Electrocatalytic Self-Assembled Nanoarchitectonics for Clean Energy Conversion Applications

Abstract

The general trends in the construction of highly active electrode devices are focused on the science of materials. These are useful for developing 2D nanostructured electrodes, with well-defined active sites, which are excellent approaches for understanding the fundamentals of electrocatalytic reactions. Here we present an overview of the experimental self-assembled molecular catalyst configurations to develop excellent electrode materials containing molecular catalysts for energy conversion device applications. First, by applying well-known reactivity descriptors for electrocatalysis, nanoarchitectonics, and the self-assembled concept, we summarize the main molecular building blocks to achieve a technology system for arranging by a rational design, nanoscale structural units configuration that promotes electrocatalytic reactions such as oxygen reaction reduction (ORR) and water-splitting reactions. We focus the discussion on the MN4 molecular catalyst linked to electrode surfaces with the help of the axial blocks, bio-inspired self-assembled approaches such as biomimetic models of metalloenzymes active sites, and molybdenum sulfide clusters for hydrogen evolution reaction (HER). We briefly discuss the advantages of developing host-guest self-assembled molecular catalyst systems based on cyclodextrins anchored to electrodes to get well-defined active sites with local environment control.

1. Introduction

Climate change is one of the worse problems humanity is facing and despite different measures that different countries are taken to counteract this problem but the situation is not improving. One of the main causes of global warming is the widespread use of fossil fuels, among other factors like cattle farming. Thus, there is an urgent need to find alternative non-polluting energy sources i.e., renewable sources. Among these new energy sources, electrochemical systems appear to be very interesting. For example, a water electrolyzer uses clean electricity to produce high uruty electrolytic hydrogen which can be used as a fuel that upon combustion is pollution free as it forms water. Hydrogen can also be used in a fuel cell producing electricity with high efficiency [1, 2]. For optimum operation of electrolyzers and H₂/O₂ fuel cells, it is necessary to develop highly active catalytic materials to be used in all different electrodes [3, 4]. The development and optimization of these systems are part of the energy revolution that is taking place in the world (UN 2030 agenda), with particular emphasis on the development of new technologies that work more efficiently to promote the decarbonization of the global energy matrix.

Among the most promising alternative energy devices are electrolyzers, fuel cells, as mentioned above, and rechargeable metal-air batteries [5]. These electrochemical systems generally have two electrodes separated by a polymeric membrane, called anode and cathode (Figure 1a). Four fundamental electrochemical reactions occur in them for their operation: hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and oxygen reduction reaction (ORR). In the water electrolyzers, the H₂O molecule is splitter to give H₂ and O₂ fuels by applying a potential difference between the cathode (HER) and the anode (OER). On the other hand, fuel cells, use H₂ and O₂ to generate electrical energy through the HOR and ORR reactions that occur at the anode and cathode, respectively Figure 1a.

Figure 1.

(a) Scheme of energy conversion devices: Fuel cells and electrolyzer. In the polymer electrolytic membrane (PEM) fuel cell and electrolyzer, the cathode and anode systems are separated by a proton-conducting membrane. Shown on the left is the fuel cell, which is supplied with hydrogen fuel by oxidizing at the anode, producing H⁺ and e⁻. while, at the cathode, the oxygen in the air reacts with H⁺ and e⁻. on the right side, an electrolyzer is shown in which the operation is reversed, i.e., at the anode, the water splitting reaction leads to the production of O₂, while the H₂ and electrons at the cathode side are used. To convert H⁺ to H₂ reprinted by permission from [springer nature customer service Centre GmbH]: [springer nature] [nature materials] [1] (energy and fuels from electrochemical interfaces, V.R. Stamenkovic, D. Strmcnik, P.P. Lopes, N.M. Markovic), [COPYRIGHT] (2017). (b) HER volcano plot for metals and MoS₂. (c) ORR volcano plot for metals. (d) OER volcano plot for metal oxides. Reprinted by permission from [the American Association for the Advancement of Science AAAS]: [science] [2] (combining theory and experiment in electrocatalysis: Insights into materials design, Z.W. Seh, J. Kibsgaard, C.F. Dickens, I. Chorkendorff, J.K. Nørskov, T.F. Jaramillo), [COPYRIGHT] (2017). (e) Schematic representation of MN4-modified ordinary pyrolytic graphite (OPG) electrode. (f) Self-assembled system made up of electrocatalyst attached by a wire attached to the gold substrate. Reprinted (adapted) with permission from the journal of physical chemistry C [6]. Copyright (2012) American Chemical Society.

