Chemistry
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Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment
Chapters
Summary December 3rd, 2019
Efficient solar-hydrogen production has recently been realized on functionalized semiconductor-electrocatalyst systems in a photoelectrochemical half-cell in microgravity environment at the Bremen Drop Tower. Here, we report the experimental procedures for manufacturing the semiconductor-electrocatalyst device, details of the experimental set-up in the drop capsule and the experimental sequence during free fall.
Transcript
This protocol includes a step-by-step procedure of how constructing nanostructured photoelectrodes for efficient light-assisted hydrogen production in microgravity environments. It also includes an electrode testing procedure of how to test these photoelectrodes at the Bremen Drop Tower where 10 to the minus six g can be generated in 9.2 seconds of free fall. It is observed by us and other research teams that electrochemically generated gas bubbles stick to the electrode surface in microgravity conditions due to the absence of buoyancy.
catalyst generates catalytic hotspots which improves the detachment of gas bubbles and the overall efficiencies. The procedures will be demonstrated by Omer Akay, a graduate student in our laboratory at FU Berlin. To begin, apply silver paste to attach the OMEC contact to a thin plated copper wire.
Thread the wire to a glass tube and use black chemical-resistant epoxy to encapsulate the P-type indium phosphide sample and seal it to the glass tube. Under a fume hood, place the 0.5 square centimeter polished indium face of P-type indium phosphide in 10 milliliters of bromine/methanol solution for 30 seconds to etch for removing native oxides. Then rinse the surface with ethanol and ultrapure water for 10 seconds each and dry the sample under nitrogen flux.
Use a borosilicate glass cell with a quartz window as a photoelectrochemical cell and place the P-type indium phosphide electrode in the cell in a standard three-electrode potentiostatic arrangement. Use a white light tungsten halogen lamp to illuminate the sample during the conditioning procedure. Adjust the light intensity with a calibrated silicon reference photodiode.
After purging the hydrochloric acid with nitrogen, photoelectrochemically condition the sample with potentiodynamic cycling at a scan rate of 50 millivolts per second for 50 cycles under continuous illumination. In this procedure, shadow nanosphere lithography is applied for forming rhodium nanostructures on the P-type indium phosphide electrode. Obtain a ready-made mixture of 300 microliters of 784 nanometer polystyrene beads and 300 microliters of ethanol containing one weight volume percent styrene and 0.1%sulfuric acid.
Use a Pasteur pipette with a curved tip to apply the solution onto the water surface. Turn the Petri dish gently to increase the area of the monocrystalline structures. Carefully distribute the solution to cover 50%of the air-water interface with a hexagonal closed packed monolayer.
Protect the copper wire of the photoelectrochemically conditioned P-type indium phosphide electrodes with parafilm. Carefully tape them to a microscope slide and place the electrodes under the floating close packed polystyrene sphere mask. Prevent the samples from rotating.
After that, use a pipette to gently remove residual water and wait for it to dry. This causes the mask to be subsequently deposited onto the electrode surface. Then take the electrode out of the Petri dish and gently dry the surface with nitrogen.
Store the electrode under nitrogen in a desiccator. To photoelectrochemically deposit rhodium nanoparticles, place the electrode in an electrolyte solution containing rhodium chloride, sodium chloride and isopropanol. With a potentiostat, apply a constant potential of 0.01 volts for five seconds under simultaneous illumination with a tungsten iodine lamp.
Then rinse the photoelectrode with ultrapure water and dry it under a gentle flow of nitrogen. To remove the polystyrene spheres from the electrode surface, place the electrodes in a beaker with 10 milliliters of toluene and gently stir for 20 minutes. Subsequently, rinse the electrode with acetone and ethanol for 20 seconds each.
For gas bubble investigations, attach two cameras to each photoelectrochemical cell to record gas bubble evolution via optical mirrors and beam splitters. Mount the photoelectrochemical setup and the cameras onto an optical board and attach it to one of the middle boards of the capsule. Use the remaining boards for installment of additional equipment such as potentiostats and shutter controls, light sources and the board computer.
Attach a battery supply at the bottom board of the capsule to power the setup during free fall. Write an automated drop sequence for the experimental steps to be controlled and carried out in microgravity environment. Maintain the experiment in free fall as well as the 45-minute capsule recovery after the drop.
To investigate light-assisted hydrogen production on the samples, carry out cyclic voltammetry and chronoamperometry. In cycling voltammetry experiments, set up scan rates for the two potentiostats at 218 millivolts per second to 235 millivolts per second for optimal resolutions in photocurrent voltage measurements. Use voltage ranges of 0.25 volts to negative 0.3 volts versus the silver-silver chloride reference electrode.
Employ the initial potential at 0.2 volts verus the silver-silver chloride reference electrode and a finishing potential at 0.2 volts versus silver-silver chloride reference electrode. In chronoamperometric measurement, use the timescale of the generated microgravity environment 9.3 seconds to record the photocurrent produced by the sample. Apply potential ranges of negative 0.3 volts to negative 0.6 volts versus silver-silver chloride reference electrode to compare produced photocurrent.
Run three scan cycles. To compare the recorded photocurrent voltage measurements, take the second scan cycle of each experiment for analysis. After retrieving the capsule from the deceleration container, remove the capsule protection shield.
Remove the samples from the pneumatic stative, rinse them with ultrapure water and dry them under gentle nitrogen flux. Store them under nitrogen atmosphere until optical and spectroscopic investigations are carried out. Tapping mode atomic force microscopy of the P-type indium phosphide surface before modification, after etching in bromine/methanol solution, and after electrochemical cycling in hydrochloric acid show the etching procedure removes the native oxide remaining on the surface.
Electrochemical cycling in hydrochloric acid causes a considerable increase in the fill factor of the cell performance accompanied by a flat band shift of the P-type indium phosphide from 0.56 volts to 0.69 volts. The deposited polystyrene particle monolayer on the P-type indium phosphide substrate and the surface after deposition of rhodium and removal of the polystyrene particles are shown through AFM topography. The application of shadow nanosphere lithography results in a nanosized two-dimensional periodic rhodium structure with a homogenous array of holes in the metallic transparent rhodium film.
The high resolution AFM image illustrates the hexagonal unit cell structure with recognizable grains of rhodium. The height profile of three spots on the electrode surface shows that the rhodium mesh is homogeneously distributed on the P-type indium phosphide surface with a height of about 10 nanometers forming a catalytic layer. The photocurrent voltage characteristics and chronoamperometric measurements of nanostructured P-type indium phosphide rhodium photoelectrodes in terrestrial and microgravity environments do not show significant differences.
Most importantly, before actually closing the drop capsule, remove loose screws and cables which are not tightened. Remaining screws can destroy the experimental setup and the instruments during free fall. rhodium, one could This results in different electrocatalytic nanostructures resulting in different efficiencies conditions.
This is most likely the same case in microgravity environments. The demonstration of efficient photoelectrochemical hydrogen production in microgravity environment opens up new pathways for the production of oxygen and solar fuels in space and could contribute to the realization of long-term space travel.
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