Solar CO2 Research Articles

Enhancement of photosynthetic isobutanol production in engineered cells of Synechocystis PCC 6803

BackgroundCyanobacteria, oxygenic photoautotrophic prokaryotes, can be engineered to produce various valuable chemicals from solar energy and CO2 in direct processes. The concept of photosynthetic production of isobutanol, a promising chemical and drop-in biofuel, has so far been demonstrated for Synechocystis PCC 6803 and Synechococcus elongatus PCC 7942. In Synechocystis PCC 6803, a heterologous expression of α-ketoisovalerate decarboxylase (Kivd) from Lactococcus lactis resulted in an isobutanol and 3-methyl-1-butanol producing strain. Kivd was identified as a bottleneck in the metabolic pathway and its activity was further improved by reducing the size of its substrate-binding pocket with a single replacement of serine-286 to threonine (KivdS286T). However, isobutanol production still remained low.ResultsIn the present study, we report on how cultivation conditions significantly affect the isobutanol production in Synechocystis PCC 6803. A HCl-titrated culture grown under medium light (50 μmol photons m−2 s−1) showed the highest isobutanol production with an in-flask titer of 194 mg l−1 after 10 days and 435 mg l−1 at day 40. This corresponds to a cumulative isobutanol production of 911 mg l−1, with a maximal production rate of 43.6 mg l−1 day−1 observed between days 4 and 6. Additional metabolic bottlenecks in the isobutanol biosynthesis pathway were further addressed. The expression level of KivdS286T was significantly affected when co-expressed with another gene downstream in a single operon and in a convergent oriented operon. Moreover, the expression of the ADH encoded by codon-optimized slr1192 and co-expression of IlvC and IlvD were identified as potential approaches to further enhance isobutanol production in Synechocystis PCC 6803.ConclusionThe present study demonstrates the importance of a suitable cultivation condition to enhance isobutanol production in Synechocystis PCC 6803. Chemostat should be used to further increase both the total titer as well as the rate of production. Furthermore, identified bottleneck, Kivd, should be expressed at the highest level to further enhance isobutanol production.

(Invited) Gas-Diffusion Electrodes for CO2 Reduction

One of the main challenges towards achieving high-efficiency solar-fuel generators performing CO2 reduction are the mass-transport limitations in traditional aqueous systems due to CO2 interactions/solubility and boundary-layer thickness. To circumvent these issues, gas-diffusion electrodes (GDEs) are considered wherein the CO2 is fed in a vapor form into a porous electrode. In this talk, we will explore the various tradeoffs endemic in GDE architectures. This is accomplished through multiphysics modeling of the cells including breakdowns of the various limiting phenomena. In addition, experimental data on the GDEs for CO2 reduction will be presented using both Ag and Cu-based catalysts. Key is understanding the potential distributions with the GDE and how this along with the observed higher current densities impact product selectivity with Cu-based catalysts. Finally, comments will be made concerning the possibilities of light inclusion in GDEs to enable solar CO2 reduction as well as the benefits of GDEs that extend beyond higher current-density operation including unprecedented control of the local environment compared to traditional, aqueous planar designs. AcknowledgementsThe material is based on work performed at the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, through the Office of Science of the U.S. Department of Energy under award no. DE-SC0004993

Efficient Visible-Light-Driven CO2 Reduction Mediated by Defect-Engineered BiOBr Atomic Layers.

Solar CO2 reduction efficiency is largely limited by poor photoabsorption, sluggish electron-hole separation, and a high CO2 activation barrier. Defect engineering was employed to optimize these crucial processes. As a prototype, BiOBr atomic layers were fabricated and abundant oxygen vacancies were deliberately created on their surfaces. X-ray absorption near-edge structure and electron paramagnetic resonance spectra confirm the formation of oxygen vacancies. Theoretical calculations reveal the creation of new defect levels resulting from the oxygen vacancies, which extends the photoresponse into the visible-light region. The charge delocalization around the oxygen vacancies contributes to CO2 conversion into COOH* intermediate, which was confirmed by in situ Fourier-transform infrared spectroscopy. Surface photovoltage spectra and time-resolved fluorescence emission decay spectra indicate that the introduced oxygen vacancies promote the separation of carriers. As a result, the oxygen-deficient BiOBr atomic layers achieve visible-light-driven CO2 reduction with a CO formation rate of 87.4 μmol g-1 h-1 , which was not only 20 and 24 times higher than that of BiOBr atomic layers and bulk BiOBr, respectively, but also outperformed most previously reported single photocatalysts under comparable conditions.

