Towards a paradigm shift in electrocatalysis using complex solid

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Towards a paradigm shift in electrocatalysis using complex solid solution nanoparticles Tobias Löffler, Alan Savan, Alba Garzon Manjon, Michael Meischein, Christina Scheu, Alfred Ludwig, and Wolfgang Schuhmann ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00531 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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ACS Energy Letters

Towards a paradigm shift in electrocatalysis using complex solid solution nanoparticles Tobias Löfflera, Alan Savanb, Alba Garzón-Manjónc, Michael Meischeinb, Christina Scheuc,d*, Alfred Ludwigb*, Wolfgang Schuhmanna*

a

Analytical Chemistry – Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universitätsstr. 150, D-44780 Bochum, Germany

b

Institute for Materials, Faculty of Mechanical Engineering, Ruhr University Bochum, Universitätsstr. 150, D-44780 Bochum, Germany

c

Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, D-40237 Düsseldorf, Germany

d

Materials Analytics, RWTH Aachen University, Kopernikusstr. 10, 52074 Aachen, Germany

Corresponding authors

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[email protected]; [email protected]; [email protected]


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Abstract 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 since 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 non-typical 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.

TOC graphic



Up to the present, technological alloys in general and catalysts in particular are based on one, two or occasionally three principal elements, sometimes with the addition of several more in low to dopant-level concentrations. More recently, multinary alloys composed of 5 or more elements in equiatomic or near-equiatomic concentrations were proposed, which remain in a single solid solution constitution rather than segregating into different intermetallic phases.1–3 An early hypothesis rationalizing this approach was that the high entropy of mixing due to the large number of elements may result in alloys with unusual properties due to maximization of configurational entropy. Calculations indicate that this effect, which is the basis for so-called “high-entropy alloys” (HEA), can become dominant when the number of elements is 5 or more.4,5 While this can be observed for a number of 5-element alloy compositions where singleACS Paragon Plus Environment

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phase solid solutions are found in which all elements are homogeneously mixed, more recent work questions if the maximization of configurational entropy is decisive for phase formation.6 Hence, such complex mixtures are more generally designated as multi-principal element alloys (MPEAs) or complex solid solutions (CSS), even though the denomination “HEA” is still widely applied.

CSS present new opportunities for the interplay between composition and microstructure as a basis for novel material properties. For mechanical properties, metallurgical experience with binary and ternary alloys suggested that highly multinary element mixtures would lead to the formation of many phases leading to precipitation of intermetallic compounds, which would result in brittle materials with limited functional value. However CSS such as CrMnFeCoNi (“Cantor alloy”) exhibit a balance of high hardness and strength with good oxidation and corrosion resistance,7 and CSS with unusual combinations of functional thermal, magnetic and electrical properties have been discovered as well.8 The investigation of CSS properties in nanomaterials has only recently started. It is of interest to which extent metallurgical concepts (size, lattice constants, electronegativity, binding character of constituent atoms), which were



applied for bulk CSS, scale for the respective nanoscale counterpart. Computational methods were applied to search for new CSS9 and predicted candidates could be synthesized in bulk, thin film and NP form to study their properties. As crystal structures and properties can change from bulk to nanoscale, interesting effects are expected here. Exemplary questions would be e.g. if the fcc or bcc structure of bulk CSS changes at the nanoscale, or if the stability of CSS is different at the nanoscale. Here, we focus on unusual effects that CSS can have on catalytic properties which to date is widely unexplored. The special phase constitution of CSS - single solid solution and highly symmetric, typically fcc crystal structure combined with a complex mixture of elements - offers opportunities for favorable chemical as well as functional properties. Applications of different CSS have been proposed for catalysis of methanol oxidation10–13, the hydrogen evolution reaction14, and dye degradation15. Moreover, noble-metal based CSS nanoparticles (NPs) were applied for ammonia oxidation.16 In the following, a few examples highlighting different synthetic routes for fabrication of CSS NPs and their applications in catalysis are highlighted. The goals for CSS NP


