**5. Photoelectrochemical decomposition of hydrogen sulfide**

In addition to the photocatalytic decomposition of H2S alone, sometimes photochemical method is combined with electrochemical method for the decomposition of H2S, that is, the photoelectron-chemical (PEC cell) decomposition of H2S. Actually, the colloidal semiconduc‐ tor photocatalyst system mentioned above could also be seen as some kind of short-circuit PEC cell, in which both anodic and cathodic reaction occurs on the surface of semiconductors at the same time (similar to the "photochemical diodes" developed by Nozik [30]). In this section, traditional PEC cells (with separated anode and cathode connected by wires) would be mainly focused. These cells could not only decompose hydrogen sulfide but also generate electricity. In addition, voltage bias could be applied to the cells if the drive force of light is not enough for hydrogen sulfide decomposition.

In 1987, Kainthla and Bockris reported a PEC cell for the decomposition of H2S based on CdSe anode and Pt cathode [55]. CdSe film was directly grown on Ti substrate. Using polysulfide (prepared by H2S dissolution in NaOH and subsequent addition of sulfur) as the electrolyte, an open circuit voltage of 0.62 V and short circuit current of 8.82 mA cm–2 could be achieved. H2 bubbles could be observed to leave the Pt cathode when photocurrent flows through the cell and a Faraday efficiency of 0.97 is calculated. With the gradual accumulation of polysufide during the reaction, elemental sulfur would precipitates from the solution when polysulfide reaches its solubility limit. Stability of the cell is also tested and short circuit drops less than 10% with continuous illumination of 2 weeks. The total cell conversion efficiency (*ε*) given as the ratio of the recoverable energy to the input energy is calculated based on Eq. (26):

(which competes not only with catalyst from light absorption but also with protons for reduction by electrons), but the acidity necessary for the release of S from the obtained thiosulfate would greatly reduce the photocatalytic activity of the catalysts. In their study, Linkous pointed out that if the depth of reaction solution in photoreactor is less than 1 cm, in order to reduce the light absorption of polysufide, S2– alone as the electron donor for photo‐ catalytic hydrogen evolution is probably more suitable for the cyclic sulfur release in a CdS/Pt involved system. Additionally, another problem of this design is that if the commonly studied suspension system is used for photoreaction, photocatalyst could not be easily separated with the solution. Therefore, catalyst may need to be immobilized for circulating.

**Figure 4.** Generalized scheme for light-driven H2S decomposition using an immobilized photocatalyst. Reprinted from

In addition to the photocatalytic decomposition of H2S alone, sometimes photochemical method is combined with electrochemical method for the decomposition of H2S, that is, the photoelectron-chemical (PEC cell) decomposition of H2S. Actually, the colloidal semiconduc‐ tor photocatalyst system mentioned above could also be seen as some kind of short-circuit PEC cell, in which both anodic and cathodic reaction occurs on the surface of semiconductors at the same time (similar to the "photochemical diodes" developed by Nozik [30]). In this section, traditional PEC cells (with separated anode and cathode connected by wires) would be mainly focused. These cells could not only decompose hydrogen sulfide but also generate electricity. In addition, voltage bias could be applied to the cells if the drive force of light is not enough

In 1987, Kainthla and Bockris reported a PEC cell for the decomposition of H2S based on CdSe anode and Pt cathode [55]. CdSe film was directly grown on Ti substrate. Using polysulfide (prepared by H2S dissolution in NaOH and subsequent addition of sulfur) as the electrolyte,

**5. Photoelectrochemical decomposition of hydrogen sulfide**

reference [39], Copyright (1995), with permission from Elsevier.

284 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

for hydrogen sulfide decomposition.

$$
\varepsilon(\%) = (0.171I + IV\_{\text{cell}}) \times 100 \, / \, W\_{\text{light}} \tag{26}
$$

where 0.171*I* is the chemical energy storage for H2S decomposition into H2 and S, *IV*cell is the electrical energy generated from the cell, and *W*light is the intensity of light. Maximum light to chemical energy storage, light to electrical energy, and total cell conversion efficiency occurs at cell voltage of 0, 0.3, and 0.275 V, with the corresponding efficiency to be 1.5%, 1.8%, and 2.85%, respectively. In this regard, the PEC cell can be operated in a manner that electrical energy or chemical energy can be selectively collected.

It is noteworthy that for eliminating the competition of polysulfide with proton for reduction in this cell, which is also a big problem in suspension systems, anode and cathode are placed in two compartments, and Nafion membrane is used to prevent the contact of polysulfide with cathode. If there is no Nafion membrane, only polysulfide is reduced into sulfide, and no H2 could be detected from the system. Under this circumstance, no net chemical reaction happens in the cell, and light energy could only be converted into electrical energy.