Overproduction of the cyanobacterial hydrogenase and selection of a mutant thriving on urea, as a possible step towards the future production of hydrogen coupled with water treatment.

Using a combination of various types of genetic manipulations (promoter replacement and gene cloning in replicating plasmid expression vector), we have overproduced the complex hydrogenase enzyme in the model cyanobacterium Synechocystis PCC6803. This new strain overproduces all twelve following proteins: HoxEFUYH (hydrogen production), HoxW (maturation of the HoxH subunit of hydrogenase) and HypABCDEF (assembly of the [NiFe] redox center of HoxHY hydrogenase). This strain when grown in the presence of a suitable quantities of nickel and iron used here exhibits a strong (25-fold) increase in hydrogenase activity, as compared to the WT strain growing in the standard medium. Hence, this strain can be very useful for future analyses of the cyanobacterial [NiFe] hydrogenase to determine its structure and, in turn, improve its tolerance to oxygen with the future goal of increasing hydrogen production. We also report the counterintuitive notion that lowering the activity of the Synechocystis urease can increase the photoproduction of biomass from urea-polluted waters, without decreasing hydrogenase activity. Such cyanobacterial factories with high hydrogenase activity and a healthy growth on urea constitute an important step towards the future development of an economical industrial processes coupling H2 production from solar energy and CO2, with wastewater treatment (urea depollution).

RETRACTED: Calculating potential of solar energy and CO2 emissions reduction for cityscale buildings based on 3D remote sensing technologies

Solar energy utilization will play an increasingly important role in sustainable urban development because it will help motivate low-carbon development. This study proposed a framework that calculates the solar irradiance on roofs and facades of city-scale buildings, as well as the potential reductions in energy consumption and CO2 emissions that can be derived from solar energy utilization. This framework consolidates data acquired using 3D remote sensing techniques (e.g., light detection and ranging (LiDAR) data and digital photogrammetry) with existing 3D building vector models from urban planning and building design. To further the economical use of solar energy, the framework provides a new strategy based on three aspects (threshold, structure, and orientation), for a more realistic analysis of the available area of roofs and facades, by combining street view images and point clouds. Nanjing, China, was selected as a study area for validating and analyzing the proposed framework. The experimental area covers approximately 30 km² and 5216 buildings. The reliability of the proposed method was validated by comparing the results with the computation modules of mainstream commercial programs and previous international studies. The following conclusions were drawn from the experimental area. (1) Annual solar energy on the roofs of all buildings was 9647.89 GWh, corresponding to a PV yield of 1447.18 GWh, which equates to reductions of 584.66 kt in standard coal consumption and 1261.94 kt in CO2 emissions. The annual solar energy on the facades was 5364.63 GWh, corresponding to a PV yield of 804.69 GWh, which equates to reductions of 325.10 kt in standard coal consumption and 701.69 kt in CO2 emissions. (2) Based on the calculated potential solar energy utilization of the building envelopes, it was shown that the utilization of solar radiation on roofs and facades would promote low-carbon urban development. The utilization of solar energy on the facades of high buildings will contribute to the supply of energy for these buildings and help balance their energy consumption. (3) As the solar incidence angle changes over time, the reductions in energy consumption and CO2 emissions related to PV power generation are maximal in summer for building roofs, whereas no significant seasonal differences were observed for facades. (4) Incorporating the available area translates the utilization of solar energy and potential CO2 reductions from theory to practical application by reducing inefficient and invalid areas. The percentages of available area of roof and facade were estimated at 47% and 16% by comprehensive calculations considering three aspects (threshold, structure, and orientation). The corresponding potential of solar energy on the roofs and facades in the validation area B are 167.18 GWh and 53.63 GWh, respectively, compared to the original theoretic values of 316.97 GWh and 237.98 GWh.

