Solar Hydrogen Production System Research Articles

Sustainable and Tunable Electrochemical CO2 Conversion to Chemicals and Fuels at Industrial Scale Using Metal-Oxide-Based Materials

The most challenging deal we face these years is the need to lower greenhouse gas (GHG) emissions and tackle climate change; though calls to reduce it are growing louder yearly, emissions remain high. CO2 is the key contributor. For this reason, the synthesis of high-added-value products from CO2 conversion is a promising approach to mitigate the problem.1 Among the different alternatives, exploiting CO2 via electrochemical reduction under mild conditions (ambient pressure and temperature) represents an opportunity to support a low-carbon economy.2 The electrocatalytic (EC) CO2 reduction (CO2R) driven by renewable energy can be exploited for the future energy transition, for the carbon storage into valuable products like syngas (H2/CO mixtures), organic acids (formic acid) and chemicals/fuels (C1+ alcohols).3 A big challenge for the industrialisation of this technology is to find low-cost electrocatalysts, efficient reactors and process conditions. In the efforts to develop efficient, selective and stable materials, we have exploited the current knowledge of thermocatalytic CO2 hydrogenation to develop noble-metal-free CO2R electrocatalysts.4 For instance, Cu/Zn/Al synthesised catalyst producing methanol and CO from the CO2 thermocatalytic (TC) hydrogenation (at H2 pressure (P) of 30 bar and temperature (T) > 200 oC) promotes the formation of methanol (⁓32% of FE) during the EC CO2R in a gas-diffusion-electrode system; while operating in the liquid phase, the same catalyst produces syngas with a tunable composition (95% of FE at the most positive applied potential) and other liquid C2+ products (in both cases at ambient T, P).4 On the other hand, Cu/ZnO electrocatalyst has also been tested at industrially relevant current densities in liquid phase configuration.5 We demonstrated through ex-situ characterisations that the presence of ZnO nanoparticles in the mixed Cu/ZnO catalyst plays an important role in forming and stabilising mixed oxidation states of copper and Cu1+/Cu0 interfaces in the electrocatalyst (in bulk and surface). These interfaces seem to promote CO dimerisation to ethanol. Indeed, ethanol was produced with the Cu/ZnO catalyst, reaching ethanol productivity of about 5.3 mmol∙gcat -1∙h-1 in a liquid-phase configuration at ambient conditions. The Cu/Zn/Al and Cu/ZnO electrocatalysts also have been tested in a catholyte-free configuration with an increased selectivity to ethylene, reaching approx. 60% and 70% of FE, respectively. Here, Cu catalyst structure transformed, on average, completely to metallic with a very thin layer of Cu1+ during testing, which seems to promote the selectivity towards C2H4, demonstrating that the reaction pathways for EC CO2R are largely determined by transport limitations rather than only by the intrinsic properties of the electrocatalysts. Our results open a promising path for the prospective implementation of metal-oxide nanostructures for CO2 conversion to the chemicals and fuels of the future. Acknowledgements This work has received financial support by the EU H2020 SunCOChem Project, grant agreement: 862192. References Guzmán, H., Russo, N. & Hernández, S. CO2 valorisation towards alcohols by Cu-based electrocatalysts: challenges and perspectives. Green Chem. 23, 1896–1920 (2021).Romero Cuellar, N. S., Wiesner-Fleischer, K., Fleischer, M., Rucki, A. & Hinrichsen, O. Advantages of CO over CO2 as reactant for electrochemical reduction to ethylene, ethanol and n-propanol on gas diffusion electrodes at high current densities. Electrochim. Acta 307, 164–175 (2019).Guzmán, H., Farkhondehfal, M. A., Rodulfo Tolod, K., Russo, N. & Hernández, S. Photo/electrocatalytic hydrogen exploitation for CO2 reduction toward solar fuels production. in Solar Hydrogen Production Processes, Systems and Technologies 560 (Elsevier Inc., 2019). doi:10.1016/C2017-0-02289-9.Guzmán, H. et al. How to make sustainable CO2 conversion to Methanol: Thermocatalytic versus electrocatalytic technology. Chem. Eng. J. 417, 127973 (2021).Guzmán, H. et al. CO2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes. ACS Appl. Mater. Interfaces 14, 517−530 (2022).

