Photoelectrochemical Oxidation of Amines to Imines and Production of Hydrogen through Mo-Doped BiVO4 Photoanode.

Imines are important multifunctional intermediates for the synthesis of pesticides, pharmaceuticals, biologics, and fine chemicals. The direct photoelectrochemical (PEC) oxidation of amines to imines is a highly selective, efficient, green, and gentle method. Interestingly, the constructive merging of the PEC oxidation of amines with the production of hydrogen can accelerate hydrogen evolution due to the less challenging oxidation of amines such as benzylamine (BN) in comparison to sluggish water oxidation. Herein, Mo-doped BiVO4 photoanodes were prepared and first applied to simultaneously oxide benzylamine (BN) to N-benzylidenebenzylamine (BI) and produce hydrogen in a closed two-chamber, three-electrode PEC cell After illumination at a bias of 1.3 V vs SCE for 3 h, the 3% Mo-doped BiVO4 photoanode achieved a maximum yield of ∼94 μmol h-1 at a 1 × 1 cm2 area with a BN to BI selectivity of almost 100% and a Faradaic efficiency of 98.4%. Our electrode presented enhanced photocorrosion resistance in acetonitrile solvent. Additionally, the PEC oxidations of benzylamine derivatives with different substituents (-F, -Cl, -Br, -CH3, -OCH3) to the corresponding imines were also investigated. The results indicated that the Mo-doped BiVO4 photoanode exhibited an excellent performance in the oxidation of these benzylamine derivatives with corresponding amine to imine selectivities of almost 100% and Faradaic efficiencies of >95%.

Introduction

The selective oxidation of amines to the corresponding imines is an important transformation reaction in organic chemistry due to the wide applications of the product imines and their derivatives in the fields of pharmaceuticals, pesticides, biologics, and fine chemicals.[1−6] Traditionally, imines are produced by the condensation of primary amines with carbonyl compounds.[7−10] However, toxic oxidants, expensive dehydrating agents, high temperature, and unstable aldehydes or ketones are usually involved in these processes, which are environmentally hazardous, uneconomical, uncontrollable, and inefficient. Thus, the exploration of new strategies that can directly oxidize amines to imines in a more controllable, efficient, and eco-friendly manner is highly desired.[11−13]

From the perspective of green chemistry, the direct photocatalytic or electrocatalytic oxidation of amines is a more attractive method. However, both methods suffer from defects. On the basis of sunlight and molecular oxygen, the direct photocatalytic oxidation of amines is green, mild, and economical. Despite the high product selectivity (>90%) achieved, the poor activity remains a headache. Long hours of illumination are usually indispensable for an ideal conversion rate and yield.[14−17] Different from photocatalysis, electrocatalysis can actively oxide amines with an external energy input. Unfortunately, nitriles tend to be formed due to the peroxidation of unstable imine intermediates, resulting in a poor product selectivity.[18−20]

As the merging of photocatalysis and electrocatalysis, a photoelectrochemical (PEC) catalysis is expected to combine the advantages of both methods, which has been proven to be an effective method for the oxidation of some organic substrates with high conversion efficiencies and selectivities.[21] Additionally, the substitution of the sluggish water oxidation with the less challenging oxidation of organic substrates could subsequently promote hydrogen evolution at the counter electrode.[22−25] For instance, researchers have carried out the TEMPO-mediated PEC oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA),[26−28] the PEC coupling of 5-amino-1H-tetrazole (5AT) to 5,5′-azotetrazolate (SZT),[29,30] and the PEC selective oxidation of glycerol to 1,3-dihydroxyacetone (DHA).[31−33] Thus, a PEC process is also expected to be able to convert amines to imines with high conversion efficiency and selectivity. Moreover, the constructive merging of the less challenging oxidation of amines on a photoanode with the production of hydrogen on a photocathode in one PECcell is beneficial, which can not only produce high-value-added imines but also achieve the high-rate production of hydrogen—an emerging energy with great application potential in the future.[34−37]