On the influence of solar cycle lengths and carbon dioxide on global temperatures

By combining Solar Cycle Lengths (SCL) and CO2 this paper predicts a global average surface temperature (GAST) anomaly of 1.5K in the year 2100 compared to 0.42K in 1996–2006. This assumes a continuing CO2 increase of 2 ppm per year and our derived form of Transient Climate Response (TCR), whose value 1.93 ± 0.26 K (K) per CO2 doubling would be 1.23 times higher if the Sun were ignored. After the CO2 effect has been subtracted out, the SCL explains a healthy 55% of the remaining variance. It also estimates that 37% of the recent warming from 1980 to 2001 was due to solar effects. We then compare with models created from Scafetta (2010, 2013) (the first of which has the best fit of all) and from radiative forcings estimated by Myhre et al. (2001) and Skeie et al. (2011). The latter confirms the solar contribution to 1980–2001 warming as 33%, in contrast to the negligible value given by Benestad & Schmidt (2009). It also gives a TCR of 1.3K if only CO2 continues to rise, and 2.0K if CH4 and NO2 also rise proportionately. Likewise this model estimates the ratio between the sensitivities of forcings from the Sun and greenhouse gases as 2.9 (versus 1.0 for Benestad & Schmidt (2009)). We develop a negative exponential model for post-forced warming to derive a ratio between Equilibrium Climate Sensitivity (ECS) and TCR, estimated to be 1.15. Two statistical novelties of the paper are the computation of the exact left tail probability of the Durbin-Watson statistic, and the demonstration of an approximate relationship between the Akaike Information Criterion and the tail probability of the F-statistic.

Re-direction of carbon flux to key precursor malonyl-CoA via artificial small RNAs in photosynthetic Synechocystis sp. PCC 6803

BackgroundPhotosynthetic cyanobacteria have attracted a significant attention as promising chassis to produce renewable fuels and chemicals due to their capability to utilizing solar energy and CO2. Notably, the enhancing supply of key precursors like malonyl-CoA would benefit the production of many bio-compounds. Nevertheless, the lacking of genetic tools in cyanobacteria, especially the knockdown strategies for essential pathways, has seriously restricted the attempts to re-direct carbon flux from the central carbohydrate metabolism to the synthesis of bioproducts.ResultsAiming at developing new genetic tools, two small RNA regulatory tools are reported for the model cyanobacterium Synechocystis sp. PCC6803, based on paired termini RNAs as well as the exogenous Hfq chaperone and MicC scaffold (Hfq-MicC) previously developed in Escherichia coli. Both regulatory tools functioned well in regulating exogenous reporter gene lacZ and endogenous glgC gene in Synechocystis sp. PCC6803, achieving a downregulation of gene expression up to 90% compared with wildtype. In addition, the Hfq-MicC tool was developed to simultaneously regulate multiple genes related to essential fatty acids biosynthesis, which led to decreased fatty acids content by 11%. Furthermore, aiming to re-direct the carbon flux, the Hfq-MicC tool was utilized to interfere the competing pathway of malonyl-CoA, achieving an increased intracellular malonyl-CoA abundance up to 41% (~ 698.3 pg/mL/OD730 nm) compared to the wildtype. Finally, the Hfq-MicC system was further modified into an inducible system based on the theophylline-inducible riboswitch.ConclusionsIn this study, two small RNA regulatory tools for manipulating essential metabolic pathways and re-directing carbon flux are reported for Synechocystis sp. PCC6803. The work introduces efficient and valuable metabolic regulatory strategies for photosynthetic cyanobacteria.

Comparative study of the anchorage and the catalytic properties of nanoporous TiO2 films modified with ruthenium (II) and rhenium (I) carbonyl complexes

In this article we study the anchoring of cis-[Ru(bpyC4pyr)(CO)2(CH3CN)2]2+, cis-[Ru(bpy)2(CO)2]2+ and cis-[Ru(bpyac)(CO)2Cl2], onto nanoporous TiO2 employing electropolymerization, electrostatic interaction and chemical bonding. Also, the [Re(bpyac)(CO)3Cl] rhenium(I) complex for chemical anchorage was analyzed. The characterization of TiO2/Ru(II) and TiO2/Re(I) nanocomposite films was performed by field emission scanning electron microscopy (FESEM), electron dispersive X-ray spectroscopy (EDS) and Raman spectroscopy. In addition, for the more stable nanocomposites obtained, the catalytic properties (solar energy conversion and CO2 reduction) were evaluated. The efficiency improvement in redox process derived from the (photo)electrochemical evidence indicates that modified nanoporous TiO2 structures enhance the rate of charge transfer reactions.

