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The electrolyzing and fuel cell (FC) modes appear in a unitized regenerative FC. Understanding the operating condition impacts on the dynamic behaviors during mode-switching procedure facilitates better handling of cell working states and increasing the performance behaviors. In the present paper, a two-dimensional transient numerical model is employed to discuss the influence of operating conditions on two-phase species distributions when switching the cell mode into FC. According to the numerical results, it is concluded that when the cell switches from electrolytic cell to the mode of FC, reducing the operating cell voltage can significantly improve the cell performance. At the same time, concentration polarization and temperature heterogeneity are serious. Increasing the gas inlet velocity can improve the cell performance. Providing the reaction gas with the same concentration as the FC mode in advance can improve the temperature uniformity and cell current density values when starting switching the mode but hardly impacts the parameter value and currents after stabilization. [ABSTRACT FROM AUTHOR]

The gas flow channels inside unitized regenerative fuel cells display obvious effects on relevant parameter changing behaviors such as species concentration, electrical signal and thermal signal during mode switching. In previous study, detailed transportation behaviors inside a cell constructed with streamlined channels have not been investigated. In our present study, a two-dimensional, transient model coupling non-isothermal characteristics is employed to study the changing characteristics of the cell with typical streamlined channel and comparatively analyze the channel structure impacts on the working states inside the operating cell regions. The simulated results indicate that when turning the mode from electrolyzer to fuel cells, the changing response occupied time of each parameter is required more, compared with the stabilizing procedure after entering the electrolytic cell procedure. For different oxygen-side channels, when reducing the channel depth, the current density output is raised, and the needed time-span for the stabilizing the working state is reduced. The present simulation results facilitate better understanding mass transportation behaviors inside unitized regenerative fuel cells when the mode switches from an electrolytic cell to fuel cell. [ABSTRACT FROM AUTHOR]

Mode switching between fuel cell and electrolytic mode is inevitable for actual applications. During the mode switching, complicated electrical response and gas-liquid phase change occur due to the different operation conditions in these two modes. To explore the effect of water removal method on electrical performance before fuel cell mode startup in mode switching process and get better overall cell performance. In this paper, the continuous mode switching within three cycles is explored by experimental works. It is found that the voltage decreases slightly with the cycle running, and the two-phase flow in the corresponding mode of each cycle is similar through analyzing the two-phase flow phenomenon of the two modes during mode switching. It is also found that bubbles are generated during electrolytic cell, the water is consumed and there is only water mist at the transparent end plate during electrolytic cell mode without water. A handful of liquid water gathers at the bottom of the flow channel during fuel cell mode. Experiments of the cycle test process are also conducted. It is shown that with the cell mode switching periodically, the cell voltages of the electrolytic cell and fuel cell both decrease periodically. The fuel cell performance changes more, especially under high current density. [ABSTRACT FROM AUTHOR]

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The research on temperature and heat flux during mode switching process of unitized regenerative fuel cells is importance. Thus, in this paper, the local temperature and heat flux are measured during mode switching using self-made thin film sensors, and heat transfer coefficient is also obtained. The results of the experiment exhibit that the temperature and heat flux change significantly during mode switching, and periodically change with periodic mode switching, the heat generation is greater than the heat dissipation inside the cell. During mode switching, the temperature and heat flux are higher in upstream, and the temperature and local heat transfer coefficient increase along main gas flow direction. The fuel cell current density has a great influence on heat transfer when cell mode switches. At different current density, the temperature and heat flux show different changing trend when the cell switches from electrolytic cell mode to the fuel cell mode. These experimental results can provide guidance to make fuel cell run more efficiently in practical application. • Temperature and heat flux are measured during mode switching. • Effect of fuel cell current density on heat transfer during mode switching is investigated. • Temperature and heat flux change periodically with the periodic mode switching. • Fuel cell current density has a great influence on heat transfer during mode switching. [ABSTRACT FROM AUTHOR]

Understanding the variation in heat and mass transfer during mode switching of a unitized regenerative fuel cell is essential for designing efficient flow channels and operational strategies. In this study, dynamic variation of current density, temperature, and species distribution during mode switching is investigated in a unitized regenerative fuel cell with a stepped flow channel. This work is conducted by a transient multi-physics model coupled with electrochemical reaction, heat and mass transfer, and phase change. Results indicate that the response time of current and temperature is prolonged as the mode-switching frequency rises. Using stepped channels instead of conventional straight channels can promote mode-switching and shorten the response time. Specifically, the stepped structure promotes airflow disturbance in flow channels, and the induced forced convection promotes heat and mass transfer. Besides, the stepped structure accelerates the stabilization of current density, temperature, and species during mode switching, leading to a more uniform current density distribution. In addition, the stepped structure promotes water discharge and convective heat transfer, improving the temperature uniformity of the catalyst layer. This study offers valuable insights for improving the durability of the membrane electrodes and improving the operational stability of unitized regenerative fuel cells during mode switching. • A transient model is developed to describe the mode-switching stage of URFCs • The heat and mass transfer variations in URFCs during mode switching are analyzed • Adopting a stepped channel instead of a straight channel enhances response speed • A stepped channel promotes water discharge and convective heat transfer in FC mode [ABSTRACT FROM AUTHOR]

The gas purging states affect electricity output and energy storage capacity of unitized regenerative fuel cells. In this study, a model of unitized regenerative fuel cell is established. Cell voltages and operating temperatures influences on the dynamic distribution of thermal fluid during purging process and the discharge of residual liquid water in electrolytic cell mode are investigated. The motivation of the present study is better understanding the gas purging characteristics and its effect on reaction behaviors of unitized regenerative fuel cells. Simulation results reveal a significant influence of purging gas temperature on the water flooding and a great effect of operating voltage on the water diffusion. The operating temperature of electrolytic cell model almost has little effect on purging results at different cell temperature and the same purging gas temperature. When the purging gas temperature is changed, higher temperatures of cell and purging gas facilitate liquid water discharging out from the cell regions. In cell water flooding situation, when having large liquid content, the purging gas has little effects on the water expelling process. • EC temperature exhibits obscure effect on water transportation under same purging temperature. • Higher cell temperature and purging temperature facilitate expelling water from the cell. • Purging temperature hardly affects water expelling during the cell flooding processes. • Liquid water remaining in CLs is less under higher cell voltage conditions. [ABSTRACT FROM AUTHOR]