To address the above assumption, Mo-doped BiVO4, as one of the most promising materials with an appropriate band gap and high theoretical photocurrent,[38−40] was selected as the photoanode. The direct oxidation of benzylamine (BN) to N-benzylidenebenzylamine (BI) was chosen as the model reaction. The experimental results indicated that the selective oxidation of BN to BI with high efficiency along with high-rate hydrogen evolution could be achieved over a Mo-doped BiVO4 photoanode under the continuous irradiation of simulated solar light. Additionally, the use of the PEC oxidation of benzylamine derivatives containing different substituents, i.e., −F, −Cl, −Br, −CH3, and −OCH3, was also investigated, in which benzylamine derivatives could be oxidized into the corresponding imines over a 3% Mo-doped BiVO4 photoanode. Our results indicate that PEC organic synthesis may offer an ideal method for the selective oxidation of amines to imines on a large scale in the future.

Results and Discussion

Pristine BiVO4 and Mo-doped BiVO4 electrodes were fabricated by spin coating followed by a subsequent annealing treatment (Figure S1). The concentrations of Mo, defined as atomic percent, were respectively 1%, 3%, 5%, and 7% with respect to V content. The structures and morphologies of the as-prepared electrodes were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). As shown in Figure a, all of the XRD patterns of the prepared BiVO4 or Mo-doped BiVO4 electrodes can be indexed to monoclinic BiVO4 (JCPDS No. 14-0688) in addition to some diffraction peaks corresponding to the FTO (F:SnO2) substrate (JCPDS No. 46-1088). The similar XRD patterns of all electrodes indicates that the addition of trace Mo has little effect on the crystalline structure. SEM images (Figure b) show that both the pristine BiVO4 and 3% Mo-doped BiVO4 electrodes are composed of irregular nanoparticles with a diameter of 150–200 nm, which indicates that the morphology of BiVO4 was not changed by the doping of trace Mo. The mapping images of SEM (Figure S2) shows that the four elements Bi, V, Mo, and O are evenly distributed in the 3% Mo-doped BiVO4 electrode, which proves the successful doping of Mo. High-resolution elemental scans for the 3% Mo-doped BiVO4 electrode were executed by XPS. Two characteristic peaks corresponding to the 3d3/2 and 3d5/2 orbitals of Mo6+, respectively, can be observed, which proves the existence of Mo in a hexavalent state (Figure c). The diffuse reflectance absorption spectrum of as-prepared electrodes was measured with a UV–vis spectrophotometer. All electrodes exhibit almost identical absorption curves with absorption edges at around 480 nm, and the band gaps are estimated to be approximately 2.52 eV (Figure d), which indicates that the doping of Mo6+ did not change the band gap and light absorption properties of the materials.

Figure 1

(a) XRD patterns and (b) SEM images of as-prepared pristine BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes. (c) Mo 3d XPS elemental spectrum of the 3% Mo:BiVO4 electrode. (d) Diffuse reflectance absorption spectrum of pristine BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes. The inset gives the Tauc plots of all electrodes.