Experimental screening of perovskite oxides as efficient redox materials for solar thermochemical CO2 conversion

Perovskites and parent Ruddlesden–Popper structures were proved to be suitable redox materials for two-step solar thermochemical CO2 splitting.

Mimicking tetravalent dopant behavior using paired charge compensating dopants to improve the redox performance of ceria for thermochemically splitting H2O and CO2

A novel doping strategy based on paired charge compensating dopants is proposed to improve the solar thermochemical H2O and CO2 splitting performance of ceria. Density functional theory based ab initio calculations show that by co-doping trivalent and pentavalent cations into ceria the behavior of the trivalent dopant resembles that of tetravalent dopants. In this study, which investigated combinations of group IIIA and VA elements, it was found that the trivalent dopant identity has the largest effect on the reduction energy, while the pentavalent dopant only slightly modifies it. The trivalent dopant in these materials suppresses the reduction energy due to decreased ionic attraction of the trivalent cation and O2− as compared to Ce4+ and O2−. We predict relative reducing capabilities in the following decreasing order: Hf-CeO2 > ScX-CeO2 > Zr-CeO2 > YX-CeO2 > LaX-CeO2 > undoped CeO2 (where X = V, Nb or Ta). Experimental thermogravimetric analysis confirms the computational predictions.

Chitin synthesis by Chlorella cells infected by chloroviruses: Enhancement by adopting a slow-growing virus and treatment with aphidicolin

Chlorella viruses or chloroviruses contain a gene that encodes an enzyme that catalyzes chitin synthesis. This gene is expressed early in viral infections to produce chitin on the outside of the Chlorella cell wall. Interestingly, chitin synthesis by microalgal Chlorella cells in combination with chloroviruses represents a unique eco-friendly process for converting solar energy and CO2 into useful materials. However, during the final viral infection stage, the host cells are completely lysed, so chitin should be harvested before cells lyse. To increase chitin yields, slow-growing chlorovirus isolates were adopted and the viral replication process was modified with an inhibitor of DNA synthesis. The accumulation of chitin on the surface of Chlorella cells infected with one of nine chlorovirus isolates carrying the chitin synthase gene was compared with that of CVK2 (a standard virus)-infected cells. Chlorella cells infected with CVNF-1 (a slow-growing virus) accumulated chitin over the entire cell surface within 15min post-infection (p.i.), and chitin continued to accumulate for up to 8h p.i. before cells lysed. This was 2-fold longer than the chitin-accumulation period for cells infected with CVK2. The addition of aphidicolin delayed the progression of the virus replication cycle and extended the chitin-accumulation period of CVNF-1-infected cells to 12h p.i. before cells lysed. Additionally, chitin production in the aphidicolin-treated CVNF-1-infected cells was approximately 6-fold higher than in CVK2-infected cells not treated with aphidicolin. Thus, chitin synthesis in a Chlorella-virus system may be prolonged by using slow-growing viral isolates treated with aphidicolin.

Chlorophyll sensitized BiVO4 as photoanode for solar water splitting and CO2 conversion

Converting solar energy into valuable hydrogen and hydrocarbon fuels through photoelectrocatalytic water splitting and CO2 reduction is highly promising in addressing the growing demand for renewable and clean energy resources. However, the solar-to-fuel conversion efficiency is still very low due to limited light absorption and rapid bulk recombination of charge carriers. In this work, we present chlorophyll (Chl) and its derivative sodium copper chlorophyllin (ChlCuNa), as dye sensitizers, modified BiVO4 to improve the photoelectrochemical (PEC) performance. The photocurrent of BiVO4 is surprisingly decreased after a direct sensitization of Chl while the sensitization of ChlCuNa obviously enhances photocurrent of BiVO4 electrodes by improved surface hydrophilicity and extended light absorption. ChlCuNa-sensitized BiVO4 achieves an improved H2 evolution rate of 5.43μmolh−1cm−2 in water splitting and an enhanced HCOOH production rate of 2.15μmolh−1cm−2 in CO2 PEC reduction, which are 1.9 times and 2.4 times higher than pristine BiVO4, respectively. It is suggested that the derivative ChlCuNa is a more effective sensitizer for solar-to-fuel energy conversion and CO2 utilization than Chl.