In this study, a two-dimensional, two-phase, non-isothermal, and un-steady state unitized regenerative fuel cell model is established. The effect of various operating conditions on the variation trend of parameters and cell performance during mode switching from fuel cell to electrolytic cell mode is studied when using an elliptically shallow flow channel. Three operating parameter conditions are discussed, including inlet gas velocity, electrolytic cell voltage and water supply velocity. The effects of various operating conditions on cell performance and parameters are compared. The results show that different operating conditions have different effects on cell performance. When switching from fuel cell mode to electrolytic cell mode, changing the stoichiometric ratio of inlet gas velocity does not affect the cell performance after mode switching. The cell performance can be improved gradually with the increase in electrolytic cell voltage. Increasing the flow rate of liquid water can effectively speed up the stabilization time, however, the cell performance is slightly reduced, and it is easy to accumulate oxygen at the outlet. [ABSTRACT FROM AUTHOR]

Oxygen side flow channel structures have significant influence on the dynamic response of relevant parameters in operation mode switching procedures of a unitized regenerative fuel cell. In this paper, an unsteady, non-isothermal, two-phase, two-dimensional unitized regenerative fuel cell model is established to study the dynamic response of two-phase species concentration distributions, current density distributions etc. when the cell mode swiches from the fuel cell mode to the electrolyzer when using orientational flow channel at the oxygen side. The influence of eight oxygen-side channels on the mode switching process is compared. Results show that the required time for each parameter reaching the stable state in the fuel cell mode is longer than the dynamic response time after switching to electrolytic cell mode when using orientational flow channels. In addition, different flow channel structures at the oxygen-side affect the time requirement for stabilizing each parameter during mode switching, while they do not change the characteristics of the stable state mode. • An un-steady state model of URFCs with orientational channels is developed. • Dynamic response of operation parameters vary under channel structure effects. • Decreasing channel depth percentage enhances cell performance. • Orientational channels facilitate stabilizing operation parameters of URFCs. [ABSTRACT FROM AUTHOR]

The present paper presents one- and two-step approaches for electrochemical Pt and Ir deposition on a porous Ti-substrate to obtain a bifunctional oxygen electrode. Surface pre-treatment of the fiber-based Ti-substrate with oxalic acid provides an alternative to plasma treatment for partially stripping TiO2 from the electrode surface and roughening the topography. Electrochemical catalyst deposition performed directly onto the pretreated Ti-substrates bypasses unnecessary preparation and processing of catalyst support structures. A single Pt constant potential deposition (CPD), directly followed by pulsed electrodeposition (PED), created nanosized noble agglomerates. Subsequently, Ir was deposited via PED onto the Pt sub-structure to obtain a successively deposited PtIr catalyst layer. For the co-deposition of PtIr, a binary PtIr-alloy electrolyte was used applying PED. Micrographically, areal micro- and nano-scaled Pt sub-structure were observed, supplemented by homogenously distributed, nanosized Ir agglomerates for the successive PtIr deposition. In contrast, the PtIr co-deposition led to spherical, nanosized PtIr agglomerates. The electrochemical ORR and OER activity showed increased hydrogen desorption peaks for the Pt-deposited substrate, as well as broadening and flattening of the hydrogen desorption peaks for PtIr deposited substrates. The anodic kinetic parameters for the prepared electrodes were found to be higher than those of a polished Ir-disc. [ABSTRACT FROM AUTHOR]

[Display omitted] • OMNs Pt-Ru-Ir catalyst shows improved oxygen bifunctional electrocatalysis. • Superior ORR activity (0.21 mA/µg & 0.33 mA/cm2) with extended durability was found. • Excellent OER activity with low Tafel slope and enhanced durability obtained for Pt 70 Ru 25 Ir 5. • Synergistic and mesoporous morphology with trace Ir boosting the bifunctional oxygen kinetics. An efficient oxygen bifunctional catalyst Pt-Ru-Ir with ordered mesoporous nanostructures (OMNs) was successfully synthesized by chemical reduction using KIT-6 mesoporous silica as a template. The crystallographic behavior, electronic effects, and microstructure of the catalysts were investigated by XRD, XPS, SEM, and TEM analysis. The influence of OMNs and the effect of Ir content in Pt-Ru-Ir catalyst on both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were investigated. The synergistic and electronic effects play an important role in electrocatalytic performance through the electronic coupling between Pt, Ru and Ir followed by the alloy formation with different lattice strain percentages. Amongst, the OMNs Pt 70 Ru 25 Ir 5 catalyst exhibits the highest mass activity of 0.21 mA µg−1 and specific activity of 0.33 mA cm−2 for ORR, which are nearly 5-fold greater than those for benchmark Pt/C catalyst. Furthermore, the Pt 70 Ru 25 Ir 5 demonstrated enhanced OER activity with an overpotential of 470 mV at 10 mA cm−2, an onset potential of 1.70 V, and a Tafel slope of 118 mV dec-1, outperforming commercial IrO 2. In addition, the durability of the Pt 70 Ru 25 Ir 5 catalyst for ORR and OER are found to be extended in comparison with that of other catalysts reported in this work after 6000 cycles. These results demonstrate that the ordered OMNs Pt-Ru-Ir with low Ir content (∼5 wt%) could be a promising oxygen bifunctional catalyst for electrochemical energy conversion and storage applications. [ABSTRACT FROM AUTHOR]

