Electrochemical Energy Conversion on Intermetallic Compounds â a

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Electrochemical Energy Conversion on Intermetallic Compounds – a Review Leonard Rößner, and Marc Armbrüster ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04566 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Electrochemical Energy Conversion on Intermetallic Compounds – a Review Leonard Rößner, Marc Armbrüster* Faculty of Natural Sciences, Institute of Chemistry, Materials for Innovative Energy Concepts, Chemnitz University of Technology, 09107 Chemnitz, Germany *[email protected]

Keywords: Electrocatalyst, Intermetallic Compound, Electronic Effect, Geometric Effect, Bifunctional Mechanism, Ordered Crystal Structure, Energy Conversion, Fuel Cell.

Abstract: Structurally ordered intermetallic compounds possess unique chemical and physical properties, making them an interesting class of materials for application in electrocatalytic reactions. This review comprises the work on intermetallic compounds used for energy relevant electrocatalysis and is structured by the reactions in scope, which are the hydrogen evolution reaction (HER), electrochemical carbondioxide reduction reaction (eCO2RR), oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR) as well as the oxidation reactions of formic acid (FAOR), methanol (MOR) and ethanol (EtOR). Optimization pathways for electrocatalysts, based on the adjustability of the intermetallic materials, are highlighted and experimental data are provided in a comparative manner, to provide an overview, foundation and reference for further development.

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Electrochemical Energy Conversion on Intermetallic Compounds – a Review .............................1 1.

Introduction.....................................................................................................................................4

2.

Stability............................................................................................................................................9

3.

Optimization Strategies .................................................................................................................10

4.

Electroreduction............................................................................................................................12

5.

4.1.

Hydrogen Evolution Reaction (HER) ......................................................................................12

4.2.

Carbon Dioxide Reduction Reaction (eCO2RR) ......................................................................23

4.3.

Oxygen Reduction Reaction (ORR) ........................................................................................29

Electrooxidation ............................................................................................................................46 5.1.

Hydrogen Oxidation Reaction (HOR) .....................................................................................46

5.2.

Formic Acid Oxidation Reaction (FAOR) ................................................................................50

5.3.

Methanol Oxidation Reaction (MOR) ....................................................................................60

5.4.

Ethanol Oxidation Reaction (EtOR)........................................................................................71

5.5.

Other Oxidation Reactions ....................................................................................................78

6.

Summary .......................................................................................................................................81

7.

References.....................................................................................................................................82


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1. Introduction The increasing energy demand of mankind 1 and the expected depletion of fossil energy carriers within the next centuries, 2,3 urge the exploration of alternative and more sustainable ways to provide energy. Furthermore, the continuous release of carbon dioxide into the atmosphere needs to be prevented where it is possible. 4 Already today wind and solar power, as ubiquitous convertible energy forms, not only satisfy the people’s desire for ecological technologies 5 but offer countries like Denmark, Spain and Germany the possibility to increase their share of renewably produced electrical energy (50.9% 6, 22.5% 6 and 38.2% 7, respectively). As this energy supply is strongly fluctuating, it needs to be balanced by intermediate storage to suit a constant demand. 8 This can be realized by different approaches, .i.e. conventional batteries 9, redox-flow batteries

10,11

or electrocatalytic energy conversion. The latter,

comprising the conversion of electrical energy into chemical energy and vice versa, like hydrogen or small organic molecules (e.g. formic acid or methanol), might be a suitable approach for small- as well as the large scale applications. 12 The resources to form these energy carriers are almost omnipresent in form of water, air and carbon dioxide. Storing electrical energy in form of hydrogen, formic acid, methanol or ethanol is realized by (co-) electrolysis, i.e. the hydrogen evolution reaction (HER) and the electrochemical carbon dioxide reduction reaction (eCO2RR). These reactions take place on the cathode, while oxygen is evolved on the anode (oxygen evolution reaction, OER). These energy carriers may be used for chemical synthesis, burnt conventionally or be oxidized electrochemically in a fuel cell, to release the energy more efficiently in comparison to the Carnot cycle taking place in combustion engines. While hydrogen oxidizes to water (HOR), formic acid (FAOR), methanol (MOR) and ethanol ideally yield only water and CO2 as products. Oxygen serves as oxidant and gets reduced at the cathode (ORR). To obtain maximum efficiencies of these conversion reactions (summarized in Scheme 1) and minimize energy losses, optimal electrocatalysts are required.


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Scheme 1: Electrochemical series: First oxidation potential of the elements vs. equilibrium potential of the conversion reactions. For example, the use of lead (Pb) in the HER at pH = 0 (red Pb - below H2 line) will lead to surface oxidation of lead, while elemental lead is expected at pH = 14 (blue Pb - above H2 line). Values for the redox potentials of the elements were taken from Darchen 13.

For most of the mentioned reactions, platinum is the most efficient catalyst among the elements, which is why catalyst development was dominated by modification of platinum and platinum-group metals (PGMs) as shown in Chapters 4 & 5. Platinum is a scarce and costly metal, thus substitution by cheaper elements and combinations of those while maintaining the activity is required. To find suitable catalysts, numerous investigations were conducted. To extend this empirical approach, reactions were modelled on the basis of experimental results to find optimal parameters, enabling predictions of promising materials, as in the case of MoS2 for the HER. 14 This development indicates that a deeper understanding of the ongoing processes is necessary for an efficient catalyst development. Knowing the needs of a reaction, a suitable model material with adjustable properties is beneficial for the next step, the optimization.


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Intermetallic compounds offer this kind of adjustability, making this class ideal platform materials with high application potential. What makes this class of materials different from metals and substitutional alloys? Upon the formation of an intermetallic compound from the elements – which is often very exothermic – a combination of ionic (partial charge transfer) and covalent bonding results. The proportion of the two types of bonding varies from compound to compound, as does the number of conduction electrons, which allows to use this class of compounds as electrocatalysts. Since the charge transfer is usually less than in oxides or hydrides, the chemical potential of the involved elements can be adjusted between these two classes of compounds. As an example, the catalytic properties of a negatively charged Pd can be tested using the intermetallic compound Ga+0.5Pd-0.5.

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The chemical

bonding results in more or less brittleness as well as crystal structures which are very different from the closed-packed cubic or hexagonal structures observed for the elements. In addition, the chemical bonding results in a strong site preference, since the interchange of cations and anions is energetically not favourable. The covalent interactions lead to directional bonding strengthening the site preference and often resulting in beautiful and complex crystal structures. So, the main difference between substitutional alloys and intermetallic compounds is the ordered crystal structure of the latter. The ordered crystal structure also results in a new “scaffold” of the electronic structure. Since the electronic structure – besides the arrangement of the atoms – determines the adsorption properties, the catalytic properties of intermetallic compounds are very much different to the ones of the corresponding elements or alloys. Tailoring of electronic properties is feasible by isostructural substitutional series where one of the elements is substituted by an element with a different number of valence electrons without changing the crystal structure (e.g. Ga1-xSnxPd2 16). Geometric influences on the catalysis can be addressed by selecting appropriate structural motifs. In both cases, however, the stability of the materials under operando conditions has to be proven. By this, another strength of the ordered structure for catalysis research can be used. Since the environment of the atoms is homogeneous, the surface potentially holds a rather limited variety of potential reaction sites. A bonus is the large number of available intermetallic compounds known so far. More than 6.000 binary and a much larger number of compounds of more than two elements are known, allowing for testing a huge number of different crystal and electronic structures. Unfortunately, deep understanding of the observed catalytic properties and elementary steps on these structurally complex surfaces by quantum chemical calculations comes at the price of a much higher calculational effort due to the more complex crystal structures. 17 Within this review, an intermetallic compound is strictly defined as having an at least partially ordered crystal structure, which is different from the constituting elements. 18 The ordered crystal structures are either identified by diffraction patterns (e.g. selected area electron diffraction (SAED) or X-ray diffraction (XRD)) or from lattice spacings in high-resolution transmission microscopy (HR-TEM) 5 ACS Paragon Plus Environment

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micrographs within the publications included in this review. As result, publications dealing with alloys, i.e. materials consisting out of several metallic phases, or substitutional alloys, i.e. materials having a structure of one of the constituting elements and random occupancy of the crystallographic sites by the elements, are not considered. The scope of this contribution is to review the experimental work on intermetallic compounds tested for energy-relevant electrocatalysis. The manuscript is separated into two major parts. The first part (Chapter 2 & 3) introduces factors influencing the stability of intermetallic compounds and optimization strategies to increase the catalytic activity. In the second part (Chapter 4 & 5) the intermetallic compound catalysts will be reviewed ordered by oxidation and reduction reactions, which are subdivided into the individual reactions. For better guidance of the reader, general principles for electrocatalyst optimization are highlighted at the beginning of each subchapter. Open questions, which will need deeper investigations for a profound understanding in the future, are summarized after the literature survey of each subchapter. The reviewed systems will be introduced within the same sequence as the row and column labelling in Figure 1, i.e. according to the atomic number, starting with the rows as primary criteria. All publications considered for this review fulfil the requirement that intermetallic compounds are involved based on the definition given above. Few exceptions were made, when the discussion of the results and the research background of the authors was related to intermetallic compounds. The type of crystal structure of each of the compounds is given (in brackets). Figure 1 summarizes the systems and reactions which are addressed and covers the timeframe from the first investigations in 1970 until end of September 2018. For all studies in which sufficient information was provided, catalytic properties are summarized in tables at the end of every chapter enabling qualitative comparison.


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Figure 1: Overview of bimetallic systems and electrochemical reactions contained in this review. Bold elements appear in rows and columns to enable a concise scheme. RE stands for rare earth elements. “Other” reactions are electrochemical oxidation reactions of higher alcohols, BH4-, hydrazine, methane and the oxygen evolution reaction (OER).

As many of the elements used in the intermetallic compounds of this review are prone to potential oxidation under the chosen reaction conditions (compare Scheme 1), the stability of the catalysts is a fundamental issue.