HER

$$2H^{+}+2e^{-}\rightleftharpoons H_2\text{acidic electrolytes}$$E1

OER

$$2H_{2}O\rightleftharpoons O_2+4H^{+}+4e^{-}\text{acidic electrolytes}$$E2

$$4O{H}^{-}\rightleftharpoons 2H_{2}O+O_2+4e^{-}\text{neutral or alkaline electrolytes}$$E3

HOR

$$H_2\rightleftharpoons 2H^{+}+2e^{-}\text{acidic electrolytes}$$E4

ORR

$$O_2+4H^{+}+4e^{-}\rightleftharpoons 2H_{2}O\text{acidic electrolytes}$$E5

$$O_2+2H^{+}+2e^{-}\rightleftharpoons H_2{O}_2$$E6

$$O_2+2H_{2}O+4e^{-}\rightleftharpoons 4O{H}^{-}\text{alkaline electrolytes}$$E7

$$O_2+H_{2}O+2e^{-}\rightleftharpoons H{O}_2^{-}+O{H}^{-}$$E8

The above reactions occur by multi-electron pathways and, therefore, can proceed via several mechanisms. For example, in acid electrolyte environments, HER follows two elemental reactions, proton adsorption on the active site of electrode surface (M) by the Volmer mechanism, and desorption of the H₂ molecule step by Heyrovsky or Tafel mechanisms. Thus, if the M-H interaction is too weak, the adsorption step will limit the overall reaction rate, and if the M-H interaction is too strong, the reaction − desorption step will limit the overall reaction rate [7]. As a result, optimal catalytic surfaces exhibit a Gibbs free energy of electrochemical H-adsorption (ΔGH) close to zero (Figure 1b) as Parson proposed in 1958 [8]. On the other hand, HOR proceeds by the same steps but reversely for HER [2, 5].

$$H^{+}+e^{-}+M\rightleftharpoons M-H\text{Volmer step}$$E9

$$M-H+H^{+}+e^{-}\rightleftharpoons H_2+M\text{Heyrovsky step}$$E10

$$2M-H\rightleftharpoons H_2+2M\text{Tafel step}$$E11

In this direction, the ORR can proceed by the four-electron pathway (4e−/4H+), which is desirable for fuel cells and air batteries; the two-electron pathway (2e−/2H+) promotes H₂O₂ as a product. Therefore, from the energy conversion challenges, an efficient catalyst for ORR is the one that achieves to promotes a selectivity for 4e−/4H+ on 2e−/2H+.

The HER, HOR, OER, and ORR reactions occur through an internal sphere electron transfer process and involve electrocatalytic phenomena. Thus, their efficiency is directly related to the electrodic materials used as anode and cathode [3, 9, 10]. Therefore, it requires electrodes with specific activity for the reaction involved (Figure 1b–d). However, in most known electrode materials, these reactions are slow and require specific catalysts to reach rates compatible with their applicability at the industrial level, which are based on expensive platinum-based materials or precious metal oxides such as IrO₂ or RuO₂ [2, 5, 11]. Noble metals are scarce and expensive, preventing the mass use of these devices. In this context, new alternatives to using inexpensive materials have been proposed. Among them, the graphite materials modified with the MN4 molecular catalysts, such as metallophthalocyanines, metalloporphyrins, and their derivatives complexes of the metal transition (Figure 1e).

