Tailored electrocatalysts by controlled electrochemical deposition and surface nanostructuring

Paula Sebastián-Pascual , Inês Jordão Pereira and María Escudero-Escribano *
Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark. E-mail: maria.escudero@chem.ku.dk

Received 9th September 2020 , Accepted 16th October 2020

First published on 16th October 2020


Abstract

Controlled electrodeposition and surface nanostructuring are very promising approaches to tailor the structure of the electrocatalyst surface, with the aim to enhance their efficiency for sustainable energy conversion reactions. In this highlight, we first summarise different strategies to modify the structure of the electrode surface at the atomic and sub-monolayer level for applications in electrocatalysis. We discuss aspects such as structure sensitivity and electronic and geometric effects in electrocatalysis. Nanostructured surfaces are finally introduced as more scalable electrocatalysts, where morphology, cluster size, shape and distribution play an essential role and can be finely tuned. Controlled electrochemical deposition and selective engineering of the surface structure are key to design more active, selective and stable electrocatalysts towards a decarbonised energy scheme.


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Paula Sebastián-Pascual

Paula Sebastian-Pascual graduated in Chemistry at the University of Valencia (Spain) and obtained the PhD degree in Electrochemistry from the University of Alicante and the University of Barcelona (Spain). She is currently a postdoctoral researcher at at the NanoElectrocatalysis and Sustainable Chemistry group at the University of Copenhagen, UCPH (Denmark). Her research interests focus on fundamental understanding of electrochemical reactions onto single crystalline surfaces and metal electrodeposition.

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Inês Jordão Pereira

Inês Jordão Pereira graduated in Chemistry at the Faculty of Science of Lisbon University (Portugal) while being a volunteer collaborator at the Interfacial Electrochemistry Group. She is currently a PhD student at the NanoElectrocatalysis and Sustainable Chemistry group at the University of Copenhagen (Denmark). Her research interests include structure manipulation to tune electrocatalysts activity towards selective green chemical synthesis and energy conversion.

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María Escudero-Escribano

María Escudero-Escribano is an Assistant Professor of Chemistry at the University of Copenhagen (Denmark) since 2017. She obtained her PhD in Chemistry from the Autonomous University of Madrid (Spain) and carried out her postdoctoral research at the Technical University of Denmark and Stanford University (US). At UCPH, María leads the NanoElectrocatalysis and Sustainable Chemistry group, which investigates tailored interfaces for electrochemical energy conversion and electrosynthesis of renewable fuels and chemicals. She has received numerous awards at national and international levels in recognition of her scientific contributions.


1. Introduction

The urgent need to develop a decarbonised economy explains the increased attention that clean energy conversion and electrosynthesis processes have recently attracted.1,2 Electrocatalytic reactions involve the transfer of electrons between the electrode and the electrolyte containing reactants, and are catalysed by electrode materials in contact with conductive liquid electrolytes.3–5 In an electrocatalytic reaction, the driving force is a potential difference necessary for the electron transfer between electrocatalyst and reactants, thereby allowing catalysis of clean energy conversion reactions and produce value-added chemicals.

Representative examples of electrocatalytic reactions include those reactions that take place in energy conversion devices such as electrolysers and fuel cells. Herein, we will mainly focus on electrocatalysts for applications in water electrolysis, electrochemical CO2 reduction and fuel cells. Water electrolysers, coupled to renewable sources such as wind or solar energy, enable the production of green hydrogen, a promising energy carrier for a sustainable future.6,7 Developing stable and active electrocatalysts for both the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode is vital to improve the efficiency of water electrolysers.8 On the other hand, in CO2 electrolysers, electrocatalysts can reduce CO2 into renewable fuels and high value chemicals.9–12 However, CO2 reduction involves a complex reaction mechanism which results in poor selective production of compounds at the expense of a high energetic cost. Thus, more efficient and selective electrocatalysis are needed to make this process economically viable.9 Finally, fuel cells are electrochemical devices that convert the chemical energy from a fuel like hydrogen directly into electricity. In polymer electrolyte membrane fuel cells, the hydrogen oxidation reaction (HOR) occurs at the anode and the oxygen reduction reaction (ORR) takes place at the cathode.13 Alternatively to the use of H2 as a fuel, small organic molecules such as ethanol, methanol or formic acid14 can be oxidised at the anode of the fuel cell.

In the solid electrode–electrolyte interface, both the structure of the electrode material and the electrolyte composition affect the performance of the electrocatalytic reaction,11 which accounts for:

(1) The activity, which typically refers to the recorded current densities measured as a result of the applied potential and/or stirring conditions to overcome the mass diffusion limitations.