The integration of solar-hydrogen hybrid renewable energy systems in oil and gas industries for energy efficiency: Optimal sizing using Fick's Law optimisation Algorithm

The global demand for sustainable energy solutions in the oil and gas industry has stimulated interest in the integration of renewable energy sources. This paper investigates the techno-economic and environmental aspects of incorporating a solar-hydrogen-based hybrid renewable energy system (SHRES) into oil and gas processing facilities.The proposed SHRES, comprising Solar PV modules, hydrogen production and storage systems, and a gas turbine generator (PV-FC-EL-Tank-GT), aims to enhance energy efficiency and reduce the carbon footprint of energy-intensive operations. The study utilises a multidisciplinary approach, encompassing engineering, economics, and environmental analysis. The techno-economic evaluation employs detailed modelling and optimisation using Fick's Law Algorithm, ensuring efficient energy production and cost-effectiveness. Ecological assessments quantify potential reductions in greenhouse gas emissions and other environmental impacts.The technical and economic evaluation identifies the electrolyser system as the most significant cost contributor (57%). The optimised system balances energy sources with 45% from PV modules, 35% from fuel cells, and 20% from gas turbines, showcasing substantial reliance on renewable energy while maintaining required power generation.Addressing the urgent need for sustainable energy solutions in the oil and gas industry, the study highlights the potential of SHRES integration. By employing Fick's Law Algorithm, alongside with 80 % of renewable energy integration, the research optimises system components to optimise the annualised cost to be $57,539.85, decrease to zero the Loss of Power Supply Probability (LPSP), and mitigate CO2 emissions to a lowest value of 0.147 gCO2eq/kWh (by 80 %). The electrolyser system's pivotal role and the distribution of power generation sources are emphasised.This study presents a novel approach to integrating solar-hydrogen systems into oil and gas processing facilities, aiming for sustainability. The findings underscore the significance of the electrolyser system and the distribution of power generation sources in optimising the SHRES. Recommendations for renewable energy integration and potential reductions in CO2 emissions and costs are discussed, providing valuable insights for future energy policies.

All-perovskite-based unassisted photoelectrochemical water splitting system for efficient, stable and scalable solar hydrogen production

All-perovskite-based unassisted photoelectrochemical water splitting system for efficient, stable and scalable solar hydrogen production

Research on Hydrogen Production System Technology Based on Photovoltaic-Photothermal Coupling Electrolyzer

Solar hydrogen production technology is a key technology for building a clean, low-carbon, safe, and efficient energy system. At present, the intermittency and volatility of renewable energy have caused a lot of “wind and light”. By combining renewable energy with electrolytic water technology to produce high-purity hydrogen and oxygen, which can be converted into electricity, the utilization rate of renewable energy can be effectively improved, while helping to improve the solar hydrogen production system. This paper summarizes and analyzes the research status and development direction of solar hydrogen production technology from three aspects. Energy supply mode: the role of solar PV systems and PT systems in this technology is analyzed. System control: the key technology and system structure of different types of electrolytic cells are introduced in detail. System economy: the economy and improvement measures of electrolytic cells are analyzed from the perspectives of cost, consumption, efficiency, and durability. Finally, the development prospects of solar hydrogen production systems in China are summarized and anticipated. This article reviews the current research status of photovoltaic-photothermal coupled electrolysis cell systems, fills the current research gap, and provides theoretical reference for the further development of solar hydrogen production systems.

