Highlights from Faraday Discussion: Artificial Photosynthesis, Cambridge, UK, March 2019

Abstract

This Faraday Discussion was held on March 25–27th, 2019 at Murray Edwards College, Cambridge, UK and was attended by 160 delegates from over 20 countries. The attendees represented the cross-disciplinary nature of the field, with biologists, engineers, material scientists, theoreticians and experimental chemists of all experience levels coming together to discuss the state of the art. The meeting captured how rapidly the field of artificial photosynthesis has progressed in a short time and highlighted how far we still have to go. In this conference report, the topics of discussion will be outlined with a brief description of the papers presented and a summary of the conference events.

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Introduction

One of the most pressing global crises we face is our increasing reliance on fossil-fuel-based energy sources and the impact this has on our climate. Scientists are looking for new approaches to mitigate increasing carbon dioxide emissions and to find sustainable energy sources. Artificial photosynthesis has come to the forefront as a potentially transformative technology to address these challenges. As such, over 160 delegates arrived from across the globe to attend the Artificial Photosynthesis Faraday Discussion from March 25–27th, 2019. This was the 301st Faraday Discussion, but the third focussed on Artificial Photosynthesis, following the success of those held in Edinburgh, Scotland (2011) and Kyoto, Japan (2017).

Murray Edwards College, Cambridge was the ideal venue for the conference and their floral displays were a happy reminder of the photosynthesis research that the field tries to mimic. The unseasonably good weather was a second reminder of the onset of global climate change that the delegates are devoted to address with their scientific advances.

The format of the Faraday Discussions conference is unique, in that all contributions are published in a journal collection at the close of the conference. During each session, 3–4 speakers present a very brief summary of their major findings and the panel of speakers collectively address the delegates and answer both technical and general questions about their contributed manuscripts, which are distributed to delegates in advance of the meeting. The delegates also have the opportunity to query the panel on their vision for the future of the field as a whole. The collection of contributed manuscripts is accompanied by a transcript documenting the extensive scientific discussion held following each session. In essence, the peer-review process is done in person, with broader discussions about the overarching goals of this transformative research evolving from these extended and cross-cutting discussions. Faraday Discussions have been held for over 100 years, and are regarded as landmark meetings in physical chemistry due to the emphasis placed on discussion and scientific dialogue between world leaders in their respective fields.

Contributions were focussed around four themes: (1) biological approaches to artificial photosynthesis; (2) synthetic approaches to artificial photosynthesis; (3) demonstrator devices for artificial photosynthesis and (4) technologies beyond artificial photosynthesis that push our sights beyond the current state-of-the-art fuel targets.

The conference kicked off with a welcome address by the Chair of the Scientific Committee, Prof. Erwin Reisner (Cambridge University, UK), in which he encouraged participants to make the best use of the format and really get involved in the discussions. RSC representatives, Colin King and Lorna Arens gave the audience a rundown of the unique conference format, to foster a lively and challenging discussion and to keep the panellists on their toes.

Session 1: Biological approaches to artificial photosynthesis

The first session centred on the study of artificial photosynthesis with components taken directly from nature. Both living cells and isolated proteins were used to investigate the combination of light-absorption, charge-separation and charge-transfer processes necessary for efficient photosynthetic function. The breadth of these studies stirred discussion relating to the scale at which biological approaches can be implemented into final devices to best benefit from their existing optimization.

Discussions began with a paper from Prof. Shelley Minteer (University of Utah, USA) entitled ‘Tuning purple bacteria salt-tolerance for photobioelectrochemical systems in saline environments’ (DOI: 10.1039/C8FD00160J). Her paper discusses the use of anoxygenic photosynthetic microorganisms to undertake light-driven decontamination of solutions containing organic species. The study reported that purple bacteria were able to adapt to variable solution salinity (up to 25 g L⁻¹), while maintaining their bio-electrochemical function. These results have exciting consequences for the treatment of salt water and further discussion of the paper centred on the future use of biofilms loaded onto electrodes and their applicability in real-world environments (such as their compatibility with pollutants and other living species).