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synthesis are to achieve control over their multinary composition, their size, morphology and avoid any contamination. Co-deposition by magnetron sputtering from several elemental sources is a wellknown technique for growing atomically mixed multinary materials.17 By using ionic liquids (ILs) in place of the usual solid substrate, multinary NPs can be synthesized. Composition and size of the NPs can be controlled by the individual deposition rates of the sputtered elements and the used IL, respectively.18 The IL is stabilizing the sputtered NPs, so no surfactants are necessary. For co-sputtering from two (or more) opposing targets, multinary NP libraries can be obtained, without the need of any precursor chemistry.19 With this approach many multinary NP systems can be rapidly fabricated. In each NP library, the compositional variation for a given system is tailorable and large. While Pt is the current state of the art catalyst for the oxygen reduction reaction (ORR) in terms of selectivity, activity and long-term stability20, it suffers from high cost due to its scarcity. A CSS comprising only earth-abundant transition metal elements, namely CrMnFeCoNi, was deposited from elemental targets into the IL [Bmim][(Tf)2N],21 and the resulting NPs were recovered and evaluated.22 In addition to the CrMnFeCoNi NPs, quaternary and binary NPs consisting of the same



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elements as well as Pt NPs, used as reference sample, were synthesized. The catalytic activity was evaluated by immersing a carbon nanoelectrode into the sputtered NP/IL suspension followed by potential-assisted NP immobilization on the nanoelectrode surface23,24 and measuring the ORR-induced current using linear sweep or cyclic voltammetry. The participation of the NPs was evaluated by subtracting the current of the bare electrode followed by normalization.25 The intrinsic electrocatalytic activity of the quinary CSS (CrMnFeCoNi) was comparable to and exceeding that of Pt for the ORR, while all of the possible quaternary and binary NPs showed significantly lower activity. Additionally, tuning the elemental contents in the NPs revealed that with increasing Mn amount the catalytic activity was further improved. The advantage of sputtering NPs into inert ILs is that only high purity Ar and pure target elements or alloys are present during the high vacuum process. However, a challenge with this method is to separate the NPs from the IL, for the envisaged application, as well as sputtering of CSS NPs with high yields. Concerning sputtering of NPs in IL, high-yield fabrication processes could be achieved by bringing IL as close as possible to the sputter

source

as

the

deposition

rate

is

inversely

proportional

to

the

source-to-substrate distance squared. The source power density should be optimized


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for maximum deposition rates. Instead of one cathode, several cathodes could be used to co-deposit NPs. Finally, the IL could be continuously transported through the sputter zone enabling a continuous synthesis process. A surface inorganic chemistry method using heterometallic double complex salt (DCS) precursors was applied to form NPs by means of sequential addition of metal anion and cation complexes onto a prepared surface. This allowed synthesis of bimetallic NPs in systems in which the metals are immiscible otherwise.26 The opposite electrostatic charges of the complexes arrange the two metals in a well-defined and equal distribution and also eliminate disadvantageous solubility impediments for DCSs. The obtained NPs have a narrow size and uniform distribution with sizes from 1 nm to 3 nm. The catalytic properties of the supported binary NPs as compared with single metals composing the binary NPs were investigated with respect to acetylene hydrogenation. Higher catalytic activity and reduced alkane selectivity was noted for binary NPs. This synthesis strategy may be extended to more elements obtaining CSS NPs in the future. An issue with this method is the chemical complexity of the process and purity of the final product, whereas no expensive vacuum and plasma apparatus is necessary.



Another preparation method is based on scanning-probe block copolymer lithography (SPBCL)27–30 to deposit very low amounts of metal precursor loaded block copolymers in attoliter volumes on a desired spot on a surface. In this way, unary to quinary NPs composed of the elements Au, Ag, Co, Cu and Ni with adjustable elemental compositions were synthesized by reducing the polymer in a two-step annealing process. Size control is achieved by adjusting the volume and metal loading of single polymer droplets. NPs were chosen using elements that are (I) only miscible, (II) two miscible and one immiscible, (III) only immiscible and (IV) two immiscible elements and one element which is miscible with both. For type II compositions, the two miscible elements were mixed in the NP while being clearly separated from the immiscible element. The three immiscible elements for type III NPs formed clearly separated areas in the NPs. For type IV, the amount of the miscible element was relevant for the NP structure with separation of the immiscible elements when the miscible element amount was < 10 at.-% in the initial mixture. The findings of the ternary NP formation were beneficial for understanding the formation of quaternary and quinary NPs, which show similar structural separation of the different miscible elements from the immiscible counterparts. Additionally, the structure of those NPs


