Poly(3-alkylthiophene) Films as Solvent-Processable Photoelectrocatalysts for Ef ﬁ cient Oxygen Reduction to Hydrogen Peroxide

a central role in various organic devices due to their solvent processability and their remarkable electrical and optical properties. The (photo)electrocatalytic abilities of unsubstituted and solvent-insoluble polythiophenes in the reduction of O 2 to H 2 O 2 in a basic aqueous electrolyte have recently emerged as an advanced function. Herein, the electrocatalytic and photoelectrocatalytic abilities of solvent-processable poly(3-alkylthiophene) ﬁ lms at pH 12 are demonstrated, as well as their characteristics are re-examined from the viewpoints of the polymer structure, electrochemistry, photochemistry, and ﬁ lm nanostructure. A comparison of the above characteristics reveals the requirements for effective (photo)electrocatalytic O 2 reduction to H 2 O 2 production. In addition, the addition of an organic salt to the polymer solution changes the formed ﬁ lm characteristics. The thin ﬁ lm of the regioregular poly(3-hexylthiophene-2,5-diyl) containing a small amount of tetramethylam-monium bis(tri ﬂ uoromethanesulfonyl) imide is easily formed by a solvent-based process and features lower crystallinity, a porous ﬁ lm nanostructure, and high conductivity. This polymer acts as a robust photoelectrocatalyst for the reduction of O 2 to H 2 O 2 with a conversion rate of 3.9 (cid:3) 10 3 mg (H 2 O 2 ) g photocat 1 h (cid:4) 1 or (cid:1) 0.040 mg (H 2 O 2 ) cm (cid:4) 2 h (cid:4) 1 and a high Coulombic ef ﬁ ciency of > 95% at 0.1 V bias from the theoretical potential.

DOI: 10.1002/aesr.202100103 Poly(3-alkylthiophene) films play a central role in various organic devices due to their solvent processability and their remarkable electrical and optical properties. The (photo)electrocatalytic abilities of unsubstituted and solvent-insoluble polythiophenes in the reduction of O 2 to H 2 O 2 in a basic aqueous electrolyte have recently emerged as an advanced function. Herein, the electrocatalytic and photoelectrocatalytic abilities of solvent-processable poly(3-alkylthiophene) films at pH 12 are demonstrated, as well as their characteristics are re-examined from the viewpoints of the polymer structure, electrochemistry, photochemistry, and film nanostructure. A comparison of the above characteristics reveals the requirements for effective (photo)electrocatalytic O 2 reduction to H 2 O 2 production. In addition, the addition of an organic salt to the polymer solution changes the formed film characteristics. The thin film of the regioregular poly(3-hexylthiophene-2,5-diyl) containing a small amount of tetramethylammonium bis(trifluoromethanesulfonyl) imide is easily formed by a solvent-based process and features lower crystallinity, a porous film nanostructure, and high conductivity. This polymer acts as a robust photoelectrocatalyst for the reduction of O 2 to H 2 O 2 with a conversion rate of 3.9 Â 10 3 mg (H 2 O 2 ) g photocat À1 h À1 or %0.040 mg (H 2 O 2 ) cm À2 h À1 and a high Coulombic efficiency of >95% at 0.1 V bias from the theoretical potential.
electrochemical or photochemical reduction of O 2 to produce H 2 O 2 on polythiophene has been reported to provide a low-concentration aqueous H 2 O 2 solution. [5d] As the required reactants are only O 2 and water, in situ H 2 O 2 production can be applied in any locations, which may lead to a breakthrough in H 2 O 2 production. However, for the practical application, it is desirable that the catalyst itself is commercially available and facilely solvent processable. Herein, we focus on solvent-processable poly(3-alkylthiophene) films, which can be formed on a wide area uniformly, coated on various substrates, and whose thickness can be adjusted easily. The characteristics of poly(3-alkylthiophene)s with different regioregularities and alkyl chain lengths are re-examined by unified methods from the viewpoint of the polymer structure, electrochemistry, photochemistry, and film nanostructure. Their (photo)electrocatalytic abilities for the reduction of O 2 to produce H 2 O 2 are demonstrated. We clarify the correlation between their physical properties and catalytic abilities and introduce a facile strategy to improve their catalytic abilities, for efficient H 2 O 2 production.

