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Artificial leaves using sunlight to produce fuels
C H A P T E R
21 Artificial leaves using sunlight to produce fuels Siglinda Perathonera,*, Gabriele Centib a
Department of ChiBioFarAm, Industrial Chemistry, ERIC aisbl and INSTM/CASPE, University of Messina, Messina, Italy; bDepartment of MIFT - Industrial Chemistry, ERIC aisbl and INSTM/CASPE, University of Messina, Messina, Italy * Corresponding author. e-mail address: [email protected]
1. Introduction Artificial photosynthesis, or artificial leaves, are emerging as one of the key concepts and technologies of the general objective of realizing the next generation of clean and renewable fuels. The first attempts toward artificial photosynthesis took place decades ago (beginning 1970s), but only recently have science and technology developments and the change of boundary conditions, i.e., the general effort to convert from fossil fuels to renewable energy and alternative C sources [1e3], including CO2, given new impetus to this topic. This is attested, for example, by the fast-growing number of publications. SciFinder reported in June 2019, for the search term “artificial photosynthesis” (as entered or as concept), 80 entries in 2003, 611 in 2013, and 791 in 2018. Many reviews have also been published on this topic, a selection of some of which is presented by Refs. [4e25] and
Catalysis, Green Chemistry and Sustainable Energy https://doi.org/10.1016/B978-0-444-64337-7.00021-5
related citations, although not exhaustive of the many relevant contributions to the area. These reviews and cited works discuss important aspects of the light-harvesting mechanism, of charge separation and transfer, and in general the reaction mechanism and the nature of the active elements; but often the studies on artificial photosynthesis are limited to H2O conversion to H2 and O2 rather than considering the more challenging (but also more relevant) conversion of CO2. However, there is increasing effort being put on devices for CO2 conversion recently. These studies contribute significantly to the advance of knowledge, but many of the proposed devices cannot operate in CO2 conversion rather than in water splitting [23e25]. The production of H2 by photochemical routes is a valuable target, but for the problems of storage, distribution, safety, etc., hydrogen should be considered an intermediate rather than the final product for chemical or energy
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uses. Hydrogen economy was popular in the early 2000s, but quite limited progress was made, and today scientific and industrial attention, notwithstanding the still active lobby on hydrogen, is given to solar fuels and chemicals that can be easily stored and transported. Thus, in the production of solar fuels and chemicals using artificial leaf-type devices, it is preferable also to investigate directly the production of the final fuels or chemicals deriving, for example, from CO2 conversion, rather than the production of the intermediate H2. Furthermore, several of the studies on artificial photosynthesis or artificial leaves deal with specific molecular elements of the complex multistep process machinery, with increasing attention paid to investigating the supramolecular aspects, i.e., the connection between single elements, but still often not having the necessary system approach to address the topic correctly. Furthermore, it is necessary to turn the approach toward realizing artificial leaf-type devices [26] to account for the design of the elements of robustness and practical implementation necessary to accelerate the passage of these devices from the laboratory to application.
1.1 Principles of artificial photosynthesis and leaves Artificial photosynthesis deals with the understanding of the molecular aspects of mimicking the different steps present in natural photosynthesis to use sunlight in driving the conversion of CO2 and H2O to (typically) carbohydrates and oxygen. Molecular assemblies bioinspired from photosynthesis have been studied by various research groups, for example, Djokic and Soo [9], Wang et al. [10], Whang and Apaydin [11], and Barber [17]. These systems contain a chromophore, such as a porphyrin, which performs the first step of light harvesting. These molecules are covalently linked to one or more electron acceptors, such as fullerenes or quinones, and secondary electron donors. After
chromophore excitation, the photoinduced electron transfer generates a primary chargeseparation state. Electron transfer chains spatially separate the redox equivalent and reduce electronic coupling, slowing recombination of the charge-separated state to make possible their use in the redox catalytic processes. Fig. 21.1 summarizes schematically these elements necessary in the artificial photosynthesis process. The reaction rates of the redox catalytic step are typically two or even more orders of magnitude slower than the charge creation/separation processes. This aspect is the critical issue in artificial photosynthesis processes, because these charged species, if not quickly consumed in the redox processes, can recombine, reducing the overall efficiency, or may instead favor side reactions, including the degradation of the components of the artificial leaf device itself. For these reasons, more robust inorganic systems have to be preferred over organic complexes and supramolecular systems. These allows a better understanding, but still, over 20 years of studies have failed to produce practically applicable artificial leaf devices, e.g., systems with a high enough rate of conversion and stability. The contributions of these studies to understanding relevant aspects, such as (1) antenna effects, by using chromophores that absorb light throughout the whole visible spectrum and then transfer electrons or energy to the charge-separation component, to increase overall efficiency, and (2) advanced catalytic redox centers with high turnover number, have to be remarked. Progress has also been made in supramolecular assembling of these components and mimicking of natural photochemical systems, but still progress is limited in crucial elements like those for the control and photoprotective elements borrowed from photosynthesis. These are the key elements for the self-regeneration behavior of natural systems and thus a key factor in the long-term stability. The concept of artificial leaves is instead focused on the system architecture as the basis for the design of the devices, rather than on the
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FIGURE 21.1 Scheme of a multifunction assembly for artificial photosynthesis, with a photosensitizer (P) linked in tandem to an oxygen-evolving (water oxidation) catalyst (OEC) and a hydrogen-evolving catalyst (HEC) or a carbon dioxideereduction electrocatalyst (CEC). Electrons flow from the OEC to the HEC/CEC when catalysis occurs, due to the potential difference. A membrane separates the photoanodic from the electrocatalytic cathodic zone. Credit: G. Centi, S. Perathoner S, Artificial leaves, in Kirk-Othmer Encyclopedia, Wiley & Son, Hoboken, NJ, 2013. Copyright: Wiley & Sons, 2013.
development of the single elements and their subsequent assembling into a functional device. The different perspective implies a higher consideration of the system functionality, rather than that of the single elements, but in artificial leaf devices there is a mutual influence between these elements determining the overall behavior. The system performance is not derived by simply putting in sequence the single components. In addition, a system approach emphasizes aspects such as charge transport and system reliability, which are instead less investigated in an artificial photosynthesis approach. These aspects are amplified in importance in moving from water splitting to the more challenging conversion of CO2. On the other hand, the complexity of the problems to be addressed indicates the need to simplify the problem by analyzing single aspects, while having a clear picture of the full device, and especially of the constraints related to its applicability, in terms of materials, cost effectiveness, possibility of recovering the products, etc. For this reason, we prefer to speak about artificial photosynthetic leaves (APLs), i.e., the need to combine the two types of approaches.
bioinspired approach, which has been typically highlighted in most of the cited reviews. Nature has taken a very long time to develop and optimize the highly complex machinery present in leaves that enables the use of sunlight to oxidize water and produce the electrons/protons used in a different part of the cell to reduce CO2. The products of carbon dioxide reduction, however, are the elements necessary for plant life and the synthesis of all plant components from carbohydrate to lipid and other organic species. In APLs the aim is instead to produce fuels or chemicals, which often are toxic to natural leaves. In addition, plants are characterized by a series of mechanisms that allow the plant to limit the rate of the processes (for example, light absorption), which otherwise would damage the plant itself. In artificial leaves it is instead necessary to intensify the processes, at least by one or more orders of magnitude, to improve the cost effectiveness. The development of efficient, cost-effective artificial systems requires a new functional and robust design to realize two goals: -
-
1.2 Biomimicking approach to developing artificial leaves
intensify the process, thus allowing a higher productivity and efficiency in converting sunlight; use solid components that keep functionalities, but are more robust, scalable, and cost effective. This concept is exemplified in Fig. 21.2.
Another introductory comment regards the necessity or not to have a biomimicking or
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H
NADPH
CO
((CH O) Dark reac ons
Light reac ons PQH
Energy
Light
ADP Light
ATP P700
HO P680
FIGURE 21.2 Going from natural to artificial leaves requires using a radically different system design taking inspiration from nature, but developing conceptually new and robust devices that overcome the limit of natural leaves. “2H2” indicates the four protons and electrons produced in the water oxidation step (they are equivalent to two hydrogen molecules), while (CH2O) indicates in general the products of CO2 hydrogenation, which include C1 and >C1 hydrocarbons, alcohols, and other organic species. PSI and PSII, photosystems I and II.
