Perspectives on the Development in the Selective Oxidation of Glycerol to Value-Added Chemicals via Photoelectrocatalysis Coupled with Hydrogen Evolution

Glycerol oxidation reaction involves complex multielectron transfer processes across multiple reaction pathways, leading to a diverse and intricate product distribution and a low selectivity for targeted GOR products. Selective PEC glycerol oxidation aims to selectively oxidize glycerol into value-added liquid products while simultaneously generating high purity and large quantities of H₂, which relies not only on the catalyst nature but also on how the reaction is conducted. Great efforts such as elemental doping, interface engineering, and surface modification have been made to improve the product selectivity and yield in GOR from various perspectives, such as enhancing adsorption properties, improving electron conduction properties, adjusting the energy band structure, and increasing light absorption [14,21,22,23,24]. As displayed in Figure 4, the catalyst nature and the reaction conditions, such as the concentration of reactants, the type and pH of electrolyte used, and the mass-transfer conditions, can synergistically influence the overall performance of glycerol oxidation [25,26]. In the following, we will introduce the recent progress in view of the product types, considering these factors mainly focusing on the most potential BiVO₄-based, TiO₂-based, and WO₃-based photoanodes. 3.1. BiVO₄-Based Photoanodes Among various semiconductors, bismuth-based photoanodes are considered one of the most potential photoanode materials for selective PEC oxidation of glycerol. Table 1 shows the representative BiVO₄-based photoanodes for the PEC glycerol oxidation. The GOR products based on single BiVO₄ photoanodes can be dominated either by FA or by DHA. The specific dominant product depends on the type of electrolyte solution and its pH value, as well as the electronic structure of the catalyst surface. Huang et al. reported GOR over BiVO₄ photoanode with glycerol being oxidized to valuable products of 15% dihydroxyacetone (DHA) and 85% formic acid in a mixed electrolyte solution composed of 0.1 M Na₂B₄O₇ (pH = 9.4) and 0.1 M glycerol under 1 sun illumination [15]. Liu et al. [32] investigated the effect of electrolyte pH on the selectivity and production rate of PEC GOR over porous BiVO₄ nanoarrays (Figure 5a,b). The selectivity and production rate of DHA reaches ~50% and 200 mmol·m⁻²·h⁻¹ at 1.2 V vs. RHE at pH = 2 from ~ 30% with pH = 5 and 7, respectively. The results demonstrate that lower pH enables easier glycerol adsorption on BiVO₄, which results in an easier charge transfer and enhanced catalysis to PEC glycerol conversion when reaction pH is decreased. Besides, an acidic environment can also suppress the reaction towards acid products, which leads to a high selectivity of DHA. Further, Cui et al. [26] investigated the effect of electrolyte type on the performance of PEC glycerol oxidation over BiVO₄ photoanode. It is found that the photocurrent density in H₂SO₄ is obviously higher than that in HNO₃. The total production rate increased with applied potential, and the yield in HNO₃ consistently exceeded that in H₂SO₄. At 1.4 V vs. RHE, the production rate of FA rose from 305.75 mmol·m⁻²·h⁻¹ in H₂SO₄ to 436.45 mmol·m⁻²·h⁻¹ in HNO₃, indicating that NO₃⁻ anions promote FA production. Additionally, a high selectivity for FA of 78.1% was achieved in HNO₃. The results highlight the significant influence of anions on the PEC performance. After deposition of FeOOH cocatalyst, highly selective conversion of glycerol into valuable GLD is obtained over BiVO₄/FeOOH with a thin amorphous layer of FeOOH onto the surface of BiVO₄. The GLD production rate reached 709 mmol·m⁻²·h⁻¹ with a selectivity of 63.3% using a Na₂SO₄ solution under AM1.5G (100 mW·m⁻²) light intensity at 1.23 V vs. RHE [33]. It is reported that the bismuth (Bi) atom exhibits a heightened electrostatic adsorption capacity for the secondary hydroxyl of glycerol [27]. Lu et al. [34] synthesized Bi-rich BiVO_4−x and investigated its PEC GOR performance (Figure 6a,b). The results demonstrate that the photocurrent density of Bi-rich BiVO_4−x photoanode in GOR increases from 1.42 to 4.26 mA·cm⁻² at 1.23 V vs. RHE under AM 1.5 G illumination, accompanied by a selectivity increase of DHA product from 54% to 80.3%, finally achieving a DHA conversion of 361.9 mmol·m⁻²·h⁻¹ (Figure 6c). The high selectivity for DHA can be ascribed to the Bi-rich surface, which is favorable for secondary hydroxyl adsorption for glycerol (Figure 6d). The high conversion rate is due to the additionally introduced oxygen vacancies (Ov), which enhance the interaction frequency of Bi atoms at the interface, leading to an improvement in surface charge transport efficiency. On the other hand, Han et al. [28] reported that the BiVO₄ photoanode modified by phenolic ligands (tannic acid, TA) and Ni²⁺/Fe²⁺ ions (denoted TANF) (Figure 7a–i) shows a high production rate of 573 mmol·m⁻²·h⁻¹ toward FA with high selectivity of 85% in a 0.1 M Na₂SO₄/H₂SO₄ electrolyte (pH = 2) (Figure 7j–k). Therefore, we can conclude that the surface structure, the pH, the type of electrolyte, and the electronic structure of the catalyst surface can significantly influence the PEC GOR performance of BiVO₄-based photoanodes. 