Characterization of Poly(3-alkylthiophene) Films
The characteristics of poly(3-alkylthiophene)s with different regioregularities and alkyl chain lengths (i.e., rr-P3BT, rr-P3HT, rra-P3HT, and rr-P3OT) were re-examined from the viewpoints of the polymer structure, electrochemistry, photochemistry, and the film nanostructure. Although there are numerous reports for each poly(3-alkylthiophene), [4c] the use of different measurement methods resulted in significant variations in the characteristics. For example, the HOMO level of rr-P3HT has been reported to range from À4.6 to À5.2 eV. [9] Therefore, this study aimed to make a legitimate comparison of the physical characteristics of such materials using the unified method, first of all. The regioregularity, molecular weight, band structure, and conductivity are shown in Table 1.
The X-ray diffraction (XRD) patterns showed crystalline signals for thin films of rr-P3BT, rr-P3HT, and rr-P3OT; no peak observed for the rra-P3HT thin film indicated its amorphous characteristic ( Figure S1, Supporting Information). From the surface scanning electron microscope (SEM) images ( Figure S2a, S2b, and S2d, Supporting Information) of rr-P3BT, rr-P3HT, and rr-P3OT, it was observed that rr-P3BT ( Figure S2a, Supporting Information) exhibited surface roughness and cracks, whereas the longer alkyl chains associated with rr-P3HT and rr-P3OT resulted in fewer cracks and a reduced roughness. In the case of rra-P3HT ( Figure S2c, Supporting Information), similar cracks, for rr-P3BT ( Figure S2a, Supporting Information), were observed, whereas the roughness was reduced ( Figure S2b, Supporting Information).
From the photoelectron spectroscopy in air ( Figure S3, Supporting Information) and the UVÀvis spectra ( Figure S4, Supporting Information) obtained for poly(3-alkylthiophene)s, their HOMO level, bandgap, and lowest unoccupied molecular orbital (LUMO) level were estimated (Table 1). Although the HOMO levels of rr-P3BT, rr-P3HT, and rr-P3OT were similar (i.e., %À4.68 eV), the HOMO level of rra-P3HT was slightly deeper, at À4.73 eV. In terms of the bandgap, rr-P3BT, rr-P3HT, and rr-P3OT presented Determined from the NMR spectra (dichloromethane-d 2 ) of the poly(3-alkylthiophene)s. ( Figure S8, Supporting Information); b) Determined by GPC (N,Ndimethylformamide (DMF)). The weight-average molecular weights and dispersities of the polymers were calculated based on linear polystyrene standards; c) Estimated by photoelectron spectroscopic in air ( Figure S3, Supporting Information); d) Estimated from the UVÀvis spectra ( Figure S4, Supporting Information); e) The four types of poly(3-alkylthiophene) film with similar thicknesses (%50 nm) showed comparable conductivity values in the order of 10 À4 S cm À1 . similar values of %1.9 eV with some fluctuation, whereas rra-P3HT exhibited a higher value of 2.25 eV. This difference can be visually observed in the film colors of rr-P3HT and rra-P3HT ( Figure S4 inset, Supporting Information). In the context of the absorbance, rr-P3HT exhibited the strongest absorbance, whereas rra-P3HT exhibited a weak absorbance that was approximately two-thirds that of rr-P3HT. This was presumably ascribed to the dense and semicrystalline structure (the πÀπ overlap) of rr-P3HT ( Figure S1, Supporting Information), and amorphous (unpacked) structure of rra-P3HT ( Figure S1, Supporting Information).
Based on these results, it was considered that a comparison of the catalytic abilities of these poly(3-alkylthiophene) films, which have been shown to possess slightly different physical properties, would reveal the requirements for the efficient reduction of O 2 to H 2 O 2 .