2. Components of an artificial leaf The previous section has outlined the functionalities necessary in an artificial photosynthesis scheme, while we discuss here the components necessary for the design of an efficient, cost-effective artificial device. The components of the leaf-type hierarchical structure should have the same functionalities of the natural ones, e.g., capture sunlight photons, realize long-living electron-hole separation and energy transduction into the final product, etc., but at the same time a new (more robust) system is necessary. The APL devices are currently typically based on a photoelectrochemical (PEC) configuration, which comprises the following main elements: (1) an anode, (2) a cathode, and (3) a membrane separating the two compartmental zones. An electrolyte is used to close the circuit. Although variations exist even around this apparently simple scheme, the most typical is that the anode
acts as a light-harvesting element, to realize the charge separation (holes and electrons) and the reaction of holes with water to generate protons (and evolve gaseous O2). The electrons and protons are transported to the cathodic compartment zone to be used to reduce CO2. Note that in both anodic and cathodic zones, the crucial factor is the catalytic reaction of water oxidation by holes and CO2 (selective) reduction by protons and electrons. For this reason, often the term photoelectrocatalysis is used rather than photoelectrochemistry.
2.1 Anode The anode is the part exposed to sunlight and containing a photocatalyst able to oxidize water, but typically supported on a conductive substrate to allow the fast collection of the photogenerated electrons and limit their recombination with holes. There are different possible configurations in PEC devices, as discussed later, but
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2. Components of an artificial leaf
(A)
FIGURE 21.3
419
(B)
(A) Conventional design of photoelectrochemical solar cells with respect to (B) the compact design. GDL, gas-
diffusion layer.
for practical implementation, it is necessary to have compact devices, minimizing (or better avoiding, as commented on later) the electrolyte. In this compact configuration, the membrane necessary to separate the anodic from the cathodic zone should be thus the interface between them. This concept is presented in Fig. 21.3B, in comparison with the conventional (Fig. 21.3A), and used in most of the cases, PEC configuration. The compact design, analogous to that used in the PEM (proton-exchange membrane) fuel cell, shows various advantages with respect to the conventional one, particularly for the industrial development of the cell, as discussed in more detail later. It must be noted, however, that a compact design is not a simple engineering of the conventional cell, which may thus take full advantage of the materials developed in the latter, which use is easier by largely avoiding the issues related to proton transfer limitations and due to the interface between the electrodes and the membrane. In fact, in the compact design, the critical factor for the development of the anode is the need of an efficient collection of electrons and at the same time fast transport of the photogenerated protons through the anode itself and
then at the interface with the membrane. At the cathode side, the protons must then again cross the interface and reach the (electro)catalytic sites to reduce CO2. Thus, the behavior of the anode is largely influenced by how efficient the transport of the electrons and protons to the cathode side (through external/internal electron conduction and a proton-permeable membrane) is. Thus, the anode should have a specific nanostructure and characteristics to make these processes efficient and avoid limitation in the overall rate of the process. The anode materials selected in PEC devices with the conventional design may thus not be suitable for the compact design. The anode should have three main functionalities, in addition to characteristics suitable for fast transport of the photogenerated electrons and protons: -
-
an efficient light-harvesting capability; generation of charge separation, e.g., electrons and holes, with simultaneous inhibition of the charge recombination processes, e.g., long life for the photogenerated charges; the presence of catalytic active centers able to provide fast oxidation of water (through the
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photogenerated holes) to generate gaseous oxygen and protons.
for the operation of an electrocatalyst for water oxidation.
There are other characteristics necessary, like stability in the presence of strong oxidants, because the oxidation of water may lead to the formation of hydrogen peroxide and hydroxyl radicals. In particular the latter are among the stronger oxidants known, and thus organic molecules may easily degrade, particularly under full load and extended operations as required for application. It should be also commented that under full sunlight operations, the PEC solar cells may reach temperatures easily above 50e60 C. This is another reason the use of a liquid electrolyte under practical operations creates severe issues, including the enhanced rate of photooxidation and photocorrosion. Thus, the anode must contain quite robust elements, an aspect often not considered with sufficient attention in the development of the electrodes of PEC devices. There are two possible options to realize light harvesting and charge separation:
In the second case, although used often in the literature, the compact design may be quite difficult to realize, because the PV component does not have the necessary proton transport characteristics. Although the use of a PV component allows one to obtain higher efficiency in light harvesting and charge separation, the two main issues are related to (1) how to add the catalytic elements for water oxidation in a stable form and (2) especially how to realize stable systems with respect to photocorrosion. The concept of integrating a PV element to realize a PEC device was presented by Nocera [27] in 2011, receiving large attention, but even if many research groups have further explored this concept, the problems of stability are still largely unsolved, even by adding various protective layers. In addition, this type of cell is not effective in CO2 PEC reduction, other than in water splitting, as designed. Fig. 21.4 shows the simplified scheme of this type of solar fuel cell. A solar PV unit provides the photocurrent (electron and holes), which is used on the two sides by suitable catalysts: on the anode side to oxidize water using an OEC (oxygen-evolving catalyst) and on the cathodic side by a hydrogen-evolving catalyst to form H2. Charge separation and cell design may provide wireless electron transport, but an external
-
-
A semiconductor thin film is used, having the nanostructure suitable for efficient light harvesting and proton transport and supported over a conductive substrate for efficient electron collection and transport to the other side of the cell. A conventional photovoltaic (PV) cell is used, which generates the electrons and potential
FIGURE 21.4 Simplified conceptual scheme of an artificial leaf (solar fuel cell). HEC, hydrogen-evolving catalyst; OEC, oxygen-evolving catalyst; PV, photovoltaic.
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2. Components of an artificial leaf
wire is necessary for more efficient transport. The protons are transported through the electrolyte. In this type of cell, there is no generation of H2 and O2 in physically separated compartments, a requirement for safety and to avoid the costs of downstream separation. When a membrane is inserted to obtain this separation, performance worsens. The crucial element is the OEC component. Nocera et al. [27] used a Co-phosphate (CoPi) OEC they had developed earlier [28]. Fig. 21.5A shows schematically this Nocera cell design, which is largely similar to that developed a decade before by Turner [29] and schematically shown in Fig. 21.5B. In the Nocera cell, there is a single-junction Si nppþ cell (where nnpþ indicates a single photovoltaic cell with n, p indicates the n- and p-type semiconductor layer and pþ the doped p-type semiconductor layer), with the pþ side created by depositing a 1-mm film of silicon-doped (1%) Al and then thermally annealing. The space charge region in the pþ layer is thin enough to act as a tunneling layer. On the n side of the PV element, a metal front contact is deposited, protected from the solution by a 10-mm layer of
(A) photoresist
421
photoresist. The Si is protected from oxidation by sputtering on the pþ side of the junction a 50-nm film of ITO (indium tin oxide), which serves as an ohmic contact for hole transport from the buried Si junction. The Co-OEC is electrodeposited on the ITO barrier layer. This single-junction cell generates 0.57 V, and thus an additional nppþ Si solar cell should be connected in series to generate the potential for water splitting. To avoid an externally applied potential, it is necessary to use a triple-junction (3jn) PV cell. The 3jn-a-Si produces 8 mA/cm2 of current at 1.8 V at an overall efficiency of 6.2%. The alternative approach is to use a semiconductor, with the presence of a cocatalyst able to oxidize water. This is a more robust approach allowing one to also realize (in principle) more productive anodes and with the characteristics suitable for a compact design, as illustrated in Fig. 21.3B. TiO2 is perhaps the best known semiconductor, having the great advantage of very robust characteristics and a large set of knowledge on how to control its nanostructure in thin photoactive films [30e32]. By anodic oxidation of Ti foils, a photoanode characterized by an array of vertically aligned
ePV cell
(B)
ITO
OEC
metal front contacts
e- h+ n-Si n
2H2O + 4h+
2H2
O2 + 4H+
4H+
p+-Si
p-Si
HEC
single junc on
FIGURE 21.5
(A) Simplified scheme of the Nocera photoelectrochemical (PEC) cell obtained by depositing an oxygenevolving catalyst (OEC) on a single amorphous Si photovoltaic (PV) cell modified to allow protection of the PV cell by a transparent indium tin oxide (ITO) film (on which the OEC is then electrodeposited) and by a photoresist layer (after depositing metal front contacts) on the other side. (B) Schematic of the monolithic PEC/PV device. HEC, hydrogen-evolving catalyst. Adapted from O. Khaselev, J.A. Turner, A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting, Science 280 (1998) 425e427.
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TiO2 nanotubes can be produced. This type of nanostructured photoanode: -
-
has excellent properties of light harvesting; shows reasonably good electron transport and fast collection, which may be improved by doping; permits a good Hþ diffusion to the underlying proton-conduction membrane.
