- Hicham Idriss received his BSc, MSc, PhD and Habilitation (Dr. Science) from the University of Strasbourg (France). P... moreHicham Idriss received his BSc, MSc, PhD and Habilitation (Dr. Science) from the University of Strasbourg (France). Prior to joining KIT, he was Fellow at SABIC Corporate Research center at KAUST.edit
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Among the major challenges in solar water, splitting to molecular hydrogen and oxygen is making a stable and affordable system for largescale applications. In this work we present results of the design, fabrication, and testing a... more
Among the major challenges in solar water, splitting to molecular hydrogen and oxygen is making a stable and affordable system for largescale applications. In this work we present results of the design, fabrication, and testing a photoelectrochemical reactor composed of the following. 1) An integrated device to reduce the balance of the system cost. 2) A concentrated sunlight to reduce the photoabsorber cost. 3) An alkaline electrolyte to reduce catalyst cost and eliminate external thermal management needs. The system consists of an III-V-based photovoltaic cell integrated with Ni foil as catalyst for oxygen production that also protects the cell from corrosion. At low light concentration and without the use of optical lenses, the solar-to-hydrogen (STH) efficiency was found to be 18.3%, while at high light concentration (up to 207 suns) with the use of optical lenses, the STH efficiency was 13%. Catalytic tests conducted for over 100 hours at 100–200 suns showed no sign of degradation nor deviation from product stoichiometry (H2/O2=2). Further tests projected a system stability of over nine years. Figure 1
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ABSTRACT The reaction of ethanol for the production of hydrogen has been studied over a series of metal supported CeO2 catalysts. The study is conducted by TPD, steady state reaction, XPS, TEM, and infrared spectroscopy. TPD gave evidence... more
ABSTRACT The reaction of ethanol for the production of hydrogen has been studied over a series of metal supported CeO2 catalysts. The study is conducted by TPD, steady state reaction, XPS, TEM, and infrared spectroscopy. TPD gave evidence for the role of Rh in dissociating the carbon–carbon bond needed for efficient production of hydrogen molecules. IR of CO adsorption at 90K revealed that Rh particles are most likely in very small clusters as evidenced by a single OC–Rh IR band at 2020cm−1. TEM did not show conclusive evidence for the presence of the metal on-top of the CeO2 support, yet the Rh-Pd/CeO2 used catalyst has features that might be attributed to epitaxial growth of the noble metal along the (111) surface of the CeO2 support. Considerable reconstruction of the CeO2 support is seen for the used catalysts, in addition. Reforming of ethanol to hydrogen using (3 moles of water per mole of ethanol) was very efficient particularly above 650K where hydrogen selectivity reaches 60vol.%. At these temperatures hydrogen production from reforming of methane takes place.
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Surface and subsurface oxygen vacancies play a crucial role in catalysis. Their formation requires considerable energy, which is mostly provided by reducing chemical compounds such as CO or hydrocarbons in catalytic oxidation and water... more
Surface and subsurface oxygen vacancies play a crucial role in catalysis. Their formation requires considerable energy, which is mostly provided by reducing chemical compounds such as CO or hydrocarbons in catalytic oxidation and water gas shift reactions. This article covers three main points related to oxygen vacancies in catalysis by metal oxides: their formation, their stability, and the way their effect can be studied. Most of the information given is from spectroscopic and computation results on well-defined oxides and metal-metal oxides interfaces. The intention is to share these results with researchers working on applied systems such as water splitting to H 2 and O 2 and CO 2 reduction to CO. An emphasis on the catalytic cycle is considered because their role in photo-and photo-electrocatalytic reactions, in these two sought-after reactions, has been invoked recently. Deviation from the catalytic cycle would result in materials instability owing to corrosion, driven by system thermodynamics.