Photo-and Electrochemical Properties of Poly(3-alkylthiophene) Films
As a typical example, the electrochemical properties of rr-P3HT (thickness 50 AE 10 nm on a glassy carbon substrate) in a basic aqueous electrolyte (pH 12) in the dark were investigated ( Figure 1a). rr-P3HT showed almost no reduction current under an argon atmosphere, although a reduction current was observed in air (see the current density at À0.4 V vs Ag/AgCl in Figure 1a).
Quantification of the amount of H 2 O 2 produced after long-term chronoamperometric measurements at À0.4 V also confirmed that H 2 O 2 production was successful with a high Coulombic efficiency of 96%. It was demonstrated that this reduction current under air can be ascribed to the reduction of O 2 to produce H 2 O 2 , and that the solvent-processable rr-P3HT acts as an O 2 reduction electrocatalyst with a low overpotential. Strictly speaking, at a higher pH than the pK a of H 2 O 2 (11.6), O 2 reduction follows Equation (1) to form HO 2 À with the theoretical potential (E ). [10]  Similarly, rr-P3BT, rra-P3HT, and rr-P3OT (thickness 50 AE 10 nm) also demonstrated an electrocatalytic ability at pH 12 in the dark ( Figure 1b). Among these species, rra-P3HT acted as an exceptionally active electrocatalyst with small overpotential (see the blue line in Figure 1b). Considering that rr-P3BT, Figure 1. a) Linear sweep voltammograms of rr-P3HT in argon (dotted blue trace) and in air (dotted red trace) in the dark and in air under light irradiation (solid red trace) recorded at 10 mV s À1 and pH 12. b) Linear sweep voltammograms of rr-P3BT (black trace), rr-P3HT (red trace), rra-P3HT (blue trace), and rr-P3OT (green trace) in air and in the dark recorded at 10 mV s À1 and pH 12. c) Linear sweep voltammograms of rr-P3BT (black trace), rr-P3HT (red trace), rra-P3HT (blue trace), and rr-P3OT (green trace) in air under light irradiation recorded at 10 mV s À1 and pH 12. d) Open-circuit potentials of rr-P3HT (red trace) and rra-P3HT (blue trace) recorded at pH 12. J (μA cm À2 ) is reduction current.
www.advancedsciencenews.com www.advenergysustres.com rr-P3HT, and rr-P3OT had almost the same overpotential %0.15 V), these relatively large overpotentials were presumably ascribed to a highly stacked structure (see the XRD pattern in Figure S1, Supporting Information), which inhibited the catalytic cycle [5] (the attachment of O 2 to the thiophene structure or the detachment of O 2 H À ) and enhanced these required activation energies. rra-P3HT also showed the steeper slope of the current trace (see in Figure 1b), which was ascribed to favorable reactant diffusion conditions or exposure of more catalytic sites due to its amorphous and porous film structure (see the XRD pattern in Figure S1, Supporting Information, and the SEM images in Figure S2c and S2g, Supporting Information) and low reaction resistance due to its high conductivity (see Table 1). Under light irradiation, rr-P3BT, rr-P3HT, rra-P3HT, and rr-P3OT demonstrated photoelectrocatalytic abilities (Figure 1c, positive shift of onset potential and increase in reduction current) ( Figure 2). It was found that rra-P3HT showed a high photovoltage (Figure 1d) corresponding to its deep HOMO level, but other three types of poly(3-alkylthiophene) showed a low photovoltage (Figure 1c,d), not corresponding to their HOMO levels, which indicates that the semicrystalline structure (brought by the alkyne chains) inhibits or limits the photovoltage. This should be ascribed to an interface barrier between crystalline and amorphous regions for excitons in the poly(3-alkylthiophene) film with high regioregularity, which corresponded to %0.3 eV as an energy gap in the previous literature [11] and was consistent with the difference of opencircuit potentials between rra-P3HT and rr-P3HT (see Figure 1d). While rra-P3HT is mainly composed of the amorphous region, the others are composed of both crystalline and amorphous regions [11] (see Figure S1, Supporting Information), and the excitons photoinduced in rr-P3HT have to be transported between crystalline and amorphous regions in most cases, which presumably limits its photochemical properties.