Introduction of CuO as cocatalyst allows one to make the systems active also in the visible light region, in addition to promoting solar-tochemical conversion efficiency [33]. A solar-tohydrogen efficiency of over 2% in the full PEC cell is observed under the best conditions. IPCE (incident photon-to-current conversion efficiency) measurements clearly evidence that the presence of CuO nanoparticles induces an enhanced IPCE in the 300e340 nm region. The increase in the performance of water splitting is mainly associated with the transient generation of a pen junction between the CuxO nanoparticles and TiO2 nanotubes upon illumination, which enhances photocurrent density by promoting charge separation. The design of this
compact-type cell, based only on earthabundant components, is presented in Fig. 21.6. An alternative approach to the PEC configuration discussed before is to realize a two-photon process, with thus light absorption at both the anode and the cathode sides. This two-step process, schematically presented in Fig. 21.7A, is called a Z scheme and is analogous to that utilized in natural photosynthesis. Fig. 21.7B shows how this concept can be implemented in principle in a compact PEC device, but still requires an electrolyte containing the redox mediator to close the electronic cycle, In addition, to realize a transparent membrane as indicated in Fig. 21.7B is a significant challenge. On the other hand, realizing, in practical devices, solar illumination on the two sides of the cell is also technically challenging and difficult to implement.
2.2 Membrane The compact-type PEC design as presented earlier requires a membrane enabling the fast transport of protons, while electrons have to be transported along a different path, because
FIGURE 21.6
Simplified scheme of compact-type cell, based only on earth-abundant components, used in water splitting. The photoanode is based on an ordered array of TiO2 nanotubes (TNTs), prepared by anodic oxidation of a Ti sheet and modified by electrodepositing CuO nanoparticles. In the inset, a transmission electron image of the junction between the CuxO nanoparticles and the TNTs is shown. The cathode is composed of a gas-diffusion layer (GDL) electrocatalyst put in contact with the Nafion membrane. Credit: Graphical abstract in J.F. de Brito, F. Tavella, C. Genovese, C. Ampelli, M.V.B. Zanoni, G. Centi, S. Perathoner, Role of CuO in the modification of the photocatalytic water splitting behavior of TiO2 nanotube thin films, Applied Catalysis B: Environmental 224 (2018) 136e145. Copyright: Wiley & Sons, 2018.
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2. Components of an artificial leaf
(A)
(B)
photoac ve HERC/CEC
H+
Transparent Membrane
photoac ve OEC
(bias)
porous conduc ve layer FIGURE 21.7 (A) Photoelectrochemical (PEC) tandem cell for the Z-scheme approach. (B) PEC artificial leaf design with double photoactive layer. CEC, carbon dioxide reduction electrocatalyst; HERC, hydrogen-evolving reduction catalyst; OEC, oxygen-evolving catalyst. Credit: Adapted from G. Centi, S. Perathoner S, Artificial leaves, in Kirk-Othmer Encyclopedia, Wiley & Son, Hoboken, NJ, 2013. Copyright: Wiley & Sons, 2013.
electron and proton transport should occur along different paths to minimize their recombination. The membrane has the critical function of separating physically the anodic and cathodic processes, thus avoiding the recombination of the oxidation and reduction products (eliminating crossover is thus a crucial aspect for efficiency) and producing in separate fluxes the oxidation and reduction products (O2 and H2 in water splitting). This is a key function, for safety (H2 and O2 mixtures, for example, are explosive in a wide range of compositions) and cost (downstream separation of the products of reaction). Thus, while a lot of research has been done, even on devices without formation of oxidation and reduction products in separate zones, this is an intrinsic necessary aspect to consider at the initial stage of development. Considering
the PEC device downstream operations, in particular aspects such as how to recover and separate the reaction products, is a key element to include at the stage of initial screening, because otherwise the results may be not relevant [26]. The membrane should be tailored to provide the minimum transfer resistance to protons and thus have a minimum thickness (in principle some micrometers), but also at the same time to hamper oxygen permeation to preserve CO2 reduction activity at the cathode. The membrane should also be in good contact with the anode and cathode sites and it is necessary to minimize the resistance at the interface. In the PEC scheme discussed before, the photoactive side is the anode, and thus water oxidation occurs on an n-type semiconductor.
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21. Artificial leaves using sunlight to produce fuels
(A)
n-type semiconductor electrode e-
(B)
e- (bias)
e- (bias)
H2
CB
e-
CB
H2O h O2 H2O
H+
h
OH-
O2 p-type H2O or VB semiconductor VB electrode h+ n-type h+ p-type OH- -membrane OEC H+ -membrane electrocatalyst on conduc ve support H2
FIGURE 21.8 Photoelectrochemical solar cell with (A) an n-type oxide semiconductor and (B) a p-type semiconductor electrode as the photoactive element. CB, conduction band; OEC, oxygen-evolving catalyst; VB, valence band. Credit: C. Ampelli, C. Genovese, G. Centi, R. Passalacqua, S. Perathoner, Nanoscale engineering in the development of photoelectrocatalytic cells for producing solar fuels, Topics in Catalysis 59 (2016) 757e771. Copyright: Springer, 2016.
Protons then cross the membrane to react with CO2 on the cathodic side. A different scheme, however, is also possible, in this case with a p-type semiconductor where the CO2 is directly photoreduced at the light-illuminated side. An OH-transporting membrane is necessary in this case, as schematically presented in Fig. 21.8. Because of the difficulties in having both OHtype membranes and p-type semiconductors, the PEC scheme shown in Fig. 21.8B is essentially not studied in the literature, although attractive.
2.3 Cathode A cathode should contain a conductive substrate (in contact with the membrane and permeable to protons) on which the active centers for CO2 adsorption and electrocatalytic conversion in the presence of protons are present (Fig. 21.9). The main issue is to realize a selective conversion, because the potentials of reduction of CO2 to different types of products are quite
similar, as shown in Table 21.1 [34]. In addition, the side reaction of recombination of protons with electrons is also present, and thus, obtaining high Faradaic selectivities and control of the selectivity in CO2 electrocatalytic reduction to the target product is a high challenge, requiring an electrocatalytic rather than electrochemical approach. It is also evident that to avoid the formation of high-energy intermediate products a multielectron/multiproton simultaneous transfer is necessary, an aspect common to other electrocatalytic reactions [35]. One-electron reduction of carbon dioxide to •CO 2 anion radical is an easy reaction, but the high potential needed to form this product indicates the need to realize instead a one-step (direct) rather than a sequential electron/proton addition, e.g., a multielectron transfer to form the intermediate to the product of possible interest. As an example, the CO2 reduction to methanol involves 6e and to isopropanol 18e. Even if difficult, and productivity is low,
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2. Components of an artificial leaf
FIGURE 21.9 Photoelectrochemical solar cell (compact design) with schemes of the photocatalytic reactions at the anode (left) and those of electrocatalytic CO2 reduction at the cathode (right). (CH2O) indicates the products of hydrogenation of CO2. TABLE 21.1
Electrode potentials for half-reactions of electrochemical CO2 reduction.
Possible half-reactions of electrochemical CO2 reduction e
Electrode potentials (V vs. SHE) at pH 7
e
CO2 (g) þ e / *COO þ
1.90
e
CO2 (g) þ 2H þ 2e / HCOOH (l) e
0.61
e
e
CO2 (g) þ H2O (l) þ 2e / HCOO (aq) þ OH þ
0.43
e
CO2 (g) þ 2H þ 2e / CO (g) þ H2O (l) e
0.53 e
CO2 (g) þ H2O (l) þ 2e / CO (g) þ 2OH þ
0.52
e
CO2 (g) þ 4H þ 2e / HCHO (l) þ H2O (l) e
0.48 e
CO2 (g) þ 3H2O (l) þ 4e / HCHO (l) þ4OH þ
0.89
e
CO2 (g) þ 6H (l) þ 6e / CH3OH (l) þ H2O (l) e
0.38
e
CO2 (g) þ 5 H2O (l) þ 6e / CH3OH (l) þ 6OH þ
0.81
e
CO2 (g) þ 8H þ 8e / CH4 (g) þ 2H2O (l) e
0.24 e
CO2 (g) þ 6H2O (l) þ 8e / CH4 (g) þ 8OH þ
0.25
e
2CO2 (g) þ 12H þ 12e / C2H4 (g) þ4H2O (l) e
0.06 e
2CO2 (g) þ 8H2O (l) þ 12e / C2H4 (g) þ 12OH þ
0.34
e
2CO2 (g) þ 12H þ 12e / CH3CH2OH (l) þ3H2O (l) e
0.08 e
2CO2 (g) þ 9H2O (l) þ 12e / CH3CH2OH (l) þ 12OH (l)
0.33
SHE, standard hydrogen electrode. Credit: W. Zhang, Y. Hu, L. Ma, G. Zhu, Y. Wang, X. Xue, R. Chen, S. Yang, Z. Jin, Progress and perspective of electrocatalytic CO2 reduction for renewable carbonaceous fuels and chemicals. Advancement of Science 5 (2017) 1700275. Copyright: Wiley, 2018.
even this highly challenging CO2 to isopropanol reaction can be realized with selectivity [36,37], thus indicating how the catalyst itself plays a very crucial role in determining the selective path, which may be virtually impossible based only on electrochemical considerations.
