Optofluidic microreactor for the photocatalytic water splitting to produce green hydrogen

The microfluidic devices can effectively be used for the renewable energy conversion, such as solar to chemical (e.g., H₂) energy, to meet the global energy demand. The microchannel design plays a vital role in improving the mass transfer in photocatalytic processes. In this study, a simple, rapid, and inexpensive adhesive tape-based method was used to fabricate the serpentine, planar and micropillared optofluidic microreactors with sharp edges without any wall irregularities. The sol-gel method was used for the CdS catalyst coating in the microreactors. The effect of liquid flow rate (0.05–1 mL min^?1) and sacrificial reagent (Na₂SO₃/Na₂S) concentration (0.05–0.5 M) on the hydrogen generation under visible light was studied. A higher H₂ production rate was observed in the serpentine microreactor as compared to that in planar and micropillared microreactors. The serpentine microreactor, having higher surface-to-volume ratio, induced the micromixing that enhanced the mass transfer of the sacrificial reagent and formed H₂ gas. A maximum H₂ production rate of 2.65 μmol h^?1 cm^?2 was observed at a flow rate of 1.0 mL min^?1 and a sacrificial reagent concentration (Na₂SO₃/Na₂S) of 0.5 M. The new approach developed in this study is a step forward in fabricating highly efficient and inexpensive optofluidic microdevices for the hydrogen production from solar energy.     

A novel thermally autonomous methanol steam reforming microreactor using SiC honeycomb ceramic as catalyst support for hydrogen production

Methanol steam reforming (MSR) is an attractive option for in-situ hydrogen production and to supply for transportation and industrial applications. This paper presents a novel thermally autonomous MSR microreactor that uses silicon carbide (SiC) honeycomb ceramic as a catalyst support to enhance energy conversion efficiency and hydrogen production. The structural design and working principle of the MSR microreactor are described along with the development of a 3D numerical model to study the heat transfer and fluid flow characteristics. The simulation results indicate that the proposed microreactor has a significantly low drop in pressure and more uniform temperature distribution in the SiC ceramic support. Further, the microreactor was developed and an experimental setup was conducted to test its hydrogen production performance. The experimental results show that the developed microreactor can be operated as thermally autonomous to reach its target working temperature within 9 min. The maximum energy efficiency of the microreactor is 67.85% and a hydrogen production of 316.37 ml/min can be achieved at an inlet methanol flow rate of 360 μl/min. The obtained results demonstrate that SiC honeycomb ceramic with high thermal conductivity can serve as an effective catalyst support for the development of MSR microreactors for high volume and efficient hydrogen production.     

Numerical modeling of microchannel reactors with gradient porous surfaces for hydrogen production based on fractal geometry

A microchannel reactor with a porous surface catalyst support has been applied to methanol steam reforming (MSR) for hydrogen production. The fluid flow, heat transfer, and hydrogen production efficiency of the microchannel reactor are significantly affected by the fabricated porous surface support, such as the pore sizes and their distributions. This paper presents a novel microchannel reactor with a gradient porous surface as the reaction substrate to enhance the performance of the microreactor for hydrogen production. Numerical modeling of the gradient porous surface is developed based on fractal geometry, and three different types of porous surfaces as the catalyst supports (two gradient porous surfaces and one uniform pore-size surface) are investigated. The fluid flow and heat transfer characteristics of these three types of microchannel reactors are studied numerically, and the results showed that the microreactor with a positive gradient pore sized surface exhibited relatively better overall performance. Experimental setups and tests were performed and the results validate that the microchannel reactor with a positive gradient porous surface can increase the heat transfer performance by up to 18% and can decrease the pressure drop by up to 8% when compared to a microreactor with a uniform pore sized surface. Hydrogen production experiments demonstrated that the microreactor with positive gradient pore sizes has the highest methanol conversion rate of 56.3%, and this rate is determined to be 6% and 9% higher than that of microreactors with reverse gradient porous surfaces and uniform pore sized surface, respectively.     

A safe and clean way to produce H2O2 from H2 and O2 within the explosion limit range

The direct synthesis of hydrogen peroxide (DSH) from hydrogen and oxygen is an attractive production route due to its green nature. However, it faces multiple technical challenges, the biggest being the explosion risk of the flammable gas mixture. Herein we have used microreactors to perform the reaction in an inherently safer way which allows the hydrogen concentration to fall within the explosion limit range. For the first time, we have studied the flame propagation phenomena inside a microreactor to determine the optimum channel dimension for DSH. A mechanism of “fast synthesis and slow destruction” has been proposed via investigation on the influence of channel length and liquid flow rate. Besides, a variety of reaction parameters including gas flow rate, oxygen: hydrogen ratio, catalyst composition and gas pressure have been studied carefully. The successful employment of a microreactor in this case has indicated the potential of using microreactors to inhibit the explosion risks of hazardous processes.     