Numerical simulation and performance analysis of solar hydrogen production using anion exchange membrane electrolyzer and blending with natural gas

Hydrogen production using solar energy and blending in a natural gas pipeline is a cost-effective alternative to hydrogen storage and transmission that leads to lower CO2 emissions caused by the energy infrastructure. In this paper, energy, exergy, and Exergoeconomic analyses of a solar hydrogen production system and its blending with natural gas in a city gate station of Isfahan city are performed in three days with minimum, average, and maximum solar irradiations. The system includes photovoltaic arrays, anion exchange membrane electrolyzer cells (AEMECs), a hydrogen compressor, and a system blending. The AEMEC is used due to its less expensive catalysts, non-acidic electrolyte, and high efficiency at high pressures. A three-dimensional numerical model is developed to determine the AEMEC’s polarization curve, accurately. The results show that with increasing solar irradiation and decreasing ambient temperature, the maximum power of the PV increases and the energy and exergy efficiency of the PV decreases. By increasing the injection of hydrogen into natural gas from 1% to 10% vol., the lower heating value of the fuel and the Wobbe Index decrease by 6% and 1.55%, respectively. By injection of 10% vol. hydrogen gas into natural gas compared to the case of using natural gas without hydrogen, the exergy cost of natural gas blended with hydrogen increases by 5.9%. Considering the environmental benefits of using hydrogen in combination with natural gas, including the reduction of greenhouse gases, and the fact that blending hydrogen up to 10% vol. with natural gas does not require any change in urban gas supply facilities, this process is feasible. In the case of using high-pressure AEMEC and removing the compressor to produce hydrogen at high pressure, the exergy cost of high-pressure hydrogen reduces by 29.3%.

Ru Cluster Incorporated NiMoO(P)4 Nanosheet Arrays as High-Efficient Bifunctional Catalyst for Wind/Solar-To-Hydrogen Generation Systems.

Developing cost-efficient bifunctional water splitting catalysts is crucial for sustainable hydrogen energy applications. Herein, ruthenium (Ru)-incorporated and phosphorus (P)-doped nickel molybdate (Ru-NiMoO(P)4 ) nanosheet array catalysts are synthesized. Due to the synergy of Ru clusters and NiMoO(P)4 by the modulated electronic structure and the rich active sites, impressively, Ru-NiMoO(P)4 exhibits superior OER (194mV @ 50mA cm-2 ) and HER (24mV @ 10mA cm-2 ) activity in alkaline media, far exceeding that of commercial Pt/C and RuO2 catalysts. Meanwhile, as bifunctional catalyst, to drive the overall water splitting at the current density of 10mA cm-2 , Ru-NiMoO(P)4 requires only 1.45V and maintaining stable output for 100h. Furthermore, Ru-NiMoO(P)4 also possesses excellent capability for seawater electrolysis hydrogen production. Moreover, the successful demonstration of wind and solar hydrogen production systems provide the feasibility of the ultra-low Ru loading catalyst for large-scale hydrogen production in the future.

Solar photovoltaic–thermal hydrogen production system based on full-spectrum utilization

Full-spectrum high-temperature water electrolysis enables efficient conversion from solar to hydrogen. However, the supply of electric and thermal energy derived from solar energy does not match the demand for electric and thermal energy in high-temperature water electrolysis, resulting in significant energy losses within the system. In this study, a solar photovoltaic-thermal hydrogen production system based on full-spectrum utilization is proposed. The concentrated sunlight is divided into two parts based on wavelength. Longer-wavelength sunlight is directed to the reactor, where it is converted into thermal energy to directly supply the endothermic water electrolysis reaction. Shorter-wavelength sunlight is directed to photovoltaic cells to generate electrical power for the reaction. To achieve a match between the supply and demand of electric and thermal energy, system parameters are optimized for the proposed system. The results demonstrate that the mismatch losses of the proposed system are reduced by 42.8 percentage points compared to a conventional photovoltaic water electrolysis system. The proposed system achieves a solar-to-hydrogen efficiency of 39.0% under optimum conditions with a cutoff wavelength of 1000 nm, surpassing that of photovoltaic water electrolysis and water-splitting thermochemical cycle systems by 19.0 and 21.6 percentage points, respectively. Based on the proposed method, the solar-to-hydrogen efficiency can be improved.

