1. Preparation of p-InP photoelectrodes
2. Fabrication of rhodium nanostructures
3. Photoelectrodeposition of rhodium nanoparticles
4. Photoelectrochemical experiments in microgravity
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 considerable increase in the fill factor of the cell performance, accompanied by a flat band shift of the p-InP from +0.56 V to +0.69 V15. Furthermore, the passive layer formation during cyclic polarization in HCl photocathodically protects the InP surface from anodic corrosion. After the conditioning procedure, the self-assembly of 784 nm polystyrene latex nanospheres on the p-InP surface is employed for the formation of a colloidal particle monolayer which further on serves as a lithographic mask during the Rh deposition process (Figure 3A,B). Figure 3B shows an AFM image of the photoelectrode after removal of the PS spheres. The application of SNL results in a nanosized, two-dimensional periodic Rh structure with a homogenous array of holes in the metallic, transparent Rh film. The high-resolution AFM image (Figure 3C) illustrates the hexagonal unit cell structure with recognizable grains of Rh. Cross-section profiles in Figure 3D show that the rhodium mesh is homogenously distributed on the p-InP surface with a height of about 10 nm, forming a catalytic layer. Combined high-resolution TEM and FFT analysis were used to determine the lattice plane spacing, the distance of the diffraction points representing the reciprocal lattice space. Our calculations show that the lattice plane is in the order of 2.17 to 2.18 Å, validating the (111) cubic structure of the deposited rhodium (Figure 4). X-ray photoelectron spectroscopy reveals that the nanostructured p-InP-Rh electrode contains an InOx/POx layer, with evidence being provided by the larger InP signal at 128.4 eV. This is not surprising due to the open InP areas resulting from the removal of the PS spheres; here, the InP is directly exposed to the environment (i.e., air and the electrolyte (Figure 5)).
The microgravity environment has been shown to have a significant impact on the electrolysis of water which has been known since the 1960s and the effect of reduced gravity on the motion of bubbles and drops is well-documented (see e.g., reference 16). Studies have been carried out especially within the frame of developing a life support system for space travel which includes a water-electrolyzing component.
Hitherto investigations of water electrolysis under microgravity environment in 'dark' experiments resulted in a stable gas bubble froth layer formation in proximity to the electrode surfaces and the accompanying ohmic resistance increased linearly to the froth layer thickness in both acidic and alkaline electrolytes17,18,19. Additionally, the diameter of the gas bubbles increased and the bubbles adhered to the membrane separating the two half-cells20,21. Furthermore, it was demonstrated that bubble-induced microconvection dominates the mass transfer in microgravity environments8,21 and it has been suggested that mass transfer of the substrate water to the electrode surface controls the process of water electrolysis, which is controlled under normal gravity conditions by the electrode reaction22.
The here employed nanostructured p-InP-Rh photoelectrodes manufactured via SNL could overcome this problem: the photocurrent-voltage measurements do not show significant differences between terrestrially tested samples in 1 M HClO4 and samples tested in 9.3 s of microgravity environment at the Bremen Drop Tower (Figure 6A,B)6. J-V characteristics (Figure 6A), additionally, chronoamperometric measurements (Figure 6B) of the nanostructured samples are nearly identical in terrestrial and microgravity environment. The difference in the open circuit potential (VOC) is attributed to performance differences of the photoelectrodes as shown earlier6. The introduced rhodium catalytic 'hot spots' on the p-InP surface by SNL allow the formation of gas bubbles to occur at distinct spots on the photoelectrode surface, preventing bubble coalescence and increasing the yield of gas bubble release. The addition of 1% (v/v) isopropanol to the electrolyte decreases the surface tension of the electrolyte furthermore, also leading toward favored gas bubble detachment from the electrode surface.
Figure 1: Scheme of the experimental set-up of the electrochemical experiments in microgravity environment. The images show the equipped drop capsule (A) and details of the photoelectrochemical set-up on the second platform of the drop capsule (B). The capsule contains batteries for power supply during free fall (platform 5), the capsule control system for experimental control (platform 4), two W-I light sources and a board computer (platform 3, see Table of Materials), the photoelectrochemical setup including four digital cameras (platform 2), and two potentiostats and two shutter control boxes (platform 1). The four digital cameras in the photoelectrochemical setup (platform 2) allows recording gas bubble formation on the photoelectrode from the front of each electrochemical cell through beam splitters and from the side through mirrors. The photoelectrodes were illuminated through the beam splitters in front of the cell. Via a pneumatic lifting ramp, the photoelectrodes are immersed in the electrolyte immediately before reaching microgravity conditions. This figure has been modified from Brinkert et al. (2018)6. Please click here to view a larger version of this figure.
Figure 2: Tapping-mode AFM topography images of the p-InP surface before and after the surface modifications steps. Panel A shows the p-InP surface before modification procedures, (B) after etching the surface in bromine/methanol solution and (C) after conditioning the sample in HCl. (D) Histogram analysis of the height distribution of terraces on the p-InP sample (blue line), after etching in bromine/ methanol (yellow line) and after conditioning in HCl (red line). Please click here to view a larger version of this figure.
Figure 3: Tapping-mode AFM topography images (see Table of Materials) of the p-InP surface after application of shadow nanosphere lithography. (A) The deposited polystyrene particle monolayer on the p-InP substrate. (B,C) The surface after deposition of rhodium and removal of the polystyrene particles at two magnifications. (D) A height profile of three different spots on the electrode surface was generated to allow the further characterization of the deposited Rh mesh. Please click here to view a larger version of this figure.
Figure 4: High-resolution TEM analysis of photoelectrodeposited rhodium grains on the p-InP electrode at different magnifications (see Table of Materials). The 2D-Fourier transformation images show the corresponding diffraction pattern with a lattice plane spacing of 2.2 Å (111), typical for cubic structures. Please click here to view a larger version of this figure.
Figure 5: X-ray photoelectron spectra of the nanostructured p-InP-Rh photoelectrodes. (A) In 3d core levels; (B) P 2p core levels and (C) Rh 3d core levels. The color coding under the lines refers to the respective composition as illustrated in the legend. This figure has been modified from Brinkert et al. (2018)6. Please click here to view a larger version of this figure.
Figure 6: Results of the photoelectrochemical experiments in microgravity environment. (A) J-V measurements of nanostructured p-InP-Rh photoelectrodes in terrestrial (1 g, red) and microgravity environments (10-6 g, blue) in 1 M HClO4 with the addition of 1% (v/v) isopropanol to the electrolyte at 70 mW/cm2 illumination with a W-I lamp. Differences in the VOC of the nanostructured samples in terrestrial and microgravity conditions are subject to performance differences of the photoelectrodes as shown earlier6. (B) Chronoamperometric measurements of the nanostructured p-InP-Rh photoelectrodes in terrestrial (red) and microgravity environment (10-6 g, blue) in 1 M HClO4 with the addition of 1% (v/v) isopropanol to the electrolyte at 70 mW/cm2 illumination with a W-I lamp. The applied potential was set to -0.09 V vs RHE. The increased signal-to-noise ratio at the end of the measurements is due to the deceleration of the drop capsule after 9.3 s. Please click here to view a larger version of this figure.
Table 1: Detailed experimental sequence for photoelectrochemical hydrogen production hydrogen production in microgravity environment at the Bremen Drop Tower. This table has been modified from Brinkert et al. (2018)6. Please click here to download this file.
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 |
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.
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.
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.