The difficulties of the benzylamine oxidation reaction (BOR) and oxygen evolution reaction (OER) were first compared with a three-electrode PEC cell with the as-prepared BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes as working electrodes, a platinum sheet (Pt) as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. We explored the OER performances of our electrodes in a 0.1 M potassium phosphate aqueous solution. The J–V curves (Figure a) show a significant increase of the photocurrent density in Mo-doped BiVO4 electrodes with a slightly higher onset potential of ∼0.7 V vs RHE with respect to the pristine BiVO4 electrode. Among them, the 3% Mo-doped BiVO4 electrode obtains the optimum photocurrent density of up to 1.98 mA/cm2 at a bias of 1.23 V vs RHE, while the pristine BiVO4 electrode merely has a 0.84 mA/cm2 density at the same bias. However, when the doping concentration of Mo6+ continues to increase, the photocurrent density decreases. Then, we explored the BOR performances of our electrodes in a 0.1 M potassium phosphate aqueous solution containing 0.05 M BN. As shown in the J–V curves (Figure b), our electrodes have an onset potential of ∼0.4 V vs RHE and achieve an obviously negative shift of ∼300 mV relative to that of the OER. The change in the J–V curves is consistent with the above test results, but the photocurrents of all electrodes increase to some extent in comparison with the OER. This experimental phenomenon preliminarily proves that the BOR is less challenging to carry out on the BiVO4 electrodes in comparison to the sluggish water oxidation. The PEC stability of the 3% Mo-doped BiVO4 electrode was tested at a constant bias of 1.3 V vs SCE in a 0.1 M potassium phosphate aqueous solution containing 0.05 M BN under simulated AM1.5G sunlight irradiation for 3 h. Unfortunately, an obvious attenuation of photocurrent density along with time can be observed from the I–t curves (Figure c, orange), which results from the high-rate photocorrosion of BiVO4 electrodes in the aqueous phase. Acetonitrile has recently emerged as a promising solvent in organic systhesis due to its two key properties: (a) an excellent ability to dissolve considerable polar and nonpolar solutes and (b) relative inertia to avoid unfavorable side reactions.[41−44] As shown in Figure S3, BN exhibits superior solubility in acetonitrile in comparison to that in water. Moreover, the photocurrent density of the 3% Mo-doped BiVO4 electrode in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile solution without the addition of BN is very low (Figure S4), which indicates that the competitive effect of acetonitrile as solvent on the BOR can be ignored. Thus, using acetonitrile as an alternative solvent is an attractive strategy. Furthermore, the photocorrosion rates of 3% Mo-doped BiVO4 electrodes in water and acetonitrile were investigated. The 3% Mo-doped BiVO4 electrodes were immersed in a 0.1 M potassium phosphate aqueous solution or a 0.1 M TBAPF6 acetonitrile solution and exposed to irradiation for 36 h at biases of 0.5 and 1.3 V vs SCE, respectively. The J–V curves (Figure d) show that the photocurrent density of the electrode after PEC electrolysis in water is severely attenuated while that of the electrode after PEC electrolysis in acetonitrile decays only slightly. The absorption spectra of 3% Mo-doped BiVO4 electrodes after PEC electrolysis were also measured. As shown in the inset of Figure d, a more significant attenuation in intensity over the range of 400–500 nm in water in comparison to that in acetonitrile can be observed. The PEC stability of the 3% Mo-doped BiVO4 electrode was further measured at a constant bias of 1.3 V vs SCE in 0.1 M TBAPF6 acetonitrile solution containing 0.05 M BN under simulated AM1.5G sunlight irradiation for 3 h. Only a slight attenuation of photocurrent density along with time was observed from the I–t curve (Figure c, gray). The experimental results prove that the high-rate photocorrosion of the BiVO4 electrode in water can be effectively suppressed by the use of acetonitrile as the solvent. In addition, acetonitrile as an alternative solvent results in an enlarged range of organic substrates due to its excellent solvation properties.

Figure 2

J–V plots of the as-prepared pristine BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes measured in a 0.1 M potassium phosphate electrolyte (a) without and (b) with 0.05 M BN upon illumination with an AM 1.5G filtered solar simulator. (c) I–t curves of the 3% Mo:BiVO4 electrode measured at a constant bias of 1.3 V vs SCE in a 0.1 M potassium phosphate electrolyte with 0.05 M BN (orange) and in a 0.1 M TBAPF6 acetonitrile solution with 0.05 M BN (gray) under continued light irradiation. (d) J–V plots of the 3% Mo:BiVO4 electrode before (solid line) and after (dashed line) PEC electrolysis for 36 h in water (orange) or acetonitrile (gray). The inset shows the absorption spectra of the 3% Mo:BiVO4 electrode before (black) and after PEC electrolysis for 36 h in water (orange) or acetonitrile (gray).