Due to competition with the solvent and the presence of the double layer, CO2 electroreduction in water or organic solvents mainly leads to products such as formic and oxalic acids, with methane and traces of C2 hydrocarbons at high potential [38]. These conditions lead, however, to fast deactivation [39]. However, if
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operating in the gas phase and with metal nanoparticles supported over carbon nanotubes, the possibility of also forming >C2 products from CO2 electroreduction, including isopropanol, has been shown [37]. Note that the term “gas phase” indicates operations without the presence of a liquid electrolyte as in conventional cells (Fig. 21.3A), with the membrane itself acting as the electrolyte. To avoid dehydration of the membrane, the GDL type of electrode (similar to those of PEM fuel cells) has to be used. Under these “gas-phase” operations, the behavior is different from that of the same electrodes operating in the presence of the liquid electrolyte, e.g., in conventional cells [40,41]. This is due to the higher surface coverage by chemisorbed CO2 over the electrode. By enhancing this effect, for example, by introducing a component able to enhance the adsorption of CO2 in the GDL, it is possible to increase the formation of C2 products in the electrocatalytic reduction of CO2 even on electrocatalysts that are considered unable to form a CeC bond during the reduction of CO2 [42]. This result introduces a change in perspective about the catalytic electrochemistry of CO2 conversion. While there are many claims of the unique properties of copper, in particular of specific crystalline phases like Cu(100), in forming C2 products [43e45], the cited result highlights that the surface concentration of CO2 at the electrode surface determines the pathway of transformation. This depends on the characteristics of the electrocatalyst, but especially on the specific conditions of operations. This influences also the reaction mechanism. While the formation of surface CO dimers is considered the key step in forming CeC bonds during the electrocatalytic reduction of CO2 [44e46], experimental studies on the mechanism of CO2 electrocatalytic reduction to acetate (thus a C2 product) prove the presence of a different reaction mechanism [47] involving the need for having dissolved CO2 to form acetic acid, likely via the reaction of the •CO anion radical with surface2
adsorbed eCH3-like species. The pathway toward formic acid is instead different from the route of the formation of acetic acid [47]. The cell concept presented in Fig. 21.6 of a compact-type cell may be extended by developing copper-based electrocatalysts for the reduction of CO2. By preparing Cu2O/GDL electrodes by electrodeposition, and coupling with the CuO/NtTiO2 photoanode presented in Fig. 21.6, a compact-design full PEC cell can be realized [48] using only noncritical raw materials and low-cost, easily scalable procedures. The scheme of this PEC solar cell for water oxidation on the photoanode side and CO2 reduction on the cathode side is shown in Fig. 21.10. Note that it operates without external bias and use of sacrificial agents. In the PEC experiments without external bias and use of sacrificial agents and 24-h operation, the total carbon Faradaic selectivity was about 90%, with about 75% Faradaic selectivity to acetate, the other product being formate. A stable current density of 0.2 mA was observed for 24-h operation. There is an in situ transformation of the Cu2O/GDL electrode leading to the formation of a hybrid Cu2OeCu/GDL system. The same electrode gives, in electrocatalytic tests of CO2 reduction at 1.5 V, high total Faradaic selectivity (>95%), but forming selectively formate (about 80% selectivity), rather than acetate. The difference between the electrocatalytic tests and those in the PEC cell derives from the different potential applied to the cathode, being 1.5 V in the electrocatalytic tests and lower, determined from the photoanode, in the PEC tests. This result also highlights how the performance of the same electrodes may change when tested separately or integrated within a full PEC device. Thus, there are still several question marks on the reaction mechanism of CO2 electrocatalytic reduction, but a general observation is that the reaction paths are clearly highly dependent on the surface population of species adsorbed on the electrocatalyst in operation, which in turn highly depends on the specific conditions of the
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FIGURE 21.10 Scheme of a photoelectrochemical full cell reactor with compact design used for water oxidation on the photoanode side and CO2 reduction on the cathode side without external bias and use of sacrificial agents. The electrodes are based on earth-abundant materials. Credit: Graphical abstract in J. Ferreira de Brito, C. Genovese, F. Tavella, C. Ampelli, M. V. Boldrin Zanoni, G. Centi, S. Perathoner, CO2 reduction of hybrid Cu2OeCu/gas diffusion layer electrodes and their integration in a Cu based photoelectrocatalytic cell, ChemSusChem 12 (2019) 4274e4284. https://doi.org/10.1002/cssc.201901352. Copyright: Wiley VCH, 2019.
electrocatalytic operations. Without considering these aspects, many of the current mechanistic studies, including by theoretical approach, fail in understanding the effective possibilities in controlling the electrocatalytic chemistry. At the same time, it is necessary to consider these aspects in the design and operations of PEC solar cells, and conventional types of cells may be not able to take advantage of these aspects, resulting in the evaluation of materials under incorrect conditions in terms of both applicability, as remarked above and discussed in a more detail elsewhere [26], and exploiting the full possibilities given by the surface electrocatalytic chemistry discussed above.
3. Conclusions and future trends There are many scientific and technological challenges in developing artificial leaf-type devices: -
The full sunlight spectrum has to be used to realize devices that should be competitive with respect to the simpler approach of coupling a PV unit with an electrocatalytic unit. In this case, the overall efficiency is already over 10% for water splitting, but still significantly lower for CO2 conversion. In principle, the integrated PEC system offers potentially lower costs and better efficiencies, but current PEC devices are still largely less efficient.
IV. Selected examples and case history
428 -
-
-
21. Artificial leaves using sunlight to produce fuels
Robust devices with long operational times must be realized. Robustness under practical operations, which include temperatures up to about 100 C, is a crucial aspect, but still largely missing from many of the artificial photosynthesis or leaf devices and related components proposed. Except in terms of understanding, it is not possible to dedicate a large effort to materials that cannot have the necessary robustness for practical operations [26]. Product recovery and artificial leaf downstream operations should be part of the design. The products of oxidation (like O2) and reduction (H2 or those deriving from CO2 conversion) should be produced in separate streams. If products, even in part, remain in the electrolyte, recovery can be expensive in terms of energy and cost. Even if they are produced in minor amounts, they accumulate in the electrolyte and must be separated. Avoiding the presence of the electrolyte, in a compact design as shown in Fig. 21.3B (indicated as electrolyte-less operations [26,40,41]), could greatly facilitate continuous operation and easy recovery of the products of reaction. Note also that if the O2 produced cannot be utilized, e.g., the most common situation in the practical use of PEC solar devices, then the efficiency of using solar light is about halved. Thus, alternative PEC device approaches using both anodic and cathodic reactions to produce valuable chemicals have to be developed. This opens new perspectives in PEC device applications. Noncritical raw and cheap materials should be used, but also in conjunction with methodologies of preparation that can be scaled up to produce devices on a large scale. There are several progressing in the area, including the preparation of electrodes by advanced printing methods.
Thus, while progress has been made in developing the components for artificial
photosynthesis or leaf devices, still a large effort is necessary to realize systems and components that are both cost efficient and robust and meet the criteria discussed above. As remarked elsewhere, a “turning perspective in photoelectrocatalytic cells for solar fuels” [26] is necessary to accelerate the progress in this area.
List of abbreviations and acronyms APL CEC GDL HEC IPCE ITO OEC PEC PEM PV SHE STH TNT
artificial photosynthetic leaf carbon dioxide reduction electrocatalyst gas-diffusion layer hydrogen-evolving catalyst incident photon-to-current conversion efficiency indium tin oxide oxygen-evolving (water oxidation) catalyst photoelectrochemical proton-exchange membrane photovoltaic standard hydrogen electrode solar-to-hydrogen TiO2 nanotubes
Acknowledgments This work has been realized in the frame of the PRIN2017 projects 2017WR2LRS “CO2 as only source of carbons for monomers and polymers: a step forwards circular economy” and 20179337R7 “Multielectron transfer for the conversion of small molecules: an enabling technology for the chemical use of renewable energy,” which are gratefully acknowledged. The FET ProActive A-LEAF “An Artificial Leaf: a photo-electrocatalytic cell from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels” project (grant agreement ID: 732840) is also gratefully acknowledged.