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2. Stability Intermetallic compounds can suffer from decomposition under reaction conditions, thus stability investigations are important to be in the position to assign the observed properties as intrinsic to the intermetallic compound, its decomposition products or both. Factors determining the (electro-)chemical stability can be divided into material-specific properties and reaction conditions. Both can vary to a large extent and the first is the result of the chemical bonding, which is rather complex in intermetallic compounds. In few special cases bonding theories, e.g. the Zintl-Klemm concept 19,20 , Hume-Rothery rules 21, the Laves concept 22 or the Brewer-Engel 23 theory, can be applied. However, in the overwhelming number of compounds the chemical bonding has to be analysed individually, e.g. by means of quantum chemical calculations 24. The chemical bonding accounts for changes of the chemical potential and the chemical activity of the involved elements. This implies that thermodynamic stability, i.e. a low Gibbs energy, is not necessarily congruent to electrochemical stability, as the latter depends upon the reaction conditions. Examples for the destabilization of noble metals, as a consequence of intermetallic compound formation with a lesser noble metal, are available. 25–27 While neglecting these (de-)stabilization effects for a first order approximation, the choice of elements for a potentially stable intermetallic compound electrocatalyst can be done according to the electrochemical series and the potential window of the targeted reaction. Scheme 1 may help to make a rough estimation for this purpose, already considering some external factors like the pH value and the applied potential. In addition to this, operating temperature as well as type and concentration of the reactants and electrolyte affect the stability of a catalyst. By definition, a catalyst increases the rate of a reaction without modifying the standard Gibbs energy change, and appears as a reactant as well as a product. 28 To fulfil this requirement, the chosen material must not be prone to the before mentioned factors. To participate in a reaction, the catalyst needs to change at least its electronic and geometric structure on the atomic scale, 29otherwise an interaction with the reactants would not be possible. Thus, it is crucial to keep a balance between a fully inert and an unstable material thus retrieving a material which maintains its activity under reaction conditions over an extended timeframe.


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3. Optimization Strategies Within this chapter the most common effects which are held responsible for activity, selectivity and/or stability gains of the surveyed electrocatalysts will be introduced using the simplified Scheme 2. The terms below will be used consistently throughout this review.

Scheme 2: Summarized optimization strategies, i.e. ligand effects (a), strain effects (b), geometric effects (c) and the bifunctional mechanism (d). Geometric effects (c) are divided into two parts. The first is the site-isolation effect, accompanied by different adsorption possibilities of a molecule, i.e. in the hollow site (Ia), on the bridge site (Ib) or on top of an atom (Ic). The second is the equality of sites (II). The bifunctional mechanism (d) proceeds via an initial spill-over of OH* (red arrow).

Going from an elemental metal M to a bimetallic alloy or intermetallic compound MM´, the coordination of the M atoms usually changes. Replacing M atoms by atoms of the second element M’ will alter the electronic environment of both metals, due to an overlap of bands.

30

In addition,

electrons are transferred partially to the element with the higher electronegativity (M´), resulting in a shift of the d-band centre (εd) relative to the Fermi edge (Ef, Scheme 2a). Upon the formation of intermetallic compounds, the crystal structure changes, thus the alteration of the electronic structure is more severe than for substitutional alloys. Overall, these influences are considered as electronic effects, the so-called ligand effect.

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The ligand effect is considered to act rather locally and

corresponds to the inductive effect in organic or coordination chemistry. The formation of core-shell particles is a common optimization strategy employed on intermetallic compounds. Leaching of the less-noble metal from the near-surface region under reaction conditions leaves behind a shell of the more-noble metal. Due to the interactions between the atoms in the shell and the underlying intermetallic compound, the atomic layers will adapt towards the underlying crystal structure. This results in either longer or shorter distances between atoms within the shell compared 9 ACS Paragon Plus Environment

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to the corresponding bulk element, thus in tensile or compressive strain (Scheme 2b).

31

While the

strain might be recognised as a geometric effect, the small difference in distances is not considered to be of significant influence on how a molecule will be oriented when approaching the surface during the adsorption.

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The much more pronounced effect is of an electronic nature and results from a

down- or up-shifted d-band centre with respect to the Fermi energy, as the d-band becomes more or less dispersed, respectively. 31,32 The effect decreases with the thickness of the shell, as the atoms can relax the strain with increasing distance. 33 The first two effects show very clearly that geometric and electronic influences usually come hand in hand, making it hard to address them separately. Besides the electronic effects described above, geometric effects can also be employed to optimize materials. Any intermetallic compound has an ordered structure, thus the surface sites are also regularly occupied by the different atoms M and M’ (if the structurally ordered surface is present under reaction conditions!). The strong site-preference of the atoms should result in rather similar active sites, which is called site equality (Scheme 2c - II).

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The crystal structure of the intermetallic

compound can also be chosen in such a way that some adsorption configurations are not possible any more. Isolating the atoms M more or less by substituting the nearest neighbours with M’ reduces the possibilities on how atoms or molecules adsorb on the surface. An isolated atom can only adsorb atoms or molecules on top (Scheme 2c - Ia). Two atoms can lead to bridged adsorption (Scheme 2c - Ib), i.e. between the atoms, and an ensemble of three atoms allows for adsorption in a so-called hollow site (Scheme 2c - Ic), i.e. between the three atoms. As each of those adsorption sites provides different adsorption strengths and configurations, this concept of more or less pronounced site isolation can be used to alter reaction pathways. 15,35 If one of the involved metals is more oxophilic than the others, it will have a higher affinity to adsorb e.g. OH* from the electrolyte. This might be beneficial, as it may provide the adsorbed hydroxyl for a consecutive reaction of an intermediate (Scheme 2d - red arrow), adsorbed on a neighbouring M’ atom. This would otherwise happen at much higher potentials, where M is also able to adsorb OH* (Scheme 2d - yellow arrow). This OH*-spillover or bifunctional mechanism is well known for Pt-Ru anodes in direct methanol fuel cells (DMFCs)

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and may be exploited when using metals which are

able to reversibly change from an oxidic to an hydroxidic state by the uptake/liberation of water 37,38.


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4. Electroreduction 4.1. Hydrogen Evolution Reaction (HER) The reaction taking place at the cathode of a water electrolyser is the hydrogen evolution reaction. The HER has two elementary steps. First a proton must be adsorbed on the surface by taking up an electron (1). The second step can proceed via two different mechanisms. Either two adsorbed hydrogen atoms recombine (2), following a Langmuir mechanism, or one adsorbed hydrogen atom reacts with a dissolved proton or water by taking up one electron (3) according to an Eley-Rideal mechanism. The latter path is usually accompanied by a higher apparent activation energy than the recombination on the surface. Based on the Tafel parameter b, determined from polarization curves, it is possible to determine which mechanism is predominant, i.e. low values ~30 mV/decade correspond to a Volmer-Tafel mechanism, whereas ~120 mV/decade correspond to a VolmerHeyrovsky mechanism.

Volmer-step:

M + H2O + e-



MHads + OH-

(1)

Tafel-step:

MHads + MHads



H2 + 2 M

(2)

Heyrovsky-step:

MHads + H2O + e-



H2 + M + OH-

(3)

Which physicochemical parameters influence the HER? In 1949 Leidheiser discovered that the HER overvoltage on d-metals depends on the interatomic spacing and the crystal structure.

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Lowest

overvoltage was extrapolated to interatomic distances of 2.7 Å and bcc metals revealed an overpotential which is ~200 mV higher compared to ccp metals. Platinum with an interatomic spacing of 2.774 Å

40

is known as the most active HER catalyst among the elements. Recently Escudero-

Escribano et al. determined an optimal interatomic spacing of 2.62 Å on a Pt overlayer on top of the intermetallic compound Pt5Tb (Cu5Ca type of crystal structure) for optimal hydrogen adsorption properties.

33

The study was aiming to tune the strain of a platinum surface making use of the

lanthanide contraction and its influence on the ORR activity. Besides this concept of using strain to tune the d-band shift, the altered band-structure scaffold is used to optimize the hydrogen adsorption energy on a catalyst’s surface. For applications, high surface area catalysts can be prepared by e.g. using aluminium and other non-noble metal-based intermetallic compounds for dealloying in alkaline media as in the case of Al7Cu4Ni 41 ((Cu0.80Ni0.20)2.53Al3.5 type of crystal structure) or Raney-nickel 42–44. Hydroxides, formed during this process, can even serve as co-catalysts (bifunctional mechanism) in alkaline conditions and thus improve the sluggish kinetics in comparison to the HER in acidic environments. 45 11 ACS Paragon Plus Environment

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The B-Mo system recently attracted a lot of interest, regarding its application potential in the HER. A comparative study revealed that the HER activity decreases in the order α-MoB2 ≫ β-MoB2 > MoB > Mo2B (AlB2, MoB2, MoB and CuAl2 types of crystal structure, respectively), with α-MoB2 showing exceptional activity of 1 A/cm² at 334 mV overpotential and no degradation over 15.000 cycles between -0.6 – 0 V vs. the reversible hydrogen electrode (RHE). 46 The high activity is related to a high density of active sites, whereas the high stability arises from covalently bonded borophene, i.e. flat 6³ nets (hexagonal net where each atom is part of three hexagons) consisting of boron atoms, which penetrate the three-dimensional lattice of metallically bonded Mo atoms. Results from a different group confirm the activity trend but report a lower durability of α-MoB2 , 47 which might be related to the different preparation method and inclusion of impurity phases. In both studies bulk preparation methods, i.e. 5.2 GPa at 1800 °C 46 and arc melting 47, were used, resulting in low surface area materials. To produce nanoparticulate samples a solid-state metathesis approach was used, resulting in sub 60 nm α-MoB2 particles with small molybdenum and MoB impurities after temperature treatment at 650 °C.

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There is a debate concerning the catalytically active centres based on density functional

theory (DFT) results, which are proposed to be Mo-terminated (001) and (100) facets 46 or B-B bridge sites on B-terminated (001) facets. A chemical vapor deposition approach, depositing boron and boronoxide on Mo foils, resulted in Mo3B thin films (6.48 nm, ordered hexagonal crystal structure) after consecutive reduction and annealing at 900 °C. 49 The thin films exhibit an annealing temperature dependant activity and durability. Mo2B4 (Mo2B4.65 type of crystal structure) which somehow resembles the structural properties of α-MoB2 and β-MoB2 as it has alternating flat 6³- and puckered 6³ nets, respectively, was synthesized using a tin flux as template and diffusion enhancer.

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Since only

structures with flat 6³ netted boron layers are very active and thus only half of the very active reaction sites are present, the surface-specific activity is lower in Mo2B4. This is compensated by a five times higher surface area compared to bulk α-MoB2 and results in a comparable activity at low overpotentials. MoB (MoB type of crystal structure) was used in a Schottky junction along with g-C3N4. 51

A Schottky junction is an interface between a metal and a semiconductor, leading to a charge

redistribution in the metal, which might be beneficial for catalysis. 51 Here, the authors were able to prove that the semiconductor-metal mix has a five times higher specific activity at -152 mV vs. RHE compared to MoB alone.