3.2. TiO₂-Based Photoanodes TiO₂ is also one of the prominent semiconductors frequently explored in PEC energy conversions. Table 2 shows the representative TiO₂-based photoanodes for the PEC glycerol oxidation. Similarly, the GOR products based on single TiO₂ photoanodes can be dominated either by FA or by DHA. The specific dominant product also depends on the type of electrolyte solution and its pH value, as well as the electronic structure of the catalyst surface. Lu et al. [8] investigated the microscale fluid effect on the selectivity of the GOR products over TiO₂ nanotube photoanode. They provided an alternative way to regulate the intrinsic selectivity of TiO₂ photoanode by altering the microenvironment (Figure 8). It is found that the action of microfluidics can effectively inhibit the overoxidation of glycerol due to the rapid mass transfer, which enables the quick desorption and discharge of multiple carbon products (Figure 8a). The TiO₂ nanotubes with a diameter of 50 nm show a great selectivity of 60.9% for GLD (Figure 8b,c). As demonstrated above, the surface structure plays a crucial role in determining the selectivity of GOR. Therefore, introducing cocatalysts is expected to alter the types of GOR products formed. Duan et al. [13] designed a heterogeneous photoanode of Bi₂O₃ nanoparticles on TiO₂ nanorod arrays (Bi₂O₃/TiO₂) (Figure 9a–c). The selectivity of DHA can be maintained at ∼65% under a relatively high conversion of glycerol (∼50%) (Figure 9d–f). In a self-powered PEC system, a DHA productivity of 1.04 mg·cm⁻²·h⁻¹ with >70% selectivity and a H₂ productivity of 0.32 mL·cm⁻²·h⁻¹ were achieved. The authors attribute the enhancement to the existing p−n junction between Bi₂O₃ and TiO₂, which promotes charge transfer and thus guarantees high photocurrent density, and the introduced Bi₂O₃, which prefers to interact with the middle hydroxyl of glycerol, facilitating the high selectivity of DHA. Li et al. [23] investigated the effect of loading of WO₃ nanosheets on TiO₂ nanorod arrays on the PEC glycerol conversion performance. The optimized composite photoanode achieved a selectivity of 61% for GLD and 31% for DHA at 1.23 V vs. RHE in 0.1 M Na₂SO₄ and 0.05 M glycerol (pH = 7) with formation rates of 319 and 160 mmol·m^-2·h⁻¹, respectively. The results demonstrate that single TiO₂ tends to over-oxidize glycerol into lower-value products, and the selectivity of WO₃/TiO₂ towards higher-value products (GLD and DHA) is significantly enhanced after loading of WO₃ due to its good adsorption of glycerol and desorption of GLD and DHA. In addition, the selectivity for GLD and DHA remains almost unchanged over a 10 h period, suggesting good stability. Liu et al. [3] reported the Ag nanoparticle-supported layered double hydroxide (LDH) nanosheets on TiO₂ (denoted Ag@LDH@TiO₂) for the glycerol selective oxidation to DHA (Figure 10a). The Ag@LDH@TiO₂ photoanode exhibited a DHA selectivity of 72% with a production rate of ~310 mmol·m⁻²·h⁻¹ at 1.2 V vs. RHE, which is obviously higher than that of pure TiO₂ (23.5%, less than 200 mmol·m⁻²·h⁻¹) (Figure 10b,c). This is due to the fact that the LDHs and Ag nanoparticles both enhanced the selective adsorption of secondary hydroxyl of glycerol, leading to its selective oxidation to DHA. Though the selectivity of TiO₂ can be significantly modified by introducing cocatalysts, its PEC activity still falls short of meeting the requirements for practical applications. CdS is a typical semiconductor with a narrow band gap and high absorption coefficient, which has been widely investigated in PEC water splitting by constructing TiO₂/CdS heterojunction. However, the hole-induced photocorrosion of CdS brings big trouble due to poor stability. Jiang et al. [37] investigated the TiO₂/CdS heterojunction photoanode in the application of GOR (Figure 11a,b). As shown in Figure 11c, the photocurrent density of the optimized TiO₂/CdS photoanode is 18.8 mA cm⁻² (1.23 V vs. RHE), which is about 10.6 times higher than that of the pristine TiO₂. The TiO₂/CdS photoanode shows a high selectivity of 49.84% for FA with a production rate ~1574.5 mmol·m⁻²·h⁻¹. After loading IrO₂ nanoparticles, the selectivity for FA is enhanced to 53.22%, with the production rate reaching 2345.2 mmol·cm⁻²·h⁻¹ due to the improved stability (Figure 11d). This is the highest production rate ever reported. The enhanced PEC performance and stability of the TiO₂/CdS/IrO₂ photoanode can be ascribed to the greatly enhanced electrode/electrolyte interfacial carrier injection efficiency caused by the fast glycerol oxidation dynamics and intimate contact. 