Artificial Photosynthesis Devices Research Articles

Rational Design of Improved Ru Containing Fe-Based Metal-Organic Framework (MOF) Photoanode for Artificial Photosynthesis.

Metal-Organic Frameworks (MOFs) recently emerged as a new platform for the realization of integrated devices for artificial photosynthesis. However, there remain few demonstrations of rational tuning of such devices for improved performance. Here, a fast molecular water oxidation catalyst working via water nucleophilic attack is integrated into the MOF MIL-142, wherein Fe3O nodes absorb visible light, leading to charge separation. Materials are characterized by a range of structural and spectroscopic techniques. New, [Ru(tpy)(Qc)(H2O)]+ (tpy = 2,2':6',2″-terpyridine and Qc = 8-quinolinecarboxylate)-doped Fe MIL-142 achieved a high photocurrent (1.6 × 10-3 A·cm-2) in photo-electrocatalytic water splitting at pH = 1. Unassisted photocatalytic H2 evolution is also reported with Pt as the co-catalyst (4.8 µmol g-1 min-1). The high activity of this new system enables hydrogen gas capture from an easy-to-manufacture, scaled-up prototype utilizing MOF deposited on FTO glass as a photoanode. These findings provide insights for the development of MOF-based light-driven water-splitting assemblies utilizing a minimal amount of precious metals and Fe-based photosensitizers.

The landscape of computational approaches for artificial photosynthesis.

Artificial photosynthesis is an attractive strategy for converting solar energy into fuels, largely because the Earth receives enough solar energy in one hour to meet humanity's energy needs for an entire year. However, developing devices for artificial photosynthesis remains difficult and requires computational approaches to guide and assist the interpretation of experiments. In this Perspective, we discuss current and future computational approaches, as well as the challenges of designing and characterizing molecular assemblies that absorb solar light, transfer electrons between interfaces, and catalyze water-splitting and fuel-forming reactions.

A baby step in assembling and integrating the components of an artificial photosynthesis device with forced heterojunctions towards improved efficiency

The photosynthesis concept is successfully employed to sustainably produce solar fuels/chemicals in direct sunlight.

Fuelling electrocatalysis at a single nanoparticle by ion flow in a nanoconfined electrolyte layer.

We explore the possibility of coupling the transport of ions and water in a nanochannel with the chemical transformation of a reactant at an individual catalytic nanoparticle (NP). Such configuration could be interesting for constructing artificial photosynthesis devices coupling the asymmetric production of ions at the catalytic NP, with the ion selectivity of the nanochannels acting as ion pumps. Herein we propose to observe how such ion pumping can be coupled to an electrochemical reaction operated at the level of an individual electrocatalytic Pt NP. This is achieved by confining a (reservoir) droplet of electrolyte to within a few micrometres away from an electrocatalytic Pt NP on an electrode. While the region of the electrode confined by the reservoir and the NP are cathodically polarised, operando optical microscopy reveals the growth of an electrolyte nanodroplet on top of the NP. This suggests that the electrocatalysis of the oxygen reduction reaction operates at the NP and that an electrolyte nanochannel is formed - acting as an ion pump - between the reservoir and the NP. We have described here the optically imaged phenomena and their relevance to the characterization of the electrolyte nanochannel linking the NPs to the electrolyte microreservoir. Additionally, we have addressed the capacity of the nanochannel to transport ions and solvent flow to the NP.

Improvements to artificial photosynthesis devices

By matching the voltages of photovoltaic modules and electrochemical reactors, researchers improved the energy conversion efficiency in artificial photosynthesis devices.

Free-Standing Nanoarrays with Energetic Electrons and Active Sites for Efficient Plasmon-Driven Ammonia Synthesis.

