Integrated methanol reforming and oxidation in wash-coated microreactors: A three-dimensional simulation

A microreactor consisting of two parallel channels is numerically simulated where methanol steam reforming takes place in one channel, and the required heat is supplied by methanol oxidation in the other channel. Effects of different parameters on methanol conversion, hydrogen yield and CO concentration are examined. Results from the parametric study are then used to propose conditions for high methanol conversion and hydrogen yield. A microreactor with enhanced output conditions is thus designed which is capable of producing a gas stream consisting of 74% hydrogen (dry). CO concentration in the generated synthesis gas stream is low enough to require only a PROX reactor for CO clean-up, eliminating the need for a bulky water–gas shift reactor. The produced hydrogen from an assembly of such microreactors can feed a low-power PEM fuel cell. A cluster of these microreactors would take a volume of about 91 cm³ to feed a typical 30-watt PEM fuel cell.     

High surface area optofluidic microreactor for redox mediated photocatalytic water splitting

Photocatalytic water splitting is a promising approach for hydrogen generation, but the low efficiency of current photoreactors limits its widespread exploitation and commercialization. Recent developments in optofluidic microreactors open a window for advancing photocatalytic water splitting technology. Nevertheless, existing optofluidic microreactors with the planar design show the low active surface area and rate of mass transport, thereby restricting the hydrogen production performance. In this work, we proposed an optofluidic microreactor with staggered micro-pillars in the reaction micro-chamber. Such design not only enlarges the surface area to load catalyst but also induces perturbation to the liquid flow and shortens the transport length, which increases the active surface area and enhances the mass transfer and eventually boosts the hydrogen production rate. To evaluate the performance of this new optofluidic microreactor, a redox mediated water splitting reaction was implemented. Results showed that the developed microreactor with micro-pillar structure exhibited a higher reaction rate. As compared to the conventional planar optofluidic microreactor, the maximal increment of the reaction rate could reach 56%.     

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.     

Numerical analysis of convective and diffusive fuel transports in high-temperature proton-exchange membrane fuel cells

The fuel transports in high-temperature proton-exchange membrane fuel cells have been numerically examined. Both convective and diffusive fuel transports are analyzed in detail. The former is often neglected in straight flow channel configurations while it has been reported to become important for serpentine or interdigitated flow channel configurations. By using a two-dimensional isothermal model, we have performed numerical simulations of a high-temperature proton-exchange membrane fuel cell with a straight flow channel configuration. The present results show that even in a straight flow channel configuration, the convection can play a significant role in fuel transports for the anode side. Examination of the flow field data reveals that the anode gas mixture is transported toward the catalyst layer (CL) whereas the gas mixture in the cathode channel moves away from the reaction site. It is also observed that as the flow moves downstream, the flow rate decreases in the anode channel but increases in the cathode channel. Species transport data are examined in detail by splitting the total flux of fuel transport into convective and diffusive flux components. For oxygen transport in the cathode gas diffusion layer (GDL), diffusion is dominant; in addition, the convective flux has a negative contribution to the total oxygen flux and is negligible compared to the diffusion flux. However, for hydrogen transport to the reaction site, both convection and diffusion are shown to be important processes in the anode GDL. At high cell voltages (i.e., low current densities), it is even observed that the convective contribution to the total hydrogen flux is larger than the diffusive one.     

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.     

Interaction of O2, H2O and H2 with proton-conducting oxides based on lanthanum scandates

An interaction of components from the gas phase, containing gaseous oxygen, hydrogen and water, with the La_1–xSr_xScO_3–α oxides in the temperature range of 300–950 °С and partial pressure of 6.1–24.3 kPa 8.1–50.7 kPa has been studied using a high temperature thermogravimetric analysis. The effects of partial pressure of the gaseous oxygen, water and hydrogen on the apparent uptake level of protons with oxides has been found. The studies of incorporation processes of hydrogen from molecular hydrogen atmosphere into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates were performed in the temperature range of 300–800 °C and hydrogen pressure of 0.2 kPa by means of the isotope exchange method with the equilibration of isotope composition in the gas phase. The protons and deuterons concentrations were determined for the La_0.91Sr_0.09ScO_3–α oxide. The existence of proton defects in the structure of studied oxides after the exposure in H₂O and H₂-containing atmospheres was revealed using the ¹Н NMR. The role of oxygen vacancies in the proton incorporation processes is considered in the present work.     