AlTi (CuAu type of crystal structure), AlFe and AlNi (both CsCl type of crystal structure) were investigated by Ezaki et al. 52 by means of their hydrogen overvoltage at a certain current density. The overpotential of these Al-based compounds decreases in the order of Ti > Fe > Ni. This trend is also observed for the elements. Since the stability was not investigated, it might be that aluminium was 12 ACS Paragon Plus Environment

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leached from the intermetallic compounds, leaving behind the transition elements as high surface area catalysts. Hf2Fe and Hf2Co (Ti2Ni type of crystal structure) are claimed to be superior HER catalysts due to their high hydrogen absorption capacity and a synergism caused by the interionic bonding theory, 53,54 which was proposed by Jaksic 55. Jaksic adopted the Brewer-Engel valence bond theory on electrocatalytic hydrogen evolution, stating that hypo-hyper-d element combinations, i.e. combinations of two transition metals, one having un- or semi-filled d-orbitals while the other has at least one fully filled dorbital, not only give exceptionally temperature-stable materials but also offer the best electrocatalytic properties for the HER.

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The synergism is ascribed to an electronic effect by the hyper-d element

donating electrons into the unfilled d-band of the hypo-d element. This hypothesis was corroborated by investigations of various systems, which will be referenced within this section. In a later publication, the hypothesis was refined and extended by an interionic synergism, now also including a concept for ionic activators (salts which are added to the electrolysis bath), having a huge influence on the electrocatalytic properties during the electrolysis of water. 56 Various rare earth metals (Y, Sm, Ce, Mm (Mm = mischmetal, i.e. a mixture of mainly Ce, La and Nd) have been reacted with iron, leading to iron alloys with domains of intermetallic compounds (mainly Th2Zn17 type of crystal structure), having partly improved performances compared to industrially used electrodes.

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This is ascribed to an electronic synergism based on the Brewer-Engel valence bond

theory 55. Nanostructured Co-Te catalysts were synthesized by the conversion of pre-prepared cobalthydroxycarbonate nanorods on carbon fiber paper.

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The resulting CoTe2 nanodendrites grown on

CoTe2 nanotubes (presumably FeS2 type of crystal structure), CoTe nanotubes (NiAs type of crystal structure) and nanosheets of CoTe/CoTe2 nanorods (NiAs and FeS2 types of crystal structure, respectively) were investigated in the HER. While the first two samples show a comparable activity, the overpotential of CoTe/CoTe2 nanosheets/nanotube assembly is approximately 120 mV lower. The higher activity is related to a lower charge transfer resistance, believed to arise from a synergy between CoTe and CoTe2, and a higher electrochemical active surface area (ECSA) compared to the other two investigated samples. Nickel-based systems have been investigated as potential platinum replacement. Ni3Al (Cu3Au type of crystal structure) prepared by powder metallurgical methods, shows increased long-term stability over porous nickel electrodes in alkaline media. This is attributed to the formation of a passivation layer under anodic conditions, which is easily reduced when reversing the current. Even prolonged immersion under highly caustic environments does not significantly alter the HER slope. 59,60 Different studies on the Al-Ni system, where mixtures of intermetallic compounds and alloys are formed, come 13 ACS Paragon Plus Environment

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to the conclusion that selective aluminium leaching leads to a highly porous nickel sponge with increased HER activity due to its high specific surface area. 42–44 In the Ni-Ti system, TiNi3 (TiNi3 type of crystal structure), which does not form hydrides, is a good HER catalyst, whereas TiNi (CsCl type of crystal structure), having octahedral voids in its structure 61, does form hydrides and has a higher overvoltage in the HER. The HER activity is indirectly related to the hydride formation by the adsorption strength of hydrogen.

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The Ti-Ni system is claimed to follow

Jaksic’s adaptation of the hypo-hyper-d valence bond theory. A Ni-Zn coating was prepared by co-depositing both metals out of the solution. Here the intermetallic compounds NiZn3, Ni2Zn11 and Ni3Zn22 (Zn9(Zn0.5Fe0.5)2F, Zn9(Zn0.5Fe0.5)2F, Zn22Ni3 types of crystal structure, respectively) are formed.

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The authors attribute the improved HER characteristics to a

higher surface area, which could further be increased by adding CeO2 in micro- and nanoparticulate form to the deposition bath, which was the main aim of the study. The Ni-Zr system, having several intermetallic compounds, is also claimed to be an ideal example for the hypo-hyper-d valence bond theory but no results have been published so far. 56 The Mo-Ni system attracted interest since Conway et al.

64

studied electroplated deposits as HER

catalysts in 1984. Later, bulk electrodes of Mo-Ni alloys were investigated by Chialvo et al. in the range of 0 - 25 % Mo, but only fcc substitutional alloys were obtained. 65 Jaksic’s group was the first who was able to investigate mixtures of Mo-Ni intermetallic compounds in the HER as proven by XRD data and interpreted on behalf of the hypo-hyper-d synergism.

66

Here, significantly improved activities

compared to previously reported substitutional alloys were found. The highest activity was observed for NiMo (Mo3(Mo0.8Ni0.2)5Ni6 type of crystal structure), still limited by a rate-determining Heyrovsky step, which was also observed for Ni3Mo and Ni4Mo samples (Cu3Ti and Ni4Mo types of crystal structures, respectively). Panek et al. 67 ball-milled a mixture of nickel and molybdenum and subjected them subsequently to different heat treatments. While annealing at 300 °C did not change the phase composition, annealing at 800 °C lead to the formation of the intermetallic compound NiMo and traces of MoO2. This material showed an increased intrinsic activity over the alloy, even though its apparent activity was lower due to a lower ECSA, resulting from the high temperature annealing. Csernica et al. 68 were able to synthesize single-phase NiMo as well as other Mo-Ni phases with small impurities. Their

HER investigations verified the results obtained by Jaksic et al. 66, stating NiMo having an increased intrinsic activity over Mo-Ni alloys. Schalenbach et al. 69 investigated a series of Mo-Ni alloys concerning dissolution behaviour and catalytic HER properties, with the aim to clarify the reason for the increased activity. The X-ray diffraction patterns from the study suggest that the intermetallic compound NiMo is also present in the alloys. The high activity of the alloys, however, is attributed to a high-surface area, which evolves because of selective molybdenum leaching in the alkaline electrolyte. This does 14 ACS Paragon Plus Environment

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ACS Catalysis

not necessarily hold true for intermetallic compounds, as Csernica et al. 68 were able to prove that the specific activity of the intermetallic compounds in the system is higher than the one of the alloys.

Figure 2: Synthesis scheme of Ni4Mo supported on MoO2 cuboids on nickel foam. Scale bars for Ni foam, 20 µm (top) and 1 µm (bottom); NiMoO4/Ni foam, 10 µm (top) and 2 µm (bottom); Ni4Mo/MoO2/Ni foam, 20 µm (top) and 1 µm (bottom). Reproduced from Zhang and co-workers 70.

MoNi4 nanoparticles/MoO2 cuboids supported on nickel foam (Figure 2) were reported to be a superior catalyst for the alkaline HER due to a decreased apparent activation energy of both the Volmer (1) and the Tafel (2) step. 70 The exceptional electrocatalytic performance, which exceeds the one of platinum, is attributed to both high surface area and intrinsic activity. The diffraction pattern, however, is not unambiguous. Similar results concerning the high activity of Ni4Mo on nanostructured catalysts were found for porous Ni4Mo networks, prepared by a hydrothermal synthesis using nickel foam and a molybdate precursor. 71 In addition to its good HER activity, the catalyst is also active in the OER, thus a viable catalyst for overall water splitting. Another approach for high-surface-area porous Mo-Ni catalysts (partially covered with N-doped graphene), leads to a mixture of NiMo and Ni4Mo.

72

The

interplay of graphene and the subsurface Mo-Ni was investigated in detail, revealing higher catalytic 15 ACS Paragon Plus Environment

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Page 16 of 122

activity due to a synergism of the holes in the graphene layer and the underlying partially structurally ordered catalyst. The graphene layer also acts as dissolution barrier in the acidic electrolyte, which significantly improves the stability during long-term operation compared to a non-coated Mo-Ni catalyst. To our knowledge no reports exist on the HER properties of the Ni4W intermetallic compound, which is isostructural to Ni4Mo, as mainly alloys of the nickel–tungsten system were investigated for the HER, which already got reviewed by Allahyarzadeh et al.. 73 Electrolytic Ni-Sn coatings were prepared by co-deposition of nickel and tin using different deposition current densities. 74 The lowest applied current density of -2 mA/cm² lead to a mixed formation of the compounds Ni3Sn (Cu3Au type of crystal structure) and Ni3Sn4 (Ni3Sn4 type of crystal structure) which showed the highest deviation of exchange current density compared to nickel. The material was not further investigated, due to its high Tafel slope compared to alloy-like deposits. Those had improved HER properties as a result of their higher specific surface area. A more systematic investigation of the nickel-tin system by Belanger 75,76 revealed decreased anodic dissolution behaviour for Ni3Sn2 (Ni3Sn2 type of crystal structure), compared to alloys, other intermetallic compounds and elemental nickel and tin, while exhibiting a catalytic activity comparable to nickel. The aim of the studies was to determine the general influence of intermetallic compound formation on the electrochemical behaviour in comparison to substitutional alloys. The improved stability, however, could not be explained. Intermetallic compounds can be prone to hydride formation. Well known examples are LaNi5 or MmNi5 (CaCu5 types of crystal structure). These were investigated in the HER by Kitamura et al. 77,78. Exchange current densities were evaluated and found to be comparable to palladium and platinum. In addition, increased current densities (>1 mA/cm²) lead to embrittlement, while raising the temperature makes them more stable.

79

A process for preparing a stable structurally ordered Ni-RE (RE = rare earth

element) electrode was patented in the mid 1980s. 80 A recent approach aims for a monolithic catalyst design (Figure 3) by selective aluminium leaching of a precursor with the composition Cu0.20-xNixAl0.80 consisting of three different phases, namely α-Al, CuAl2 (CuAl2 type of crystal structure) and Al7Cu4Ni ((Cu0.80Ni0.20)2.53Al3.5 type of crystal structure).