However, rra-P3HT presented a low reduction current at more positive potentials than E eq . This was attributed to its reduced level of light absorption (see Figure S4, Supporting Information) and the shallower LUMO level, that might prevent transfer of an electron to the reactant (Figure 2). At more negative potentials than E eq , a high reduction current was observed, as in the case of the electrocatalyst in the dark. On the other hand, the small differences found in the catalytic activities among these three types of poly(3-alkylthiophene) films with high regioregularity were presumably due to the strength of light absorption (as indicated in Figure S4, Supporting Information). In the chronoamperometric measurement at À0.2 V (relatively close to E eq ), rr-P3HT showed a stable reduction current of À20 μA cm À2 , which was the highest value ( Figure 3a).
In contrast, the current of rra-P3HT decreased with time due to delamination, which was presumably caused by the weak adhesiveness to the substrate (rra-P3HT film was completely peeled off after 30 min of measurement.) Subsequent optimization of the film thickness of the rr-P3HT thin film showed that the reduction current reached its highest value at 50 nm ( Figure 3b). This is supported by considering the diffusion length of excitons. [12] Furthermore, considering that the film thickness does not dramatically affect the reduction current, the reduction current, in this case, can be limited by the interface itself.
Based on the earlier results and the physical characteristics of the poly(3-alkylthiophene)s described in Section 3.1, the requirements for a highly efficient O 2 reduction reaction to produce H 2 O 2 in the dark and under light irradiation could be deduced. More specifically, in the dark, favorable reactant diffusion conditions or exposure of more catalytic sites (i.e., amorphous and porous structure) are necessary to ensure an appropriate number of reaction sites, and a high conductivity is required for efficient charge transfer; low crystallinity (amorphous) is preferred to  www.advancedsciencenews.com www.advenergysustres.com activate the catalytic ability. Under light irradiation, superior absorbance properties are necessary to enhance the effect of light energy, the film thickness must correspond to the diffusion length of the excitons, and low crystallinity (amorphous) is preferred to remove an interface barrier between crystalline and amorphous regions in the polymer for efficient exciton transport.

Improvement of the (Photo)electrocatalytic Activity by the Addition of Organic Salts
To improve the efficiency of H 2 O 2 production, the influence of organic salt addition to the rr-P3HT solution was examined. The addition of organic salt is relatively facile as many types of organic salts based on different cations and anions are commercially available, and they can be simply mixed with the polymer solution. In addition, the addition of organic salt is expected to produce polymer films with different physical properties (e.g., electrical conductivity) depending on the type and amount of organic salt. For this study, LiTFSI, TEAPF 6 , TBAPF 6 , and TBATFSI were selected as the organic salts to form a homogeneous rr-P3HT film, due to their high commercially availability and high solubility in the polymer solution (organic solvents). The additive salt concentration was fixed at 10 mol% of the rr-P3HT monomer unit, four types of rr-P3HT film containing an organic salt with similar film thicknesses (%60 nm) and chronoamperometric measurements were conducted at À0.2 V ( Figure 4a). As indicated, rr-P3HT containing 10 mol% TEAPF 6 , TBAPF 6 , or TBATFSI exhibited a higher reduction current (e.g., À28 μA cm À2 for rr-P3HT containing 10 mol% TBATFSI) than in the absence of an organic salt (i.e., À20 μA cm À2 ), whereas the use of LiTFSI gave a lower current (i.e., À16 μA cm À2 ). The low current associated with LiTFSI was presumably ascribed to formation of polymer grain boundary, which serves as resistance component by the dissolution of salt, and ascribed to the reaction inhabitation from the heat of dissolution. In contrast, TEAPF 6 , TBAPF 6 , and TBATFSI are insoluble in water, the higher reduction current was achieved in the case of TBATFSI (see Figure 4a). The salt concentration was optimized to 10 mol% (Figure 4b). From a comparison of the linear sweep voltammograms of the rr-P3HT thin films prepared in the presence and absence of TBATFSI ( Figure 5), it was apparent that there was a remarkable difference in catalytic ability in the dark. Overall, these results indicated that the overpotential was small in the presence of TBATFSI, and an enhancement in the slope of the reduction current was also observed.
