Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides (“MnOx”) for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and composition of the “MnOx” is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are especially good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been observed that the electrocatalytic performance is also largely dependent on the electrolyte’s composition and pH and that a number of equilibria accompany the catalytic process, resulting in “adaptive changes” of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidation electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the composition of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates observed for iridium, ruthenium or nickel-iron oxide anodes in acidic or alkaline solutions, respectively. So at the end of the article, two fundamental questions will be addressed: (1) are there technical applications where MnOx materials could actually be the first choice as OER electrocatalysts? and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: “Why did nature choose manganese to make oxygen?”

11.3% efficiency Cu(In,Ga)(S,Se)2 thin film solar cells via drop-on-demand inkjet printing

Although Cu(In,Ga)(S,Se)2 (CIGSe) based thin film solar cells have reached efficiencies exceeding 22% based on vacuum processed CIGSSe absorbers, the supply of indium and gallium might become an issue if CIGSSe thin-film solar cells are produced in very large volumes. It is therefore mandatory to reduce the wastage of indium and gallium during the fabrication process. In this work, we report on a highly efficient precursor utilization, and a vacuum- free, and scalable route to the deposition of Cu(In,Ga)(S,Se)2 (CIGSSe) thin films via drop-on-demand inkjet-printing. The precursor ink, which shows long- term stability in air at room temperature, is formulated by dissolving metal nitrate salts in alcohol-based solvents. Crack free CIGSSe absorbers consisting of a layer with large grains at the surface and a layer with small grains at the back have been prepared by annealing the inkjet-printed Cu–In–Ga nitrate precursors in a Se/H2S containing atmosphere. Ga accumulation has been observed within the layer with small grains. A solar cell with a total area efficiency of 11.3% under standard AM 1.5 illumination has been achieved based on the printed CIGSSe absorbers

Towards global sustainability: Education on environmentally clean energy technologies

The recent climate change agreement in Paris highlights the imperative to aggressively decarbonize the energy economy and develop new technologies, especially for the generation of electrical energy that are environmentally clean. This challenge can only be addressed by a multi-pronged approach to research and education of the next generation of scientists and engineers as well as informed public discourse. Consequently this requires the introduction of new and comprehensive education programs on sustainable energy technologies for universities and, possibly, high schools. Among others, the new programs should provide in-depth knowledge in the development of new materials for more efficient energy conversion systems and devices. The enhanced level of education is also needed for properly assessing the competing technologies in terms of their economic and social benefits. The increasing recognition of the significance of clean and efficient energy conversion indicates the need for a comprehensive education program to be developed. The purpose of the present work is to consider the structure of both an education program and the related textbook where the energy-related fundamental and applied subjects are presented in a concentrated and uniform manner. Such a textbook could be an education aid for students of energy-related courses as well as the teachers involved in the formulation of the education programs. The textbook, which should be dedicated mainly for students at the undergraduate levels at universities, and possibly high schools, should include in-depth interdisciplinary sections dedicated to energy experts and graduate students. This paper considers the present international efforts in reducing the impact of climate change and the need to develop new technologies for clean energy generation. It is argued that progress in this area requires recognition of hydrogen as the main energy carrier of the future. This work also delineates the goals of the Sustainable Energy Network, SEN, involved in the UN program of Future Earth

Structure of the catalytic sites in Fe/N/C-catalysts for O-2-reduction in PEM fuel cells

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH3 at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (FexN, with x ≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN4-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e− per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials

Size- and density-controlled deposition of Ag nanoparticle films by a novel low-temperature spray chemical vapour deposition method—research into mechanism, particle growth and optical simulation

Ag nanoparticles have attracted interest for plasmonic absorption enhancement of solar cells. For this purpose, well-defined particle sizes and densities as well as very low deposition temperatures are required. Thus, we report here a new spray chemical vapour deposition method for producing Ag NP films with independent size and density control at substrate temperatures even below 100 °C, which is much lower than for many other techniques. This method can be used on different substrates to deposit Ag NP films. It is a reproducible, low-cost process which uses trimethylphosphine (hexafluoroacetylacetonato) silver as a precursor in alcoholic solution. By systematic variation of deposition parameters and classic experiments, mechanisms of particle growth and of deposition processes as well as the low decomposition temperature of the precursor could be explained. Using the 3D finite element method, absorption spectra of selected samples were simulated, which fitted well with the measured results. Hence, further applications of such Ag NP films for generating plasmonic near field can be predicted by the simulation

Artificial Photosynthesis for Solar Fuels - an Evolving Research Field within AMPEA, a Joint Programme of the European Energy Research Alliance

On the path to an energy transition away from fossil fuels to sustainable sources, the European Union is for the moment keeping pace with the objectives of the Strategic Energy Technology-Plan. For this trend to continue after 2020, scientific breakthroughs must be achieved. One main objective is to produce solar fuels from solar energy and water in direct processes to accomplish the efficient storage of solar energy in a chemical form. This is a grand scientific challenge. One important approach to achieve this goal is Artificial Photosynthesis. The European Energy Research Alliance has launched the Joint Programme "Advanced Materials & Processes for Energy Applications” (AMPEA) to foster the role of basic science in Future Emerging Technologies. European researchers in artificial photosynthesis recently met at an AMPEA organized workshop to define common research strategies and milestones for the future. Through this work artificial photosynthesis became the first energy research sub-field to be organised into what is designated "an Application” within AMPEA. The ambition is to drive and accelerate solar fuels research into a powerful European field - in a shorter time and with a broader scope than possible for individual or national initiatives. Within AMPEA the Application Artificial Photosynthesis is inclusive and intended to bring together all European scientists in relevant fields. The goal is to set up a thorough and systematic programme of directed research, which by 2020 will have advanced to a point where commercially viable artificial photosynthetic devices will be under development in partnership with industr

Angular momentum–induced delays in solid-state photoemission enhanced by intra-atomic interactions

Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.This work was supported by the German Research Foundation (DFG) within the Collaborative Research Center (SFB) 613 (F.S., P.B., W.P., and U.H.), the Priority Programs SPP 1931 (C.S., M.H., and W.P.), and SPP 1840 (St.F., S.N., and W.P.); the Basque Government (grant IT-756-13 UPV/EHU) (V.M.S., E.E.K., R.D.M., P.M.E., and A.K.K.); and the Spanish Ministerio de Economía y Competitividad (grants FIS2016-76617-P and FIS2016-76471-P) (V.M.S., E.E.K., R.D.M., P.M.E., and A.K.K.) and Fondo Europeo de Desarrollo Regional (FEDER) (CTQ2016- 80375-P) (M.T.-S.). N.M.K. acknowledges hospitality and financial support from the theory group in cooperation with the small quantum systems (SQS) research group of European XFEL.Peer Reviewe