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Unlike thermally driven catalytic reactions by metals, the reaction rates in photo-catalysis do not scale with neither the amount of metals nor with their size. Because of the complexity of multi-component photo-catalysts in powder forms,... more
Unlike thermally driven catalytic reactions by metals, the reaction rates in photo-catalysis do not scale with neither the amount of metals nor with their size. Because of the complexity of multi-component photo-catalysts in powder forms, this phenomenon that has been routinely observed for over three decades, has so far no fundamental explanations. In order to probe into this, hydrogen production rates from ethanol over Au clusters with different sizes deposited on TiO2(110) rutile single crystal, were studied by scanning tunneling microscopy (STM) and online mass spectrometry. A non-linear increase of the rate of hydrogen with increasing surface coverage of gold was observed. While Au particles with sizes ranging from 4 to 8 Å, marginally affected the reaction rate, the inter-particle distance was found to be crucial. Increasing the separation distance resulted in increasing the normalized reaction rate. These results are explained in terms of competition between particles for exc...
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The potential of CeO2 as an epoxidation catalyst is studied for the reaction of propylene with hydrogen peroxide (H2O2) by Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). Adsorption and... more
The potential of CeO2 as an epoxidation catalyst is studied for the reaction of propylene with hydrogen peroxide (H2O2) by Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). Adsorption and decomposition of H2O2 and propylene oxide (PO) are also explored to determine their surface chemistry and thermal stability. Hydrogen peroxide adsorbed dissociatively on CeO2 forming adsorbed peroxo (O—O) species, as observed through vibrational features at 890 cm−1 and (830–855) cm−1 (FTIR). The signal at 890 cm−1 disappeared when a pulse of propylene was passed through the catalyst, and at the same time, adsorbed PO was observed (a sharp IR mode at 827 cm−1; ring deformation). The reaction between gas phase propylene and adsorbed peroxide species suggested the Eley–Rideal type mechanism. The absence of a ring opening reaction of PO at room temperature may indicate that CeO2 can be a suitable oxide for epoxidation of hydrocarbons. PO started to decompose a...
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The effect of Localized Surface Plasmon (LSP) of 5 nm mean size Au particles deposited on TiO2 P25 was investigated during the photo-thermal water gas shift reaction (WGSR). The effects of CO concentration, excitation light flux and... more
The effect of Localized Surface Plasmon (LSP) of 5 nm mean size Au particles deposited on TiO2 P25 was investigated during the photo-thermal water gas shift reaction (WGSR). The effects of CO concentration, excitation light flux and energy, and molecular oxygen addition during the reaction were investigated. The photocatalytic WGSR rate under light excitation with wavelengths extending from 320 to 1100 nm was found to be higher than the thermal reaction alone at the same temperature (85 oC). A ratio H2/CO2 of near unity was found at high concentrations of CO. Addition of molecular oxygen during the reaction resulted in a slight decrease of molecular hydrogen production while the rates of CO2 formation and that of CO consumption changed by one order of magnitude. More important, it was found that the WGSR rates were still high under only visible light excitation (600 - 700 nm). The results prove that Au LSP alone triggers this chemical reaction without the need to excite the semicond...
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Valence-Level Spectroscopy Study of the Enhanced Reduction of CeO 2 by Iron Substitution-Implications for the Thermal Water-Splitting Reaction.
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Water oxidation is one of the most important reactions needed for a transition to a green economy. The reaction relies on extracting electrons from oxygen anions and is commonly studied using homogenous catalysts based on Ru or Ir metals.... more
Water oxidation is one of the most important reactions needed for a transition to a green economy. The reaction relies on extracting electrons from oxygen anions and is commonly studied using homogenous catalysts based on Ru or Ir metals. Because of Ir scarcity and its relative instability in acidic environments, metals to replace it are sought after. In this study, we have synthesized Au-Pd based catalysts deposited on TiO2 with different ratios in order to mimic IrO2 valence orbitals (Ir5d)
by the hybrid valence orbitals of Au5d and Pd4d and compared their heterogeneous catalytic activity for the evolution of O2 from water in the presence of cerium ammonium nitrate (CAN). Au-Pd-based catalysts were found to be active at a particular nominal atomic ratio. At an atomic ratio of 1 Au to 2 Pd and 1 Au to 3 Pd, the catalysts were active and stable for oxygen production from water. Long-term runs up to 20,000 min still showed the expected stoichiometry between O2 production and CAN consumption (1 to 4). However, catalysts with a reverse ratio were not active. Also, the monometallic catalysts were found to be not active for the reaction. We link the reason for the activity of Au-Pd with this specific ratio to the shape and energy position of their valence band that might
be similar to those of IrO2 particles. While the turnover numbers of the Au-Pd-based catalysts were found to be lower than those of IrO2-based catalysts, on the same support in a heterogenous system, there is considerable potential upon further optimization for these two metals to replace IrO2 for a water oxidation reaction.
