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Sunlight drives photocatalytic and solar–photoelectrochemical processes. These play a key role in converting solar energy into chemical fuel by using semiconductors as the photoactive components. This chapter aims to provide a brief introduction to such processes, by pointing out the similarities and differences of such processes.

Notwithstanding the declaration of intent of national governments, international organizations, and public opinion (see, for example, the overwhelming success of the Fridays for Future worldwide protests),1  the fact remains that the world's energy supply is still mostly dependent on the use of non‐renewable resources. It is apparent that scalable and renewable sources of energy, and fuels, are urgently needed to ensure the sustainability of the energy supply for future generations. Indeed, with clean energy systems, the minimization of emissions and waste is a feasible undertaking by taking advantage of energy conservation and recovery.

In his keynote address at the 2011 Sixth International Green Energy Conference, Dincer2  pointed out that energy sources must exhibit different properties in order to be defined as clean, including, among the others, better effectiveness, high-cost efficiency and safety. At present, the perception that fossil fuels need to be replaced by alternative energy sources appear ingrained in the mindset of society. In this context, among the different renewable sources (including hydroelectric, geothermal, and eolic), solar energy is certainly the most promising energy source as the Sun delivers an enormous amount of energy. For instance, the energy that reaches the Earth's surface in one hour (4.3 × 1020 Joules per hour) is comparable to what is consumed globally in one year (5.76 × 1020 Joules per year)3  so that 1 h of the Sun's energy will satisfy the Earth's energy need for one whole year.

In 1980, Bard4  pointed to the key importance of suitable forms of interconvertible energy in the hope of developing “artificial photosynthetic systems for the conversion of abundant materials such as water and carbon dioxide to fuels directly or (otherwise) by electrolysis”. Among the available fuels, hydrogen has been considered the elective sustainable energy vector because of its high energy content, because it can be easily stored, and because of the absence of greenhouse gas emissions upon its use. Hay and co-workers5  reported a growth rate for hydrogen demand as 5.6% in the five years 2011–2016. Hydrogen is produced mainly via the steam reforming of fossil resources such as natural gas or liquid hydrocarbons. This process occurs at high temperatures (up to 950 °C) and pressures under nickel catalytic conditions. In contrast, the extent of generation of hydrogen via water electrolysis is fairly low at less than 4% of global annual production.6  Indeed, the water splitting reaction (H2O(l) → H2(g) + O2(g)) is a process that involves a large positive change of the Gibbs free energy for it to occur (ΔG° = 237.13 kJ mol−1). At the same time, the efficient conversion of CO2 into chemical fuels such as, among others, methane, and methanol, is limiting as the concentration of such a greenhouse gas in the atmosphere is beyond intriguing. In the most researched technique, however, the efficiency of Carbon Capture and Storage is counterbalanced by environmental and economic limitations, including the risk of leakage from geological storage sites, not least of which is the cost of compression and transportation.7  In this context, solar-induced photocatalytic and solar–photoelectrochemical processes represent key approaches in harvesting solar energy for energy storage and (solar) fuel production through the exploitation of semiconductors as the photoactive components. The two strategies (photocatalytic and photoelectrochemical) exhibit some similarities (treated together in several articles and reviews)8  and some differences (described in the next sections) in the requirements needed to optimize performances.

In 2007, Palmisano and co-workers9  defined photocatalysis as “a promising route for 21st century organic chemistry”. Initially, such a technology has been applied widely toward environmental remediation that included the photodegradation of organic and photoinduced elimination of inorganic pollutants.10,11  More rarely, however, it has been used in organic syntheses.12  Recent decades have witnessed investigations into a plethora of semiconductors that have been used as photocatalytic materials. However, from economic and photostability points of view, titania (TiO2; anatase polymorph) with a bandgap energy of 3.2 eV has been the semiconductor of choice and the one that has been examined most extensively in terms of its fundamentals and its various applications.

Three fundamental steps are involved in the conversion of sunlight energy to chemical energy: (i) the absorption of solar photons by the semiconductor photocatalysts to cause the electronic transition from the valence band (VB) to the conduction band (CB); (ii) the generation of redox equivalents (an electron in the conduction band, CB, and a positive vacancy or hole in the valence band, VB), which is then followed by; (iii) their migration to the surface to initiate the redox events (reduction and oxidation half reactions).13,14  Broadly speaking, photocatalytic processes depend both on the wavelength and the irradiance of the light source, which then determine the energy and population of the photogenerated electron–hole pairs. The VB holes (h+) typically exhibit a strong oxidation potential and can thus react with electron-rich compounds, whereas the electrons located in the CB (e) act as reductants. Obviously, the capability of both h+ and e to cause oxidations and reductions depends strictly on the corresponding potentials of the redox couples. However, recombination of the charge carriers15,16  in a photocatalytic process competes effectively with the redox reactions occurring at the semiconductor particle surface, thereby limiting the efficiency (quantum yield) of the semiconductor photocatalytic process.