(2) The selectivity or faradaic efficiency towards specific products, i.e., the fraction of total current density that involves the formation of one specific product;

(3) The energetic efficiency, which is related to the necessary applied overpotential to increase the rate of the reaction.

(4) The stability of the catalyst, i.e., the robustness or resistance of the catalyst to degradation over time.

These parameters as well as the involved preferred mechanism pathway can be rationalised through the description of the different interactions between electrode surface, electrolyte and reaction intermediates at the electrified interface.15 In addition, electrocatalytic reactions are sensitive to the structure of the electrode, i.e., the distribution of atoms at the active site positions.3,16 Size, shape, density and distribution of nanoscale arrangements at the electrode surface in real catalysts highly affect the performance of an electrocatalytic reaction.17,18 Ultimately, the catalyst surface can be selectively modified with sub-monolayer or nanoscale arrangements of other elements or molecules to improve their catalytic properties. Apart from this, electrochemical engineering of the catalyst surface also pursues to design more robust catalysts that can work under real operation conditions.10

Here, we intend to highlight recent representative work in the field of electrochemical deposition and surface modification to catalyse reactions of interest in clean energy conversion devices. This highlight is specially intended for a non-specialised readership who would like to be introduced in the fields of electrodeposition and surface nanostructuring for applications in electrocatalysis. We have divided this manuscript in three parts: first, we discuss the most relevant procedures to modify the catalyst surface by control deposition of sub-monolayers or atomic ensembles. We then discuss the benefits from nanostructuring electrode surfaces for electrocatalysis. Finally, we comment on the perspectives and opportunities on tailoring the electrocatalytically active site by metal electrodeposition.

2. Tailoring the electrocatalyst surface

2.1 First principles of electrocatalytic reactions on the electrode surface

Fundamental understanding of electrocatalytic reactions requires the design of strategies for catalyst optimization. The electrocatalytic activity can be qualitatively rationalized in terms of the Sabatier principle,19 which states that the best catalyst should bind not too strong nor too weak to the relevant adsorbed reaction intermediates. While a too weak bond results in insufficient reactant activation, a too strong bond does not allow the desorption of the products. The relation between activity and bond-strength of one of the reaction intermediates, called descriptor of the reaction, are commonly represented in a volcano-type plot such as that represented in Fig. 1. Volcano plots provide a general picture of the optimum value of catalytic activity. They have been usually employed for rationalisation of designed electrocatalysts in relevant reactions, e.g., the ORR (Section 2.2).20–22 Noteworthy, in complex electrocatalytic reactions with different intermediates and transition states, volcano plots are typically insufficient in providing the rules for catalyst optimization.
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Fig. 1 The top image is an scheme of a Sabatier volcano-type plot of the catalytic reaction rate as a function of bond strength of the reaction intermediate.19 The bottom image illustrates the reaction of two molecules (green and orange balls) and its steps.

Electrocatalytic reactions are sensitive to the electrode structure, i.e., the geometry of the active site, where the reaction occurs. Polycrystalline materials contain different crystalline domains and grains contributing to the catalytic response.23 Single-crystalline electrodes, on the other hand, are well-ordered surfaces containing a regular distribution of active sites, thus allowing decoupling the electrocatalytic response of the different facets. They are vital for the rational description of electrocatalytic reactions.24,25 Well-ordered surfaces include basal planes, which are defined by the minimum index Miller coefficients ((111), (110) and (100), as shown in Fig. 2); stepped surfaces, which are surfaces with variable terrace length and density of steps; and kinked surfaces, i.e., surfaces that have corners with well-known distribution (Fig. 2).


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Fig. 2 Scheme and atomic representation of different model surfaces from face-centred cubic structures (fcc). From top to bottom: (A) Representation of the unit cell of (fcc) group metals and lattice planes for 〈111〉, 〈110〉 and 〈100〉 orientations. Lattice planes are noted with the Miller Index: 〈hkl〉. (B) Atomic representation of flat surfaces for the basal planes. (C) Stepped surfaces and indication of step and terrace sites (D) Kinked stepped surfaces, i.e., stepped surfaces with corners. (E) Polycrystalline surface.