Graphite electrodes modified by a simple process involving the physical adsorption process of intact MN4 complexes on their surface have made it possible to perform fundamental studies to establish reactivity descriptors for specific reactions such as ORR and, with this, advance in the search for the best catalyst for this reaction through a rational design [9, 10] of the MN4 molecules. However, new experimental strategies and surface characterization techniques have allowed the development of more advanced electrode systems based on nanostructured systems of molecular catalysts as the main active component [6, 12, 13, 14]. Thus, one of the fundamental strategies to incorporate a molecular catalyst such, as MN4 complexes, on electrode surfaces is using the self-assembled process, where the molecular catalyst is anchored to the electrode surface using a molecular building block that can establish a link between surface-molecular catalysts. [15] Among these systems, the most prominent are the self-assembled organic monolayers (SAMs) on gold electrode surfaces and their subsequent functionalization with MN4 complexes that can be converted into regular molecular arrays by molecular-level nanoarchitectonics (Figure 1f) [16].

The concept of nanoarchitectonics has been proposed as a technology system for arranging nanoscale structural units in a rationally pre-designed configuration for a given function [17]. Thus, in self-assembled molecular catalyst systems on an electrode surface, each functionalized SAM molecule fulfills the function of molecular wire and, at the same time, as an axial ligand of the MN4 complex. This configuration allows the catalyst to be anchored on the surface, permitting the electronic communication between the electrode and molecular catalyst (Figure 1f). Therefore, through the self-assembly process, it is possible to obtain nanostructures with a specific functionality on the electrode that has allowed the generation of efficient electrode systems with reproducible activity for specific reactions to advance the understanding of reaction mechanisms achieving to obtain the control of one pathway reaction over other [12, 14]. Moreover, self-assembled nanostructures provide the fundamental basis for constructing supramolecular nano-devices that can be used as active components to build compact and efficient electrodic devices for energy conversion systems.

According to above, the following sections present the fundamental aspects of the construction of molecular catalysts self-assembled to electrode surfaces, which serve as self-assembled supramolecular electrocatalytic devices. For this, it is necessary to consider each molecular building block that, through a rational design, allows the formation of nanostructures guided by molecular self-organization processes on electrode surfaces; in addition, the optimization of electrocatalytic properties of the molecular catalyst for a specific reaction of energetic interest. We will focus mainly on ORR and HER processes, ending with an overview of the new perspectives to consider.

2. Modified electrodes with MN4 complexes as electrocatalytic building molecular blocks

The concept of modified electrodes refers to those electrodes whose surface has been altered by incorporating compounds that can act as mediators in a charge transfer reaction (Figure 2a-c) [18]. These compounds can be metal complexes, organometallic complexes, conductive polymeric films, metallic atoms, and self-assembled molecules confined, deposited, or adsorbed on the electrode surface. The last strategy is the first approach to incorporate, by a simple method, new active sites on the electrode surface by the physical adsorption process of molecular catalyst; in this process, the molecules are attached to the surface through weak bonds such as van der Waals forces and π-staking interactions, among others. Thus, the modified electrode can present electrocatalytic activity for a given reaction. In this case, the electrocatalytic activity is greater than that of the unmodified electrode (Figure 2f-g) [18].

Figure 2.

Examples of commonly used macrocyclic compounds: (a) Metalphthalocyanine. (b) Metalloporphyrin, (c) phenanthroline derivative, (d) dicobalt cofacial porphyrin M = Co, and (e) cobalt phthalocyanine on graphite M = Co. f) Voltammetric response for O₂ reduction for a clean and/or modified (111) gold electrode with different configurations containing MN4 complexes. (g) MN4 complexes: Iron phthalocyanine (FePc) and hexadecachlorinated phthalocyanine (16(Cl)FePc). ORR schemes in the unmodified gold electrode (i) and modified with FePc (ii) and 16(Cl)FePc (iii) measurements made in a 0.1 M NaOH solution saturated with O₂, dE/dt = 50 mV/s. reprinted (adapted) with permission from ACS catalysis [12]. Copyright (2018) American Chemical Society.

