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Open AccessCCS ChemistryRESEARCH ARTICLE1 Jun 2021Visible-Light-Driven Anti-Markovnikov Hydrocarboxylation of Acrylates and Styrenes with CO2 He Huang†, Jian-Heng Ye†, Lei Zhu†, Chuan-Kun Ran, Meng Miao, Wei Wang, Hanjiao Chen, Wen-Jun Zhou, Yu Lan, Bo Yu and Da-Gang Yu He Huang† Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Jian-Heng Ye† Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Lei Zhu† School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030 , Chuan-Kun Ran Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Meng Miao Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Wei Wang Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Hanjiao Chen Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Wen-Jun Zhou Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Yu Lan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030 College of Chemistry, and Institute of Green Catalysis, Zhengzhou University, Zhengzhou 450001 , Bo Yu Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 and Da-Gang Yu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 https://doi.org/10.31635/ccschem.020.202000374 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Light-driven carbon dioxide (CO2) capture and utilization is one of the most fundamental reactions in Nature. Herein, we report the first visible-light-driven photocatalyst-free hydrocarboxylation of alkenes with CO2. Diverse acrylates and styrenes, including challenging tri- and tetrasubstituted ones, undergo anti-Markovnikov hydrocarboxylation with high selectivities to generate valuable succinic acid derivatives and 3-arylpropionic acids. In addition to the use of stoichiometric aryl thiols, the thiol catalysis is also developed, representing the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. The UV–vis measurements, NMR analyses, and computational investigations support the formation of a novel charge-transfer complex (CTC) between thiolate and acrylate/styrene. Further mechanistic studies and density functional theory (DFT) calculations indicate that both alkene and CO2 radical anions might be generated, illustrating the unusual selectivities and providing a novel strategy for CO2 utilization. Download figure Download PowerPoint Introduction Photosynthesis is one of the most fundamental reactions in nature. For more than one century, chemists have been mimicking nature and searching for highly efficient visible-light photochemistry to realize novel chemical transformations in an environmentally friendly way.1 As diverse organic compounds cannot be excited by visible light, external photocatalysts are indispensable for most cases.2–5 Compared with widely investigated photocatalysis, visible-light-driven charge-transfer complex (CTC) or electron donor and acceptor (EDA) complex in the absence of external photocatalyst is less developed, although it is more economical and sustainable for industry.6 Notably, the light-driven CTC of thiols/disulfides and alkenes has attracted a great deal of attention in organic and polymer chemistry.7–9 For example, Wang reported a novel visible-light-driven CTC of alkenes and disulfides with alkenes as electron donors and disulfides as electron acceptors (Figure 1b).9 While few examples of CTCs of thiolates and (hetero)arenes have been reported,10–13 to the best of our knowledge, the CTCs of thiolates and acrylates/styrenes have not been reported. Figure 1 | (a) Selected examples for valuable succinic acid and 3-arylpropionic acid derivatives. (b–d) Visible-light-driven CTCs between alkenes and disulfides or thiolates and acrylates/styrenes. CTC, charge-transfer complex; 4-PE-HE, 4-phenethyl Hantzsch ester. Download figure Download PowerPoint Carbon dioxide (CO2), a well-known greenhouse gas, has gained considerable attention as an ideal one-carbon building block in chemical transformations due to its abundance, low cost, and sustainability.14–23 Hydrocarboxylations of alkenes with CO2, especially transition-metal-catalyzed hydrocarboxylations, represent efficient strategies to generate bioactive and synthetically useful carboxylic acids.24 Recently, in addition to UV-light photocatalysis,16,25–27 visible-light photocatalysis has emerged as an intriguing strategy to realize novel organic transformations with CO2.17,18,22,28–42 Notably, Iwasawa pioneered this strategy by realizing the hydrocarboxylation of alkenes with Markovnikov