Hydrogen production from cylindrical methanol steam reforming microreactor with porous Cu-Al fiber sintered felt

In this study, the porous Cu-Al fiber sintered felt (PCAFSF) was fabricated by low temperature solid-phase sintering method. The laminated PCAFSF as the catalyst support was used for cylindrical methanol steam reforming microreactor for hydrogen production. The two-layer impregnation method was employed to coat the Cu/Zn/Al/Zr catalyst on the PCAFSF. The material composition, specific surface area and catalyst loading of PCAFSF were also measured. The effect of the fiber material, surface morphology and porosity on the reaction performance of methanol steam reforming microreactor for hydrogen production was further investigated. Our results show that the PCAFSF demonstrated much higher methanol conversion and H₂ flow rate compared to the porous Cu fiber sintered felt (PCFSF) and porous Al fiber sintered felt (PAFSF) having the same porosity. Furthermore, the rough PCAFSF showed much higher methanol conversion and H₂ flow rate compared to the smooth PCAFSF. In case of the PCAFSF, the methanol conversion and H₂ flow rate were increased with the decrease of Cu fiber weight and the increase of Al fiber weight. The best reaction performance of microreactor for hydrogen production was obtained using the three layer PCAFSFs with 80% porosity and 1.12 g Cu fiber/1.02 g Al fiber.     

A mini-review on transition metals-based 1D nanotubular bifunctional electrocatalysts for overall water splitting

Research and development on bifunctional electrocatalysts for water electrolysis are essential in achieving cost reduction, simplifying electrolyzer design, and a sustainable hydrogen economy. One widely investigated approach to boosting electrocatalyst's efficiency is morphology control via one-dimensional nanostructures. Nanotubes are attractive among the different one-dimensional nanostructures due to their high surface-to-volume ratio, internal void space, low density, and fast charge/mass transport. This mini-review provides a concise account of transition metals-based nanotubular bifunctional electrocatalysts reported for overall water splitting. Nanotubes of precious and non-precious metals/alloys/compounds, their preparation methods, hydrogen and oxygen evolution reaction kinetics, overall water splitting performance, and mechanism of action are deliberated.     

Structural design of self-thermal methanol steam reforming microreactor with porous combustion reaction support for hydrogen production

To replace the traditional electric heating mode and increase methanol steam reforming reaction performance in hydrogen production, methanol catalytic combustion was proposed as heat-supply mode for methanol steam reforming microreactor. In this study, the methanol catalytic combustion microreactor and self-thermal methanol steam reforming microreactor for hydrogen production were developed. Furthermore, the catalytic combustion reaction supports with different structures were designed. It was found that the developed self-thermal methanol steam reforming microreactor had better reaction performance. Compared with A-type, the △T_max of C-type porous reaction support was decreased by 24.4 °C under 1.3 mL/min methanol injection rate. Moreover, methanol conversion and H₂ flow rate of the self-thermal methanol steam reforming microreactor with C-type porous reaction support were increased by 15.2% under 10 mL/h methanol-water mixture injection rate and 340 °C self-thermal temperature. Meanwhile, the CO selectivity was decreased by 4.1%. This work provides a new structural design of the self-thermal methanol steam reforming microreactor for hydrogen production for the fuel cell.     

Highly efficient electrocatalytic hydrogen production via MoSx/3D-graphene as hybrid electrode

Molybdenum sulfide (MoS_x) has recently emerged as a promising catalyst for the hydrogen evolution reaction (HER) in water splitting that may replace the noble metal, such as platinum, as a cost-effective and high catalytic materials. It has been reported that two-dimensional structured MoS_x exhibit significant amount of exposed S-edge, which can be an active electrocatalytic catalyst for hydrogen production. However, the current reports mainly focusing on the planar electrode, where the catalyst utilization and the number of active sites are limited due to the lower exposed specific surface area (SSA) of supporting electrodes. In this work, we utilize the freeze-drying method to produce a porous three-dimensional (3D) structure assembled by graphene flakes. The as-prepared 3D graphene scaffold shows high surface area, high porosity while low density, which makes it as an ideal conductive electrode for supporting of MoS_x catalysts. Moreover, it was found out that the crystallinity of MoS_x, controlled by thermolysis temperature of thiosalts precursor ((NH₄)₂MoS₄), shows significantly influence the performance of HER. The optimized annealing temperature for the designed hybrid electrodes (MoS_x/3D-graphene) was found to create a lot of active sites, which facilitate the electrocatalytic performance for water splitting (overpotential of 163 mV @10 mA/cm² and a Tafel slope of 41 mV/dec). The study provides a potential material, which could pave the way for future applications of hydrogen energy.     

