An organic polymer nano-photocatalyst has been developed for highly efficient light driven proton reduction.

In recent years, the global climate change is in evidence and it is almost a consensus that it is caused by the greenhouse gases emissions. An alternative to reduce these emissions is carbon capture and storage (CCS), which employs solvents based on amine compounds. In this scene, ionic liquids (IL) have been investigated to a greater extent for this application. In this work, we make an evaluation of interactions between gases (CO2, SO2, and H2S) and anion/cation from IL, as well as cation-anion interactions. For this, quantum calculations under vacuum were performed at the B3LYP/6-311+G** level of theory and using the M06-2X functional, where dispersion effects are considered. Among the well-studied systems based on imidazolium cations and fluorinated anions, we also studied the tetraalkylammonium, tetraalkylphosphonium, ether-functionalized imidazolium based systems, and tetrahexylammonium bis(trifluoromethanesulfonyl)imide, [THA][Tf2N], as a potential prototype. The ion pairs evaluated include [Tf2N](-)-based IL, with alkyl chain varying from [C1mim](+) to [C8mim](+) and [C1mim](+)-based IL. We found that the anion becomes more available to interact with gas with the weakening of the cation-anion interaction. [THA][Tf2N] has a binding energy of -274.89 kJ/mol at the B3LYP/6-311+G** level of theory, which is considered energetically interesting to gas capture applications.

A set of fluorene-based polymers with a donor−acceptor architecture has been investigated as a potential candidate for photocatalytic hydrogen evolution reaction. A design protocol has been employed based on first-principles theory and focusing on the following properties: (i) broad absorption spectrum to promote a higher number of photogenerated electron−hole pairs, (ii) suitable redox potentials, and (iii) appropriate reaction thermodynamics using the hydrogen-binding energy as a descriptor. We have found that the polymers containing a fused-ring acceptor formed by benzo(triazole-thiadiazole) or benzo(triazole-selenodiazole) units display a suitable combination of such properties and stand out as potential candidates. In particular, PFO-DSeBTrT (poly (9,9′-dioctylfluorene)-2,7-diyl-alt-(4,7-bis(thien-2yl)-2-dodecyl-benzo-(1,2c:4,5c′)-1,2,3-triazole-2,1,3-selenodiazole)) has an absorption maximum at around 950 nm for the highest occupied molecular orbital−lowest unoccupied molecular orbital transition, covering a wider range of solar emission spectrum, and a reduction catalytic power of 0.78 eV. It also displays a calculated hydrogen-binding free energy of ΔG H = 0.02 eV, which is lower in absolute value than that of Pt (ΔG H ≈ −0.10 eV). Furthermore, the results and trends analysis provide guidance for the rational design of novel photo-electrocatalysts.

The electrochemical reduction of carbon dioxide into carbon monoxide, hydrocarbons and formic acid has offered an interesting alternative for a sustainable energy scenario. In this context, Sn-based electrodes have attracted a great deal of attention because they present low price and toxicity, as well as high faradaic efficiency (FE) for formic acid (or formate) production at relatively low overpotentials. In this work, we investigate the role of tin oxide surfaces on Sn-based electrodes for carbon dioxide reduction into formate by means of experimental and theoretical methods. Cyclic voltammetry measurements of Sn-based electrodes, with different initial degree of oxidation, result in similar onset potentials for the CO2 reduction to formate, ca. −0.8 to −0.9 V vs. reversible hydrogen electrode (RHE), with faradaic efficiencies of about 90–92% at −1.25 V (vs. RHE). These results indicate that under in-situ conditions, the electrode surfaces might converge to very similar structures, with partially reduced or metastable Sn oxides, which serve as active sites for the CO2 reduction. The high faradaic efficiencies of the Sn electrodes brought by the etching/air exposition procedure is ascribed to the formation of a Sn oxide layer with optimized thickness, which is persistent under in situ conditions. Such oxide layer enables the CO2 “activation”, also favoring the electron transfer during the CO2 reduction reaction due to its better electric conductivity. In order to elucidate the reaction mechanism, we have performed density functional theory calculations on different slab models starting from the bulk SnO and Sn6O4(OH)4 compounds with focus on the formation of -OH groups at the water-oxide interface. We have found that the insertion of CO2 into the Sn-OH bond is thermodynamically favorable, leading to the stabilization of the tin-carbonate species, which is subsequently reduced to produce formic acid through a proton-coupled electron transfer process. The calculated potential for CO2 reduction (E = −1.09 V vs. RHE) displays good agreement with the experimental findings and, therefore, support the CO2 insertion onto Sn-oxide as a plausible mechanism for the CO2 reduction in the potential domain where metastable oxides are still present on the Sn surface. These results not only rationalize a number of literature divergent reports but also provide a guideline for the design of efficient CO2 reduction electrocatalysts.