A key structural parameter to consider while describing structure effects is the coordination number, which value depends on the specific geometry of the active site. The general coordination number only accounts for the number of neighbour atoms in contact.26 This parameter is well accepted to describe extended surfaces, but losses accuracy when describing nanoparticles or rough surfaces with a high density of low coordinated sites. The generalised coordination number (GCN), in contrast, introduces the effect of low coordinated nearest-neighbour atoms in second and third layers, thereby allowing for more precise description of structure effects in electrocatalysis.27

The activity and selectivity of electrocatalytic reactions typically depend on the orientation of the substrate, as that will affect the geometry of the active site. The simplest classification discriminates between reactions that are structure sensitivity towards 〈100〉 or 〈111〉 flat surfaces (Fig. 2A). A representative example is the ORR, a reaction extensively studied on Pt-based electrode surfaces.28 Interestingly, while 〈111〉 terraces are more active than 〈100〉 terraces on non-specifically adsorbed electrolytes such as perchloric acid,29–31 the trend is inverted in sulphuric acid media due to a higher deactivation of the 〈111〉 sites by the specific sulphate adsorption.32 This illustrates the importance of tuning both the structure of the electrode surface and the electrolyte composition. On the other hand, some electrocatalytic reactions are highly affected by the presence and distribution of defects on the surface. For instance, stepped surfaces are highly active for the ORR activity in acid.31,33

The CO2 reduction reaction on copper is a model structure-sensitive electrocatalytic reaction, particularly due to the presence of 〈100〉 active sites, which are more selective towards C2H4 production than Cu(111).34 Interestingly, the CO2 reduction efficiency also increases with the presence of low under-coordinated sites on Cu, which enhances the production of alcohols and oxygenates, reducing the production of hydrogen.34,35 On the other hand, the CO2 reduction to CO on gold electrodes shows 20-fold higher activity on stepped 〈110〉 than both 〈100〉 and 〈111〉 basal planes, revealing that undercoordinated sites are the active sites to the formation of CO on gold.36

The electrocatalytic properties of a catalyst are typically governed by the structure of the surface or the sub-surface layer.37 Combination of two different metals induces changes in the electronic structure of the electrocatalyst through ligand or strain effects. Ligand effects modify the electronic structure of the catalyst surface atoms due to the incorporation of a foreign metal in the subsurface.37 Strain effects occur when a metal overlayer under lateral compressive or tensile strain is formed onto a bulk alloy, thus modifying the electronic properties of the electrode surface.17,38,39 Tuning the electrocatalytic properties of real catalysts is typically the result of the interplay between ligand and strain effects. Despite that, some model studies on well-defined surfaces have been able to differentiate among these two electronic effects.28,37 Another promising strategy to tune the electrocatalytic properties consists of tuning the geometric structure of the electrode surface by means of the formation of controlled atomic ensembles.40,41

The electrolyte composition can also affect the electrocatalytic performance; ions from the electrolyte can adsorb on the surface and compete for the available active site positions, or modify the electrified double layer structure.42,43 The poisoning effect of the electrolyte anions has been demonstrated, among other reactions, for the ORR on Pt-based catalysts41 and the OER on Ir-based catalysts.44 In acidic media, both reactions show lower activities in sulphuric acid electrolytes than in perchloric acid solutions due to strong specific adsorption of sulphate anions, while the adsorption of perchlorate anions is typically negligible. Cation effects on different electrocatalytic reactions have been also widely analysed in different reactions. Some reactions occurring e.g. on Pt surfaces, such as the ORR,45 the HER46 or the CO oxidation reaction47 are highly influenced by the identity of the cation in solution. CO2RR activity and selectivity on Cu or Au electrodes are also affected by cation effects.48 In these examples, the cation can act as a promoter of the reaction as it can change the electrified double layer structure or because of more favourable interactions with the intermediates, allowing them to react more easily.

In the following section, we discuss different strategies to improve the catalytic performance of electrocatalysts, providing representative examples that highlight how ligand, strain and ensemble effects can improve the electrocatalytic properties of new catalysts.

2.2 Surface modification at monolayer and sub-monolayer level

There are two main strategies to modify the structure of the catalyst surface with adlayers of other elements: underpotential and adatom deposition. Underpotential deposition (UPD) consists of the reversible metal electrodeposition of a monolayer or sub-monolayer on the electrode surface. The UPD deposition occurs at more positive potentials than the needed for the massive deposition of the same metal (thermodynamic or Nernst potential). This is because the binding between electrode surface and deposited metal, determined by its work function differences, is stronger than the metal–metal binding itself.24 The interest on modifying surfaces by UPD lays on the fact that the deposited adlayers show different electrocatalytic properties due to the intermetallic bond.24 Different bimetallic systems prepared by UPD have been widely studied and still continue being a focus of interest in electrocatalysis. In contrast, adatom deposition refers to the irreversible deposition of adlayers of other elements. Irreversible adatom adsorption on the substrate can be performed at open circuit potential or by electrochemical cycling under certain potential limits conditions.49

Pt-based materials are the most active and stable catalysts towards the ORR for fuel-cell applications. Model studies on well-defined single-crystalline and polycrystalline Pt-based electrodes have been widely investigated for this reaction.38,50