CFD simulation of the two-phase flow for a falling film microreactor

Falling film microreactors, which provide very high specific interfacial area, have become a promising solution to the fast and strongly exothermic/endothermic gas–liquid reaction systems. A computational fluid dynamic simulation of the two-phase flow for a falling film microreactor is presented using the volume of fluid (VOF) model. The hydrodynamic characteristics, from both 2-D and 3-D simulations, including liquid film thickness, velocity, pressure and shear stress profiles, are analyzed. 2-D simulation is adopted for the study of the relationship of liquid flow rate and film thickness, as well as the effects of gas flow rate, surface tension, liquid viscosity and pressure difference on the liquid flow rate. 3-D simulation is necessary to provide the comprehensive flow profiles. Although the system is in the laminar flow regime, the liquid film features a wavy structure and the velocity profiles are complex.     

A performance study of methanol steam reforming microreactor with porous copper fiber sintered felt as catalyst support for fuel cells

A porous copper fiber sintered felt (PCFSF) as catalyst support is used to construct a methanol steam reforming microreactor for hydrogen production. The PCFSF has been produced by solid-state sintering of copper fibers which is fabricated using the cutting method. The impregnation method is employed to coat Cu/Zn/Al/Zr catalyst on the PCFSF. In this study, the effect of the porosity and manufacturing parameters for the PCFSF on the performance of methanol steam reforming microreactor is studied by varying the gas hourly space velocity (GHSV) and reaction temperature. When the 80% porosity PCFSF sintered at 800 °C in the reduction atmosphere is used as catalyst support, it is found that the microreactor shows remarkable superiority in the methanol conversion and H₂ flow rate in comparison to the ones fabricated under other manufacturing parameters. Moreover, the microreactor with this catalyst-coated PCFSF also demonstrates the excellent stability of catalytic reaction in the methanol steam reforming process.     

Three-dimensional simulation and optimization of an isothermal PROX microreactor for fuel cell applications

Three-dimensional numerical simulations of the reacting flow in rectangular micro-channel PROX reactors are performed. To solve the set of governing equations, a finite volume method is applied using an improved SIMPLE algorithm. A three-step surface kinetics for the chemical reactions is utilized that includes hydrogen oxidation, carbon monoxide oxidation, and water–gas shift reaction. The kinetics chosen are for a Pt–Fe/γ-Al₂O₃ catalyst and operating temperatures of about 100 °C. The PROX reactor is expected to remove the carbon monoxide content in a hydrogen-rich stream from about 2% to less than 10 ppm. Effects of the inlet steam content, oxygen to carbon monoxide ratio, reactor wall temperature, aspect ratio of the channel cross section, and the channel hydraulic diameter are investigated. It is found that increasing the steam content, oxygen to carbon monoxide ratio, or wall temperature may improve the performance of the microreactor. It is also shown that the rate of water–gas shift reaction or its reverse is much lower than the oxidation reactions. Finally, it is revealed that based on a modified CO yield definition, the optimum channel geometry is a square shape.     

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.     

Development of methanol steam reforming microreactor based on stacked wave sheets and copper foam for hydrogen production

Methanol steam reforming has been used for in-situ hydrogen production and supply for proton exchange membrane fuel cell (PEMFC), while its power density and energy efficiency still needs to be improved. Herein, we present a novel methanol steam reforming microreactor based on the stacked wave sheets and copper foam for highly efficient hydrogen production. The structural of stacked wave sheets and copper foam, and their roles in the microreactor are described, methanol catalytic combustion is adopted to supply heat for methanol steam reforming reaction and enables the microreactor to work automatically. For catalyst carrier, a fractal body-centered cubic model is established to study the flow characteristics and chemical reaction performances of the copper foam with coated catalyst layer. Both simulation and experimental results showed that the reformate flowrate increases with the increasing of microreactor layers and methanol solution flowrate, the discrepancies of methanol conversion between simulation and experimental tests are less than 7%. Experimental results demonstrated that the reformate flowrate of 1.0 SLM can be achieved with methanol conversion rate of 65%, the output power of the microreactor is 159 W and power density is 395 W/L. The results obtained in this study indicates that stacked wave sheets and copper foam can uniform the reactant flow and improve the hydrogen production performances.     

