A shift in energy dependence from fossil fuels to sustainable and carbon-neutral alternatives is a daunting challenge that faces the human society. Light harvesting for the production of solar fuels has been extensively investigated as an attractive approach to clean and abundant energy. An essential component in solar energy conversion schemes is a catalyst for water oxidation. Ruthenium-based catalysts have received significant attention due to their ability to efficiently mediate the oxidation of water. In this context, the design of robust catalysts capable of driving water oxidation at low overpotential is a key challenge for realizing efficient visible light-driven water splitting. Herein, recent progress in the development within this field is presented with a focus on homogeneous ruthenium-based systems and surface-immobilized ruthenium assemblies for photo-induced oxidation of water.
- water splitting
- sustainable chemistry
The search for inexpensive and renewable energy is currently one of society's greatest technological challenges. As light energy from the sun continuously strikes the earth’s surface, harnessing this energy would solve the increasing future energy demand and lead to a more sustainable society. An appealing solution would therefore be to convert light energy to storable fuels, such as hydrogen gas or reduced carbon compounds. For realizing this scenario, novel technologies have to be developed that efficiently utilize solar energy. Furthermore, such systems also need to rely on abundant and inexpensive feedstocks in order to become viable on a large scale. As water is plentiful, it would be attractive to use it as a feedstock for obtaining the necessary reducing equivalents—protons and electrons [1–6].
The natural system constitutes an excellent source of inspiration for how to design an artificial system that is capable of harnessing solar energy for fuel production. The concept of artificial photosynthesis emerged in the 1970s and is inspired by nature where light-induced charge separation events sequentially oxidize a Mn4Ca cluster (Figure 1) known as the oxygen-evolving complex (OEC) [7, 8]. After four electrons have been abstracted from the OEC, two molecules of water are oxidized to molecular oxygen, thus releasing four electrons and four protons. The natural photosynthetic apparatus subsequently utilizes the generated reducing equivalents to reduce CO2 to carbohydrates [9, 10]. However, instead of using the generated reducing equivalents to reduce CO2 to carbohydrates as in the natural photosynthetic apparatus, these “artificial leafs” would produce hydrogen gas from the protons and electrons that are liberated when water is oxidized (Eq. 1) [11, 12].
Water splitting can be divided into two half-reactions; proton reduction and water oxidation. The reductive side of water splitting involves the generation of hydrogen gas from the generated protons and electrons. In contrast to hydrocarbons, hydrogen gas is considered to be environmentally benign, as water is the only combustion product. Although deceptively simple, the other half-reaction, water oxidation (Eq. 2), is a mechanistically complex process and is currently considered as the bottleneck. The oxidation of water thus requires a single catalytic entity capable of accumulating four oxidizing equivalents, breaking several bonds and forming the crucial O–O bond. Splitting of water is an energy demanding process with a Gibbs free energy of 237.18 kJ mol–1 and a minimum electrochemical potential of 1.229 V vs. normal hydrogen electrode (NHE) is required. The basic thermodynamic requirements for splitting water suggest that any light with a wavelength shorter than 1 mm has enough energy to split a molecule of water. Consequently, this allows the use of the entire visible solar spectrum and a majority of the near-infrared spectrum, which collectively constitutes ~80% of the total solar irradiance .
The first example of photoelectrochemical water splitting was reported by Fujishima and Honda in the early 1970s. Their system consisted of a titanium dioxide (TiO2) photoanode which upon irradiation with ultraviolet (UV) light generated oxygen at the anode and hydrogen gas at an unilluminated platinum cathode . Since the seminal work by Fujishima and Honda, several research groups have attempted to improve the system in order to enable the reaction to be driven by visible light instead of UV light [15, 16].
A simple depiction of an artificial photosynthetic system is shown in Figure 2 and consists of three components: a chromophore (photosensitizer) for light-absorption, a water oxidation catalyst (WOC), and a reduction catalyst for proton reduction. The light-absorbing component, the molecular chromophore, is in general coordinated to the surface of a semiconductor, such as TiO2. The initial step in such a system involves light absorption by the photosensitizer, generating a long-lived charge-separated state by transferring an electron to the conduction band of the semiconductor. The oxidized photosensitizer subsequently recovers an electron from the covalently bound oxidation catalyst (the WOC) or from the functionalized semiconductor surface to regenerate the ground state photosensitizer. After four successive electron transfers, the highly oxidized WOC is reduced by oxidizing two molecules of water, thus releasing molecular oxygen. Although the events seem trivial, the overall process of light-driven water splitting requires interfacing of several nontrivial chemical steps such as accumulation and abstraction of several electrons at the reduction and oxidation catalyst, respectively. This requires the integration of efficient light absorption, generation of long-lived charge separation, organized proton reduction at the cathode, and fast oxidation of water at the anode [17–19].