In this paper, a hybrid renewable power plant with a storage system is designed. The benefits of sizing and energy management are assessed for a commercial building under eight different climatic conditions in the United States. In the considered system, photovoltaic panels are coupled to a unitized regenerative solid oxide fuel cell. The use of biogas to feed unitized regenerative solid oxide fuel cell is investigated, employing a detailed electrochemical model of electrolyzer and fuel cell modes. A battery pack is included in the plant as a secondary storage system, together with a diesel engine operating in backup mode. Four scenarios where biogas amount is varied together with the initial state of charge of the battery were evaluated. Results demonstrate that the power plant can operate with 100 % renewable procurement if the digester produces from 6000 to 9500 stdm3/y and the battery is completely charged at the beginning of the year. By reducing the biogas availability or starting with a low state of charge, the use of the diesel generator is inevitable. The study confirms that the proposed hybrid renewable power plant is technically feasible and can be considered a reliable and clean energy source in other areas and buildings. • Fully renewable power plant optimization/sizing for commercial buildings. • Influence of U.S. climate zones on power plant operations. • Definition of a control strategy for fossil fuel usage minimization. [ABSTRACT FROM AUTHOR]

The present paper presents one- and two-step approaches for electrochemical Pt and Ir deposition on a porous Ti-substrate to obtain a bifunctional oxygen electrode. Surface pre-treatment of the fiber-based Ti-substrate with oxalic acid provides an alternative to plasma treatment for partially stripping TiO2 from the electrode surface and roughening the topography. Electrochemical catalyst deposition performed directly onto the pretreated Ti-substrates bypasses unnecessary preparation and processing of catalyst support structures. A single Pt constant potential deposition (CPD), directly followed by pulsed electrodeposition (PED), created nanosized noble agglomerates. Subsequently, Ir was deposited via PED onto the Pt sub-structure to obtain a successively deposited PtIr catalyst layer. For the co-deposition of PtIr, a binary PtIr-alloy electrolyte was used applying PED. Micrographically, areal micro- and nano-scaled Pt sub-structure were observed, supplemented by homogenously distributed, nanosized Ir agglomerates for the successive PtIr deposition. In contrast, the PtIr co-deposition led to spherical, nanosized PtIr agglomerates. The electrochemical ORR and OER activity showed increased hydrogen desorption peaks for the Pt-deposited substrate, as well as broadening and flattening of the hydrogen desorption peaks for PtIr deposited substrates. The anodic kinetic parameters for the prepared electrodes were found to be higher than those of a polished Ir-disc.

The big challenge of global phase‐down of carbon dioxide emissions leads us to alternatives in storing electric energy from renewable sources. For the worldwide use of batteries and fuel cells very high amounts of precious electrode materials are needed. Batteries require rare earth metals and environmentally harmful Lithium, PEM fuel cells require noble platinum and iridium. Thus, an economical and eco‐friendly alternative to lithium‐ion‐batteries should be found, especially for the use in domestic homes. In the on‐hand work, a well‐known type of fuel cell is revised and considered in the new context of global material usage. Therefore, an alkaline fuel cell with the classic design of an electrolyte gap in‐between the electrodes is observed. Gas‐diffusion‐electrodes are used, based on Raney‐nickel. Results are given for different catalysts: Raney‐nickel, Raney‐silver, manganic‐oxide, carbon and ruthenium. For the analysis current‐voltage‐characteristics, chronopotentiometry and SEM micrographs are used. The results are discussed in comparison to a unitized reversible PEM fuel cell. At low current density, the alkaline cell is obtaining round‐trip efficiencies close to 70%, while the PEM cell achieves only 60% efficiency. With Silver‐catalyst clear higher efficiencies up to 80% are presentable. The PEM fuel cell shows obvious better performance at high current densities. [ABSTRACT FROM AUTHOR]

The unitized regenerative fuel cell (URFC) system has attracted significant attention and interests because of its round-trip energy conversion with high energy density. However, the identification of low-cost bipolar (BP) plate with higher corrosion resistance is required to produce a sustainable system. In this work, we investigated the possibilities of using cost-effective lead (Pb) metal-based plate as a BP plate material for URFC system. To further enhance the advantageous properties of Pb plate, silver (Ag) was coated onto the Pb plate. Different types of structural and microstructural analyses (XRD, XPS, SEM, EDAX, and mapping) were conducted to characterize the properties of Pb-based plate and the presence of Ag-coated layer on the surface of Pb plate. When compared with the Pb plate, the Ag-coated Pb plate exhibited a higher water contact angle. The obtained water contact angles of Pb and Ag-coated Pb plates were 77.83° and 92.21°, respectively. The obtained water contact angle of the Ag-coated Pb plate is quite acceptable in the URFC system because it offers exceptional advantages during the URFC operation. Furthermore, the interaction between Pb plate and Ag coated layer resulted in an efficient electrochemical performance. Based on these results, we could conclude that the Pb-based BP plate can be possibly considered for the URFC applications. • The viabilities of cost-effective Pb based plate identified as a BP plate for URFC. • A new kind of BP material for round-trip energy conversion device of URFC system. • The higher water contact angle of Ag–Pb BP plate is more desirable to the URFC. • Ag coating on Pb surface can effectively increase the corrosion resistance. [ABSTRACT FROM AUTHOR]

• A two-phase simulation in a unitized regenerative fuel cell is performed. • Relative humidity effect on purging is studied under severe water flooding condition. • Purging result shows diverse effects on cell start-up performance in fuel cell mode. A unitized regenerative fuel cell is a typical gas–liquid two-phase system that consumes oxygen and hydrogen in a fuel cell mode and liquid water in an electrolytic cell mode. Liquid water removal is crucial for a successful cell start-up after the electrolytic cell mode ends. However, investigations on two-phase transfer mechanisms for liquid water removal are limited during mode switching. To fill this research gap, a three-dimensional two-phase full-cell model is developed to describe charges, gas mixtures and liquid water transfer corresponding to operational modes of electrolytic cell, gas purging and fuel cell. The cell is assumed to being in the isothermal state. Numerical model is analyzed by using COMSOL Multiphysics 5.3a software. Furthermore, experimental and simulated results are compared to validate the proposed model. Results show that more than 84.0% of pore volume is occupied with liquid water on porous layers in an electrolytic cell mode. Although pre-switching for water removal can decrease the volume fraction of liquid water within the porous layers from 0.88 to 0.50 in a short time, more time is required for liquid water being carried away under a low level of water flooding. Purging result presents diverse influences on start-up performance in a fuel cell mode. Liquid water distribution, which is similar to that in a fuel cell mode, formed in a purging mode is encouraged for promoting a quick and stable start-up in a fuel cell mode. [ABSTRACT FROM AUTHOR]