Second in the session was Prof. Lars Jeuken (University of Leeds, UK), with his work ‘Towards compartmentalized photocatalysis: multihaem proteins as transmembrane molecular electron conduits’ (DOI: 10.1039/C8FD00163D). Photosynthetic photosystems I and II are membrane bound and undertake separate redox reactions on either side of this boundary, yet artificial photosynthesis rarely takes compartmentalisation into account on this scale. Professor Jeuken seeks to remedy this, through the use of a heterotrimeric protein, MtrCAB, encapsulated in a lipid membrane that transfers electrons from a photosensitiser to decompose a redox dye (Reactive Red 120) within the liposome (Fig. 1). Upon irradiation of the system, the reaction proceeded at a considerable rate, particularly when carbon dots were implemented as the photosensitiser. This proof-of-concept will be essential for future development, to ensure adequate separation of reactants, catalysts and products and the next steps for the system will be to replace the redox dye and sacrificial donor with catalysts to undertake the necessary fuel-generating redox reactions. Questions were raised to establish how different orientations of the membrane-bound protein and proton gradients would affect the efficiency of the system.

Fig. 1 (a) Transmembrane electron transfer in (a) the natural photosystems, (b) an artificial photosynthetic equivalent and (c) the biological model used by Prof. Lars Jeuken. (Reproduced from DOI: 10.1039/C8FD00163D with permission from the Royal Society of Chemistry.)

Finishing the session was Prof. Nicolas Plumeré (Ruhr University Bochum, Germany), who presented his group's recent research entitled ‘A kinetic model for redox-active film based bio-photoelectrodes’ (DOI: 10.1039/C8FD00168E). This work sought to model and predict the photocurrent vs. time curves that are generated from bio-photocathodes consisting of a photosynthetic protein loaded into a redox matrix. Through careful deconvolution of the recombination contributions from the desired electron transfer pathways, accurate predictions of the competition between reactant diffusion and recombination were realised. These models will be incredibly useful for the design of future bio-photocathodes and have even been compiled into an App that is available in the Supporting Information of their paper. Discussion centred on the versatility of this model for a range of different system configurations, such as the use of different catalyst types (molecular, biofilm and heterogeneous) and electrode structures.

The discussion in this session was highly animated and led to interesting questions on the extent to which artificial photosynthesis will use components from the natural systems and to what level of accuracy natural systems can be mimicked (Fig. 2).

“Long-term stability (of these photocatalytic systems) is a problem for us all. There is so much scope for the improvement of stability that it could be a whole Faraday Discussion.”

–Prof. Lars Jeuken (University of Leeds, UK)

Fig. 2 (left) Session 1 Panel: Prof. Shelly Minteer, Prof. Nicolas Plumeré, Prof. Lars Jeuken and Session Chair Dr Jenny Zhang discussing biological approaches to artificial photosynthesis. (middle) Faraday Discussion Loving Cup ceremony. PhD students Mr Amedeo Agosti (University of Bologna, Italy) watches Ms Catherine Aitchinson (University of Liverpool, UK) drink from the Loving Cup, while conference chair Prof. Erwin Reisner (Cambridge University, UK) waits for his turn. (Right) One of two poster sessions in Murray Edwards College (photo credit: Mr D. A. García).

Opening lecture

Due to some UK transportation difficulties, the opening lecture was delayed until after the first session. The presentation was very much worth the wait, and the crowd warmly welcomed this year's lecturer and winner of the Spiers Award, Prof. Mattias Beller (Leibniz-Institut für Katalyse, Germany).

Context is key for anyone in the field of artificial photosynthesis; the scale of the environmental and economic issues of the current energy infrastructure must be understood if we are to build a sustainable replacement. In this respect, the lecture served as a perfect introduction, or reminder, of our current vs. future energy needs, which are set to double by 2050. It also presented tools such as the carbon footprint calculator (available at http://www.footprintnetwork.org) to re-emphasise the unsustainable quantities of greenhouse gases we are responsible for, consciously or not, as well as the inevitable environmental consequences these emissions will have.