regioselectivity via visible-light photoredox/Rh dual catalysis.27,41 Besides Markovnikov selectivity, König et al.35 also realized a novel hydrocarboxylation of styrenes with anti-Markovnikov regioselectivity26,43–45 via photoredox/nickel dual catalysis. In these cases, both photocatalysts and transition-metal catalysts are indispensable. Moreover, CO2 is proposed to undergo reaction with highly reactive organometalic intermediates or a low-valent transition-metal catalyst, both of which are two-electron processes. In contrast, it is still highly challenging for visible-light-driven single-electron activation of CO2 to generate its corresponding radical anion, which is reactive and with selectivity that is hard to control. Anti-Markovnikov hydrocarboxylation of acrylates with CO2,46 which is in high demand to provide valuable succinic acid derivatives for pharmaceuticals (Figure 1a),47 in the organic and polymer chemistry industry,48 remains an unresolved challenge, we wondered whether we could realize this in an economical, efficient, and selective way. We envisioned that the electron-rich thiolates and electron-deficient acrylates/styrenes might form the CTC to facilitate the single-electron reduction of acrylates/styrenes or CO2 to generate radical anions, thus promoting anti-Markovnikov hydrocarboxylation of acrylates/styrenes (Figure 1c). We recognized, however, that such a strategy faced many challenges. For example, the light-driven thiol–ene reaction, oligmerization, and/or polymerization with C–S bond formation are competitively efficient.7,8,49 The thiocarboxylation33 and transesterification of acrylates may also occur, thus further complicating the reaction mixture. Herein, we report our success in realizing the first anti-Markovnikov selective hydrocarboxylation of acrylates and styrenes with CO2 under visible-light-driven photocatalyst-free conditions, enabled by a novel CTC between thiolate and acrylate/styrene (Figure 1d). Experimental Section Experimental methods To an oven-dried Schlenk tube (10 mL) equipped with a magnetic stir bar is added the alkene (0.2 mmol, 1.0 equiv for nonliquid substrates). Then the tube is moved into a glovebox and charged with NaOtBu (0.5 mmol, 2.5 equiv). The tube is sealed, evacuated, and backfilled with CO2 three times. Subsequently, the tube is opened and N-Methyl-2-pyrrolidinone (NMP) (2 mL) is added, followed by thiophenol (0.4 mmol, 2.0 equiv), tBuOH (0.4 mmol, 2.0 equiv), and alkene (0.2 mmol, 1.0 equiv for liquid substrates) via syringe under CO2. Once added, the resulting mixture is degassed by using a freeze-pump-thaw procedure (two times). Then the Schlenk tube is backfilled with CO2 and sealed at atmospheric pressure of CO2 (1 atm). The reaction is stirred and irradiated with a 30 W blue light-emitting diode (LED) lamp (1 cm away, with cooling fan to keep the reaction temperature at 25 °C and the reaction region located in the center of LED lamp) for 24 h. The resulting mixture is diluted with 3 mL EtOAc and quenched by 1.5 mL 2 N HCl, then stirred for 5 min. The reaction mixture is extracted by EtOAc six times, and the combined organic phases are concentrated in vacuo. The residue is purified by silica gel flash column chromatography (petroleum ether/EtOAc/AcOH 10/1/0.4%∼3/1/0.4%) to give the pure desired product. Further details may be found in the Supporting Information. Computational methods All the density functional theory (DFT) calculations are carried out with the Gaussian 0950 series of programs. DFT method ωB97XD51 with a standard 6–31+G(d) basis set is used for geometry optimizations. The solvent effects are considered by an universal solvation model based on solute electron density (SMD)52. Harmonic vibrational frequency calculations are performed for all of the stationary points to confirm them as local minima or transition structures, and to derive the thermochemical corrections for the enthalpies and free energies. The large basis set 6–311+G(d,p) is used to calculate the single-point energies to give more accurate energy information. Results and Discussion At the beginning of the research, we hypothesized that the addition of a proton source and the use of sterically hindered thiols might inhibit the side reactions, including thiocarboxylation and thiol–ene reactions. After systematic screening (Table 1), the selective hydrocarboxylation of methacrylate 1a proceeded in the presence of the easily