A floatable hydrogel-based photocatalytic system for large-scale solar hydrogen production

The effectiveness of scaling up photocatalysis is constrained by the agglomeration of photocatalysts. A floatable hydrogel-based photocatalytic system at the air-water interface reported by Hyeon et al. in the current issue of Nature Nanotechnology offers benefits for scaling up solar H2 production with effective light delivery and fast gas separation.

Metal-Oxide-Based Materials for the Sustainable and Tunable Electrochemical CO2 Conversion to Chemicals and Fuels at Industrial Scale

Anthropogenic activities have impacted the planet’s carbon cycle through the emissions of large amounts of greenhouse gases (GHGs), shifting the equilibrium of human history since the industrial revolution. CO2 is the key contributor to global climate change in the atmosphere. For this reason, the synthesis of high-added-value products CO2 conversion is a promising approach to mitigate climate change.1 Among the different alternatives, exploiting CO2 via electrochemical reduction under mild conditions (ambient pressure and temperature) represents an opportunity to support a low-carbon economy.2 The electrocatalytic (EC) CO2 reduction (CO2R) driven by renewable energy can be exploited for the future energy transition, for the carbon storage into valuable products like syngas (H2/CO mixtures), organic acids (formic acid) and chemicals/fuels (C1+ alcohols).3 A big challenge for the industrialisation of this technology is to find low-cost electrocatalysts, efficient reactors and process conditions. In the efforts to develop efficient, selective and stable materials, we have exploited the current knowledge of thermocatalytic CO2 hydrogenation to develop noble-metal-free CO2R electrocatalysts.4 For instance, Cu/Zn/Al synthesised catalyst producing methanol and CO from the CO2 thermocatalytic (TC) hydrogenation (at H2 pressure (P) of 30 bar and temperature (T) > 200 oC) promotes the formation of methanol (⁓32% of FE) during the EC CO2R in a gas-diffusion-electrode system; while operating in the liquid phase, the same catalyst produces syngas with a tunable composition (95% of FE at the most positive applied potential) and other liquid C2+ products (in both cases at ambient T, P).4 On the other hand, Cu/ZnO electrocatalyst also has been tested at industrially relevant current densities in liquid phase configuration.5 We demonstrated through ex-situ characterisations that the presence of ZnO nanoparticles in the mixed Cu/ZnO catalyst plays an important role in forming and stabilising mixed oxidation states of copper and Cu1+/Cu0 interfaces in the electrocatalyst (in bulk and surface). These interfaces seem to promote CO dimerisation to ethanol. Indeed, ethanol was produced with the Cu/ZnO catalyst, reaching ethanol productivity of about 5.3 mmol∙gcat -1∙h-1 in a liquid-phase configuration at ambient conditions. A Cu/ZnO electrocatalyst also has been tested in a catholyte-free configuration with an increased selectivity to ethylene, reaching approximately 70% of FE, demonstrating that the reaction pathways for EC CO2R are largely determined by transport limitations rather than only by the intrinsic properties of the electrocatalysts. Our results open a promising path for the prospective implementation of metal-oxide nanostructures for CO2 conversion to the chemicals and fuels of the future. Acknowledgements This work has received financial support by the EU H2020 Project SunCOChem (grant agreement: 862192). References Guzmán, H., Russo, N. & Hernández, S. CO2 valorisation towards alcohols by Cu-based electrocatalysts: challenges and perspectives. Green Chem. 23, 1896–1920 (2021).Romero Cuellar, N. S., Wiesner-Fleischer, K., Fleischer, M., Rucki, A. & Hinrichsen, O. Advantages of CO over CO2 as reactant for electrochemical reduction to ethylene, ethanol and n-propanol on gas diffusion electrodes at high current densities. Electrochim. Acta 307, 164–175 (2019).Guzmán, H., Farkhondehfal, M. A., Rodulfo Tolod, K., Russo, N. & Hernández, S. Photo/electrocatalytic hydrogen exploitation for CO2 reduction toward solar fuels production. in Solar Hydrogen Production Processes, Systems and Technologies 560 (Elsevier Inc., 2019). doi:10.1016/C2017-0-02289-9.Guzmán, H. et al. How to make sustainable CO2 conversion to Methanol: Thermocatalytic versus electrocatalytic technology. Chem. Eng. J. 417, 127973 (2021).Guzmán, H. et al. CO2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes. ACS Appl. Mater. Interfaces 14, 517−530 (2022).