Water splitting (not to be confused with the simple production of hydrogen) is defined as “the decomposition of water to hydrogen and oxygen in a 2 to 1 ratio”. As predicted, this process is equivalent to the electrolysis of water, whereby water undergoes reduction at the cathode with hydrogen evolution (two reducing equivalents, eqn (1.1)) while oxidation occurs at the anode with release of oxygen (four oxidizing equivalents, eqn (1.2)).17 

Equation 1.1
Equation 1.2

with the overall reaction being: 2H2O → 2H2 + O2, ΔG° = 237.13 kJ mol−1.

The measured standard free energy for water is 1.23 eV. Therefore, when both the reductive and oxidative events are within the bandgap of the photocatalyst (e.g., titania, Figure 1.1), water splitting becomes a process that is feasible thermodynamically. However, when considering thermodynamic factors, and other losses, it is likely that (electrochemically) the energy required would be greater by 0.3–0.4 eV, namely about 1.5–1.6 eV, and as pointed out by both Gerischer18  and Nozik19  adding the need for the overpotential for the oxygen evolution of ca. 1 eV would then require approximately 2.5–2.6 eV to achieve water splitting.

Figure 1.1

An illustration of the thermodynamic feasibility of splitting water into its constituent elements hydrogen and oxygen in the presence of a photoactivated semiconductor such as titania, TiO2.

Figure 1.1

An illustration of the thermodynamic feasibility of splitting water into its constituent elements hydrogen and oxygen in the presence of a photoactivated semiconductor such as titania, TiO2.

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As pointed out by Osterloh,20  only metal oxides with d0 ion configuration (Ti, Zr, Nb, and Ta) and d10 ion (Ga, In, Ge, Sn, and Sb) configuration possess the appropriate activity to allow for the overall photochemical splitting of water. The metal oxides tend to be dominant. However, nitrides and oxynitrides (GaN/ZnO and Ge3N4) have also been found to be effective in the process.

Titanium dioxide (TiO2) was the first semiconductor in photocatalytic water splitting and is still the most widely used. This is due to its efficient photocatalytic activity, its chemical stability, and the relatively low commercial cost. However, titania exhibits a low efficiency in converting solar energy into hydrogen and oxygen via the photocatalytic water splitting process and is one the principal limitations for its application. Titania crystallizes in three different polymorphic structures: rutile, anatase, and brookite that display bandgaps in the 3.04–3.23 eV range, depending on the crystal form. As a result, only the UV part of the solar emission spectrum (about 4% of the solar energy at the Earth's surface) can be exploited. Regardless, the list of photocatalysts that can be applied efficiently toward visible light water splitting without additives (e.g. electron donors/acceptor) has increased notably in the last decade.20,21  At present, aside from the historical separate stable self-driven photoelectrochemical cells consisting of the coupled n-TiO2/p-CdTe, n-TiO2/p-GaP, n-SrTiO3/p-CdTe and n-SrTiO3/p-GaP systems capable of water photoelectrolysis,22  novel ones include {Zn1.44Ge)(N2.08O0.38} with a bandgap of ca. 2.7 eV 23  and {NiO/RuO2/Ni:InTaO4},24  together with the tandem systems composed of Pt/WO3 and Pt/SrTiO3/TaON,25  the oxysulfide α-LaOInS2,26  the Milstein's ruthenium complex,27  the CdS/WO3/CdWO4 ternary composite,28  and the carbon dots/cadmium sulfide (CDs/CdS) nanocomposites.29  Along similar lines, Liu and co-workers30  proposed a thermal strategy to prepare {reduced graphene oxide/γ-Fe2O3–C3N4} S-scheme heterojunctions from MIL-101(Fe) and melamine. The rGO/γ-Fe2O3–C3N4 system presented a relatively high oxygen evolution rate (OER) of 3.85 mmol g−1 h−1 under visible light irradiation, with an overall H2O splitting activity that yielded hydrogen evolution (HER) and OER rates of 23.3 and 12 µmol g−1 h−1, respectively.

