The scan direction was from -25 mV vs. b) SEM image of the p-InP/Pt electrode after undergoing 3 CVs in (a).
Solar-driven water splitting
For the water splitting reaction, the thermodynamic standard potentials are for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Therefore, the water splitting reaction is a thermodynamically uphill process (ΔEocell>0 V) that requires external energy inputs such as solar energy and electricity.
Understanding the stability of semiconductor photoelectrodes
If the position of Eqf can be controlled under the cathodic corrosion potential, the photoelectrode can be kinetically stabilized while the photogenerated electrons will carry out the reduction of the solvent. In contrast to (b), if the cathodic corrosion potential is below that of the solvent reduction, the corrosion pathway is thermodynamically preferred leading to an intrinsic instability of the semiconductor photoelectrode.
Semiconductor photoelectrochemistry (PEC)
The planar p-Si/Pt photocathode gave a Voc of ~300 mV, which is attributed to the barrier height of the p-Si/H2O junction. Compared to the Pt dark electrode, the J-E behavior of the p-Si/Pt photocathode shifted significantly cathodically by the magnitude of Voc.
Importance of surface chemistry for PEC
However, the presence of surface states can be challenging to visualize directly experimentally, requiring systematic studies. An In- and P-rich surface of InP can produce surface states located below the conduction band minimum by ∼0.1 eV and 0.4 eV, respectively.
Currently, three different methods are used in the literature to quantify the concentration of ammonia in solution. Ion chromatography (IC) is another technique that can quantify ammonium ions (NH4+) with high sensitivity and a low limit of detection (LOD) of μM, while still not being able to determine isotopic identity.
Thesis organization
Taken together, these results showed that the electrochemical stability and physical stability of p-InP/Pt electrodes should be considered separately. 2 INVESTIGATIONS ON THE STABILITY OF ETCHED OR PLATINO PLATED P-INP(100) PHOTOCATHODES FOR SOLAR DRIVEN HYDROGEN EVOLUTION IN ACID OR ALKALINE AQUEOUS ELECTROLYTES.
Abstract
Consequently, differences in catalytic kinetics and surface stoichiometry are both important considerations for determining the corrosion chemistry and long-term operational stability of InP photoelectrodes.
Introduction
Possible oxidation of the electrode surface by ambient O2(g) after electrochemical measurements can further distort the analysis of the conditions of a pristine electrode surface. The long-term evolution of the J-E behavior of these photoelectrodes was monitored during the stability tests.
Results
XPS analysis of the thus prepared p-InP/Pt electrode showed an almost stoichiometric InP surface (In/P=1.1) with minimal POx (Figure S 2.12). A conformal overlay attributable to InOx observed in the XPS data was evident on the exposed InP surface (Figure 2.5a). In contrast, in contact with KOH, the n-InP/Pt electrode showed a stoichiometric InP surface (Figure S 2.26).
Discussion
Instead, the surface passivation of p-InP with InOx resulted in a significant deterioration of the J-E behavior of the p-InP/Pt electrode. These results reveal the importance of surface stoichiometry (In/P atomic ratio) in determining the long-term evolution of the J-E behavior of p-InP/Pt photocathodes. These results further indicate that the physical stability (dissolution) and electrochemical stability (decay in J) of the p-InP/Pt electrode should be considered separately.
Conclusions
Experimental
Electrochemical data were acquired on an SP-200 potentiostat (BioLogic Science Instruments) without compensation for the series resistance of the solution. The illumination intensity at the location of the sample in the cell was calibrated to 1 Sun (100 mW cm-2) using a Si photodiode (FDS100, Thorlabs). Electrochemical data were acquired on an MPG-2 multichannel potentiostat (BioLogic Science Instruments) without compensation for the series resistance of the solution.
Analytical Methods
After pumping down the loadlock again to 1×10-2 Torr, the gate valve to the transfer box was opened and the turbomolecular pump was turned on. After achieving a pressure of <1 x 10-6 Torr, the sample was transferred to the sample transfer chamber (base pressure <1 × 10-9 Torr) before further transfer to the analysis chamber. Scanning electron microscopy (SEM) images were obtained using a calibrated Nova NanoSEM 450 (FEI) with an acceleration voltage of 5 kV.