The work carried out by Adzic and coworkers on Pt monolayers deposited on well-defined transition metal-electrodes for ORR51–53 deserves a special mention. In particular, the ORR on bimetallic electrodes formed by a Pt monolayer (ML) on a single-crystalline (111) metal electrode (metal = Au, Ir, Ru, Rh and Pd) have been compared with pure Pt(111). PtML/Pd(111) displayed slightly higher activities than Pt(111).52,54 Furthermore, depositing a mixed monolayer of Pt with other transition metal (M) on Pd(111), and in the optimum Pt/M ratio, can also contribute to improve the ORR of this system.22 The change in the observed activity trends can be rationalized in terms of the coverage degree and adsorption strength of OH* on the surface, key descriptor of the ORR.55 The bimetallic system PtML/Pd(111), or the mixed M–PtML on Pd(111), reduces the coverage and adsorption strength of the OH on Pt(111), which contributes to the ORR activity enhancement.22,52

Bimetallic alloys of Pt and 3d-transition metals, such as Ni, Co, Fe or Cu, have been traditionally used to catalyse the ORR.28,50,56,57 The activity enhancement on Pt-alloys can be explained by a weakening of the binding energy of adsorbed OH*.20,55 The OH* binding energy can be tuned by means of ligand and/or strain effects. These two difficult to separate effects can be investigated individually on certain well-defined surfaces.28,37,38 Well-ordered Cu/Pt(111) near-surface alloys (NSAs), i.e. Pt(111) with a Cu sub-monolayer, is a model system that clearly illustrates how the OH* binding energy can be tuned by controlling the amount of Cu in the subsurface. This is allowed in two steps, as illustrated in Fig. 3A.37,56 First, Cu UPD deposition on Pt(111) is carried out with potential control. Secondly, subsurface formation of a Cu layer is induced by thermal annealing of the surface. The ORR activity follows a volcano-type relationship as a function of the OH adsorption on Cu/Pt(111) near-surface alloys (NSAs), as shown in Fig. 3B.37,56 Moreover, Pt-rare earth alloys such as Pt5Gd, Pt5Tb and Pt3Y are amongst the most active ORR catalysts.38,50,58 In this case, the activity enhancement is due to the formation of a thick Pt overlayer under compressive strain.38,39,59 This is due to the formation of kagome-type layers stabilised by larger lanthanide atoms, as demonstrated by synchrotron X-ray diffraction (XRD) experiments on Gd/Pt(111) single crystals.60In situ grazing-incidence XRD measurements on Gd/Pt(111) revealed the formation of a compressively-strained Pt overlayer of about 3 atomic layers, which remained stable when cycling in the region of relevance for ORR.59 Interestingly, a volcano relation results from controlling the Pt–Pt distance, with Pt5Gd and Pt5Tb approaching the top of the volcano.38


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Fig. 3 (A) Scheme of the preparation of bimetallic surfaces by UPD-monolayer deposition with subsequent near-surface alloy formation. (B) Experimental volcano plot for oxygen reduction activities for different Pt(111)/Cu near surface alloys. Activities measured between 0.8 and 0.9 V vs. RHE. From ref. 28.

Decorating selectively the steps or defect sites of the electrode surface is another valuable strategy to tune the catalytic properties of bimetallic materials. This strategy can serve to enhance the electrocatalytic performance at terrace sites in the vicinity of the decorated defect sites. Selective deposition of Au at step sites on Pt-stepped surfaces have been tested for the ORR. Pt step-atoms decorated by gold reduces the binding energy of adsorbed OH in the Pt sites close to the steps61,62 This reduces the needed overpotential and increases the ORR rate.

Pt single-crystalline electrodes modified with adlayers of other metals have been also widely investigated for many electrocatalytic reactions of interest beyond the ORR. As an example, the oxidation of ethanol and formic acid is highly affected by the adatom deposited on Pt.63,64 Irreversible deposition of electropositive p-block adatoms, such as Bi or Pb, enhances the formic acid oxidation activity, while Sn–Pt bimetallic system is more efficient to oxidize ethanol.64,65 Here, the oxidation of small organic molecules towards CO2 is enhanced because the adatoms reduce the formation of CO, which would poison the surface decreasing its performance.