The fast recombination of photogenerated electron–hole pairs is one of the limitations for an efficient water splitting process. Other limitations include the many factors that affect: (1) quantum yields of surface reactions such as (i) the rate constant of either charge transfer to (or from) adsorbed molecules or charge carrier trapping to surface defects to form surface-active centers, (ii) quantum yield of internal photo effects, such as the probability of charge carrier generation caused by absorption of a single photon, (iii) absorption coefficient, (iv) electric field, (v) diffusion length of the charge carriers, (vi) their lifetimes, (vii) depth of surface space charge layer, (viii) charge carrier recombination rate, (ix) size of the semiconductor particulates, (x) concentration of surface-active defects as well as of adsorbed molecules, (xi) diffusion coefficient, (xii) charge carrier mobility and (xiii) the potential surface charge;31,32  together with (2) back reactions and secondary reactions;33,34  (3) the photostability of photocatalytic materials;35  (4) the need for co-catalysts for H2 and O2 generation18,19  and (5) the need to minimize the density of defect states in metal–oxide photocatalysts.36  To overcome some of these limitations, various strategies have been proposed that include, among others, (a) the tailoring of the surface morphology, (b) engineering the bandgap, and (c) the loading of suitable co-catalysts.37  Control of the surface morphology plays a key role in both promoting charge separation and suppressing charge recombination at the surface, as demonstrated by Serpone and co-workers38–41 via the inter-particle electron and hole transfer processes between coupled semiconductor photocatalysts (Figure 1.2).

Figure 1.2

Schematic illustration of the coupling of two semiconductors displaying the vectorial inter-particle electron transfer process. Reproduced from ref. 40 with permission from American Chemical Society, Copyright 2012.

Figure 1.2

Schematic illustration of the coupling of two semiconductors displaying the vectorial inter-particle electron transfer process. Reproduced from ref. 40 with permission from American Chemical Society, Copyright 2012.

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Ternary heterostructures can also be envisaged to separate the photogenerated charge carriers and minimize their recombination as illustrated in Figure 1.3(a) in which, with proper alignment of the VB and CB bands, it is possible to use low-energy visible light to activate wide-bandgap semiconductors utilizing two narrow bandgap semiconductors sensitive to visible light.40,41  One such example of a ternary heterostructure originates from the work of Jiang and co-workers42  in which two narrow bandgap systems—graphitic carbon nitride g-C3N4 (Eg = 2.66 eV) and indium oxide (Eg = 2.93–3.02 eV)—activate the higher bandgap TiO2 (Eg = 3.2 eV) for redox reactions (Figure 1.3(b)).

Figure 1.3

Illustration showing ternary heterostructures of coupled semiconductors. (left) Reproduced from ref. 40 with permission from American Chemical Society, Copyright 2012. (right) Reproduced from ref. 42 with permission from Elsevier, Copyright 2015.

Figure 1.3

Illustration showing ternary heterostructures of coupled semiconductors. (left) Reproduced from ref. 40 with permission from American Chemical Society, Copyright 2012. (right) Reproduced from ref. 42 with permission from Elsevier, Copyright 2015.

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Controlling the morphology can be achieved by the optimization of the synthesis method as well as by adopting doping techniques and surface treatment strategies. A demonstrative example is represented by the single crystal Ta3N5 nanorods grown on a lattice-matched cubic KTaO3 that, when combined with Rh/Cr2O3 as the co-catalyst, resulted in water splitting under both visible light and simulated solar light.43  Moreover, the use of protectors has often been exploited to stabilize the surface and inhibit the formation of surface defects, thereby preventing or otherwise suppressing any back reaction. As an example, the efficiency of the Pt/SrTiO3 photocatalyst in overall water splitting has been increased dramatically by coating the bare material with a molybdenum-based layer. The so-obtained MoOx/Pt/SrTiO3 is stable in acidic media for more than 15 h and the MoOx layer hinders the gas permeation of O2 and contact with the active Pt surface.44 

The photocatalytic conversion of CO2 into solar fuels and chemicals using everlasting solar energy is a suitable approach to contemporaneously curb greenhouse gas emissions. However, to develop an efficient protocol, several limitations must be overcome, including: (1) the poor overlap between the absorption spectra of the semiconductor photocatalysts (e.g. in the case of titania) and the solar emission spectrum; (2) the inefficient charge carrier separation; (3) the back reactions observed during the CO2 reduction process; and (4) the low solubility of carbon dioxide in aqueous media (about 33 mmol mL−1 in water at 100 kPa and ambient temperature).45–47  Furthermore, the competition between CO2 conversion and oxidation of water to hydrogen (eqn (1.9) and (1.10)) must be avoided. The latter point is not a trivial issue, as the reduction of CO2 into carbon-based fuels requires electrons in the conduction band that exhibit a redox potential more negative than required for water reduction. As highlighted below (see eqn (1.3)–(1.10)), despite the formation of carbon based-fuels from carbon dioxide the process is a multielectron process with the first rate-determining step being the monoelectronic reduction of CO2 to the corresponding radical anion CO2˙, a monoelectronic reduced species isoelectronic with NO2 that exhibits a C2v geometry.48,49 