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IV. Selected examples and case history
21 Artificial leaves using sunlight to produce fuels Siglinda Perathonera,*, Gabriele Centib a
Department of ChiBioFarAm, Industrial Chemistry, ERIC aisbl and INSTM/CASPE, University of Messina, Messina, Italy; bDepartment of MIFT - Industrial Chemistry, ERIC aisbl and INSTM/CASPE, University of Messina, Messina, Italy * Corresponding author. e-mail address: [email protected]
1. Introduction Artificial photosynthesis, or artificial leaves, are emerging as one of the key concepts and technologies of the general objective of realizing the next generation of clean and renewable fuels. The first attempts toward artificial photosynthesis took place decades ago (beginning 1970s), but only recently have science and technology developments and the change of boundary conditions, i.e., the general effort to convert from fossil fuels to renewable energy and alternative C sources [1e3], including CO2, given new impetus to this topic. This is attested, for example, by the fast-growing number of publications. SciFinder reported in June 2019, for the search term “artificial photosynthesis” (as entered or as concept), 80 entries in 2003, 611 in 2013, and 791 in 2018. Many reviews have also been published on this topic, a selection of some of which is presented by Refs. [4e25] and
Catalysis, Green Chemistry and Sustainable Energy https://doi.org/10.1016/B978-0-444-64337-7.00021-5
related citations, although not exhaustive of the many relevant contributions to the area. These reviews and cited works discuss important aspects of the light-harvesting mechanism, of charge separation and transfer, and in general the reaction mechanism and the nature of the active elements; but often the studies on artificial photosynthesis are limited to H2O conversion to H2 and O2 rather than considering the more challenging (but also more relevant) conversion of CO2. However, there is increasing effort being put on devices for CO2 conversion recently. These studies contribute significantly to the advance of knowledge, but many of the proposed devices cannot operate in CO2 conversion rather than in water splitting [23e25]. The production of H2 by photochemical routes is a valuable target, but for the problems of storage, distribution, safety, etc., hydrogen should be considered an intermediate rather than the final product for chemical or energy
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uses. Hydrogen economy was popular in the early 2000s, but quite limited progress was made, and today scientific and industrial attention, notwithstanding the still active lobby on hydrogen, is given to solar fuels and chemicals that can be easily stored and transported. Thus, in the production of solar fuels and chemicals using artificial leaf-type devices, it is preferable also to investigate directly the production of the final fuels or chemicals deriving, for example, from CO2 conversion, rather than the production of the intermediate H2. Furthermore, several of the studies on artificial photosynthesis or artificial leaves deal with specific molecular elements of the complex multistep process machinery, with increasing attention paid to investigating the supramolecular aspects, i.e., the connection between single elements, but still often not having the necessary system approach to address the topic correctly. Furthermore, it is necessary to turn the approach toward realizing artificial leaf-type devices [26] to account for the design of the elements of robustness and practical implementation necessary to accelerate the passage of these devices from the laboratory to application.
1.1 Principles of artificial photosynthesis and leaves Artificial photosynthesis deals with the understanding of the molecular aspects of mimicking the different steps present in natural photosynthesis to use sunlight in driving the conversion of CO2 and H2O to (typically) carbohydrates and oxygen. Molecular assemblies bioinspired from photosynthesis have been studied by various research groups, for example, Djokic and Soo [9], Wang et al. [10], Whang and Apaydin [11], and Barber [17]. These systems contain a chromophore, such as a porphyrin, which performs the first step of light harvesting. These molecules are covalently linked to one or more electron acceptors, such as fullerenes or quinones, and secondary electron donors. After
chromophore excitation, the photoinduced electron transfer generates a primary chargeseparation state. Electron transfer chains spatially separate the redox equivalent and reduce electronic coupling, slowing recombination of the charge-separated state to make possible their use in the redox catalytic processes. Fig. 21.1 summarizes schematically these elements necessary in the artificial photosynthesis process. The reaction rates of the redox catalytic step are typically two or even more orders of magnitude slower than the charge creation/separation processes. This aspect is the critical issue in artificial photosynthesis processes, because these charged species, if not quickly consumed in the redox processes, can recombine, reducing the overall efficiency, or may instead favor side reactions, including the degradation of the components of the artificial leaf device itself. For these reasons, more robust inorganic systems have to be preferred over organic complexes and supramolecular systems. These allows a better understanding, but still, over 20 years of studies have failed to produce practically applicable artificial leaf devices, e.g., systems with a high enough rate of conversion and stability. The contributions of these studies to understanding relevant aspects, such as (1) antenna effects, by using chromophores that absorb light throughout the whole visible spectrum and then transfer electrons or energy to the charge-separation component, to increase overall efficiency, and (2) advanced catalytic redox centers with high turnover number, have to be remarked. Progress has also been made in supramolecular assembling of these components and mimicking of natural photochemical systems, but still progress is limited in crucial elements like those for the control and photoprotective elements borrowed from photosynthesis. These are the key elements for the self-regeneration behavior of natural systems and thus a key factor in the long-term stability. The concept of artificial leaves is instead focused on the system architecture as the basis for the design of the devices, rather than on the
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FIGURE 21.1 Scheme of a multifunction assembly for artificial photosynthesis, with a photosensitizer (P) linked in tandem to an oxygen-evolving (water oxidation) catalyst (OEC) and a hydrogen-evolving catalyst (HEC) or a carbon dioxideereduction electrocatalyst (CEC). Electrons flow from the OEC to the HEC/CEC when catalysis occurs, due to the potential difference. A membrane separates the photoanodic from the electrocatalytic cathodic zone. Credit: G. Centi, S. Perathoner S, Artificial leaves, in Kirk-Othmer Encyclopedia, Wiley & Son, Hoboken, NJ, 2013. Copyright: Wiley & Sons, 2013.
development of the single elements and their subsequent assembling into a functional device. The different perspective implies a higher consideration of the system functionality, rather than that of the single elements, but in artificial leaf devices there is a mutual influence between these elements determining the overall behavior. The system performance is not derived by simply putting in sequence the single components. In addition, a system approach emphasizes aspects such as charge transport and system reliability, which are instead less investigated in an artificial photosynthesis approach. These aspects are amplified in importance in moving from water splitting to the more challenging conversion of CO2. On the other hand, the complexity of the problems to be addressed indicates the need to simplify the problem by analyzing single aspects, while having a clear picture of the full device, and especially of the constraints related to its applicability, in terms of materials, cost effectiveness, possibility of recovering the products, etc. For this reason, we prefer to speak about artificial photosynthetic leaves (APLs), i.e., the need to combine the two types of approaches.
bioinspired approach, which has been typically highlighted in most of the cited reviews. Nature has taken a very long time to develop and optimize the highly complex machinery present in leaves that enables the use of sunlight to oxidize water and produce the electrons/protons used in a different part of the cell to reduce CO2. The products of carbon dioxide reduction, however, are the elements necessary for plant life and the synthesis of all plant components from carbohydrate to lipid and other organic species. In APLs the aim is instead to produce fuels or chemicals, which often are toxic to natural leaves. In addition, plants are characterized by a series of mechanisms that allow the plant to limit the rate of the processes (for example, light absorption), which otherwise would damage the plant itself. In artificial leaves it is instead necessary to intensify the processes, at least by one or more orders of magnitude, to improve the cost effectiveness. The development of efficient, cost-effective artificial systems requires a new functional and robust design to realize two goals: -
-
1.2 Biomimicking approach to developing artificial leaves
intensify the process, thus allowing a higher productivity and efficiency in converting sunlight; use solid components that keep functionalities, but are more robust, scalable, and cost effective. This concept is exemplified in Fig. 21.2.
Another introductory comment regards the necessity or not to have a biomimicking or
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H
NADPH
CO
((CH O) Dark reac ons
Light reac ons PQH
Energy
Light
ADP Light
ATP P700
HO P680
FIGURE 21.2 Going from natural to artificial leaves requires using a radically different system design taking inspiration from nature, but developing conceptually new and robust devices that overcome the limit of natural leaves. “2H2” indicates the four protons and electrons produced in the water oxidation step (they are equivalent to two hydrogen molecules), while (CH2O) indicates in general the products of CO2 hydrogenation, which include C1 and >C1 hydrocarbons, alcohols, and other organic species. PSI and PSII, photosystems I and II.