MN4-type macrocyclic complexes are well-known to exhibit electrocatalytic activity for different reactions of interest, including ORR. When incorporated into the electrode surfaces, these act as charge mediators, creating new active sites on the surface and reducing the overpotential needed to initiate the electrochemical reaction (Figure 2f-g). In this direction, the studies have focused on exploring the reactivity descriptors for a specific reaction such as ORR in ordinary pyrolytic graphite (OPG) and glassy carbon electrodes modified directly with MN4. The importance of these electrode surfaces modified with MN4 lies in the fact that, through certain experimental conditions, it is possible to achieve a high degree of control of the surface in terms of its structure and reactivity, being able to establish and control its reactivity and specificity. Most MN4 complexes studied have involved transition metal phthalocyanines (MPcs), metal porphyrins (MP), and similar complexes based on biomolecules (e.g., Vit B12, Cytochrome c, and multicopper oxidase) adsorbed directly on carbon, and pyrolytic graphite materials, among others [11]. This is due to the variety of electrochemical reactions catalyzed by these systems.

In MN4 complexes, the metal center acts as an active site; in addition, they have a flat and rigid structure with a highly delocalized electronic system (Figure 2a-c). These can undergo very fast reversible redox processes with a minimum of molecular reorganization. In addition, its redox properties can be tunned using electron donor or acceptor substituents in the cyclic ligand, thus, regulating the electron density on the metal center. The redox processes of MN4 are closely related to the energies of the frontier orbitals, which are precisely those involved in the interaction with the reacting molecule. [6, 12, 13, 14]. Among the most studied MN4 complexes are phthalocyanines and porphyrins. However, new highly active complexes for ORR have recently emerged, such as Fe phenanthroline derivatives, which mimic the M-N4 site (Figure 2c) [17].

In these systems, reactivity descriptors have been found for ORR, such as the number of “d” electrons in the metal center, the donor-acceptor intermolecular hardness, the binding energy between the metal and oxygen, and the redox potential Mⁿ⁺/M⁽ⁿ⁻¹⁾⁺; the latter two being closely related [9]. These reactivity descriptors allow the synthesis of new and optimized molecular catalysts based on these complexes. In iron phthalocyanine, it has been shown that by incorporating electron-withdrawing substituents at the periphery of the ligand macrocycle, the electrocatalytic activity for ORR increases. Electron withdrawing ligands would promote a harder active site, tuning a better Fe-O₂ interaction since the O₂ molecule is a hard molecule; the Sabatier principle supports this effect [12]. The control binding energies of the reacting molecule on an active site of the electrode surface are the key to designing materials with improved performance [2]. In this direction, multinuclear complexes onto graphite electrodes are selective for the 4e−/4H+ reduction for ORR over the 2e-/2H+ pathway; thus, a high selectivity has been obtained for dicobalt cofacial porphyrin and related complexes (Figure 2d-e) [19]. In those models, the two metal centers help to improve the electrocatalytic performance due to the interaction of the O₂ molecule linked to the multinuclear active site [19].

2.1 Molecular catalyst self-assembled to electrode surface: a experimental platform to obtain 2D nanomaterials for clean energy conversion applications

MN4 molecular catalysts and derivatives have functional electrocatalytic properties to be used as active components in constructing nanostructured electrode surfaces for energy conversion systems. As mentioned in the previous section, their electrocatalytic properties can be tuned by the action of the substituents on the cyclic ligand (Figure 2g). Moreover, molecular catalysts such as metal phthalocyanines of Fe, Co, Mn (FePc, CoPc, MnPc) can accept axial extraplanar ligands directly coordinate with the metal center. Exploiting this ability, the complexes can be anchored to electrode surfaces using an axial ligand by a bottom-up construction procedure to obtain well-defined nanostructures [12, 14].