available and bulky thiol, 2,4,6-triisopropylthiophenol, as well as NaOtBu and tBuOH to give the desired product 2a in 73% yield (Entry 1). The resulting chemo- and regioselectivities were both excellent; only very low yields of 2a′ (8% yield) and 2a″ (<5% yield) were obtained. Neither the Markovnikov-type hydro- nor thiocarboxylation was observed. Control experiments confirmed the essential roles of thiol, light, base, and CO2 (Entries 2–5). In the absence of tBuOH, 2a was obtained in a much lower yield (Entry 6). Using iPrOH instead of tBuOH gave a lower yield (Entry 7). The use of less sterically hindered thiols, such as 4-tert-butylthiophenol, resulted in poorer chemoselectivity (Entries 8–10). Other bases and solvents were tested, but they gave worse results (Entries 11 and 12). Table 1 | Optimization of the Reaction Conditions Entry Alteration Yield/2a Yield/2a′ Yield/2a″ 1 None 76% (73%) 8% <5% 2 Without ArSH N.D. N.D. N.D. 3 Without light N.D. 34% N.D. 4 Without NaOtBu N.D. 33% N.D. 5 Under N2 instead of CO2 N.D. N.D. N.D. 6 Without tBuOH 42% 6% 6% 7 iPrOH instead of tBuOH 65% 12% <5% 8 p-tBuC6H4SH as ArSH 64% 24% <5% 9 p-tBuC6H4SH, FeCl3 (5 mol %) Trace 5% 66% 10 Entry 9, tBuOH (2 equiv) Trace 16% 76% 11 K2CO3 instead of NaOtBu 46% 21% <5% 12 DMSO instead of NMP 33% 16% <5% Reaction conditions: 1a (0.2 mmol), 2,4,6-triisopropylthiophenol (ArSH, 0.4 mmol), NaOtBu (0.5 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h. NMR yields are determined by crude 1H NMR with CH2Br2 as internal standard. Yield of isolated product is provided in parenthesis. DMSO, dimethyl sulfoxide; LED, light-emitting diode; N.D., not detected; RT, room temperature. With satisfactory conditions (Condition A: Table 1, Entry 1) in hand, we aimed to investigate the scope of acrylates (Figure 2). A diversity of 2-methylacrylates bearing tertiary ( 1a– 1e), secondary ( 1f– 1h), and primary ( 1i and 1j) alkyl substituents in the ester moiety all underwent hydrocarboxylation under these conditions with high chemo- and regioselectivity. Besides methyl, other kinds of primary ( 1k– 1o), secondary ( 1p– 1r), and tertiary ( 1s) alkyl groups, as well as aryl groups ( 1t– 1w), were well tolerated at the α-position of tert-butyl acrylates. Excellent chemoselectivity was also observed in the hydrocarboxylation of 2-allylacrylate 1o. Furthermore, β-substituted acrylates also provided the corresponding products 2x– 2ba in good yields. Notably, the challenging tetrasubstituted acrylates with high steric hindrance, such as 1ca and 1da, also gave the desired products in moderate yields accompanied by recovery of the starting material (in 28% for 1ca and 44% yield for 1da, respectively) without any products of reduction, thiol–ene reaction and thiocarboxylation detected. Moreover, some acrylates ( 1ea– 1ia) with bioactive motifs, including isoborneol, L-(–)-menthol, α-terpineol, 4-carvomenthenol, and β-cholesterol, were applicable in this transformation, providing the corresponding acids in moderate to good yields. Figure 2 | Substrate scope of acrylates and styrenes. Condition A: Table 1, Entry 1, isolated yields. Condition B: 3 (0.2 mmol), p-tBuC6H4SH (0.4 mmol), NaOtBu (0.5 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h, isolated yields. Condition C: 1 or 3 (0.2 mmol), p-tBuC6H4SH (0.02 mmol), 4-PE-HE (0.6 mmol), NaOtBu (0.4 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h, isolated yields. aThe diastereoselectivity (d.r.) values are about 1∶1 by crude NMR. b3 mL of NMP was used. cd.r. = 3∶1. dd.r. = 1.8∶1. eThe Z/E ratio of acrylates 1 is 1∶1. fd.r. = 5∶1. gd.r. > 19∶1, cis-isomer (see Supporting Information for more details). h3 equiv of ArSH and 3.5 equiv of NaOtBu were used, 48 h. iGram scale. j2 equiv of p-tBuC6H4SNa was used instead of p-tBuC6H4SH, NaOtBu, and tBuOH. kThe ratio (95∶5) of 4p and 4a is determined by ultra-performance liquid chromatography (UPLC). LED, light-emitting diode; RT, room temperature; N.D., not detected; 4-PE-HE, 4-phenethyl Hantzsch ester. Download figure Download PowerPoint As 3-arylpropionic acids also are valuable compounds (Figure 1a),35,43–45 we further investigated the visible-light-driven photocatalyst-free anti-Markovnikov hydrocarboxylation of styrenes (Figure 2). Under slightly modified reaction conditions (Condition B: p-tBuC6H4SH is used instead of 2,4,6-triisopropylthiophenol in Condition A), diverse styrenes bearing either electron-neutral ( 3a– 3c, 3i, and 3m) or electron-donating ( 3d, 3f, and 3j) groups on the arene moiety all reacted smoothly to afford the desired products 4 in moderate to good yields. A diversity of functional groups and heteroarenes ( 3g and 3ba) were tolerated well. The electron-withdrawing CF3 group ( 3h) could also be compatible to give desired product in 31% yield when employing p-tBuC6H4SNa. Notably, o-allyloxystyrene 3l underwent the reaction smoothly to afford 4l with excellent chemoselectivity. 