Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Summary

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.

Abstract

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.

Introduction

Our atmosphere on Earth is formed through oxygenic photosynthesis, a 2.3-billion-year-old process converting solar energy into energy-rich hydrocarbons, releasing oxygen as a by-product and using water and CO₂ as substrates. Currently, artificial photosynthetic systems following the concept of the energetic Z-scheme of catalysis and charge transfer in natural photosynthesis are realized in semiconductor-electrocatalyst systems, showing hitherto a solar-to-hydrogen conversion efficiency of 19 %^1,2,3. In these systems, semiconductor materials are employed as light absorbers which are coated with a thin, transparent layer of electrocatalysts⁴. Intense research in this field is promoted by the global quest for renewable energy systems with hydrogen and long-chain hydrocarbons making excellent candidates for an alternative fuel supply. Similar obstacles are also faced on long-term space missions, where a resupply of resources from Earth is not possible. A reliable life-support hardware is required, employing an efficient air revitalization unit providing about 310 kg of oxygen per crew member per year, not accounting for extravehicular activities⁵. An efficient solar water-splitting device, capable of producing oxygen and hydrogen or reduce carbon dioxide solar-assisted and in a monolithic system would provide an alternative, lighter route to currently employed technologies on the ISS: the air revitalization unit is comprised of a separated system with an alkaline electrolyzer, a solid amine carbon dioxide concentrator and a Sabatier reactor for the reduction of CO₂.

Unprecedentedly, we realized efficient solar-hydrogen production in microgravity environment, provided by a 9.3 s during free-fall at the Bremen Drop Tower (ZARM, Germany)⁶. Using p-type indium phosphide as a semiconducting light-absorber^7,8 coated with a nanostructured rhodium electrocatalyst, we overcame substrate and product mass transfer limitations to and from the photoelectrode surface, which is an obstacle in reduced gravity environments due to the absence of buoyancy^9,10. The application of shadow nanosphere lithography^11,12 directly on the photoelectrode surface allowed the formation of rhodium catalytic 'hot spots', which prevented hydrogen gas bubble coalescence and the formation of a froth layer in proximity of the electrode surface.

Herein, we provide experimental details of the p-InP photoelectrode preparation including surface etching and conditioning, followed by the application of shadow nanosphere lithography on the electrode surface and the photoelectrodeposition of rhodium nanoparticles through the polystyrene spheres. Furthermore, the experimental set-up in the drop capsule at the Bremen Drop Tower is described and details of the experimental sequence during the 9.3 s of free fall are provided. Sample installment and handling before and after each drop are outlined as well as the preparation of the drop capsule and its equipment to operate illumination sources, potentiostats, shutter controls and video cameras upon command.

Protocol

1. Preparation of p-InP photoelectrodes

1. Use single crystal p-InP (orientation (111 A), Zn doping concentration of 5 × 10¹⁷ cm^-3) as the photoabsorber. For the back contact preparation, evaporate 4 nm Au, 80 nm Zn and 150 nm Au on the backside of the wafer and heat it to 400 °C for 60 s.
2. Apply Ag paste to attach the ohmic contact to a thin-plated Cu wire. Thread the wire to a glass tube, encapsulate the sample and seal it to the glass tube using black, chemical resistant epoxy.
3. In order to remove native oxides, etch the 0.5 cm² polished indium face of p-InP for 30 s in 10 mL of bromine/methanol solution (0.05 % w/v), rinse the surface with ethanol and ultrapure water for 10 s each and dry the sample under nitrogen flux. Prepare the solutions from ultrapure water and analytical grade chemicals with an organic impurity level below 50 ppb.
   CAUTION: Bromine causes acute toxicity upon inhalation, skin corrosion and acute aquatic toxicity. Wear protective equipment such as safety glasses, gloves and lab coat. Work under the fume hood. Methanol is flammable, causes acute toxicity (oral, dermal and inhalation) and is known to cause specific target organ toxicity. Wear protective equipment such as safety glasses, gloves and a lab coat. Work under the fume hood.
4. Subsequently, condition the p-InP electrode photoelectrochemically in a standard three-electrode potentiostatic arrangement. Use a borosilicate glass cell with a quartz window as a photoelectrochemical cell in order to illuminate the sample with a white-light tungsten halogen lamp (100 mW/cm²) during the procedure.
5. Adjust the light intensity with a calibrated silicon reference photodiode.
6. Prepare a 0.5 M HCl solution and purge it in the photoelectrochemical cell with nitrogen of 5.0 purity for 15 min.
7. Use potentiodynamic cycling between -0.44 V und +0.31 V at a scan rate of 50 mV s^-1 for 50 cycles to photoelectrochemically condition the sample under continuous illumination.
   CAUTION: Hydrochloric acid causes serious eye damage, skin corrosion and it is corrosive to metals. Furthermore, it possesses specific target organ toxicity following single exposure. Wear protective equipment such as safety glasses, gloves and a lab coat. Work under the fume hood.