Toward a Practical Solar-Driven CO2 Flow Cell Electrolyzer: Design and Optimization

A first-of-its-kind solar CO2 flow cell electrolyzer is reported here with a solar-to-fuel efficiency (SFE) of 6.5% at high operating currents (>1 A), orders of magnitude greater than those of other reported solar-driven devices that typically operate at currents of a few milliamperes. The approach of solar module-driven electrolysis, compared to monolithic photoelectrochemical cells, allows simpler manufacture, use of commercially available components, and optimization of the power transfer between the photovoltaic and the electrochemical systems. Employing commercial high-efficiency crystalline silicon solar cells with a large area flow cell CO2 electrolyzer (25 cm2), we present a procedure for optimizing the SFE of a decoupled photovoltaic electrolyzer by impedance matching the source and the load using their independent current–voltage characteristics. The importance of the voltage-dependent Faradaic efficiency of the electrolyzer on device performance and optimization is highlighted.

Energy analysis for an artificial tree generating polymer fuels from water and CO2

Solar energy is one of the most abundant sources of renewable energy. The utilization of solar energy and CO2 in the atmosphere to generate fuels is regarded as a great way to resolve both the problems of fossil fuel depletion and global warming. An artificial tree was constructed to generate polymer fuels from water and CO2 in the atmosphere. Solar panels acted as leaves to absorb solar energy and generate electric energy, and the CO2 in the air was absorbed by alkalized water and then converted to polymer fuels in an electrolytic cell. This tree yielded an energy efficiency of 53.3% for the generation of polymer fuels, and the ratio between the combustion heat of the polymers and the electric energy consumption was 701.6%. Additionally, the energy conversions in the process of polymer generation were investigated, and the evidences of radicals generated from water by electric field induction were provided. The results provide an alternative method to utilize solar energy.

A Facile Electrochemical Synthesis for Nanostructured Au for Improved Photo- and Electrochemical CO2 Reduction Activity