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was found to depend strongly on the elemental compositions, indicating a route to wellcontrolled tailoring of NPs. Spatial control of NP synthesis is advantageous, whereas upscaling of this method seems challenging due to the necessity of massively parallel tip arrays. Furthermore, the purity of the end product might be an issue. Up to eight immiscible elements were successfully alloyed in single-phase solidsolution NPs by a carbothermal shock (CTS) method (Figure 1).16 Metal salts are used as precursors and carbon nanofibers (CNF) are used as support. Flash heating and cooling with adjustable shock durations and ramp rates result in multinary CSS NPs with narrow size distribution, uniform dispersion on the support and consisting of up to eight uniformly mixed elements. Decreasing the cooling rate also leads to phaseseparated multinary NPs achieving controllable composition, size and phase. Longer CTS times generated bigger and less uniform NPs, while creating solid-solution or phase-separated NPs was controlled by adjusting the rate of temperature change. Catalytic experiments aiming at ammonia oxidation showed improvements for quinary CSS NPs (PtPdRhRuCe) synthesized via CTS with respect to conversion, selectivity and degradation behavior as compared with PtPdRhRuCe NPs synthesized via wet impregnation. This was attributed to the high-entropy effect stabilizing the NPs as a



single-phase solid solution and preventing phase segregation. A clear path to scale-up with low-cost equipment is advantageous, whereas high purity of the NPs could be difficult to obtain. Conclusively, there are multiple pathways reported which successfully yielded CSS materials. Still, their reliable synthesis in form of nanomaterials remains complex and new cost-efficient routes with good control over composition, size, morphology and impurities poses an important goal of future research. The complex structure of CSS catalysts sets additional demands on characterization techniques for complete structural and compositional information. Common features of interest are i) size and shape, ii) crystallinity and crystal structure, iii) elemental composition, iv) spatial distribution of elements across the NPs to confirm the single solid solution phase without intermetallic phases, v) homogeneity of these characteristics across the NP population, and ultimately vi) visualization of defects and active sites, which are of special interest for this catalyst class.


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Figure 1: a) Effect of high-entropy stabilization with single solid solution phase formation on catalytic performance for ammonia oxidation. b) EDS maps of quinary PtPdRhRuCe NPs forming a single solid solution phase. Adapted and reprinted with permission from AAAS from ref 16. In general, all these features can be analyzed via state-of-the-art aberrationcorrected (scanning) transmission microscopy ((S)TEM) at the sub-nanometer scale.31 High-resolution (S)TEM (HR(S)TEM) with corresponding Fast Fourier transformation (FFT) analysis can be used to assess the crystal structure of individual NPs.32 These imaging techniques also allow for the detection of defects such as grain boundaries or interfaces and identification of the atomic arrangement at the surface.32



Energy-dispersive X-ray spectroscopy (EDS) or electron energy-loss spectroscopy (EELS) in STEM mode can be used to determine whether the NPs have a core/shell structure or homogeneous distribution in form of a single solid solution.31,33 EDS is more frequently applied than EELS as the interpretation is more straightforward. When performing a tilt series within STEM tomography, it is further possible to obtain a 3D volume representation of the NP morphology. This is powerful in CSS characterization since sub-surface layers can be visualized to complete the analysis of possible active sites.34 However, this tool was just recently developed and not applied for sophisticated CSS NPs yet. The effect of strain between the multi-element neighboring atoms of different sizes on NP morphology could be revealed following this approach and can be goal of future studies. To circumvent beam damage by many image exposure, discrete electron tomography was introduced.35 By combination with statistical theory, even two images taken in different zone axes are enough to allow atom counting in a complete 3D particle with high precision. Following this approach atom by atom, the homogeneity of NPs can be determined.36 Local inhomogeneity of elemental distribution or specific surface facets can also be visualized by combination of EDS maps with ADF images.37 When aspiring illustration of complete active sites


