On the Limits of Photocatalytic Water Splitting

Bahar Ipek and Deniz Uner

## Abstract

The major drawbacks on the limited H2 and O2 evolution activities of one-step photocatalytic water splitting systems are given here with the emphasis on charge recombination, back-oxidation reactions, and mass transfer limitations. Suppression of these unwanted phenomena is shown to be possible with the usage of small crystal-sized photocatalysts with low defect concentrations, presence of phase junctions, selection of co-catalyst that would be active for H2 evolution but inactive for O2 reduction, coating of the co-catalyst or the whole photocatalyst with selectively permeable nanolayers, and usage of photocatalytic systems with high solid–liquid and liquid–gas surface areas. The mass transfer limitations are shown to be important especially in the liquid–gas interfaces for agitated and suspended systems with estimated H2 transfer rates in the range of 200–8000 μmol/h.

Keywords: hydrogen production, photocatalyst, water splitting, mass transfer, back-oxidation

#### 1. Introduction

Hydrogen gas is one of the best alternatives to fossil fuels since it has a high gravimetric energy density (142 MJ/kg) and it produces zero carbon upon combustion. Hydrogen is also used as a major reactant in environmentally important reactions such as carbon dioxide hydrogenation to methanol [1] or ammonia production (Haber-Bosch reaction) [2]. For hydrogen to be used as a clean energy source, its production via renewable ways is of great importance. It is conventionally produced via steam reforming of methane and fossil fuels (energy intensive, ΔH<sup>0</sup> rxn = 206 kJ/mol, 700–1100°C [3]) and coal gasification, which results in significant amounts of carbon dioxide production. The renewable ways for carbon-free production include biological sources (microalgae and cyanobacteria) and electrolysis of water using wind energy and photovoltaic cells as electricity generation sources. In addition to the mentioned renewable ways, photocatalytic water splitting/oxidation is a promising alternative, in which solar energy is used as the driving force to split water molecules to hydrogen and oxygen on the surface of a catalyst. This renewable production method of hydrogen is advantageous over other renewable methods due to the free source of energy and lower cost of the photocatalysts when compared to that of photovoltaic cells or wind turbines. Solar-driven catalytic (photocatalytic) reactions are considered to be of fundamental importance to the catalysis community since the solar energy is inexhaustible; i.e., the solar energy absorbed by the lands and oceans on an hourly basis (432 EJ/h

or 120,000 TW [4]) is comparable to the Earth's yearly energy consumption (reaching 575 EJ/year or 18 TW in 2017). However, the solar-to-hydrogen energy conversion efficiency value for photocatalytic water splitting systems is much lower (targeted to be 10%, currently reaching 1% [5]) than that of photovoltaic-assisted electrolysis (reaching 30% [6]) due to the major drawbacks in the one-step photocatalytic water splitting systems. Herein, we firstly introduce photocatalytic water splitting systems and give the major developments in materials such as visible light utilization and corresponding H2 and O2 production activity values (in Section 2). Then in Section 3, we discuss the causes of the low efficiencies in photocatalytic water splitting systems and the recent approaches in preventing energy efficiency-lowering factors such as inefficient visible light utilization, charge recombination, back-oxidation reactions, and mass transfer limitations.

the conduction band energy level should be located at a more negative potential than 0 V vs. NHE, at pH = 0 for H2 evolution (Eq. (1)), and the maximum of the valence band should be at a more positive potential than 1.23 V vs. NHE at pH = 0

Following the light absorption, photoexcited electrons are transferred to the conduction band, while a positively charged charge carrier (hole) is generated at the valence band. These charge carriers are then transferred to the catalyst surface

recombine in the bulk or on the surface (step 4). Ultimately, electrons and holes

TiO2, having a large bandgap (anatase: 3.2 eV), is the most commonly used photocatalyst due to its photostability, nontoxicity, and high activity (upon UV radiation λ < 387 nm). Following the report on water oxidation reaction [7], various photochemical reaction activities of TiO2 such as carbon dioxide reduction with H2O [17–19], alkene and alkyne hydrogenation [20, 21], CH3Cl oxidation [22], 1 octanol degradation [23], phenol degradation [24], surfactant degradation [25], and

As photostable and active TiO2 is, UV light requirement to activate the large bandgap of TiO2 motivated research for visible light active semiconductors as well as bandgap engineering for TiO2 such as nonmetal ion doping (N [29], C [30], F [31], S [32]). Substitution of lattice oxygen atoms by these anions is reported to shift

Similar to TiO2, oxides of other transition metals with d<sup>0</sup> (such as Ti4+, Zr4+, Nb5+, Ta5+, and W6+ [34, 35]) and d<sup>10</sup> electronic configurations (such as Ga3+, In3+, Ge4+, Sn4+, and Sb5 [36–38]) are shown to possess large bandgap energies (>3 eV) due to the maximum valence band levels consisting O2p orbitals located near 3 V (vs. NHE at pH = 0). These d<sup>0</sup> and d10 metal oxide catalysts are reported to show remarkable one-step photocatalytic water splitting activity under UV light irradiation [39] reaching 71% quantum yield with photocatalysts such as Al-doped SrTiO3 [40] or Zn-doped Ga2O3 [41]. The H2 and O2 evolution activity under UV radiation and the apparent quantum yields of some of these materials are given in Table 1. The apparent quantum yield is defined as the number of reacted electrons and holes divided by the number of incident photons on the photocatalysts. Table 1 is not intended to cover the whole range of particulate catalysts in literature but rather to give a selection of examples. A wider selection of d0 and d<sup>10</sup> metal oxide particulate catalysts' one-step water oxidation activity and apparent quantum yields can be found in the works of Kudo et al., Chen et al., and Domen et al. [39, 42, 43].

The most remarkable upgrades in the apparent quantum yields are achieved by material engineering such as (i) doping the metal oxides/perovskites with cations having lower valences, (ii) decreasing the crystal sizes to submicron levels, and

(step 2 in Figure 1) to be utilized in surface redox reactions, unless they

reduce/oxidize the adsorbed species on the catalyst surface (step 3), the products of which should then be desorbed from the surface to complete the

more have been reported. Detailed reviews on TiO2-based materials and

the valence band level upward and narrow the bandgap to as low as 2.25 eV

photocatalytic performances can be found in literature [26–28].

(�550 nm) with 16.5% N doping [33].

(iii) loading with H2/O2 evolution co-catalysts.

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4H<sup>þ</sup> þ 4e� ! 2H2 (1) 2H2O þ 4 h<sup>þ</sup> ! 4H<sup>þ</sup> þ O2 (2)

for oxygen evolution reaction (Eq. (2)):

On the Limits of Photocatalytic Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.89235

overall process.

2.1 Semiconductors
