CO2 Reduction System Research Articles

Dynamics of Electron Transfers in Photosensitization Reactions of Zinc Porphyrin Derivatives

Photocatalytic systems for CO2 reduction operate via complicated multi-electron transfer (ET) processes. A complete understanding of these ET dynamics can be challenging but is key to improving the efficiency of CO2 conversion. Here, we report the ET dynamics of a series of zinc porphyrin derivatives (ZnPs) in the photosensitization reactions where sequential ET reactions of ZnPs occur with a sacrificial electron donor (SED) and then with TiO2. We employed picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption (TA) measurement to investigate the fast ET dynamics concealed in the steady-state or slow time-resolved measurements. As a result, Stern-Volmer analysis of fluorescence lifetimes evidenced that the reaction of photoexcited ZnPs with SED involves static and dynamic quenching. The global fits to the TA spectra identified much faster ET dynamics on a few nanosecond-time scales in the reactions of one-electron reduced species (ZnPs•-) with TiO2 compared to previously measured minute-scale quenching dynamics and even diffusion rates. We propose that these dynamics report the ET dynamics of ZnPs•- formed at adjacent TiO2 without involving diffusion. This study highlights the importance of ultrafast time-resolved spectroscopy for elucidating the detailed ET dynamics in photosensitization reactions.

In-depth insight into the mechanism on photocatalytic selective CO2 reduction coupled with tetracycline oxidation over BiO1-xBr/g-C3N4

To effectively alleviate environmental pollution and energy shortage and simultaneously utilize the photo-induced electrons and holes in one system. Herein, a dual-purpose reaction system is structured by coupling CO2 reduction with tetracycline (TC) mineralization, which is realized using model BiOBr/g-C3N4 photocatalyst with rich oxygen vacancies (OVs) (donated as BiO1-xBr/g-C3N4). Compared to two half-reactions of the individual CO2 photoreduction and independent TC oxidation under Ar atmosphere, such designed coupled reaction system presents much superior catalytic performance than those of two half-reactions. Another crucial finding is that the concentration and molecular structure of TC directly affect the selectivity and synergistic efficiency of photocatalytic CO2 reduction to CO. The mechanism of cooperative effect is discussed deeply according to theoretical calculation and experiments. Besides, the intermediates and pathway of TC degradation are extremely inquired by LC-MS. The work is expected to provide beneficial guidance on reasonably devising the bifunctional catalysts with efficacious integrating reaction systems for coupling CO2 reduction with antibiotic removal.

Homoleptic Al(III) Photosensitizers for Durable CO2 Photoreduction

Exploiting noble-metal-free systems for high-performance photocatalytic CO2 reduction still presents a key challenge, partially due to the long-standing difficulties in developing potent and durable earth-abundant photosensitizers. Therefore, based on the very cheap aluminum metal, we have deployed a systematic series of homoleptic Al(III) photosensitizers featuring 2-pyridylpyrrolide ligands for CO2 photoreduction. The combined studies of steady-state and time-resolved spectroscopy as well as quantum chemical calculations demonstrate that in anerobic CH3CN solutions at room temperature, visible-light excitation of the Al(III) photosensitizers leads to an efficient population of singlet excited states with nanosecond-scale lifetimes and notable emission quantum yields (10-40%). The results of transient absorption spectroscopy further identified the presence of emissive singlet and unexpectedly nonemissive triplet excited states. More importantly, the introduction of methyl groups at the pyrrolide rings can greatly improve the visible-light absorption, reducing power, and durability of the Al(III) photosensitizers. With triethanolamine, BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole), and an Fe(II)-quaterpyridine catalyst, the most methylated Al(III) photosensitizer achieves an apparent quantum efficiency of 2.8% at 450 nm for selective (>99%) CO2-to-CO conversion, which is nearly 28 times that of the unmethylated one (0.1%) under identical conditions. The optimal system realizes a maximum turnover number of 10250 and higher robustness than the systems with Ru(II) and Cu(I) benchmark photosensitizers. Quenching experiments using fluorescence spectroscopy elucidate that the photoinduced electron transfer in the Al(III)-sensitized system follows a reductive quenching pathway. The remarkable tunability and cost efficiency of these Al(III) photosensitizers should allow them as promising components in noble-metal-free systems for solar fuel conversion.