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including surface and bulk atoms, these tools become especially important. The realization of such strategy was not performed yet in the CSS catalyst field, but the prospect of its possibility is a great achievement to obtain structure-activity correlations in the future. All these high-resolution techniques are at the edge of what is possible up to now and not all have been employed for CSS NPs and such application is yet to be shown. Specifically, when synthesis routes employ ILs, this may cause impurities such as organic contaminants, preventing atomic resolution in EDS maps. Additional cooling stages or low doses can be performed to minimize this effect.31 Still, novel techniques with even higher resolution such as atom probe tomography might become especially important for CSS catalysts in the future or combinations with support of theoretical predictions might find their way to supplement characterization.38 Conclusively, current characterization of CSS catalysts mainly focused on (S)TEM-EDS analysis to analyze particle size and homogeneous distribution of all elements. We believe these will remain the two most commonly analyzed features, but have to be complemented in the future by counting and identifying the chemical nature of each atom in 3D volume



with high precision. We envisaged that the technical capabilities for such goal were already shown, yet not applied to CSS NPs until now. The reported few examples of CSS NPs in electrocatalysis show the effect of single solid solution formation on catalytic activity,10–13,15,16,22 but as-yet mainly lack of an explanation of the origin. Indeed, this catalyst class behaves differently in various ways compared to conventional single element or binary catalysts and a systematic discussion becomes complex and still involves postulations due to the lack of experimental data. For example, many standard experiments may appear not promising at first sight, especially using common RDE measurements. However, deeper understanding of its origin can unveil the reasons and may allow significant improvement of performance for instance by adjusting the composition even without replacement of a single element, indicating the enormous potential in modifying properties with CSS catalysts. Some of the special features may already be rationalized and this will be done in the context of this perspective. However, an in-depth understanding of their working principle extending what can be hypothesized up to now will significantly facilitate progress in this field and studies about the working principles will likely become a very important part of future research.


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In the following, we provide a first discussion about the special features of CSS catalysts in electrocatalysis. These aspects are structured in three categories regarding: 1) their special features concerning electrocatalysis, 2) what impact this has on experimental data and thus, how to interpret these results, and finally 3) propose first strategies for future catalyst design. The special features of such complex solid solution catalysts can be highlighted with the following concepts: i) Formation of new active sites: The specific arrangement of multiple principal elements in a single solid solution phase allows interaction between neighboring atoms of different elements. In more common single element or bimetallic catalysts, the properties are predominantly set by the principal element and dopants can only finetune binding energies. Consequently, the principal element alone already has to possess very promising catalytic properties, which can only slightly be adjusted. In this case, one is limited by the given position of the principal element at the volcano plot39 for a certain reaction which for the example of the ORR implies the use of noble metals (if not solely Pt) as the main component. A stronger tuning effect may be obtained with more sophisticated structures such as complexes.



The formation of a single solid solution phase in many CSS due to the high-entropy effect means also a homogeneous distribution of all elements. The arrangement of all elements next to each other provides unique active sites consisting of multiple elements, which are otherwise not accessible. Up to now, there is no suggestion on how such an active site is defined. As shown in Figure 2, one option would be to consider a center of the active site at the NP surface, which - in the case of a single adsorption site - can be an atom for on-top adsorption or in the midst of 2 or 3 atoms for bridge or threefold adsorption as well as the impact of the nearest neighbors. The impact of atoms which are located even farther away on the activity of the active site is supposedly much weaker. For the simultaneous adsorption of multiple atoms, e.g. of a formed intermediate the number of atoms within the active sites has to increase. Still, the neighbors have to be classified in subcategories of i) indirectly contacting and at the surface, ii) directly contacting at the surface, and iii) directly contacting, but in the bulk since surface atoms are thermodynamically less stabilized and additionally in contact with the electrolyte which may modulate their properties.