Unitized regenerative fuel cells, as being able to use and regenerate the hydrogen, seem to be compact solution for the standalone systems. The cells with smaller active areas (<50 cm2) have better contact between electrode and bipolar plate due to their smaller sizes. It therefore results in very low resistance at the interface owing to high performance. However, the power produced by the cells is not generally observed to linearly follow the changes in the size of electrodes. Such losses in the performance for scaled up cells is due to the high interfacial contact resistance incurred at the interface. Such resistance could be lowered using optimized gaskets as well as applied torque. The present study evaluates different gaskets for a scaled-up version of the cell (300 cm2) as a measure of increased contact resistance when operated at high pressures during electrolysis mode of operation. The cell is modelled structurally and simulated for most available gasket materials i.e. silicon and Teflon. Average contact pressure at the interface of electrode and bipolar plate is considered as the parameter to estimate the interfacial contact resistance. Silicon is evaluated better material than Teflon and is observed to hold almost 4.5 bar of gas in electrolysis mode when clamped with 8 Nm of torque. The cell is observed to perform close to state of art system and delivered 125 A at 0.5 V during fuel cell mode and generate 500 and 250 mL/min of hydrogen and oxygen during electrolysis mode of operation. Image 1 • Dependence of average contact pressure on gasket materials is explored. • Variation in the contact pressure over torque is investigated. • Impact of operating pressure on ICR for various clamping torques for silicon is evaluated. • Characterization of URFC with optimized gasket is done and compared with present state of art system. [ABSTRACT FROM AUTHOR]

The exploitation of a bifunctional oxygen catalyst with high efficiency is crucial for a high‐performance unitized regenerative fuel cell (URFC). However, the existing bifunctional oxygen catalysts still suffer from low catalytic efficiency due to sluggish oxygen electrode reactions toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we report on a facile synthesis of nanoporous Pt‐encapsulated Ir black with Pt/Ir mass ratio of 55/45 (Pt55@Ir45) via a newly modified polyol process at room temperature with the aid of water. The resulting Pt55@Ir45 catalyst demonstrated to be highly efficient and robust for both ORR and OER, relative to a mixture of commercial Pt and Ir black with Pt/Ir mass ratio of 50/50 (Pt50/Ir50). Mass activity of Pt55@Ir45 presented an 8.7‐ and 1.6‐fold increase toward ORR and OER, respectively, compared with the Pt50/Ir50 catalyst. The enhanced bifunctional performance was rationalized in terms of the maximized Pt utilization, achieved by the excellent dispersion of Pt nanoparticles, and the nanoporous Pt layer constructing a conductive network without impeding the transport of oxygen and water molecules. Our work demonstrates an effective means to encapsulate nanoporous Pt layers on Ir black for the fabrication of bifunctional oxygen catalysts. [ABSTRACT FROM AUTHOR]

Mode switching operations between fuel cell (FC) and water electrolysis (WE) modes are indispensable to unitized regenerative FCs. Complicated electrochemical reactions and product transformations occur in the cell during mode switching. Thus, identifying dynamic behaviors during this procedure can improve cell durability and system design. In this study, the dynamic behaviors of cell voltage and electrochemical reaction during the switch from the FC mode to the WE mode are experimentally investigated. Reactant switching time significantly affects the electrochemical reactions. The water pumped into the cell in the FC mode reduces the cell voltage to a negative value and results in a hydrogen evolution reaction at the oxygen electrode side. Before FC mode voltage rapid decrease caused by supplied water, current transition could efficiently avoided the hydrogen evolution reaction at oxygen side. Ensuring that the moment water reaches the channels is close to the moment of current transition can improve the stability of unitized regenerative FCs. • The dynamic response of voltage during mode switching is investigated. • Two-phase flow in oxygen side channels is observed during mode switching. • Hydrogen evolution reaction occurs at oxygen electrode side during mode switching. • Short time interval between reactant and current switching promotes stable operation. [ABSTRACT FROM AUTHOR]

Unitized regenerative fuel cells operate in not only fuel cell but also water electrolyzer mode. Heat management is important for the stable operation of unitized regenerative fuel cells. In this work, temperature and heat flux on the surface of the gas diffusion layer at the hydrogen side of a unitized regenerative fuel cell are experimentally measured using thin film sensors. Four pairs of sensors with good linear relation coefficient are inserted in the unitized regenerative fuel cell. The variation of temperature and heat flux on the gas diffusion layer surface during mode switching is obtained. The effect of mode switching on temperature and heat flux in the unitized regenerative fuel cell is analyzed. Experimental results show that reactant switching significantly affects temperature and heat flux. Reactant switching also causes decreased temperature and variation in heat flux. Despite of the decrease of temperature caused by the low-temperature water, the temperature increases with the operation of the URFC. When the effect of reactant switching is ignored, temperature is further found to increase in fuel cell and water electrolyzer modes, and heat flux remains relatively stable. • Thin film sensors are inserted in a URFC to measure temperature and heat flux. • Reactants switching has significant effect on the temperature and heat flux. • Temperature increases and heat flux maintains steady in both FC and WE modes. [ABSTRACT FROM AUTHOR]

Summary: In this study, the cell performance of nonuniform depth and conventional straight channel in a unitized regenerative fuel cell (URFC) is compared. Various shapes of oxygen‐side channel cases are also proposed. Several parameters, such as the distribution of reactants and products and current density and powers in fuel cell (FC) and electrolytic cell (EC) modes, are investigated. A steady‐state model of two‐dimensional, two‐phase, nonisothermal, and coupled electrochemical reaction is developed. Five oxygen‐side channel shapes are also designed, in which the depth along the flow direction is narrowed. Result shows that narrowing the average channel depth can promote and guide the reactant transfer to the catalyst layer and avoid the blocking of the production. Thus, in comparison with the conventional channel, the cell performances of nonuniform depth and shallow straight channel cases are improved in both modes. In addition, with the decrease of average channel depth, the temperature uniformity gets better, which is also conductive to the improvement of cell performance. Furthermore, in FC mode at low voltage and EC mode, the cell net power basically increases with the decrease of the average channel depth ratio. And when the average channel depth is the same, the net power of straight channel is always lower than nonuniform depth case. This study introduces the round‐trip energy efficiency as an evaluation indicator of URFC. This efficiency can be increased by improving the cell performance of both modes, especially at high current density. [ABSTRACT FROM AUTHOR]