Supplementary figures and tables
The inset shows the p-InP electrode towards the CA; the whitish area was inside the O-ring and exposed to the electrolyte. The thickness of the redox-active layer on the electrode was calculated as 0.33 nm. d,e,g) CVs performed after the experiment shown in (c,f), scanning in a positive direction from the initially applied potential using a scan rate of 20 mV s-1. Comparison of linear sweeps of n-InP dark electrodes and illuminated p-InP electrodes showed positive shifts in potential from n-InP (dark) to p-InP (light) of >900 mV in 1.0 M H2SO4(aq) and of >750 revealed mV in 1.0 M KOH(aq).
Abstract
Introduction
A systematic understanding of the corrosion mechanism of p-GaAs photocathodes during HER requires further experimental studies. Here we present a systematic study of the stability of p-GaAs photocathodes for HER in 1.0 M H2SO4(aq) or 1.0 M KOH(aq), with or without HER catalysts (Pt or CoP). This work provides fundamental insights into the physical and electrochemical stability of p-GaAs photocathodes for HER, as well as the interfacial interactions between GaAs and various HER catalysts.
Results
Increased |J| in the dark parallel changes in the J-E behavior of the illuminated p-GaAs electrode over time (Figure 3.3b). The AFM image of the p-GaAs/Pted(0,5) electrode after CA showed various particles sparsely distributed over the surface (Figure 3.6h). By CA, the p-GaAs/CoP(400) electrode showed a similar morphology to the as-prepared p-GaAs/CoP(200) electrode, while the surface of both p-GaAs/CoP(50) and p-GaAs/CoP(200) electrodes showed sparsely distributed particles (Figure S 3.23-24).
Discussions
Furthermore, holding illuminated etched p-GaAs electrodes at cathodic bias (-0.2 or -0.6 V vs. RHE) again resulted in little continuous corrosion of GaAs. 3a-b by forming an interfacial galvanic pathway (Scheme 3.1d).173 The fast kinetics of HER at Pt also lowers the surface potential of p-GaAs into the E region, favoring the formation of As0. Despite the surface transformations, sustained dissolution of GaAs over time remained slow for both etched p-GaAs electrodes and those coated with Pt catalysts under cathodic bias in acidic and alkaline electrolytes.
Conclusions
Using the earth-rich CoP catalyst instead of the noble metal Pt catalyst on p-GaAs also provides a constructive example situation where an unfavorable surface conversion of GaAs can be mitigated by tailoring the morphology and composition of the HER catalyst. However, the increases in dark J of p-GaAs/CoP electrodes as the CoP loading increases indicate an unfavorable band alignment at the interface, which will require additional interfacial engineering techniques to optimize both the stability and high performance of such systems as photocathodes for solar fuel production .
Experimental
All electrolytes were degassed using a Schlenk line to remove dissolved oxygen before being transferred to the glove box. Before each experiment, the compression cell was assembled immediately after the etching of the GaAs samples and transferred to the hand box. A miniature fiber-optic adjustable arm light equipped with a 150 W halogen bulb was used as the light source and inserted from outside the glove box via an optical fiber.
Analytical Methods
The transfer case was attached to the loading slot of the Kratos Axis Ultra system. After pumping the loadlock to ~100 Torr, the slide valve to the transfer case was opened and the turbomolecular pump was turned on. After closing the gate valve to transfer the case, the sample was pumped to <1×10-8 Torr before being transferred to the analysis chamber.
Supplementary figures
RHE for 4 hours under 1 sun illumination in 1.0 M KOH(aq); the arrows indicate hourly interruptions for CV data collection (15 s at Eoc. followed by three cycles of CV at 50 mV s-1), (b) Comparison of the JE behavior of the p-GaAs electrode under illumination during the CA in (a), (c) J-E behavior for p-GaAs electrode measured in the dark in 1.0 M KOH(aq) before and after the CA in (a). The long-term stability of p-InGaP2 photocathodes, with a band gap of ~1.8 eV, for the solar-driven hydrogen evolution reaction (HER) has been systematically investigated in both acidic and alkaline aqueous electrolytes. During the stability tests, the current density potential (JE) behavior of the p-InGaP2/Pt photoelectrodes deteriorated due to the pH-dependent evolution of the surface conditions.