by the hybrid valence orbitals of Au5d and Pd4d and compared their heterogeneous catalytic activity for the evolution of O2 from water in the presence of cerium ammonium nitrate (CAN). Au-Pd-based catalysts were found to be active at a particular nominal atomic ratio. At an atomic ratio of 1 Au to 2 Pd and 1 Au to 3 Pd, the catalysts were active and stable for oxygen production from water. Long-term runs up to 20,000 min still showed the expected stoichiometry between O2 production and CAN consumption (1 to 4). However, catalysts with a reverse ratio were not active. Also, the monometallic catalysts were found to be not active for the reaction. We link the reason for the activity of Au-Pd with this specific ratio to the shape and energy position of their valence band that might
be similar to those of IrO2 particles. While the turnover numbers of the Au-Pd-based catalysts were found to be lower than those of IrO2-based catalysts, on the same support in a heterogenous system, there is considerable potential upon further optimization for these two metals to replace IrO2 for a water oxidation reaction.
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The reaction of a UO 3 thin film with atomic hydrogen was studied by He(II) ultraviolet photoelectron spectroscopy (UPS) in the temperature range 190-300 K. UO 3 reduction was instantaneously observed once it contacted H atoms at 10-7... more
The reaction of a UO 3 thin film with atomic hydrogen was studied by He(II) ultraviolet photoelectron spectroscopy (UPS) in the temperature range 190-300 K. UO 3 reduction was instantaneously observed once it contacted H atoms at 10-7 torr. The reduction was manifested by the presence of U5f 1 electrons in He(II) UPS at approximately 1.5 eV below the Fermi level. Based on the peak characteristics, the valence band shape (composed largely of O2p orbitals in addition to some contribution from U6d and U5f orbitals), and X-ray photoelectron spectroscopy (XPS) U4f lines, the reduction of U 6+ in UO 3 only results in the formation of U 5+ cations and was largely limited to those on the surface. Associated with the reduction was the formation of surface hydroxyls (-OH species) due to the transfer of a proton of the H atom (H .) to surface oxygen ions, while the electron of H. is transferred to a U5f orbital. The pseudo-first-order rate constant of the initial rate of reduction at 10-7 torr and 190 K was found to be approximately 0.01 s-1. Qualitative analysis of the valence band before and after reduction indicates that O2p hybridization with U6d and U5f orbitals leads to welldistinguished features that are characteristic of UO 3 , U 2 O 5 , and UO 2. These features, which were quantitatively reversed during the redox process, furthers the assessment of the stoichiometry of a given binary uranium oxide. KEYWORDS U 6+ reduction, ultraviolet photoelectron spectroscopy He(II), uranium 5f, U 2 O 5 , UO 3 RECEIVED
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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
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ABSTRACT Temperature-programmed desorption (TPD) after methanol and formic acid adsorption and CO&z.sbnd;H2 reaction were studied on palladium catalysts. No difference in the nature of the products after decomposition of methanol... more
ABSTRACT Temperature-programmed desorption (TPD) after methanol and formic acid adsorption and CO&z.sbnd;H2 reaction were studied on palladium catalysts. No difference in the nature of the products after decomposition of methanol on 3% Pd/SiO2 and 3% Pd/CeO2 was found. Both catalysts decompose methanol into CO+H2. After adsorption of formic acid on CeO2 alone, methanol was desorbed with a maximum at 540–550 K. This result can be interpreted by possible hydrogenation of formate species into methanol. On the other hand, no methanol was observed after adsorption of formic acid on 3% Pd/CeO2 (methane was desorbed with a maximum at 545 K). Two peaks of methanol desorption were found after CO&z.sbnd;H2 reaction on CeO2 at 383 and 535 K. The first could be formed after methoxy hydrogenation and the second may be due to hydrogenation of formate species. TPD after CO&z.sbnd;H2 reaction on 3% Pd/CeO2 gave results similar to those found after methanol adsorption. These results suggest that formyl species are involved in methanol synthesis on Pd/CeO2 catalyst, whereas on CeO2 formate species could be the main intermediate.
