Generation of solar fuels using a semiconductor photocatalyst continues to be a popular topic in light–energy conversion. Semiconductor-assisted photoelectrolysis usually includes water (or H+) reduction to produce H2 or CO2 reduction to produce value-added chemicals. (1,2) Significant strides have been made in recent years to design new photocatalysts and obtain mechanistic insights into the interfacial charge-transfer processes in semiconductor particle systems. However, semiconductors such as metal chalcogenides (e.g., CdS) are susceptible to hole-induced oxidation (anodic corrosion) when employed as photocatalysts for H2 generation. Sacrificial donors such as alcohols, amines, ascorbic acid, and EDTA are commonly employed to scavenge the photogenerated holes and, thus, maintain photocatalyst stability. (3−5) The product formed in such a process (e.g., H2) is often assumed to arise exclusively from the reduction process. Because of this popular belief, the direct involvement of sacrificial donors in the net accumulation of the product often goes unchecked. In an earlier Editorial, the importance of evaluating intermediates and products of the reduction and oxidation processes and the need to present a complete story of the photocatalysis experiment were discussed. (7) Ideally, in a water-splitting process, one would expect the products to be H2 and O2. The relatively high overpotential for the electron transfer at the semiconductor–electrolyte interface requires an additional boost through the introduction of a cocatalyst or a chemical reagent. The instant we introduce additional chemicals to facilitate a charge-transfer process (e.g., a sacrificial electron donor to promote reduction or electron acceptor to promote oxidation), the product analysis and mass balance become more complex. Despite the frequent use of sacrificial donors in photocatalysis and claims of high photoconversion efficiency (even up to 100%), (8−11) the participation of a sacrificial donor in the photocatalytic process is rarely evaluated. One cannot simply ignore the fact that sacrificial donors such as methanol, SH–, ascorbic acid, and triethanolamine can generate H2 as one of the final oxidation products. Some sacrificial donors, such as methanol, are also known as current-doubling agents in photoelectrochemistry, as the oxidation intermediates (CH2OH) can additionally serve as electron donors. (12) In a semiconductor nanocrystal suspension or a slurry system, both reduction and oxidation processes occur together when subjected to bandgap excitation. Polymer membranes (e.g., Nafion) embedded with a photocatalyst (e.g., CdS) or bipolar membranes embedded with a photocatalyst and a metal electrocatalyst can assist in separating the products formed in a photocatalytic reduction and oxidation. In a recent study, we employed a bipolar membrane containing CdS and Pd to selectively oxidize chlorophenol and reduce nitrophenol under visible light irradiation. (6) This approach of separating oxidation and reduction compartments using a bipolar photocatalytic membrane has now been utilized to assess the contribution of sacrificial donors in photocatalytic H2 production. The preparation of a photocatalytically active bipolar membrane (BPM) with CdS and Pd in the anion-exchange layer (AEL) and cation-exchange layer (CEL), respectively, is described in the Supporting Information. The cell configuration shown in Figure 1 illustrates the design strategy for water electrolysis employed in this study, and it is similar to the earlier investigations. (6,13) The photocatalytic BPM separating the CEL and AEL compartments allows isolation of the products formed in the two half-cells. Prior to irradiation, both compartments are purged with N2 and sealed with a septum. Upon excitation of the CdS in the BPM-CdS-Pd membrane, the photogenerated electrons are transferred to Pd nanoparticles, which in turn assist in reducing H+ ions or H2O to generate H2 in the AEL compartment. We employed pH 4 water in the AEL compartment to carry out reduction of H+/H2O. The sacrificial donor introduced in the CEL compartment scavenged photogenerated holes from CdS. Details on the vectorial electron transfer between CdS and metal nanoparticles within the BPM are presented elsewhere. (6,13) Figure 1. Top: Photoelectrolysis H-cell with BPM-CdS-Pd membrane separating oxidation and reduction half-cells. Bottom: Cross-section illustration showing the CdS nanoparticles embedded in the CEL layer and Pd nanoparticles embedded in the AEL layer of the BPM. Oxidation and reduction reactions were induced in these two half-cells separately following visible excitation of CdS in the membrane. H2 formed in each compartment was analyzed. Methanol oxidation in the scheme represents an example of sacrificial donor oxidation. Adapted from ref (6). Copyright 2021 The Authors, published by American Chemical Society. The reactions that follow visible light excitation of BPM-CdS-Pd can be summarized as reactions 1–3.