Another advantage for PEC cells is that some strategies for the electrochemical decomposition of H2S could be extended to PEC cell. One strategy is the indirect decomposition of H2S with the assistance of redox couple like I– /I3 – (or I– /IO3 – ) and Fe3+/Fe2+, in which the electrical energy or solar energy is first stored in the redox intermediate species, and then the intermediate could drive the following chemical reactions. Although indirect strategy may consume more additional energy for H2S decomposition from a thermodynamical point of view, it is kineti‐ cally more favored and is beneficial for the extraction of elemental sulfur from the system.

Lately, Li and Wang et al. have adopted this strategy in PEC cells for H2S decomposition and achieved good results. PEC cell with p-type Si deposited with protective TiO2/Ti n+ doping layer and H2 evolution cocatalyst Pt (Pt/TiO2/Ti/n+p-Si) as the photocathode and Pt plates as anode was reported for the decomposition of H2S [56]. In a two-compartment cell separated by Nafion membrane, freshly prepared 0.2 M of FeSO4 (or KI) in 0.5 M of H2SO4 solution and 0.5 M of H2SO4 was used as the anodic and cathodic electrolyte, respectively. After H2S bubbling into the anode compartment, S and H2 could be separately produced from the anode and the cathode under light illumination at an applied potential of 0.2 V vs RHE. In this system, the chemical redox couple is significant for the conversion of H2S into H2 and S (Eqs. (27–33)):

$$\text{Photo electrode} + 2\text{ }hv \to 2\text{ }h^{+} + 2\text{ }e^{-} \tag{27}$$

$$\text{Anode:}\,\text{Fe}^{2+} + 2\,\text{h}^{\*} \to \text{Fe}^{3+} \tag{28}$$

$$2\text{ H}\_2\text{S} + 2\text{ Fe}^{3+} \rightarrow 2\text{ Fe}^{2+} + 2\text{ H}^+ + \text{S} \text{ (chemical reaction)}\tag{29}$$

$$\text{Or anode:}\ 3\ I^- + 2\ h^+ \to \ I\_3^- \tag{30}$$

$$\text{CH}\_2\text{S} + \text{I}\_3^- \rightarrow \text{3 I}^- + 2\text{ H}^+ + \text{S} \text{ (chemical reaction)}\tag{31}$$

$$\text{Cathode:}\ 2\ \text{H}^{+}\ + 2\ \text{e}^{-}\ \rightarrow \text{H}\_{2}\tag{32}$$

$$\text{Overall reaction:}\,\mathrm{H}\_{2}\mathrm{S} + 2\,\mathrm{h}\mathrm{v} \to \mathrm{H}\_{2} + \mathrm{S}\tag{33}$$

Control experiment shows that if there is no existence of Fe2+ or I– in the electrolyte, such experiment is unsuccessful due to the low solubility of H2S in acidic solution. Besides, n-type Si coated with 3,4-ethylenedioxythiophene (PEDOT) as the anode was also tested in this system, and it turns out that Fe2+ and I– could be easily oxidized on it. Nevertheless, due to the low stability of the n-type Si anode, further study in this report is unclear.

Notably, they further developed this indirect strategy in PEC cell and have made H2O2 and S from H2S in the presence of oxygen [57]. This is quite novel because most study related to H2S decomposition is limited to H2 as the only reduced product now. In addition to the redox couple I– /I3 – in the anode compartment of the cell for S production, another redox couple anthraquinone/anthrahydroquinone (AQ/H2AQ) was introduced to the cathode cell for H2O2 production. In fact, AQ is also an important reaction substrate in Hysulf process, one indirect strategy related to the thermal decomposition of H2S. The anode reaction is still the same as Eqs. (30 and 31), but the cathode reaction and the overall reaction change as follows (Eqs. (34–36)):

$$\text{Cathode:}\,2\,\text{H}^+ + \text{AQ} + 2\,\text{e}^- \rightarrow \text{H}\_2\text{AQ} \tag{34}$$

$$\text{H}\_2\text{AQ} + \text{O}\_2 \rightarrow \text{AQ} + \text{H}\_2\text{O}\_2 \text{ (chemical reaction)}\tag{35}$$

$$\text{Overall reaction:}\,\mathrm{H}\_{2}\mathrm{S} + \mathrm{O}\_{2} + 2\,\mathrm{h}\mathrm{v} \rightarrow \mathrm{H}\_{2}\mathrm{O}\_{2} + \mathrm{S}\tag{36}$$

At zero bias, Pt/p+ n Si photoanode and Pt cathode can simultaneously oxidize I– to I3 – and reduce AQ to H2AQ, respectively. Solar to chemical conversion efficiency was estimated to be 1.1%. If Pt cathode is replaced with carbon plate, a higher photocurrent could be observed.