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A variety of OH containing molecules in their different modes of adsorption onto the rutile TiO2(110) are studied by means of density functional theory. A special focus is given to ethanol, ethylene glycol and glycerol. The different... more
A variety of OH containing molecules in their different modes of adsorption onto the rutile TiO2(110) are studied by means of density functional theory. A special focus is given to ethanol, ethylene glycol and glycerol. The different species were analyzed with respect to the adsorption energy, work function, and atomic Bader charges. Our results show that dissociated adsorption is favored in all cases. Within these modes, the strongest binding is observed in the case of bidentate fully dissociated adsorption, followed by bidentate partially dissociated then the monodentate dissociated modes. The dependence is also noted upon charge transfer analysis. Species adsorbing with two dissociated OH groups show a negative charge which is roughly twice as large compared to those exhibiting only one dissociated group. In the case of molecular adsorption, we find a small positive charge on the adsorbate. The change in work functions obtained is found to be negative in all studied cases. We obs...
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Understanding the mechanism behind the superior catalytic power of single- or few-atom heterogeneous catalysts has become an important topic in surface chemistry. This is particularly the case for gold, with TiO2 being an efficient... more
Understanding the mechanism behind the superior catalytic power of single- or few-atom heterogeneous catalysts has become an important topic in surface chemistry. This is particularly the case for gold, with TiO2 being an efficient support. Here we use scanning tunneling microscopy/spectroscopy with theoretical calculations to investigate the adsorption geometry and local electronic structure of several-atom Au clusters on rutile TiO2(110), with the clusters fabricated by controlled manipulation of single atoms. Our study confirms that Au1 and Au2 clusters prefer adsorption at surface O vacancies. Au3 clusters adsorb at O vacancies in a linear-chain configuration parallel to the surface; in the absence of O vacancies they adsorb at Ti5c sites with a structure of a vertically pointing upright triangle. We find that both the electronic structure and cluster-substrate charge transfer depend critically on the cluster size, bonding configuration, and local environment. This suggests the possibility of engineering cluster selectivity for specific catalytic reactions.
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This special issue of Catalysis Science & Technology pays tribute to the scientific work of François Gault.
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A series of Cu(OH)2–Ni(OH)2/P25 photocatalysts was prepared by co‐deposition–precipitation (total metal loading ≈1 wt %) and their performance was evaluated for H2 production. Among this series, the 0.8 Cu(OH)2–0.2 Ni(OH)2/P25... more
A series of Cu(OH)2–Ni(OH)2/P25 photocatalysts was prepared by co‐deposition–precipitation (total metal loading ≈1 wt %) and their performance was evaluated for H2 production. Among this series, the 0.8 Cu(OH)2–0.2 Ni(OH)2/P25 photocatalyst demonstrated very high H2 production rates in 20 vol % ethanol/water and 5 vol % glycerol/water mixtures (10 and 22 mmol h−1 g−1, respectively). Detailed analyses based on reaction kinetics, photoluminescence, X‐ray photoelectron spectroscopy (XPS), and charge carrier scavenging suggest that both working catalysts are composed of Cu and Ni metals in their active phases. Cu0 is produced directly by the transfer of electrons from the conduction band of TiO2 to surface Cu(OH)2 nanoclusters, whereas Ni0 is formed indirectly through a process of gradual dissolution of Ni(OH)2 to yield aqueous Ni2+ owing to the acidic environment of the medium, followed by Ni2+ reduction by electrons from the TiO2 conduction band. The high rates of H2 production that m...