The BOR performances of the electrodes were further investigated in a 0.1 M TBAPF6 acetonitrile solution containing 0.05 M BN. The change in photocurrent density obtained from the J–V plots (Figure a) is in accord with that of the photocurrent density measured previously in the aqueous phase electrolyte. Likewise, the 3% Mo-doped BiVO4 electrode achieves the highest photocurrent density of about 5.39 mA/cm2 at 1.3 V vs SCE, which is about 3 times higher than the photocurrent density of the pristine BiVO4 electrode. When the doping concentration of Mo6+ is increased to 5% or 7%, the values of the photocurrent density decrease. The monochromatic incident photon to electron conversion efficiencies (IPCEs) of pristine BiVO4 and Mo-doped BiVO4 electrodes were measured. The IPCE values of BiVO4 electrodes are increased by Mo6+ doping, and the 3% Mo-doped BiVO4 electrode achieves an optimal value about 64% while that of the pristine BiVO4 electrode is only 23% at a wavelength of 420 nm (Figure b). When the doping concentration of Mo6+ is increased to 5% or 7%, the values of IPCEs also decrease. As we all know, the IPCE value is relevant to the absorptivity and carrier separation properties of the electrodes. We have demonstrated that doping did not change the absorptivity of the electrodes; thus, the change in IPCE values is related to the carrier separation and electrical properties of the electrodes. Electrochemical impendence spectra analyses and Mott–Schottky measurements were carried out to investigate the charge separation and electrical conductivity properties of the as-prepared electrodes. The charge transfer resistances of the pristine BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes obtained from an equivalent circuit by fitting the Nyquist plots (Figure c) are estimated to be 4.4, 0.8, 0.6, 1.2, and 1.5 kΩ, respectively, which illustrates that the electrical conductivity of the pristine BiVO4 electrode can be promoted by Mo6+ doping and the change trend is consistent with the photocurrent density. The carrier concentration of pristine BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes extracted from the Mott–Schottky plots (Figure d) are calculated to be 1.41 × 1019, 5.29 × 1019, 6.12 × 1019, 5.01 × 1019, and 1.96 × 1019 cm–3, respectively, which demonstrates that the donor density can be improved by Mo6+ doping and the change trend is also similar to those of the photocurrent densities and resistances. These experimental results show that the doping of trace Mo6+ can promote electron–hole separation and increase carrier density and electrical conductivity, which can improve the PEC performances of our electrodes. However, when the Mo6+ doping amount is large enough, it can serve as a new electron–hole composite center and reduce the length of electron diffusion, which further limits the improvement in the PEC performances of our electrodes. This is in accord with previous reports.[45−47]

Figure 3

(a) J–V plots of the as-prepared electrodes measured at a scan rate of 20 mV/s upon illumination with an AM 1.5G filtered solar simulator. (b) Monochromatic incident photon to electron conversion efficiencies (IPCEs) of the as-prepared electrodes measured at a bias of 1.3 V vs SCE. (c) Nyquist plots of the as-prepared electrodes measured at a bias of 1.3 V vs SCE upon light illumination. (d) Mott–Schottky plots of the as-prepared electrodes measured at a frequency of 1 kHz in the dark. All of the tests were performed in a 0.1 M TBAPF6 acetonitrile solution containing 0.05 M BN.

The dark current of the BiVO4 electrode for BOR were measured in 0.1 M TBAPF6 acetonitrile solution containing 0.05 M BN. The J–V plots (Figure S5a) show that the electrochemical BOR occurs slightly after a bias of ∼1.5 V vs SCE. Thus, 1.3 V vs SCE was selected as the reaction bias to avoid the electrochemical BOR and give a considerable yield. Additionally, the yield and selectivity of oxidation of BN to BI by only photocatalysis or electrocatalysis were also investigated. As shown in Figure S5b, the PEC oxidation of BN to BI can result in a high yield and selectivity, which could not be achieved by single electrocatalysis or photocatalysis, and the PEC oxidation of BN to BI at 1.3 V vs SCE is the result of the synergistic effect of photocatalysis and electrocatalysis. A chronoamperometry experiment was implemented in a closed two-chamber, three-electrode PEC cell with proton transfer between the two chambers via a proton exchange membrane. Efficient H2 evolution can be observed during the reaction process (Figure S6). After a PEC reaction for 3 h, the anode products were detected by a gas chromatograph equipped with a flame ionization detector. The BI yields of pristine BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes are 29.73, 69.28, 94.01, 48.83, 38.87 μmol/(h cm2), respectively (Figure a). The 3% Mo-doped BiVO4 electrode had the best PEC performance and the maximum yield of BI. The Faradaic efficiency and selectivity of BN to BI reaction by the 3% Mo-doped BiVO4 electrode were further calculated to be 98.40% and ∼100%, respectively. (Table , entry 1). Similarly, the Faradaic efficiency for photocathode hydrogen production was also calculated to be >95%. As shown in Figure b, the ratio of BI yield to H2 yield is about 1:1, and the yield is almost equal to the theoretical value.

