Photoelectrochemical Water Splitting

Under standard conditions, the change in free energy (ΔG) associated with the conversion of one molecule of H2O into H2 and ½ O2 is 237.2 kJ/mol, which corresponds to an electrolysis cell voltage (ΔE°) of 1.23 V per electron transferred.

As shown in Figure 1-1, photoelectrochemical water splitting involves two electrodes: the anode and the cathode. The anode is an electrode based on a photoactive material or a semiconducting material. A light beam irradiates the anode during water splitting. The cathode is a counter-electrode (also called a photocathode), although it is not irradiated by light. To drive this reaction, the layer of the photoactive material on the photoanode must absorb radiant light to make

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its electrode potential higher than 1.23 V. Thus, the water molecule that can be oxidized to form O2 and proton (H⁺) can be simultaneously reduced to form H2 at the cathode. If the photoanode is irradiated by light that has an energy greater than the bandgap (EBG) of the photoactive material, then the electrons of the valence band will be excited into the conduction band (CB) while the holes remain in the valence band. These photogenerated electrons will then pass through the external load and reach the surface of the cathode to react with protons, generating H2. In addition, the holes at the photoanode will diffuse to the surface of the photoanode and oxidize the H2O to produce O2.

Figure 1-1. Sketch diagram for basic principles of water splitting on photoelectrochemical cells with an n-type semiconductor photoanode, where oxygen is evolved, and a photocathode (Pt sheet), where hydrogen is evolved.^[8]

Semiconductor electrodes used in photoelectrochemical cells must satisfy several criteria. First, as a result of the existence of kinetic sluggishness for driving

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the reactions of water splitting, energy is lost during the transfer of electrons at the photoanode/electrolyte interface. The energy required for photoelectrolysis is frequently given as at least 1.7 eV to provide the potential necessary for electrolysis and to overcome other energy losses in the system. Based on electrochemical principles, the water-splitting reaction can only be driven when the irradiation energy exceeds 1.23 eV (~1,000 nm), indicating that the energy of the light must be larger than the bandgap to separate the electrons and holes. In practical operation, the minimum energy requirements plus the thermodynamic/overpotential loss must be at least 1.7 eV to 1.9 eV for photoelectrochemical water splitting, which corresponds to an onset of light absorption at a wavelength of 730 nm.^[9,10]

In addition to the bandgap requirement, a second factor that commonly affects the use of water splitting is the energy band edge of the semiconductor. In water splitting, the bottom level of the CB must be located at a more negative potential than the reduction potential of H⁺/H2. The top of the valence band must also be more positive than the oxidation potential of H2O/O2. With respect to the fundamental requirements of conduction and valence bands, the band edge positions must straddle the hydrogen and oxygen redox potentials. Figure 1-2 illustrates the different semiconductors and their corresponding potential band edges.^[5] Some semiconductor materials can reduce but not oxidize water. However, several metal-oxide materials, such as Fe2O3, can oxidize but not reduce protons.

For the majority of such materials, an external bias is essential to reduce protons and assist in the measurement of photocurrents in the photoelectrochemical cell;

thus, the onset potential of the photoresponse from the I–V curve always shifts to a higher potential region.^[11]

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Figure 1-2. Illustration of the band edge potentials of different semiconductors.^[5]

Another critical factor is electrochemical stability or resistance to photocorrosion, which may limit the usefulness of several photocatalytic materials.

Most metal-oxide semiconductor materials are thermodynamically unstable. Thus, the photogenerated holes may oxidize themselves rather than water (photocorrosion and/or anodic photodecomposition). These undesired photodecompositions in various photoactive materials commonly depend on the pH value of electrolytes and frequently limit their utilization in certain conditions.

TiO2 and SnO2 are highly stable over a wide range of pH values in aqueous environments upon illumination. The stability of hematite strongly depends on the presence of dopants, pH values, and oxygen stoichiometry. Many non-oxide semiconductor materials may either dissolve or form a thin oxide film on their surface, preventing the transfer of electrons through the interface between semiconductor/electrolyte interfaces. Photocorrosion or anodic decomposition is

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expected to be markedly inhibited if the transfer of carriers for water oxidization through the interface is faster than a competing reaction. Thus, the development of semiconductor materials with excellent stability against photocorrosion/anodic decomposition becomes a critical issue for future applications. To prevent serious problems, scientists add redox couples into the photoelectrochemical cell to scavenge the photoexcited electron/hole pairs. This technique improves the charge separation and controls the desired product. The redox couples are also called sacrificial reagents.^[7] The mechanism of the sacrificial reagent is shown in Figure 1-3. The sacrificial reagents are used in the following conditions: (1) The CB of the semiconductor is more positive than the hydrogen redox potential, such as for Fe2O3 and WO3. The oxidizing reagents (electron acceptor), such as S^2- and SO32-, are added to the water-splitting system to scavenge the photoexcited electrons, which facilitate the diffusion of the photoexcited hole to the surface of the semiconductor and, thus, the generation of oxygen gas. (2) The valence band of the semiconductor is more negative than the oxygen redox potential, such as for silicon and InP. Reducing reagents (electron acceptor), such as Ag⁺ and Fe³⁺, are added to the water-splitting system to scavenge the photoexcited holes, which facilitate the migration of the photoexcited electrons to the surface of the semiconductor and, thus, the generation of hydrogen gas. (3) To increase the charge separation efficiency of the semiconductor materials. (4) To investigate the activity or reaction condition of the specific reaction, such as hydrogen generation.

The sacrificial reagent facilitates the electron/hole transfer. Thus, the generated photocurrent does not represent the activity of the materials, and the photocurrent cannot be used when calculating efficiency. Moreover, the reaction of the sacrificial reagent consumes the reducing/oxiding reagent. This consumption is not reversible, and the reagent cannot be recycled. As a result,

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sacrificial reagents cannot be used in practical applications.

Figure 1-3. Mechanism of the sacrificial reagent.

1.2.1 Photoelectrochemical Activity

The bandgap energy requirements of the semiconductors for photoelectrochemical water splitting must be at least 1.7 eV to 1.9 eV, which corresponds to an onset of light absorption at a wavelength of 730 nm. The intensity of the solar spectrum also dramatically falls below 350 nm, resulting in an upper limit on the bandgap of approximately 3.5 eV. For this reason, the desired optimum value of the semiconductor bandgap should be between 1.9 and 3.5 eV, which is within the viable range of the solar spectrum. In practical cases, the flux of solar photons in the wavelength range of 680 nm to 280 nm (1.8 eV–4.4 eV) represents 27.5% of the total solar photon flux, which is the maximum efficiency predicted in various investigations based on the bandgap of the semiconductor material, the solar spectrum, and various losses.^[12] However, the efficiency in

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energy conversion using a single bandgap material is too low to satisfy the requirements of actual application, even if a perfect photocatalytic material can be developed. As a result, a more efficient configuration has to be developed.

Some important factors other than bandgap energy must be considered to improve water-splitting reaction, including charge separation, charge mobility, and the lifetime of photogenerated electron–hole pairs. These factors also critically affect the photoactivity of semiconductor materials. The structural and electronic properties of co-catalysts on the surface of photoelectrodes also strongly affect the generation and separation of electron–hole pairs. In the general case, highly crystalline materials with a low density of defects are beneficial for water-splitting reaction because defects may serve as recombination centers for photogenerated electrons/holes.