3-Chlorostyrene 3q provides desired product 4q in good yield with very little dehalogenation (95∶5 ratio 4q: 4a). The styrenes bearing mono-, di-, and trisubstituted arenes as well as α-substitution all were suitable substrates under our protocol. A bioactive product 4ia, an antidiabetic GPR40 agonist, was generated in 74% yield through the hydrocarboxylation of 3ia. More challenging substrates, such as trisubstituted alkene ( 3ma), could also give the desired product in 50% yield. Notably, the gram-scale reaction of 3a went smoothly to give 4a in 78%. Interestingly, a dihydrobenzothiazole could replace the aryl thiol for the generation of 4a in 68% under otherwise identical reaction conditions (see Supporting Information for more details). Given that the aryl thiolates act as the electron donors to form disulfides as main byproducts and are not incorporated into the desired products, we hypothesized whether we could realize a thiol catalysis in the presence of stoichiometric reductant. If successful, to the best of our knowledge, such a strategy would represent the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. However, significant challenges remained due to facile side reactions of aryl thiols with alkenes, including thiol–ene reaction and thiocarboxylation, which would consume the catalyst and thus terminate the catalytic cycle. Moreover, competitive reduction of alkenes, CO2 and disulfides by the external reductant might be difficult to control. Therefore, we strove to realize this organocatalytic strategy (see Supporting Information for more details). Fortunately, we found that 4-phenethyl Hantzsch ester (4-PE-HE)53 was an ideal reductant to promote the thiol catalysis, providing the desired product 3a in 70% yield (Figure 2). Under the catalytic reaction conditions (Condition C), diverse acrylates, such as those bearing different ester moieties ( 1a, 1f, 1h, and 1i), α-alkyl ( 1k, 1r, and 1s), α-phenyl ( 1t), and β-ethyl ( 1y) substitution, all were suitable substrates. Moreover, bioactive derivatives of acrylates, such as 1ha and 1ia, could also undergo such a transformation smoothly. Besides acrylates, styrenes bearing mono- ( 3b, 3d, 3e, 3i, and 3m) and disubstituted ( 3r and 3x) arenes delivered the desired products 4 smoothly. The successful application of α-aryl ( 3ca– 3ea) and α-methyl ( 3ja) styrenes in this catalytic hydrocarboxylation was also realized. To gain more insights into the reaction mechanism, we tested various control experiments (Figure 3). When 2,2,6,6-tetramethyl-1-piperinedinyloxy (TEMPO) was added to the reaction mixture, the desired product 2a was not detected (Figure 3a). "Radical clock" experiments resulted in exclusive formation of the corresponding ring-opening products (Figure 3b). These results suggested the formation of radical intermediates. In addition, some reductive coupling products of 3a, the reduction products of 3aa, and the reductive ring-opening products of 3la were also detected under N2 or CO2 (Figures 3c–3e). Cyclic voltammetry (CV) test demonstrated that the reaction mixture is highly reductive (see Supporting Information for more details). Electron paramagnetic resonance (EPR) spectroscopy further supported the reduction of alkenes to generate alkyl radicals in this system (Figures 3f and 3g). The experiments using 13C-labeled CO2 generated the 2a, sodium formate, and sodium oxalate, all with 13C-labeling (Figure 3h). Moreover, both sodium formate and sodium oxalate were detected under the reaction conditions in the absence of alkenes (Figure 3i), which suggested that CO2 could be also reduced in such highly reductive conditions. Figure 3 | (a–i) Mechanistic studies. EPR, electron paramagnetic resonance; HRMS, high-resolution mass spectra; LED, light-emitting diode; N.D., not detected; RT, room temperature; TEMPO, 2,2,6,6-tetramethyl-1-piperinedinyloxy. Download figure Download PowerPoint To probe our hypothesis of formation of CTC, we