2. Ruthenium-Based Photosensitizers
The first step in solar energy conversion schemes involves light absorption by a chromophore. In the natural photosynthetic system, a set of specialized chlorophyll-based pigments is responsible for the absorption of visible light, and subsequently transfers the excitation energy to the reaction centers of photosystem II and I. Mimicking these events for constructing artificial photosynthetic devices is a crucial objective and requires tailored photosensitizers that are photostable and efficiently absorb photons across a wide range of wavelengths in the visible spectral region. Furthermore, they should also be easy to modify to allow for straightforward tuning of the photophysical features. The main requirement is that the reduction potential of the oxidized photosensitizer is more positive than that of the WOC and the onset potential for water oxidation (and any overpotential that is produced in the designed system) [20, 21].
2.1. Photophysical Description of Ruthenium-Type Photosensitizers
Perhaps the most extensively studied metal-based photosensitizers are the [Ru(bpy)3]2+-type complexes (Figure 3; bpy = 2,2’-bipyridine). Shortly after the seminal report on UV-light-mediated water splitting at TiO2 photoanodes by Honda and Fujishima , the basis for artificial photosynthesis appeared when it was realized that metal complexes, such as [Ru(bpy)3]2+ (
2.2. Evaluating Light-Driven Water Oxidation with Ruthenium Complexes
A three-component system is typically employed for evaluating light-driven water oxidation and consists of a photosensitizer, a water oxidation catalyst, and a sacrificial electron acceptor (Figure 5). As the sacrificial electron acceptor, sodium persulfate is usually employed since the recombination or reversed electron transfer can be ruled out, thereby simplifying the kinetic analysis of subsequent steps in the catalytic process. The three-component light-driven system using the persulfate anion (S2O8 2–) and a metal-based photosensitizer has been well documented and is believed to commence with oxidative quenching of the excited state of the photosensitizer, such as [Ru(bpy)3]2+*. This results in the generation of [Ru(bpy)3]3+, sulfate, and a sulfate radical (SO4 −•), which is a strong oxidant (E° > 2.40 V vs. NHE) and has the ability to directly oxidize a second equivalent of [Ru(bpy)3]2+ . The reduction of two equivalents affords the four equivalents of [Ru(bpy)3]3+ that are needed to oxidize the WOC, which in turn oxidizes water to molecular oxygen. The processes involved in the light-driven persulfate system are summarized by Eqs. 3–6.
3. Ruthenium-Based Water Oxidation Catalysts—Oxidatively Robust Catalytic Entities
One of the main challenges in realizing water splitting is the development of efficient and robust WOCs that possess low overpotentials and high turnover rates. The development of homogeneous WOCs is an intense and rapidly expanding research field. During the past decade, considerable progress in constructing molecular catalysts capable of oxidizing water has been made using transition metal-based catalysts in the presence of strong chemical oxidants, such as CeIV [29, 30]. Homogeneous catalysts are advantageous as they facilitate mechanistic studies, thus stimulating the design of new and improved WOCs . Owing to their high abundance and low toxicity, several WOCs based on first-row transition metals, such as cobalt (Figure 6) [32–35], copper (Figure 7) [36–38], iron (Figure 8) [39–42], and manganese (Figure 9) [43–47], have been designed. However, their design has proven particularly challenging as these WOCs suffer from insufficient stability and are rapidly deactivated/decomposed under the harsh conditions required to oxidize water. In contrast, catalysts based on the third-row transition metal ruthenium have shown to produce robust catalysts that are able to deliver high turnover numbers (TONs) and high turnover frequencies (TOFs) [29, 30]. This chapter summarizes the recent advances that have been made in designing ruthenium-based WOCs for visible light-driven water oxidation.