41

While α-Al gets completely dissolved, microstructured Cu remains from leaching of CuAl2. The intermetallic compound Al7Cu4Ni particles remain as particles covered by a fcc-Cu0.80Ni0.20 shell. The particles are seamlessly integrated in the copper matrix, thus allowing for their direct use in the HER. It is assumed that the structurally ordered core hinders the alloy-like shell’s reorganization, whereas the shell hinders further leaching. The catalyst is stable at room temperature in 0.1 M KOH solution holding a current density of -10 mA/cm² but is prone to further Al-leaching at 50 °C in a 1 M KOH solution. Even though this might hinder industrial application, the demonstrated concept of micro- and 16 ACS Paragon Plus Environment

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ACS Catalysis

nanostructured monoliths, using a nobler metal backbone and intermetallic compound crystallites, respectively, has several benefits over conventional nanoparticulate approaches, i.e. high electric conductivity of the support, strongly bonded and thus stable active material and the ease of preparation.

Figure 3: Structural characterization of the monolithic Al7Cu4Ni@Cu80Ni20/Cu. X-ray diffraction pattern (A). SEM micrograph showing the nanoporous microstructure (B). Crystal structure representations of Cu80Ni20 and Al7Cu4Ni (C). HR-TEM micrographs indicating characteristic lattice spacings, as well as FFT patterns (D). Reprinted with permission from Sun and co-workers 41. Copyright (2018) Wiley.

A screening of a huge variety of Bi-Pd thin films for their properties in the HER, prepared by high throughput physical vapour deposition (PVD), revealed that an atomic ratio of roughly 70/30 is the most active. This could be attributed to the presence of monoclinic PdBi2 (PdBi2 type of crystal structure). 81 Dealloyed PdBi2 nanoparticles offer increased stability over platinum and higher activity than elemental Pd/C for the HER, 82 whereas the monoclinic low-temperature phase offers better HER characteristics than the tetragonal high-temperature phase (CuZr2 type of crystal structure). Fully ordered PtFe (CuAu type of crystal structure) was tested in the HER under acidic reaction conditions and was found to have a higher activity and durability compared to Pt/C. As the catalyst was meant to be used in the ORR, no further investigations and interpretations were made. 83 Ge2Pt and GePt3 nanoparticles (CaCl2 and GePt3 types of crystal structure) were prepared by stepwise reduction of the precursors and consecutive annealing.

84

While Ge2Pt shows no appreciable HER

activity, GePt3 is comparable to elemental platinum, showing no signs of degredation over 10.000 CV cycles as well as a 12 h chronoamperometric HER test. 17 ACS Paragon Plus Environment

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Page 18 of 122

Several compositions in the Mo-Pt system were investigated for their HER properties in the light of the hypo-hyper-d valence bond theory adapted by Jaksic, but the use of single-phase intermetallic compounds was not proven. 85 Further publications, investigating similar Mo-Pt compositions for their energy consumption to produce a certain amount of hydrogen, without 86 and with 87 ionic activators, claim but do not prove the use of intermetallic compounds. DyPt and HoPt (both FeB type of crystal structure) were synthesized by arc-melting of the elements and tested in the HER. 88 Both catalysts have comparable Tafel slopes to platinum but higher exchange current densities. This is explained by hydride formation, as well as the impact of Engel-Brewer theory on the electrocatalytic properties. Table 1 summarizes all relevant publications and gives a comparison of the different materials – if sufficient data is provided in the corresponding references. In conclusion, most studies on nonprecious metals try to resemble the structural and/or electronic properties of platinum, which is the most active element in the HER. While hydride formation may be beneficial for the HER, it can also cause problems like in the case of LaNi5 or elemental palladium. As selectivity is of no concern in this reaction, geometric properties of altered crystal structures seem to be of minor importance compared to electronic effects, arising from the chemical bonding within the intermetallic compounds. The ligand effect which describes partial electron transfers from one atom to one of another kind, thus covering the hypo-hyper-d valence bond theory, can shift the d-band centre, alter the adsorption properties of hydrogen and thus directly influences the catalytic activity of a material as shown by volcano plots. 66 This can be also achieved by strain effects as they are observed for core-shell assemblies. The bifunctional mechanism, making use of the OH* adsorption properties of less-noble metals like molybdenum is also beneficial, as water can be split more readily if the H-OH bond is already weakened.


Page 19 of 122

Q

η(10mA/cm²) vs. RHE

Electrolyte

j0

State

b

Preparation

j

Compound/System

M

type

pH

mV

mA/cm²

mV/dec.

mA/cm²

kWh/m³

mV

Ref.

Table 1 Catalytic properties of intermetallic compounds in the hydrogen evolution reaction (HER). EWE vs. RHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

ACS Catalysis

AlFe

arc-melting, annealing

pellet

0.5

H2SO4

0.0

-346

10

52

AlNi


pellet

0.5

H2SO4

0.0

-176

10

52

AlTi


pellet

0.5

H2SO4

0.0

-556

10

52

Fe-Y

induction melting

pellet

1

NaOH

14.0

-470

250

-125

0.001

57

Ce-Fe

induction melting

pellet

1

NaOH

14.0

-400

250

-100

0.044

57

Fe-Sm

induction melting

pellet

1

NaOH

14.0

-450

250

-115

0.005

57

FeHf2

induction melting

pellet

9.96

NaOH

15.0

α-MoB2

mixing, 5.2 GPa @ 1800 °C

pellet

0.5

H2SO4

0.0

-334

1000

-74.2

α-MoB2


80% towards CO2, as a consequence of the already mentioned bifunctional mechanism, the mixed system revealed an increased activity of approximately 1.8. This is ascribed to more favourable adsorption of methanol on three neighbouring Pt sites, which are not present in PtSn. Similar conclusions were also made by Borbath et al., investigating a series of Pt-Sn materials, amongst them the intermetallic compound Pt3Sn. 299 PtSb (NiAs type of crystal structure), prepared by arc-melting of pressed powders, shows a significantly enhanced onset potential of more than 100 mV, increased current densities and a higher ratio between the peak current density of the forward and backward CV scan (If/Ib), which is believed to be an indicator for complete oxidation of carbonaceous species. 300 The authors ascribe the rise in activity to the higher Pt-Pt distance (active-site isolation), which hinders strong CO adsorption on bridged- and hollow-sites as well as electronic effects. The latter alter the adsorption properties, resulting from a charge transfer from Pt to Sb, thus creating d-band vacancies on the platinum-sites. Matsumoto screened a series of bulk intermetallic compounds for their application in the MOR, from which he selected PtPb, PtBi and PtBi2 (NiAs, NiAs and FeS2 types of crystal structures) for further characterization, as the others did not show appreciable activity. 301 Gregoire et al. tested a sputter-deposited library of the Pt-Ta system with varying compositions in the MOR. 255 Intrinsic catalytic properties could not be assigned, as only mixtures of different phases could be tested with the applied methodology. However, the catalytic activity is promoted by the presence of tantalum oxides. Out of the investigated intermetallic compounds from the screening of Casado-Rivera et al., only PtIn (CuAl type of crystal structure) and PtPb (NiAs type of crystal structure) showed a remarkable peak current density in the MOR. 259 However, a more recent publication reports on PtBi (NiAs type of crystal structure) nanoplatelets having a higher MOR activity and long-term (4.000 s CA) stability over elemental platinum due to less favourable adsorption of CO as a consequence of isolated platinum sites. 268 Additionally, bismuth atoms on the surface provide hydroxyl groups to adjacent Pt sites, which makes CO oxidation possible at lower potentials. PtBi@Pt (PtBi: NiAs type of crystal structure) nanoplatelets produced via co-reduction in the presence of capping agents, were tested in the MOR 65 ACS Paragon Plus Environment

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Page 66 of 122

and exhibit a 7.4 times increased specific activity over Pt/C, while the mass-specific activity is 3.7 times higher. 220 PtPb/C nanorods prepared by co-reduction of acetonates in the presence of capping agents were tested in the MOR and had a 1.6 times higher peak mass current density compared to a commercial PtRu/C.

302

Liu et al. prepared PtPb nanoparticles by a pyrolysis route of acetylacetonates in the

presence of oleic acid and oleylamin, to evaluate half-cell tests for the MOR, besides CV and CA studies. 275 As expected, PtPb shows higher power densities of 75 mW/cm² compared to the 55 mW/cm² of the Pt-Ru benchmark. Another approach synthesized PtPb by a microwave assisted polyol method and ascribes the high durability and activity to the presence of a high surface coverage with oxidic species, facilitating CO oxidation.

276

Defect engineered PtPb@Pt core/shell hexagonal

nanoplates, prepared via co-reduction and consecutive C+ sputtering, were tested in the MOR, evaluating the influence of the defect density. 216 The highest activity was obtained with nanoplates having a partially amorphous structure, besides dislocations and subgrain boundaries in the ordered crystal structure. To shed light on the activity improvement over samples having a completely ordered NiAs type of crystal structure and samples being totally amorphous, a subsequent study aimed to optimize the interface between crystalline and amorphous domains. 303 The outcome were hexagonal nanoplates having structurally ordered centres, while the outer spheres of the plate were amorphous. Those optimized nanoplates have an increased mass-specific activity by factors of 1.4, 1.7 and 3.6 over fully ordered PtPb@Pt/C, fully amorphous PtPb/C and Pt/C, respectively. The activity enhancement based on density functional theory results is related to a promoted bond activation of C-H and O-H, along with more favourable adsorption energies of OH and CO. Those effects arise from strain-, ligandand geometric effects, which offer optimal properties at the interface of crystalline and amorphous domains. A spillover of co-adsorbed OH* from amorphous Pb sites towards interfacial Pt sites (bifunctional mechanism) is also likely. Pt3Pb@PtPb nanoparticles prepared by co-reduction, were reported to be more active in the MOR than nanoparticles of the corresponding single-phase intermetallic compounds. The activity gain over PtPb nanoparticles is explained by a stretched shell, hindering adsorption of CO.

304

PtPb hexagonal

nanoplatelets covered with a Pt shell were synthesized in a solution of octadecene and oleylamine by co-reduction.