2. Components of an artificial leaf The previous section has outlined the functionalities necessary in an artificial photosynthesis scheme, while we discuss here the components necessary for the design of an efficient, cost-effective artificial device. The components of the leaf-type hierarchical structure should have the same functionalities of the natural ones, e.g., capture sunlight photons, realize long-living electron-hole separation and energy transduction into the final product, etc., but at the same time a new (more robust) system is necessary. The APL devices are currently typically based on a photoelectrochemical (PEC) configuration, which comprises the following main elements: (1) an anode, (2) a cathode, and (3) a membrane separating the two compartmental zones. An electrolyte is used to close the circuit. Although variations exist even around this apparently simple scheme, the most typical is that the anode
acts as a light-harvesting element, to realize the charge separation (holes and electrons) and the reaction of holes with water to generate protons (and evolve gaseous O2). The electrons and protons are transported to the cathodic compartment zone to be used to reduce CO2. Note that in both anodic and cathodic zones, the crucial factor is the catalytic reaction of water oxidation by holes and CO2 (selective) reduction by protons and electrons. For this reason, often the term photoelectrocatalysis is used rather than photoelectrochemistry.
2.1 Anode The anode is the part exposed to sunlight and containing a photocatalyst able to oxidize water, but typically supported on a conductive substrate to allow the fast collection of the photogenerated electrons and limit their recombination with holes. There are different possible configurations in PEC devices, as discussed later, but
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2. Components of an artificial leaf
(A)
FIGURE 21.3
419
(B)
(A) Conventional design of photoelectrochemical solar cells with respect to (B) the compact design. GDL, gas-
diffusion layer.
for practical implementation, it is necessary to have compact devices, minimizing (or better avoiding, as commented on later) the electrolyte. In this compact configuration, the membrane necessary to separate the anodic from the cathodic zone should be thus the interface between them. This concept is presented in Fig. 21.3B, in comparison with the conventional (Fig. 21.3A), and used in most of the cases, PEC configuration. The compact design, analogous to that used in the PEM (proton-exchange membrane) fuel cell, shows various advantages with respect to the conventional one, particularly for the industrial development of the cell, as discussed in more detail later. It must be noted, however, that a compact design is not a simple engineering of the conventional cell, which may thus take full advantage of the materials developed in the latter, which use is easier by largely avoiding the issues related to proton transfer limitations and due to the interface between the electrodes and the membrane. In fact, in the compact design, the critical factor for the development of the anode is the need of an efficient collection of electrons and at the same time fast transport of the photogenerated protons through the anode itself and
then at the interface with the membrane. At the cathode side, the protons must then again cross the interface and reach the (electro)catalytic sites to reduce CO2. Thus, the behavior of the anode is largely influenced by how efficient the transport of the electrons and protons to the cathode side (through external/internal electron conduction and a proton-permeable membrane) is. Thus, the anode should have a specific nanostructure and characteristics to make these processes efficient and avoid limitation in the overall rate of the process. The anode materials selected in PEC devices with the conventional design may thus not be suitable for the compact design. The anode should have three main functionalities, in addition to characteristics suitable for fast transport of the photogenerated electrons and protons: -
-
an efficient light-harvesting capability; generation of charge separation, e.g., electrons and holes, with simultaneous inhibition of the charge recombination processes, e.g., long life for the photogenerated charges; the presence of catalytic active centers able to provide fast oxidation of water (through the
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photogenerated holes) to generate gaseous oxygen and protons.
for the operation of an electrocatalyst for water oxidation.
There are other characteristics necessary, like stability in the presence of strong oxidants, because the oxidation of water may lead to the formation of hydrogen peroxide and hydroxyl radicals. In particular the latter are among the stronger oxidants known, and thus organic molecules may easily degrade, particularly under full load and extended operations as required for application. It should be also commented that under full sunlight operations, the PEC solar cells may reach temperatures easily above 50e60 C. This is another reason the use of a liquid electrolyte under practical operations creates severe issues, including the enhanced rate of photooxidation and photocorrosion. Thus, the anode must contain quite robust elements, an aspect often not considered with sufficient attention in the development of the electrodes of PEC devices. There are two possible options to realize light harvesting and charge separation:
In the second case, although used often in the literature, the compact design may be quite difficult to realize, because the PV component does not have the necessary proton transport characteristics. Although the use of a PV component allows one to obtain higher efficiency in light harvesting and charge separation, the two main issues are related to (1) how to add the catalytic elements for water oxidation in a stable form and (2) especially how to realize stable systems with respect to photocorrosion. The concept of integrating a PV element to realize a PEC device was presented by Nocera [27] in 2011, receiving large attention, but even if many research groups have further explored this concept, the problems of stability are still largely unsolved, even by adding various protective layers. In addition, this type of cell is not effective in CO2 PEC reduction, other than in water splitting, as designed. Fig. 21.4 shows the simplified scheme of this type of solar fuel cell. A solar PV unit provides the photocurrent (electron and holes), which is used on the two sides by suitable catalysts: on the anode side to oxidize water using an OEC (oxygen-evolving catalyst) and on the cathodic side by a hydrogen-evolving catalyst to form H2. Charge separation and cell design may provide wireless electron transport, but an external
-
-
A semiconductor thin film is used, having the nanostructure suitable for efficient light harvesting and proton transport and supported over a conductive substrate for efficient electron collection and transport to the other side of the cell. A conventional photovoltaic (PV) cell is used, which generates the electrons and potential
FIGURE 21.4 Simplified conceptual scheme of an artificial leaf (solar fuel cell). HEC, hydrogen-evolving catalyst; OEC, oxygen-evolving catalyst; PV, photovoltaic.
IV. Selected examples and case history
2. Components of an artificial leaf
wire is necessary for more efficient transport. The protons are transported through the electrolyte. In this type of cell, there is no generation of H2 and O2 in physically separated compartments, a requirement for safety and to avoid the costs of downstream separation. When a membrane is inserted to obtain this separation, performance worsens. The crucial element is the OEC component. Nocera et al. [27] used a Co-phosphate (CoPi) OEC they had developed earlier [28]. Fig. 21.5A shows schematically this Nocera cell design, which is largely similar to that developed a decade before by Turner [29] and schematically shown in Fig. 21.5B. In the Nocera cell, there is a single-junction Si nppþ cell (where nnpþ indicates a single photovoltaic cell with n, p indicates the n- and p-type semiconductor layer and pþ the doped p-type semiconductor layer), with the pþ side created by depositing a 1-mm film of silicon-doped (1%) Al and then thermally annealing. The space charge region in the pþ layer is thin enough to act as a tunneling layer. On the n side of the PV element, a metal front contact is deposited, protected from the solution by a 10-mm layer of
(A) photoresist
421
photoresist. The Si is protected from oxidation by sputtering on the pþ side of the junction a 50-nm film of ITO (indium tin oxide), which serves as an ohmic contact for hole transport from the buried Si junction. The Co-OEC is electrodeposited on the ITO barrier layer. This single-junction cell generates 0.57 V, and thus an additional nppþ Si solar cell should be connected in series to generate the potential for water splitting. To avoid an externally applied potential, it is necessary to use a triple-junction (3jn) PV cell. The 3jn-a-Si produces 8 mA/cm2 of current at 1.8 V at an overall efficiency of 6.2%. The alternative approach is to use a semiconductor, with the presence of a cocatalyst able to oxidize water. This is a more robust approach allowing one to also realize (in principle) more productive anodes and with the characteristics suitable for a compact design, as illustrated in Fig. 21.3B. TiO2 is perhaps the best known semiconductor, having the great advantage of very robust characteristics and a large set of knowledge on how to control its nanostructure in thin photoactive films [30e32]. By anodic oxidation of Ti foils, a photoanode characterized by an array of vertically aligned
ePV cell
(B)
ITO
OEC
metal front contacts
e- h+ n-Si n
2H2O + 4h+
2H2
O2 + 4H+
4H+
p+-Si
p-Si
HEC
single junc on
FIGURE 21.5
(A) Simplified scheme of the Nocera photoelectrochemical (PEC) cell obtained by depositing an oxygenevolving catalyst (OEC) on a single amorphous Si photovoltaic (PV) cell modified to allow protection of the PV cell by a transparent indium tin oxide (ITO) film (on which the OEC is then electrodeposited) and by a photoresist layer (after depositing metal front contacts) on the other side. (B) Schematic of the monolithic PEC/PV device. HEC, hydrogen-evolving catalyst. Adapted from O. Khaselev, J.A. Turner, A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting, Science 280 (1998) 425e427.
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21. Artificial leaves using sunlight to produce fuels
TiO2 nanotubes can be produced. This type of nanostructured photoanode: -
-
has excellent properties of light harvesting; shows reasonably good electron transport and fast collection, which may be improved by doping; permits a good Hþ diffusion to the underlying proton-conduction membrane.