Equation 1.3
Equation 1.4
Equation 1.5
Equation 1.6
Equation 1.7
Equation 1.8
Equation 1.9
Equation 1.10

* In aprotic media. E = −2.21 V in water.

Once formed, the intermediates can undergo bimolecular decay to form oxalate and formate anions.50  The pH of the aqueous solution also plays a key role in determining the efficiency of the process, since at pH > 5 carbon dioxide is also present in solution as the carbonate anion (CO32−) that acts as an electron donor species toward the photogenerated holes. Recently, different efforts have been carried out to investigate in detail the mechanism of the photochemical process, in order to properly design photocatalysts with improved stability, activity, and high selectivity for CO2 reduction.

To the extent that the reduction of CO2 under heterogeneous catalytic conditions occurs at the interface between the catalyst(solid) and the gas, the design of an appropriate photocatalyst plays a fundamental role in the conversion of CO2 under heterogeneous photocatalytic conditions. In the last decade a plethora of photocatalytic materials has been investigated in this field, including metal oxides (e.g., TiO2, ZnO, Fe2O3, WO3) and metal sulfides (e.g., CdS), transition metals (Cu, Ni, and Fe), noble metals or alloys thereof (Pt, Au, Rh, Ru, PtCu, AuCu), and several carbon-based materials (e.g., graphene derivatives and the carbon nitride C3N4).51  To date, CO2 can be photoconverted into a plethora of products (Figure 1.4) that include, among others, CO, alkanes (methane and ethane), olefins (ethylene and propylene), alcohols (methanol and ethanol) as well as organic acids (formic, acetic and malonic acid).51 

Figure 1.4

Schematic of the PEC CO2 conversion into fuels and value-added products under solar light irradiation. Reproduced from ref. 51 with permission from American Chemical Society, Copyright 2020.

Figure 1.4

Schematic of the PEC CO2 conversion into fuels and value-added products under solar light irradiation. Reproduced from ref. 51 with permission from American Chemical Society, Copyright 2020.

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Research in the field of photoelectrocatalysis is considered to have started with the 1972 seminal paper by Fujishima and Honda.52  This research reported on the light-driven electrolysis of water to molecular hydrogen and oxygen by means of a photoelectrochemical cell equipped with an n-type rutile-phase titanium dioxide (TiO2) working electrode under UV irradiation and a platinum (Pt) counter electrode. This ground-breaking study, along with the earlier reports on the electrochemical behavior of metal semiconductors under illumination dating back to the 1950s, have inspired several pioneering studies, starting with the short-circuited photoelectrochemical cell based on TiO2 and Pt designed by Bard that de facto represented the first heterogeneous photocatalytic system.53,54 

There are significant analogies between photocatalysis (PC) and photoelectrocatalysis (PEC). Indeed, both approaches proceed primarily through three sequential pathways that are fundamental to the solar-to-chemical energy conversion. These are (a) the electron–hole pair generation following the absorption of photons with energies greater than the bandgap energy (Eg) of the photocatalyst employed, (b) the electron–hole separation phenomenon whereby the charges are moved on the surface of the photocatalytic system, and (c) the redox event(s) occurring on the surface of the photocatalyst. Again, the performance of the two systems depends mainly on charge separation and transfer kinetics, which still represent the bottleneck for a large diffusion of the approach for applications such as the solar-to-hydrogen conversion for which the efficiency barely reaches the 10% value.55–57  In any case, light harvesting and electron transfer occur on the same components of the system for both PC and PEC.

The main difference between the two approaches is a practical one, since the former process is performed in a reaction vessel wherein the (heterogeneous) photocatalytic system is suspended in the reaction solution, while in the second system two separate electrolyte solutions (for reduction and oxidation, respectively) are wired to an external bias to favor the electron transfer (Figure 1.5).

Figure 1.5

Schematic representation of a PEC cell composed of a photoanode electrode equipped with a nanostructured photocatalyst (1) and a cathode (counter) electrocatalyst (2). Reproduced from ref. 57 with permission from Elsevier, Copyright 2017.