conducted the UV–vis absorption spectra tests. A slight bathochromic shift was observed when we tested the mixture of p-tBuC6H4SNa with either 1a or 3a (Figures 4a and 4b). When we used a more electron-deficient 4-(trifluoromethyl)styrene, the bathochromic shift was more significant (Figure 4c). To seek more evidence for the CTC formation, we carried out 19F-NMR experiments, mixing a certain amount of 4-(trifluoromethyl)styrene with increasing amounts of p-tBuC6H4SNa. The chemical shift of the CF3 group distinctly moved downfield (Figure 4d), indicating the increased electron density on the 4-(trifluoromethyl)styrene via electron donation from strongly reductive thiolates (−2.66 V vs Ag/Ag+; see Supporting Information for more details) to styrenes. All these results indicated the formation of CTC between electron-rich thiolates and electron-deficient styrenes. Figure 4 | UV–vis measurements and DFT calculations on the CTC between thiolate and acrylate/styrene. (a) UV–vis measurement of 1a. (b) UV–vis measurement of 3a. (c) UV–vis measurement of 4-(trifluoromethyl)styrene. (d) Correlation between the chemical shift of the CF3 group of 4-(trifluoromethyl)styrene and the relative p-tBuC6H4SNa concentration by 19F-NMR. (e) The energy of HOMO of CTC between thiolate and 1a is −6.45 eV. (f) The energy of LUMO of CTC between thiolate and 1a is +0.86 eV. (g) ESP surface of singlet state of CTC. (h) ESP surface of triplet state of CTC. CTC, charge-transfer complex; DFT, density functional theory; ESP, electrostatic potential; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital. Download figure Download PowerPoint Besides the experimental evidence, we further investigated the formation of CTC between thiolate and acrylate 1a using DFT calculations at the ωB97XD/6–31+G(d) level of theory. The energy gap was reduced as 7.31 eV (Figures 4e and 4f) compared with 1a and thiolates ( Supporting Information Figures S18a–S18d), which might arise from the electron donation from the highest occupied molecular orbital (HOMO) of thiolate to the lowest unoccupied molecular orbital (LUMO) of 1a. The reduced HOMO–LUMO gap explained well the bathochromic shift of absorption of CTC. Moreover, DFT calculations of electrostatic potential (ESP) surface of singlet and triplet states showed that the electron donation from thiolates to 1a take place in the excited state (Figures 4g and 4h). Although CO2 might also act as an electron acceptor, the DFT calculations indicated the formation of CTC between thiolate, and CO2 might be less favored because of its higher energy gap (8.06 eV) and excited energy (65.5 kcal/mol; Supporting Information Figures S21 and S23) than that of thiolate–alkene CTC (7.31 eV; 56.5 kcal/mol; Supporting Information Figure S23) in our reaction system. Based on the experimental and computational results, we proposed possible pathways for the hydrocarboxylation of 3a (Figure 5a). Irradiation of the CTC 5 might generate styrene radical anion 6 (Path a) or CO2 radical anion 7 (Path b) as well as thiol radical 8. reactions of 6 with CO2 and 7 with 3a could generate the 9, which might undergo to would to desired product Besides the desired hydrocarboxylation to give and thiocarboxylation are also possible via the intermediates 11 and which might be generated from the reaction of 8 and 3a or Figure 5 | and corresponding DFT CTC, charge-transfer complex; DFT, density functional theory. Download figure Download PowerPoint To probe the of such pathways (Figure a and we further DFT The CTC at triplet state could the electron to styrene or CO2 resulting styrene radical anion 6 or CO2 radical anion 7 suggested that CO2 reduction might be more (Figure Moreover, the density in styrene 3a and acrylate 1a radical anion was also with higher density in which explained the high regioselectivity (see Supporting Information for more details). We further investigated the of chemoselectivity by DFT calculations (Figure The radical addition of 3a with 7 through transition state the radical 9 with an energy of to the However, the corresponding energy for the radical addition of 3a with thiol which to the was to The generated radical 11 was also more than Therefore, the of hydrocarboxylation was and more than In addition, an radical addition of 3a with 7 through resulted in Markovnikov However, results showed that Markovnikov selectivity was due to its high energy in with anti-Markovnikov from 9, a higher