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Efficient artificial photosynthesis systems are currently realized as catalyst- and surfacefunctionalized photovoltaic tandem- and triple-junction devices, enabling photoelectrochemical (PEC) water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. Although PEC systems also bear advantages for the activation of dinitrogen - such as a high system tunability with respect to the electrocatalyst integration and a directly controllable electron flux to the anchoring catalyst through the adjustability of incoming irradiation - only a few PEC devices have been developed and investigated for this purpose. We have developed a series of photoelectrodeposition procedures to deposit mixed-metal electrocatalyst nanostructures directly on the semiconductor surface for light-assisted dinitrogen activation. These electrocatalyst compositions containing Co, Mo and Ru in different atomic ratios follow previously made recommendations of metal compositions for dinitrogen reduction and exhibit different physical properties. XPS studies of the photoelectrode surfaces reveal that our electrocatalyst films are to a large degree nitrogen-free after their fabrication, which is generally difficult to achieve with traditional magnetron sputtering or e-beam evaporation techniques. Initial chronoamperometric measurements of the p-InP photoelectrode coated with the Co-Mo alloy electrocatalyst show higher photocurrent densities in the presence of N2(g) than in the presence of Ar at -0.09 V vs RHE. Indications of successful dinitrogen activation have also been found in consecutive XPS studies, where both, N 1s and Mo 3d spectra, reveal evidence of nitrogen-metal interactions.

Electrocatalytic reduction of dinitrogen (N2) to ammonium (NH4+) in acidic aqueous solutions was investigated at ambient temperature and pressure using a cobalt–molybdenum (CoMo) thin-film electrode prepared by magnetron reactive sputtering. Increased concentrations of ammonium ions (NH4+) were consistently detected in the electrolyte using ion chromatography (IC) after constant-potential electrolysis at various potentials (≤−0.29 V vs. RHE). Using a newly developed analytical method based on ammonia derivatization, performing the experiments with 15N2-labelled gas led however to the detection of increased 14NH4+ concentrations instead of 15NH4+. X-ray photoelectron spectroscopic (XPS) analysis of the electrode surface revealed the presence of Mo[triple bond, length as m-dash]N and Mo–NHx species. Several contamination sources were identified that led to substantial increases in the concentration of ammonium ions, including 15NH3 impurities in 15N2 gas. The observed ammonium concentrations can be consistently ascribed to leaching of nitrogen (14N) impurities incorporated in the CoMo film during the sputtering process. Researchers in the field are therefore urged to adopt extended protocols to identify and eliminate sources of ammonia contamination and to very carefully monitor the ammonium concentrations in each experimental step.\ud \ud

Photoelectrochemical devices integrate the processes of light absorption, charge separation, and catalysis for chemical synthesis. The monolithic design is interesting for space applications, where weight and volume constraints predominate. Hindered gas bubble desorption and the lack of macroconvection processes in reduced gravitation, however, limit its application in space. Physico-chemical modifications of the electrode surface are required to induce gas bubble desorption and ensure continuous device operation. A detailed investigation of the electrocatalyst nanostructure design for light-assisted hydrogen production in microgravity environment is described. p-InP coated with a rhodium (Rh) electrocatalyst layer fabricated by shadow nanosphere lithography is used as a model device. Rh is deposited via physical vapor deposition (PVD) or photoelectrodeposition through a mask of polystyrene (PS) particles. It is observed that the PS sphere size and electrocatalyst deposition technique alter the electrode surface wettability significantly, controlling hydrogen gas bubble detachment and photocurrent–voltage characteristics. The highest, most stable current density of 37.8 mA cm−2 is achieved by depositing Rh via PVD through 784 nm sized PS particles. The increased hydrophilicity of the photoelectrode results in small gas bubble contact angles and weak frictional forces at the solid–gas interface which cause enhanced gas bubble detachment and enhanced device efficiency.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

The tyrosine residue D2-Tyr160 (Tyr(D)) in photosystem II (PSII) can be oxidized through charge equilibrium with the oxygen evolving complex in PSII. The kinetics of the electron transfer from Tyr( ...