Figure 1.5

Schematic representation of a PEC cell composed of a photoanode electrode equipped with a nanostructured photocatalyst (1) and a cathode (counter) electrocatalyst (2). Reproduced from ref. 57 with permission from Elsevier, Copyright 2017.

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PEC water splitting is one of the most promising strategies for the generation of hydrogen in a sustainable and economic manner, in view of the small thermodynamic potential required (1.23 eV; however, see prior discussion) and zero CO2 emissions. As hinted previously in the PEC approach, the photocatalyst is initially prepared on a conductive substrate after which a bias is applied to the resulting photoelectrode. When the cell is irradiated with natural sunlight, the photogenerated charge carriers are generated and separated by the electric field in the space-charge region and the minority charges (holes for an n-type photoanode and electrons for a p-type photocathode) then migrate to the semiconductor electrode–liquid interface for the reaction to occur. One of the main advantages, with respect to the photocatalytic approach, is that no gas separation is needed because the generation of hydrogen and oxygen take place in different electrode compartments (Figure 1.6).58,59 

Figure 1.6

Energy diagram for the PEC water splitting at a hematite photoanode with oxygen evolution. Reproduced from ref. 59 with permission from American Chemical Society, Copyright 2012.

Figure 1.6

Energy diagram for the PEC water splitting at a hematite photoanode with oxygen evolution. Reproduced from ref. 59 with permission from American Chemical Society, Copyright 2012.

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The materials used in the preparation of photocathodes (where the reduction of water takes place) should obviously exhibit an edge potential for the conduction band more negative than the hydrogen redox potential and an enhanced resistance in aqueous media. In this case, different materials have been tested, including silicon, III–V group semiconductors (InP, GaP, the former treated with a protective layer of TiO2 to prevent corrosion) and more recently, monometallic oxides (cuprous and cupric oxide, nickel oxide, the latter often sensitized with CdSe quantum dots), bimetallic oxides (CuFeO2, LaFeO3) and chalcogenides (CuInS2, CuGaS2). Photoanodes (where oxygen generation occurs) should be made of a water-stable, photo-responsive n-type semiconductor with a bandgap that allows absorption of a wide range of wavelengths. Different materials were found suitable for this target, including again the monometallic and bimetallic oxides (among others, TiO2 is the one used mostly, despite its low absorption in the solar emission spectrum), and metal chalcogenides and nitrides (see Table 1.1).60 

Table 1.1

Photo-responsive materials for water splitting via PEC

MaterialsBandgap (eV)
Photocathode 
Cu21.9–2.2 
CuO 1.2–1.8 
NiO 3.6–4.0 
CuFeO2 1.5–1.6 
LaFeO3 2.3–2.4 
CaFe2O4 1.9 
CuInS2 1.5 
CuGaS2 2.4 
Silicon 1.1 
GaP 2.2–2.3 
InP 1.35 
Photoanode 
TiO2 3.2 
ZnO 3.2 
WO3 2.5–2.8 
α-Fe2O3 2.0–2.2 
BiVO4 2.84 
CuWO4 2.3 
TaON 2.5 
Ta3N5 2.1 
CdS 2.4 
ZnS 3.6 
MaterialsBandgap (eV)
Photocathode 
Cu21.9–2.2 
CuO 1.2–1.8 
NiO 3.6–4.0 
CuFeO2 1.5–1.6 
LaFeO3 2.3–2.4 
CaFe2O4 1.9 
CuInS2 1.5 
CuGaS2 2.4 
Silicon 1.1 
GaP 2.2–2.3 
InP 1.35 
Photoanode 
TiO2 3.2 
ZnO 3.2 
WO3 2.5–2.8 
α-Fe2O3 2.0–2.2 
BiVO4 2.84 
CuWO4 2.3 
TaON 2.5 
Ta3N5 2.1 
CdS 2.4 
ZnS 3.6 

The PEC reduction of carbon dioxide aims to combine the advantages of both photocatalysis and electrolysis, on the route toward the solar-driven generation of chemical fuels (including, among others, methanol, ethanol, formic acid, formaldehyde, methane, carbon monoxide) from CO2, thanks to low cost and easy accessible apparatuses.61  In addition, the overpotential observed at the electrodes can be reduced smoothly by applying a bias potential that can also be exploited to tune the selectivity of the process and minimize the electron–hole recombination process. A traditional PEC apparatus typically consists of two compartments containing the (photo)electrode separated by a Nafion™ membrane that allows for proton exchange. The carbon dioxide reduction occurs at the cathode, whereas oxidation of the water reaction takes place in the anodic compartment; the protons that are generated in the latter move from the anodic compartment to the cathodic one through the Nafion™ membrane by exploiting the gradient concentration.