Hydrogen production from methane and natural gas steam reforming in conventional and microreactor reaction systems

Ni-based (over MgO and Al₂O₃) and noble metal-based (Pd and Pt over Al₂O₃) catalysts were prepared by wet impregnation method and thereafter impregnated in microreactors. The catalytic activity was measured at several temperatures, atmospheric pressure and different steam to carbon, S/C, ratios. These conditions were the same for conventional, fixed bed reactor system, and microreactors. Weight hourly space velocity, WHSV, was maintained equal in order to compare the activity results from both reaction systems. For microreactor systems, similar activities of Ni-based catalyst were measured in the steam methane reforming (SMR) activity tests, but not in the case of natural gas steam reforming tests. When noble metal-based catalysts were used in the conventional reaction system no significant activity was measured but all catalysts showed some activity when they were tested in the microreactor systems. The analysis by SEM and TEM revealed a carbon-free surface for Ni-based catalyst as well as carbon filaments growth in case of noble metal-based catalysts.     

Chemical looping gasification of biomass char for hydrogen-rich syngas production via Mn-doped Fe2O3 oxygen carrier

Because of its low cost, an iron-based oxygen carrier is a promising candidate for hydrogen-rich syngas production from the chemical looping gasification of biomass. However, it needs modification from a reactivity point of view. In this study effect of Mn doping on Fe₂O₃ has been investigated for hydrogen-rich syngas production from biomass char at different temperatures (700–900 °C) and steam flow rates (60–100 μL/min). Several techniques (XRD, XPS, BET, and TPR-H₂) have been utilized to characterize fresh and spent oxygen carriers. The result demonstrated Mn-doing boosted the redox activity and the amount of oxygen vacancies, which increased hydrogen gas generation. Hydrogen production displayed different behavior across temperatures due to detecting Fe₂O₃ and MnFeO₃ phases for spent oxygen carriers. For the Fe₂O₃ oxygen carrier hydrogen gas yield is 1.67 Nm³/kg which is due to reduction of Fe₂O₃ phase to Fe₃O₄. However, the MnFe₂O₄ spinel phase detected in the spent MnFeO₃ oxygen carrier is a reason for improving hydrogen gas yield to 1.84 Nm³/kg. Change reaction temperature from 900 °C to 850 °C reduced hydrogen gas yield from 1.84 Nm³/kg to 1.83 Nm³/kg for with MnFeO₃ oxygen carrier. Regarding different steam flows, the proper flow rates that can maintain the formed phases and obtained best hydrogen gas yield are 80 and 90 μL/min, respectively. Meanwhile, the best hydrogen gas yield (2.21Nm³/kg) are obtained with MnFeO₃ oxygen carrier at optimum conditions (850 °C and 90 μL/min).     

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.     

Electrocatalytic reduction of O2 and H2O2 by adsorbed cobalt tetramethoxyphenyl porphyrin and its application for fuel cell cathodes

In this paper, the mechanism and kinetics of oxygen and hydrogen peroxide electrochemical reduction that is catalyzed by an adsorbed cobalt tetramethoxyphenyl porphyrin (CoTMPP) on a graphite electrode were investigated using cyclic voltammetry (CV) and the rotating disk electrode (RDE) technique. The temperature and anion effects on O₂ and H₂O₂ electroreduction processes were also studied. The pH dependencies of cobalt redox centers, and oxygen and hydrogen peroxide reductions were measured for the purpose of exploring the reaction mechanism. In neutral solutions, the oxygen reduction reaction was observed to be a two-electron process, producing H₂O₂ in the low potential polarization range. In the high potential polarization range, an overall four-electron reduction of O₂ to H₂O was found to be the dominating process. The kinetic parameters obtained from the RDE experiments indicate that in a neutral solution, the reduction rate at the step from H₂O₂ to H₂O is faster than that seen from O₂ to H₂O₂. Carbon particle-based air cathodes catalyzed by CoTMPP were fabricated for metal-air fuel cell application. The obtained non-noble catalyst content cathodes show considerably improved performance and stability.     