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ABSTRACT Photo-catalytic H2 production from water has been studied over Au–Cu2O nanoparticle deposited on TiO2 (anatase) in order to probe into both the plasmon resonance effect (Au nanoparticles) and the pn-junction at the Cu2O–TiO2... more
ABSTRACT Photo-catalytic H2 production from water has been studied over Au–Cu2O nanoparticle deposited on TiO2 (anatase) in order to probe into both the plasmon resonance effect (Au nanoparticles) and the pn-junction at the Cu2O–TiO2 interface. The Au–Cu2O composite is in the form of ∼10 nm Au nanoparticles grown on ∼475 nm Cu2O octahedral nanocrystals with (1 1 1) facets by partial galvanic replacement. X-ray Photoelectron Spectroscopy (XPS) Cu2p and Auger L3M4,5M4,5 lines indicate that the surface of Cu2O is mainly composed of Cu+. The rate for H2 production (from 95 water/5 ethylene glycol; vol.%) over 2 wt.% (Au/Cu2O)–TiO2 is found to be ∼10 times faster than that on 2 wt.% Au–TiO2 alone. Raman spectroscopy before and after reaction showed the disappearance of Cu+ lines (2Eu) at 220 cm−1. These observations coupled with the induction time observed for the reaction rate suggest that in situ reduction from Cu+ to Cu0 occurs upon photo-excitation. The reduction requires the presence of TiO2 (electron transfer). The prolonged activity of the reaction (with no signs of deactivation) despite the reduction to Cu0 indicates that the latter takes part in the reaction by providing additional sites for the reaction, most likely as recombination centers for hydrogen atoms to form molecular hydrogen. This phenomenon provides an additional route for enhancing the efficiency and lifetime of Cu2O–TiO2 photocatalytic systems, beyond the usually ascribed pn-junction effect.
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Defects (oxygen vacancies and interstitial cations) in oxide semiconductors have recently been invoked as a key property behind increased photocatalytic reaction rates. In this work, we have monitored by transient absorption spectroscopy... more
Defects (oxygen vacancies and interstitial cations) in oxide semiconductors have recently been invoked as a key property behind increased photocatalytic reaction rates. In this work, we have monitored by transient absorption spectroscopy (TAS) excited electrons in the conduction band decaying into the invoked traps to extract their lifetime using a rutile single crystal instead of the more conveniently used powder homologue. This is preferred in order to rule out grain boundary, degree of crystallinity, and size effects among other parameters that would obscure the results. It was found, in the energy region investigated (1.3−1.8 eV), that the lifetime of excited electrons is about four times shorter for the bulk defect crystal when compared to the fresh one. This indicates that the created defects (mostly oxygen defects and interstitial Ti cations) are unlikely to contribute to reaction rate enhancement.