As implied previously, the reaction needs the proper combination of reductant equivalents for the carbon dioxide reduction and positive charges (holes) for the water splitting process. For this reason, the choice of (photocatalytic) materials for the photoanode and photocathode is a key issue in the development of a PEC apparatus. At first, the conduction band position of the photoactive semiconductors should be closed to that of the reduction energy level of CO2 for product selectivity, whereas the valence band position should be more positive than the potential for water oxidation. As in the case of water splitting, the PEC system for CO2 reduction consists of both working and counter (photo)electrodes. A reference electrode is often employed at the laboratory scale, in order to verify the exact reduction potential of the working electrode in the system. In most cases, n-type semiconductor materials are employed for the photoanode (e.g., TiO2, BiVO4), whereas photocathodes are constituted by p-type semiconductors. The irradiation strategy determines the overall mechanism. Indeed, when the photoanode is illuminated, the electrons are transferred (after charge separation) toward the cathode (where reduction occurs) while the generated holes are responsible for the oxidation reaction on the surface. However, in a system equipped with a photocathode, the photogenerated electrons will be employed for reducing CO2 on the surface of the electrode. When light irradiates both photoanode and photocathode, the electrons generated in the photoanode will be transferred to the photocathode via the external bias and combine with the electrons produced at the photocathode surface. Recently, Zhou et al.62  replaced the Nafion™ membrane with a bipolar membrane (BPM) composed by an anion-exchange layer and a cation-exchange layer that allows the transfer of protons and hydroxide anions to the cathode and the anode, respectively, so as to maintain the pH in each compartment. In this way, each electrolyte will work under optimal conditions.

Along with the development of PC and PEC, the conversion of sunlight to electricity based on solar cells has experienced a continuing development. Currently, the power conversion efficiency (PCE) of a single crystal Si solar cell is about 26%.63  Such impressive success laid the foundation of an alternative approach to solar hydrogen production, namely the photovoltaic (PV) driven electrolysis (PV-EC), and more in general, to the PV-driven electrochemical processes. Contrary to PC and PEC, the PV-EC process is based on separated modules of the PV part (light harvesting and solar-to electricity conversion) and EC part for the redox event(s).

A recent editorial in the journal ACS Sustainable Chemistry & Engineering64  noted that migration from fossil resources to renewable energies is vital if we wish to relieve the impact of energy shortages and environmental costs, particularly as photovoltaic and eolic power have experienced rapid growth and the associated drop in electricity costs might make it possible to replace traditional fossil fuels. This notwithstanding, however, the intermittent nature of many renewable energy sources (e.g., solar and wind power) makes the task of integrating the renewable electricity into existing power grids somewhat challenging. Nonetheless, chemicals and fuels will continue to be needed for some time to come as they are needed for heating, transportation, and industrial production so that conversion and storage of renewable electricity into some suitable form makes sense if the task is to build an energy system based on 100% renewable energy resources (Figure 1.7).64  Therefore, the introduction of the so-called concept Power-to-X (P2X),65  also dubbed as P2X or PtX, is defined as “a bundle of pathways for the conversion, storage, and reconversion of electric power, especially that generated by renewable energy.” Such an umbrella concept, under which X can be taken as heat or such chemicals as hydrogen, syngas, and synthetic fuels, among others, comprises the extent to which most P2X technologies aim at converting renewable electricity into fuels and/or chemicals. Therefore, electrocatalysis plays a key role in the development of P2X strategies.

Figure 1.7

Schematic illustration of the Power-to-X umbrella, where X can be hydrogen, syngas, ammonia, methanol, etc. Reproduced from ref. 64 with permission from American Chemical Society, Copyright 2021.

Figure 1.7

Schematic illustration of the Power-to-X umbrella, where X can be hydrogen, syngas, ammonia, methanol, etc. Reproduced from ref. 64 with permission from American Chemical Society, Copyright 2021.