Prof. Beller used these unsettling truths as justification for mimicking photosynthesis, as our scientific progress is most efficient when we are inspired by nature. This was undertaken through analogy to the evolution of human flight across the 20th century through study of the aerodynamics of birds. His key message, that what is unthinkable for artificial photosynthesis now, could be a ubiquitous technology within a few decades, definitely acts a motivator for anyone in the field, particularly for the younger delegates.

He then presented a sample of his work that had earned him the Spiers award. He showed recent data on molecular catalysts for a number of CO₂ transformations, even combining industrially relevant C and N coupling reactions towards energy-dense fuels and high-added-value fine-chemical feedstocks. He finished this discussion by emphasizing the importance of spin-offs, as the economic and engineering constraints they impose call for necessary multi-scale innovation far beyond the limits of the laboratory framework. However, he reminded the audience that many of these pearls of wisdom can only survive thanks to continued support from government subsidies and therefore major advances in this field will require a common and earnest effort from politicians, engineers and scientists.

Session 2: Synthetic approaches to artificial photosynthesis

The second session focused purely on synthetic components designed to mimic the key light-harvesting and catalytic transformations found in natural photosynthetic machinery. Numerous challenges remain in this field, such as the rational design of high-performance catalysts, efficient light harvesting and fast electron transfer between photocatalytic building blocks. To this end, interesting routes were presented, ranging from bioinspired catalyst design, semi-conductor optimization and mechanistic investigations of multi-metallic catalytic processes.

Session 2 began with some sophisticated organic chemistry, as Prof. Andrew Cooper (University of Liverpool, UK) discussed ‘Photocatalytically active ladder polymers’ (DOI: 10.1039/C8FD00197A). The study described two new ladder polymers bearing dibenzothiophene and dibenzothiophene sulfone subunits (Fig. 3). In the presence of a sacrificial electron donor, the polymers were capable of light-driven H₂ evolution. Questions still remain about whether the catalytic activity is attributed to the residual Pd remaining from their synthetic process, or if activity is a result of catalysis by the polymer itself. Activity could be pushed even higher by the addition of a small quantity of Pt as a co-catalyst, particularly when using light of longer wavelengths (λ > 420 nm). The structure of the polymer has a strong influence over charge-transfer dynamics, illustrating the tunability of this class of organic photosensitizer, and provides a range of possibilities for future polymer design.

Fig. 3 Activity and structures of polymeric photocatalysts for H₂ evolution. (Reproduced from DOI: 10.1039/C8FD00197A with permission from the Royal Society of Chemistry.)

Following the theme of light-absorbing organic polymers, Prof. Martijn Zwijnenburg (University College London, UK) presented his work on ‘Computational high-throughput screening of polymeric photocatalysts: exploring the effect of composition, sequence isomerism and conformational degrees of freedom’ (DOI: 10.1039/C8FD00171E). The research details the development of a DFT-based computational screening that, for a given polymer photosensitizer, can predict key parameters, such as ionization potential, electron affinity and electronic band gap. A vast pool of conjugated monomers was considered, including species such as thiophenes, pyridines and pyrroles, to create a variety of homo- and co-polymers and to subsequently predict their activity. Combined with the preceding paper in this session, this highlight shows that work in this area is booming and the chances of discovering highly active photocatalytic polymers are better than ever.

The day's discussion wrapped up with Prof. Ferdi Karadaş (Bilkent University, Turkey), who presented the first look into water oxidation of the conference in his paper ‘Visible light-driven water oxidation with a ruthenium sensitizer and a cobalt-based catalyst connected with a polymeric platform’ (DOI: 10.1039/C8FD00166A). This work describes a dyad that makes use of a Ru tris-bipyridine type chromophore linked to a Co/Fe hexacyanometallate using poly(vinylpyridine). The dyad catalyzed light-driven water oxidation for 6 h, demonstrating a vastly increased stability compared to the bimolecular system. The increased stability can be attributed to the pairing of the components into a dyad, facilitating electron transfer. This work draws significant parallels with the highly-tuned environment of natural photosynthesis.