Highlights • A 3D simulation is performed to study cell performance during mode switching. • Local water starvation causes reduction on current density and hydrogen generation. • High inlet velocity and low fuel cell voltage effectively improve cell performance. Abstract Mode switching is essential in a unitized regenerative fuel cell system. The switching of electricity, reactants, and reversible electrochemical reactions occurs, thereby affecting cell performance. During model switching, insufficient reactant supply may cause severe concentration, especially along the width of the catalyst layer which is not currently considered in a low-dimensional model. To fill this gap, a three-dimensional simulation in a proton exchange membrane unitized regenerative fuel cell is performed and validated to investigate the dynamic responses of current density and mass transfer. Results certify that reductions on current density and hydrogen generation are severe, as the cell is switched to a high electrolysis voltage. The analysis of species transfer along the width of the porous layers indicates that the deterioration is attributed to the occurrence of serious concentration polarization which is caused by the severe local water starvation on the catalyst layer, especially below the rib. A high inlet velocity for reactant and low fuel cell voltage are conducive for remitting local water starvation to improve cell performance effectively. However, a slight deterioration cannot be avoided completely before the water supply arrives, which is limited by the high rates of electrochemical reactions at a high electrolysis voltage. [ABSTRACT FROM AUTHOR]

Highlights • Gas purge on URFC voltage during mode switching from EC to FC mode is studied. • High purge gas flow rate is preferred than long purge time during gas purge. • Gas purging in hydrogen side is essential for mode switching from EC to FC mode. • FC mode startup current and EC mode operation current affect mode switching. Abstract In unitized regenerative fuel cells, residual water at the end of electrolysis cell mode greatly affects the cell voltage during mode switching from electrolysis cell to fuel cell mode. Therefore, the residual water removing is essential to realize the mode switching, which requires the knowledge on gas purge effect on cell voltage dynamic response. In this study, gas purge effect in oxygen and hydrogen sides on cell voltage dynamic response is investigated, respectively. In addition, the effects of fuel cell mode startup current density and electrolysis cell mode operating current density are analyzed. The results show that: the increases of gas purging time and purging gas flow rate promote the mode switching from electrolysis cell to fuel cell modes. High flow rate of purging gas is preferred than long gas purging time during mode switching when the same amount of purging gas is used. In addition, gas purging in hydrogen side is necessary. The influence of residual water in hydrogen side increases with the startup current density of FC mode, and long purging time is required for high startup current density of FC mode. Electrolysis cell mode operating current density increase leads fuel cell mode performance decrease under the same gas purge condition. [ABSTRACT FROM AUTHOR]

Highlights • Photovoltaic stand-alone microgrid with a Unitized Regenerative Fuel Cell. • Strip mall building in eight Köppen-Geiger climate zones as case study. • Matlab/Simulink® self-made simulation tool for the power plant management. • Technical, economic and environmental Simulink results comparison against HOMER and grid. Abstract Small commercial buildings in the United States consume 47% of the total primary energy of the buildings sector and, to save energy and mitigate the environmental impact of electricity consumption, distributed generation, involving renewable energy sources, might be a promising solution. In the present paper, a self-made simulation tool has been developed using Matlab/Simulink® to model a stand-alone polygeneration power plant for a strip mall. A photovoltaic panel array has been coupled to a battery and a unitized regenerative polymer electrolyte membrane fuel cell as primary storage/backup systems and a diesel generator as secondary backup system. The possibility of applying the same plant layout and control strategy to eight different climate zones in eight different States has then been assessed. Results show that even in the most favorable conditions in terms of daily radiation and average temperature, such as Miami, Las Vegas or Houston, the cost of electricity of the utility makes the implementation of these systems still inconvenient, unless a reduction in the initial investment costs of above 60% is pursued by means of incentives or a further establishment of the proposed technologies. [ABSTRACT FROM AUTHOR]

Abstract The process during mode switching from electrolysis cell mode to fuel cell mode is unclear. In this work, dynamic responses under different mode switching strategies from electrolysis cell mode to fuel cell mode are investigated using a unitized regenerative fuel cell with an oxygen side transparent window. Effects of time interval between reactant and current switching, gas flow rate, fuel cell mode startup current density, and gas purging time on the cell voltage are studied. The experimental results indicate that pre-reactant switching is an effective way to consume the residual water at the end of an electrolysis cell mode, and a sufficient time interval between reactant and current transition can promote the smooth mode switching under low current density. Oxygen flow rate increase can promote smooth startup of FC mode when the time interval between reactants and current transitions is not long enough, and hydrogen flow rate has little effect on the mode switching from EC mode to FC mode. In addition, the method, combining electrolysis reaction and gas purge, can efficiently eliminate the residual water and prevent water-starved condition when FC mode starts up. Highlights • Influence factors on mode switching from EC to FC modes are experimentally tested. • Pre-reactant switching is suitable for FC mode startup under low current density. • Oxygen flow rate increase promotes mode switching and hydrogen flow rate has no help. • Combine electrolysis reactions and gas purge is an efficient way for mode switching. [ABSTRACT FROM AUTHOR]