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We report outdoor experimental results of PV-electrolysis systems using a series of proton exchange membrane (PEM) electrolyzers powered directly either through InGaP/GaAs/Ge based concentrated (750 sun) or Si based non-concentrated (1... more
We report outdoor experimental results of PV-electrolysis systems using a series of proton exchange membrane (PEM) electrolyzers powered directly either through InGaP/GaAs/Ge based concentrated (750 sun) or Si based non-concentrated (1 sun) PV modules. At power matched conditions, solar to hydrogen (STH) conversion efficiency of 18-21% with a production rate of 0.8-1.0 L/(min-m 2 module area) of H 2 was obtained using concentrated PV modules (~28-30% efficiency). Conventional Si module (~17.5% efficiency), on the other hand, generated about 0.3 L/(min-m 2 module area) of H 2 Average with a STH Average of 9.4%. While scaling up of the electrolyzers by increasing the size of their electrodes resulted in a similar voltage losses due to mismatching of their power point with that of PV modules (when coupled without power electronics), scaling them out in series maximizes H 2 production by utilizing the maximum power available from PV. Unlike, PV-electrolysis operated via power electronics, direct integration of PV modules with electrolyzers leads to effective utilization of PV power, with STH values closer to the theoretical limit of the coupled system (STH = PV module efficiency × Electrolyzer efficiency). The H 2 production rate and STH using a concentrated system with a solar tracker remained constant and high during the measurement period (5-6 h). On the other hand, the production rate as well as STH varied with time on a conventional Si module without a tracker under 1 sun, with a significantly low average H 2 production rate. Keeping cost factor aside, PV-electrolysis under concentration outweighs the performance of non-concentrated PV based system by a factor between 1.5 and 3.0.
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A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO 2 (110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype... more
A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO 2 (110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype hole-scavenger molecule, benzoic acid, over stoichiometric (s) s-TiO 2 , Au 9 /s-TiO 2 , and reduced (r) Au 9 /r-TiO 2. STM results show that benzoic acid adsorption displaces a large fraction of Au clusters from the terraces toward their edges. DFT calculations explain that Au 9 clusters on stoichiometric TiO 2 are distorted by benzoic acid adsorption. The influence of sub-monolayers of Au on the UV/visible photoreaction of benzoic acid was explored at room temperature, with adsorbate depletion taken as a measure of activity. The empty sites, observed upon photoexcitation, occurred in elongated chains (2 to 6 molecules long) in the [11̅ 0] and [001] directions. A roughly 3-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate. To our knowledge this is the first time that atomic-scale directionality of a chemical reaction is reported upon photoexcitation of the semiconductor.
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The elusive photocatalytic water splitting reaction using sunlight on suspended nanoparticles: is there a way forward? Hicham Idriss ab For many decades hydrogen production by photocatalytic methods has been pursued over a variety of... more
The elusive photocatalytic water splitting reaction using sunlight on suspended nanoparticles: is there a way forward? Hicham Idriss ab For many decades hydrogen production by photocatalytic methods has been pursued over a variety of semiconductors with probably over a thousand formulations of powder catalysts in many structures and compositions. Yet, with the exception of a few reports, water splitting to molecular hydrogen and oxygen has remained elusive. The only reproducible results are those involving other additives to water: electron donors or acceptors yielding either hydrogen or oxygen, but not both. The consequence of this is a system unrelated to water splitting but simply driven by the organic or inorganic redox potential. One may argue that thermodynamic limitations indicate that an inorganic semiconductor with a band gap within the spectrum of sunlight, and that is stable in water, cannot split water. Otherwise, it would not have existed on earth.
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Solar to hydrogen (STH) efficiency of photovoltaic-electrolysis (PV-E) setups is a key parameter to lower the cost of green hydrogen produced. Commercial c-Si solar cells have neared saturation with respect to their efficiency, which... more
Solar to hydrogen (STH) efficiency of photovoltaic-electrolysis (PV-E) setups is a key parameter to lower the cost of green hydrogen produced. Commercial c-Si solar cells have neared saturation with respect to their efficiency, which warrants the need to look at alternative technologies. In this work, we report a concentrator photovoltaic-electrolysis (CPV-E) setup with STH efficiency of 28% at 41 suns (without the use of Fresnel lenses); the highest reported efficiency using an alkaline system to date. Using this as a base case, we carried out a detailed techno-economic (TEA) analysis, which showed that despite the high cost associated with CPV cells, levelized cost of hydrogen (LCOH) is at $ 5.9/kg, close to that from c-Si solar farms ($ 4.9/kg), primarily due to the high STH efficiency. We also report sensitivity analysis of factors affecting both CPV and alkaline electrolyser systems such as the CPV module efficiency and installed capacity, electrolyser stack lifetime, operating current density, and working hours. Our results indicate that in a scenario where the installed capacity of CPV technology matches that of silicon and with electrolyser operating current density of ~ 0.7 A/cm 2 , the LCOH from CPV-electrolysis systems can be < $ 2/kg. These results demonstrate the potential of CPV technology for large-scale green hydrogen production to replace that obtained from fossil fuels.
