Metal complexes and inorganic materials for solar fuel production

During the last hundred years, about 80% of the world's energy consumption has been based on fossil fuels, including coal, oil and natural gas. However, humanity now faces the consequences of this dependence on fossil fuels. World energy consumption is expected to increase by more than 50% by the mid-2000s.¹ With fossil fuels nearing the end, it is very challenging to ensure that this demand can be met, not least because of possible political tension and other potential energy problems. Fortunately, our global energy market is progressing through an important historical transition. This transition is mainly driven by the search for new ways to gain access to new energy sources, geopolitical events, and a slight increase in political and financial responses to research in the field.²

In particular, increased interest in climate change has led to an understanding of the basis of global warming and serious pollution, essentially linked to the use of fossil fuels that generate large quantities of emissions as oxides such as CO₂ and SO₂.³ Recent reports by the Intergovernmental Panel on Climate Change have increasingly highlighted the need to reduce CO₂ emissions on a global scale.⁴ The combination of these arguments makes the development of sustainable energy technologies – possibly carbon-free or carbon utilizing – one of the most pressing challenges facing humanity. The most abundant renewable energy source on the planet is solar energy, which has always been the basis of the energy resources at our disposal. The use of a resource such as solar light to quickly obtain alternative energy sources is one of the most urgent needs for mankind, thanks to its extensive availability. The sunlight that strikes the Earth's surface every hour represents a quantity of energy that is greater than the world's energy consumption over a whole year.⁵ The decision to start investing in the field of direct conversion of solar energy through the use of photovoltaic panels and cells, as well as using an indirect form of solar energy through wind turbines or tides, has certainly had an effective input in letting alternative energies enter the daily energy market. Alongside these strategies, which are in fact very interconnected with financial market policies, complementary technologies require funding dedicated to basic research so that chemists can study and produce efficient, durable and low-cost materials.

Chemists, thanks to collaboration with other sciences such as physics and biology, learned to exploit photons, the primary vectors of solar energy, collecting them through chromophores capable of absorbing sunlight and converting it by hole–electron separation. This step is fundamental, for example, in the field of photovoltaic cells that are capable of converting sunlight to electrical energy that can be used in everyday life. But, in reality, electricity is not the only form of energy that can replace the energy provided by fossil fuels; it has a much lower energy-storage density compared to chemical fuels. New discoveries and a deeper understanding of the structural and physical–chemical properties of materials have lowered the costs of solar and electrical energy conversion and storage technologies, and just as chemistry has been fundamental in many of these past developments, so it will continue to be. The work carried out by chemists, with biologists and physicists, has made it possible to understand the hidden secrets of the processes that nature has always carried out to fix solar energy, and therefore to understand what components, processes and structures are necessary to transform the energy and information contained in photons into chemical energy, a more easily transportable and versatile form of energy – because it is contained in chemical bonds – than electrical energy: solar fuel. Solving the structure of the components that participate in the natural photosynthetic process has paved the way for the realization of systems able to mimic it through artificial photosynthesis. Chemists have understood how to optimize the absorption of sunlight, building antenna systems able to mimic the performance of natural systems, to convey the absorbed energy to an energy trap and from this point optimize the processes of electronic transfer. The realization of charge separation can lead to the accumulation of holes and electrons on appropriate catalysts capable of carrying out catalytic processes to fix solar energy in the bonds of new species (molecular energy vectors). In order to carry out this process in an efficient way it is necessary to consider several strategies using heterogeneous inorganic systems and molecular systems. In the last decades, hundreds of catalysts have been developed and studied for the reactions involved in artificial photosynthesis. However, it seems that devices assembled from molecular catalysts are less stable and work less efficiently than those made from inorganic materials. Nevertheless, new advances have made it clear that molecular catalysts are very promising; subsequent developments concern efficiency improvements, the heterogenization of molecular catalysts and their engineering in applicable devices. Scientists are working very hard to develop high performance catalysts for oxidation and water reduction, as well as CO₂ reduction.

This themed collection aims to provide a brief overview of the latest generation of inorganic systems (molecular systems or materials or a combination) applied to energy science. This small collection is obviously far from an exhaustive account of the latest news in the field, but we want to provide a series of milestones offered by the research of some of the greatest experts in the field.

The articles in this collection discuss most of the factors related to inorganic photocatalysis processes, such as the amount of light needed to optimize the water photooxidation process using Ir cyclometallate complexes as catalysts and [Ru(bpy)₃]²⁺ as a sensitizer in sacrificial cycles (Volpe et al., DOI: 10.1039/c9dt04841c). In this article the need for photon management as an important parameter to optimize the quantum yield of the system is highlighted. It also discusses the effect of the nature of NHC ligands and in particular the donor character of carbenes on the catalytic properties of the iridium complexes used as catalysts.

The use of a bridge ligand to connect catalytic and photosensitizer subunits is investigated using the well-known 2,3-dpp ligand (de Palo et al., DOI: 10.1039/c9dt04815d). Although the catalysts discussed in this paper perform less well than other Ru(II) water oxidation catalysts that appear in the literature, the reported results confirm the possibility to build up multinuclear photosensitizer-catalyst assemblies without affecting the activity of the catalytic subunit. Moreover, while for most of the mono-site catalysts the activity is demonstrated to depend on the initial chloride/water substitution, the results suggest that, due to the presence of the tripodal tpm ligand, H₂O coordination probably occurs after metal oxidation in these cases.