One strategy to immobilize MN4 complexes on electrode surfaces is functionalizing self-assembled organic monolayers, SAMs; among the most used surfaces to carry out this process are gold surfaces.

SAMs are a set of molecular building blocks spontaneously formed in a 2D arrangement (D = dimension) on an electrode surface, forming films of nanometric height [15, 20]. The building blocks that form the SAMs have a well-defined structure to give them a certain chemical functionality, which consists of a “head” and a “terminal” group, which binds strongly to the substrate at one extreme and, another extreme, limit toward the outer surface in contact with the electrolytic solution, respectively. Connecting the head and terminal groups is carried out by a spacer or backbone of variable structure and length [15]. Thus, if SAMs on an electrode surface are considered 2D nanomaterials according to their dimensions, the functionalization of the SAMs with molecular catalyst represents an excellent platform for the construction of electrocatalytic patterned surfaces to apply to the fabrication of compact and efficient electrodic devices (Figure 3a).

Figure 3.

(a) Schematic representation of 3D nanostructured gold electrode obtained by the bottom-up construction of self-assembled monolayers functionalized with FePc. (b) Lineal sweet voltammetry for ORR on Au(111) modified with different configurations of pyridiniums molecules with a electron-withdrawing groups into molecular backbone. Performed on 0.1 M NaOH O₂ saturated, dE/dt = 0.05 V s⁻¹. Reprinted (adapted) with permission from Electrochimica Acta [14]. Copyright (2018) Elsevier. (c) Optimized molecular structures of FePc-py-SWCNT. Reprinted (adapted) with permission from Electrochimica Acta [21]. Copyright (2021) Elsevier. (d) Construction of the electrode bearing the biosynthetic model. Adapted from Figure 2 in ref. [22].

In the self-assembled arrays, there is a synergic contribution at the molecular level by a self-controlled organization. Therefore, by a predesignated configuration, self-assembled FePc among others, and related metal complexes systems have been developed on gold surfaces for ORR. In this surface configuration, at the nanoscale, each electrocatalytic nanostructure has a molecular catalyst with an active site conformed by a five-coordinated metal center forming an umbrella-like configuration (Figure 3b). As mentioned above, axially coordinated molecular catalysts present higher performance activity in comparison with the adsorbed systems. So, the axial ligand apart from serving as a molecular anchor is not innocent and affects the electron density on the metal center. Thus, in well-established electrocatalytic molecular platforms involving this type of self-assembled nanoarchitectonic for FePc and related complexes, the activity for ORR is enhanced by the axial ligand action compared to the FePc directly adsorbed on the electrode surface. Similar self-assembled systems have been carried out on single-walled carbon nanotubes, SWCNTs, where the axial ligand corresponds to a building block anchored to the carbon surface by a covalent bond, SWCNTs-Py systems (Figure 3c). However, FePc is self-assembled in SWCNTs-Py systems by spontaneous interaction between Fe and the axial ligand available in SWCNTs-Py systems [21].

In a broad scene, three components are fundamental for obtaining self-assembled molecular catalyst arrays:

1. The electrode surface, which is the crucial material for electrochemical systems represents the source and sink of the electrons. The nature of the surface (such as graphite, gold, and Ag) depends on the type of molecular building block and, therefore, the kind of interactions that drive the bottom-up self-assembly process (e.g., van der Waals interaction, π-stacking, among others).