Biogas steam and oxidative reforming processes for synthesis gas and hydrogen production in conventional and microreactor reaction systems

In this work, a renewable source, biogas, was used for synthesis gas and hydrogen generation by steam reforming (SR) or oxidative reforming (OR) processes. Several Ni-based catalysts and a bimetallic Rh–Ni catalyst supported on magnesia or alumina modified with oxides like CeO₂ and ZrO₂ were used. For all the experiments, a synthetic biogas which consisted of 60% CH₄ and 40% CO₂ (vol.) was fed and tested in a fixed bed reactor system and in a microreactor reaction system at 1073 K and atmospheric pressure. The catalysts which achieved high activity and stability were impregnated in a microreactor to explore the viability of process intensification. For the SR process different steam to carbon ratios, S/C, varied from 1.0 to 3.0 were used. In the case of OR process the O₂/CH₄ ratio was varied from 0.125 to 0.50. Comparing conventional and microreactor reaction systems, one order of magnitude higher TOF and productivity values were obtained in the microreactors, while for all the tested catalysts a similar activity results were achieved. Physicochemical characterization of catalysts samples by ICP-AES, N₂ physisorption, H₂ chemisorption, TPR, SEM, XPS and XRD showed differences in chemical state, metal–support interactions, average crystallite sizes and redox properties of nickel and rhodium metal particles, indicating the importance of the morphological and surface properties of metal phases in driving the reforming activity.     

In-situ investigation and modeling of electrochemical reactions with simultaneous oxygen and hydrogen microbubble evolutions in water electrolysis

The development of water electrolyzer is challenging as we approach theoretical limits arising from electrochemical reactions and micro-scale bubble dynamics. In this research, two-phase flow and bubble dynamics are in-situ studied in a special designed single-channel electrolyzer. The devices fabricated by a 3D printer provide a whole vision of the electrochemical reaction within the channel. In-situ observations of channel-scale hydrogen and oxygen micro-bubbles dynamics are conducted, and the whole process of hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs) are simultaneously studied. The results indicate that all bubbles generate at the interface between the proton exchange membrane and the electrode wire, and the operating conditions have a great impact on the micro bubble evolution process. The bubble detachment diameter is inversely proportional to the flow velocity, but is in direct proportion to the current density. Finally, a mathematic model has been developed, and shows a good agreement with experimental data. Those results could help to better understand the bubble evolution mechanism, in order to further understand the electrochemical reaction.     

The experimental analysis of a dead-end H2/O2 PEM fuel cell stack with cascade type design

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode and cathode can reach high hydrogen and oxygen utilization by a relatively simple system. Nevertheless, the accumulation of the water in the anode and cathode channels can lead to a local fuel starvation deteriorating the performance and the durability of PEMFCs. In this study, a novel design for a polymer electrolyte membrane (PEM) fuel-cell stack was presented which could achieve higher fuel utilization without using hydrogen and oxygen recirculation devices such as hydrogen pumps or ejectors that consume parasitic power and require additional control schemes. The basic concept of the innovatively proposed design was to divide the cells of a stack into several stages by conducting the outlet gas of each stage to a separator and reentering it into the next stage; thereby, a multistage anode and cathode system was prepared. In this relatively ingenious design, a higher gaseous flow rate was maintained at the cell outlet, even under dead-end conditions resulted in a reduced purge-gas emission by avoiding the accumulation of liquid water in the cells. The results revealed that proposed design had the same polarization curve as the open-end mode, leading to an enhanced PEMFC performance.     

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.     

Modeling and experimental assessment of the novel HI-I2-H2O electrolysis for hydrogen generation in the sulfur-iodine cycle

To improve the sulfur-iodine (SI or IS) cycle for renewable hydrogen production, direct electrolysis of HIx solution (HI-I₂-H₂O) from Bunsen reaction has been recently proposed. This work concerns the detailed microscopic physical performance and electrolytic processes of HIx electrolysis through theoretical simulation and experimental exploration. A two-dimensional mathematical model of the electrolytic cell for HIx electrolysis was developed, and was verified by the relevance between the simulated and experimental hydrogen production. The concentration, electric and flow field distributions were characterized. The decrease of anodic HI and increase of cathodic H₂ were found along the flow direction. The local potential distribution declined from anode to cathode region. Higher pressure drop from the inlet to outlet of channel as well as the pumping power were required for anolyte than catholyte. More detailed electrolytic processes along the flow channel at different operating conditions were analyzed. Increasing temperature from 303 K to 343 K improved the H₂ production rate. A low anode flow rate of 0.2 m/s was favorable for achieving high conversion rate of reactants and low consumed pumping power. The developed models and the clarified characteristics of HIx electrolysis will be further applied to the improved SI cycle.