Introduction
Further progress in improving electrode durability requires a rational and quantitative understanding of the surface corrosion chemistry of p-InGaP2. Herein, we systematically investigated the stability of p-InGaP2 photocathodes under the HER conditions in both 1.0 H2SO4(aq) and 1.0 M KOH(aq), with and without Pt catalyst. This work provides a rational understanding of the electrode stability of p-InGaP2 photocathodes for HER, benefiting further stabilization strategies.
Results
However, the periodically measured J-E behavior of the p-InGaP2/Pt electrode exhibited significant cathodic shifts during the first 4 h of CA (Figure 4.6b). The XP spectra of the Ga and P regions showed only peaks attributed to Ga3+ cations and P3- anions of p-InGaP2. These results together indicated an enrichment of InOx on the surface of the p-InGaP2/Pt electrode resulting from a selective leaching of Ga ions.
Discussion
This result suggested a selective and self-limited leaching of Ga ions from the surface of the p-InGaP2/Pt electrode. In contrast, no metallic In0 was formed on the surface of the p-InGaP2/Pt electrodes under CA at 0 V vs. At pH=0, the degraded J-E behavior of the p-InGaP2/Pt electrode after 23 h probably correlated with its continuous dissoln.
Conclusions
Comparison of the initiation potentials of corrosion reactions and fuel formation reactions is useful for identifying opportunities to inhibit adverse corrosion by kinetic control; (2) for compound semiconductors, the J-E behavior of photoelectrodes can be affected by surface stoichiometry, especially during long-term operation in electrolytes.98 Changes in J-E behavior and electrode surface conditions should be closely related before and after electrochemical tests;. 3) physical breakdown may not necessarily result in a degradation in J during CA of the photoelectrodes. This material is based on work conducted by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Center supported by the US Office of Science. The research was conducted in part at the Molecular Materials Research Center (MMRC) of the Beckman Institute of the California Institute of Technology.
Experimental
Before each experiment, the cell was collected immediately after p-InGaP2 etching and transferred to the glove box. After each experiment, the cell was separated inside the glove compartment and the electrode sample was rinsed with deionized water, dried with nitrogen and stored inside the glove compartment until XPS analysis. A miniature fiber optic light with adjustable arm equipped with a 150 W halogen bulb was used as the illumination source and was introduced from the outside of the glove compartment into the glove compartment via a fiber optic.
Analytical Methods
The single peak of GaAs in the Ga 2p spectra was fitted using the asymmetric Lorentz function. The surface atomic ratios were calculated using the relative sensitive factors (RSF) in the Kratos instrument database. Atomic force microscopy (AFM) images were acquired on a Bruker Dimension Icon using Bruker ScanAsyst-Air probes (silicon tip, silicon nitride cantilever, spring constant: 0.4 N m-1, frequency: 0.50-90 kHz), operating in ScanAsyst mode.
Supplementary figures
Abstract
Introduction
Our results are consistent with other reports where the addition of active HER catalysts improved the operational stability of these electrodes in acidic electrolytes. Moreover, both semiconductors exhibit pH-dependent solvation and surface changes during long-term operation of the HER. Therefore, investigating how the corrosion of the emitter layer affects the photoelectrochemical performance of the entire photoelectrode can identify the limiting factors in the device stability for long-term operation. Potentiostatic long-term experiments were performed at two different potentials to study their influence on the corrosion of underlying GaAs substrates.
Results
The pn+-InGaP2 epilayer consists of a p-type InGaP2 (2 μm) base light absorber, together with an n+ type InGaP2 emitter layer (25 nm) to obtain a higher photovoltage (Voc). In the first 20 h, the J-E behavior of the pn+-InGaP2/Pt electrode showed a cathodic shift, while the onset E gradually decreased to +0.6 V vs. RHE under 1-sun illumination; the pn+-InGaP2 epilayer was grown on a p+-GaAs (111B) substrate by a. d-e) Corrosion thicknesses of (d) InGaP2 and (c) GaAs as a function of time, based on the concentrations of dissolved ions in electrolyte measured by ICP-MS.
Discussion
Conclusion
Supplementary figures
Introduction
Results and discussion
Conclusion
Experimental
Supplementary figures and tables
Abstract
Introduction
Results and Discussion
Concluding Remarks
Experimental
Supplementary figures and tables
Fabrication of dual-working-electrode (DWE) of p-InP
DWE photoelectrochemistry of p-InP photocathodes