(1)(2)(3) The BPM-CdS-Pd membrane was excited from the CEL side with visible light (λ > 390 nm) for 120 min (Figure 1). We analyzed the headspace for hydrogen produced in the CEL and AEL compartments using a TRACE GC-ULTRA (Thermo Fisher Scientific) gas chromatograph. The details on the procedure and calibration are given in the Supporting Information. We introduced an aqueous solution containing individual sacrificial donors, viz., 50% methanol (pH 7), 0.1 M Na2S (pH 12.4), 0.1 M ascorbic acid (pH 2.8), 0.1 M Na2SO3 (pH 7), and 0.1 M triethanolamine (TEOA, pH 10.7) in water, into the CEL compartment. The AEL compartment contained an aqueous solution (pH 4). The pH difference between the two compartments induces a bias potential within the BPM, which in turn provides additional driving force to achieve charge separation. Since the oxidation and reduction processes can be selectively carried out in two half-cells separated by BPM-CdS-Pd, we were able to examine the amount of H2 produced by direct reduction of H+ (H2O) in the AEL compartment and oxidation of sacrificial donor in the CEL compartment. The analyses for the H2 produced in the AEL and CEL compartments for different sacrificial donors are shown in Figure 2. The data for TEOA as a sacrificial donor are presented in Figure S2 in the Supporting Information. Figure 2. H2 formation in the AEL (reduction) and CEL (oxidation) compartments during visible light (λ > 390 nm) excitation of the BPM-CdS-Pd membrane. The AEL compartment contained water (pH 4), and the CEL compartment contained different sacrificial donors dissolved in water: (A) 50% methanol, (B) 0.1 M Na2S, (C) 0.1 M ascorbic acid (C6H8O8), and (D) 0.1 M Na2SO3. H2 produced in the AEL side is due to proton reduction, whereas H2 produced in the CEL side originates from oxidation of sacrificial donors. As expected from reaction 2, the formation of H2 is seen in the AEL compartment (the reduction half-cell). We observed about 0.8 μmol of H2 in 2 h in the AEL compartment. This amount was nearly half of when Na2SO3 was used as a sacrificial donor. The rate at which holes are scavenged in the CEL compartment has a direct influence on the electron accumulation in Pd nanoparticles, which in turn dictates the rate of H2 formation in the AEL compartment. This observation further highlights how the kinetics of hole transfer from an excited semiconductor to a sacrificial donor can dictate the electron transfer to metal nanoparticles. We also observed a plateau in the H2 generation at long times, which we attribute to the depletion of H+ from the AEL compartment. When the AEL compartment was recharged with a fresh pH 4 solution, we could again see the generation of H2 with a similar trend. The H2 formation in the CEL compartment (oxidation half-cell), on the other hand, corresponds to the oxidation of the sacrificial donor as it scavenges away photogenerated holes from the CdS surface (reaction 3). It is evident from Figure 2 that the oxidation of different sacrificial donors (except Na2SO3) yields a significant amount of H2 (reactions 4–8).(4)(5)(6)(7)(8) The oxidation of Na2SO3 is the sole example in which there is no H2 production during the oxidation process (reaction 7). In all other cases, the amount of H2 generated in the CEL compartment varied between 0.3 and 0.6 μmol during 2 h of irradiation (Figure 2A–C). This variation for H2 produced during sacrificial donor oxidation shows the complexity of the multistep oxidation leading to the formation of H2. Another important point to note is the chemical identity of the dissolved sacrificial donor species. Na2S is an unusual sacrificial donor in this regard since it does not inherit hydrogen in the solid form. However, when dissolved in aqueous solution, it is converted to Na+ and SH– (and not S2–) ions. Upon dissolution of Na2S, S2– quickly gets protonated to form SH– (equilibrium 9). The existence of this equilibrium is realized from the increase in pH (12.4) resulting from the increase in OH– concentration.