3.1. Dinuclear Ruthenium Complexes Capable of Mediating Light-Driven Water Oxidation
Intensive attempts to develop efficient WOCs have been made since the first homogeneous ruthenium catalyst, the “blue dimer” (
The groups of Sun and Åkermark designed a dinuclear ruthenium complex (
Three different [Ru(bpy)3]2+-type photosensitizer derivatives were employed for visible light-driven water oxidation . It could be shown that the TONs and TOFs increased with the increasing potential of the photosensitizer, which is ascribed to the stronger driving force. When employing the [Ru(bpy)(deeb)2]2+ photosensitizer, a TON of 370 and a TOF of 0.26 s–1 was obtained for ruthenium complex
Subsequent studies by the group of Sun and Åkermark focused on the dinuclear ruthenium complex
A dinuclear ruthenium complex (
Several redox active species were observed in the electrochemical studies in phosphate buffer at pH 7.2. Oxidation peaks at potentials of approximately 0.05, 0.38, 0.70, 0.90, and 1.20 V vs. NHE were observed, with a catalytic current for water oxidation appeared at an onset potential of 1.20 V vs. NHE. The photochemical oxidation of water was initially carried out at pH 7.2, using [Ru(bpy)2(deeb)]2+ as photosensitizer. At a 3.0 μM catalyst concentration, the amount of evolved oxygen corresponded to a TON of 830. Lowering the pH led to a slight increase of the TON (890); however, a further decrease of the pH to 5.2 resulted in a substantial reduction in oxygen production, presumably due to the lower driving force. An increase of the pH to 8.2 also resulted in a significantly lower oxygen formation and was ascribed to the decomposition of the ruthenium-based photosensitizer . A subsequent study has suggested that the designed ligand scaffold in dinuclear ruthenium complex
Llobet and coworkers reported a pyrazolate bpy-based dinuclear ruthenium complex
The catalytic activity for light-driven water oxidation was measured at pH 7, using the [Ru(bpy)(deeb)2]2+ photosensitizer (
The activity of
The catalytic efficiencies for the developed ruthenium-based complexes housing carboxylate-functionalized ligands and related complexes containing negatively charged ligand backbones highlight the robustness of these WOCs and suggest that such catalysts can be further heterogenized and applied onto photoanodes, which constitute an important part for the fabrication of devices for artificial photosynthesis.
3.2. Light-Driven Water Oxidation Catalyzed by Single-Site Ruthenium Complexes
Thummel and coworkers demonstrated that the ruthenium(II) polypyridyl complex
Although a plethora of single-site ruthenium-based WOCs have been designed, only a handful has been shown to catalyze light-driven water oxidation. A majority of the ruthenium complexes that have been successful in driving water oxidation with visible light contain negatively charged ligand scaffolds, which are essential for lowering the redox potentials, allowing water oxidation to be driven by [Ru(bpy)3]2+-type photosensitizers.
The [Ru(bda)(pic)2(OH2)] complex (
The two structurally related ruthenium complexes
Åkermark and coworkers have recently reported on the two single-site ruthenium complexes [Ru(Hhpbc)(pic)3]+ (
O–O bond formation. This could thus provide the foundation for new pathways for the activation of small molecules such as water.
4. Molecular Chromophore–Catalyst Assemblies for Water Oxidation
Due to the progress that has been made in developing ruthenium-based catalysts for water oxidation, increased attention has recently been given to constructing supramolecular dyads where a photosensitizer and a WOC are covalently linked. These assemblies can subsequently be grafted onto solid electrodes, which would ultimately allow for visible light-driven water splitting [68, 69]. This chapter highlights some of the representative ruthenium-based assemblies and devices that have been constructed for photochemical and photoelectrochemical water oxidation.
4.1. Chromophore–Catalyst Assemblies for Homogeneous Water Oxidation
As a result of the synthetic complexity associated with linking a chromophore to a catalyst, there are only a limited number of examples of reported chromophore–catalyst assemblies. Here, the design of an appropriate linker is essential as these supramolecular assemblies have been shown to suffer from fast back electron transfer from the excited photosensitizer to the oxidized WOC entity .
Based on the promising results obtained with the [Ru(bda)(pic)2(OH2)] complex
4.2. Surface-Bound Chromophore–Catalyst Assemblies
Dye-sensitized photoelectrochemical cells (DSPECs) for water splitting consist of a photoanode, which is connected via an external circuit to a cathode. A characteristic feature is also the use of a membrane, which is supposed to prevent mixing of the generated gaseous products. The membrane should also be permeable to proton diffusion, in order to allow equilibration between the cell compartments. In a majority of water-splitting cells based on TiO2, an external bias of ~0.2 V is needed for efficient reduction of the produced protons to hydrogen gas and maximize water splitting [74–76].
At the core of DSPECs for water splitting, is the chromophore–catalyst assembly that is supposed to mediate water oxidation. Important characteristics of this are 1) robust anchoring to the electrode surface, which also allows electron transfer events to take place through electronic orbital coupling, 2) chromophores that display broad absorption of visible light that 3) upon excitation undergo electron injection into the conduction band of the photoanode, and 4) a stable and efficient WOC entity, having catalytic rates that exceed the rate of solar insolation. It is also essential that the oxidized chromophore is rapidly reduced by the WOC. The ruthenium-based chromophores are in general highly unstable in their oxidized state, which facilitates degradation via nucleophilic attack of water or buffer anions, and back electron transfer events can occur from the semiconductor, thus regenerating the reduced state of the chromophore resulting in low quantum yields [74–77].