Energy and exergy analysis of a novel solar-hydrogen production system with S–I thermochemical cycle

A novel hydrogen production system based on a sulfur-iodine (S–I) thermochemical water-splitting cycle driven by 100% solar energy is proposed in this paper. A solar power tower (SPT) with thermal energy storage (TES) is used to supply heat stably. Considering the large heat recovery potential in the S–I cycle, a supercritical carbon dioxide (S–CO2) Brayton cycle is used to improve the system performance. A complete thermodynamic model and detailed description of the energy and exergy flow from the sun to SPT, TES, S–I cycle, S–CO2 Brayton cycle and hydrogen is obtained. The system performance in the base case and actual weather conditions are obtained. The influence of major parameters on the system performance is investigated. The results show that the hydrogen production rate is 3.52 mol s−1 in the base case. Using the S–CO2 Brayton cycle, the overall energy and exergy efficiency can be increased from 10.6%, 11.1% to 30.5%, 32.4%, respectively. The solar part accounts for 48.2% of the total energy loss and 76.1% of the exergy destruction. In addition, increasing DNI, the maximum reaction temperature, the regeneration ratio of the S–CO2 Brayton cycle (RR) can improve the system performance, and the system efficiency is more sensitive to RR.

Synergistic effect of internal electric field and ligand-to-metal charge transfer in Z-scheme CuPc/ZnIn2S4 for boosting photocatalytic hydrogen evolution

The construction of an efficient and stable photocatalytic water splitting system is the critical path and arduous challenge to convert solar energy into green hydrogen energy. Herein, a novel CuPc/ZnIn2S4 organic–inorganic hybrid photocatalyst was designed and used for photocatalytic hydrogen production from water. The separation of photogenerated electrons and holes was greatly enhanced due to the formation of the compact organic–inorganic heterointerface and the Z-scheme charge transport mechanism between CuPc and ZnIn2S4. Meanwhile, CuPc can broaden the light absorption of CuPc/ZnIn2S4 to the near infrared region, which enhances the utilization of sunlight. Moreover, the Cu metal center of CuPc could serve as the electron acceptor to accumulate photogenerated electrons via the ligand-to-metal charge transfer pathway, which further accelerates the separation of photogenerated charge carriers and enhances the photocatalytic hydrogen production efficiency. The optimal CuPc/ZnIn2S4 photocatalyst exhibited a hydrogen production rate of 151.2 μmol h−1, which was 8.1 times higher than that of pristine ZnIn2S4 under visible light irradiation. This work provides an idea for the construction of an organic–inorganic photocatalytic system for solar hydrogen production.