energy of thiocarboxylation ( was which was to a for hydrocarboxylation with lower energy ( Further DFT calculations for different thiol catalysis and were also investigated in our reaction system for (see Supporting Information for more details). We have a novel CTC between thiolate and which is supported by UV–vis measurements, NMR and computational Based on the CTC, the first visible-light-driven photocatalyst-free anti-Markovnikov hydrocarboxylation of acrylates and styrenes with CO2 is realized to generate valuable succinic acid derivatives and 3-arylpropionic unusual and excellent and Moreover, this system is also and for diverse acrylates and styrenes with different including the challenging tri- and tetrasubstituted Notably, aryl thiol be used as a catalyst, representing the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. Further mechanistic studies and DFT calculations indicate that both alkene and CO2 radical anions might be generated in the reaction mixture. As this photocatalyst-free and photochemistry great potential for application in organic and the polymer further investigations and of this system are in our Supporting Information Supporting Information is of The the A on this is with the support was provided by the the of and the Sichuan and Technology and and the for the in Chen Yu light-driven organic in in König a in in in 8. Wang Wei Wang Wang of and of an C–S via of via Electron

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The electrochemical reduction of CO2/H2O using solid oxide electrolysis cells (SOECs) is one of the most promising approaches to achieve a carbon-neutral society. Although SOECs can produce syngas (H2/CO) from H2O and CO2, they have not received much attention for several decades because of the low price of fossil fuels; however, SOECs have regained global interest because of concerns about global warming and the fossil energy shortage. SOECs allow the use of pure CO2 to produce CO and O2, with potential applications in the production of oxygen to support life and exploration on Mars, where CO2 is abundant. Cathodic reaction process plays an important role in H2/CO production. Electrode structures such as metal catalyst, electrolyte structure may influence cathodic reaction process involving carbon deposition; however, little effort has been done to study cathodic reaction process using DFT (Density Functional Theory) calculation. DFT could offer a better atomic-scale understanding of this electrode. In this study, cathodic reaction process during CO2 electrolysis in SOECs was studied with experiments and DFT calculation.In the experiment, an electrolyte-supported disk-type cell, which was composed of YSZ (8mol% Y2O3-stabilized ZrO2), was used. A paste of NiO/YSZ (50/50 vol%) or NiO/GDC (50/50 vol%) was coated on the one side of the disk. The disk was dried, and then sintered at 1573 K for 4 hr. Using the similar procedure, a paste of LSM((La0.80Sr0.20)0.95MnO3-d)/YSZ (50/50 vol%) was coated on the other side of the disk. The disk was dried and sintered at 1473 K for 4 hr. These cells were defined as Ni/YSZ and Ni/GDC cells. The current–voltage (I–V) measurements were conducted at 1073 K. The cell was initially tested in SOFC mode, with humidified H2 and O2 gases introduced to the anode side (Ni/YSZ or Ni/GDC) and cathode side (LSM/YSZ), respectively. Then, the cell polarization was switched to SOEC mode, with humidified H2/CO2 (H2/CO2 = 7.5 vol%) and Ar gases introduced to the cathode (Ni/YSZ or Ni/GDC) and anode side (LSM/YSZ), respectively. The H2O/CO2 ratio was set to 1.8 vol%. In addition to electrochemical I–V measurements, the H2 and CO generated during constant electrolysis at 0.42 A/cm2 was withdrawn from the gas line using a gas-tight syringe and injected into a gas chromatograph (GC-8A, TCD, Shimadzu Corp.) for analysis. The Faradaic efficiency of H2 and CO production (h) was determined from the measured and theoretical gas production rates. The theoretical H2 and CO production rates were calculated from the current density on the basis of the reaction scheme (CO2+2e-→CO+O2−, H2O+2e-→H2+O2-). After electrolysis, heating was stopped, and the gas in the gas line was switched to Ar to terminate carbon deposition. Then, the Ni/YSZ and Ni/GDC surfaces were analyzed via scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy. In the DFT calculation, As a result, Ni/GDC outperformed Ni/YSZ in both SOFC and SOEC