The covalent attachment of chromophores and/or WOC to the high surface area of the oxide-based semiconductor is a critical feature that needs to be targeted in order to prevent detachment of the molecular entity from the oxide surface. A variety of strategies for anchoring molecular chromophores, WOCs, and chromophore–catalyst assemblies to oxide surfaces have therefore been evaluated. Reported examples include the use of acetylacetonates , alkoxides , hydroxamates , and siloxanes . However, the most widely used functional groups are carboxylic (–COOH) and phosphonic acid (–PO3H2) derivatives. Carboxylic acids are the most frequently used linking groups in dye-sensitized solar cells (DSSCs) where they provide strong electronic coupling for ultrafast electron injection from the excited state of the dye. Under nonaqueous conditions, surface detachment is not a limiting factor; however, photoanodes for water splitting operate in aqueous solutions, thus causing irreversible hydrolysis of the covalently attached molecular scaffold and limiting the lifetime of the assemblies. An attractive alternative is the use of phosphonate linkers (Figure 21), which provide more robust surface binding and are typically resistant to hydrolysis/desorption at pH > 5 with added buffer bases [82–85].
It has been suggested that the presence of molecular oxygen facilitates the photodesorption of phosphonate-linked dyes in aqueous solutions, presumably due to the formation of superoxide ions through back electron transfer from TiO2. This problem can partially be circumvented by constructing chromophores with multiple ligating phosphonate units. However, the electron injection ability of the chromophores decreases with increasing number of phosphonate groups, highlighting that additional work on designing chromophores for surface stabilization is needed .
From the aforementioned discussion, it is apparent that fast electron transfer between the WOC and the oxidized photosensitizer is vital for achieving efficient and robust water-splitting cells. Recent work has shown that the use of phosphonate linkages has limited impact on the reactivity or properties of surface-bound assemblies, thus maintaining the reactivity observed for the homogeneous system .
Meyer and coworkers have designed several chromophore–catalyst assemblies that upon attachment to oxide surfaces are able to mediate electrocatalytic water oxidation [88–90]. For the chromophore–catalyst
An attractive approach for constructing catalytic assemblies for light-driven water splitting involves the layer-by-layer addition of a chromophore followed by a catalyst overlayer. Such co-loading strategies allow for straightforward, and potentially practical, approaches for surface attachment and are considered to be general and simple alternatives for producing functioning DSPEC photoanodes [92–94].
An example of such an approach can be seen in Sun and coworkers’ photoanode where a derivative of a previously developed ruthenium-based WOC
As a consequence of the depleting energy resources and the increasing need for energy, modern society faces a daunting task of realizing sustainable, carbon-neutral alternatives. In this context, the development of solar-to-fuel conversion technologies by mimicking the natural photosynthetic apparatus is considered as an attractive solution. While significant effort has been devoted to designing artificial systems for solar energy conversion, replicating the essential functions of natural photosynthesis has proved to be intricate.
An essential component in light-harvesting devices is a catalyst that is able to oxidize water to molecular oxygen, thereby providing the necessary reducing equivalents for reduction of protons to hydrogen gas. Photo-induced charge separation and buildup of multiple redox equivalents is also an essential part of solar-to-fuel conversion schemes. The catalysts reviewed here represent the state of the art and are hence capable of mediating homogeneous light-driven water oxidation through the accumulation of multiple redox equivalents at the catalytic center. The envisioned assemblies for water splitting consist of three parts—a chromophore, a catalyst for water oxidation, and a semiconductor—and have mainly been developed separately. The current limitation of these assemblies is related to the inefficient coupling of the individual catalytic events, which is necessary for the development of efficient artificial photosynthetic systems.
Considerable progress has been made during the last decade in constructing photochemical cells capable of splitting water. However, the fundamental aspects, including their synthesis, their long-term durability, and the mechanistic understanding, are far from resolved and are of significant concern. Further elaboration and assembling of all of the integral components through cooperative interplay will certainly continue, thereby realizing efficient artificial photosynthesis in a not too distant future.
Financial support from the Swedish Research Council (637-2013-7314, 2015-04995 and 621-2013-4872), Swedish Foundation for Strategic Research, Stiftelsen Olle Engkvist Byggmästare, the Knut and Alice Wallenberg Foundation, and the Carl Trygger Foundation is gratefully acknowledged.
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