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Figure 2: a) Schematic visualization of quinary CSS NPs. Arbitrary active sites at planes, edges and corners are highlighted with the active site center (#), nearest neighbors at the surface which are in direct contact with the center (orange), non-direct contact (green), and direct contact in bulk (blue). b) + c) Analogous illustration of active sites for multi-fold coordination centers (bridge (b) and threefold (c)) as center of active site. In this environment, the properties of the active site are predominantly governed by the electronic interaction of neighboring atoms. Due to the heterogeneity of these atoms in CSS in terms of electronic properties, a high variability of electronic states of different active sites is obtained which depends on the choice of elements and their constitution in those sites. Moreover, different atom sizes induce lattice distortions and thus strain effects, which in turn affect the local electronic properties. These modulations may alter adsorption energies of the active sites and impact interatomic distances, which play an important role for simultaneous adsorption of two or more



atoms. Ultimately, multi-element interactions cause a strong divergence of the properties of the center atom(s) as compared to a single element surface. Therefore, the reaction is not any longer limited by the properties of one principal element and its position in volcano plots, which is a very important feature when pursuing to replace noble-metal based catalysts. ii) High flexibility. CSS possess enormous potential in adjusting configuration (i.e. which elements are used) and composition (i.e. ratio of elements) which allows to tailor the properties of the active sites. Additionally, the high number of different active sites, which are simultaneously present justifies a concept of a continuum of properties due to the very small differences between active sites which just differ by one atom. This concept was very recently proposed by Batchelor et. al40 based on theoretical calculations, where the mountain-tale shaped distribution of energy levels is discussed in more detail demonstrating that the number of “mountains” equals the number of principal elements (Figure 3).


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Figure 3: Distribution of *OH adsorption energies for the MPEA IrPdPtRhRu forming a single CSS for a) equiatomic composition and b) Ir10.2Pd32Pt9.3Rh19.6Ru28.9. The respective volcano plot as a descriptor for energies yielding highest activity is shown and served as benchmark for optimization of the composition. Adapted and reprinted with permission from Elsevier from ref 40. The challenge here lies in finding a configuration set and optimal composition where a well-pronounced “mountain peak” coincides with the binding energy optimum predicted by the volcano plot. However, achieving a good fit is theoretically not limited to only one certain element configuration, but may be fulfilled by completely different elemental combinations as well. This flexibility in element selection allows more options in combining two aspired properties such as optimizing two individual adsorption energies or additionally improving mechanical properties such as thermal stability or conductivity. The universality of these concepts further enlarges the



applicability of this material class to basically any reaction by exploring CSS catalysts of different configuration and composition with adsorption energies specific for the reaction. iii) Enhancing cascade reactions and providing a platform for breaking scaling relations. The presence of neighboring active sites with different properties allows for follow-up reactions of primary reaction products to occur using closely localized active sites with ideal properties for both reactions (Figure 4). The farther the distance between the active site centers, the less atoms are shared at both sites and the more individual the properties of both sites can be.

Figure 4: Illustration of a cascade reaction at the surface of a CSS catalyst of two consecutive reduction reactions with single coordinating intermediates, which occur at directly neighboring active sites of different properties with a) atoms as center of active sites, b) threefold coordinated adsorption, and c) bridge coordinated adsorption. The proposed contributing atoms forming the active site are highlighted. ACS Paragon Plus Environment

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This feature can be exploited for reactions in which products are desired which form after multiple cascade reaction steps (CO2 reduction, ORR with 2x2 electron transfer pathway, …). One requirement for achieving the cascade product is a slower diffusion away from the CSS surface compared to the reaction rate at the neighboring site. Such prerequisite may be facilitated in closed environments like channels/pores as present in so-called nanozymes.41 Another specific potential of CSS catalysts is related to the scaling relation issue for multistep reactions such as ORR and OER. As amongst others described by Koper, there has to be a minimum overpotential for multi-step reactions due to variations in optimal binding energies for each reaction step.42 This conclusion is drawn from the scaling relation problem applicable to many multistep reactions where multiple intermediates have to be stabilized during the reaction and at least two of them bind to the catalyst surface in the same manner. If the adsorption energy of one active site is optimized for one of those intermediates, it simultaneously weakens stabilization of the other intermediate (Figure 5). Hence, changing the catalyst only induces a shift on the scaling line, but when this line does not intercept with the optimum regarding binding energies, a minimum overpotential cannot be circumvented.



