Protein film voltammetry and spectroelectrochemistry of the electron acceptor site in Photosystem II

The key reaction of oxygenic photosynthesis, the light-driven oxidation of water, is carried out by Photosystem II (PSII), a light-driven water-plastoquinone oxidoreductase. Photosystem II is a multi-cofactor protein and its energetic characterization is quite a difficult undertaking. Electrochemical approaches play an important role as a tool for completing the energetic picture and for carrying out fundamental studies of the enzyme. In this work, two electrochemical methods have been applied to study the electron acceptor site of the Photosystem II: spectroelectrochemistry and protein film voltammetry. The first part focuses on the re-measurement of the midpoint potential of the primary quinone electron acceptor, QA, in PSII core complexes isolated from Thermosynechococcus elongatus and in PSII enriched membranes from spinach using an optical transparent thin layer (OTTLE) cell. The obtained results show that the bicarbonate anion, ligated to the non-heme iron at the electron acceptor site in close proximity to QA, plays a significant role in controlling the redox properties of the QA/QA--couple. This finding explains various controversies about existing literature values of the QA/QA-midpoint potential. The second part describes investigations of photocurrents generated by Photosystem II in metal oxide hybrid systems. PSII isolated from T. elongatus was immobilized onto nanostructured titanium dioxide/indium tin oxide electrodes (TiO2/ITO) and the origin of photocurrent upon illumination was studied. Using conditions in which PSII was immobilized as a monolayer, it was demonstrated that direct electron transfer occurs from the redox cofactor QA to the electrode surface, but that the electron transfer through the metal oxide is the rate-limiting step. Redox mediators enhance the photocurrent by taking electrons from the TiO2 surface to the ITO and not from PSII. Furthermore, the origin of cathodic photocurrents (i.e. electron flow from the electrode) was investigated. The results indicate that a one-electron reduction of oxygen to the superoxide anion radical (O2•-) occurs at the ITO surface in darkness when an external bias lower than +300 mV vs NHE was applied. The findings can explain the light-driven and catalytic nature of this current by the fact that the reduction of O2•- occurs at the non-heme iron, which is driven by the photo-reduction of QA.Open Acces

Fundamentals and future applications of electrochemical energy conversion in space

Long-term space missions require power sources and energy storage possibilities, capable at storing and releasing energy efficiently and continuously or upon demand at a wide operating temperature range, an ultra-high vacuum environment and a significantly reduced buoyant force. Electrochemical energy conversion systems play already a major role e.g., during launch and on the International Space Station, and it is evident from these applications that future human space missions - particularly to Moon and Mars - will not be possible without them. Here, we will provide an overview of currently existing electrochemical conversion technologies for space applications such as battery systems and fuel cells and outline their role in materials design and fabrication as well as fuel production. The focus lies on the current operation of these energy conversion systems in space as well as the challenges posed on them by this special environment. Future experiment designs which could help elucidating and optimizing their key operating parameters for an efficient and long-term operation are discussed

Magnetic phase separation in microgravity

The absence of strong buoyancy forces severely complicates the management of multiphase flows in microgravity. Different types of space systems, ranging from in-space propulsion to life support, are negatively impacted by this effect. Multiple approaches have been developed to achieve phase separation in microgravity, whereas they usually lack the robustness, efficiency, or stability that is desirable in most applications. Complementary to existing methods, the use of magnetic polarization has been recently proposed to passively induce phase separation in electrolytic cells and other two-phase flow devices. This article illustrates the dia- and paramagnetic phase separation mechanism on MilliQ water, an aqueous MnSO4 solution, lysogeny broth, and olive oil using air bubbles in a series of drop tower experiments. Expressions for the magnetic terminal bubble velocity are derived and validated and several wall–bubble and multi-bubble magnetic interactions are reported. Ultimately, the analysis demonstrates the feasibility of the dia- and paramagnetic phase separation approach, providing a key advancement for the development of future space systems

Magnetic phase separation in microgravity

The absence of strong buoyancy forces severely complicates the management of multiphase flows in microgravity. Different types of space systems, ranging from in-space propulsion to life support, are negatively impacted by this effect. Multiple approaches have been developed to achieve phase separation in microgravity, whereas they usually lack the robustness, efficiency, or stability that is desirable in most applications. Complementary to existing methods, the use of magnetic polarization has been recently proposed to passively induce phase separation in electrolytic cells and other two-phase flow devices. This article illustrates the dia- and paramagnetic phase separation mechanism on MilliQ water, an aqueous MnSO4 solution, lysogeny broth, and olive oil using air bubbles in a series of drop tower experiments. Expressions for the magnetic terminal bubble velocity are derived and validated and several wall–bubble and multi-bubble magnetic interactions are reported. Ultimately, the analysis demonstrates the feasibility of the dia- and paramagnetic phase separation approach, providing a key advancement for the development of future space systems

application of surface transformation films and nanosphere lithography

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 potential of catalysis for closing the loop in human space exploration

This mini-review provides a perspective on how catalysis can contribute to the challenge of human space exploration, i.e. to the establishment of a space habitat on the Moon in the near future and the realization of a journey to Mars in the far future. The role of catalysis for space exploration is reviewed along elemental resource provisions for human life that are based on catalytic chemical processes. As a secondary effect, learning about catalysis under the high constraints of the sustainability requirements inherent to long-term space missions can be beneficial for the advancement of currently developed catalytic processes for a green, circular economy on Earth. In-depth research into catalysis for space applications could therefore also provide answers to urgent industrial, environmental and social sustainability challenges on our planet e.g., as expressed by the United Nations Sustainability Development Goals. Even for today’s short-term space missions, catalysis already plays a key role in the Environmental Control and Life Support System (ECLSS) on the International Space Station (ISS) and moreover, in fuel and propellant synthesis as well as the recycling of combustion products such as CO2

Efficient solar hydrogen generation in microgravity environment

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

Releasing the Bubbles: Nanotopographical Electrocatalyst Design for Efficient Photoelectrochemical Hydrogen Production in Microgravity Environment

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

Cathodic NH₄⁺ leaching of nitrogen impurities in CoMo thin-film electrodes in aqueous acidic solutions

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 N and Mo–NH_x 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

Cathodic NH₄⁺ leaching of nitrogen impurities in CoMo thin-film electrodes in aqueous acidic solutions

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 N and Mo–NH_x 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