Figure 4

(a) Yields of BI obtained by the pristine BiVO4 and X% Mo:BiVO4 (X = 1, 3, 5, 7) electrodes after an AM 1.5G filtered solar simulator illumination for 3 h. (b) Actual yields and theoretical yields of H2 and BI derived from a chronoamperometric experiment at a bias of 1.3 V vs SCE by the 3% Mo:BiVO4 electrode under an AM 1.5G filtered solar simulator illumination for 3 h. (c) J–V plots of the 3% Mo:BiVO4 electrodes measured under different atmospheres. (d) Yields of BI and the Faradaic efficiencies of the coupling reaction obtained by the 3% Mo:BiVO4 electrodes under different atmospheres.

Table 1

Oxidative Coupling of Various Amines to Corresponding Imines through a 3% Mo:BiVO4 Electrode

entry	substrate	product	t (h)	Q (C)	ne– (μmol)	yield (μmol)	FE (%)	SEL (%)
1	X = H	X = H	3	55.23	573.22	282.03	98.40	>99
2	X = p-F	X = p-F	3	54.83	569.13	279.21	98.12	>99
3	X = p-Cl	X = p-Cl	3	56.19	583.20	283.52	97.23	>99
4	X = p-Br	X = p-Br	3	55.99	581.16	282.85	97.34	>99
5	X = p-CH3	X = p-CH3	3	53.69	557.27	273.23	98.06	>99
6	X = p-OCH3	X = p-OCH3	3	49.58	514.59	247.47	96.18	>99

There are usually two mechanisms of BN photooxidation. One is an aerobic mechanism, where oxygen is reduced to a superoxide radical by photogenerated electron and then participates in the oxidation process.[48−51] The other is an anaerobic mechanism, where the whole process is led by the photogenerated holes on the valence band of photoanode without the participation of superoxide radicals.[52,53] The previous PEC coupling of BN to BI over our electrodes was performed without venting the air; thus, we could not rule out the effect of oxygen in the solution and above the liquid level. Therefore, we investigated the PEC coupling of BN to BI over 3% Mo-doped BiVO4 electrodes under different atmospheres. Before all of the experiments, nitrogen or oxygen was used to vent the air, and oxygen was kept flowing over the course of the experiment to prevent oxygen shortages due to excessive consumption. As shown in Figure c, the J–V curves measured under different atmospheres exhibit almost the same trend. Chronoamperometry experiments were also performed in a closed two-chamber, three-electrode reactor with a proton exchange membrane at a bias of 1.3 V vs SCE under irradiation for 3 h. The yields of BI and the Faraday efficiencies of coupling BN to BI after reaction for 3 h obtained by 3% Mo-doped BiVO4 electrodes under N2 and O2 atmospheres have very little difference from the experimental results obtained in an air atmosphere (Figure d). This proves that superoxide radicals were not involved in the reaction and the PEC oxidative coupling of BN to BI over Mo-doped BiVO4 electrodes was carried out by an anaerobic mechanism. The detailed oxidation process is shown in Figure . First, BN is oxidized to generate a nitrogen-centered radical cation intermediate and the intermediate is deprotonated to form a neutral carbon radical intermediate. Then, the intermediate is oxidized and deprotonated to form an imine followed by a simple condensation reaction with another BN, producing BI. Each BN molecule is produced with the consumption of two holes, while the cathode consumes two electrons to produce one molecule of hydrogen.

Figure 5

Proposed mechanism of PEC coupling of amines to imines over a Mo:BiVO4 electrode.