2. A functional molecular building block to act as an axial ligand to the metal center of the molecular catalyst. The general rational design of this building block must consider incorporating an “anchor” functional group at one end, which has a chemical affinity to bind to the electrode surface (e.g., SH and -SCH₃ for gold and pyrene for graphite surfaces). Also, a coordinating functional group binds to the metal center of an MN4 complex (e.g., Pyridine and NH₂, it contains atoms that can donate a pair of electrons to the metal center). Moreover, the rational design must be considered the reactivity descriptors for obtaining a molecular backbone where it aids in exploiting the crucial factors to improve the activity of the self-assembled systems for an interesting reaction. Thus, for ORR carried out on FePc, the activity is increased when the molecular catalyst is anchored to the gold surface by a building block with electron-withdrawing groups into the molecular backbone (Figure 3b). This effect can be related to the donor − acceptor Fe − O₂ intermolecular hardness. The electron-withdrawing groups, although they decrease the electron density on the metal center, favor the donor−acceptor electronic coupling, and increase the catalytic activity of FePc for the ORR. In similar highly catalytic systems, FePc-Py-SWCNTs the presence of pyridine neutral axial ligand decreases the electron density on the Fe site compared with that of FePc adsorbed on the SWCNTs surface [12, 21].

3. An electrocatalytic molecular building block, such as MN4 complexes. In the case of incorporating the complex to the surface in an umbrella-like configuration (Figure 3b), it is required that the central metal (Fe, Co, Mn) can accept extraplanar ligands. In addition, as mentioned in the previous point and Section 2, a variety of reactivity descriptors have been proposed for the MN4 complexes for the ORR. In addition, as mentioned in the previous point and Section 2, various reactivity descriptors have been proposed for the MN4 complexes for the ORR. Among them, the M(III)/(II) formal potential of the catalysts is an excellent reactivity predictor that requires an accurate determination of the surface modified with MN4 complexes. Furthermore, these complexes present reversible redox peaks that have been assigned to M(II)/(I) and M(III)/(II) processes supported by spectroscopic evidence [12].

The bioinspired nanostructured electrodes are an excellent experimental platform for mimi kg the efficiency and selectivity of natural metalloenzymes (Figure 4a). Thus, the electrode bearing the biosynthetic model has been constructed for electrochemical ORR by “exploiting” the bio-architecture of the active site configuration. As mentioned earlier, multinuclear complexes are selective for the four-electron reduction for ORR over the two-electron pathway. Thus, the biomimetic models of Cytochrome c oxidase (CcO) have been carried out for ORR, where the active site corresponds to a heteronuclear complex (Fe heme, a distal copper, and tyrosine residues) anchored to a gold electrode surface by the action of imidazole ligand linked to alkanethiols SAMs on gold electrodes. In those systems, the synergic contribution of axial ligand and multinuclear active sites helps to improve the electrocatalytic performance for ORR due to the improved interaction of the O₂ molecule linked to the multinuclear active site [11, 23]. The same principle is fulfilled in self-assembled monolayers functionalized by covalently attached mutant myoglobin based on the biosynthetic model of CcO to carry out ORR where both the distal Cu and the redox-active tyrosine residue can help both electron and proton transfer during O₂ reduction [22].

Figure 4.

(a) Schematic representation of SAMs functionalized with synthetic CcO model. Adapted from Figure 3 in ref. [23]. (b) in situ STM images of [Mo₃S₄]⁴⁺ monolayer on Au(111) in 0.1 M HClO₄ 10x10 nm². Adapted from Figure 7 in ref. [24].

In search of functional molecular building blocks to obtain a low-cost catalyst for the hydrogen evolution reaction (HER), sulfur-rich transition metal clusters have been used to modify graphite and gold electrodic surfaces due to containing groups that resemble the active core groups in redox metalloenzymes, such as iron–sulfur cluster-based enzymes as well as Mo-enzymes; in truth, many processes in nature also rely on metal sulfide clusters [8, 24]. Thus, self-assembled inorganic monolayers of incomplete cubane type [Mo₃S₄]⁴⁺ discrete clusters have been developed on the gold electrode surface by direct interaction between atom sulfur linked to Au(111) surfaces (Figure 4b). The [Mo₃S₄]⁴⁺ unit comprises three molybdenum atoms in the oxidation state +4 positioned in a triangular plane capped by a μ₃-S²⁻ entity. The four sulfur atoms in the cluster are in the reduced state −2 like –SH “head” functional group in organic thiols [8]. Therefore, in the self-assembled [Mo₃S₄]^4+/Au(111) systems, the most stable configuration would be three sulfur atoms acting as linkers to gold and one sulfur atom exposed between the solution electrode interface (Figure 4b). This configuration has been characterized by different experimental techniques such as cyclic and lineal voltammetry, XPS, and in-situ scanning tunneling microscopy, STM [7, 24]. This system works efficiently for electrocatalysis for hydrogen evolution reaction, where the onset potential (the potential at which catalytic current is first observed) is shifted 200 mV, in an acidic environment, to favorable potentials concerning the gold surface without inorganic SAMs [24].