Direct ammonia (NH3 ) synthesis from water and atmospheric nitrogen using sunlight provides an energy-sustainable and carbon-neutral alternative to the Haber-Bosch process. However, the development of such a route with high performance is impeded by the lack of effective charge transfer and abundant active sites to initiate the nitrogen reduction reaction (NRR). Here, the authors report efficient plasmon-induced photoelectrochemical (PEC) NH3 synthesis on the hierarchical free-standing Au/Kx MoO3 /Mo/Kx MoO3 /Au nanoarrays. Endowed with energetically hot electrons and catalytically active sites, the plasmonic nanoarrays exhibit an efficient PEC NH3 synthesis rate of 9.6µg cm-2 h-1 under visible light irradiation, which is among the highest PEC NRR systems. This work demonstrates the rationally designed plasmonic nanoarrays for highly efficient NH3 synthesis, which paves a new path for PEC catalytic reactions driven by surface plasmons and future monolithic PEC devices for direct artificial photosynthesis.

The Challenge to Develop an Artificial Photosynthesis Device that Fixes CO2 Using the Sun

The Challenge to Develop an Artificial Photosynthesis Device that Fixes CO2 Using the Sun

Foam-like 3D Graphene as a Charge Transport Modifier in Zinc Oxide Electron Transport Material in Perovskite Solar Cells

The effect of foam-like 3D graphene (3DG) in an electron transport material (ETM), viz. ZnO thin film, on the steady-state photoluminescence (PL), light-harvesting efficiency (LHE), photocurrent density (JSC), photovoltage (VOC), and charge transport parameters of perovskite solar cells (PSCs) are systematically investigated. The ETM is developed by spin coating a ZnO precursor solution containing varying amounts of 3DG on conducting glass substrates and appropriate annealing. A significant improvement in the photoconversion efficiency of PSCs is observed for a low concentration of 3DG in ZnO. The current–voltage and electrochemical impedance spectroscopy measurements show that the addition of 3DG enhances the VOC due to efficient electron–hole separation and charge transport compared to the pristine ZnO. These studies offer a route for further advances in enhancing the optoelectronic properties of ETM for artificial photosynthesis and photocatalysis devices.

Bismuth Vacancy-Induced Efficient CO2 Photoreduction in BiOCl Directly from Natural Air: A Progressive Step toward Photosynthesis in Nature.

Photocatalytic CO2 conversion into carbonaceous fuels through artificial photosynthesis is beneficial to global warming mitigation and renewable resource generation. However, a high cost is always required by special CO2-capturing devices for efficient artificial photosynthesis. For achieving highly efficient photocatalytic CO2 reduction (PCR) directly from natural air, we report rose-like BiOCl that is rich in Bi vacancies (VBi) assembled by nanosheets with almost fully exposed active {001} facets. These rose-like BiOCl with VBi assemblies provide considerable adsorption and catalytic sites, which hoists the CO2 capture and reduction capabilities, and thus expedites the PCR to a superior value of 21.99 μmol·g-1·h-1 CO generation under a 300 W Xe lamp within 5 h from natural air. The novel design and construction of a photocatalyst in this work could break through the conventional PCR system requiring compression and purification for CO2, dramatically reduce expenses, and open up new possibilities for the practical application of artificial photosynthesis.

Efficient workflow for the investigation of the catalytic cycle of water oxidation catalysts: Combining GFN-xTB and density functional theory.

Photocatalytic water oxidation remains the bottleneck in many artificial photosynthesis devices. The efficiency of this challenging process is inherently linked to the thermodynamic and electronic properties of the chromophore and the water oxidation catalyst (WOC). Computational investigations can facilitate the search for favorable chromophore‐catalyst combinations. However, this remains a demanding task due to the requirements on the computational method that should be able to correctly describe different spin and oxidation states of the transition metal, the influence of solvation and the different rates of the charge transfer and water oxidation processes. To determine a suitable method with favorable cost/accuracy ratios, the full catalytic cycle of a molecular ruthenium based WOC is investigated using different computational methods, including density functional theory (DFT) with different functionals (GGA, Hybrid, Double Hybrid) as well as the semi‐empirical tight binding approach GFN‐xTB. A workflow with low computational cost is proposed that combines GFN‐xTB and DFT and provides reliable results. GFN‐xTB geometries and frequencies combined with single‐point DFT energies give free energy changes along the catalytic cycle that closely follow the full DFT results and show satisfactory agreement with experiment, while significantly decreasing the computational cost. This workflow allows for cost efficient determination of energetic, thermodynamic and dynamic properties of WOCs.