Efficient expression of heterologous protein by engineered Komagataella phaffii by harnessing a bioelectrical CO2 reduction system

Methylotrophic yeast Komagataella phaffii (syn. Pichia pastoris) is an important host for heterologous protein expression although its traditional inducer methanol is toxic and flammable. Using green fluorescent protein (GFP) as a model of heterologous protein in this research, recombinant K. phaffii was constructed through wild-type formate dehydrogenase gene knockout, intracellular and extracellular expression of Candida boidinii formate dehydrogenase gene under sorbitol dehydrogenase promoter (PSDH) respectively to reduce carbon dioxide (CO2) to formate in a two-chamber H-shaped three-electrode bioelectrical reactor equipped with a solar panel stage by stage. Generated formate in the bioelectrical reactor was used as a safe inducer of engineered K. phaffii strains expressing GFP successfully. Further, the electrical potential was optimized and three cathodes, graphite disk electrode, graphite rod electrode, and glassy carbon electrode, were used and modified subsequently with poly(neutral red) to obtain the highest volumetric GFP intensity and specific GFP intensity as 4424.5 a.u. and 384.0 a.u./OD600, which was 1.7- and 3.2- fold of that of methanol induction respectively, 3.9-fold of that formate induction. Therefore, a safe recombinant K. phaffii system was developed successfully for heterologous protein expression, and demonstrated a new approach to utilize CO2.

Trade-Off of Metal Sites in Fe-Ni Bimetal Metal-Organic Frameworks for Efficient CO2 Photoreduction with Nearly 100% CO Selectivity.

This work disclosed the trade-off effect of two metal sites, which display distinct, key functionalities in naturally occurring and artificial catalysts for developing an advanced CO2 reduction system. To exploit the metal-organic frameworks (MOFs) as advanced catalysts, we prepared a series of Prussian blue analogues (FeNix PBAs) of tunable Ni/Fe molar ratio without changing the oxidation state of Fe and Ni for use as a photocatalyst in the CO2 reduction reaction (CRR). The FeNi0.66 PBA gives a superior CO yield rate (14.28 mmol·g-1·h-1) with nearly 100% CO selectivity, but the PBA would be basically CRR-inactive without either Ni or Fe. Experimental and calculation studies demonstrate that Fe and Ni display distinct functionalities. Specifically, Fe is an efficient mediator that boosts the electron transfer both from the photosensitizer to FeNix PBA and from FeNix PBA to CO2, and Ni serves as the active site for CO2 adsorption and reduction. Intriguingly, when there is already sufficient Ni in the catalyst, further increase of the Ni content gives marginal gains in the CO2 adsorption affinity that cannot offset the weakened electron transfer due to the Ni excess. The findings can help advance the design of bimetallic MOF catalysts that mimic naturally occurring bimetallic catalysts.

Modulating CoFeOX Nanosheets Towards Enhanced CO2 Photoreduction to Syngas: Effect of Calcination Temperature and Mixed-Valence Multi-Metals.

CoFeOX nanosheets were synthesized by a facile coprecipitation and calcination method. The effect of calcination temperature on the crystal texture, morphology and surface areas of CoFeOX were fully explored. CoFeOX sample calcined at 600 °C (CoFeOX -600) showed superior catalytic performance for the reduction of CO2 under visible light. Compared with the pure Ru(bpy)3 2+ -sensitized CO2 reduction system, the CoFeOX -added system achieved 19-fold enhancement of CO production (45.7 μmol/h). The mixed valence state and nanosheet-like structure of CoFeOX cocatalyst support its ultra-high charge transfer and abundant CO2 active adsorption sites exposure, which promote the separation of photogenerated charges, and thus improve the photocatalytic CO2 reduction activity. Carbon source of CO from CO2 was verified by 13 CO2 isotopic labelling experiment. Repeated activity experiments confirmed the good stability of CoFeOX in the CO2 photoreduction system. This work would provide prospective insights into developing novel cost-effective, efficient, and durable non-precious metal cocatalysts to improve the efficiency of photocatalytic reduction of CO2 .