A brief overview of solar and wind-based green hydrogen production systems: Trends and standardization

Coupling water electrolyzers with solar and wind sources may be a promising solution in the near future for utilizing excess renewable energy. Indeed, many researchers have investigated various green hydrogen production systems, but a systematic approach should be developed to compare them. In addition, it is crucial to understand which solar and wind-based green hydrogen production systems have been studied and the literature gap on this topic. This review presents the latest research on green hydrogen production systems integrated with solar and wind technologies. One of the most significant novelties of this paper is to suggest a preliminary pathway and standardization strategy for the comparison of different green hydrogen production systems based on solar and wind sources. Various data and information are extracted from the most relevant papers in this field, such as renewable systems investigated, the location of the studied system, connection with the electrical grid, research type developed, final utilization of hydrogen considered, and performance indicators employed. This work will be an introductory guide for the community interested in green hydrogen production systems.

Kilowatt-scale solar hydrogen production system using a concentrated integrated photoelectrochemical device

The production of synthetic fuels and chemicals from solar energy and abundant reagents offers a promising pathway to a sustainable fuel economy and chemical industry. For the production of hydrogen, photoelectrochemical or integrated photovoltaic and electrolysis devices have demonstrated outstanding performance at the lab scale, but there remains a lack of larger-scale on-sun demonstrations (>100 W). Here we present the successful scaling of a thermally integrated photoelectrochemical device—utilizing concentrated solar irradiation—to a kW-scale pilot plant capable of co-generation of hydrogen and heat. A solar-to-hydrogen device-level efficiency of greater than 20% at an H2 production rate of >2.0 kW (>0.8 g min−1) is achieved. A validated model-based optimization highlights the dominant energetic losses and predicts straightforward strategies to improve the system-level efficiency of >5.5% towards the device-level efficiency. We identify solutions to the key technological challenges, control and operation strategies and discuss the future outlook of this emerging technology.

Efficient hydrogen production system with complementary utilization of methane and full-spectrum solar energy

Solar hydrogen production by integration of photovoltaic cells (PV) and electrolytic water splitting offers the ability to simultaneously store intermittent solar energy and produce hydrogen. However, the solar-to-hydrogen efficiency is limited by the inability of PV cells to utilize solar energy in the long-wavelength spectrum. On the other hand, solar energy in the short-wavelength spectrum with high activation ability is underutilized in solar thermochemical methane steam reforming (MSR). To address these issues, this paper proposes a hydrogen production system that integrates electrolytic water splitting with a thermochemical MSR and employs a Rankine Cycle (RC) for balancing electricity and heat for water electrolysis. This system enables the efficient conversion of full-spectrum solar energy into hydrogen while minimizing the underutilization of high-grade solar energy. The thermodynamic and environmental performances of the proposed system are evaluated and compared with those of a reference system - a full-spectrum solar hydrogen production system without fossil fuels. Results show that the proposed system produces 2.21–2.71 times more hydrogen than the reference system when solar direct normal irradiance (DNI) ranges from 200–1000 W/m2. It exhibits higher energy efficiency, exergy efficiency, and solar-to-hydrogen efficiency than the reference system. Furthermore, the proposed system can reduce approximately 15.0 kg of CO2 emissions when producing 1 kg of hydrogen. This study provides a promising approach to efficiently using solar energy and fossil fuels for generating hydrogen.

Design of gas-liquid two-phase separation device with application in solar hydrogen production system

Currently, solar thermochemical energy conversion technology has attracted more and more attention because of the increasing global need for clean fuel and chemical energy demand. The thermochemical hydrogen production technology uses solar radiation with concentrated high-density as a high-temperature heat source to produce hydrogen by decomposing water through two-step cycle redox reactions. It is an environment-friendly technology based on artificial photosynthesis utilizing water vapor as the reaction gas. However, limited by the separation efficiency, current gas-liquid separation devices cannot output stable high-purity water vapor and sophisticate the experimental processes. In this research, intensive numerical investigations were carried out on the widely-used corrugated plate steam separators. The structural parameters are designed by the multi-objective optimization method combining the artificial neural network and the genetic algorithm. Nine optimized structures are designed by adding hydrophobic hooks, streamlined, and reducing the plate distance optimization methods. The results give engineering application conditions of different optimized separators. The streamlined single-hook corrugated plate separator with rounded corners exhibited the best separation efficiency under the heat state condition, which is 97.27%. It can also completely separate droplets over 28.5 μm. The applicability of the cyclone steam separators indicated that it is more suitable for pretreatment. The benchmark experimental system was built for the measurement of the separation efficiency resulting in only a 0.73% error. The designed steam separator was successfully applied to the solar thermochemical hydrogen production experiment, which ensures its practicability and pertinence in the process of solar fuel synthesis.