Direct CO2 conversion using renewable energy source (e.g. solar energy) to value-added chemicals has attracted much attention for alternative fuel production. An efficient and durable electrocatalyst is essential to achieve practical level of CO2 conversion. In the recent years, nanostructuring of metal catalysts (e.g. Au, Ag, Cu) has been reported for enhancing CO2 reduction reaction (CO2RR) activity in aqueous solutions. In particular, nanostructured Au formed on a thick Au foil by electrochemical treatment has been intensively focused due to the superior properties regarding low overpotential and enhanced stability.[1] However, the utilization of thick Au foils can increase the cost and is also unavailable for photoelectrochemical (PEC) CO2RR which is the simplest approach for solar CO2 conversion. Accordingly, we recently demonstrated that mild electrochemical treatment enabling the formation of nanoporous Au thin film on Si photocathode for PEC CO2RR.[2] Although it shows exceptional CO2RR activity with only 20 nm-thick nanoporous Au layer, stability issue still remains to be solved for practical CO2 conversion. Here, we propose a novel electrochemical synthesis strategy to form Au nanostructured thin film for further improved CO2RR activity. The electrochemical synthesis is simply performed with 160 nm-thick Au thin film on Si substrate by anodization of Au thin films to grow Au oxide at constant potential (2.5 V (vs. RHE) for 40 min) followed by electro-reduction of the grown Au oxide at constant current density (-0.5 mA cm-2 for 10 min) in 0.2 M KHCO3 solution. Nanoporous Au nanostructures with height of ~180 nm were formed as a result of the electrochemical treatment. The CO2RR activities (overpotential, durability) for the nanostructured Au is significantly improved as compared to bare Au thin film. For example, our CO selectivity is to 66% and 98% at -0.35V and -0.39V, respectively, (bare Au: 0 and 20%, respectively). Nanostructured Au also shows better stability at -0.49V (~2 hours for over 90% selectivity) as compared to bare Au (20% selectivity at an hour). In order to further improve the stability of our nanostructured Au during CO2RR, we introduce additional electrochemical oxidation and reduction cycles. Interestingly, declined CO2RR activity (CO selectivity: ~70%) is revived to have 97% CO selectivity after an additional cycle. Furthermore, after the 4th electrochemical cycles, our Au nanostructures are stable over 6 hours while producing over 90% CO selectivity. We performed a systematic morphological and microstructural investigation of Au nanostructures during the repeated electrochemical treatment cycles. In particular, the height of Au nanostructured increases to about 390 nm and roughness factor is doubled after the 4th treatment. TEM investigations confirmed that remarkably increased number of grain boundaries (GBs) on the 4th treated Au samples. In addition, most GBs disappear after 15 hours of CO2RR. These results indicate that extended stability by repeating cycle of electrochemical synthesis is attributed to increased GBs on a roughened Au nanostructure. Finally, we applied our Au nanostructures to Si photocathodes for PEC CO2RR with Si photocathodes by forming Au micro-mesh. Micro-mesh Au on Si enables electrochemical treatments selectively on Au surface without deteriorating the underlying Si. Our Si photocathode with nanostructured Au co-catalyst remarkably achieves ~82% CO selectivity at -0.07 V which is more positive than CO2/CO redox potential under 1 sun illumination with extended stability. Therefore, our electrochemical synthesis would provide an efficient strategy to improve and recycle nanostructured Au activity for photo- and electrochemical CO2reduction. [1] Yihong Chen, Christina W. Li, and Matthew W. Kanan, J. Am. Chem. Soc. 134, 19969 (2012) [2] Jun Tae Song, Hyewon Ryoo, Minhyung Cho, Jaehoon Kim, Jin-Gyu Kim, Sung-Yoon Chung, and Jihun Oh, Adv. Energy Mater. 7, 1601103 (2017)

Novel system integrations of 1000 MW coal-fired power plant retrofitted with solar energy and CO2 capture system

The energy penalty of MEA based post-combustion CO2 capture technology will reduce the efficiency of coal-fired power generation significantly. The introduction of solar energy to the capture system can compensate the CO2 capture penalty. This paper proposed three integrations of 1000MW coal-fired power plant retrofitted with solar energy and post-combustion CO2 capture system (PP+Solar+PCC), which the solar energy is used to replace the 1st extraction to heat the feedwater or provide the MEA regeneration heat demand is the main difference in three PP+Solar+PCC both in thermal and economic. The 1000MW coal-fired power plant, solar aided coal-fired power system and coal-fired power plant with post-combustion CO2 capture system (PP+PCC) are selected as the reference cases. By comparison of six systems, it was concluded that the integration of using solar energy to replace the first high pressure extraction and taking a part of intermediate pressure cylinder exhaust to provide the reboiler heat demand is the best in three strategies of PP+Solar+PCC. In addition, this paper analyzed the evaluation indicators variation with the changes of CO2 removal rate, solar collector field area, DNI, solar collector field cost, coal cost and carbon tax.

Inorganic Colloidal Perovskite Quantum Dots for Robust Solar CO2 Reduction.

Inorganic perovskite quantum dots as optoelectronic materials have attracted enormous attention in light-harvesting and emitting devices. However, photocatalytic conversion based on inorganic perovskite halides has not been reported. Here, we have synthesized colloidal quantum dots (QDs, 3-12 nm) of cesium lead halide perovskites (CsPbBr3 ) as a new type of photocatalytic material. The band gap energies and photoluminescence (PL) spectra are tunable over the visible spectral region according to quantum size effects on an atomic scale. The increased carrier lifetime revealed by time-resolved PL spectra, indicates the efficient electron-hole separation and transfer. As expected, the CsPbBr3 QDs with high selectivity of greater than 99 % achieve an efficient yield of 20.9 μmol g-1 towards solar CO2 reduction. This work has opened a new avenue for inorganic colloidal perovskite materials as efficient photocatalysts to convert CO2 into valuable fuels.