Figure 5: Illustration of the scaling relation limitation for finding optimal binding energies for multiple intermediates in multi-step reactions for e.g. the oxygen reduction reaction. For each reaction step, the equilibrium potential is plotted as a function of the free binding energy of oxygen. Since they scale differently, a volcano-shaped (thick lines) correlation for the thermodynamic overpotential as the distance to the formal potential of the ORR (black line) is obtained while this distance can never become 0. Reprinted with permission from Elsevier from ref 42. This limitation could not be overcome yet with conventional catalysts offering only a limited number of different active sites. The group of Nørskov introduced possible strategies capable of solving this scaling relation challenge, which allow moving away from the predefined scaling line.43,44 One strategy is the utilization of bi-/multifunctional active sites, which allow different interactions with the related intermediates. For the ACS Paragon Plus Environment

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example of the OOH/OH intermediates responsible for the scaling relation in ORR and OER (both adsorb via a single oxygen atom), a neighboring atom of different properties related to the adsorption site has to be able to interact with the pending oxygen. Since this is only present in the OOH intermediate, only the binding property of the OOH is selectively affected. To enhance the catalyst/pending oxygen interaction, 3D structures were proposed. In common electrocatalysts, the presence of intermetallic phases implies the absence of such bi/multifunctional active sites or their low presence at the intermetallic interface with limited functionality and this strategy could not be exploited. For CSS catalysts, the single solid solution phase across the whole NP may provide the aspired platform to diminish the scaling relation challenge by exploiting the discussed strategy. Interactions can be tailored by rational choice of elemental configuration and composition. Furthermore, strain effects induce slight lattice distortions yielding a non-perfectly planar surface and design of more sophisticated 3D structures should not be restricted for this catalyst class. Hence, the volcano principle in finding the best compromise of binding energies, but implying a minimum overpotential might become obsolete for CSS catalysts. However, the discussed concepts are based on theoretical considerations and their applicability is neither



experimentally shown, nor confirmed in calculations yet and have to be seen as speculations at this point of time. Specific features of CSS electrocatalysts a) Evaluation of catalytic properties. Generally, in electrocatalysis the reaction is assumed to only occur at one active site at a relevant rate implying a consistent current increase with increasing overpotential according to the current-overpotential equation, which simplifies to the Butler-Volmer equation in the kinetic region. For CSS, activities of different active sites may be quite close to each other and the resulting continuum of active site properties causes a high probability of active site limitation for each individual site due to their low presence within the diversified energy landscape. This enables covering “ideal” properties and the presence of very active sites, but simultaneously implies only a low amount of these sites while also exposing sites with continuously lower activity. Hence, multiple active sites of similar activity contribute to the overall observed current before mass transport limitation may be reached (Figure 6). Consequently, the observed current is the sum of all significantly contributing active site currents implying a deviation of the ideal, consistently exponentially growing activity curve. The shape of the current over potential curve is still preserved for each


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active site contribution, but the sum of the contributions from all active sites is represented by multiple wave-shaped segments. When the activity differences between the relevant active sites are high, higher overpotentials are required to reach a sufficient overall current and active site limitation might be present when the mass transport current limit is high. Accordingly, a Tafel plot cannot be generated for the overall current, but only for each kinetic region of the respective contributing active sites, which should be considered for the interpretation now including a sequential consideration of each active site in addition to the behavior of their sum.

Figure 6: Catalytic current curve over the applied potential for a reduction reaction utilizing a CSS-based catalyst, which exhibits three relevant active sites of different



activity and active site limited current. The observed current is the sum of all individual active site currents. b) Stability. CSS theoretically exhibit high stability due to entropic stabilization leading to a thermodynamic stable state of a single CSS preventing sufficient driving forces for degradation processes such as dealloying or the Kirkendall effect.45 Additionally, the atoms are strongly connected via metallic bonds which can be seen as an advantage over macrocyclic complexes with rather loose and thus reactive coordinated ligands. Excellent structural stability10 and corrosion7 as well as oxidation resistance46 were already shown for some CSS in the bulk state. However, CSS can be in a metastable state at application temperatures. The nanoscale cantor alloy was shown to decompose rapidly into competing phases at elevated temperatures between 300 °C and 450 °C, compared to the bulk counterpart.47,48 For catalytic applications at temperatures below 200 °C this may not be a problem, but the stability of new nanoscale CSS must be studied in detail what was not done under harsh electrocatalytic conditions yet. The stability against decomposition or oxidation of CSS can be rapidly investigated by the combinatorial processing platform approach47,49 using combinatorial co-deposition and processing on pre-sharpened Si tips for atom