3.3. WO₃-Based Photoanodes WO₃ stands out as another prominent semiconductor frequently explored in PEC glycerol oxidation for C3 products. Table 3 shows the representative WO₃-based photoanodes for the PEC glycerol oxidation. For example, Yu et al. [41] reported the PEC behavior of monoclinic WO₃ in oxidizing glycerol to higher-value C3 products, such as DHA and glyceraldehyde. The monoclinic WO₃ showed a high and stable selectivity for glyceraldehyde and DHA, with selectivity between 87 and 95% within a potential range of 0–1.5 V vs. Ag/AgCl under ambient conditions and near-neutral pH, outperforming those of commercially available WO₃/TiO₂ (DTW5) catalyst. The high selectivity of the WO₃ photoanode, as well as DTW5 for C3 compounds and, specifically, for glyceraldehyde, is likely influenced by the strong acidity of WO₃, which can enhance the desorption of these products from the catalyst surface, as previously reported on glycerol oxidation and dehydrogenation reactions. Besides, the high selectivity towards primary C3 products of WO₃ is also linked to its high activity for the PEC oxygen evolution, which could promote the direct glycerol oxidation mechanism. It is reported that the exposed facets determine the surface structure of a photocatalyst and, therefore its physicochemical properties and photocatalytic activities [42]. Xiao et al. [38] reported the selective PEC glycerol oxidation to glyceric acid (GA) on (002) facets exposed WO₃ nanosheets in 0.5 M H₂SO₄ solution (Figure 12a,b). As shown in Figure 12c,d, GA is the primary product under all applied potentials with the selectivity approaching 75% on WO₃ nanosheet photoanode with preferentially exposed (002) facets (002-W) at 0.7 V_RHE, and 70% on WO₃ nanorod photoanode with preferentially exposed (200) facets (200-W) at 1.0 V_RHE. The 002-W shows higher PEC oxidation activity than the commonly studied 200-W, achieving GA production rate of 32.3 mmol·m⁻²·h⁻¹ at 0.9 V_RHE under AM 1.5G illumination. The total selectivity of C3 products (GA + DHA) reaches as high as 96% over 002-W. It is disclosed that the abundant surface states on (002) facets of WO₃ can dramatically enhance charge separation and increase the carrier lifetime. Moreover, the 002-W exhibits a lower energy barrier for glycerol oxidation to GA as compared to 200-W [43]. The comparison between the {202} facets and the commonly studied {200} facets of WO₃ was also investigated (Figure 13a,b) [13]. As shown in Figure 13c–e, the WO₃ {202} photoanode displays a high 80% selectivity to glyceraldehyde (GLD) with a production rate of 462 mmol·m⁻²·h⁻¹, about 2.7 times higher than that of WO₃ {200} photoanode (173 mmol·m⁻²·h⁻¹) with a selectivity of 71%. The enhanced PEC performance for GOR can be attributed to the fact that the WO₃ predominated with the {202} facet has a lower band gap, higher carrier concentration, and facilitated interface charge transfer compared with the WO₃ photoanode predominated with the {200} facet. Furthermore, the {202} crystal facet is beneficial to the adsorption and activation of glycerol via the terminal hydroxyl groups as well as the desorption of glyceraldehyde. Feng et al. [40] investigated the effect of Pt single atom (SA) doping and oxygen vacancies on the PEC glycerol oxidation performance of WO₃ photoanode (Figure 14a). The results demonstrate that the coupled Pt-SA/WO_x photoanode has a highest photocurrent density of 2.85 mA·cm⁻² at 1.2 V versus RHE with a DHA selectivity of 60.2%, higher than that of WO₃ (48.6%) and WO_x with rich oxygen vacancies (55.5%) (Figure 14b,c). The enhanced PEC activity and DHA selectivity are ascribed to the regulated band structure, built-in electric field, and surface charge density of Pt-SA/WO_x. As mentioned above, the semiconductor surface chemical structures play a key role in the inherent thermodynamic adsorption of glycerol [24,44,45]. Consequently, even minor changes in the surface structure can significantly affect the adsorption behavior of semiconductors to the electrolyte and, finally, the selectivity of the PEC upgrading anode reaction[29,46]. Therefore, facet manipulation and cocatalyst decoration are considered promising approaches for enhancing the selectivity of GOR products in the future [47,48,49].