The physical properties of the rr-P3HT thin film containing 10 mol% TBATFSI were evaluated (Figure 6 and S5, Supporting Information), and it was possible to describe the improvement in the catalytic ability based on the discussion in Section 2.2. The small overpotential is presumably ascribed to lower crystallinity by adding TBATFSI (see Figure S5c, Supporting Information). TBATFSI should dilute rr-P3HT concentration in the film (see Figure S5a, Supporting Information) and prevent the stacking and aggregation of rr-P3HT. The enhancement in the slope of the reduction current by adding salt is supported by the favorable diffusion condition or due to low crystallinity (see Figure S5c, Supporting Information) and a porous film structure similar to that of Emmental cheese (see Figure 6b) and the higher conductivity of 4.3 Â 10 À4 S cm À1 . It is also noteworthy that the addition of organic salt can change Figure 4. Photoelectrochemical properties of the rr-P3HT/organic salt films. a) Chronoamperometric measurements for rr-P3HT (black trace), rr-P3HT þ TBATFSI (red trace), rr-P3HT þ TBAPF 6 (blue trace), rr-P3HT þ TEAPF 6 (green trace), and rr-P3HT þ LiTFSI (light blue trace) carried out at À0.2 V versus Ag/AgCl and pH 12. b) Chronoamperometric measurements for rr-P3HT þ TBATFSI (1 mol%, black trace), P3HT þ TBATFSI (5 mol%, blue trace), P3HT þ TBATFSI (10 mol%, red trace), P3HT þ TBATFSI (20 mol%, green trace), and P3HT þ TBATFSI (50 mol%, light blue trace) carried out at À0.2 V versus Ag/AgCl and pH 12. Linear sweep voltammograms of rr-P3HT þ TBATFSI recorded at 10 mV s À1 and pH 12. www.advancedsciencenews.com www.advenergysustres.com the crystallinity, the film morphology, and the conductivity. Also, the more amorphous structure may expose more catalytic sites (thiophene groups) to the water interphase, suggesting that the addition of the salt facilitates a better interface. Indeed, under light irradiation, the superior reduction current at potentials below À0.1 V was observed, which could be ascribed to the improved electrocatalytic ability, resulting from the addition of an organic salt. However, the onset potential (photovoltage) did not change. It should be noted that the lower crystallinity affects the electrocatalytic ability, but does not significantly affect the photocatalytic ability, especially onset potential (photovoltage). This can be ascribed to the existence of the interface barrier between crystalline and amorphous regions even after reducing the crystallinity. Very recently, the 3D-structured and hydrophilized rr-P3HT film was reported for the photochemical oxygen reduction to produce H 2 O 2 at low pH. [13] However, the photocurrent remained <50 μA cm À2 even supplying pure oxygen, and the plasmatreated film was degraded or delaminated from the substrate after 10 h.
In this study, long-term measurement over several hours at 0.080 V bias from E eq under light irradiation ( Figure S6, Supporting Information) gave a H 2 O 2 production rate of 3.9 Â 10 3 mg (H 2 O 2 ) g photocat À1 h À1 or %0.040 mg (H 2 O 2 ) cm À2 h À1 , in addition to a high Coulombic efficiency of 95%, with no degradation ( Figure S7, Supporting Information).