Earlier, we mentioned the three possible reaction steps involved in the mechanism for HER (reactions 9, 10, and 11). Whether the reaction proceeds via the Volmer − Heyrowsky mechanism or the Volmer − Tafel mechanism, the reaction occurs by hydrogen atoms adsorbed on the electrode surface, M-H (where M = active site on electrode surface) [7]. On the other hand, these inorganic clusters consist of small molecular units of molybdenum sulfide with undercoordinated sulfur abounding at its surface, and it is generally assumed that hydrogen binds to under coordinated sulfur edge sites. Therefore, this self-assembled discrete unit on electrode surfaces provides an efficient active site for carrying out the HER process, where the overall reaction rate is influenced by the free energy of hydrogen adsorption, ΔGH (Figure 1b) [7].

Because molybdenum sulfide clusters have shown great promise for electrocatalysis for HER to generate carbon-free fuel from water, different experimental techniques have been proposed to incorporate them on electrode surfaces. Among these techniques, straightforward methods involving Mo-S clusters are supported on various substrates by simple drop-casting from a specific solution. Thus, the highly oriented pyrolytic graphite (HOPG) electrode surfaces have been modified with [Mo₃S₄]⁴⁺ molecular clusters by physical adsorption or electrostatic interaction on anodized surfaces. In this configuration, it was possible to obtain a structure–function relationship of these electrocatalytic building blocks at the single-molecule level; this latter, by combining electrochemistry and STM experiments. In those systems, the molecule cluster activity was higher than in nonprecious metal HER catalysts, such as Ni, Cu, or W. Similar behavior has shown [Mo₃S₁₃]²⁻ nanoclusters [7]. In this direction, Mo-S represents promising building block entities to obtain electrocatalytic nanostructured electrode surfaces in well-established arrays directed by different self-assembled ways such as quasi-covalent interaction (S-Au), van der Waals interaction, electrostatic interaction (between surface groups and cluster unit), and in-situ ligand substitution to obtain electrocatalytic linked blocks arrays [7].

2.2 Electrocatalysis based on host-guest systems

Recently, a new perspective has emerged in the development of nano-device with potential applications in energy conversion reactions, which is the formation of host-guest systems at the electrode interface [25, 26]. Within the wide range of hosts, molecules are cyclodextrins, calixarenes, and cucurbit[n]urils, among others [27, 28], which are based on the formation of inclusion complexes through non-covalent interactions [29].

Cyclodextrins (CDs) are functional building blocks corresponding to a family of macrocyclic oligosaccharides composed of D-(+)-glucopyranose subunits linked through an α-1,4-glucosidic bond. The most common CDs are composed of 6, 7, or 8 glucopyranose subunits and are called α -CD, β-CD, and γ-CD, respectively [30]. The CDs exhibit two types of –OH groups at three different positions: primary –OH groups at the C₆ position and secondary –OH groups at C₂ and C₃ positions. These –OH groups are located on the edges providing a hydrophilic exterior, whereas C–H units at the inside create a hydrophobic [31, 32]. Among the native CDs, the most widely used due to its greater accessibility and functionalization of its chemical structure is β-CD. This CD can form inclusion complexes with a series of aromatic and heterocyclic compounds [33]. In this direction, as an experimental platform, these entities are suitable building blocks to obtain well-structured arrays in 2D conformation on electrodes surfaces to study, at the single-molecule level, the influence of the local environment hydrophobicity/hydrophilicity on the electron-transfer process, across the self-assembled molecular catalyst for ORR, HER, OER, and HOR [31]. Therefore, the host-guest self-assembled molecular catalyst could provide a directed via to develop a fundamental understanding of the intrinsic activity of catalytic sites in the interface’s electrode solutions, which is necessary to design highly active surfaces whit well-defined active sites.