Fabrication of porous metal by selective laser melting as catalyst support for hydrogen production microreactor

To improve the hydrogen production performance of microreactors, the selective laser melting method was proposed to fabricate the porous metals as catalyst supports with different pore structures, porosities, and materials. The influence of the porous structures on the molecule distribution after passing through the porous metals was analyzed by molecular dynamics simulation. The developed porous metals were then used as catalyst supports in a methanol steam reforming microreactor for hydrogen production. Our results show that the porosity of the porous metal had significantly influence on the catalyst infiltration and the reaction process of hydrogen production. A lower degree of catalyst infiltration of the porous metal was obtained with lower porosity. A copper layer-coated stainless-steel porous metal with a staggered structure and gradient porosity of 80%–60% exhibited much larger methanol conversion and H₂ flow rate due to its better heat and mass transfer characteristic. Methanol conversion and H₂ flow rates could reach 97% and 0.62 mol/h, respectively. Finally, it was found that the experimental results were in good agreement with the simulation results.     

Porous copper fiber sintered felts with surface microchannels for methanol steam reforming microreactor for hydrogen production

In this study, a laser micro-milling technique was introduced into the fabrication process of surface microchannels with different geometries and dimensions on the porous copper fiber sintered felts (PCFSFs). The PCFSFs with surface microchannels as catalyst supports were then used to construct a new type of laminated methanol steam reforming microreactor for hydrogen production. The microstructure morphology, pressure drop, velocity and permeability of PCFSF with surface microchannels were studied. The effect of surface microchannel shape (rectangular, stepped, and polyline) and catalyst loading amount on the reaction performance of methanol steam reforming microreactor for hydrogen production was further investigated. Our results show that the PCFSF with rectangular microchannels demonstrated a lower pressure drop, higher average velocity and higher permeability compared to the stepped and polyline microchannel. Furthermore, the PCFSF with rectangular microchannels also exhibited the highest methanol conversion and H₂ flow rate. The best reaction performance of methanol steam reforming microreactor for hydrogen production was obtained using PCFSF with rectangular microchannels when 0.5 g catalyst was loaded.     

A review on nickel cobalt sulphide and their hybrids: Earth abundant,pH stable electro-catalyst for hydrogen evolution reaction

The production of hydrogen from electrocatalytic water splitting using a clean energy source has become a sustainable route to overcome the problems related to fossil fuels. Therefore, it is essential to develop a non-precious, stable and highly efficient electrocatalyst. Nickel and cobalt sulphide have gained much attention as hydrogen evolution reaction (HER) catalysts due to their pH stability, low cost and high activity. But the application of these sulphides is limited due to high over-potentials, low surface area and less catalytic active sites available for HER. Furthermore, Nickel and cobalt sulphide can be coupled with different functional components to enhance their catalytic activity. This comprehensive review focuses on the progress made so far to enhance the electrochemical properties of nickel and cobalt sulphides and their composites with various active materials. The comparative survey of their HER activities is made in terms of their electrocatalytic performance parameters.     

A perspective on the fabrication of heterogeneous photocatalysts for enhanced hydrogen production

This perspective provides an insight to the possibility of adopting hydrogen as a key energy-carrier and fuel source, through Photocatalytic water splitting in the near future. The need of green and clean energy is increasing to overcome the growing demand of sustainable energy throughout globe, owing to CO₂ emission using fossil fuels. To generate highly efficient and cost-competitive hydrogen, the semiconductor based heterojunction nanomaterials have gained tremendous consideration as a promising way. Currently, the efficiency for hydrogen generation through UV–Vis active photocatalysts is relatively low. The key issues are found to be poor separation of photogenerated electron/hole, less surface area, and low absorption region of electromagnetic spectrum. Such issues arise due to inappropriate band edge potentials and large bandgap of present catalyst. A lot of schemes has been devoted to design and fabricate efficient photocatalysts for improved photocatalytic performance in recent years. However, it seems still a challenge and imperative to greatly comprehend the fundamental aspects, photocatalysis and transfer mechanisms for complete deployment of electron/hole pairs. Further, to produce hydrogen to a larger extent through photocatalytic water splitting, the photocatalyst has been modified through co-catalysts/dopants using numerous techniques including the Z-scheme, hybridization, crystallinity, morphology, tuning of band edge positions, reduction of the band gap, surface structure etc., such that these heterogeneous photocatalysts may have ability to absorb enough light in the UV-VIS-IR region. This type of heterogeneous photocatalysts has the ability to improve the rate of efficiency for hydrogen evolution through absorption of sufficient light of solar spectrum and enhance the separation of charge-carriers by inhibiting recombination of electron/hole pairs. We surmise that taking into account the aforesaid factors should support in scheming an efficient photocatalysts for hydrogen production through water splitting, eventually prompting technological developments in this field.     