Enwrapping g-C3N4 on In2O3 hollow hexagonal tubular for photocatalytic CO2 conversion: Construction, characterization, and Z-scheme mechanism insight

The reduction of CO2 achieved by photocatalysis can simultaneously alleviate the energy crisis and solve environmental issues. Nevertheless, it remains challenging for the rational design of photocatalysts with high-efficiency carrier migration ability. Herein, the Z-scheme g-C3N4/In2O3 (CN/INO) heterostructure was fabricated via metal–organic frameworks (MOFs) assisted thermal deposition which could form a fully encapsulated hollow tubular structure. The unique structure was based on the MOFs-derived hollow hexagonal In2O3 tubular integrated with ultrathin g-C3N4. The Z-scheme CN/INO heterojunction exhibited a larger specific surface area and excellent charge separation efficiency. Benefiting from the above features, the Z-scheme CN/INO heterojunction demonstrated superior performance on photocatalytic CO2 reduction. The formation of CO and CH3OH over the optimized CN/INO-2 catalyst was 7.94 and 1.44 µmol⋅g−1⋅h−1, respectively. Moreover, the density functional theory (DFT) calculations and Kelvin probe force microscopy (KPFM) was carried out to further investigate the situation of charge transfer on the interface of CN/INO. The in-situ Fourier transform infrared spectroscopy (FTIR) was measured to confirm the immediate products and the possible mechanism of photocatalytic CO2 reduction was proposed. This work provided a MOFs-assisted strategy to construct a Z-scheme system for photocatalytic CO2 reduction.

Sustainable Carbon Dioxide Reduction of the P3HT Polymer-Sensitized TiO2/Re(I) Photocatalyst.

In this study, a p-type π-conjugated polymer chain, poly(3-hexylthiophene-2,5-diyl) (P3HT), was physically adsorbed onto n-type TiO2 nanoparticles functionalized with a molecular CO2 reduction catalyst, (4,4-Y2-bpy)ReI(CO)3Cl (ReP, Y = CH2PO(OH)2), to generate a new type of P3HT-heterogenized hybrid system (P3HT/TiO2/ReP), and its photosensitizing properties were assessed in a heteroternary system for photochemical CO2 reduction. We found that P3HT immobilization on TiO2 facilitated photoinduced electron transfer (PET) from photoactivated P3HT* to the n-type TiO2 semiconductor via rapid interfacial electron injection (∼65 ps) at the P3HT and TiO2 surface interface (P3HT* → TiO2). With such effective charge separation, the heterogenization of P3HT onto TiO2 resulted in a steady electron supply toward the co-adsorbed Re(I) catalyst, attaining durable catalytic activity with a turnover number (TON) of ∼5300 over an extended time period of 655 h over five consecutive photoreactions, without deformation of the adsorbed P3HT polymer. The long-period structural stability of TiO2-adsorbed P3HT was verified based on a comparative analysis of its photophysical properties before and after 655 h of photolysis. To our knowledge, this conversion activity is the highest reported so far for polymer-sensitized photochemical CO2 reduction systems. This investigation provides insights and design guidelines for photocatalytic systems that utilize organic photoactive polymers as photosensitizing units.