On the design and performance of InGaN/Si double-junction photocathodes

Through a combined theoretical and experimental study, we have investigated the synthesis and performance characteristics of InGaN/Si double-junction photoelectrochemical (PEC) water splitting devices, which promise a theoretical solar-to-hydrogen conversion efficiency ∼30% under AM 1.5G one-sun illumination. The double-junction photocathodes consist of a p+-InGaN top light absorber and a Si bottom p-n junction, which are connected through a nanowire tunnel junction. The effect of indium composition of the top light absorber as well as the impact of p-type Mg dopant incorporation on the PEC performance was studied. Experimentally, the sample with 32% indium composition showed a maximum photocurrent density of ∼9 mA/cm2 at 0.4 V vs reverse hydrogen electrode (RHE) with applied bias photon-to-current efficiency (ABPE) of ∼9.5%. An optimum p-type doping level ∼1 × 1017 cm−3 was also identified, which results in the best device performance as a result of optimum surface band bending as well as vertical charge carrier (hole) transport. These results also show a good agreement with our theoretical analysis. This work provides significant insights in advancing the design and development of high efficiency PEC devices for artificial photosynthesis using industry ready materials, e.g., Si and GaN, to achieve large-scale, low-cost onsite hydrogen fuel production.

Structural and dynamic insights into Mn4Ca cluster-depleted Photosystem II.

In the first steps of natural oxygenic photosynthesis, sunlight is used to oxidize water molecules to protons, electrons and molecular oxygen. This reaction takes place on the Mn4Ca cluster located in the reaction centre of Photosystem II (PSII), where the cluster is assembled and continuously repaired through a process known as photoactivation. Understanding the molecular details of such a process has important implications in different fields, in particular inspiring synthesis and repair strategies for artificial photosynthesis devices. In this regard, a detailed structural and dynamic characterization of Photosystem II lacking a Mn4Ca cluster, namely apo PSII, is a prerequisite for the full comprehension of the photoactivation. Recently, the structure of the apo PSII was resolved at 2.55 Å resolution [Zhang et al., eLife, 2017, 6, e26933], suggesting a pre-organized structure of the protein cavity hosting the cluster. Anyway, the question of whether these findings are a feature of the method used remains open. Here, by means of classical Molecular Dynamics simulations, we characterized the structural and dynamic features of the apo PSII for different protonation states of the cluster cavity. Albeit an overall conformational stability common to all investigated systems, we found significant deviations in the conformation of the side chains of the active site with respect to the X-ray positions. Our findings suggest that not all residues acting as Mn ligands are pre-organized prior to the Mn4Ca formation and previous local conformational changes are required in order to bind the first Mn ion in the high-affinity binding site.

Ultrafast terahertz spectroscopy provides insight into charge transfer efficiency and dynamics in artificial photosynthesis.