Enhanced photoelectrocatalytic hydrogen production performance of porous MoS2/PPy/ZnO film under visible light irradiation

Photoelectrochemical (PEC) water splitting provides a “green” approach for hydrogen production. However, the design and fabrication of high-efficient catalysts are the bottleneck for PEC water splitting owing to the involved thermodynamic and kinetic challenges. Herein, we report a new strategy for constructing a porous MoS₂/PPy/ZnO thin film photocatalyst with large specific surface area and excellent conductivity to achieve photoelectrochemical water splitting under visible light irradiation. Porous PPy/ZnO was synthesized via template-assisted electrodeposition, and MoS₂ was further electrodeposited to construct porous MoS₂/PPy/ZnO thin film photocatalyst. The hydrogen evolution rate of MoS₂/PPy/ZnO exhibits about 3.5-fold increase to 40.22 μmol cm⁻² h⁻¹ under visible light irradiation. The enhancement for photoelectrochemical hydrogen production is not only ascribed to enlarged specific surface area of the porous structure, but also attributed to the synergistic effects of MoS₂ and porous PPy/ZnO, which could dramatically improve its visible light absorption capacity and enhance the separation and transfer of photogenerated charges. Thus, more abundant photogenerated electrons and holes participate in photoelectrochemical process, which significantly enhances its photoelectrochemical hydrogen production performance.     

Kinetic investigations of the hydrogen production step of a thermochemical cycle using mixed iron oxides coated on ceramic substrates

A two‐step thermochemical cycle for solar hydrogen production using mixed iron oxides as the metal oxide redox system has been investigated. The ferrite is coated on a honeycomb structure, which serves as the absorber for solar irradiation and provides the surface for the chemical reaction. Coated honeycomb structures have already been tested in a solar receiver reactor in the solar furnace of DLR in Cologne with respect to their water splitting capability and their long‐term stability. The concept of this new reactor design has proven feasible and constant hydrogen production during repeated cycles has been shown. For a further optimization of the process and in order to gain reliable performance predictions more information about the process especially concerning the kinetics of the oxidation and the reduction step are essential. To examine the hydrogen production during the water splitting step a test rig has been built up on a laboratory scale. In this test rig small coated honeycombs are heated by an electric furnace. The honeycomb is placed inside a tube reactor and can be flushed with water vapour or with an inert gas. A homogeneous temperature within the sample is reached and testing conditions are reproducible. Through analysis of the product gas the hydrogen production is monitored and a reaction rate describing the hydrogen production rate per gram ferrite can be formulated. Using this test set‐up, SiC honeycombs coated with zinc ferrite have been tested. The influences of the temperature and the water concentration on the hydrogen production during the water splitting step have been investigated. An analysis of the ferrite conversion was performed using the Shrinking Core Model. A mathematical approach for the peak reaction rate at the beginning of the water splitting step was formulated and the activation energy was calculated from the experimental data. An activation energy of 110 kJ mol⁻¹ was found. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of the optimal active area for proton exchange membrane fuel cells with parallel,interdigitated or serpentine designs

Effects of active area size on steady-state characteristics of a working PEM fuel cell, including local current densities, local oxygen transport rates, and liquid water transport were studied by applying a three-dimensional, two-phase PEM fuel cell model. The PEM fuel cells were with parallel, interdigitated, and serpentine flow channel design. At high operating voltages, the size effects on cell performance are not noticeable owing to the occurrence of oxygen supply limit. The electrochemical reaction rates are high at low operating voltages, producing large quantity of water, whose removal capability is significantly affected by flow channel design. The cells with long parallel flow field experience easy water accumulation, thereby presenting low oxygen transport rate and low current density. The cells with interdigitated and serpentine flow fields generate forced convection stream to improve reactant transport and liquid water removal, thereby leading to enhanced cell performance and different size effect from the parallel flow cells. Increase in active area significantly improves performance for serpentine cells, but only has limited effect on that of interdigitated cells. Size effects of pressure drop over the PEM cells were also discussed.     