Regarding CO2 electroreduction, different bimetallic systems combining Cu and other transition metals have been widely assessed. Due to the expanding research in the field, here we also provide some illustrative examples of bimetallic systems for the CO2RR.9 Controlled deposition of a specific number of Cu adlayers on Pt allowed tuning the selectivity towards the formation of both C2H4 and CH4. This could be explained by strain effects on the Cu overlayer.66 For low coverages of Cu on Pt, the CO2RR was highly hindered due the high activity of exposed Pt towards the formation of hydrogen. Varela et al.67 showed that the Cu adlayers deposited on Pt single-crystalline electrodes reorganise under reaction conditions, forming nanoclusters and leaving high areas of bare Pt uncovered on which hydrogen evolved while reducing the CO2RR efficiency. These results highlight the vital importance of assessing the stability of the modified bimetallic surfaces by means of in situ characterization techniques that allow to monitor the structure changes during the electrocatalytic reactions.68

Combining Cu with other transition metals aim to tune the selectivity and efficiency of the Cu catalyst by simply modifying the binding energy of adsorbed CO*, while balancing the HER rates. CO* is a key descriptor of the CO2RR, and the main reaction intermediate to produce hydrocarbons and oxygenates.26 Unlike Pt, both Ag and Au are highly selective towards the formation of CO but present low activity towards the HER. In fact, it has been observed that Cu–Ag and Cu–Au systems for instance increase the production of ethanol and oxygenates, although at low overpotential.69–71 Both Pd and Au are also selective towards CO but they bind CO with different strength. Indeed, Pd–Au alloys can produce a low amount of hydrocarbons.26 These works illustrate the importance of modifying the electronic structure of the catalyst surface in electrocatalysis. It is worth to remark that selective decoration of specific active sites, such as steps sites, by non-active elements can be used as a valuable strategy to tune the activity of the CO2RR. Selective blockage of Au defect sites with non-active Pb causes a dramatic decrease of the CO2 reduction towards CO on Au(100) and Au(111), thus evidencing that low-coordinated sites are the active positions for the CO2 reduction on gold electrodes.36

2.3 Atomic ensemble control

A promising strategy to modify the electrocatalytic properties of an electrode surface consists of tuning the geometric structure of the active surface site at the atomic level. This can be achieved by controlling the atomic ensembles, i.e. the smallest group of atoms with a specific geometric configuration which acts as active site for the adsorption of a reactant or a reaction step to occur.40 These arrangements of active sites can go from single atoms to clusters of few atoms.72,73 Tailoring the geometric atomic ensembles on the catalyst surface is highly relevant to understand reaction mechanisms as well as tune the electrocatalytic activity, reactivity and/or selectivity.74 By means of atomic ensemble control, we can leave the electrocatalytically active sites available while blocking other positions that are not desirable for the reaction to occur. We can therefore modify the reaction mechanism so that it proceeds via the desired pathway. Thus, controlling the geometric atomic ensembles allows for fine tuning of the electrocatalytic efficiency and selectivity.

The use of well-defined surfaces such as single-crystalline electrodes is desirable to control the size and geometric structure of the different surface sites. The methods for preparation of atomic ensembles on extended electrode surfaces can be divided into two main strategies:

(i) Formation of ordered bimetallic surfaces by means of e.g. deposition of regular distribution of ensembles of a foreign metal onto a metallic surface.

(ii) Modification of the surface in such a way that some kind of sites are selectively blocked without altering the electronic structure of the rest of the surface.

An important contribution to the field of atomic ensemble effects in electrocatalysis was made by Behm and co-workers by means of electrodeposition of Pd atomic ensembles on Au(111) electrodes.73Fig. 4 A shows electrochemical scanning tunnelling microscopy (STM) images of Pd/Au(111) electrodes. This system was used to study the adsorption of carbon monoxide and hydrogen. Interestingly, while CO adsorption and oxidation can take place on isolated Pd atoms, dimers of Pd atoms are necessary for hydrogen adsorption.73 Using model surfaces such as Au(111) single crystals allows electrochemists to align the experimental knowledge with theoretical calculations, as the latter are typically performed assuming that the reactions occur on well-ordered surfaces.75


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Fig. 4 (A) Electrochemical STM images with atomic resolution of one monolayer PdAu (left: Pd7Au93, and right: Pd15Au85) alloys with the stripped palladium electrodeposited on Au(111). From ref. 73. (B) Three-dimensional model of CN-Pt(111); grey balls correspond to Pt atoms, dark grey balls to carbon and blue to nitrogen. From ref. 76. (C) Proposed model for the effect of adsorbed cyanide molecules on Pt(111) electrodes for oxygen reduction. From ref. 41.