Design and Implementation of High Voltage Photovoltaic Electrolysis System for Solar Fuel Production from CO2

ABSTRACTGrowing interest in the use of CO2 as a feedstock for fuel generation has led to increased interest in solar CO2 electrolysis for renewable fuel generation which has a variety of applications ranging from providing renewable sources for energy-dense carbon fuels, to curbing high-density emissions from power plants, industries and automobiles. The challenges of integrated solar-to-carbon fuel converters, where the photovoltaic (PV) material is immersed in the electrolyte, are well-known: the need for unique PV cell designs; material incompatibility; corrosion; and optical losses. In this paper, a PV-electrolysis system is presented, where a flow-cell electrolyzer is power-matched to a high-performance solar PV module array which has two system design advantages: 1) use of standard PV cells external to the electrolyzer, which allows de-coupling the design, fabrication and operation of the PV system from that of the electrolyzer; and 2) enabling optimization of the PV configuration to maximize power coupling efficiency to the specific electrolyzer Tafel curve, with or without the use of electronic power-conditioning devices. The implemented system resulted in a peak SFE of 6.5%, a competitive solar-to-fuel efficiency (SFE) figure to those reported in literature.

Defect-Mediated Electron-Hole Separation in One-Unit-Cell ZnIn2S4 Layers for Boosted Solar-Driven CO2 Reduction.

The effect of defects on electron-hole separation is not always clear and is sometimes contradictory. Herein, we initially built clear models of two-dimensional atomic layers with tunable defect concentrations, and hence directly disclose the defect type and distribution at atomic level. As a prototype, defective one-unit-cell ZnIn2S4 atomic layers are successfully synthesized for the first time. Aberration-corrected scanning transmission electron microscopy directly manifests their distinct zinc vacancy concentrations, confirmed by positron annihilation spectrometry and electron spin resonance analysis. Density-functional calculations reveal that the presence of zinc vacancies ensures higher charge density and efficient carrier transport, verified by ultrafast photogenerated electron transfer time of ∼15 ps from the conduction band of ZnIn2S4 to the trap states. Ultrafast transient absorption spectroscopy manifests the higher zinc vacancy concentration that allows for ∼1.7-fold increase in average recovery lifetime, confirmed by surface photovoltage spectroscopy and PL spectroscopy analysis, which ensures promoted carrier separation rates. As a result, the one-unit-cell ZnIn2S4 layers with rich zinc vacancies exhibit a carbon monoxide formation rate of 33.2 μmol g-1 h-1, roughly 3.6 times higher than that of the one-unit-cell ZnIn2S4 layers with poor zinc vacancies, while the former's photocatalytic activity shows negligible loss after 24 h photocatalysis. This present work uncovers the role of defects in affecting electron-hole separation at atomic level, opening new opportunities for achieving highly efficient solar CO2 reduction performances.

Efficient solar light photoreduction of CO2 to hydrocarbon fuels via magnesiothermally reduced TiO2 photocatalyst

Elevated atmospheric CO2 levels are recognized as a key driver of global warming. Making use of sunlight to photoreduce CO2, in turn fabricating hydrocarbon fuels compatible with the current energy infrastructure, is a compelling strategy to minimize atmospheric CO2 concentrations. However, practical application of such a photocatalytic system requires significant efforts for improved photoreduction performance and product selectivity. Herein, we investigate the performance of our newly developed reduced TiO2, prepared by a reduction process using Mg in 5% H2/Ar, for photoconversion of CO2 and water vapor to hydrocarbons, primarily CH4. Using Pt nanoparticles as a co-catalyst, under simulated solar light irradiation the reduced anatase TiO2 exhibits a relatively stable performance with a threefold increase in the rate of CH4 production (1640.58ppmg−1h−1, 1.13μmolg−1h−1) as compared to anatase TiO2 nanoparticles (546.98ppmg−1h−1, 0.38μmolg−1h−1). The improved photocatalytic performance is attributed to enhanced light absorption, suitable band edge alignment with respect to the CO2/CH4 redox potential, and efficient separation of photogenerated charges. Our results suggest that the Pt-sensitized reduced TiO2 can serve as an efficient photocatalyst for solar light CO2 photoreduction.