Designing of CoP4/FeP4 with hollow nanocages as bifunctional catalysts for overall water splitting

Efficient and stable electrocatalysts are essential for water splitting. Feasible structural design can facilitate electron transport and increase specific surface area. Herein, the porous CoP₄/FeP₄ hollow cubes are synthesized by two steps: synthesizing the Co–Fe prussian blue analogues via co-precipitation and phosphating it by calcination. The construction of heterojunction in CoP₄/FeP₄ not only accelerates electronic transmission but also provides active sites, which acts synergistically on the oxygen and hydrogen evolution reactions. Therefore, the CoP₄/FeP₄ hollow cubes with the exist of mesoporous exhibit the promising performance for water splitting. The enhanced performance that basically originates from bimetallic synergy and unique morphological structure is acquired with a low overpotential of 270 mV at 10 mA cm^?2 and the Tafel slope of 42.4 mV dec^?1 towards the oxygen evolution reaction (OER). Electrolyzer with two-electrode system assemble by utilizing the CoP₄/FeP₄ hybrid as anode and cathode exhibits a cell voltage of 1.74 V to achieve 10 mA cm^?2 for overall water splitting. This study that provides a simple strategy to design and construct the heterogeneous interface may promote the development of non-noble metal for HER and OER.     

Novel MoSe2–Ni(OH)2 nanocomposite as an electrocatalyst for high efficient hydrogen evolution reaction

Nowadays, there is a great demand for low-cost and highly active electrocatalyst for the production of clean renewable energy. However, most of the electrocatalysts are noble metal-based which are very costly and unstable. To counter this, electrochemical water splitting in energy storage systems is been widely applied, using non-noble metal-based nanostructured electrocatalysts. In this work, a novel noble metal-free MoSe₂–Ni(OH)₂ nanocomposite electrocatalyst is synthesized using a multi-step hydrothermal technique for efficient hydrogen evolution reaction (HER). The morphology, structural, chemical composition, and functional features of the synthesized nanomaterials were characterized using different techniques that include scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), and Raman analysis. The new developed MoSe₂–Ni(OH)₂ nanocomposite combines a high active surface area with a high chemical stability, generating a novel material with a synergistic effect that enhances water splitting process performance. Thus, an outstanding low Tafel slope of 54 mV dec⁻¹ is accomplished in the hydrogen evolution reaction.     

Effective surface roughening of three-dimensional copper foam via sulfurization treatment as a bifunctional electrocatalyst for water splitting

Cost effective electrocatalysts in water splitting reaction are critically important for the practical application of hydrogen fuel. The surface of three-dimensional copper foam is successfully roughened via one-step sulfurization reaction, and cuprous sulfide is formed on copper foam accordingly, which is denoted as Cu₂S@Cu. The as-prepared Cu₂S@Cu electrocatalyst exhibits remarkable performance on oxygen evolution reaction in basic solution, with a low overpotential of 345 mV to achieve 20 mA cm⁻². Cu₂S@Cu also shows enhanced performance on hydrogen production, compared to the original copper foam. Furthermore, Cu₂S@Cu can work as both cathode and anode in full water splitting, with superior activity to the noble metal-based electrocatalysts under large current densities. This study demonstrates that surface roughening technique on copper foam by sulfurization reaction can be valuable for developing novel copper-based electrocatalysts for water splitting.     

Performance assessment study of photo-electro-chemical water-splitting reactor designs for hydrogen production

Solar water splitting is considered a greatly promising technique for producing clean hydrogen fuel. However, limited studies have paid attention to the designs of photo-electrochemical (PEC) reactors. In this regard, two different designs of PEC reactor are proposed and studied numerically in the present paper. The effects of important design parameters on the system performance are also investigated. The PEC governing equations of transport phenomena related to water splitting reactor are developed and numerically solved. According to the current results, the rate of the hydrogen volume production and the solar - to - hydrogen conversion efficiency increase as an applied solar incident flux increases for both proposed designs. The solar - to - hydrogen conversion efficiencies are calculated to be 12.65% for design 1 and 12.48% for design 2. The hydrogen volume production rate is performed to achieve 78.3 L/m² h by design 1, and 74.8 L/m² h by design 2.