An interesting approach to study atomic ensemble effects in electrocatalysis is to selectively block one kind of sites of the electrode surface without modifying the electronic structure of the rest of the surface (the so-called third body effect).40,76 This can be achieved by molecular patterning of electrode surfaces. In particular, a self-ordered molecular pattern, cyanide-modified Pt(111) (see Fig. 4B), was used to study different electrocatalytic reactions such as the oxidation of methanol and formic acid and the reduction of oxygen.41,77,78 Cyanide groups form an ordered structure76,79 that blocks all the three-fold hollow adsorption sites as well as the reaction sites composed of three contiguous Pt atoms.76,80 Interestingly, the work on CN-Pt(111) surfaces show that at least 3 continuous Pt atoms are needed to form adsorbed CO (the poisoning species) during the oxidation of formic acid and methanol.77,78 Thus, CN-Pt(111) electrodes can be used to channel these reactions through the desired pathway. Regarding the ORR, catalysing this fuel-cell reaction in the presence of anions from the electrolyte (such as anions present in sulphuric acid and phosphoric acid media) is very challenging, as these anions strongly adsorb on Pt and act as poisons. Notably, cyanide groups effectively blocked the adsorption of sulphate and phosphate anions from the electrolyte while leaving unaffected the Pt surface sites needed for the ORR to take place (see Fig. 4C). As a result, CN-Pt(111) surfaces present a great ORR activity enhancement over unmodified Pt(111) electrodes.41

These examples illustrate how atomic ensemble control is very useful to understand the mechanism of electrocatalytic reactions at the atomic level. Ultimately, the conclusions from single-crystal studies can be extended to the rational design and development of realistic nanoparticulate catalysts with enhanced activity and selectivity towards the desired reaction. Examples of some of these works will be discussed in the following section.

3. Nanostructured catalysts

3.1 Electrocatalysis on nanostructured surfaces

Engineering the electrocatalytically active site on well-defined surfaces such as single-crystalline electrodes allows for rational description of the reaction. However, real catalyst surfaces are typically polycrystalline and contain a high density of defects and undercoordinated sites. Thus, nanostructured catalysts allow to investigate the electrocatalytic properties of more realistic systems.16,17,81,82 The shape of the nanoparticle is representative of the main crystalline domain contribution to the catalytic response. In this section, we will provide examples that show the enhancement of some electrocatalytic reactions induced by surface nanostructuring.

Palladium ensembles decorating gold nanoparticles have been recently investigated for the selective reduction of CO2 to CO as a function of the Pd content.83 The work on PdAu nanoparticles offers a way of engineering the active site to tune the CO2RR activity and selectivity on nanoparticulate catalysts based on the work by Behm and co-workers.75,83 Density-functional theory (DFT) calculations showed that Pd dimers were preferential in this type of reaction since the CO* poisoning is limited when comparing with bigger atom clusters of Pd. Interestingly, geometric atomic ensemble effects have been also assessed in other complex reactions such as ethanol oxidation. Recently, Ji et al.84 showed that the synergetic effects between neighbouring Pd and Hg can greatly enhance the electrocatalytic performance of this reaction. These two metals were chosen for the preferential binding of OH* to Pd and advantageous desorption of the same species on Hg.84 This system had also been used for the selective electrochemical production of H2O2 by Siahrostami et al.72 In these works on bimetallic alloys, the enhancement in the electrocatalytic performance can be attributed to a combination of geometric ensemble effects and electronic effects.

Transferring the results obtained on Pt-based extended surfaces to nanostructured catalyst has the aim to reduce loadings of high expensive noble metals while maintaining high activity and stability. Pt-based nanoparticles with different shape and morphology have been assessed. The field is quite broad and a detailed description of the topic can be found in ref. 14, 17 and 28. The influence of the size on shaped Pt nanoparticles on mass activities for the ORR has been investigated, assessing the effect of generalised coordination number (GCN) on active sites (Fig. 5A).85 Calle-Vallejo et al. reported that atoms at the active sites on Pt catalysts with a GCN around 8 are more active than active sites on extended Pt(111) surfaces, with a GCN of 7.5.27 Nanoparticles of 1.1 nm had the highest relation active site/mass loading, showing an increase of the mass activities 2.5 times higher than commercial Pt nanoparticles (Fig. 5B). Regarding the hydrogen evolution reaction, modification of Pt surfaces with nanoclusters of 3d-metal hydroxides (M(OH)n, where M = Ni, Co, Fe) reduces the overpotential needed for the HER in alkaline media.86–88


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Fig. 5 Top panel: Scheme of shaped-nanoparticles with 〈111〉, 〈110〉 and 〈100〉 active sites. (A) Optimal Pt nanoparticles are identified at diameters of 1.1 nm, 2.07 nm, and 2.87 nm. The ratio between active sites with a GCN between 7.5–8.3 (highlighted in yellow) and the total number of sites is given for all illustrated nanoparticles. (B) Specific activity of Pt nanoparticles compared with the commercial Pt/C catalyst at 0.9 V vs. RHE regarding to the oxygen reduction reaction. From ref. 85.