Boosting photocatalytic CO2 reduction by tuning photogenerated carrier kinetics in two-dimensional WOx/BiOCl S-scheme heterojunction with oxygen vacancies

Photocatalytic CO2 reduction driven by sustainable solar energy is a highly desirable route to achieve carbon recycling, to which charge separation in photocatalysts holds the key. In this work, two-dimensional (2D) WO3·H2O nanosheets with rich oxygen vacancies (Vo-WOx) were combined with 2D BiOCl nanosheets to form a 2D/2D Vo-WOx/BiOCl S-scheme heterojunction by a self-assembly process. The 2D/2D stacked structure combined with the introduction of oxygen vacancies in Vo-WOx could improve the light harvesting of the designed photocatalytic system. The dual transfer pathways in the S-scheme junction were induced by the fact that the photogenerated electrons on the conduction band and defect energy level of the Vo-WOx transfer across the interface to combine with the BiOCl, leading to an improved kinetic process of photogenerated carriers. Moreover, the S-scheme configuration can retain a strong redox potential for satisfying the thermodynamic requirement of CO2 reduction. As a result, the optimized 30Vo-WOx/BiOCl composite can achieve the highest CO yield rate of 8.82 μmol h−1 from CO2 reduction without any sacrifice reagents, which was 4.69 and 3.08 times higher than that of pure BiOCl and WO3/BiOCl without oxygen vacancy. This work provides a new view of defect engineering in S-scheme heterojunction for pursuing a highly efficient photocatalytic CO2 reduction system.

(Invited, Digital Presentation) Interface Rich Catalyst Design for Enhanced CO2 Conversion

The efficient electrochemical conversion of CO2 to fuels or stock chemicals with high-energy density would be a major step forward in the introduction of a carbon neutral energy cycle. Especially, understanding the role of electrocatalysts, supports, and electrolytes that can efficiently reduce CO2 to fuels with high selectivity is a subject of significant interest. Copper is the only known catalyst for producing a reasonable quantity of hydrocarbons, which means that designing proper electrode interfaces would modulate the catalytic reactivity and product selectivity. One of the observations on copper catalyst interface is that copper catalyst with a specific atomic-scale gap accelerates the reaction kinetics and selectivity to C2+ products, by confining CO2 and reaction intermediates within a sub-nanoscale reactor in which the reaction energy for CO formation and subsequent C-C coupling can be accelerated. However, the selectivity control toward a target product such as ethylene or ethanol is a remaining issue to be resolved. Recently, we have designed interface rich CuO-Al2CuO4 catalyst and were able to reach about 80% FE of ethylene. In-situ ATR-FTIR and DFT calculation supported that CO2 was selectively converted to CO on CuO and subsequently CO was coupling on Al2CuO4 to ethylene. Therefore, designing interface rich catalysts offers efficient, yet cheap electrochemical CO2 reduction systems.

Nanomaterials Facilitating Conversion Efficiency Strategies for Microbial CO2 Reduction.

Microbial electro- and photoelectrochemical CO2 reduction represents an opportunity to tackle the environmental demand for sustainable fuel production. Nanomaterials critically impact the electricity- and solar-driven microbial CO2 reduction processes. This minireview comprehensively summarizes the recent developments in the configuration and design of nanomaterials for enhancement of the bacterial adhesion and extracellular electron transfer (EET) processes, based on the modification technologies of improving chemical stability, electrochemical conductivity, biocompatibility, and surface area. Furthermore, the investigation of incorporating non-photosynthetic microorganisms using advanced light-harvesting nanostructured photoelectrodes for solar-to-chemical conversion, as well as the current understanding of EET mechanisms occurring at photosynthetic semiconductor nanomaterials-bacteria biohybrid interface is detailed. The crucial factors influencing the performance of microbial CO2 reduction systems and future perspectives are discussed to provide guidance for the realization of their large-scale application.

Photocatalytic Hydrogen Production under Water Vapor Feeding─A Minireview

Recent advances in photocatalytic water splitting in particulate semiconductor systems have been tremendous in terms of efficiency and scalable photoreactor design. In general, photocatalyst powders are immersed in liquid water, leading to higher efficiency than gas-fed water splitting systems. Conducting a gas-phase photocatalytic reaction under water vapor, however, has several advantages versus liquid water, such as easy scalability and suppression of leaching of the catalyst components. Here, we offer a historical overview of photocatalytic water splitting under vapor feeding. Moreover, the principles of water adsorption are reviewed to understand the basic phenomena occurring on the surface of semiconductor powders. The critical role of relative humidity is identified, including water on the surface for ion conductivity, which can close the electric circuit between cathodic and anodic reaction sites. Finally, on the basis of the cross-sectional discussion of reported works in both water vapor splitting and physical property of the adsorbed water layer, how to develop the photocatalytic water vapor splitting system and the perspective of the CO2 and N2 reduction system using water vapor as an electron donor are provided.