Efficient artificial photosynthesis systems are currently realized as catalyst- and surface-functionalized photovoltaic tandem and triple junction devices [1,2] enabling photoelectrochemical water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible. We recently demonstrated in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel [3]. Current limiting factors such as the absence of macroconvection processes were overcome by controlling the micro- and nanotopography of the electrocatalyst using shadow nanosphere lithography (SNL), generating so-called catalytic ‘hot-spots’ on the electrode surface which prevent gas bubble coalescence [3,4]. We found that the J-V characteristics of the half-cell and the overall device efficiency in microgravity environment are significantly affected by alterations in the electrocatalyst nanotopography [5]. By varying the shape and distance of catalytic ‘hot-spots’ on the electrode surface, we could control the gas bubble radius upon detachment from the electrode surface and the light absorption properties of the semiconductor in free fall. Shadow nanosphere lithography can therefore be used as a prosperous tool to develop custom-tailored electrocatalyst nanostructures of high fidelity on a light-absorbing semiconductor surface for an optimized device performance in microgravity and terrestrial applications. References [1] Young J. L., Steiner M. A., Döscher H., France R. M., Turner J. A., Deutsch T. G. (2017). “Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures”, Nat. Energ. 2. (17028). [2] Cheng W. H., Richter M. H., May M. M., Ohlmann J., Lackner D., Dimroth F., Hannappel T., Atwater H. A., Lewerenz H. J (2018). “Monolithic Photoelectrochemical Device for 19% Direct Water Splitting”, ACS Energy Lett. 3, 8, 1795-1800. [3] Brinkert K., Richter M. H., Akay Ö., Liedtke J., Gierisig M., Fountaine K. T., Lewerenz H. J. (2018). Efficient Solar Hydrogen Production in Microgravity Environment. Nat. Commun. 9 (2527). [4] Patoka P., Giersig M. (2011). “Self-assembly of latex particles for the creation of nanostructures with tunable plasmonic properties”, J. Mater. Chem. 21, 16783-16796. [5] Brinkert K., Richter M. H., Akay Ö., Giersig M., Fountaine K. T., Lewerenz H.-J. (2018). “Advancing semiconductor-electrocatalyst systems: application of surface transformation films and nanosphere lithography”, Faraday Discuss. 208, 523-535.

Efficient artificial photosynthesis systems are currently realized as catalyst- and surface-functionalized photovoltaic tandem and triple junction devices [1,2] enabling photoelectrochemical water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible. We recently demonstrated in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel [3]. Current limiting factors such as the absence of macroconvection processes were overcome by controlling the micro- and nanotopography of the electrocatalyst using shadow nanosphere lithography (SNL), generating so-called catalytic ‘hot-spots’ on the electrode surface which prevent gas bubble coalescence [3,4]. We found that the J-V characteristics of the half-cell and the overall device efficiency in microgravity environment are significantly affected by alterations in the electrocatalyst nanotopography [5]. By varying the shape and distance of catalytic ‘hot-spots’ on the electrode surface, we could control the gas bubble radius upon detachment from the electrode surface and the light absorption properties of the semiconductor in free fall. Shadow nanosphere lithography can therefore be used as a prosperous tool to develop custom-tailored electrocatalyst nanostructures of high fidelity on a light-absorbing semiconductor surface for an optimized device performance in microgravity and terrestrial applications. [1] Young J. L., Steiner M. A., Döscher H., France R. M., Turner J. A., Deutsch T. G. (2017). “Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures”, Nat. Energ. 2. (17028). [2] Cheng W. H., Richter M. H., May M. M., Ohlmann J., Lackner D., Dimroth F., Hannappel T., Atwater H. A., Lewerenz H. J (2018). “Monolithic Photoelectrochemical Device for 19% Direct Water Splitting”, ACS Energy Lett. 3, 8, 1795-1800. [3] Brinkert K., Richter M. H., Akay Ö., Liedtke J., Gierisig M., Fountaine K. T., Lewerenz H. J. (2018). Efficient Solar Hydrogen Production in Microgravity Environment. Nat. Commun. 9 (2527). [4] Patoka P., Giersig M. (2011). “Self-assembly of latex particles for the creation of nanostructures with tunable plasmonic properties”, J. Mater. Chem. 21, 16783-16796. [5] Brinkert K., Richter M. H., Akay Ö., Giersig M., Fountaine K. T., Lewerenz H.-J. (2018). “Advancing semiconductor-electrocatalyst systems: application of surface transformation films and nanosphere lithography”, Faraday Discuss. 208, 523-535.

Long-term space missions require extra-terrestrial production of storable, renewable energy. Hydrogen is ascribed a crucial role for transportation, electrical power and oxygen generation. We demonstrate in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel. The photoelectrochemical cell consists of an integrated catalyst-functionalized semiconductor system that generates hydrogen with current densities >15 mA/cm2 in the absence of buoyancy. Conditions are described adverting the resulting formation of ion transport blocking froth layers on the photoelectrodes. The current limiting factors were overcome by controlling the micro- and nanotopography of the Rh electrocatalyst using shadow nanosphere lithography. The behaviour of the applied system in terrestrial and microgravity environment is simulated using a kinetic transport model. Differences observed for varied catalyst topography are elucidated, enabling future photoelectrode designs for use in reduced gravity environments., While renewable energy production is a terrestrial concern, far less attention is devoted to solar-to-fuel conversion for long-term space missions. Here, the authors explore photoelectrochemical hydrogen generation in microgravity and overcome microgravity’s limitations by electrode nanostructuring.