215

The MOR mass-specific activity is 2.4 and 7.9 times higher compared to PtBi

nanoparticles and Pt/C, respectively. The numerical values for the catalytic studies presented in this section are summarised in Table 6, if sufficient data was given in the publication. Methanol is readily (electro)oxidized on elemental platinum. Overpotentials and reaction rates, however, are improvable. The most common interpretation of increased activities and durabilities is 66 ACS Paragon Plus Environment

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ACS Catalysis

the bifunctional mechanism, aiding in further oxidation of reaction intermediates, especially CO. However, the atomic arrangement of the ordered compounds helps by site isolation and may improve the adsorption rate of methanol on the catalyst, which preferably happens on the hollow site of three neighbouring Pt atoms. Electronic effects from strain and ligands alter the adsorption energies of intermediates, leading to increased reaction rates or might completely prevent CO formation during the reaction.


ACS Catalysis

peak masscurrent density

CA current density

CA mass-current density

CA measured @ vs. RHE

State

peak current density

Preparation

peak curren potential vs. RHE

Compound /System

M

M

type

pH

mV

mV

mA/cm²

mA/mg (PGM)

mA/cm²

mA/mg (PGM)

mV

0.5

0.1 H2SO4

0.7

290

640

0.09

rpm MeOH

2000

Electrolyte

Ref.

Table 6 Catalytic properties of intermetallic compounds in the methanol oxidation reaction (MOR). Oxidation Onset vs. RHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 68 of 122

Pt3Ti

co-reduction

NPs/C

Pt3Ti

co-reduction

NPs/C

1

0.1 HClO4

1.0

328

Pt8Ti

pyrolysis

NPs/TiO2/N-doped CNTs

0.5

0.5 HClO4

0.3

340

Pt3Ti


Bulk

2000

0.5

0.1

KOH

13.0

578

1.23

301

PtTi


Bulk

2000

0.5

0.1

KOH

13.0

588

0.12

301

Pt3V

co-reduction

NPs/C

1

0.1 HClO4

1.0

328

0.03

149

PtFe

annealed in MCM41, unleached

NPs/C

1

0.1 HClO4

1.0

4.40

PtFe

annealed in MCM41, AcOH leached

NPs/C

1

0.1 HClO4

1.0

PtFe

annealed in MCM41, HCl leached

NPs/C

1

0.1 HClO4

PtFe

annealed in MCM41, HF leached

NPs/C

1

PtFe

impregnation, annealing (H2)

NPs/C

PtFe0.7Cu0.3


PtFe0.5Cu0.5

1000

246

0.04

200

0.19

29

39 6.00

249

500

288

858

249

629

950

198

4.62

879

950

198

1.0

4.89

939

950

198

0.1 HClO4

1.0

4.00

1435

950

198

1

0.5 H2SO4

0.0

3.56

800

~13

800

290

NPs/C

1

0.5 H2SO5

0.3

2.53

637

~68

800

290


NPs/C

1

0.5 H2SO6

0.3

2.06

563

~18

800

290

PtCu

co-reduction

NPs/C

1

0.1 HClO4

1.0

2.50

PtZn

vapor solid reaction

NPs/C

0.2

0.1 H2SO4

0.7

Pt3Zn

co-reduction, annealing

NPs/C

0.5

0.1 H2SO4

0.7

1600

46

858

183

500

292

960

260

294


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ACS Catalysis

Pt3Ga

co-reduction

NPs/C

1

0.5 H2SO4

0.0

Pt3Ge2


Bulk

2000

PtZr


Bulk

Pt3Nb


Bulk

PtCd


NPs/C

PtIn


Bulk

Pt2In3

solvothermal

NPs/C

0.5

0.1

KOH

13.0

568

0.78

301

2000

0.5

0.1

KOH

13.0

588

0.10

301

2000

0.5

0.1

KOH

13.0

508

0.57

301

0.5

0.1 H2SO4

0.7

520

840

2.59

253

0.125 0.1 HClO4

1.0

588

868

1.10

259

907

0.76

PtIn2


Bulk

PtSn


Bulk

Pt3Sn


Bulk

Pt2Sn3


Bulk

PtSb


PtSb

1

7.20

0.5 HClO4

0.3

0.5

0.1

13.0

598

0.5

0.1 H2SO4

0.7

540

0.5

0.1

KOH

13.0

0.5

0.1

KOH

Bulk

0.25

arc-melting, sintering

Bulk

PtSb


Bulk

Pt3Ta


Pt2Ta

2000

326

1.10

849

~0.5

296

817

297

0.42

301

0.11

259

568

0.35

301

13.0

588

0.70

301

0.1 H2SO4

0.7

670

860

0.04

259

0.5

0.5 H2SO4

0.0

570

860

0.45

2000

0.5

0.1

KOH

13.0

548

0.05

301

Bulk

2000

0.5

0.1

KOH

13.0

588

0.56

301


Bulk

2000

0.5

0.1

KOH

13.0

668

0.47

301

PtPb


Bulk

0.2

0.1 H2SO4

0.7

530

8.00

259

PtPb

co-reduction

nanoplatelets/C

0.1

0.1 HClO4

1.0

1500

170

703

215

PtPb

co-reduction

NPs/C

0.1

0.1 HClO4

1.0

625

25

703

215

PtPb

co-reduction

NPs/C

0.5

0.5 H2SO4

0.0

299

~780

80

599

275

PtPb

microwave

NPs/C

1

0.5 H2SO4

0.0

580

1139

53

580

276

2000 2000

KOH

1094

840

870

910

~8

0.37

300


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

PtPb


Bulk

PtPb

co-reduction

nanorods/C

PtPb

co-reduction

NPs/C

PtBi

co-reduction with capping agents

PtBi

2000

0.5

0.1

0.5

0.1 H2SO4

0.7

0.5

0.1 H2SO4

0.7

nanoplatelets/C

0.1

0.1 HClO4

1.0


Bulk

0.5

0.5 HClO4

0.3

PtBi


Bulk

0.5

0.1

KOH

13.0

558

4.27

301

PtBi2


Bulk

0.5

0.1

KOH

13.0

568

2.05

301

PtBi

co-reduction

NPs/C

0.5

0.1 H2SO4

0.7

540

2000

2000 2000 2000

KOH

Page 70 of 122

13.0

478

10.00 -

540

873

301

700

302

1.85 3.18

877

305

1100 470

60

670

220

867

268

305


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ACS Catalysis

5.4.Ethanol Oxidation Reaction (EtOR) Within the scope of this review, the ethanol oxidation reaction (EtOR) is the most challenging reaction, due to the necessity to cleave a C-C bond to achieve a full oxidation leading to CO2 formation. To achieve this, most of the herein reviewed contributions will introduce a more oxophilic element to form a palladium- or platinum-based intermetallic compound. This might facilitate a bifunctional mechanism, providing activated *OH for the formation of C-O bonds. It is also reported that the selectivity of early hydrogen abstraction, i.e. from the α-C or β-C, is decisive on whether a C-C cleavage takes place or not. 306 A series of Cu-Pd alloy nanoparticles with and without partially doping by ~20 At.-% Ni or Co are converted into structurally ordered nanoparticles with the CsCl type of crystal structure and a Pdenriched shell by thermal treatment, leading to nominal compositions of Pd38Cu42Ni20 and Pd39Cu42Co19.

128

As a result, their EtOR (mass-)activity (in the sequence PdCu < PdCuNi < PdCuCo =

7.72 A/mgPd) and long-term stability are increased. The increased long-term stability is attributed to an increased CO-poisoning resistivity arising from the structural ordering, which was derived from the forward/backward peak ratios. Also, a higher activity and structural stability compared to the disordered counterparts could be demonstrated. Pd2Ge (Fe2P type of crystal structure) nanoparticles were synthesized by a solvothermal method and the influence of reaction time on crystallinity and activity in the EtOR was studied. 307 While samples heated for 24 h, which results in a highly defective crystal structure, already offer a higher activity than Pd/C, heating for 36 hours led to full site occupation and even higher activity. The catalytic activity is related to partial dealloying under reaction conditions. In-Pd catalysts of different compositions were prepared by precipitation of their salts on silica, reduced at 300 °C under hydrogen stream, and etched in caustic solution to remove silica. 285 Since a mixture of intermetallic compounds (InPd3, InPd and In7Pd3 with TiAl3, CsCl and Ru3Sn7 types of crystal structure, respectively) is obtained, no further conclusions despite a positive effect of indium addition to the EtOR is reported. Ordered Pd2Sn (Co2Si type of crystal structure) nanoparticles offer increased activity and durability in the EtOR compared to the substitutional fcc alloy as well as elemental palladium, ascribed to more favourable adsorption energies of intermediates on the intermetallic catalyst, because of an electron transfer from Pd to Sn.

308

Nonetheless, the reaction is still limited by the conversion of adsorbed

acetaldehyde to acetic acid, which however is promoted by hydroxyl species spilled over from SnO2 patches, emerging on the surface. Pd2Sn nanoribbons grown along [010] with confined size were synthesized by co-reduction of the precursors in the presence of capping agents.

309

Based on DFT 71

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Page 72 of 122

calculations, the presence of predominantly (100) and (001) facets is considered to be responsible for the increased activity compared to Pd2Sn nanoparticles and Pd/C and could be optimized by tuning the size of the nanoribbons. In fact, the highest surface-specific as well as mass-specific peak current densities were achieved for 9 x 26 nm ribbons being both higher by a factor of 9 compared to Pd/C and nanoribbons in other dimensions. The high durability was proven in a 2 h CA test where the current stabilized on a value of 10.4 mA/cm² while the nanoparticles only achieved 0.8 mA/cm². Pd3Pb (Cu3Au type of crystal structure) nanowire networks show an enhanced catalytic activity compared to Pd black in the EtOR by a factor of two.

134

The authors claim the higher stability and

activity to be due to an increased number of reaction sites and less favourable adsorption of reaction intermediates. The latter is assumed to arise from improved geometric and electronic effects of the intermetallic compound compared to elemental palladium. Ghavidel et al. investigated a series of initially disordered Pt25Mn75 nanoparticles, prepared by coreduction of the precursors, which were annealed at different temperatures and times. 310 The best result for the EtOR was achieved for samples treated at 700 °C for 4 h, as they formed predominantly PtMn (CuAu type of crystal structure) along with PtMn3 (Cu3Au type of crystal structure) and a MnOxshell, the latter dissolved by contact with the acidic solution. The improved performance is believed to substantiate from an electronic effect. A consecutive publication by the same authors report on the addition of sodium citrate during the co-reduction process, to improve the dispersion and control the size of the nanoparticles, which in turn is improving the peak current density of the particles by 30%, reaching 8.4 mA/cm². 311 The data are not contained in Table 7 as the phase composition of the catalyst is not clear. Nanoparticles of Pt3Co (Cu3Au type of crystal structure) covered with a few layers of platinum were tested in the EtOR and compared with commercial Pt/C in terms of activity and selectivity.