Advances and challenges in developing cocatalysts for photocatalytic conversion of carbon dioxide to fuels

The global adoption of efficient sustainable energy sources is a crucial step toward meeting energy demands while achieving carbon emission reduction targets. Solar energy has become a clean and cost-competitive alternative to traditional fossil fuels, but the intermittent nature of sunlight results in challenges associated with energy storage and transport. Photocatalytic carbon dioxide reduction intends to mimic natural photosynthesis for utilizing sunlight to chemically convert water and CO2 into fuels. In this process, the solar energy is captured and stored in fuels, so-called solar fuels, for widespread on-demand use. Heterogeneous solar fuel production systems are multi-component, comprising light-harvesting (photosensitizer) and catalytic (cocatalyst) units. Cocatalysts are indispensable for photocatalytic CO2 reduction systems, which promote charge carrier separation and transport, reduce the reaction activation energy, and alter the reaction route, thereby enhancing the activity and selectivity of the photocatalytic reactions. This review presents a comprehensive summary of the recent advancements in cocatalysts for photocatalytic CO2 reduction reaction (CO2RR), with the purpose of providing new insights and guidance to the field with regard to research directions and best practices. We summarize how various cocatalysts including inorganic nanoparticles, metal complexes, enzymes, and bacteria can be combined with semiconductor photosensitizer for light-driven photocatalytic CO2RR. Side-by-side comparisons reveal the strengths and limitations of each kind of cocatalysts and how lessons extracted from studying natural photosynthetic systems can be applied to investigations of artificial photosynthesis, presenting an outlook discussing possible future concepts for a more effective photocatalytic CO2 reduction process.

First-principles studies of monolayers MoSi2N4 decorated with transition metal single-atom for visible light-driven high-efficient CO2 reduction by strain and electronic engineering

Solar-driven CO2 reduction into value-added hydrocarbons could promote CO2 utilization and solve related environmental issues, while the key lies in the modulation of the active site’s orbital electronic state. By performing first-principles calculations, we systematically explore MoSi2N4 decorated with 3d and 4d period transition metal single atoms (TMSAs) as efficient CO2 reduction photocatalysts and find that product selectivity is highly correlated with the d-band center of TMSAs. Compressive and tensile strains in mutually perpendicular directions are introduced in the horizontal plane to improve the catalytic performance. After filtering, V, Mn, and Ni were found to be the best TMSAs for CO, HCOOH, and CH4 production, respectively. Ni/MoSi2N4 under −6% tensile strain exhibited the lowest CO2 reduction limiting potential (UL) of 0.24 V. Notably, the time-dependent ab initio nonadiabatic molecular dynamics simulations were carried out to elucidate the photogenerated electron transport properties, where the photo-generated electrons can be efficiently transferred from MoSi2N4 to TMSAs active sites. We also identify that the applied strains affect the CO2 reduction performance by modulating the d-band center and valence electrons. These findings provide pioneering design schemes of precisely tailored photocatalytic systems for CO2 reduction under visible light.

Metal-Free Semiconductor-Based Bio-Nano Hybrids for Sustainable CO2 -to-CH4 Conversion with High Quantum Yield.

Bio-nano hybrids with methanogens and nano-semiconductors provide an innovative strategy for solar-driven CO2 -to-CH4 conversion; however, the efficiency mismatch between electron production and utilisation results in low quantum yield and CH4 selectivity. Herein, we report the integration of metal-free polymeric carbon nitrides (CNx ) decorated with cyanamide (NCN) groups and Methanosarcina barkeri (M. b). The self-assembled M. b-NCN CNx exhibited a quantum yield of 50.3 % with 92.3 % CH4 selectivity under illumination, which outperforms other reported bio-nano hybrid systems and photocatalytic systems for CO2 reduction. This excellent performance was attributed to the distinct capacitance and conductive effects of NCN CNx , which promoted electron storage and redistribution at the biotic-abiotic interface to alleviate recombination losses and side reaction. This study provides new design guidelines for bio-nano hybrids for the sustainable photocatalytic reduction of CO2 into fuels.