Day 2 of the conference continued the papers from Session 2, which was kicked off by Dr Wendy Shaw (Pacific Northwest National Laboratory, USA), who presented her paper ‘Evaluating the impacts of amino acids in the second and outer coordination spheres of Rh-bis(diphosphine) complexes for CO₂ hydrogenation’ (DOI: 10.1039/C8FD00164B). This work used 5 different complexes bearing ligands to mimic amino acids with neutral, acidic and basic groups, to reproduce the coordination sphere around naturally occurring CO₂-hydrogenation enzyme active sites (Fig. 4). Their study showed that a catalyst bearing a N atom with a methyl group showed the highest activity for formate production as this coordination sphere could move to deliver protons to the bound substrate more rapidly compared to the species with the larger side chains. Further study of various catalysts with more electron-withdrawing properties presented large changes in activity depending on the amino acids in the secondary coordination sphere, showing the synergistic influence of all aspects of the ligand sphere for activity. This study perfectly illustrates the intricate balance of properties that must be taken into account when designing a molecular catalyst and how challenging it is to include such features in rational design.

Fig. 4 PNP ligands screened in the Rh-catalyzed CO₂ hydrogenation to formate by Shaw and co-workers (reproduced from DOI: 10.1039/C8FD00164B with permission from the Royal Society of Chemistry).

Discussion of metal bis(diphosphine) complexes was continued by Prof. Peter Brueggeller (University of Innsbruck, Austria), who presented his paper ‘Performance of enhanced DuBois type water reduction catalysts (WRC) in artificial photosynthesis – effects of various proton relays during catalysis’ (DOI: 10.1039/C8FD00162F). In this case, the study centered on modification of the metal center and the number of proton relays around the metal. Light-driven H₂ evolution showed particularly high activity with a Pd metal center, while a need for compromise was found between fast-reacting asymmetrical complexes and more robust, but slower homoleptic complexes. Their additional optimization showed a large catalyst backbone, a high proton relay density and a complex stabilized by solvent molecules (rather than halides) produced the most active complexes, which will be key to future design of this promising family of catalysts.

“Don’t just look under the lamp post; don’t work on what everyone else is (already) working on.”

–Prof. Leif Hammarström (Uppsala University, Sweden)

The morning's session came to a close with Dr Sergei Shylin (Uppsala University, Sweden), who presented his paper on ‘Photoinduced hole transfer from tris(bipyridine)ruthenium dye to a high-valent iron-based water oxidation catalyst’ (DOI: 10.1039/C8FD00167G). In this study, an Fe-clathrochelate molecular complex was used for light-driven water oxidation with a sacrificial electron donor and a Ru photosensitizer. This served as a model system for a nanosecond transient-absorption spectroscopic investigation into the electron transfer kinetics of this important reaction. The catalyst showed the fastest reported hole quenching of a [Ru(bipyridine)₃]²⁺ photosensitizer, consistent with the high reported activity of this interesting range of catalysts.

The synthetic approaches then turned towards heterogeneous systems, beginning with Prof. Wilson Smith (Delft University of Technology, Netherlands) who presented work on ‘Light induced formation of a surface heterojunction in photocharged CuWO₄ photoanodes’ (DOI: 10.1039/C8FD00179K). This work explores the interface between semiconductor surfaces and electrolytes in photoanodes. This highlights the importance of surface states generated from ions in the electrolyte and their role in facilitating charge separation. This was illustrated by photocharging a CuWO₄ photoanode under open-circuit potential and the subsequent formation of a Cu borate complex layer that provides a heterojunction that slows charge recombination. This work corroborates previous phenomena observed on other photoanodes, such as BiPO₄ layers on BiVO₄, and calls for a systematic consideration of the contribution of such layers to the activity of all metal oxide semiconductor surfaces used for water oxidation.

This was followed by Ms Aubrey Paris (Princeton University, USA) who presented her work entitled ‘Mechanistic insights into C₂ and C₃ product generation using Ni₃Al and Ni₃Ga electrocatalysts for CO₂ reduction’ (DOI: 10.1039/C8FD00177D). In this article, the ability of these catalysts to go beyond smaller CO₂ reduction products, without use of Cu, is highlighted. Comparison of the alloyed catalyst¹ to an electrode containing two strips of monometallic surfaces confirmed the importance of the bimetallic phase in the mechanism. The choice of carbon support was also emphasized and the combination of these factors led to the formation of interesting products, such as formate, methanol and propanol (Fig. 5). Further experiments undertaken in deuterated aqueous electrolyte compromised formation of C₂ and C₃ products, implying that a different mechanism from classical ‘Fischer–Tropsch’ style activity is taking place on these alloys.