Abstract The unitized regenerative fuel cell (URFC) is a round-trip energy conversion device for efficient energy storage systems that offers promising electrochemical energy conversion and environmentally friendly features. The electrocatalyst is a key component for operating URFC unit cell devices. Optimal electrocatalyst materials should be bi-functional with catalytic activity for the oxygen reduction and oxygen evolution reactions (ORR and OER). Over the past few decades, platinum has been recognized as a promising bi-functional electrocatalyst material for the URFC system. However, the ORR and OER activity of Pt is inadequate during the round-trip energy conversion process due to the formation of an oxide layer (PtO x) and the high onset potential for H 2 evolution. To address these issues, extensive effort has been made to enhance the OER performance without affecting the ORR performance. The most efficient alternative electrocatalyst materials comprise combinations of platinum group metals (PGMs) and their oxides, especially Pt Ir, Pt Ir Ru, Pt IrO 2 , Pt Ir IrO 2 , and Pt IrO 2 RuO 2. This comprehensive review emphasizes the potential of various bifunctional electrocatalyst materials for renewable energy generation in the URFC system. Herein, we discuss the limitations of Pt electrocatalysts in the URFC-OER process based on the reaction mechanism. The classification of different bifunctional electrocatalysts is extensively reviewed and highlighted based on the structural, microstructural, fuel cell-ORR, and water electrolysis-OER characteristics, round-trip energy conversion efficiency, inadequacies, and advantages. Taking these features into account, we discuss the possibilities and performance of cost-effective bifunctional electrocatalyst materials for the ORR/OER electro-catalytic process in advanced URFC systems. This review presents an exclusive vision for the development of bifunctional electrocatalyst materials and should stimulate research on bifunctional electrode-based URFC systems. Highlights • URFC is an efficient round-trip energy conversion device with high specific energy. • Bifunctional electrocatalyst is a key component for operating URFC unit cell. • Pt suffered by formation of PtO x layer and high onset potential for H 2 evolution. • Pt/IrO 2 is superior electrocatalyst material for efficient ORR/OER performances. • Cost-effective and novel structured electrocatalysts are an alternative to PGM. [ABSTRACT FROM AUTHOR]

Summary: In this work, a proton exchange membrane unitized regenerative fuel cell with a 25 cm2 active area and a transparent window was designed to study the influence of mode switching from the fuel cell mode to the electrolysis cell mode on the cell voltage and the gas‐liquid two‐phase flow behaviors in the oxygen flow channels. Results indicate that: the growth rate of electrolysis cell voltage before the water pumped to the oxygen flow channels decreases with the increase of the fuel cell current density; while the growth rate of electrolysis cell voltage before the water pumped to the oxygen flow channels increases with the cell temperature; the voltage of electrolysis cell mode before the water pumped to the oxygen flow channels decreases with the increase of water flow rate; the different voltage reduction speeds are attributed to the different water flow rates. The water temperature has an obscure influence on the cell voltage of electrolysis cell mode before the water pumped to the oxygen flow channels. [ABSTRACT FROM AUTHOR]

Summary: Mode switching between fuel cell and water electrolyzer of a unitized regenerative fuel cell alters two‐phase flow dynamics in the channels. To fully understand the mode switching, the observation of two‐phase flow is necessary. In this work, experiments are conducted to investigate the two‐phase flow in the flow field of a unitized regenerative fuel cell during mode switching. A transparent window is assembled at the oxygen side to allow a direct view of the serpentine flow field. The two‐phase flow are captured by a high‐speed camera. Water has a significant effect on the mode switching. The switching from water electrolyzer to fuel cell is difficult because of the flooding problem. High temperature causes insufficient water in water electrolyzer mode without water supply and membrane dehydration in fuel cell mode. The voltage in fuel cell mode decreases more rapidly with fuel cell current density during mode switching. [ABSTRACT FROM AUTHOR]

Summary: Mode switching is an important process in unitized regenerative fuel cells. The complex interactions of heat and mass transfer during the operation of mode switching have a significant effect on cell performance. Twelve different ways of mode switching were proposed by controlling inlet boundary conditions of supplies and operating voltage. Numerical simulations were applied to analyze the dynamic response of heat and mass transfer as well as electrochemical signals under the different ways of mode switching. Current density increased with mass fraction of reactants. Cell heat source had an instant response to current density, but the temperature was slow to respond to the heat source. Hydrogen‐side inlet velocity had minimal impact on mode switching. The time for cell reaching stability increased with the increase of voltage change time, and the time for current density, mass transfer, and temperature reaching stable values increased in order. Unitized regenerative fuel cell had similar dynamic response in the 2 period: cell temperature increased in the fuel cell mode and decreased in the water electrolysis mode after mode switching. [ABSTRACT FROM AUTHOR]

In the present work, a set of five Layered Double Hydroxides, LDH, containing Co and Mn derived electrocatalysts was prepared for the oxygen electrode, evaluating their performances in ORR and OER. A LDH with a Co/Mn ratio of 4, CoMn4, was the reference electrocatalyst of the set, presenting the typical hydrotalcite structure with a trace of MnCo 2 O 4 , with a surface area of 76 m 2 g −1 . HNO 3 oxidized carbon nanotubes (CNT_HNO 3 ) and CNT_HNO 3 with a low content of carboxylic acids (CNT_ HNO 3 tt350), were incorporated into the CoMn4 synthesis, obtaining CoMn4@CNT_HNO 3 and CoMn4@CNT_HNO 3 tt350 with surface areas of 111 and 167 m 2 g −1 , respectively. According to X-ray diffraction (XRD), the oxide phase of CoMn4@CNT_HNO 3 is composed by a mixture of LDH with MnCo 2 O 4 , while the low%COOH on the carbon surface promoted the synthesis of LDHs as the only oxide structure at CoMn4@CNT_HNO 3 tt350. Moreover, after a thermal treatment of the latter electrocatalyst, the LDHs were totally converted into MnCo 2 O 4 . All electrocatalysts showed to have activity over both oxygen reactions when the KOH solution was saturated with O 2 . A detailed ORR study showed that the LDH structures mixed with CoMn oxides, present at CoMn4@CNT_HNO 3 , play a relevant role in ORR, exhibiting an onset potential, E ORR , of −0.274 V. On the other hand, CoMn4@CNT_HNO 3 tt350 led to a four-electron mechanism for ORR, similar to the Pt/C standard. The best OER potential (0.636 V) was also obtained for CoMn4@CNT_HNO 3 tt350. Regarding the oxygen electrode bifunctionality, a good balance between ORR and OER was observed for the CoMn4@CNT_HNO 3 tt350_N 2 composite. [ABSTRACT FROM AUTHOR]