Metal‐Free Semiconductor‐Based Bio‐Nano Hybrids for Sustainable CO2‐to‐CH4Conversion with High Quantum Yield

AbstractBio‐nano hybrids with methanogens and nano‐semiconductors provide an innovative strategy for solar‐driven CO2‐to‐CH4conversion; however, the efficiency mismatch between electron production and utilisation results in low quantum yield and CH4selectivity. Herein, we report the integration of metal‐free polymeric carbon nitrides (CNx) decorated with cyanamide (NCN) groups andMethanosarcina barkeri(M. b). The self‐assembledM. b‐NCNCNxexhibited a quantum yield of 50.3 % with 92.3 % CH4selectivity under illumination, which outperforms other reported bio‐nano hybrid systems and photocatalytic systems for CO2reduction. This excellent performance was attributed to the distinct capacitance and conductive effects ofNCNCNx, which promoted electron storage and redistribution at the biotic–abiotic interface to alleviate recombination losses and side reaction. This study provides new design guidelines for bio‐nano hybrids for the sustainable photocatalytic reduction of CO2into fuels.

A critical review on advances in TiO2-based photocatalytic systems for CO2 reduction

The growing world population and uncontrolled industrial growth have resulted in elevated energy consumption and harsh emission of toxic agents tearing down the ecosystem. The transformation of a greenhouse gas like CO2 into energy sources, i.e., CO, CH3OH, CH4, etc. is a promising strategy to alleviate the interlinked issues of environmental pollution, global warming and climate change. This article presents an overview of the fundamentals of photocatalytic systems and the latest developments in CO2 chemical reduction towards solar fuels over titanium dioxide (TiO2). The basic principle of photocatalysis states new photocatalyst synthesis and how the current materials can be used to improve the existing catalysts, focusing on the surface area, electrical conductivity, and chemical stability of TiO2. In the study of the state of the art it was observed that at a lower CH4/CO2 ratio, more H2 was produced, while a higher feed ratio promoted CO production. The maximum production rate reported for CH4 was 0.64 mmol g-1h−1, for CO was 158 mmol/h/gcat, and 102 mmol h-1Kgcat_1. Detailed insights for enhanced performance, especially factors affecting mass transfer, thermodynamics, selectivity, and reaction mechanism are provided and discussed. This review summarizes and analyzes research work on the photoreduction of CO2 in the presence of TiO2 over the past ten years. State of the art in photocatalytic CO2 chemical reduction over TiO2 emphasizes material design and reactor configurations. One section is dedicated to reactor design optimization and studying the synergic effect in photocatalysis. Also, brief perspectives and the main challenges in photocatalytic CO2 reduction are outlined, which are vital for improving the conversion efficiency of CO2. The use of solar energy to drive CO2 photocatalytic reactions and the recent progress in this area are also explored and discussed.

Understanding Active Sites in Molecular (Photo)Electrocatalysis through Vibrational Spectroelectrochemistry