Introduction of CuO as cocatalyst allows one to make the systems active also in the visible light region, in addition to promoting solar-tochemical conversion efficiency [33]. A solar-tohydrogen efficiency of over 2% in the full PEC cell is observed under the best conditions. IPCE (incident photon-to-current conversion efficiency) measurements clearly evidence that the presence of CuO nanoparticles induces an enhanced IPCE in the 300e340 nm region. The increase in the performance of water splitting is mainly associated with the transient generation of a pen junction between the CuxO nanoparticles and TiO2 nanotubes upon illumination, which enhances photocurrent density by promoting charge separation. The design of this
compact-type cell, based only on earthabundant components, is presented in Fig. 21.6. An alternative approach to the PEC configuration discussed before is to realize a two-photon process, with thus light absorption at both the anode and the cathode sides. This two-step process, schematically presented in Fig. 21.7A, is called a Z scheme and is analogous to that utilized in natural photosynthesis. Fig. 21.7B shows how this concept can be implemented in principle in a compact PEC device, but still requires an electrolyte containing the redox mediator to close the electronic cycle, In addition, to realize a transparent membrane as indicated in Fig. 21.7B is a significant challenge. On the other hand, realizing, in practical devices, solar illumination on the two sides of the cell is also technically challenging and difficult to implement.
2.2 Membrane The compact-type PEC design as presented earlier requires a membrane enabling the fast transport of protons, while electrons have to be transported along a different path, because
FIGURE 21.6
Simplified scheme of compact-type cell, based only on earth-abundant components, used in water splitting. The photoanode is based on an ordered array of TiO2 nanotubes (TNTs), prepared by anodic oxidation of a Ti sheet and modified by electrodepositing CuO nanoparticles. In the inset, a transmission electron image of the junction between the CuxO nanoparticles and the TNTs is shown. The cathode is composed of a gas-diffusion layer (GDL) electrocatalyst put in contact with the Nafion membrane. Credit: Graphical abstract in J.F. de Brito, F. Tavella, C. Genovese, C. Ampelli, M.V.B. Zanoni, G. Centi, S. Perathoner, Role of CuO in the modification of the photocatalytic water splitting behavior of TiO2 nanotube thin films, Applied Catalysis B: Environmental 224 (2018) 136e145. Copyright: Wiley & Sons, 2018.
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2. Components of an artificial leaf
(A)
(B)
photoac ve HERC/CEC
H+
Transparent Membrane
photoac ve OEC
(bias)
porous conduc ve layer FIGURE 21.7 (A) Photoelectrochemical (PEC) tandem cell for the Z-scheme approach. (B) PEC artificial leaf design with double photoactive layer. CEC, carbon dioxide reduction electrocatalyst; HERC, hydrogen-evolving reduction catalyst; OEC, oxygen-evolving catalyst. Credit: Adapted from G. Centi, S. Perathoner S, Artificial leaves, in Kirk-Othmer Encyclopedia, Wiley & Son, Hoboken, NJ, 2013. Copyright: Wiley & Sons, 2013.
electron and proton transport should occur along different paths to minimize their recombination. The membrane has the critical function of separating physically the anodic and cathodic processes, thus avoiding the recombination of the oxidation and reduction products (eliminating crossover is thus a crucial aspect for efficiency) and producing in separate fluxes the oxidation and reduction products (O2 and H2 in water splitting). This is a key function, for safety (H2 and O2 mixtures, for example, are explosive in a wide range of compositions) and cost (downstream separation of the products of reaction). Thus, while a lot of research has been done, even on devices without formation of oxidation and reduction products in separate zones, this is an intrinsic necessary aspect to consider at the initial stage of development. Considering
the PEC device downstream operations, in particular aspects such as how to recover and separate the reaction products, is a key element to include at the stage of initial screening, because otherwise the results may be not relevant [26]. The membrane should be tailored to provide the minimum transfer resistance to protons and thus have a minimum thickness (in principle some micrometers), but also at the same time to hamper oxygen permeation to preserve CO2 reduction activity at the cathode. The membrane should also be in good contact with the anode and cathode sites and it is necessary to minimize the resistance at the interface. In the PEC scheme discussed before, the photoactive side is the anode, and thus water oxidation occurs on an n-type semiconductor.
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21. Artificial leaves using sunlight to produce fuels
(A)
n-type semiconductor electrode e-
(B)
e- (bias)
e- (bias)
H2
CB
e-
CB
H2O h O2 H2O
H+
h
OH-
O2 p-type H2O or VB semiconductor VB electrode h+ n-type h+ p-type OH- -membrane OEC H+ -membrane electrocatalyst on conduc ve support H2
FIGURE 21.8 Photoelectrochemical solar cell with (A) an n-type oxide semiconductor and (B) a p-type semiconductor electrode as the photoactive element. CB, conduction band; OEC, oxygen-evolving catalyst; VB, valence band. Credit: C. Ampelli, C. Genovese, G. Centi, R. Passalacqua, S. Perathoner, Nanoscale engineering in the development of photoelectrocatalytic cells for producing solar fuels, Topics in Catalysis 59 (2016) 757e771. Copyright: Springer, 2016.
Protons then cross the membrane to react with CO2 on the cathodic side. A different scheme, however, is also possible, in this case with a p-type semiconductor where the CO2 is directly photoreduced at the light-illuminated side. An OH-transporting membrane is necessary in this case, as schematically presented in Fig. 21.8. Because of the difficulties in having both OHtype membranes and p-type semiconductors, the PEC scheme shown in Fig. 21.8B is essentially not studied in the literature, although attractive.
2.3 Cathode A cathode should contain a conductive substrate (in contact with the membrane and permeable to protons) on which the active centers for CO2 adsorption and electrocatalytic conversion in the presence of protons are present (Fig. 21.9). The main issue is to realize a selective conversion, because the potentials of reduction of CO2 to different types of products are quite
similar, as shown in Table 21.1 [34]. In addition, the side reaction of recombination of protons with electrons is also present, and thus, obtaining high Faradaic selectivities and control of the selectivity in CO2 electrocatalytic reduction to the target product is a high challenge, requiring an electrocatalytic rather than electrochemical approach. It is also evident that to avoid the formation of high-energy intermediate products a multielectron/multiproton simultaneous transfer is necessary, an aspect common to other electrocatalytic reactions [35]. One-electron reduction of carbon dioxide to •CO 2 anion radical is an easy reaction, but the high potential needed to form this product indicates the need to realize instead a one-step (direct) rather than a sequential electron/proton addition, e.g., a multielectron transfer to form the intermediate to the product of possible interest. As an example, the CO2 reduction to methanol involves 6e and to isopropanol 18e. Even if difficult, and productivity is low,
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2. Components of an artificial leaf
FIGURE 21.9 Photoelectrochemical solar cell (compact design) with schemes of the photocatalytic reactions at the anode (left) and those of electrocatalytic CO2 reduction at the cathode (right). (CH2O) indicates the products of hydrogenation of CO2. TABLE 21.1
Electrode potentials for half-reactions of electrochemical CO2 reduction.
Possible half-reactions of electrochemical CO2 reduction e
Electrode potentials (V vs. SHE) at pH 7
e
CO2 (g) þ e / *COO þ
1.90
e
CO2 (g) þ 2H þ 2e / HCOOH (l) e
0.61
e
e
CO2 (g) þ H2O (l) þ 2e / HCOO (aq) þ OH þ
0.43
e
CO2 (g) þ 2H þ 2e / CO (g) þ H2O (l) e
0.53 e
CO2 (g) þ H2O (l) þ 2e / CO (g) þ 2OH þ
0.52
e
CO2 (g) þ 4H þ 2e / HCHO (l) þ H2O (l) e
0.48 e
CO2 (g) þ 3H2O (l) þ 4e / HCHO (l) þ4OH þ
0.89
e
CO2 (g) þ 6H (l) þ 6e / CH3OH (l) þ H2O (l) e
0.38
e
CO2 (g) þ 5 H2O (l) þ 6e / CH3OH (l) þ 6OH þ
0.81
e
CO2 (g) þ 8H þ 8e / CH4 (g) þ 2H2O (l) e
0.24 e
CO2 (g) þ 6H2O (l) þ 8e / CH4 (g) þ 8OH þ
0.25
e
2CO2 (g) þ 12H þ 12e / C2H4 (g) þ4H2O (l) e
0.06 e
2CO2 (g) þ 8H2O (l) þ 12e / C2H4 (g) þ 12OH þ
0.34
e
2CO2 (g) þ 12H þ 12e / CH3CH2OH (l) þ3H2O (l) e
0.08 e
2CO2 (g) þ 9H2O (l) þ 12e / CH3CH2OH (l) þ 12OH (l)
0.33
SHE, standard hydrogen electrode. Credit: W. Zhang, Y. Hu, L. Ma, G. Zhu, Y. Wang, X. Xue, R. Chen, S. Yang, Z. Jin, Progress and perspective of electrocatalytic CO2 reduction for renewable carbonaceous fuels and chemicals. Advancement of Science 5 (2017) 1700275. Copyright: Wiley, 2018.
even this highly challenging CO2 to isopropanol reaction can be realized with selectivity [36,37], thus indicating how the catalyst itself plays a very crucial role in determining the selective path, which may be virtually impossible based only on electrochemical considerations.