Techno-economic analysis for clean hydrogen production using solar energy under varied climate conditions

The paper discusses the feasibility of the use solar energy into hydrogen production using a photovoltaic energy system in the four main cities of Iraq. An off-grid photovoltaic system with a capacity of 22.0 kWp, an 8.0 kW alkaline electrolyser, a hydrogen compressor, and a hydrogen tank were simulated for one year in order to generate hydrogen. A mathematical model of the proposed system behavior is presented using MATLAB/Simulink, considering nine years from the 2021 to 2030 project span using hourly experimental weather data. The outcomes demonstrated that the annual hydrogen production ranged from 1713.92 kg up to 1891.12 kg, oxygen production ranged from 1199.74 to 1323.78 kg, and water consumption ranged from 7139.91 L to 7877.29 L. The hydrogen evaluated costs equal to $3.79/kg. The results show that the optimum site for solar hydrogen production systems can be established in the midwest of Iraq and in other cities with similar climates, especially those that get a lot of sunlight.

Modelling and analysis of green hydrogen production by solar energy

Abstract In the article, the viability of adopting photovoltaic energy systems to convert solar energy into hydrogen in Iraqi four main cities are examined. A 22 kWp off-grid solar system, an 8 kW alkaline electrolyzer, a hydrogen compressor, and a hydrogen tank were modeled for an entire year in order to produce hydrogen. Using hourly experimental weather data from 2021 to 2030, MATLAB/Simulink is used to create a mathematical model of the recommended system behavior. The results revealed a range of annual hydrogen production from 1713.92 to 1891.12 kg, annual oxygen production from 1199.74 to 1323.78 kg, and annual water consumption from 7139.91 to 7877.29 L. Each kilogram of hydrogen costs $3.79. The results indicate that the optimal location for solar hydrogen production systems might be constructed in the central region of Iraq and in other regions with comparable climatic characteristics, particularly those with high radiation levels.

(Invited) Solar Hydrogen Production with Particulate Photocatalysts

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen on a large scale. Both the efficiency and scalability of water splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of the solar energy. In this context, overall water splitting using particulate photocatalysts has been attracting growing interest, because such systems can be spread over wide areas by inexpensive processes potentially [1], although it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH) of particulate photocatalysts and develop suitable reaction systems. In my talk, recent progress in photocatalytic materials and reaction systems will be presented.The author’s group has studied various semiconductor oxides, (oxy)nitrides, and (oxy)chalcogenides as photocatalysts for water splitting [2]. SrTiO3 is an oxide photocatalyst that has been known to be active in overall water splitting under ultraviolet irradiation since 1980 [3]. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [4]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY has been improved to more than 90% at 365 nm [5], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.The author's group has also been developing panel reactors for large-scale applications. Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultraviolet irradiation even when the water depth is merely 1 mm [6]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [7]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. More recently, a solar hydrogen production system based on 100-m2 arrayed photocatalytic water splitting panels and an oxyhydrogen gas-separation module was built, and its performance and system characteristics including safety issues was reported [8].For practical solar energy harvesting, it is essential to develop photocatalysts that are active under visible light irradiation. Ta3N5 and Y2Ti2O5S2 photocatalysts are active in overall water splitting via one-step excitation under visible light irradiation [9,10]. Particulate photocatalyst sheets efficiently split water into hydrogen and oxygen via two-step excitation, referred to as Z-scheme, regardless of size. In particular, a photocatalyst sheet consisting of La- and Rh-codoped SrTiO3 and Mo-doped BiVO4 splits water into hydrogen and oxygen via the Z-scheme, showing a STH exceeding 1.0% [11,12]. Some other (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths can also be applied to Z-scheme photocatalyst sheets. Hisatomi et al., Nat. Catal. 2019, 2, 387.Chen et al., Nat. Rev. Mater. 2017, 1, 17050.Domen et al., J. Chem. Soc., Chem. Commun. 1980, 543Ham et al., J. Mater. Chem. A 2016, 4, 3027.Takata et al., Nature 2020, 581, 411.Goto et al., Joule 2018, 2, 509.Lyu et al., Chem. Sci. 2019, 10, 3196.Nishiyama et al., Nature 2021, 598, 304.Wang et al., Nat. Catal. 2018, 1, 756.Wang et al., Nat. Mater. 2019, 18, 827.Wang et al., Nat. Mater. 2016, 15, 611.Wang et al., J. Am. Chem. Soc. 2017, 139, 1675.