Molecularly defined (photo)electrocatalysts have great potential to perform important energy conversion reactions with high efficiency and selectivity. A particular advantage is their modular design, which allows their reactivity to be tailored to a specific reaction. However, to advance their rational development, a deep mechanistic understanding of the catalytic reactions is required. In situ vibrational spectrochemistry, in particular confocal Raman and infrared absorption spectroscopy, is an extremely powerful method to gain deep insights into the structure and reactivity of electrode-immobilized molecular catalytic systems under operando conditions [1]. Since both vibrational spectroscopic techniques have different selection rules, complementary information about the system under operando conditions can be obtained. In my talk, selected research examples will be used to present the different aspects and features of (photo)electrocatalysis that can be visualized using vibrational spectrochemistry. The target systems presented include, on the one hand, porphyrin- and phthalocyanine-based systems for electrocatalytic oxygen and CO2 reduction [2,3] and, on the other hand, conjugated acetylene polymers for photoelectrocatalytic hydrogen evolution [4]. Spectra evaluation under operando conditions allowed us to demonstrate charge separation and ligand binding within these catalysts, identify the active sites during catalysis, and gain insight into the redox properties, reaction intermediates as well as synergistic effects of dual active sites.[1] H. K. Ly, I. M. Weidinger, Chem. Commun. 2021, 57, 2328-2342[2] T. R. Ramuglia, V. Budhija, K. H. Ly, M. Marquardt, M. Schwalbe, I. M. Weidinger, ChemCatChem, 2021, 13, 3934.[3] H. Zhong, M. Ghorbani-Asl, K. H. Ly, J. Zhang, J. Ge, M. Wang, Z. Liao, D. Makarov, E. Zschech, E. Brunner, I.M. Weidinger, J. Zhang, A. V. Krasheninnikov, S. Kaskel, R. Dong, X. Feng, Nat. Comm. 2020, 11, 1409.[4] M. Borelli, C. J. Querebillo, D. L. Pastoetter, T. Wang, A. Milani, C. Casari, H. K. Ly, F. He, Y. Hou, C. Neumann, A. Turchanin, H. Sun, I. M. Weidinger, X. Feng, Angew. Chemie Int. Ed. 2021, 60 (34), 18876-18881.

(Invited) Experimental Measurement of Spatial Activity on CO2 & CO Reduction Gas Diffusion Electrodes

Over the past decade, the accessible current densities of electrochemical CO2 reduction (CO2R) and CO reduction (COR) systems have increased by orders of magnitude, following the adaptation of the Gas Diffusion Electrode (GDE) architecture. As GDEs increase in size from cm2 to m2, cell stability and performance can change due to physical and chemical spatial variations within the cell, which can be exacerbated by the chemical complexity of carbonaceous electrochemical systems. Here, we present two approaches to measuring spatial activity: testing iteratively larger electrodes and harmonizing those results with a multi-port sampling reactor capable of measuring product concentration along the length of the reactor. This dual-pronged approach allows for benchmarking of electrochemical performance and selectivity down the channel as reaction conditions vary. We examine selectivity changes through the lens of fundamental electrochemical properties and provide subsequent recommendations for device operation as devices approach > kW scale. Experimental results are aligned with a 2-D (path length and electrode thickness) transport model to compare physical simulations with measured electrochemical signatures. Feedback loops between experiment and simulation were developed to incorporate cell inhomogeneities into the transport model. We identify potential causes for poor catalyst utilization in large electrodes for both COR and CO2R and provide recommendations for translating bench-top scale results to pilot scale results.

Photoelectrochemical CO2 Reduction to Formate over Hybrid System of CdS Photoanode and Formate Dehydrogenase Under Visible Light Irradiation