modes. Especially, Ni/GDC well outperformed Ni/YSZ during CO2/H2O electrolysis. Fig. 1 shows H2 and CO Faradaic efficiency during CO2/H2O electrolysis in Ni/YSZ and Ni/GDC at 0.42 A/cm2. Faradaic efficiencies of H2 and CO were decreased to approximately 60 % in Ni/YSZ, whereas those were kept at approximately 90 % in Ni/GDC. This indicated an input energy was converted to other products such as carbon in Ni/YSZ at high current densities. SEM observation showed severe carbon deposition was observed in Ni/YSZ, whereas no carbon was found in Ni/GDC.In the DFT calculation, the adsorption energies of H2O and CO2 were 0.32 eV and 0.65 eV for Ni/GDC. Those were 0.42 eV and 0.56 eV for Ni/YSZ. In triple phase boundary model, an energy difference greater than 0.2 eV was considered noteworthy [1]. Therefore, Ni reactivity was almost the same between Ni/YSZ and Ni/GDC. Therefore, it was suggested that GDC surfaces as double phase boundary caused the difference between Ni/YSZ and Ni/GDC cells.Reference[1] Watanabe, T. Ogura, Mechanisms of the carbon deposition at the Ni/YSZ interface: A combination study of microscopic observation and first-principles calculation, Int. J. Hydrogen Energy, 47 (2022) 29027-29036. Figure 1

A new theoretical model for inelastic tunneling in realistic systems : comparing STM simulations with experiments

Introduction Li2S-P2S5 glass has been attracted attention as the solid electrolyte for lithium ion battery because of its high ionic conductivity and wide electrochemical potential window. It is known that the ratio of Li2S and P2S5 affects Li-ion conductivity, and suggested that the atomic structure is related to Li ion conductivity. Atomic structures of Li2S-P2S5 glasses calculated by Reverse Monte Carlo (RMC) modeling previously reported.1,2RMC calculation results in structures satisfying the requirement for experimental data in totality, however, its local structure might have unreasonable configuration. In this study, we calculated reasonable Li3PS4, Li7P3S11 and Li4P2S7glass structure using combination of RMC calculations and density functional theory (DFT) calculations. Method Cubic cell whose lattice constant is about 22.5 Å and number of atoms is 567 were used for the initial structure of Li7P3S11 glass structure. Density is equal to experimental result. Li atoms, PS4 and P2S7molecules were randomly allocated in the cell. At first, RMC calculation was performed for the initial random structure. RMC calculation was carried out for the two structure-factor data S(Q) of Li7P3S11 glass using RMC++ code. S(Q) data were obtained from neutron diffraction and X-ray diffraction. Secondly, DFT calculation was performed for RMC structure using VASP code. In DFT part, internal atomic positions optimized until residual forces become less than 0.02 eV/Å. These RMC and DFT calculations were repeated until the difference of atomic structures between RMC and DFT become to be less than 0.1 Å. For comparison, smaller cells, 10 types of 325-atom cells and 100 types of 105-atom cells were calculated in the same way. Results and Discussion The structures calculated by RMC had good agreement with S(Q) in any cases as shown in Fig. 1, however, the densities of states are obviously different. While the last RMC structure had about 1.5 eV band gap (shown in blue line in Fig. 2), the 1st RMC structure was metallic (shown in red line in Fig. 2). The largest difference between 1st and last RMC structure is configuration of Li atoms. Figure 3 shows the pair distribution function of Li and Li. The last RMC g(r) is flat around 1, which means that Li atoms exist homogeneously in the cell. In contrast, the 1st RMC g(r) has a peak at 2.5 Å which is equal to the constraint distance between Li atoms setup condition in RMC. This suggests that Li configuration does not affect to the S(Q) in this system even if neutron diffraction is considered. Figure 4 shows the relative energies of Li7P3S11 based on Li2S and P2S5 energies against band gaps. The energies of glass structures are higher than crystal and lower than the mixture of Li2S and P2S5. Most energy fell within 0.03 eV/atom and band gaps fell within 0.6 eV. In summary, the combination of RMC and DFT calculation results proper atomic structures satisfying experimental data within satisfactory accuracy. Acknowledgment This work was supported by the RISING project of the New Energy and Industrial Technology Development Organization (NEDO).