Conclusion
We clarified the electrocatalytic and photoelectrocatalytic abilities of solvent-processable poly(3-alkylthiophene)s in the reduction of O 2 to H 2 O 2 in a basic aqueous electrolytic solution. Our results indicated that a catalyst film can be easily produced using a commercially available polymer through a solvent-processable route. A comparison of the catalytic abilities of the prepared poly(3-alkylthiophene) films, which had slightly different physical properties, revealed some requirements to achieve an efficient catalytic O 2 reduction to H 2 O 2 production. More specifically, in the dark, favorable reactant diffusion conditions, a high conductivity, and low crystallinity are required, whereas under light irradiation, superior light absorbance properties, an appropriate film thickness related to the diffusion length of the excitons, and amorphous property to remove the interface barrier between crystalline and amorphous regions are necessary. It was also found that the addition of an organic salt to the rr-P3HT solution alters the formed film properties, such as porous surface, high conductivity, and low crystallinity, thereby enhancing the performance of the electrocatalyst to give a stable and high H 2 O 2 production rate of 3.9 Â 10 3 mg (H 2 O 2 ) g photocat À1 h À1 (at 0.080 V bias from E eq ). The addition of organic salt, which changes their crystallinity, film morphology, and conductivity, is extremely facile and practical, not only in the context of this study into π-conjugated polymers for catalytic applications, but also for investigations into π-conjugated polymers for other organic devices. This strategy will therefore be expected to accelerate research aimed at improving the characteristics of π-conjugated polymers.
Material Characterization: For the legitimate evaluation of the catalytic effect of an organic material, it should always be confirmed that no residual metal is present that could affect the catalytic activity. [14] Hence, by inductively coupled plasma mass spectrometric measurements, it was confirmed that the poly(3-alkylthiophene)s, which were used in this work, contained low levels of precious metals with catalytic ability (i.e., platinum, gold, and mercury [10a] ) below the detection limit (0.3 ppm). It was also confirmed that the electrochemical behaviors discussed in Sections 2.2 and 2.3 were completely different from those of such conventional metal catalysts. We prepared commercially available poly(3-alkylthiophene)s with almost the same number average molar mass (M n ) as much as possible. Gel permeation chromatography (GPC) confirmed M n of about 2 Â 10 4 À4 Â 10 4 (Table 1). Even considering the degree of dispersion, they possessed the same order of the molecular weight (10 4 ). The regioregularity of each poly(3-alkylthiophene) was determined by 1 H NMR spectroscopy from the integral ratios of the signals originating from two types of proton in the alkyl chain at %2.5-3.0 ppm (Table 1 and Figure S8, Supporting Information, as a representative example). The 1 H NMR signal at www.advancedsciencenews.com www.advenergysustres.com 2.8 ppm was assigned to the head-to-tail polymeric structure, whereas that at 2.6 ppm was assigned to the head-to-head structure. [15] Based on the obtained results, rr-P3BT, rr-P3HT, and rr-P3OT were found to possess a high regioregularity of >95%, whereas rra-P3HT had a low regioregularity of 59%. Substrates: Each substrate was sonicated in chlorobenzene for 15 min and then rinsed with chlorobenzene prior to the film formation. Glassy carbon, fluorine-doped tin oxide (FTO)-coated glass, and indium tin oxide (ITO)-coated glass were purchased from Alfa Aesar (Production code: 38023-GH), Nippon Sheet Glass (Production code: FTN 1.6), and Matsunami Glass Ind., Ltd. (Production code: S9226), respectively.
Film Formation in the Absence of an Organic Salt: A chlorobenzene solution of poly(3-alkylthiophene) (20 mg mL À1 , to yield a 50-60 nm thickness of rr-P3HT) was prepared and spin coated onto a glassy carbon (GC) substrate (after ozone cleaning) or a glass substrate (after cleaning with 2propanol) at 4000 rpm for 20 s. Subsequently, the substrate was heated and dried on a hot plate at 85 C for 15 min and stored in nitrogen atmosphere. Thin films with different thicknesses were prepared by changing the solution concentration. The film thickness measurement had an error %AE10 nm.
Film Formation in the Presence of an Organic Salt: A chlorobenzene solution of poly(3-alkylthiophene) and the desired organic salt (mol%) was prepared and spin coated onto a GC substrate (after ozone cleaning) or a glass substrate (after cleaning with 2-propanol) at 4000 rpm for 20 s. Subsequently, the substrate was heated and dried on a hot plate at 85 C for 15 min and then stored in nitrogen atmosphere. The film thickness measurement had an error of %AE10 nm.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.