In addition, the organic substrates could be extended to benzylamine derivatives containing electron-withdrawing substituents (−F, −Cl, −Br) or electron-donating substituents (−CH3, −OCH3). The 3% Mo-doped BiVO4 electrode also exhibited good PEC performances with respect to these benzylamine derivatives. As shown in Figure S7a, the J–V curves of the 3% Mo-doped BiVO4 electrode for the oxidation of different benzylamine derivatives exhibit almost the same onset potential of ∼−0.3 V vs SCE with a photocurrent density higher than 5 mA/cm2 at 1.3 V vs SCE. Chronoamperometry experiments were further implemented at a bias of 1.3 V vs SCE upon irradiation with simulated sunlight for 3 h. The 3% Mo-doped BiVO4 electrodes show good stability in electrolytes containing various benzylamine derivatives during the PEC reaction process (Figure S7b–f). All of the benzylamine derivatives could be converted to the corresponding imines through a coupling reaction in considerable yields (>80 μmol/(h cm2)), and the Faradaic efficiencies for the coupling of various benzylamine derivatives to the corresponding imines were all more than 95% and the selectivities were near 100% (Table , entries 2–6). These experimental results preliminarily prove that this PEC oxidative coupling reaction through the Mo-doped BiVO4 photoanode has a certain universality.

Conclusions

The oxidation of BN and the production of H2 were performed simultaneously in a closed two-chamber, three-electrode PEC cell using a series of Mo-doped BiVO4 as the working electrode, a conventional Pt as the counter electrode and the SCE as the reference electrode. Doping with 1–7% Mo6+ significantly improved the PEC performances by increasing the carrier concentration and enhancing the conductivity, of which the 3% Mo-doped BiVO4 electrode achieved the optimal PEC performance. An ∼94.01 μmol/(h cm2) yield of BI could be achieved along with the production of homologous stoichiometric H2 with the 3% Mo-doped BiVO4 electrode. Both the oxidation of BI and the production of H2 display more than 95% Faradaic efficiencies and nearly 100% selectivities. This coupling reaction has a certain universality, and the organic substrates can be extended to some benzylamine derivatives. This work achieves both the efficient PEC production of H2 and high-value-added products, which indicates that PEC may offer a mild, green, and efficient pathway for the large-scale production of imines in the future.

Experimental Section

Photoanode Preparation

BiVO4 and Mo:BiVO4 electrodes were fabricated by spin coating with a subsequent annealing treatment. For the BiVO4 precursor solution, 0.4 M Bi(NO3)3·5H2O was dissolved in 1.8 mL of glacial acetic acid and 0.06 M vanadyl acetylacetonate was dissolved in 12 mL of acetylacetone, and the two solutions were mixed in a Bi:V molar ratio of 1:1. For the Mo:BiVO4 precursor solutions, molybdenyl acetylacetonate was used as the Mo source for dissolution in the above precursor solution. The concentrations of Mo, defined as atomic percent, were respectively 1%, 3%, 5%, and 7% with respect to V content and the Bi:(V + Mo) molar ratio in the Mo:BiVO4 precursor solution was kept at 1:1. The precursor solutions were obtained after stirring for 20 min and an ultrasonication treatment for 4 h at room temperature. The above precursor solutions were spin-coated on FTO substrates at 700 rpm for 10 s. The FTO substrates were then dried at 150 °C for 10 min and calcined at 470 °C for 30 min. The above procedures were repeated for four cycles.

Characterization

The structures and morphologies of the as-prepared electrodes were characterized by X-ray diffraction (Bruker D8 Advance with Cu Kα irradiation), scanning electron microscopy (Hitachi S-4800), and X-ray photoelectron spectroscopy (Thermo Fisher Scientific Escalab 250). The diffuse reflectance spectra were measured with a UV–vis spectrophotometer equipped with an integrating sphere (Shimadzu UV 2550).

Photoelectrochemical Measurement

The PEC measurements were performed in a three-electrode PEC cell with the as-prepared electrode as the working electrode, Pt as the counter electrode, and the SCE as the reference electrode with a Princeton Applied Research EG&G 263A potentiostat. Experiments were conducted under AM1.5G simulated sunlight with a 300 W xenon arc lamp (Perfectlight, Beijing, Co. Ltd.) coupled with an AM1.5G global filter. The anode electrolyte was a 0.1 M potassium phosphate aqueous solution or a 0.1 M TBAPF6 acetonitrile solution containing 0.05 M of the organic substrates; the cathode electrolyte was a 0.1 M HClO4 aqueous solution. The PEC oxidation of organic substrates and the production of H2 were performed in a closed two-chamber, three-electrode PEC cell with a constant bias of 1.3 V vs SCE. Experimental details are provided in the Supporting Information.