Fig. 5 C₂ and C₃ products formed using alloyed bimetallic species (reproduced from DOI: 10.1039/C8FD00177D with permission from the Royal Society of Chemistry).

Following on, Prof. Chia Yu Lin (National Cheng Kung University, Taiwan) presented a paper ‘Iron phosphate modified calcium iron oxide as an efficient and robust catalyst in electrocatalyzing oxygen evolution from seawater’ (DOI: 10.1039/C8FD00172C). This study seeks to utilize the abundant, but challenging media of seawater as an electrolyte for water oxidation. In such media, chloride oxidation to the corrosive hypochlorite and other fouling metals ions (such as Mg²⁺) is detrimental to typical Fe-based catalysts. To overcome these hurdles, this article discusses the use of a CaFeO_x anode that is coated in FePO₄, the latter surface layer being responsible for the stable activity of the electrode. The resultant catalyst operated for 10 h (η_{10 mA cm⁻²} = 710 mV), which represents an improvement over catalysts under similar conditions.

The session was rounded off by Prof. Ulf-Peter Apfel (Ruhr University Bochum, Germany) who presented his paper entitled ‘Fe_xNi_9−x (x = 3–6) as potential photocatalysts for solar-driven hydrogen production?’ (DOI: 10.1039/C8FD00173A). These catalysts are naturally occurring minerals that comprise the fundamental components of the active site of hydrogenase enzymes and, as such, present a theoretically ideal combination for H₂ evolution catalysis. The article details the generation of particle catalysts through ball-milling and novel sol–gel strategies with various Ni : Fe ratios that have conduction bands at the thermodynamic limit for H₂ evolution. The particles were then used as photosensitizers coupled to a molecular DuBois-type catalyst² or Pt. Interestingly, replacing the S with O in the structure, showed no activity, confirming the importance of these photosensitizers’ composition for provision of the ideal conduction band edge for photocatalysis.

The general discussion at the close of this session was enthusiastic for the future of this field. Key debate centered on questions such as: Should we expect our computational screening to predict the perfect structure or instead highlight the key parameters for enhancing activity? Is photocatalytic screening appropriate when sacrificial donors are used? On the subject of oxidation, is the only possible reaction water oxidation or are there equally useful, but more simple processes? Prof. Erwin Reisner put forward a suggestion to replace sacrificial donors with redox mediators, which would allow the artificial systems to more closely resemble natural systems.

Session 3: Demonstrator devices for artificial photosynthesis

Transitioning from the synthetic approaches, which generally focus on the intricate mimicry of a subsection of the photosynthetic process, Session 3 moved towards full photo-driven devices. Engineering efforts were devoted to the coupling of the individual components to produce measurable solar-to-fuel efficiencies for industrial benchmarking, as well as continued tuning of catalytic selectivity.

The third session began with by Prof. Michael Grätzel (École Polytechnique Fédérale de Lausanne, Switzerland) who presented a demonstrator device in his paper ‘Sequential catalysis enables enhanced C–C coupling towards multi-carbon alkenes and alcohols in carbon dioxide reduction: a study on bifunctional Cu/Au electrocatalysts’ (DOI: 10.1039/C8FD00219C). The work presented takes advantage of the C₂/C₃-forming CO reduction pathways on Cu in combination with the low overpotential generation of CO from CO₂ on Au. In contrast to the article in Session 2 by Paris, Bocarsly and co-workers, the independent properties of the metal phases are exploited, which decrease the required overpotential required to generate ethylene, ethanol and n-propanol on Cu, along with an increased current density. This strategy to increase CO coverage on Cu surfaces represents a promising route towards CO₂ reduction catalysts with higher activity and higher selectivity for valuable products.

“There are so many (photoactive) materials. I wouldn’t get depressed…it's exciting! When I was a student, all we had was TiO ₂ .”