The 21st century confronts us with significant challenges, summarized by the UN Sustainability Goals.

The application of electrolysis or photoelectrolysis of water to generate oxygen and hydrogen gas could provide a scalable mechanism to store intermittent renewable energy.

The dream of realising artificial photosynthesis can probably be best compared to the dream of flying at the turn of the 19th century; early aviation pioneers looked at birds, the function of the wings, the tail, fuselage and the aerodynamics of flying to come up with biomimetic aeroplane design features.

Photoelectrochemical (PEC) cells offer the possibility of carbon-neutral solar fuel production through artificial photosynthesis. The pursued design involves technologically advanced III–V semiconductor absorbers coupled via an interfacial film to an electrocatalyst layer. These systems have been prepared by in situ surface transformations in electrochemical environments. High activity nanostructured electrocatalysts are required for an efficiently operating cell, optimized in their optical and electrical properties. We demonstrate that shadow nanosphere lithography (SNL) is an auspicious tool to systematically create three-dimensional electrocatalyst nanostructures on the semiconductor photoelectrode through controlling their morphology and optical properties. First results are demonstrated by means of the photoelectrochemical production of hydrogen on p-type InP photocathodes where hitherto applied photoelectrodeposition and SNL-deposited Rh electrocatalysts are compared based on their J–V and spectroscopic behavior. We show that smaller polystyrene particle masks achieve higher defect nanostructures of rhodium on the photoelectrode which leads to a higher catalytic activity and larger short circuit currents. Structural analyses including HRSEM and the analysis of the photoelectrode surface composition by using photoelectron spectroscopy support and complement the photoelectrochemical observations. The optical performance is further compared to theoretical models of the nanostructured photoelectrodes on light scattering and propagation.

The increased understanding of photosynthetic energy conversion and advances in chemical synthesis have made it possible to create artificial biomimetic systems and semibiological hybrids which are able to carry out parts of the natural photosynthetic process.

The sun provides more than 100,000 TW to the surface of the earth. The evolution of photosynthesis occurred based on maximizing the productive process of generating new components and energy for biological activities.

Oxygenic photosynthesis is the process in which oxygen and carbohydrates are produced from water and carbon dioxide utilizing sunlight. With the ability of converting solar energy into chemical energy, oxygenic photosynthesis remains one of the most important biological processes on earth, producing directly or indirectly the building blocks of all living organisms and also of oil, gas and coal. In contrast to some anaerobic photosynthetic organisms which use H2S as a reducing agent and produce sulphur as a major product, oxygenic photosynthesis utilizes H2O as the reducing agent.

The high demand for the integration of sustainable topics into university curricula presents new challenges for the way chemistry is traditionally taught. New teaching concepts are required that consider and connect different disciplines to achieve a higher student awareness of the importance of these topics for humanity, the environment, and the future of our planet. This article describes a uniquely multidisciplinary graduate course on solar energy; the course may serve as an example of how to incorporate sustainable topics into chemistry programs, and as a starting point from which to build for others. It combines different scientific, industrial, political, and humanitarian approaches to the topic and provides a broad overview of one of the main future energy resources. New teaching methods are introduced, combined, and evaluated with respect to their effectiveness for a sustainable education within science and, in particular, chemistry. The success of this new course concept is demonstrated by high student satisfaction in a trial course evaluation and impressive exam results.

Efficient artificial photosynthesis systems follow the concept of the Z-scheme of natural photosynthesis. They are realized as catalyst- and surface-functionalized photovoltaic tandem devices [1,2] enabling photoelectrochemical water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible. We demonstrate in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel [3]. The photoelectrochemical cell consists of an integrated catalyst-functionalized semiconductor system that generates hydrogen with current densities >15mAcm-2 in the absence of buoyancy. Conditions are described adverting the resulting formation of ion transport blocking froth layers on the photoelectrodes. The current limiting factors were overcome by controlling the micro- and nanotopography of the electrocatalyst using shadow nanosphere lithography [4]. We show that shadow nanosphere lithography can be used as a prosperous tool to obtain desired catalyst nanostructures of high fidelity on a light-absorbing semiconductor surface with tunable optical properties showing significant advantages for photoelectrochemical hydrogen production in microgravity and terrestrial applications [5]. References [1] Young J. L., Steiner M. A., Döscher H., France R. M., Turner J. A., Deutsch T. G. (2017). “Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures”, Nat. Energ. 2. (17028). [2] Cheng W. H., Richter M. H., May M. M., Ohlmann J., Lackner D., Dimroth F., Hannappel T., Atwater H. A., Lewerenz H. J (2018). “Monolithic Photoelectrochemical Device for 19% Direct Water Splitting”, ACS Energy Lett. 3, 8, 1795-1800. [3] Brinkert K., Richter M. H., Akay Ö., Liedtke J., Gierisig M., Fountaine K. T., Lewerenz H. J. (2018). Efficient Solar Hydrogen Production in Microgravity Environment. Nat. Commun. 9 (2527). [4] Patoka P., Giersig M. (2011). “Self-assembly of latex particles for the creation of nanostructures with tunable plasmonic properties”, J. Mater. Chem. 21, 16783-16796. [5] Brinkert K., Richter M. H., Akay Ö., Giersig M., Fountaine K. T., Lewerenz H.-J. (2018). “Advancing semiconductor-electrocatalyst systems: application of surface transformation films and nanosphere lithography”, Faraday Discuss. 208, 523-535.