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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
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Surface and subsurface oxygen vacancies play a crucial role in catalysis. Their formation requires considerable energy, which is mostly provided by reducing chemical compounds such as CO or hydrocarbons in catalytic oxidation and water... more
Surface and subsurface oxygen vacancies play a crucial role in catalysis. Their formation requires considerable energy, which is mostly provided by reducing chemical compounds such as CO or hydrocarbons in catalytic oxidation and water gas shift reactions. This article covers three main points related to oxygen vacancies in catalysis by metal oxides: their formation, their stability, and the way their effect can be studied. Most of the information given is from spectroscopic and computation results on well-defined oxides and metal-metal oxides interfaces. The intention is to share these results with researchers working on applied systems such as water splitting to H 2 and O 2 and CO 2 reduction to CO. An emphasis on the catalytic cycle is considered because their role in photo-and photo-electrocatalytic reactions, in these two sought-after reactions, has been invoked recently. Deviation from the catalytic cycle would result in materials instability owing to corrosion, driven by system thermodynamics.
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The effect of electrode area, electrolyte concentration, temperature, and light intensity (up to 218 sun) on PV electrolysis of water is studied using a high concentrated triple-junction (3-J) photovoltaic cell (PV) connected directly to... more
The effect of electrode area, electrolyte concentration, temperature, and light intensity (up to 218 sun) on PV electrolysis of water is studied using a high concentrated triple-junction (3-J) photovoltaic cell (PV) connected directly to an alkaline membrane electrolyzer (EC). For a given current, the voltage requirement to run an electrolyzer increases with a decrease in electrode sizes (4.5, 2.0, 0.5, and 0.25 cm 2) due to high current densities. The high current density operation leads to high Ohmic losses, most probably due to the concentration gradient and bubble formation. The EC operating parameters including the electrolyte concentration and temperature reduce the voltage requirement by improving the thermodynamics, kinetics, and transport properties of the overall electrolysis process. For a direct PV−EC coupling, the maximum power point of PV (P max) is matched using EC I−V (current−voltage) curves measured for different electrode sizes. A shift in the EC I−V curves toward open-circuit voltage (V oc) reduces the P op (operating power) to hydrogen efficiencies due to the increased voltage losses above the equilibrium water-splitting potential. The solar-to-hydrogen (STH) efficiencies remained comparable (∼16%) for all electrode sizes when the operating current (I op) was similar to the short-circuit current (I sc) irrespective of the operating voltage (V op), electrolyzer temperature, and electrolyte concentration.
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Abstract In recent years there has been a large number of articles wrongly attributing, an XPS O1s signal at about 531–532 eV to oxygen vacancies. These studies are based on ex situ measurements of powder materials. By definition,... more
Abstract In recent years there has been a large number of articles wrongly attributing, an XPS O1s signal at about 531–532 eV to oxygen vacancies. These studies are based on ex situ measurements of powder materials. By definition, photoelectron spectroscopy relies on measuring the kinetic energy of an electron removed from the core or valence levels of an atom in a compound. Therefore, a photoelectron signal originating from a missing oxygen atom is not possible. The signal attributed to oxygen vacancies is simply that of adventitious hydroxyls of water that are inevitably adsorbed on oxides in ambient conditions. XPS O1s of reduced and non-reduced single crystals and thin films have been studied in details for decades in surface science, some of which are given here. In addition, a binary or mixed metal oxide material or catalyst containing surface oxygen vacancies will be oxidized instantaneously in ambient conditions due to the strong adsorption energy of water (typically about 1 eV) and its high sticking probability (typically equal 1) on defected oxides) and therefore ex-situ measurements of surface oxygen vacancies are not possible.