–Prof. Michael Grätzel (École Polytechnique Fédérale de Lausanne, Switzerland)

Prof. Yi-Hsuan Lai (National Sun Yat-sen University, Taiwan) presented her paper entitled ‘A tandem photoelectrochemical water splitting cell consisting of CuBi₂O₄ and BiVO₄ synthesized from a single Bi₄O₅I₂ nanosheet template’ (DOI: 10.1039/C8FD00183A). The article presents an interesting approach to produce both a photoanode and a photocathode from similar inexpensive components. Bias-free water splitting was achieved with a photocurrent of 36 μA cm⁻² and solar-to-H₂ efficiency of 0.04% through the implementation of a Janus-type Co-based co-catalyst photodeposited onto the electrode surfaces. The current density of the system was limited by the photocathode, motivating future efforts to focus attention on this half reaction.

This was followed by a talk by Prof. Akihiko Kudo (Tokyo University of Science, Japan) entitled ‘Z-scheme photocatalyst systems employing Rh- and Ir-doped metal oxide materials for water splitting under visible light irradiation’ (DOI: 10.1039/C8FD00209F). The article describes how impurities of Ir³⁺ and Rh³⁺ in metal oxide materials allow the use of long-wavelength light and the subsequent use of the materials in Z-schemes to enable water splitting. The mediation of electron transfer between H₂- and O₂-evolving photocatalysts was systematically explored ceteris paribus to compare strategies, such as interparticle electron transfer, ionic electron mediation or electron mediation through a solid support (such as reduced graphene oxide). The efficiencies of the particles show room for improvement, but the article provides a comprehensive overview of key parameters in the design of high-performance photocatalyst Z-schemes.

The final contribution in this session was by Mr Evangelos Kalamaras (Heriot-Watt University, UK) with his work on ‘A microfluidic photoelectrochemical cell for solar-driven CO₂ conversion into liquid fuels with CuO-based photocathodes’ (DOI: 10.1039/C8FD00192H). This article presents the interesting use of a microfluidic flow cell containing a photocathode (Fig. 6). The authors found that a p–n junction attained through addition of CuO over a layer of hematite enabled the electrode to reach higher photocurrents for CO₂ reduction to formate. The cell containing this p–n junction achieved a solar-to-product efficiency of 0.48% and was even able to produce the high-value product, MeOH. This work highlights the importance of cell engineering to unlock routes to industrialize artificial photosynthesis, which have been relatively unexplored until now.

Fig. 6 PEC cell engineering and resultant chopped light J–V curve (reproduced from DOI: 10.1039/C8FD00192H with permission from the Royal Society of Chemistry).

Discussion in this session was focused on the technological readiness of these devices, as would be expected. Prof. Michael Grätzel made a clear argument that the fastest route to high solar-to-fuel efficiencies for the CO₂ reduction reaction will be achieved by coupling highly-selective gas-diffusion-electrode devices to the most efficient photovoltaic cells. The integrated photoelectrochemical cells, which more closely mimic natural photosynthesis, will then arise after further maturation of the science. Other discussion speculated over the most valuable products to focus on, with a consensus that n-propanol production from CO₂ represents an ambitious, but necessary goal. Separation of the oxidation and reduction reactions was also discussed, as light-transparent bipolar membranes, able to separate a high pH gradient, become more prevalent. Finally, the congregation was reminded of the 2 $ per kg target price for H₂ and Prof. Grätzel highlighted that our current problems will require complicated solutions and a substantial amount of work will be required, i.e. no pain, no gain.

Session 4: Beyond artificial photosynthesis

The use of solar energy to decarbonize industrial chemical production and move away from fossil-fuel-based chemical sources has long been discussed as a natural next step. Such “solar-driven” chemical synthesis would find use as a source of base raw materials, including olefins, methanol and ammonia.³ The final scientific session highlighted some of the goals of this forward-looking field and extended the reach of artificial photosynthetic tools and techniques beyond the classic targets of water splitting and CO₂ reduction.

