscale or even atomic resolution. Due to recent marked advancement in electron microscopy equipment such as aberration corrections and monochromators, such insightful information is now accessible in many institutions around the world and provides huge benefit to everyone using electron microscopy characterization in general. Classical ex situ TEM characterization of random catalyst locations however suffers from two limitations regarding catalysis. First, the necessary low operation pressures in the range of 10(-6) to 10(-9) mbar for TEM are not in line with typical reaction conditions, especially considering electrocatalytic solid-liquid interfaces, so that the active state cannot be assessed. Second, and somewhat related, is the lack of time resolution for the evaluation of alterations of the usually highly heterogeneous nanomaterials. Two methods offer a solution to these shortcomings, namely, identical location TEM (IL-TEM) and electrochemical in situ liquid TEM. The former is already well established and has delivered novel insights particularly into degradation processes; however, characterization is still performed in vacuum. The latter circumvents this issue by using dedicated in situ TEM holders but introduces extremely demanding technical challenges. Although the introduction of revolutionizing thin SiN window cells, which elegantly confine the specimen from vacuum, has allowed demonstration of the potential of the in situ approach, the reproducibility and data interpretation is still limited predominately due to the strong interaction of the electron beam with the supporting electrolyte and electrode material. Because of the importance of understanding the nanoelectrochemical structure-function relationship, this Account aims to convey a timely perspective on the opportunities and particularly the challenges in electrochemical identical location TEM and in situ liquid cell TEM with a focus on electrochemical energy conversion.

For a successful replacement of Pt, tremendous efforts have hitherto been made to develop high-performing Fe-N-C catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). In comparison to the remarkable progress in activity, the stability of Fe-N-C catalysts still remains critical, however. Fe demetallation in acidic medium is hypothesized to be one critical factor for the overall lifetime. In contrast to the general belief, we herein demonstrate using an operando spectroscopic analysis that catalytically inactive Fe particles exposed to acid electrolytes cannot be fully removed by acid washing due to a relatively high open circuit potential (ca. 0.9 VRHE) leading to the formation of insoluble ferric species, whereas these particles dissolve under PEMFC operating conditions (Ecathode < 0.7 VRHE) due to operando reduction to soluble ferrous cations. To overcome this issue, we demonstrate two approaches: (i) synthesis of Fe-N-C catalysts free of Fe particles and (ii) postsynthesis removal of exposed Fe particles through the control of potential using an external potentiostat or an internal reducing agent (i.e., SnCl2). Operando spectroscopic analyses verified that Fe demetallation during a given voltammetric protocol was dramatically decreased for both synthetically and postsynthetically modified Fe-N-C catalysts, while the initial ORR activity did not significantly change. However, all of these catalysts showed similar performance decay over short-term PEMFC durability tests, demonstrating the lack of a role played by ferrous cations leached from inactive Fe particles on catalyst deactivation. This supports the view that the activity is mainly imparted by FeNxCy moieties. Nevertheless, the presented guidelines are generally applicable to the whole class of Fe-N-C catalysts in order to minimize Fe demetallation in PEMFCs, which provides important advances for the future design of stable electrocatalytic systems for long-term operation.