306

The

intermetallic compound has a higher 2.5 times increased mass-specific activity over the benchmark. However, based on DFT results the splitting of the α-C-H bond is preferred over β-C-H cleavage on the surface of the Pt3Co@Pt, thus the ratio between acetic acid and CO2 as final products is severely higher compared to Pt/C, as observed in FTIR spectra. Pt3Nb (Cu3Ti type of crystal structure) nanoparticles, synthesized by co-reduction of the precursors and consecutive annealing, were tested in the EtOR and a higher catalytic activity compared to platinum was found. 312 The activity gain is attributed to a d-band shift in Pt3Nb with respect to platinum, making the intermetallic compound less prone to CO poisoning. ZrPt3 nanoparticles annealed at 900 and 1000 °C offer different catalytic activities, similar and twice as high with respect to Pt, respectively. 251 While the former one adopts a cubic structure of the Cu3Au type of crystal structure, the latter forms a hexagonal structure (TiNi3 type of crystal structure). The 72 ACS Paragon Plus Environment

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ACS Catalysis

differences in catalytic activity and durability are explained on the basis of the crystal structures. The TiNi3 type, according to hard X-ray photo emission spectroscopy (HX-PES), leads to a higher binding energy for Zr and Pt. A couple of intermetallic compounds (PtBi, PtPb, PtIn, PtMn, PtSn, PtSb, with NiAs, NiAs, CuAl, CuAu, NiAs and NiAs type of crystal structure, respectively)) were tested in the EtOR by Casado-Rivera et al., the first three showing the highest activity. However, full oxidation of ethanol was not observed and the reaction most likely stopped at the intermediate acetic acid, as C-C cleavage did not occur. 259 Single-phase In2Pt3 (Tl2Pt3 type of crystal structure) synthesized by solvothermal reduction shows a higher EtOR activity and also a significantly improved stability compared to elemental Pt.

297

As no

further product analysis was done, a final reduction product is not stated and it cannot be judged whether a full oxidation to CO2 was achieved. Pt3Sn (Cu3Au type of crystal structure) nanoparticles, prepared by co-reduction and consecutive annealing, were investigated in detail by means of DEMS and IRRAS investigations.

313

The authors

come to the conclusion that, even though the oxidation rate of ethanol on the intermetallic nanoparticles is higher compared to platinum, the selectivity towards CO2 is only 3.7%, compared to 8.5% on platinum, as C-C cleavage does hardly occur and the main product is acetaldehyde. A study of Kwak et al. confirms the high oxidation rate in reference to platinum, however, their carbon supported Pt3Sn nanoparticles show significant differences in the onset potential of the EtOR, compared to the aforementioned results313.

314

The difference might be due to the different reaction conditions but

could also arise from the absence of an ordered structure, which cannot be concluded based on the presented diffraction patterns. Pt3Ta (NbPt3 type of crystal structure) nanoparticles prepared by co-reduction were reported to have superior activity compared to state-of-the-art Pt3Sn/C and Pt/C (Figure 12). 315 This, however, only holds true for surface-specific activity, as the mass-specific activity is much smaller compared to the benchmarks. Nevertheless, full oxidation of ethanol to CO2 is reported based on IRRAS spectra. Facilitated C-C cleavage leads to the formation of adsorbed CO and the high CO oxidation activity to CO2 on the surface of Pt3Ta arises from a high surface coverage of oxygenated Ta species. Single-cell tests for the direct ethanol fuel cell (DEFC) were also conducted and resulted in a CO poisoning tolerance comparable to the state-of-the-art catalysts Pt3Sn (Cu3Au type of crystal structure), while obtaining 50% of the power density. The authors note, however, that the particle size in comparison to Pt3Sn is 100 times higher.


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Figure 12: Full oxidation of ethanol on Pt3Ta. (a) and (b) cyclic voltammetry profiles for Pt3Ta, Pt and Pt3Sn/C in 1.5 M ethanol solution. (c) Variation of ECSA of Pt3Ta and Pt as a function of potential cycles. (d) I–V profiles and power density profiles of Pt3Ta and Pt, obtained at room temperature in 1 M ethanol solution. In situ IRRAS spectra (1900 to 2500 cm-1) of Pt3Ta (e) and Pt. (f) In situ IRRAS spectra (2200 to 2500 cm-1) of Pt3Ta (g) and Pt (h). The broken line in (h) located at 2350 cm-1 corresponds to the peak position for atmospheric CO2. Adapted with permission from Kodiyath and co-workers 315. Copyright (2015) Royal Society of Chemistry.

Pt3Pb@PtPb nanoparticles prepared by a facile co-reduction method, were reported to be more active in the EtOR than nanoparticles only containing the intermetallic compounds.

304

The activity gain is

explained by a high fraction of OH* groups adsorbed on Pb-surface atoms. PtPb (NiAs type of crystal structure) nanoplatelets with hexagonal morphology and covered by a Pt-shell were synthesized in a solution of octadecene and oleylamine via co-reduction. 215 The material shows an EtOR mass-specific activity which is 2.5 and 8.8 times higher compared to PtBi (NiAs type of crystal structure) nanoparticles and Pt/C, respectively. Comparing hexagonal shaped PtPb@Pt/C nanoplatelets with and without defects, which were introduced by C+ sputtering, the defect-rich samples, comprising partly amorphous regions, had a higher mass-specific activity towards the EtOR by a factor 1.85.

216

Both

samples have a better activity than Pt/C. The activity gain is attributed to the interface between crystalline and amorphous domains, being more active than pure amorphous or crystalline samples of Pb-Pt. Whether a full oxidation towards carbon dioxide was achieved was not investigated. 74 ACS Paragon Plus Environment

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ACS Catalysis

As most studies using intermetallic compounds in the EtOR focus on the catalytic activity but not on the product distribution, a general concept on the influence of electronic or geometric parameters on the product formation cannot be deduced. In general, higher current densities compared to elemental platinum or disordered alloys are obtained. This may be interpreted as a great achievement but could also mean, that the first reaction step is proceeding faster. The dehydrogenation of ethanol to acetic aldehyde is the most dominant and often also exclusive reaction happening on the electrodes’ surface, as was concluded by Herranz et al. 313. Quick liberation of this reaction intermediate might mimic the behaviour of a non-poisoned catalyst, which fully oxidises the ethanol. Increased desorption rates for acetic aldehyde may arise from altered adsorption properties, as a consequence of electronic effects. To facilitate C-C cleavage, the supply of readily available OH* species over a bifunctional mechanism might be crucial, allowing for further oxidation of acetic aldehyde. Intermetallic compounds containing highly oxophilic elements are able to provide such properties and thus are able to fully oxidize ethanol, as was demonstrated for Pt3Ta

315.

Nevertheless, catalytic results retrieved from CVs and

chronoamperometric measurements were collected if retrievable and summed up in Table 7 to allow for a quick comparison, even if this does not tell about the final product of the oxidation of ethanol.


ACS Catalysis

M

M

type

pH

mA/cm² mA/mg (PGM) mA/cm² mA/mg (PGM)

Ref.

CA measured @ vs. RHE

mV

CA mass-current density

mV

CA current density

Electrolyte

peak mass-current density

State

peak potential vs. RHE

Preparation

Onset vs.. RHE

Compound/System

specific peak current density

Table 7 Catalytic properties of intermetallic compounds in the ethanol oxidation reaction (EtOR).

Ethanol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 76 of 122

mV

PdCu


NPs/C

1

1

KOH

14.0

~730

5170

128

Pd(Cu,Co)


NPs/C

1

1

KOH

14.0

~730

7720

128

Pd(Cu,Ni)


NPs/C

1

1

KOH

14.0

~730

6170

128

Pd2Ge

solvothermal

NPs

1

1

KOH

14.0

Pd2Sn


NPs/C

0.1

0.5

KOH

13.7

Pd2Sn

co-reduction with capping agents

nanoribbon [010]/C

0.5

0.5

KOH

13.7

Pd3Pb

co-reduction with PVP

nanowire network

1

1

KOH

14.0

PtMn


polished pellet

PtMn


NPs/C

0.1

0.5 H2SO4

Pt3Co


NPs/C

0.1

Pt3Zr - hexagonal


NPs/C

Pt3Zr - cubic


NPs/C

PtIn


polished pellet

Pt2In3

solvothermal

NPs/C

1

0.5 HClO4

0.3

PtSn


polished pellet

0.5

0.1 H2SO4

0.7

0.125 0.1 H2SO4

0.7

406

438

852

4.1

756

12.68

968

900

45.6

447

950 440

2

0.0

898

6.49

0.1 HClO4

1.0

903

1

0.5 H2SO4

0.0

~870

~1

1

0.5 H2SO4

0.0

~890

~0.5

0.125 0.1 HClO4

1.0

908

1.3

927

0.48

630

0.075

340

826

307

0.273

600

308

800

309

820

134

3200

890

538

1.57

200

259

310

790

~0.1

~500

753

306

251

251

259

206.3

817

297 259


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ACS Catalysis

Pt3Sn


NPs

0.5

0.5 HClO4

0.3

170

460

350

Pt3Sn

thermal decomposition

NPs/C

2

0.1 HClO4

1.0

~660 1008

PtSb


polished pellet

0.5

0.1 H2SO4

0.7

480

760

0.063

Pt3Ta

co-reduction

NPs/C

1.5

0.5 H2SO4

0.0

469

879

2.3

~0.4

PtPb

co-reduction

nanoplates/C

0.1

0.1 HClO4

1.0

2.5

1400

706

PtPb

co-reduction

NPs/C

0.1

0.1 HClO4

1.0

1.3

560

706

PtPb

co-reduction

nanoplates/C

0.1

0.1 HClO4

1.0

2.9

2360

PtPb

co-reduction, C+ sputtering

defect. nanoplates/C

0.1

0.1 HClO4

1.0

5.48

4370

PtPb


polished pellet

0.25

0.1 H2SO4

0.7

370

900

3

PtBi


polished pellet

0.125 0.1 HClO4

1.0

578

818

2.2

3.5

~0.25

~760

313

314 259 315 215

215

216

216

259

259


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Page 78 of 122

5.5. Other Oxidation Reactions Thin films of CoTe nanorods (NiAs type of crystal structure) were prepared by the tellurization of cobalt-hydroxycarbonate nanorods. 316 With an OER onset of 1.56 V vs. RHE, the intermetallic nanorods reach a current density of 10 mA/cm² at an overpotential of 370 mV, with respect to the thermodynamically required 1.23 V. Related studies from the same group investigated the influence of the annealing temperature on the as-deposited films with respect towards the OER activity, finding an optimum at 200 °C.