Due to competition with the solvent and the presence of the double layer, CO2 electroreduction in water or organic solvents mainly leads to products such as formic and oxalic acids, with methane and traces of C2 hydrocarbons at high potential [38]. These conditions lead, however, to fast deactivation [39]. However, if
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21. Artificial leaves using sunlight to produce fuels
operating in the gas phase and with metal nanoparticles supported over carbon nanotubes, the possibility of also forming >C2 products from CO2 electroreduction, including isopropanol, has been shown [37]. Note that the term “gas phase” indicates operations without the presence of a liquid electrolyte as in conventional cells (Fig. 21.3A), with the membrane itself acting as the electrolyte. To avoid dehydration of the membrane, the GDL type of electrode (similar to those of PEM fuel cells) has to be used. Under these “gas-phase” operations, the behavior is different from that of the same electrodes operating in the presence of the liquid electrolyte, e.g., in conventional cells [40,41]. This is due to the higher surface coverage by chemisorbed CO2 over the electrode. By enhancing this effect, for example, by introducing a component able to enhance the adsorption of CO2 in the GDL, it is possible to increase the formation of C2 products in the electrocatalytic reduction of CO2 even on electrocatalysts that are considered unable to form a CeC bond during the reduction of CO2 [42]. This result introduces a change in perspective about the catalytic electrochemistry of CO2 conversion. While there are many claims of the unique properties of copper, in particular of specific crystalline phases like Cu(100), in forming C2 products [43e45], the cited result highlights that the surface concentration of CO2 at the electrode surface determines the pathway of transformation. This depends on the characteristics of the electrocatalyst, but especially on the specific conditions of operations. This influences also the reaction mechanism. While the formation of surface CO dimers is considered the key step in forming CeC bonds during the electrocatalytic reduction of CO2 [44e46], experimental studies on the mechanism of CO2 electrocatalytic reduction to acetate (thus a C2 product) prove the presence of a different reaction mechanism [47] involving the need for having dissolved CO2 to form acetic acid, likely via the reaction of the •CO anion radical with surface2
adsorbed eCH3-like species. The pathway toward formic acid is instead different from the route of the formation of acetic acid [47]. The cell concept presented in Fig. 21.6 of a compact-type cell may be extended by developing copper-based electrocatalysts for the reduction of CO2. By preparing Cu2O/GDL electrodes by electrodeposition, and coupling with the CuO/NtTiO2 photoanode presented in Fig. 21.6, a compact-design full PEC cell can be realized [48] using only noncritical raw materials and low-cost, easily scalable procedures. The scheme of this PEC solar cell for water oxidation on the photoanode side and CO2 reduction on the cathode side is shown in Fig. 21.10. Note that it operates without external bias and use of sacrificial agents. In the PEC experiments without external bias and use of sacrificial agents and 24-h operation, the total carbon Faradaic selectivity was about 90%, with about 75% Faradaic selectivity to acetate, the other product being formate. A stable current density of 0.2 mA was observed for 24-h operation. There is an in situ transformation of the Cu2O/GDL electrode leading to the formation of a hybrid Cu2OeCu/GDL system. The same electrode gives, in electrocatalytic tests of CO2 reduction at 1.5 V, high total Faradaic selectivity (>95%), but forming selectively formate (about 80% selectivity), rather than acetate. The difference between the electrocatalytic tests and those in the PEC cell derives from the different potential applied to the cathode, being 1.5 V in the electrocatalytic tests and lower, determined from the photoanode, in the PEC tests. This result also highlights how the performance of the same electrodes may change when tested separately or integrated within a full PEC device. Thus, there are still several question marks on the reaction mechanism of CO2 electrocatalytic reduction, but a general observation is that the reaction paths are clearly highly dependent on the surface population of species adsorbed on the electrocatalyst in operation, which in turn highly depends on the specific conditions of the
IV. Selected examples and case history
3. Conclusions and future trends
427
FIGURE 21.10 Scheme of a photoelectrochemical full cell reactor with compact design used for water oxidation on the photoanode side and CO2 reduction on the cathode side without external bias and use of sacrificial agents. The electrodes are based on earth-abundant materials. Credit: Graphical abstract in J. Ferreira de Brito, C. Genovese, F. Tavella, C. Ampelli, M. V. Boldrin Zanoni, G. Centi, S. Perathoner, CO2 reduction of hybrid Cu2OeCu/gas diffusion layer electrodes and their integration in a Cu based photoelectrocatalytic cell, ChemSusChem 12 (2019) 4274e4284. https://doi.org/10.1002/cssc.201901352. Copyright: Wiley VCH, 2019.
electrocatalytic operations. Without considering these aspects, many of the current mechanistic studies, including by theoretical approach, fail in understanding the effective possibilities in controlling the electrocatalytic chemistry. At the same time, it is necessary to consider these aspects in the design and operations of PEC solar cells, and conventional types of cells may be not able to take advantage of these aspects, resulting in the evaluation of materials under incorrect conditions in terms of both applicability, as remarked above and discussed in a more detail elsewhere [26], and exploiting the full possibilities given by the surface electrocatalytic chemistry discussed above.
3. Conclusions and future trends There are many scientific and technological challenges in developing artificial leaf-type devices: -
The full sunlight spectrum has to be used to realize devices that should be competitive with respect to the simpler approach of coupling a PV unit with an electrocatalytic unit. In this case, the overall efficiency is already over 10% for water splitting, but still significantly lower for CO2 conversion. In principle, the integrated PEC system offers potentially lower costs and better efficiencies, but current PEC devices are still largely less efficient.
IV. Selected examples and case history
428 -
-
-
21. Artificial leaves using sunlight to produce fuels
Robust devices with long operational times must be realized. Robustness under practical operations, which include temperatures up to about 100 C, is a crucial aspect, but still largely missing from many of the artificial photosynthesis or leaf devices and related components proposed. Except in terms of understanding, it is not possible to dedicate a large effort to materials that cannot have the necessary robustness for practical operations [26]. Product recovery and artificial leaf downstream operations should be part of the design. The products of oxidation (like O2) and reduction (H2 or those deriving from CO2 conversion) should be produced in separate streams. If products, even in part, remain in the electrolyte, recovery can be expensive in terms of energy and cost. Even if they are produced in minor amounts, they accumulate in the electrolyte and must be separated. Avoiding the presence of the electrolyte, in a compact design as shown in Fig. 21.3B (indicated as electrolyte-less operations [26,40,41]), could greatly facilitate continuous operation and easy recovery of the products of reaction. Note also that if the O2 produced cannot be utilized, e.g., the most common situation in the practical use of PEC solar devices, then the efficiency of using solar light is about halved. Thus, alternative PEC device approaches using both anodic and cathodic reactions to produce valuable chemicals have to be developed. This opens new perspectives in PEC device applications. Noncritical raw and cheap materials should be used, but also in conjunction with methodologies of preparation that can be scaled up to produce devices on a large scale. There are several progressing in the area, including the preparation of electrodes by advanced printing methods.
Thus, while progress has been made in developing the components for artificial
photosynthesis or leaf devices, still a large effort is necessary to realize systems and components that are both cost efficient and robust and meet the criteria discussed above. As remarked elsewhere, a “turning perspective in photoelectrocatalytic cells for solar fuels” [26] is necessary to accelerate the progress in this area.
List of abbreviations and acronyms APL CEC GDL HEC IPCE ITO OEC PEC PEM PV SHE STH TNT
artificial photosynthetic leaf carbon dioxide reduction electrocatalyst gas-diffusion layer hydrogen-evolving catalyst incident photon-to-current conversion efficiency indium tin oxide oxygen-evolving (water oxidation) catalyst photoelectrochemical proton-exchange membrane photovoltaic standard hydrogen electrode solar-to-hydrogen TiO2 nanotubes
Acknowledgments This work has been realized in the frame of the PRIN2017 projects 2017WR2LRS “CO2 as only source of carbons for monomers and polymers: a step forwards circular economy” and 20179337R7 “Multielectron transfer for the conversion of small molecules: an enabling technology for the chemical use of renewable energy,” which are gratefully acknowledged. The FET ProActive A-LEAF “An Artificial Leaf: a photo-electrocatalytic cell from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels” project (grant agreement ID: 732840) is also gratefully acknowledged.
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IV. Selected examples and case history