Recent advances in operando-synchrotron-based X-ray techniques are making it possible to address fundamental questions related to complex proton-coupled electron transfer reactions, for instance, the electrocatalytic water splitting process. However, it is still a grand challenge to assess the ability of the different techniques to characterize the relevant intermediates, with minimal interference on the reaction mechanism. To this end, we have developed a novel methodology employing X-ray photoelectron spectroscopy (XPS) in connection with the liquid-jet approach to probe the electrochemical properties of a model electrocatalyst, [Ru II (bpy) 2 (py)-(OH 2 )] 2+ , in an aqueous environment. There is a unique fingerprint of the extremely important higher-valence ruthenium−oxo species in the XPS spectra along the oxidation reaction pathway. Furthermore, a sequential method combining quantum mechanics and molecular mechanics is used to illuminate the underlying physical chemistry of such systems. This study provides the basis for the future development of in-operando XPS techniques for water oxidation reactions.

Polymeric materials containing an extended π-conjugated backbone have shown a wide range of applicability including photocatalytic activity for the hydrogen evolution reaction (HER). The latter requires highly efficient materials with optimal light absorption and thermodynamic driving force for charge transfer processes, properties that are tailored by linking chemical units with distinct electron affinity to form a donor−acceptor architecture. Here, this concept is explored by means of ab initio theory in benzothiadiazole-based polymers with varying electron-rich moieties, viz., fluorene (PFO), cyclopentadithiophene (CPT), methoxybenzodithiophene (O-BzT), thiophenebenzodithiophene (T-BzT), and thiophene (T, VT)and thienethiophene (TT, VTT)-based units. All materials exhibit a red-shifted absorption spectrum with respect to the reference polymer (PFO-DT-BT) while keeping the catalytic power for hydrogen production almost unchanged. In particular, a displacement of Δλ = 167 nm in the first absorption maximum has been achieved upon combination of chemical units with high donating character in CPT-VTT-BT. Furthermore, the exciton binding energies (E b ) have been systematically investigated to unveil the effects of geometry relaxation, environment polarity, and finite temperature contributions to the free energy. For instance, we show a significant change in E b when going from the gas phase (E b = 1.43−1.85 eV) to the solvent environment (E b = 0.29−0.54 eV in 1-bromooctane with ε = 5.02). Furthermore, we have found a linear correlation between the lowering of exciton binding energies and the increasing of the ratio between donor and acceptor contributions to the HOMO orbital. This is a consequence of increased donating ability and enhanced spatial separation of electron−hole pairs, which weakens their interaction. Finally, our findings reveal that the donor unit plays a crucial role in key properties that govern the photocatalytic activity of donor−acceptor polymers contributing to the development of a practical guideline to design more efficient photocatalysts for the HER. This goes through a proper combination of electron-rich moieties to tune the optical gap, favor thermodynamic driving force for charge transfer, and lower exciton binding energies.

Small-molecules (SM) have attracted a great deal of attention in the field of solar energy conversion due to their unique properties compared to polymers, such as well-defined molecular weights and lack of regio-isomeric impurities. Furthermore, these materials can be synthesized in a variety of configurational architectures, representing an opportunity for tailoring chemical and optical properties that could lead to a better photocatalytic efficiency for hydrogen generation. Here, we evaluate, by means of density functional theory (DFT) and time-dependent DFT methods, a set of smallmolecules with A-D-A architecture (A, acceptor; D, donor) based on well-known building blocks like thiophene (T), cyclopentadithiophene (CPT), and benzothiadiazole (BT) as potential candidates for photocatalytic hydrogen evolution reaction (HER). We also propose (i) the replacement of the thiophene unit by 3,4ethylenedioxythiophene (EDOT) to form with a CPT unit an extended donor core, (ii) an additional acceptor unit, the 1,3,4-thiadiazole (Tz), in the extremities, and (iii) insertion of the difluoromethoxy (DFM) as a substituent in the BT unit. Our outcomes reveal that these materials have a broad absorption spectrum with λ = 318 to 719 nm being the most intense absorption peak originated from an electronic transition with charge-transfer nature, as the spatial distribution of LUMO is concentrated on the acceptor units for all materials. Moreover, these small-molecules not only present catalytic power or thermodynamic driving force to carry out the chemical reactions involved in the process of hydrogen production but also can be coupled in cooperative photocatalytic systems to promote intramolecular charge transfer that is expected to boost the overall photocatalytic efficiency of these materials.

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