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probe tomography. The implementation of transition metals in acidic medium remains an issue and the effect of oxide formation at high anodic potentials has yet to be explored. c) Competition for adsorption of reacting species. The coexistence of various active sites further causes sites of different adsorption strengths to be located next to each other. Even if an optimal adsorption energy is present for a given catalyst, reacting species might more likely adsorb at neighboring sites of higher adsorption strength but lower activity. However, due to the lower reactivity (longer adsorption time) of the higher adsorbing site it becomes blocked for further adsorption and the next reacting species may then be converted at the sites with higher activity. Proposed strategies to achieve progress in this field 1) First guideline for promising CSS compositions. Proposing a promising catalyst design, equiatomic CSS may possess too many different sites of equal number, which reduces the number of relevant sites and simultaneously increases the number of competing sites. The number of different exact arrangements of all statistically possible active sites is highest for an equiatomic CSS composition. Based on this consideration, increasing the relative amount of 1 or 2 elements might be a favorable strategy as long



as the formation of a single solid solution is maintained. In this case, the active sites consist of more atoms of the enriched elements and the number of possible arrangements with this elemental ratio is decreased, statistically increasing the number of each possible active site. The limits a of single solid solution phase in CSS can be explored by combinatorial thin film libraries50 or be theoretically assessed.51 NP libraries also allow the identification of optimum compositions for a given reaction. 2) Challenges in future CSS catalyst design. The high number of possible configurations and compositions enables high flexibility for tailoring properties, but also generates a vast number of different catalysts, which need to be investigated. To address this large number of necessary experiments, high-throughput techniques are required. Additionally, the intrinsic catalytic activity is advantageously examined using isolated NPs in the absence of any film effects, which are implied using conventional catalyst matrix inks. Hence, experimentally addressing all possible catalyst configurations and even more all possible varying compositions of the same elemental configuration is still limited and requires novel high-throughput nano-electrochemical tools.


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In case of computational methods, the postulated capability to act as platform for breaking the scaling relation problem for multistep reactions should be considered. Hence, it is not only possible to optimize a catalyst configuration with a certain composition in terms of finding the best fit with the top of the volcano, but also exploring possible different interactions between active sites and intermediates, which adsorb in the same manner (Figure 3). Hence, optimizing intensity and position of CSS adsorption peaks regarding the intersection of adsorption energy scaling lines of possible rate limiting steps with the formal potential could yield even better catalysts beyond the minimum overpotential as predicted by scaling relations in case decoupling of intermediates turns out to be possible. The accuracy of the calculation of specific active sites correlates with the specificity of input data. For a straightforward approach, consideration of the nearest neighbors provides a valuable first information. To reduce the complexity of implementing the exact arrangement, all sites with the same ratio of elements could be grouped together regardless of their exact structural arrangement and the most promising ratio can then be correlated to the composition, which can be experimentally investigated. For more specific data, the distance of the considered atoms to the center of the active site, their



position at the surface or in the bulk and the fact that these atoms are also affected by interaction with their own neighbors and possibly with the electrolyte should be considered. CSS are an emerging class of potential electrocatalysts, offering promise for a paradigm change. However, they pose many challenges in all fields of synthesis, characterization and analysis of electrochemical data combined with a very high number of possible combinations to explore. Yet, this is just the point that makes them so special and provides their high potential in the field of electrocatalysis due to the intrinsic possibility to tailor their catalytic properties coupled with their unique ability to overcome limitations of a single-element catalyst. Hence, they may provide a longsought solution for reactions where only scarce and expensive catalysts show reasonable activities. Furthermore, their unique structure introduces additional advantages such as the theoretical possibility to act as a platform to break the scaling relation limitation and enabling cascade reactions. Ultimately, the minimum required overpotential could be significantly reduced while only using abundant elements. Up to now, their universal applicability is not yet shown and there remain many open questions, which have to be investigated and solved before they may replace well-


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established catalysts. We highlighted such challenges for future progress in this field and provided first guidelines for analysis and concepts of CSS catalysts, which aim to support research in this emerging complex but highly promising field.