1. IntroductionPhotocatalytic and photoelectrochemical (PEC) CO2 reductions using semiconductor materials under solar light have attracted significant attention. The development of CO2-reduction systems operating under visible-light irradiation is essential for the practical use of solar energy because visible light takes up almost half of the solar radiation spectrum. Most metal sulfides, such as CdS, have suitable conduction band levels for CO2 reduction and narrow bandgaps, enabling visible-light absorption. However, most metal sulfides are unstable in aqueous solution under photoirradiation due to the occurrence of self-oxidative deactivation (photocorrosion) by photogenerated holes (e.g., CdS + 2h+ → Cd2+ + S). CO2 reduction in an aqueous solution competes with H2 evolution because the CO2 reduction potential is more negative compared to the water reduction potential. Therefore, the catalysts that can selectively reduce CO2 have been utilized. Enzymes are biocatalysts that only act on specific substrates. For example, formate dehydrogenase (FDH) can reduce CO2 to formate with 100% selectivity by accepting electrons from coenzyme NADH. We have recently reported PEC reduction of CO2 to formate using a sacrificial reagent-free system consisting of a TiO2 photoanode and FDH.1 However, this system cannot utilize visible light owing to the wide bandgap of TiO2. In this study, we attempted to fabricate stable CdS photoanode and construct a hybrid system consisting of the CdS photoanode, NADH, and FDH for the reduction of CO2 to formate under visible-light irradiation.2. ExperimentalThe CdS photoanode was fabricated by the chemical bath deposition (CBD) method according to a previous report.2 The CdS electrode was calcined in N2 flow at 200, 300, and 400 °C for 30 min. The electrochemical cell used for photocurrent measurements comprised a prepared electrode, a counter electrode (Pt wire), an Ag/AgCl reference electrode, and a borate buffer solution (pH 8) containing K4[Fe(CN)6]•3H2O. The electrodes were irradiated using a 300 W Xe lamp fitted with an L-42 cut-off filter. CO2 reduction was carried out using two-electrode system (counter electrode: carbon paper) in phosphate buffer solution (pH 7) under CO2 bubbling. Rh complex [Cp*Rh(bpy)(H2O)]2+,3 NAD+, and FDH was added in the counter side.3. Results and discussionThe stabilities of the photocurrents of the CdS electrodes were evaluated at a fixed potential (–0.5 V vs. Ag/AgCl) under continuous visible light irradiation. As shown in Fig. 1a, for CdS electrodes with and without calcination at 200 and 300 °C, the photocurrent densities gradually decreased with the start of light irradiation. After the reaction, large cubic particles with a particle size of approximately 100–300 nm, reflecting the crystal structure of K2Cd[Fe(CN)6], were observed on the electrodes. XRD analysis also confirmed the formation of K2Cd[Fe(CN)6]. As previously reported, K2Cd[Fe(CN)6] forms spontaneously on the CdS surface during photoirradiation by reaction of dissolved Cd2+ cations via photocorrosion and [Fe(CN)6]4− anions in the solution.4 Therefore, this gradual decrease in photocurrent density was mainly due to the photocorrosion of CdS. In contrast, calcination at 400 °C positively influenced the CdS electrode, gradually increasing the photocurrent density. The number of electrons passing through the outer circuit for over 1 h (19.7 C, corresponding to 205 μmol) exceeded the number of moles of CdS (approximately 22 μmol) in the electrode. Although large K2Cd[Fe(CN)6] particles were observed on the CdS electrode after the reaction, the amount was significantly lower than that observed on the other electrodes. Instead, fine particles of K2Cd[Fe(CN)6] (particle size ~10 nm) were densely formed on the CdS surface. Therefore, fine K2Cd[Fe(CN)6] particles generated on the surface effectively scavenged photogenerated holes in CdS and enabled the oxidation of [Fe(CN)6]4− to [Fe(CN)6]3−.4 Reduction of NAD+ to NADH using [Cp*Rh(bpy)(H2O)]2+ as an electron mediator was examined in two-electrode system consisting of the CdS photoanode (calcined at 400 °C) and a carbon paper counter electrode with no applied bias (0 V). Relatively stable photocurrent (0.2 mA cm–2) was observed for 60 min. UV-vis spectroscopy and liquid chromatography analyses revealed that the amount of 1,4-NADH, which functions as a coenzyme, was increased with progression of reaction by reduction of NAD+ over the carbon electrode accepting electrons from CdS photoanode. The PEC reduction of CO2 to formate was attempted by introducing FDH in the system. As shown in Fig. 1b, the amount of produced formate was increased with the increasing irradiation time and reached 1285 nmol, which exceeded that of FDH (64 nmol). Ishibashi, M. Higashi, S. Ikeda, Y. Amao, ChemCatChem, 2019, 11, 6227.Ikeda, T. et al, ChemSusChem, 2011, 4, 262.Ruppert, et al, Tetrahedron Lett., 1987, 28, 6583.Shirakawa, M. Higashi, O. Tomita, R. Abe, Sustain. Energy Fuels, 2017, 1, 1065. Figure 1