Komatsu. et al. reported the formation of a host-guest system building from α-CDs units and porphyrin-pyridinium complexes with an iron penta-coordinated metal center. In this system, a stable adduct with an oxygen molecule forms through the direct interaction of Fe-O2, Figure 5a [34]. Other host-guest systems between thiolated cyclodextrins (β-CDSH) and iron porphyrin (FeTMPyP) have been developed to promote the ORR via the 4-electrons reduction process [36]. In these self-assembled systems, the MN4 complex is linked to the electrode surface modified with CD by the favorable interaction of the axial ligand into de hydrophobic cavity into the CD. On the other hand, cobalt porphyrins with different substituents on the periphery of the macrocycle rings ((TPPS)Co, (TMPyP)Co, (TPP)Co)) were used to form inclusion complexes with β-CD polymers immobilized on gold to evaluate electrocatalysis of ORR (Figure 5b) [35, 37]. This reaction at the electrode coated with a polymer film, hosting the cobalt porphyrin catalyst, involves a 2e-/2H+ pathway resulting in the formation of hydrogen peroxide.

Figure 5.

(a) Schematic representation of possible dominant αααβ structure of the deoxygenated host-guest ensemble. Adapted from reference [34]. (b) Scheme of electrocatalytic dioxygen sensing at the gold electrode coated with the β-CD film containing cobalt porphyrin (Co(TPP)). Adapted from reference [35].

Besides, the host-guest inclusion complexes have been carried out with ruthenium electrocatalysts and per-thiolated β-CDs [31] anchored on gold electrode surfaces. This self-assembled configuration favors the ammonia electrooxidation reaction (AOR) since ammonia (NH₃) shows promise as a renewable hydrogen storage medium, i.e., by electrochemical decomposition for on-site hydrogen generation or by direct use of ammonia as fuel in a fuel cell. This shows that the development of AOR electrocatalysts is essential to obtain a high yield in HER and ORR.

3. Conclusion

We have described the crucial role of rational design in obtaining molecular building blocks as active components for constructing a self-assembled molecular catalyst for the electrocatalysis of the energetic reactions interest, focusing on the ORR and HER process. These reactions occur through an inner-sphere electron transfer process and involve electrocatalytic phenomena; therefore, their efficiency is directly related to the electrodic materials used as cathode. We summarized the reactivity descriptors applied to build self-assembled molecular catalysts to promote electrocatalytic reactions in the central molecular configurations to gain a fundamental understanding of obtaining nanostructured electrode surfaces. We make the description for different experimental approaches to achieve self-assembled arrays with synergic contribution, at the molecular level, by a self-controlled organization. Among this configuration, MN4 molecular catalyst linked to gold electrodes and graphite surfaces by the axial ligands have been developed to improve the activity for ORR compared with catalyst adsorbed directly on electrode surfaces. Furthermore, we described a few bio-inspired self-assembled approaches based on biomimetic architecture to emulate similar metalloenzymes active sites. Finally, we describe the advantages of developing host-guest self-assembled molecular catalyst systems based on cyclodextrins anchored to gold electrodes to obtain highly active surfaces whit well-defined active sites with local environment hydrophobicity/hydrophilicity control on the electron-transfer process.

Acknowledgments

This work has been financed by the Fondecyt Project 1211351. Fondecyt Project and 1221798. Ring Project ACT 192175 and ANID-Fondecyt Postdoctoral 3220728.