The session began with Prof. Burkhard König (University of Regensburg, Germany), who presented his paper ‘Utilising excited state organic anions for photoredox catalysis: activation of (hetero) aryl chlorides by visible light-absorbing 9-anthrolate anions’ (DOI: 10.1039/C8FD00176F). He outlined the methodology whereby aryl chlorides undergo photomediated reduction and loss of Cl⁻ to promote C–H arylation. The single-electron transfer (referred to as SET) reactivity occurs through excitation of an organic anion as a photocatalyst. Subsequent electron transfers then complete the catalytic cycle to regenerate the original photocatalyst. A unique feature of this methodology is the second SET from the reduced radical anion intermediate back to the oxidized photocatalyst, which also forms the product. The researchers presented evidence to support their mechanistic hypothesis in the form of radical trapping experiments. In addition, the metal-free reaction was shown to be reasonably broad in its substrate scope and generated decent yields under mild conditions. This work demonstrates the potential for continued development of light-driven SET reactivity towards the formation of complex value-added chemicals.

Prof. Marta C. Hatzell (Georgia Institute of Technology, USA) then presented her work on the ‘Influence of carbonaceous species on aqueous photo-catalytic nitrogen fixation by titania’ (DOI: 10.1039/C8FD00191J). The paper outlines her group's investigation into claims that photocatalytic nitrogen fixation by TiO₂ in aqueous environments is possible, despite the mismatching redox potentials of N₂ reduction and TiO₂. Using rotating-ring-disc electrochemistry, her work showed that adventitious carbon contamination of TiO₂ produces N₂-adsorption sites that undertake N₂ reduction to NH₃. The electrochemical techniques presented in this paper also allow the detection of ammonia at low levels, which may be useful as the field progresses and photocatalytic nitrogen fixation becomes viable.

Prof. Joost Reek (University of Amsterdam, Netherlands) then presented his paper on ‘p-Type dye-sensitized solar cells based on pseudorotaxane mediated charge transfer’ (DOI: 10.1039/C8FD00169C). The design of this system relies upon a supramolecular dye that was hypothesized to decrease occurrence of charge recombination in the DSSC, which often limits forward electron transfer and overall efficiency (Fig. 7). Synthesis of the dye involved the threading of a viologen-ring rotaxane, which holds a 4+ charge in its oxidized state, onto the photosensitizer. Electron transfer to the ring decreases the charge and induces de-threading, releasing the reduced ring into solution to propagate charge transfer. Curiously, they noted that upon addition of methyl viologen into the solution, their electron transfer efficiency increased. However, the methyl viologen may have been reduced by the electrode or the photosensitizer directly without requiring the ring molecule. The presented system sparked much interest and debate from the audience, as the use of supramolecular systems for solar fuels are relatively rare. Several comments from the audience pointed out that the effect observed might have been due to a static electron shuttling mechanism occurring in a system that has so many ‘moving parts’.

Fig. 7 Pseudorotaxane based p-type DSSC making up the supramolecular charge transfer on a Ni–O electrode (reproduced from DOI: 10.1039/C8FD00169C with permission from the Royal Society of Chemistry).

The final contribution was by Mr Stelios Gavrielides (Heriot-Watt University, UK) who presented his paper ‘Photo-generation of cyclic carbonates using hyper-branched Ru–TiO₂’. (DOI: 10.1039/C8FD00181B). In this work, CO₂ is converted into cyclic carbonates by a photo-driven catalytic reaction with epoxides, using hyper-branched nanorods of TiO₂ loaded with RuO₂ nanoparticles. Current industrial processes to carry out such transformations employ temperatures exceeding 200 °C and very high pressures (50–100 bar). The methodology presented here is less energy intensive, requiring only 55 °C and 2 bar pressure. This is due to an additional driving force provided by light, which facilitates an alternative light-driven reaction pathway. Such a unique approach not only makes a valuable organic chemical but has the added benefit of fixing gaseous CO₂; a potentially useful reaction for our atmosphere.