2. Fabrication of rhodium nanostructures

1. Employ shadow nanosphere lithography (SNL)^11,12 for the formation of rhodium nanostructures on the p-InP photoelectrode. In order to create the polystyrene masks on the p-InP electrode, obtain mono-dispersed beads of polystyrene (PS) sized 784 nm at a concentration of 5% (w/v) and dissolve them in ultrapure water.
2. To obtain the final volume of 600 μL, mix 300 μL of the polystyrene bead dispersion with 300 μL of ethanol containing 1% (w/v) styrene and 0.1% sulphuric acid (v/v).
3. Apply the solution onto the water surface using a Pasteur pipette with a curved tip. In order to increase the area of the monocrystalline structures, turn the Petri dish gently. Carefully distribute the solution to cover 50% of the air-water interface with a hcp monolayer. Leave place for stress relaxation and avoid forming cracks in the lattice during the next preparation steps.
4. Protect the Cu wire of the photoelectrochemically conditioned p-InP electrodes with parafilm. Place them delicately under the floating closed packed PS sphere mask by carefully taping them to a microscope slide, preventing the samples from rotating. Gently remove residual water with a pipette and by evaporation, causing the mask to be subsequently deposited onto the electrode surface.
5. Take the electrode out of the Petri dish and gently dry the surface with N₂. Store the electrode under nitrogen until rhodium photoelectrodeposition (e.g., in a desiccator).
   NOTE: The protocol can be paused here for up to one week.

3. Photoelectrodeposition of rhodium nanoparticles

1. For the photoelectrochemical deposit of rhodium nanoparticles through the PS sphere mask, place the electrode in an electrolyte solution containing 5 mM RhCl₃, 0.5 M NaCl and 0.5% (v/v) 2-propanol and apply a constant potential of V_dep = +0.01 V for 5 s under simultaneous illumination with a W-I lamp (100 mW/cm²). Electrochemical specifications such as the electrochemical cell, reference and counter electrode are the same as for the photoelectrochemical conditioning procedure.
2. Rinse the photoelectrode with ultrapure water and dry it under a gentle flow of N₂.
3. In order to remove the PS-spheres from the electrode surface, place the electrodes for 20 min under gentle stirring in a beaker with 10 mL of toluene (the electrode should be covered with toluene). Subsequently, rinse the electrode with acetone and ethanol for 20 s each.
4. Remove residual carbon from the surface by O₂-plasma cleaning for 6 min at a process pressure of 0.16 mbar, 65 W and gas inflows of O₂ and Ar of 2 sccm and 1 sccm, respectively.
5. Prepare the samples up to one week prior to tests in the drop tower and store them until the experiments under N₂ atmosphere in the dark (e.g., in a glove bag or desiccator).
   NOTE: The protocol can be paused here for about 1-2 weeks.

