four-electron/four-proton transfer, which causes sluggish kinetics. Pt-based materials are the state-of-the-art electrocatalysts for ORR due to their outstanding catalytic activity, selectivity, and long-term stability under operating conditions. [1] However, their scarcity and, thus, high cost limit their applications.Accordingly, research on non-noble metal catalysts increased substantially over the past decades. The most promising material classes so far are transitionmetal-nitrogen-carbon complexes [2][3][4] with Fe or Co as the metal center, [5,6] metal-N 4 organometallic complexes [7] or metal-free catalysts such as nitrogen-doped carbon species utilizing carbon nanotubes or graphene. [8][9][10] These catalysts show promising activity; however, they still fall short in their overall performance compared to Pt. Hence, new materials capable of replacing Pt-based catalysts for ORR are yet to be found.One approach in exploring novel materials is to increase their chemical complexity. Multinary metal alloys require a more challenging synthesis pathway, but enable a virtually unlimited amount of different compositions. For instance, over 2 × 10 6 combinations for quinary alloys selected from a list of 50 elements are possible, each with a different composition. Therefore, selection of candidate materials to be tested is necessary, e.g., based on abundance and toxicity of elements. Some of these largely unexplored multinary compositions might show unique physical and chemical properties. For many alloys, comprising typically five principal elements or more, an unexpected stability as a single solid solution phase was observed in spite of their chemical complexity. This stability is usually explained by the so-called high-entropy effect. [11][12][13] The proposed underlying rational include i) stabilization of the multinary single phase due to increasing entropy with an increasing number of constituents, ii) a lattice distortion effect, and iii) sluggish diffusion. [14] This special state of a complex solid solution may result in unusual properties which are not observed for heterogeneous multiphase alloys comprising intermetallic phases. Their advantages in mechanical, physical, and chemical properties such as structural stability [15] as well as corrosion [16] and oxidation resistance have been evaluated over the past years. [17] Potential applications in (electro)catalysis have only been reported for

Complex solid solution (CSS) nanoparticles were recently discovered as efficient electrocatalysts for a variety of reactions. As one of many advantages, they exhibit the potential to replace noble-metal catalysts with multinary combinations of transition metals because they offer formation of new unique and tailorable active sites of multiple elements located next to each other. This Perspective reports on the current state and on challenges of the (combinatorial) synthesis of multinary nanoparticles and advanced electron microscopy characterization techniques for revealing structure–activity correlations on an atomic scale. We discuss what distinguishes this material class from common catalysts to highlight their potential to act as electrocatalysts and rationalize their nontypical electrochemical behavior. We provide an overview about challenges in synthesis, characterization, and electrochemical evaluation and propose guidelines for future design of CSS catalysts to achieve further progress in this research field, which is still in its infancy.

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Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.