317

The thermally treated nanorods were used in overall water splitting. The

higher activity was related to a higher ECSA, as well as to the formation of CoTe2 (FeS2 type of crystal structure). 318 The intermetallic compound ZnNi (CuAu type of crystal structure) was tested as anode in a direct hydrazine fuel cell. 319 Even though the oxidation onset was higher than in the case of an α-Ni-Zn-alloy, the activity at higher potentials was the same. As zinc is inactive for hydrazine oxidation the authors hypothesized that the activity gain might be related to an electronic effect, which was supported by DFT calculations. Ni3Sn2 (Ni3Sn2 type of crystal structure) was used as anode in a direct methane solid oxide fuel cell (SOFC) working at 800 °C. 320 In comparison to an elemental nickel anode, no carbon formation was observed after a durability test, even though the intermetallic compound decomposes reversibly under reaction conditions to Ni3Sn (Mg3Cd type of crystal structure) and SnO2. Also, the effect of a small addition of Sn or Sb to NiO anodes used in direct methane SOFCs working at 650 °C was investigated. 321

Under reaction conditions, the intermetallic compounds Ni3Sn (tin-addition) and Ni3Sb, Ni5Sb2 and

NiSb (antimony-addition, Mg3Cd, Cu3Ti, Ni5Sb2 and NiAs type of crystal structures, respectively), depending on the atomic ratio of the elements, are formed. In all cases, carbon formation during the fuel cell operation was suppressed which was attributed to the chemical bonding between nickel and Sn or Sb, thus preventing an interaction with the 2p electrons of carbon. Also, the formation of hydroxyl groups on the surface of the intermetallic compound might restrict carbon agglomeration, by making oxidation pathways more likely to be favoured. A series of MmM´5 compounds (M´: Ni, Al, Mn, Co, B and mixtures thereof, with a CaCu5 type of crystal structure) were tested as anodes in alkaline direct borohydride fuel cells. 322 A reason for the choice of these intermetallic compounds and an explanation for electrocatalytic differences was not given, however, the highest power output of 150 mW/cm² (1 mg Pt/cm², 70 °C) was achieved using MmNi3.55Al0.3Mn0.4Co0.75 as anode catalyst. Rods of Ni2Ta (MoSi2 type of crystal structure) were prepared by arc melting and studied in the OER. 323 For better comparison to high surface area catalysts, pellets of sintered powder containing Ni2Ta were 78 ACS Paragon Plus Environment

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ACS Catalysis

prepared along with Co2Ta (MgCu2-type) and Fe2Ta (MgZn2-type). Even though the required overpotential to produce a current density of -10 mA/cm² on Ni2Ta rods was 980 mV (610 mV for Ni rod), the intermetallic catalyst was less prone to dissolution in the acidic electrolyte compared to elemental nickel. The reason for the good durability is the formation of a non-dissolvable Ta2O5 layer on the surface after initial nickel dissolution, accompanied by a decrease of the ECSA. Whether subsurface atoms catalyse the OER could not be clarified. A promoting spillover mechanism of activated water from TaOx towards the catalytically active centres, as discussed by Jaksic et al. 37, might compensate for the decreased ECSA. Pd3Pb@Pd core/shell aerogels (Cu3Au type of crystal structure) were used in the electrochemical oxidation of ethylene glycol. 324 The 3D structure has a 5.8 times higher mass-specific activity over Pdblack, which is attributed to favourable electronic and geometric effects as well as to a high defect density, due to the presence of twin domains. In alkaline media, Fe3Pt@Ni3FeN (Cu3Au type of crystal structure) core/fcc-shell nanoparticles were demonstrated to be active and stable in the OER. 204 The specific activity decreased only by 5.2% during an ADT of 1.000 cycles between 1.3 and 1.6 V vs. RHE, compared to the iridium benchmark loosing 22.1% of its initial activity. However, the good OER activity of 10 mA/cm² at η = 365 mV is stated to originate from the Ni3FeN shell. PtSb (NiAs type of crystal structure) nanoparticles, synthesized via the polyol route and the additional reducing agent NaBH4, were applied for the electrooxidation of glycerol, showing comparable activity to Pt/C in acidic but enhanced performance in alkaline media. In the latter, an improved C-C bond cleavage is observed by means of spectro-electrochemical results, indicating a positive effect of Sb on Pt, most likely due to electron transfer. 325 It is also likely that Sb is providing activated OH* species via a spillover mechanism, as the catalyst already contains Sb2O5, which may serve as OH* storage, as proposed by Jaksic et al. 37. PtPb (NiAs type of crystal structure) prepared by co-reduction of either dissolved precursor salts (route A) or reduction of a lead precursor on already supported Pt/C nanoparticles (route B) were tested in the oxidation of ethylene glycol and 2-propanol. 326 XPS results indicate a relatively high fraction of Pb2+ species on the surface of materials synthesized by route A, which turned out to be beneficial for the oxidation activity of ethylene glycol as more OH* groups are provided for the oxidation of carbonaceous species. In addition, the structural ordering is more pronounced than in materials synthesized by route B. This indicates a stronger interaction between Pt and Pb, thus modifying the electronic structure of Pt and Pb to a higher extent. It is believed that this modification is responsible for a stronger interaction between carbon and Pt, allowing for an acceleration of C-C cleavage at lower potentials, which is rate determining in the oxidation of 2-propanol. A similar explanation was brought 79 ACS Paragon Plus Environment

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Page 80 of 122

forward for the higher activity of PtBi (NiAs type of crystal structure) compared to Pt/C in the oxidation of 2-propanol. 327


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ACS Catalysis

6. Summary Intermetallic compounds used as electrocatalysts are a rich and exciting playground, with more than 50 binary systems of M-M’ already tested. This review provides a systematic overview on the most important reactions for electrochemical energy conversion in which intermetallic compounds have been applied as electrocatalysts – directly or as precursors. Synergies and principles to improve catalytic performance are pointed out: The ordered crystal structure of intermetallic compounds offers superior activity and durability compared to disordered substitutional alloys. In most cases, the ordering comes along with a negative reaction enthalpy, as the entropy in the material decreases and free energy is released. This leads to a stabilization of the compound compared to the constituting elements, making it less prone towards (partial) dissolution, sintering and particle growth under reaction conditions. However, the initiation of the ordering process often requires elevated temperatures. Facile strategies were developed to mitigate this effect. Encapsulation of nanoparticles in inorganic salts prior to annealing prevents sintering and yields unsupported structurally ordered nanoparticles. Pre-deposition of the particles or even direct precipitation of the precursors onto high surface area supports also prevents particle growth during the temperature program. Unique properties arise from the formation of crystal structures different from closed-packed structures, because of the altered electronic structure and the presence of covalent and ionic interactions between the atoms, while a metallic state maintaining electrical conductivity is still present. The changed crystal structure may prevent bulk leaching of the less noble species, provide isolated reactive sites and equal catalytic centres, while the change in electronic structure increases the corrosion resistance and alters the adsorption properties of molecules as a result of ligand and strain effects. The latter deviation arises from shifted d-band centres with respect to the individual elements. Water activation over a bifunctional mechanism is observed to be beneficial in certain oxidation reactions. Also, the exposure of specific facets, production of defect-rich structures and formation of highly irregular morphologies, i.e. a high concentration of kinks, edges and corners, allows to increase the activity, as intermetallic compounds provide increased stability over alloys to prevent spontaneous surface reconstruction and rapid particle agglomeration. Profound knowledge and the ability to alter and tune those effects to a desired extent, allows to tailor durable catalysts with exceptionally high activities and selectivities. Even though the highest activities are achieved by complex, nanostructured materials, a proper development ideally starts from bulk samples, as this allows the determination of intrinsic catalyst properties without any influences from support and particle size.


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Environ. Sci. 2015, 8, 1685–1689. (316) Patil, S. A.; Kim, E. K.; Shrestha, N. K.; Chang, J.; Lee, J. K.; Han, S. H. Formation of Semimetallic Cobalt Telluride Nanotube Film via Anion Exchange Tellurization Strategy in Aqueous Solution for Electrocatalytic Applications. ACS Appl. Mater. Interfaces 2015, 7, 25914–25922. (317) Kim, E. K.; Bui, H. T.; Shrestha, N. K.; Shin, C. Y.; Patil, S. A.; Khadtare, S.; Bathula, C.; Noh, Y. Y.; Han, S. H. An Enhanced Electrochemical Energy Conversion Behavior of Thermally Treated Thin Film of 1-Dimensional CoTe Synthesized from Aqueous Solution at Room Temperature. Electrochim. Acta 2018, 260, 365–371. (318) Gao, Q.; Huang, C. Q.; Ju, Y. M.; Gao, M. R.; Liu, J. W.; An, D.; Cui, C. H.; Zheng, Y. R.; Li, W. X.; Yu, S. H. Phase-Selective Syntheses of Cobalt Telluride Nanofleeces for Efficient Oxygen Evolution Catalysts. Angew. Chem. Int. Ed. 2017, 56, 7769–7773. (319) Martinez, U.; Asazawa, K.; Halevi, B.; Falase, A.; Kiefer, B.; Serov, A.; Padilla, M.; Olson, T.; Datye, A.; Tanaka, H.; Atanassov, P. Aerosol-Derived Ni1−xZnx Electrocatalysts for Direct Hydrazine Fuel Cells. Phys. Chem. Chem. Phys. 2012, 14, 5512–5517. (320) Bogolowski, N.; Iwanschitz, B.; Drillet, J.-F. Development of a Coking-Resistant NiSn Anode for the Direct Methane SOFC. Fuel Cells 2015, 15, 711–717. (321) Yoon, D.; Manthiram, A. Ni–M (M = Sn and Sb) Intermetallic-Based Catalytic Functional Layer as a Built-in Safeguard for Hydrocarbon-Fueled Solid Oxide Fuel Cells. J. Mater. Chem. A 2015, 3, 21824–21831. (322) Choudhury, N. A.; Raman, R. K.; Sampath, S.; Shukla, A. K. An Alkaline Direct Borohydride Fuel Cell with Hydrogen Peroxide as Oxidant. J. Power Sources 2005, 143, 1–8. (323) Mondschein, J. S.; Kumar, K.; Holder, C. F.; Seth, K.; Kim, H.; Schaak, R. E. Intermetallic Ni2Ta Electrocatalyst for the Oxygen Evolution Reaction in Highly Acidic Electrolytes. Inorg. Chem. 2018, 57, 6010–6015. (324) Zhu, C.; Shi, Q.; Fu, S.; Song, J.; Du, D.; Su, D.; Engelhard, M. H.; Lin, Y. Core–shell PdPb@Pd Aerogels with Multiply-Twinned Intermetallic Nanostructures: Facile Synthesis with Accelerated Gelation Kinetics and Their Enhanced Electrocatalytic Properties. J. Mater. Chem. A 2018, 6, 7517–7521. (325) Scachetti, T. P.; Angelo, A. C. D. Ordered Intermetallic Nanostructured PtSb/C for Production of Energy and Chemicals. Electrocatalysis 2015, 6, 472–480. (326) Feng, Y.; Yin, W.; Li, Z.; Huang, C.; Wang, Y. Ethylene Glycol, 2-Propanol Electrooxidation in Alkaline Medium on the Ordered Intermetallic PtPb Surface. Electrochim. Acta 2010, 55, 6991– 6999. (327) Feng, Y.; Li, Z.; Huang, C.; Wang, Y. Ordered Intermediate Compound PtBi-Modified Pt/C Catalyst for 2-Propanol Electrooxidation in Alkaline Medium. Ionics 2011, 17, 617–625.