AUTHOR INFORMATION

Tobias Löffler: [email protected] Alan Savan: [email protected] Alba Garzón-Manjón: [email protected] Michael Meischein: [email protected] Corresponding authors: Christina Scheu: [email protected] Alfred Ludwig: [email protected] Wolfgang Schuhmann: [email protected]

Notes The authors declare no competing financial interest.

Author biographies: Tobias Löffler: https://orcid.org/0000-0002-3919-6464



Tobias Löffler is a Ph.D. student in the Center of Electrochemistry (CES) at Ruhr University Bochum, Germany. He received his M.Sc. (Chemistry) at RUB in 2016 utilizing nanometric tools for lithium-ion batteries. His current research in electrocatalysis focusses on the development of tools for assessment of intrinsic electrocatalytic activity and exploring multinary alloys regarding their capability to act as novel electrocatalysts. Alan Savan: https://orcid.org/0000-0003-0559-3290 Alan Savan is a research staff member in the Institute for Materials, Faculty of Mechanical Engineering at the Ruhr University Bochum. His current research is in the growth and characterization of thin films of multifunctional materials, particularly using physical vapor deposition to make combinatorial libraries followed by data-based high-throughput measurements. Alba Garzón-Manjón: Alba Garzón-Manjón is a Postdoctoral Researcher at Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany. She got her Ph.D. in chemistry at Universitat Autònoma de Barcelona in 2016. Her present research is focused on the characterization of multinary alloy systems using aberration corrected high resolution (scanning) transmission electron microscopy. Michael Meischein: Michael Meischein is a Ph.D. student at Ruhr-University Bochum (RUB). He received his M.Sc. degree in physics in 2017, working on the morphological investigation of Cu nanoparticles deposited via inverse micelle encapsulation on rutile titanium dioxide. His actual research concentrates on the investigation of metal nanoparticles stabilized in ionic liquids via physical vapor deposition. Christina Scheu: https://orcid.org/0000-0001-7916-1533


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Christina Scheu is the head of the independent research group Nanoanalytics and Interfaces at the Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany, and holds on the same time a full professorship at the RWTH Aachen University. Her research activities are related to unravel the atomic arrangement of energy materials by advanced (scanning) transmission electron microscopy and to correlate the findings to physical and chemical properties. Link to webpage: https://www.mpie.de/person/43227/2281 Alfred Ludwig: https://orcid.org/0000-0003-2802-6774 Alfred Ludwig is a full professor of the institute for materials in the mechanical engineering department of RUB, Germany, as well as the director of the ZGH, RUB, Germany. His research is focused on the discovery of multifunctional and interface-dominated materials using highthroughput experimentation combined with atomic-scale analysis and materials informatics. Link to webpage: https://www.ruhr-uni-bochum.de/wdm/index.html.en Wolfgang Schuhmann: https://orcid.org/0000-0003-2916-5223 Wolfgang Schuhmann is professor for Analytical Chemistry at the Ruhr-University Bochum since 1996. His research covers different fields of electrochemistry, including micro- and nanoelectrochemistry, scanning electrochemical microscopy, bioelectrochemistry, electrocatalysts for energy conversion, catalyst nanoparticles and noble-metal free electrocatalysts. Link to webpage: https://www.ruhr-uni-bochum.de/elan/ ACKNOWLEDGMENT

The authors acknowledge funding by the BMBF in the framework of the project “NEMEZU” (03SF0497B) as well as the DFG projects LU1175/23-1 and SCHE634/21-



1. T. Löffler acknowledges the “Fonds der chemischen Industrie” for a “Chemiefonds-Stipendium”.

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Quotes from perspective to be shown as highlights

1) Multiple pathways for the successful synthesis of CSS materials are reported. However, their reliable synthesis in form of nanomaterials remains complex and new routes with good control over composition, size, morphology and impurities poses an important goal of future research.

2) Using CSS materials as electrocatalysts is the basis that the reaction is not any longer limited by the properties of one principal element and its position in volcano plots, which is a very important feature when pursuing to replace noble-metal based catalysts.

3) CSS possess enormous potential in adjusting configuration (i.e. which elements are used) and composition (i.e. ratio of elements) which allows to tailor the properties of the active sites of related electrocatalysts.

4) For CSS catalysts, the single solid solution phase across the whole NP may provide a platform to diminish the scaling relation challenge.


Towards a paradigm shift in electrocatalysis using complex solid

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