A novel Z-scheme NH2-MIL-125(Ti)/Ti3C2 QDs/ZnIn2S4 photocatalyst with fast interfacial electron transfer properties for visible light-driven antibiotic degradation and hydrogen evolution

Herein, a novel all-solid-state Z-scheme NH2-MIL-125(Ti)/Ti3C2 MXene quantum dots/ZnIn2S4 (Ti-MOF/QDs/ZIS) photocatalyst was prepared, then applied for photocatalytic antibiotic (tetracycline and sulfamethazine) degradation and hydrogen evolution under visible light. The Z-scheme Ti-MOF/QDs/ZIS photocatalyst afforded very efficient degradation of tetracycline (TC, 96% in 50 min) and sulfamethazine (SMZ, 98% in 40 min), as well as a high rate of hydrogen generation (2931.9 μmol g-1 h−1) under visible light irradiation. Trapping studies showed superoxide radicals (•O2−) to be the main active species for antibiotic degradation, with the intermediates formed during antibiotic photo-oxidation identified by liquid chromatography-mass spectrometry. Photocatalyst recycling studies showed the Z-scheme composite photocatalyst to be stable during both antibiotic photodegradation and hydrogen generation experiments. The Ti3C2 MXene quantum dots facilitated electron transfer between the semiconducting components of Z-scheme composite photocatalyst. This work identifies a promising new Z-scheme photocatalyst system for water purification and solar hydrogen production.

Synergistic tailoring of band structure and charge carrier extraction in “green” core/shell quantum dots for highly efficient solar energy conversion

Environment-friendly colloidal core/shell quantum dots (QDs) with controllable optoelectronic characteristics are promising building blocks for future commercial solar technologies. Herein, we synergistically tailor the electronic band structure and charge carrier extraction of eco-friendly AgInS2 (AIS)/ZnS core/shell QDs via Mn-alloying and Cu-doping in the core and shell, respectively. It is demonstrated that the Mn-alloying in AIS core can broaden the band gap to facilitate delocalization of photogenerated electrons into the shell and further incorporation of Cu in the ZnS shell enables the creation of Cu-related states that capture the photogenerated holes from core, thus leading to charge carrier recombination and accelerated transfer of photogenerated electrons in the core/shell QDs. As-prepared Mn-AIS/ZnS@Cu QDs were assembled as light harvesters in a photoelectrochemical (PEC) device for light-driven hydrogen evolution, delivering a maximum photocurrent density of ∼ 6.4 mA cm−2 with superior device stability under standard one sun irradiation (AM 1.5G, 100 mW cm−2). Our findings highlight that simultaneously engineering the band alignment and charge carrier dynamics of “green” core/shell QDs endow the feasibility to design future high-efficiency and durable solar hydrogen production systems.