Terahertz (THz) spectroscopy provides a noncontact method to measure the ultrafast dynamics and photoconductivity of mobile carriers in semiconducting materials. This has proven useful in studying artificial photosynthesis devices which use semiconductor photoelectrodes. We present a brief introduction to optical-pump THz-probe (OPTP) spectroscopy, a technique that provides unique and useful insight into interfacial electron transfer (from the surface-attached dye to the conduction band of the semiconductor) in dye-sensitized photoelectrochemical cells. Compared with more familiar methods like visible transient absorption spectroscopy, OPTP spectroscopy stands out in offering both sub-picosecond time resolution as well as sensitivity to mobile carriers (electrons and holes) in the semiconductor portion of artificial photosynthesis devices. The mobile carriers are crucial to device performance as only they pass to the other half cell to complete the reaction. In order to highlight these advantages and illustrate the types of questions OPTP spectroscopy can address, we discuss three case studies. In the first, OPTP spectroscopy is used to measure the injection rates from a set of six different dyes, revealing the effect of the energetics and lifetimes of the dye excited states on interfacial electron transfer. The subsequent case studies investigate the influence of varying the moieties which bind to the surface (anchors), as well as the moieties that connect the chromophore with these anchors (linkers). OPTP spectroscopy was used to measure the interfacial electron transfer rate as these moieties were varied.

Directing charge transfer in perylene based light-harvesting antenna molecules.

Directing energy and charge transfer processes in light-harvesting antenna systems is quintessential for optimizing the efficiency of molecular devices for artificial photosynthesis. In this work, we report a novel synthetic method to construct two regioisomeric antenna molecules (1-D2A2 and 7-D2A2), in which the 4-(n-butylamino)naphthalene monoimide energy and electron donor is attached to the perylene monoimide diester (PMIDE) acceptor at the 1- and 7-bay positions, respectively. The non-symmetric structure of PMIDE renders a polarized distribution of the frontier molecular orbitals along the long axis of this acceptor moiety, which differentiates the electron coupling between the donor, attached at either the 1- or the 7-position, and the acceptor. We demonstrate that directional control of the photo-driven charge transfer process has been obtained by engineering the molecular structure of the light-harvesting antenna molecules.

Multicomponent transport of alcohols in Nafion 117 measured by in situ ATR FTIR spectroscopy

Proton exchange membranes are vital components for many applications including energy applications such as fuel cells and electrolyzers. Artificial photosynthesis devices are one type of fuel cell which converts CO2 into useful high-value products via catalytic reduction. In this, and other, applications the transport behavior of small molecules and ions through the polymeric membrane is critical. In this work, the transport behavior of the commercial cation exchange membrane Nafion® 117 is examined to four CO2 reduction products (methanol, ethanol, n-propanol, and acetone) and their complex mixtures. Here, we use in situ attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectroscopy to characterize the co-transport behavior of complex solute mixtures without the need for aliquotic sampling and ex situ analysis. Methanol (MeOH), ethanol (EtOH), n-propanol (PrOH), and acetone (Ace) permeabilities are determined via a custom-built diffusion cell in single, double, triple and quadruple component aqueous permeability experiments. Overall, differences in solute transport behavior are observed across these complex mixtures. Importantly, the relative permeability (permselectivity) of Nafion® to solute pairs was found to vary by up to 60% when comparing the multicomponent selectivity to the single component selectivity for solute pairs. Changes in solute permeabilities are discussed in the context of measured solute solubilities and calculated solute diffusivities in Nafion® 117. The observed large changes in permeability and, consequently, permselectivity for multicomponent mixtures highlights the importance of directly investigating transport behavior for complex systems.

Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes.

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal-ligand bonds pose important challenges. Herein, we explore a copper complex (12-) featuring an extended, π-delocalized, tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst and compare its activity to analogous systems with lower π-delocalization (22- and 32-). Their characterization evidences a special metal-ligand cooperativity in accommodating the required oxidative equivalents using 12- that is absent in 22- and 32-. This consists of charge delocalization promoted by easy access to different electronic states at a narrow energy range, corresponding to either metal-centered or ligand-centered oxidations, which we identify as an essential factor to stabilize the accumulated oxidative charges. This translates into a significant improvement in the catalytic performance of 12- compared to 22- and 32- and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a kobs of 140 s-1 at an overpotential of only 200 mV. In contrast, 22- degrades under oxidative conditions, which we associate to the impossibility of efficiently stabilizing several oxidative equivalents via charge delocalization, resulting in a highly reactive oxidized ligand. Finally, the acyclic structure of 32- prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.