The product distribution and efficiency of CO2 reduction can be also tuned by means of surface nanostructuring, particularly to increase the formation of alcohols and oxygenates at lower overpotentials.89,90 A pulse potential-step technique induces the appearance of defects on the surface. This is achieved by switching the applied potentials towards values at which Cu oxidation occurs, and then switching again to potential at values from the pseudocapacitive potential region.35,91,92 On Cu single-crystalline electrodes, potential control inducement of surface defects considerable increases the faradaic efficiency towards the formation of oxygenates and hydrocarbons.35 Noteworthy, oxide-derived Cu surfaces present also promising properties for selective CO2 reduction, likely because of their high roughness factor and high density of defects.93,94 The employed electrolyte can also induce nanostructuring on Cu surfaces, for instance, through electrochemical cycling of the Cu surface in solutions containing halides95 or organic additives96. The specific halide anion in solution (Cl, Br, I) induces different morphologies on nanostructured Cu, which clearly affect the product distribution besides increasing the activity (Fig. 6A and B).95 The presence of organic halides additives in solution during the reaction has been also observed to promote the formation of nanocubes on the surface which remains relatively stable under conditions of reduction, allowing to minimize the HER and increase the production of hydrocarbons.96 Stability of nanostructures under reductive conditions is crucial to shed lights into the structure–activity-selectivity relationships of the CO2RR. More recently, the nucleation and growth of Cu nanocubes from electrodeposition methods, and under the presence of chloride in solution, have been imaged by using in situ transmission electron microscopy (TEM). Stability of formed nanocubes under reductive conditions were evaluated, observing the loosening of the cubic shape and development of dendrites in presence of CO2, most likely due to an increase of Cu surface diffusion.97


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Fig. 6 Top: Scheme of a nanostructured surface. (A) Non-cubic (blue solid line) and cubic (red solid line) particle size evolution of the selected particles marked in the STEM image with a circle of the corresponding colour as a function of time and applied potential in a 5 mM CuSO4 plus 5 mM KCl solution. From ref. 97. (B) C2+ products, as a function of the applied potential after 1 h of CO2RR in a CO2-saturated 0.1 M KHCO3 solution and in different Cu foils prepared by electrochemical cycling in solutions containing halides. From ref. 95.

In nanoparticulate systems, the size is a critical factor for efficient CO2RR on Cu. Nanoparticles with a size below 20–30 nm, produce more hydrogen and CO,98 while on Au nanoparticles below 10 nm are also favourable for the HER.99 Here, the distribution of GCN on active sites of Cu and Au nanoparticles must have a key role in the electrocatalytic performance. The enhancement of electrocatalytic conversion of CO2 on nanostructures catalysts, as well as other above-mentioned reactions, is the result of balancing the size, shape, and morphology effects.

3.2 Electrochemical deposition of nanostructures

In this section, we aim to introduce some electrochemical procedures to prepare nanostructured catalysts. We note that there are different methodologies to prepare nanostructured surfaces, involving vapour deposition processes or synthesis of nanoparticles. Physical and chemical vapour deposition methods produce well-defined and homogenous distribution of the coatings at the nano and micro scale, but the procedure requires a high input energy and is lowly scalable.69–71 Synthesis of nanoparticles frequently employ colloid-chemical routes, i.e., it uses a reducing agent to reduce the metal salt precursor in fine particles that later growth into nanoparticles. Parameters of relevance in nanoparticle synthesis are size, shape, crystallinity and distribution. Synthesis of nanoparticles with different shapes usually requires organic additives, surfactant agents or templates that, however, are difficult to eliminate during the cleaning of the nanoparticles. The reader can find more information about the nanoparticle synthesis field in the following ref. 14 and 100.

Metal electrodeposition, on the other hand, is a promising and relatively inexpensive way to prepare new materials at room temperature.101 The electrodeposition process is initiated with the formation of stable nucleus (nucleation step) that activate the electrode surface thereby accelerating the growth of the new metallic phase (Fig. 7A). Selection of the electrolyte composition is very important because it highly influences morphology, size, shape of grains, porosity and adherence of the deposit. Traditionally, metal electrodeposition has been carried out in aqueous electrolytes. However, the narrow potential window of water is a limitation for those metals which equilibrium potential of deposition is more negative than the HER reaction. To avoid the interference of the parasitic HER during the metal electrodeposition, which affect both the quality of the coating and the efficiency of the process, organic additives, surfactant or levelling agents are added to stabilize the growth of the deposit.101


image file: d0cc06099b-f7.tif
Fig. 7 (A) Cyclic voltammetry for the metal electrodeposition that follows nucleation and growth mechanism. The arrows indicate the direction of the scan. Adapted from ref. 106. (B) AFM image of the deposition of Ni particles on Pt(111) in DES solvent at low applied overpotential. From ref. 105.