Constant electrode (CE) operation of a unitized regenerative fuel cell (URFC) facilitates the optimization of each electrode. In this work, bipolar plates (BPPs) with different flow fields (FFs) are assessed, and a graphite BPP is compared with a titanium BPP in the cathode. Platinum coated titanium felt and carbon cloth and paper gas diffusion layers (GDLs) are tested in the cathode. The carbon BPP and GDLs operated with higher ohmic resistance than the titanium alternatives. However, the titanium GDL caused flooding in FC mode. Carbon GDLs can be applied in the cathode, where carbon paper operated with higher performance than carbon cloth. Titanium GDL is still required in the anode to resist the corrosive environment. A double-serpentine FF in the cathode and a parallel design in the anode were applied. Optimization of the URFC operating conditions under FC mode provided the best results at 80 °C, 2 bar abs , with saturated air and dry hydrogen and stoichiometry ratios of 2 and 8. This resulted in a round-trip efficiency of 56% at 0.1 A cm−2, 48% at 0.3 A cm−2 and 22% at 1 A cm−2. In comparison, 56%, 52% and 37% were obtained with a discrete regenerative fuel cell (DRFC). [ABSTRACT FROM AUTHOR]

Inkjet printing is evaluated as a feasible technique for unitized regenerative fuel cells (URFCs) electrode fabrication and to study the optimal ionomer and catalyst loading. A physical mixture of Pt, IrO x , and Nafion ionomer was inkjet printed directly on a Nafion membrane to fabricate varying loading bifunctional oxygen electrodes for a URFC and characterized via scanning electrode microscopy and cyclic voltammetry. The results show that the two catalyst do not interfere with one another and that an optimal electrode requires low ionomer loading, i.e., 10 wt%, and significantly lower amount of the IrO x catalyst compared to platinum, i.e., 3:1 Pt-IrO x ratio. A catalyst loading study shows that the highest round-trip efficiency is obtained at around 0.67 mg Pt+IrOx /cm2 and further increasing the amount of catalyst does not result in a significant cell performance improvement either in water electrolysis or fuel cell mode. Overall, inkjet printing technique was used to fabricate thin bifunctional electrodes reaching round-trip efficiencies of up to 50% at 500 mA/cm 2 using a bifunctional oxygen electrode with only 0.67 mg Pt+IrOx /cm2 PGM catalyst loading, resulting in the highest efficiency by amount of catalyst reported to date. [Display omitted] • Low-loading bifunctional mixed Pt-IrO x electrodes fabricated by inkjet printing. • Active area was similar for single-and mixed-layer catalysts. • Round-trip efficiency (RTE) of 50% at 0.5 A/cm2. • Electrode with 0.67 mg/cm2 catalyst, 3:1 Pt to IrO x ratio and 10%wt ionomer. [ABSTRACT FROM AUTHOR]

Severe crisis of energy that our planet is going to face in the near future demands rapid development of alternative energy harnessing and storing devices. Various alternative energy devices have been developed so far, including solar cells, batteries and fuel cells, in order to compete and replace the traditionally used fossil fuel based energy technologies. A very recent addition to the list of such devices is the unitized regenerative fuel cell (URFC), which can function as a dual, i.e., electrolyzer-cum-fuel cell, device. In remote areas, where utilization of conventionally used energy sources results in high expense, URFC can serve as the best energy option, due to its relatively low cost, high efficiency and light weight. URFC utilizes bifunctional electrodes, which function in a dual mode of water electrolysis and fuel cell. The prime focus of URFC research has been to develop various low cost and highly efficient catalysts and catalyst supports towards the fabrication of alternative bifunctional electrodes and to fabricate strong and efficient bipolar plates. The aim of this review is to highlight the main outcomes of these aspects of research on URFCs. [ABSTRACT FROM AUTHOR]

In part A of this review, we have introduced the progress of the research and the application status of unitized regenerative proton exchange membrane fuel cells. In addition to this Proton Exchange Membrane (PEM)-based Unitized Regenerative Fuel Cell (URFC), other URFC technologies with different electrolytes have also been reported in the literature, which form the basis for emphasis in this part of the review. Unitized Regenerative Alkaline Fuel Cells (UR-AFC) have long been utilized for aerospace applications, while the recent development of Anion Exchange Membrane (AEM) has stimulated their further development, especially on the AEM-based UR-AFCs. Vast research works have been reported on the bifunctional oxygen catalyst development, while the latest UR-AFC prototypes are also being briefly introduced. Despite their potential cost-efficiency and better reactivity, cell performance and round-trip efficiency of the current UR-AFCs are still lower than their PEM-based counterparts. Unitized regenerative solid oxide fuel cell, which is more commonly cited as Reversible Solid Oxide Fuel Cell (RSOFC), is a high-temperature URFC technology with superior performance and reversibility. Review works conducted on this type of URFC are separated into two categories, that is, RSOFC with oxygen ion conducting electrolyte and RSOFC with proton ion conducting electrolyte. Despite the highest efficiency among various URFC technologies, the application of RSOFCs, however, is restricted by their limited long-term stability and poor cycle ability. Unitized regenerative microfluidic fuel cell, also referred to as the reversible microfluidic fuel cell, is a newly-emerging URFC research trend which benefits a lot from its membraneless configuration. However, limited research works have been conducted on this new technology. [ABSTRACT FROM AUTHOR]

From extensive reported analyses, we reviewed the limitations, challenges, and advanced developments of the materials and components mainly used in the unitized regenerative fuel cell (URFC) system. URFC is a viable energy storage system owing to its high specific packaged and theoretical energy densities of 400–1000 Wh/kg and 3660 Wh/kg, respectively. Nevertheless, during the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), the stability and durability of the URFC unit cell was severely affected by various degradation factors in the stacked cell. The certain issues are related to the (i) electrocatalysts (high cost, aggregation, migration, and supportive material corrosion), (ii) dissolution and cracks in the Nafion binder, (iii) physical degradation and higher cost of polymer membrane, and severe carbon corrosion in (iv) gas diffusion packing and (v) bipolar plates. Among these factors, the critical challenges are the severe carbon corrosion and durability of the membrane in the unit cell regions. The degradation occurs in the supporting material of the electrocatalyst, gas diffusion packing, and bipolar plate owing to carbon corrosion because of the high applied potential in the water electrolyzer mode. Recent developments are significantly enhancing the durability and overcoming the limitations in the URFC system. In this comprehensive review, we have pointed out the limitations, challenges, and critical developments in URFC systems. Furthermore, built on our experimental and intellectual awareness in the context of URFC system developments, new strategies have been suggested to prepare novel structured materials and composites for advanced URFC applications. [ABSTRACT FROM AUTHOR]