4. Photoelectrochemical experiments in microgravity

1. For the experiments in microgravity environment, contact one of the major drop tower facilities, (e.g., the Centre of Applied Space Technology and Microgravity (ZARM), Bremen Germany).
   NOTE: By employing a catapult system, 9.3 s of microgravity environment can be generated at ZARM with an approached minimum g-level of about 10^-6 m·s^{-2 13}. A hydraulically controlled pneumatic piston-cylinder system is used to launch the drop capsule (Figure 1A) upwards from the bottom of the tower. The capsule is decelerated again in a container which is placed onto the cylinder system during the time of free fall.
2. Use a two-compartment photoelectrochemical cell (filling volume of each cell: 250 mL) for the photoelectrochemical experiments in order to carry out two experiments in microgravity environment in parallel. The front of each cell should consist of an optical quartz glass window (diameter: 16 mm) for illuminating the working electrode (see Figure 1B).
3. Employ a three-electrode arrangement in each cell for the photoelectrochemical measurements with a Pt counter electrode and an Ag/AgCl (3 M KCl) reference electrode in HClO₄ (1 M). Add 1% (v/v) isopropanol to the electrolyte in order to reduce the surface tension and enhance gas bubble release. Use a W-I white light source for illuminating each cell compartment through the optical windows.
   CAUTION: Concentrated perchloric acid is a strong oxidizer. Organic, metallic and non-organic salts formed from oxidation are shock sensitive and pose a great fire and explosion hazard. Wear safety glasses, gloves and a protective lab coat. Work under the fume hood and minimize bench top storage time.
4. For gas bubble investigations, attach two cameras to each cell via optical mirrors and beamsplitters (e.g., a color camera at the front and a monochromatic camera at the side, see Figure 1) to record gas bubble evolution during free fall of the experiment. For each drop, store the recorded data on an integrated board computer in the drop capsule. Record single pictures at a frame rate of (e.g., 25 fps (color camera) and 60 fps (monochromatic camera)).
5. The drop capsule is equipped with several boards (Figure 1). Mount the photoelectrochemical set-up and the cameras onto an optical board and attach it to one of the middle boards in the capsule. Use the remaining boards for installment of additional equipment such as potentiostats, light sources, shutter controls and the board computer. Attach a battery supply at the bottom board of the capsule to power the set-up during free fall (Figure 1).
6. Write an automated drop sequence for the experimental steps which should be controlled and carried out in microgravity environment. The program should be started prior to each drop. Upon reaching microgravity environment, the sequence should automatically start cameras, illumination sources and the electrochemical experiment for the duration of 9.3 s while simultaneously immersing the working electrode into the electrolyte using a pneumatic system (see Figure 1, Table 1).
7. Investigation of light-assisted hydrogen production on the samples in photoelectrochemical measurements (e.g., cyclic voltammetry and chronoamperometry).
   1. Control the electrochemical parameters by the two potentiostats in the capsule. For optimal resolutions in J - V measurements, use scan rates (dE/dt) of 218 mV/s to 235 mV/s in order to run 3 scan cycles in cycling voltammetry experiments, using voltage ranges of +0.25 V to -0.3 V vs Ag/AgCl (3 M KCl). Employ the initial potential, E_i = +0.2 V vs Ag/AgCl (3 M KCl) and the finishing potential, E_f = +0.2 V vs Ag/AgCl (3 M KCl). To compare the recorded J - V measurements, take the second scan cycle of each experiment for analysis.
   2. In chronoamperometric measurements, use the time scale of generated microgravity environment, 9.3 s, to record the photocurrent produced by the sample. Apply potential ranges of -0.3 V to -0.6 V vs Ag/AgCl (3 M KCl) to compare produced photocurrents.
8. At the end of each drop, when the drop capsule is decelerated again to zero velocity, use the drop sequence to let the sample be removed from the electrolyte and cameras, potentiostats and illumination sources be switched off.
9. 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 N₂ atmosphere until optical and spectroscopic investigations are carried out.
10. Exchange the electrolyte in the two cells, ensure the function of all instruments before equipping the cells with new samples and prepare the capsule for another drop experiment.

Results

Etching the p-InP surface in Br₂/ methanol for 30 s with consecutive photoelectrochemical conditioning of the sample by cycling polarization in HCl is well established in the literature and discussed (e.g., by Schulte & Lewerenz (2001)^14,15). The etching procedure removes the native oxide remaining on the surface (Figure 2) and electrochemical cycling in HCl causes furthermore a conside...

Discussion

For the preparation of photoelectrodes, it is important to minimize oxygen exposure between the etching and conditioning procedure and to purge the 0.5 M HCl before usage for about 10 - 15 min with nitrogen. Once the samples are conditioned, they can be stored under nitrogen atmosphere in 15 mL conical tubes for a few hours to allow sample transport and/ or preparation time of the polystyrene particle masks. In order to achieve a homogenous arrangement of PS spheres on the electrode substrate, it is important to form a c...