Close modal

The most mature P2X, namely P2H2, route is water electrolysis, which relies on the use of renewable electricity to drive catalytic water splitting into H2 and O2.66  Generally, three kinds of systems are applied for the electrolysis of water: (1) alkaline electrolysis cell (AEC); (2) solid oxide electrolysis cell (SOEC); and (3) polymer electrolyte membrane electrolysis cell (PEMEC). Liu and co-workers66  reported, for the first time, the transformation of commercially available stainless steel into useful electrocatalysts for both H2 and O2 evolution reactions (HER and OER, respectively) after a facile modification of the surface (Figure 1.8). Specifically, they sulfurized a stainless steel foil (SSFS) and achieved a catalytic current density of 10 mA cm−2 at overpotentials of 136 and 262 mV for HER and OER, respectively, in 1.0 M KOH. A water splitting current density of 10 mA cm−2 and a robust durability was observed at 1.64 V when the SSFS was employed as the electrocatalytic material for both the cathode and the anode. Figure 1.9 illustrates the stoichiometric quantities of H2 and O2 evolved after 167 min in comparison to those values estimated on the basis of charge passed during electrolysis. The nearly perfect overlap between the experimentally measured and the calculated amounts of both gases strongly validated the near unity Faradaic efficiency of their SSFS catalyst couple for overall water splitting.66 

Figure 1.8

System used for the electrocatalytic water splitting for clean H2 and O2 production with a modified surface of commercially available stainless steel as the electrocatalyst. Reproduced from ref. 66 with permission from American Chemical Society, Copyright 2017.

Figure 1.8

System used for the electrocatalytic water splitting for clean H2 and O2 production with a modified surface of commercially available stainless steel as the electrocatalyst. Reproduced from ref. 66 with permission from American Chemical Society, Copyright 2017.

Close modal
Figure 1.9

Comparison between the amount of chromatography-measured gases and theoretically calculated gases observed in the overall water splitting by the SSFS couple in 1.0 M KOH after 167 min. Reproduced from ref. 66 with permission from American Chemical Society, Copyright 2017.

Figure 1.9

Comparison between the amount of chromatography-measured gases and theoretically calculated gases observed in the overall water splitting by the SSFS couple in 1.0 M KOH after 167 min. Reproduced from ref. 66 with permission from American Chemical Society, Copyright 2017.

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The central P2F process involves the electrocatalytic reduction of CO2, which has seen significant recent studies and applications. In this regard, power-to-fuels (P2F) processes have recently emerged as a potential long-term solution for the storage of surplus renewable electricity through the conversion of carbon dioxide into the production of the gaseous fuel methane.67,68  Such processes are significant for future strategies of energy storage as they address two fundamental issues: (1) electrical grid stability with high share of renewable sources and (2) decarbonization of high-energy density fuels for the transportation sector. A large number of pathways exist to transform the energy from renewable sources into gaseous/liquid fuels through their combination with residual carbon dioxide that have been reviewed in some details by Bailera et al. (Figure 1.10).69 

Figure 1.10

Illustration of some renewable energy and CO2 hybrid storage techniques. Reproduced from ref. 69 with permission from Elsevier, Copyright 2016.

Figure 1.10

Illustration of some renewable energy and CO2 hybrid storage techniques. Reproduced from ref. 69 with permission from Elsevier, Copyright 2016.

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In addition, one application example that was noted by Bailera and co-workers69  in their extensive 2017 review of laboratory/pilot/demonstrable plants for storing renewable energy through the transformation of carbon dioxide is the 6-MW Audi e-gas plant, which is currently converting biomass-derived CO2 to CH4 at an efficiency of 54% with an expected production of ca. 1000 tons per year. The process is based on the catalytic methanation of pure hydrogen and carbon dioxide carried out in a single isothermal fixed-bed reactor; the hydrogen originated from 3 × 2.0 MWe alkaline electrolyzers powered by an offshore wind park in the North Sea, and equipped with a 4 × 3.6 MWe turbine, while the required CO2 was separated from the raw biogas of a neighboring biomethane plant.

As pointed out by Thema and co-workers,70  the coupling of P2F (or in their words, power-to-gas PtG process) and energy storing technology has recently received considerable attention from various areas toward: (1) an integrated future energy systems architecture; (2) focus on the technology; (3) social acceptance; (4) marketing; and (5) policy makers. These authors have described various pilot and demonstration projects because of a significant rise in interest in the PtG technology as it provides an opportunity to convert electricity into chemical energy that can be stored as a combustible gas such as hydrogen and methane. Hydrogen is used either directly or otherwise fed into a downstream methanation process depending on the requirements of the embedding energy system such as hydrogen tolerance of gas networks, gas buffering, mobility and/or heat applications.70 