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Acknowledgement: LR acknowledges the financial support from the European Social Fund (ESF, Project No. 100284169) for his PhD scholarship.


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For Table of Contents only:


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Overview of bimetallic systems and electrochemical reactions contained in this review. Bold elements appear in rows and columns to enable a concise scheme. RE stands for rare earth elements. “Other” reactions are electrochemical oxidation reactions of higher alcohols, BH4-, hydrazine, methane and the oxygen evolution reaction (OER). 84x86mm (300 x 300 DPI)


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Synthesis scheme of Ni4Mo supported on MoO2 cuboids on nickel foam. Scale bars for Ni foam, 20 µm (top) and 1 µm (bottom); NiMoO4/Ni foam, 10 µm (top) and 2 µm (bottom); Ni4Mo/MoO2/Ni foam, 20 µm (top) and 1 µm (bottom). Reproduced from Zhang and co-workers 70. 177x184mm (300 x 300 DPI)


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Structural characterization of the monolithic Al7Cu4Ni@Cu80Ni20/Cu. X-ray diffraction pattern (A). SEM micrograph showing the nanoporous microstructure (B). Crystal structure representations of Cu80Ni20 and Al7Cu4Ni (C). HR-TEM micrographs indicating characteristic lattice spacings, as well as FFT patterns (D). Reprinted with permission from Sun and co-workers 41. Copyright (2018) Wiley. 177x102mm (300 x 300 DPI)


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Selectivities of Ga-Ni intermetallic compounds in the eCO2RR. Potential-dependent Faradaic efficiencies (solid lines) and current densities (dotted line) for CO2 reduction in 0.1 M Na2CO3 acidified to pH 6.8 with 1 atm CO2 (g) (aq) to methane (△), ethane (×) and ethylene (□). Adapted with permission from Torelli and coworkers 91. Copyright (2016) American Chemical Society. 177x59mm (300 x 300 DPI)


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SEM (a), TEM (b, c) and HAADF-STEM (d) images of Ag76Sn24, EELS mapping (e−h) of the selected region showing elemental distribution of Sn (e), Ag (f), O (g), their overlay (h), the faradaic efficiency vs. applied potential (i) and durability in eCO2RR (j). Adapted with permission from Luc and co-workers 100. Copyright (2017) American Chemical Society. 84x69mm (300 x 300 DPI)


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ORR activity and MOR immunity of 2D-nanosquares (a-d) and 3D-nanoplate assemblys (e-h) of Pd3Pb, XRD patterns (a,e), TEM images (b,f), ORR polarization curves in O2-saturated 0.1 M KOH + 0.5 M CH3OH (c) and after 1.000 and 5.000 cycles for the Pd3Pb nanosquares (d), CVs in 1 M KOH + 1 M methanol (g) and MOR activities of 3D-nanoplate assemblies. Adapted with permission from Wang and co-workers 136, Copyright (2018) Wiley and Bu and co-workers 137, Copyright (2018) American Chemical Society. 84x124mm (300 x 300 DPI)


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Schematic illustration of the structure evolution before and after longer electrochemical cycles. (a) PtNi substitutional alloy and (b) (Pt1−xNix)3Al intermetallic compound. ORR polarization curves of NP (Pt1−xNix)3Al/C catalyst in a longer cycling test of 50.000 potential cycles collected at a rotation rate of 1.600 rpm and a scan rate of 10 mV s−1 in O2-saturated 0.1 M HClO4 (c) and Pt mass-specific activity retentions for (Pt1−xNix)3Al/C, Pt60Ni40/C, and Pt/C as a function of cycle number (d). Adapted with permission from Han and co-workers 141. Copyright (2016) American Chemical Society. 84x200mm (300 x 300 DPI)


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FePt@Au core-shell nanoparticles used in the FAOR. Schematic illustration of the structural change of the FePtAu nanoparticles upon annealing. When annealed at 400 °C, the FePtAu nanoparticles are structurally disordered, but at 600 °C, the ordered FePtAu structure is formed, with Au segregating on the nanoparticle surface (a), J−V curves of Fe43Pt37Au20 annealed at different temperatures (b), J−V curves of the specific activity of ordered Fe43Pt37Au20, ordered Fe55Pt45 and commercial Pt catalysts (c), J−V curves of the massspecific activity of ordered Fe43Pt37Au20 (d), J− V curves of ordered Fe43Pt37Au20 before and after a 13 h I−t stability test (e). The studies were conducted in 0.5 M H2SO4 + 0.5 M HCOOH solutions (for J−V curves). Adapted with permission from Zhang and co-workers 247. Copyright (2012) American Chemical Society. 114x48mm (300 x 300 DPI)


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In situ electrochemical ATR-FTIR spectra for formic acid oxidation (0.1 M formic acid + 0.1 M H2SO4) on Pt particles (a) and nanodendrites of PtPb (50:50) (b). Adapted with permission from Wang and co-workers 279. Copyright (2009) American Chemical Society. 84x48mm (300 x 300 DPI)


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Model of Pt8Ti-TiO2/CNTs (top) and polarization and power density curves for direct methanol fuel cells using Pt8Ti-TiO2/C (0.091 mgPtcm−2) and Pt/C as anode (0.189 mgPtcm−2) catalysts (bottom). The cathode was 40% Pt/C (0.4 mgPtcm−2, Johnson Matthey) with 2 M methanol and O2-feeding mode operated at 333 K. Reproduced with permission from Sanetuntikul and co-workers 288. Copyright (2015) American Chemical Society. 84x115mm (300 x 300 DPI)


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ZnPt nanoparticles (3.2 nm)/MWCNTs with high activity towards the MOR. High Resolution HAADF STEM image of a ZnPt nanoparticle (a) and the catalytic activity towards the MOR for ZnPt (3.2 nm), Pt and ZnPd (27 nm) in 0.1 M KOH and 0.5 M CH3OH solution (b). Reprinted with permission from Qi and co-workers 293. Copyright (2017) American Chemical Society. 84x42mm (300 x 300 DPI)


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Full oxidation of ethanol on Pt3Ta. (a) and (b) cyclic voltammetry profiles for Pt3Ta, Pt and Pt3Sn/C in 1.5 M ethanol solution. (c) Variation of ECSA of Pt3Ta and Pt as a function of potential cycles. (d) I–V profiles and power density profiles of Pt3Ta and Pt, obtained at room temperature in 1 M ethanol solution. In situ IRRAS

spectra (1900 to 2500 cm-1) of Pt3Ta (e) and Pt. (f) In situ IRRAS spectra (2200 to 2500 cm-1) of Pt3Ta (g) and Pt (h). The broken line in (h) located at 2350 cm-1 corresponds to the peak position for atmospheric CO2. Adapted with permission from Kodiyath and co-workers 315. Copyright (2015) Royal Society of Chemistry. 84x108mm (300 x 300 DPI)


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Electrochemical series: First oxidation potential of the elements vs. equilibrium potential of the conversion reactions. For example, the use of lead (Pb) in the HER at pH = 0 (red Pb - below H2 line) will lead to surface oxidation of lead, while elemental lead is expected at pH = 14 (blue Pb - above H2 line). Values for the redox potentials of the elements were taken from Darchen 13. 84x74mm (300 x 300 DPI)


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Summarized optimization strategies, i.e. ligand effects (a), strain effects (b), geometric effects (c) and the bifunctional mechanism (d). Geometric effects (c) are divided into two parts. The first is the site isolation effect, accompanied by different adsorption possibilities of a molecule, i.e. in the hollow site (Ia), on the bridge site (Ib) or on top of an atom (Ic). The second is the equality of sites (II). The bifunctional mechanism (d) proceeds via an initial spill-over of OH* (red arrow). 84x52mm (300 x 300 DPI)


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Reduction pathway for CO2. Modified after Jones and co-workers 89. Copyright (2014) Wiley. 84x66mm (300 x 300 DPI)


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The reaction paths involved in the electrocatalytic oxidation of formic acid to CO2. Direct path (centre) to CO2, indirect path through CO (bottom), and formate path though the formation of the formate intermediate (top). Schematic follows that proposed by Behm et al. 242. Experimental evidence also suggests the possible exchange of formate with electrolyte in solution. Reproduced with permission from Neurock and co-workers 241. Copyright (2009) Royal Society of Chemistry. 84x48mm (300 x 300 DPI)


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Possible reaction pathways of the MOR. 284 Red and blue arrows stand for reactions in acidic and alkaline media, respectively. Black arrows are used for reactions that are not pH-dependent. Dotted arrows stand for reactions where oxygen is introduced to the molecule. 84x65mm (300 x 300 DPI)


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