Materials

Name	Company	Catalog Number	Comments
12.7 mm XZ Dovetail Translation Stage with Baseplate, M4 Taps (4 x)	Thorlabs	DT12XZ/M
Beam splitters (2 x)	Thorlabs	CM1-BS013	50:50 400-700nm
Beamsplitters (2 x)	Thorlabs	CM1-BS014	50:50 700-1100nm
Ohmic back contact: 4 nm Au, 80 nm Zn, 150 nm Au	Out e.V., Berlin, Germany	https://www.out-ev.de/english/index.html	Company provides custom made ohmic back contacts
Glass tube, ca. 10 cm, inner diameter about 4 mm	E.g., Gaßner Glasstechnik		Custom made
p-InP wafers, orientation 111A, Zn doping concentration: 5 x 10^17 cm^-3	AXT Inc. Geo Semiconductor Ltd. Switzerland		Custom made
Photoelectrochemical cell for terrestrial experiments	E.g., glass/ materials workshop		Custom made
Matrox 4Sight GPm (board computer)	Matrox imaging		Ivy Bridge, 7 x Cable Ace power I/O HRS 6p, open 10m, Power Adapter for Matrox 4sight GPm, Samsung 850 Pro 2,5" 1 TB, Solid State Drive in exchange for the 250Gb hard drive
2-propanol	Sigma Aldrich	I9516-500ML
35mm Kowa LM35HC 1" Sensor F1.4 C-mount (2 x)	Basler AG
Acetone	Sigma Aldrich	650501-1L
Ag/AgCl (3 M KCl) reference electrode	WPI	DRIREF-5
Aluminium breadboard, 450 mm x 450 mm x 12.7mm, M6 Taps (2 x)	Thorlabs	MB4545/M
Beaker, 100 mL	VWR	10754-948
Black epoxy	Electrolube	ER2162
Bromine	Sigma Aldrich	1.01945 EMD Millipore
Colour camera (2 x)	Basler AG	acA2040-25gc
Conductive silver epoxy	MG Chemicals	8331-14G
Copper wire	E.g., Sigma Aldrich	349224-150CM
Ethanol	Sigma Aldrich	459844-500ML
Falcon tubes, 15 mL	VWR	62406-200
Glove bags	Sigma Aldrich	Z530212
Hydrochloric acid (1 M)	Sigma Aldrich	H9892
Magnetic stirrer	VWR	97042-626
Methanol	Sigma Aldrich	34860-100ML-R
Microscope slides	VWR	82003-414
MilliQ water
NIR camera (2 x)	Basler AG	acA1300-60gm
Nitrogen, grade 5N	Airgas	NI UHP300
Ø 1" Stackable Lens Tubes (6 x)	Thorlabs	SM1L03
O2 Plasma Facility
OEM Flange to SM Thread Adapters (4 x)	Thorlabs	SM1F2
Parafilm	VWR	52858-000
Pasteur pipette	VWR	14672-380
Perchloric acid (1 M)	Sigma Aldrich	311421-50ML
Petri dish	VWR	75845-546
Photoelectrochemical cell for microgravity experiments	E.g., glass/ materials workshop
Polystyrene particles, 784 nm, 5 % (w/v)	Microparticles GmbH	0.1-0.99 µm size (50 mg/ml): 10 ml, 15 ml, 50 ml
Potentiostats (2 x)	Biologic	SP-200/300
Pt counter electrode	ALS-Japan	12961
Rhodium (III) chlorid	Sigma Aldrich	520772-1G
Shutter control system (2 x)
Silicon reference photodiode	Thorlabs	FDS1010
Sodium chlorid	Sigma Aldrich	567440-500GM
Stands and rods to fix the cameras	VWR
Sulphuric acid (0.5 M)	Sigma Aldrich	339741-100ML
Telecentric High Resolution Type WD110 series Type MML1-HR110	Basler AG
Toluene	Sigma Aldrich	244511-100ML
Various spare beakers and containers for leftover perchloric acid etc for the drop tower	VWR
W-I lamp with light guides (2 x)	Edmund Optics	Dolan-Jenner MI-150 Fiber Optic Illuminator
CM-12 electron microscope with a twin objective lens, CCD camera (Gatan) system and an energy dispersive spectroscopy of X- rays (EDS) system)	Philips
Dimension Icon AFM, rotated symmetric ScanAsyst-Air tips (silicon nitride), nominal tip radius of 2 nm	Bruker