Over-expression of the C82-methyltransferase BchQ in mutant strains of the green sulfur bacterium Chlorobaculum limnaeum for synthesis of C8-hyper-alkylated chlorosomal pigments

The pigment 8-isobutyl-12-ethyl-([,][I,E])bacteriochlorophyll (BChl) c is key to chlorosomal self-aggregation structures in vivo and was found to produce an organogel in carbon tetrachloride in vitro. Therefore, [I,E]BChl species have been the focus of both fundamental and applicational studies. Since these pigments are very rare in naturally occurring bacterial cells, a method was designed for large-scale production. Here, mutants over-expressing C82-methyltransferase BchQ were constructed using the wild-type and mutant stains of the green sulfur bacterium Chlorobaculum limnaeum. The C8-hyper-alkylated BChls c, d, e, and f, as well as [I,E]BChl species were synthesized by cultured cells. The results showed that more than 95 % of C8-hyper-alkylated BChl c and BChl d were successfully produced in bacterial cells. These findings should prove useful for further development of new chlorosomal models and antenna devices for artificial photosynthesis.

Rational Design of Ion Exchange Membrane Material Properties Limits the Crossover of CO2 Reduction Products in Artificial Photosynthesis Devices.

Efficient operation is crucial for the deployment of photoelectrochemical CO2 reduction devices for large-scale artificial photosynthesis. In these devices, undesired transport of CO2 reduction products from the reduction electrode to the oxidation electrode may occur through a liquid electrolyte and an ion exchange membrane, reducing device productivity and increasing the energy required for product purification. Our work investigated the CO2 reduction product crossover through ion exchange membranes separating the cathode and anode compartments in CO2 reduction cells. The concentrations of liquid products produced by CO2 reduction on copper foil were measured. A systematic approach for the investigation of product crossover was developed. The crossover of products was analyzed over a range of working electrode potentials (-1.08 V vs RHE to -0.88 V vs RHE) in cells employing a commercial Selemion AMV membrane and a new poly(vinylimidazolium) family of ion exchange membranes with variable chemical and structural properties. We found that product loss due to electromigration of charged species in the device was more significant than product loss due to diffusion of uncharged species. To reduce the crossover of CO2 reduction products, the influence of membrane properties such as the ionic conductivity and water volume fraction was investigated for the Selemion AMV membrane and poly(vinylimidazolium) membranes with variable material properties. We show that the water volume fraction and, by extension, ionic conductivity of the membrane may be controlled to reduce product crossover in CO2 reduction artificial photosynthesis devices.

Transport of Neutral and Charged Solutes in Imidazolium-Functionalized Poly(phenylene oxide) Membranes for Artificial Photosynthesis

Anion exchange membranes (AEMs) play an essential role in artificial photosynthesis devices, which photoelectrochemically convert CO2 and water into useful products. AEMs allow the transport of cha...

Toward a New Energy Era: Self‐Driven Integrated Systems Based on Perovskite Solar Cells

Integrated smart portable devices (e.g., self‐powered devices) that utilize the environment‐friendly energy (e.g., solar energy) by means of photovoltaic technology (e.g., solar cell) are a popular concept in the current technological development trend. As a key component of integrated devices, photovoltaic devices acting as a bridge between solar energy and working devices play an important role in the whole system performance. The emergence of perovskite solar cells (PSCs) with high power conversion efficiencies (over 25%) allows for the possibility and appearance of many multifunctional self‐powered integrated devices. In this review, a systematic overview of self‐powered integrated devices based on PSCs that are reported so far is provided, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices and performance are also discussed to further the understanding of fundamental device physics. Finally, the current challenging issues and future perspective are provided to promote the development of self‐powered integrated devices based on PSCs in the near future.