Their 2019 research paper70  gave an exhaustive overview of the active power-to-gas projects for the production of hydrogen or else a renewable substitute natural gas with a particular focus on 153 projects in 22 countries either completed, recent or planned since 1988 in central Europe. The projects were discussed on the basis of plant allocation and size, installed power development, shares, and yields of hydrogen or otherwise substitute natural gas production and product utilization phases. Projects showed various applications in diverse fields, from early research and development level to the adoption of pilot and industrial scale power conversion plants. As well, most projects that were examined in some details were located in Germany, Denmark, the USA, and Canada with the larger number of projects located in Germany and Denmark (see Figure 1.11). Figure 1.12 shows the projects examined on a global scale. Also analyzed were the development costs for electrolysis and carbon dioxide methanation with a projection to 2030 and with an outlook toward the magic year 2050.70  Results of their analyses showed substantial reduction of the costs for electrolysis and methanation, with a further price decline to less than 500 Euros per KW electric power input for both technologies to 2050, with current projects in 2019 operating at 39 MW.

Figure 1.11

PtG project allocation and number of active/inactive (light grey) methane projects and hydrogen projects (black) in various countries. Reproduced from ref. 70, https://doi.org/10.1016/j.rser.2019.06.030, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Figure 1.11

PtG project allocation and number of active/inactive (light grey) methane projects and hydrogen projects (black) in various countries. Reproduced from ref. 70, https://doi.org/10.1016/j.rser.2019.06.030, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Close modal
Figure 1.12

PtG project allocation differentiated according to the target products hydrogen and methane as well as activity/inactivity status. Dark green: active PtG with biological CO2-methanation; light green: inactive PtG with biological CO2-methanation; red: active PtG with chemical CO2-methanation; orange: inactive PtG with chemical CO2-methanation; dark blue: active PtG without methanation; light blue: inactive PtG without methanation. Reproduced from ref. 70, https://doi.org/10.1016/j.rser.2019.06.030, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Figure 1.12

PtG project allocation differentiated according to the target products hydrogen and methane as well as activity/inactivity status. Dark green: active PtG with biological CO2-methanation; light green: inactive PtG with biological CO2-methanation; red: active PtG with chemical CO2-methanation; orange: inactive PtG with chemical CO2-methanation; dark blue: active PtG without methanation; light blue: inactive PtG without methanation. Reproduced from ref. 70, https://doi.org/10.1016/j.rser.2019.06.030, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Close modal

Along similar lines, Golling and co-workers71  proposed a roadmap for Germany on the implementation of the PtG technology in which they showed a pathway toward short-term reductions in greenhouse gas emissions of ca. 40% until 2020, ca. 55% by 2030 and from 80 to 95% in the long term. Such a proposal considered international CO2-trade, implementation of the hydrogen infrastructure, the intersectoral planning of infrastructure, and the presence of export markets for the PtG technology. The global focus of research and application of the PtG technology rests mostly in Europe, although the USA appears to be catching up.

Table 1.2 summarizes the status of active projects as of three years ago (2019) with regard to conversion efficiencies from PtG in projects that produce hydrogen and methane, as well as the number of plants that feed in their products; also shown are the electrical load of methanation projects related to the electrolysis power needed to feed the methanation unit.70 

Table 1.2

Summary of the status of active projects as of three years ago (2019)70 

Hydrogen productionMethane production
Feed-in projects 21 36 
Number of active projects (2019) 56 38 
Installed production capacity 6205 m3 h−1 590 m3 h−1 
18.6 MWch,LHV-H2 6 MWch,LHV-CH4 
Installed electrical load 24.1 MWel 14.5 MWel 
Efficiency electricity-to-gas 77% 41% 
Hydrogen productionMethane production
Feed-in projects 21 36 
Number of active projects (2019) 56 38 
Installed production capacity 6205 m3 h−1 590 m3 h−1 
18.6 MWch,LHV-H2 6 MWch,LHV-CH4 
Installed electrical load 24.1 MWel 14.5 MWel 
Efficiency electricity-to-gas 77% 41% 

This chapter has described, albeit not in great detail, the production of solar fuels (hydrogen and methane) via the water-splitting process and the methanation of carbon dioxide through the use of three essential technologies: (1) photocatalysis involving photoactivated semiconductor photocatalysts; (2) photoelectrocatalysis also involving semiconductor materials; and (3) photovoltaic-electrocatalysis to drive the formation of solar fuels with particular attention to the Power-to-X technology. It is evident that much remains to be done in these technologies to diminish the use of non-renewable resources (e.g., fossil fuels) toward sustaining a clean and non-polluting energy supply for years to come.

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