As alternative to surfactant agents and organic additives in traditional aqueous baths, green ionic liquids have been proposed for the metal electrodeposition. They have enough conductivity and can solubilize most of the metals.102 Some ionic liquids such as deep eutectic solvent (DES) are sustainable and non-toxic while being easy to prepare.103 They are formed by eutectic mixtures of a quaternary ammonium salt and a proton donor molecule. Although less electrochemically stable than other ionic liquids, they have other benefits such as high stability towards moisture, are affordable and clean while still offering wide potential limits for the deposition of most of the metals. Nanostructures of Pd, Cu, Ag, Ni or Au have already deposited using deep eutectic solvents.104 In all these works, it has been evidenced that DES chemistry plays a key role in the stabilization of the growth of the nanoparticles preventing from dendritic growth. Deposition of Ni nanostructures with triangular shape on Pt(111) has been obtained in DES (Fig. 7B), showing the combined effect of the substrate orientation and DES composition on the tuning of the final morphology of the deposit.105 Under the lights of previous reports, we remark that electrodeposition in sustainable organic solvents is a promising methodology to prepare nanostructured materials with tuneable composition and morphology by simply controlling specific electrochemical parameters.

To reach an in-depth understanding of the structure sensitivity and the mechanism of electrocatalytic reactions on tailored electrocatalysts, it is crucial to perform in situ monitoring of the changes of the electrified interface structure under reaction conditions. Surface electrodes are very dynamic, i.e., they can re-structure, react or dissolve, which ultimately alter the catalytic performance at the active site positions in the course of the reaction.107,108

In situ scanning probe microscopy (SPM) such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) allows visualizing the surface at the atomic to nano-scale. High-speed STM has represented a technological progress by providing videos of the atomic-scale structure and the molecule-dynamics at the surface and in real time.109,110 Other in situ microscopy techniques, such as transmission electron microscopy (TEM) are highly valuable to assess the stability and monitor the morphological changes, growth and particle coalescence of deposited nanoparticles, e.g., during the HER or reduction of CO and CO2.97In situ/operando X-ray synchrotron-based techniques are essential to study the structural and chemical properties of the catalyst surface. In situ surface X-ray diffraction (SXRD)111 and X-ray absorption spectroscopy (XAS)107,112 techniques are having an increasing attention due to its enormous potential to elucidate surface structure and chemical state of the catalysts, as well as shed lights into the specific active site for different electrocatalytic reactions in situ.107,111 These techniques can be key, for instance, to investigate degradation mechanisms of the catalysts, such as Pt alloys during the ORR.28,113 Quasi in situ X-ray photoelectron spectroscopy (XPS) can provide information in relation to the chemical composition and oxidation states of the catalyst surface, before and after electrochemistry, without exposing the surface electrode to air.35 Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) allows to carry out the measurement at pressure magnitude-values near to ambient pressure, being a tool with a huge potential in probing the chemical composition of the electrified interface at relevant conditions, and with a depth penetration of a few nanometers. Despite its benefits, the technique is constrained to solid–gas and liquid–gas interfaces while the interface solid–liquid still remains challenging.114In situ infrared and Raman spectroscopy, along with electrochemical mass spectrometry115 allow the detection of the reaction intermediates and products at different stages of the reaction, being crucial to understand the electrocatalytic reaction mechanism.108,115,116

4. Future perspectives and concluding remarks

In this highlight, we have discussed different electrochemical strategies for tailoring and nanostructuring the catalyst surface for sustainable energy conversion. Preparation of bimetallic catalysts through adlayer deposition, monolayer underpotential deposition or metal alloying is carried out to tune the electronics of the employed catalyst. This allows to modify the electrocatalytic reactivity at the active site positions towards selective production of desirable products. Importantly, both engineering of the surface active site together with optimization of electrolyte composition is required for improved activity and selectivity at the electrified interfacial region. We have also highlighted that atomic ensemble control allows to modify the electrocatalytic performance through geometric effects, being a valuable tool to tune selectivity. Herein, well-defined studies under potential control and the use of single-crystalline electrodes are vital to rationally design the catalyst surface with controlled geometric structures at the atomic level.

Results obtained on model surfaces can be translated into more realistic catalysts such us nanostructured materials. Optimization of shape, size and distribution of nanostructures plays a key role in the enhancement of electrocatalytic reactions. In situ characterization techniques are vital to assess changes on the morphology or surface stability under reaction conditions, allowing for more detailed explanation of structure effects in electrocatalysis. Optimization of new routes that allows the preparation of more robust nanostructures while being sustainable and affordable is relevant in the development of nanotechnologies. Electrodeposition in green solvents allows to prepare nanostructures with tuneable morphology and composition, which makes this technique promising to develop novel electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the Villum Foundation for the award of a Villum Young Investigator Grant (project number: 19142).

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