On the difference in cycling behaviors of lithium-ion battery cell between the ethylene carbonate- and propylene carbonate-based electrolytes

Electrolytes, comprising salts, solvents, diluents, and various additives, must form a stable solid electrolyte interphase (SEI) to ensure the performance and durability of lithium (ion) batteries. However, the structure of the electric double layer (EDL) near charged surfaces remains unresolved, despite its critical role in determining which species can be reduced to form the SEI near the negative electrode. A recent hierarchical modeling framework [1] was developed to address two fundamental questions: (1) Which species accumulate at the negatively charged electrode surface in a complex multicomponent electrolyte? (2) At what voltage do these local EDL species undergo reduction, contributing to SEI formation? The first question is answered using classical molecular dynamics (MD) to simulate EDL structures. Statistically representative local solvation clusters are then used as input for density functional theory (DFT) calculations to address the second question. As the number of electrolyte components increases, the combinatorial growth in possible clusters within the EDL requiring DFT calculations creates a computational bottleneck. To overcome this, a gradient boosting regression model, initialized with a linear regression model, is implemented to accelerate the reduction voltage predictions with balanced interpolation and extrapolation performance. This DFT–MD–data hierarchical model has been successfully applied to various electrolytes, including 1 M carbonate-based systems,[1] low- and high-concentration ether electrolytes, localized high-concentration electrolytes (LHCEs),[2.3] and seven fluorinated formulations, demonstrating its utility in guiding SEI design through EDL understanding.[1] J. Am. Chem. Soc. 145, 4, 2473–2484 (2023)[2] Nature Materials 22, 1531–1539 (2023)[3] Energy Environ. Sci., 18, 3036-3046 (2025)

Molecular Simulations of Nanoscale Transformations in Ionic Semiconductor Nanocrystals

Most of the anodes of the state-of-the-art lithium ion batteries are made of graphite due to its specific properties such as relatively low cost, abundance, high energy density, power density, very long cycle life and hierarchical structure1. Because of the relatively wide inter space between two adjacent graphene layers, lithium ions easily intercalate into the graphite anode material, thus avoiding the structure, shape and size variations of the electrode material during the charge–discharge process2. Besides the advantages of using graphite as anode material, quite a few drawbacks of graphite anodes such as safety issues, low specific capacity and rate capacity have led to more research on the performance enhancement of carbon-based material anodes3. In our study we focused on enhancement the specific capacity of graphite. To enhance the specific capacity of the graphite electrode, modifying carbon electrode surfaces with heteroatom doping is an effective strategy4. In this work, we investigated the Li intercalation into germanium (Ge) doped graphite electrodes via density functional theory (DFT) calculations. The van der Waals density functional (vdW-DF) has been used to describe weak interaction between graphene layers. Also DFT calculations have been carried out in combination with the effective screening medium (ESM) to compute the surface that is charged up and/or exposed to an electric field5. For the DFT calculations with ESM, bilayer graphene slab model which is periodic in the direction parallel to the surface but is not periodic in the perpendicular direction, has been adopted to represent the graphite electrode. Sandwich slab model between two semi-finite media such as vacuum, an electrode, or an electrolyte has been formed. Charged C6LiC6 slab model is shown in Figure 1. We investigate both charge density difference, ∆ρ, and the electrostatic potential difference, ∆V of our lithium intercalated bilayer graphene model under charge addition and removal. In addition, our results indicate high Li capacity of heteroatom doped bilayer graphene electrodes compared to undoped one for Li-ion batteries.

Lithium ion batteries are widely employed in energy storage, but the connection between the molecular interactions in their electrolytes and the macroscopic properties remains elusive. Across three vastly different electrolytes, speciation and dynamics were studied via linear and nonlinear infrared spectroscopy to shed light on this relationship. The impact of mixed solvation on ionic speciation was studied from the perspective of the anion, which revealed a significant energetic favorability for the formation of contact ion pairs in linear carbonate solvents over cyclic carbonates. Infrared spectroscopy and density functional theory calculations described a complete inversion of the speciation due to solvent composition from 74% free anion in the cyclic carbonate to 95% contact ion pair in the linear carbonate. The impact of adding a highly-fluorinated additive on the speciation of two different electrolytes was studied via linear infrared spectroscopy. These fluorinated co-solvents are typically considered inert species with no impact on the speciation, but the research presented opposite trends in infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, electrochemistry, and conductivity for the two different electrolytes, indicating a change in speciation with co-solvent concentration. To explain the interesting dependence of the change in speciation on the anion, density function theory (DFT) calculations showed the formation of weak hydrogen bonds between the TFSI- anion and the co-solvent; this interaction is not observed with the PF6- anion. Finally, a polyacrylonitrile polymer gel electrolyte was investigated. The results from linear spectroscopy showed different interactions in the electrolyte with increasing polymer concentration; polymer addition also raised the viscosity of the sample by orders of magnitude, changing the sample composition from a liquid electrolyte to a room-temperature gel. Interestingly, the dynamics determined from 2DIR spectroscopy are similar across sample. A molecular picture was proposed of the nitrile side- vii chains interacting at the highest polymer concentration, forming channels to facilitate the flow of ions; these interactions were confirmed via differential scanning calorimetry. This synergistic approach enabled the complete characterization of these complex systems from multiple perspectives to fully understanding the unintuitive way the molecular interactions can alter the macroscopic properties.

Site preference-based luminescence studies in Eu doped calcium magnesium silicate phosphor: A combined experimental and DFT approach