(9)Thus, the oxidation of SH– leads to the production of H2 (reaction 5) in the CEL compartment. Similar oxidation of H2S and SH– leading to the production of H2 has been reported earlier. (14−16) We employed ferrioxalate actinometry (17) to determine the photoconversion efficiency (quantum yield × 100) of H2 generation in the two compartments separated by the BPM-CdS-Pd membrane in the H-cell. The details of the procedure can be found in the supporting information of our previous work. (18) The photoconversion efficiency of H2 generation after 30 min of visible light irradiation is compared in Figure 3. The net efficiency for H2 generation was ∼3.0 ± 0.1% when we employed Na2S, ascorbic acid, and TEOA as the hole scavengers in the CEL compartment. Since we were able to isolate the H2 formed in the reduction and oxidation half-cells, it is possible to estimate the contribution of H2 produced from sacrificial donor oxidation. For example, the contribution of Na2S, ascorbic acid, or TEOA to the net H2 generation is >30%. When Na2SO3 was used as a sacrificial donor, the H2 generation efficiency in the CEL compartment was nearly 0%. Thus, the choice of sacrificial donor and its oxidation in photocatalysis is important in determining the photocatalytic efficiency, as it can significantly alter the measurements of H2 production in semiconductor-assisted photocatalysis. Figure 3. Dependence of the photoconversion efficiency of H2 generation on the type of sacrificial donor, as measured after 30 min of irradiation with white light. The efficiency of net H2 yield shows the contributions from the AEL (reduction) and CEL (oxidation) parts of the BPM-CdS-Pd membrane. The percentage value indicates the fraction of H2 produced during sacrificial donor oxidation (CEL compartment) with reference to net H2 production. The sacrificial donor oxidation can vary depending on the medium, type of photocatalyst, oxidation mechanism, and other experimental conditions. In a recent study, (19) the interference from direct photolysis of solvents and sacrificial donors in the estimation of products of the photocatalytic reduction of CO2 is discussed. Interference from such sacrificial donor oxidation can lead to erroneous claims while reporting the mechanism of CO2 reduction. Hence, one should not assume sacrificial donors employed in photocatalysis or photoelectrocatalysis to be neutral participants with no impact on the identified products. The results presented here show how oxidation of sacrificial donors at a semiconductor surface can generate H2 as part of the oxidation process. Since a major fraction of the observed product (viz., H2 in water photoelectrolysis) in a photocatalysis experiment may come from oxidation of the sacrificial donor, it is important to account for this contribution while estimating the photoconversion efficiency or kinetics of H2 evolution. It is recommended that researchers who are interested in establishing H2 generation efficiency should independently verify the products resulting from sacrificial donor oxidation. Alternatively, one can compare the sacrificial donor performance with that of Na2SO3 in photocatalytic process since it does not produce H2 as part of the oxidation step. Simply ignoring the participation of a sacrificial donor in a photocatalysis experiment and making claims such as “high photon conversion efficiency” or “highly efficient photocatalyst” undermine the research advances. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.1c02487.Additional supporting results related to the H2 calibration curve using GC analysis, H2 evolution with TEOA sacrificial donor, and actinometry (PDF) Additional supporting results related to the H2 calibration curve using GC analysis, H2 evolution with TEOA sacrificial donor, and actinometry (PDF) Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html. Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. P.V.K. acknowledges support by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, of the U.S. Department of Energy (award DE-FC02-04ER15533), and F.C. acknowledges the support of Catholic University of Sacred Heart (UCSC, Italy) and Italian Institute of Technology (IIT, Italy) to conduct research at the University of Notre Dame. This is NDRL 5342 from the Notre Dame Radiation Laboratory. This article references 19 other publications.