Significance The E m of Q A / Q A − • is the reference pegging the thermodynamics of Photosystem II (PSII) to an absolute value. This work resolves a long-standing discrepancy in the literature, removing this ambiguity at the heart of PSII bioenergetics. The discrepancy in the literature reflects the loss of bicarbonate in some titrations. This surprising finding shows that bicarbonate binding controls the E m of Q A / Q A − • , and thus the redox state of Q A influences the binding of bicarbonate. This process constitutes a previously unrecognized regulation of PSII: Q A − • triggers bicarbonate release, slowing electron transfer and tuning the potential of Q A to protect PSII against photodamage. This work provides an explanation for the existence of bicarbonate in PSII, a mystery for more than half a century.

THE WORLD ENERGY PROBLEM AND THE BIOFUEL ENERGY PROBLEM Oxygenic photosynthesis has been promoted as a system for fuel production on a global scale to replace fossil fuels. The fundamental requirement for this to be viable is that the energy output of the system must be greater than the energy input from fossil fuels. For biofuel production, this criterion is not always met. This issue is often dodged because life-cycle analyses are complex (and thus disputed) and future technological innovations can always be invoked. The second requirement is a sufficiently high rate of solar energy conversion to make the process feasible in terms of the time and space needed to produce fuel on a relevant scale. Both requirements are closely linked to the photosynthetic efficiency; i.e., the conversion efficiency of solar energy to organic material (sugar, biomass, hydrocarbons, etc.). Here, we discuss limitations on photosynthetic efficiency and approaches suggested to overcome them. We focus on biofuels produced by photosynthetic microbes as they are often considered the fuels of the future for their year-round cultivation, non-competition with food crops, higher reported photosynthetic yields, and the potential for genetic engineering to produce fuels directly (Brennan and Owende, 2010). The energy investment required for biomass production (e.g., water, nutrients, fertilizers, stirring, bubbling, containment, harvesting, processing) cancels out some or all of the energy gained from sunlight (Slade and Bauen, 2013). This is described by the energy returned as a proportion of energy invested (EROI), and this factor is the key measure of energy sustainability in life-cycle analysis (Murphy and Hall, 2010). If the EROI is >1, the system produces a fuel with net solar energy content; if 1 and a high rate of solar energy conversion) on a pilot scale seems advisable before scaling-up is considered. The energetic prerequisites rely fundamentally on the efficiency of photosynthesis. Calculations for theoretical photosynthetic efficiency agree on a maximum value for solar energy to carbon–carbon bonds in glucose of around 13%, falling to around 5% of solar energy to biomass for C3 plants, considering photorespiratory and respiratory losses (Zhu et al., 2010). The highest efficiency reported for photosynthetic microbes under controlled lab conditions is 3% for light-to-biomass [Melis, 2009; also Cuaresma et al. (2009)]. Under growth conditions more relevant to industrial settings, the efficiency is stated to be significantly lower than this (Melis, 2009). Efforts are thus being made to find ways of improving photosynthesis itself.

Artificial photosynthesis systems, which follow the concept of the Z-scheme of natural photosynthesis, are presently being realized as catalyst-functionalized photovoltaic tandem devices for the photoelectrochemical oxidation of water and the simultaneous generation of hydrogen as a so-called “solar fuel”. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible. Here, we present the photoelectrochemical production of hydrogen in microgravity environments on p-type indium phosphide electrodes with deposited rhodium electrocatalysts. Our findings indicate that microgravity has a significant impact on the gas bubble evolution behaviour and the mass transfer rate of the evolved hydrogen gas on the electrode surface. Furthermore, microgravity influences the current-voltage characteristics and the overall solar-to-hydrogen efficiency of the catalyst functionalized semiconductor-based half-cell. Further experiments with nanostructured rhodium catalysts fabricated by shadow nanosphere lithography on the InP surface suggest that the structure of the electrode surface plays a significant role for the gas bubble evolution behaviour and for the further the development of efficient prototypes for solar-assisted water splitting and hydrogen production that operate in micro- and hypergravity environments.