Spiers memorial award presentation and conference dinner

Spirits were high during the conference dinner, held in the Murray Edwards dining hall on the Tuesday evening. The Chair of the Faraday Division Awards Committee, Prof. Claire Vallance (University of Oxford, UK), was on hand to present Prof. Mattias Beller with the 2019 Spiers Memorial Award, which includes a Medal and certificate (Fig. 8). This award commemorates Frederick S. Spiers, who worked as Secretary of The Faraday Society, which he helped found in 1902. The award was given to Prof. Beller for the development of practical homogeneous and heterogeneous catalysts for sustainable chemical transformations. His scientific contributions exemplify thoughtful and transformative developments in sustainable catalysis. Prof. Vallance also introduced delegates to a special Faraday Discussions tradition, in which ‘The Loving Cup’, a silver-engraved goblet is filled with port. The Cup is passed around from one delegate to the next, with each delegate reciting a verse that honors the benefactors of the Faraday Discussion, before taking a (small) sip of the wine, followed by a bow to the person on their left and a bow to the person on their right. The refrain “in piam memoriam of G. S. Marlow and Angela & Tony Fish” was thus heard approximately 200 times as the cup travelled around the room. Tradition says that the two people flanking the cup were there to protect the drinker from harm while their sword hand was occupied. En garde!

Fig. 8 (left) Spiers Award winner Prof. Matthias Beller receiving his medal and certificate from Prof. Claire Vallance. (right) The closing photo of conference delegates (photo credit: Mr D. A. García).

Poster sessions

Enthusiasm for the poster session was high and there were over 80 posters on display, so many posters in fact, that the session spanned two evenings. Leading experts gave insight to the contemporary findings in several rooms overflowing with young scientists at Murray Edwards College. Two deserving students, Mr Joshua Lawrence (University of Cambridge, UK) and Ms Tessel Bowens (University of Amsterdam, The Netherlands) were awarded the prizes for their excellent research and poster presentation.

Concluding remarks and wrap-up

Prof. James Durrant (Imperial College London, UK) provided some concluding remarks, effectively summarizing the key work presented in each session. He was also able to capture the true driving motivations of all scientists working towards the ultimate goal we call ‘Artificial Photosynthesis’: the storage of solar energy in chemical bonds. The question of whether we can move to more cost-effective methods to compete with fossil fuels was raised and he highlighted that there are many ideas and many paths forwards, with many synergistic links between them. We comprise a diverse field and the individuals from each specialization must meet and learn from one another.

Prof. Durrant challenged scientists to push beyond boundaries. He emphasized that leadership and direction from industrial partners is crucial, as we inch ever closer to commercially viable technologies. There was no shortage of optimism in the room, but it is clear that these efforts will require global communication and collaboration which could, and should, result in many scientific breakthroughs. Much is at stake, but there is much to be gained should we succeed.

“As a community, we must speak clearly with one voice, to guide our mission”

–Prof. James Durrant (Imperial College London, UK)

Continuing meetings

A full week of discussions on Artificial Photosynthesis continued after the close of the Faraday Discussion, with several delegates participating in the Christian Doppler Laboratory Symposium on Solar Fuels, as well as the UK Solar Fuels Network's annual meeting. There was much buzz about the potential location and host venue for the next Artificial Photosynthesis Faraday Discussion. Keep your eyes open for the future announcement.

Overall, the three events acted as a venue for deep scientific discussion, networking and sparking new collaborative efforts in the push towards solar-driven generation of sustainable fuels.

Acknowledgements

David Wakerley and Sarah Lamaison acknowledge funding from the Collège de France for their attendance at the conference. Christine Caputo acknowledges financial support from the Department of Chemistry, University of New Hampshire. David Wakerley and Christine Caputo also acknowledge the RSC for travel funding.

Notes and references

1. A. R. Paris and A. B. Bocarsly, ACS Catal., 2017, 7, 6815–6820.
2. A. D. Wilson, R. H. Newell, M. J. McNevin, J. T. Muckerman, M. Rakowski DuBois and D. L. DuBois, J. Am. Chem. Soc., 2006, 128, 358–366.
3. (a) P. Lanzafame, S. Abate, C. Ampelli, C. Genovese, R. Passalacqua, G. Centi and S. Perathoner, ChemSusChem, 2017, 10, 4409–4419; (b) M. Beller, G. Centi and L. Sun, ChemSusChem, 2017, 10, 6–13.

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Footnote

† These authors made equal contributions to the manuscript.

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