The studies that aim to assemble the performance of molecular catalysts with that of metal semiconductors led Tsubonouchi et al. (DOI: 10.1039/c9dt04442f) to use a multi-potential-step chronocoulospectrometry technique (MPSCCS) to immobilize a new mononuclear Ru(II) catalyst on a mesoporous ITO electrode. Studies on the electrocatalytic oxidation of water by MPSCCS have highlighted its versatility and importance for the detection of redox reactions and catalytic aspects of molecular catalysts immobilized on electrodes, providing a better understanding of the mechanisms in molecule-based electrocatalytic systems.

The performances of these hybrid systems have also been tested on systems obtained by doping layered double hydroxide electrodes with very low amounts of Ir(III) and Ru(III) metal centers (<2 mol%) (Fagiolari et al., DOI: 10.1039/c9dt04306c). The results confirm that if these species are immobilized on carbon paste electrodes (low-cost material), the performance obtained, in terms of water oxidation, is much higher than that of the reference IrO₂.

Modifications on inorganic materials often lead to electronic disturbances that result in completely different properties compared to the original. Cr₂O₃ is a p-type semiconductor with a negative conduction band position suitable for photo-cathodic H₂ generation. The efficiency and stability of the photocurrent generated by a Cr₂O₃ electrode are poor, but by modifying its surface with n-TiO₂, and by depositing Pt nanoparticles, a Pt/TiO₂/Cr₂O₃ electrode capable of generating H₂ through water splitting is obtained (Sekizawa et al., DOI: 10.1039/c9dt04296b).

The methods of deposition of inorganic catalysts on electrodes are numerous; this collection includes an article (Li et al., DOI: 10.1039/c9dt03983j) describing a new method of flame-assisted deposition (FAD) to generate and immobilize cobalt oxide (CoO_x) on the surface of FTO or hematite modified with TiO₂ for electrochemical or photoelectrochemical water oxidation, respectively.

Part of this collection is also a beautiful perspective (Gotico et al., DOI: 10.1039/c9dt04709c) on the use of the porphyrinic platform in this research field. In particular, the focus of this work is on metalloporphyrins and their second coordination sphere, access to which would allow the elimination of excess potential and improve catalytic rates and product selectivity. The metalloporphyrins have also attracted a lot of attention in terms of the development of hybrid materials for heterogeneous catalysis. In fact, as mentioned above, research at the interface between fundamental and applied research can provide rapid solutions to address problems related to the manufacture of hybrid materials capable of driving the formation of carbon–carbon bonds in which to store energy.

Since the conversion of small molecules requires the transfer of more than one electron and the breaking/formation of new covalent bonds, it is necessary to design and implement systems capable of operating both these processes. From this point of view, multinuclear complex systems can be promising candidates to realize such reactions, for instance a pentanuclear Co₅ system that reduces CO₂ to CO under radiation in the presence of a photosensitizer (Akai et al., DOI: 10.1039/c9dt04684d).

The efficiency of these processes, as well as the characteristics of the photosensitizer and catalyst, is a function of the chemical environment. A detailed study in this regard (Asai et al., DOI: 10.1039/c9dt04689e) comments on the effect of the nature of ionic liquids on the photophysical and photochemical properties of Ir and Re complexes, showing that ionic liquids have great potential as solvents in modulating the photocatalytic CO₂ reduction efficiency.

One of the fundamental parameters for scientists working in the field of photocatalysis is the identification of the products obtained. In particular, while gaseous products (e.g. CO, CH₄) are typically detected and quantified by well-defined gas chromatography protocols, highly reduced products are for the most part liquids and are more difficult to characterize using reliable techniques and protocols. This collection also addresses this problem, and in particular a methodology for the detection and quantification of liquid CO₂RR (CO₂ reduction reaction) products based on a simple and widely available NMR technique is reported (Chatterjee et al., DOI: 10.1039/c9dt04749b).

All of the authors, as well as the reviewers, are acknowledged for their efforts and relevant contributions. We hope that all the articles in this themed collection make it clear that recent discoveries in chemistry, and inorganic chemistry in particular, are crucial for progress in the field of solar fuels. All of these articles demonstrate not only the intensity of research in the field, but also progress in creating a bridge between fundamental chemistry (understanding the principles of how these processes operate) and applied science, using fundamental knowledge to obtain more efficient systems for extracting and converting energy.

References

1. IEA, World Energy Outlook 2019, IEA, Paris, 2019, https://www.iea.org/reports/world-energy-outlook-2019.
2. N. Armaroli and V. Balzani, Energy for a sustainable world-from the oil age to a sun powered future, Wiley, Weinheim, 2011.
3. IEA, CO₂ Emissions from Fuel Combustion 2019, IEA, Paris, 2019, https://www.iea.org/reports/co2-emissions-from-fuel-combustion-2019.
4. IPCC, Global Warming of 1.5 °C, ed. V. Masson-Delmotte, P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor and T. Waterfield, 2018.
5. REN Renewables, Global status report, 2019, https://www.ren21.net/gsr-2019/.

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