Unitized regenerative fuel cell (URFC) is considered to be the compact solution to generate and utilize hydrogen. It possesses combined capabilities of operating in fuel cell and electrolyser modes. In the present study, the performance of a URFC in electrolyser mode is modelled and also experimentally validated. The performances are being modelled using a combination of structural and CFD analysis tool. The effect of the operating gas pressure on the variation in the contact pressure between GDL and BPP on the performances are studied. The clamping pressure, as well as the operating pressure of the electrolyser, are seen to have a high impact on the contact resistance and thereby the performance as well. It is observed that the simulated polarization behavior is in good agreement with the experimental results. To restrict the area specific resistance below 150 mΩ cm 2 the operating pressure should be maintained below 5.9 bar at clamping pressure of 1.5 MPa. [ABSTRACT FROM AUTHOR]

[Display omitted] • The electricity and mass transfer of URFC are analyzed by experiments combined with simulation methods. • T -type flow channels are proposed and the effects on performance are comparatively discussed. • Start-up performance of URFC is various when using channels having different cross-sections. • Two-phase mass transportation inside URFCs is various and related with performance changing. Different flow channel structures of unitized regenerative fuel cells have great effects on the flow process of reactive gas, liquid product and flowing purge gas inside channels and the transport process of them in the porous media layer. In this study, the unitized regenerative fuel cell with three different kinds of channel structures are designed and manufactured based on the numerical simulation. By utilizing the unitized regenerative fuel cell testing set-up, the output electricity performance test of the single cell in the fuel cell mode, electrolytic cell mode and mode switching process are realized, and effective verification data are provided for the two-phase model verification of unitized regenerative fuel cells. The results show that different channel configurations result in various performance behaviors during different procedures of the cell. Using the channel having 1 mm square cross-section structures can perform better start-up performance, compared to the T -wide and T -narrow section channels. When entering into electrolysis cell mode, compared to the cell with T -section channels, the output performance of the cell constructed with 1 mm square section channel is decreased obviously at the initial stage of start-up because of the residual water affecting. The present experiments combined with simulation works facilitate better understanding how the performance and two-phase species changing at the operating procedures of unitized regenerative fuel cells with different flow channels. [ABSTRACT FROM AUTHOR]

Energy storage and conversion is a very important link between the steps of energy production and energy consumption. Traditional fossil fuels are natural and unsustainable energy storage medium with limited reserves and notorious pollution problems, therefore demanding for a better choice to store and utilize the green and renewable energies in the future. Unitized regenerative fuel cell (URFC), a compact version of regenerative fuel cell with only one electrochemical cell, is one of the competent technologies for this purpose. A URFC can produce hydrogen fuel through an electrolysis mode to store the excess energy, and output power in a fuel cell mode to meet different consumption requirements. Such a reversible system possesses several distinctive advantages such as high specific energy, pollution-free, and most importantly, the decoupled energy storage capacity with rated power. Based on the different electrolytes utilized, current available URFC technologies include the most common proton exchange membrane (PEM)-based URFC, and other types of URFC such as the alkaline, solid oxide and microfluidic URFCs. This part of the URFC review emphasizes on the PEM-based URFC. Specifically, the research progress on both cell components and systematic issues is introduced. Benefiting from its fairly mature technology stage, the PEM-based URFC has already been applied in aerospace and terrestrial areas. However, for large-scale application, their cost and efficiency are still the obstacles when competing with other energy storage technologies. As for the alkaline, solid oxide and microfluidic types of URFC, their research progress is reported independently in part B of this review. [ABSTRACT FROM AUTHOR]

The oxygen electrodes in unitized regenerative fuel cells (URFC) must have high activities towards oxygen reduction reaction (ORR) as well as oxygen evolution reaction (OER), thus requiring high loading of noble metal electrocatalysts. In this study, porous Pt/IrO 2 /carbon paper (CP) electrocatalysts were developed to reduce the metal loading. The Pt/IrO 2 /CP electrodes were fabricated by sequential formation of IrO 2 layers (loading 0.1 mg cm − 2 ) and porous Pt layers (0–0.3 mg cm − 2 ) on CP substrates by electrodeposition and spraying techniques, respectively. The fuel cell (FC) performances increased linearly up to 0.69 A cm − 2 with increasing Pt loading (up to ~ 0.3 mg cm − 2 ) at 0.6 V, whereas the water electrolysis (WE) activity was highest at Pt loading of 0.2 mg cm − 2 . The current densities in the FC and WE modes and round-trip efficiency of the developed Pt/IrO 2 /CP electrodes with the oxygen electrocatalysts loadings of 0.3 and 0.4 mg cm − 2 were higher or comparable to previously reported values with higher loading (1.5–4.0 mg cm − 2 ). These high performances with low loading are probably due to the facile oxygen and water transport through well-developed macropores originating from the open CP structures, providing effective utilization of the IrO 2 and Pt electrocatalysts towards OER and ORR, respectively. [ABSTRACT FROM AUTHOR]

In this paper the losses in coulombic efficiency are investigated for a vanadium/air redox flow battery (VARFB) comprising a two-layered positive electrode. Ultraviolet/visible (UV/Vis) spectroscopy is used to monitor the concentrations c V 2 + and c V 3 + during operation. The most likely cause for the largest part of the coulombic losses is the permeation of oxygen from the positive to the negative electrode followed by an oxidation of V 2+ to V 3+ . The total vanadium crossover is followed by inductively coupled plasma mass spectroscopy (ICP-MS) analysis of the positive electrolyte after one VARFB cycle. During one cycle 6% of the vanadium species initially present in the negative electrolyte are transferred to the positive electrolyte, which can account at most for 20% of the coulombic losses. The diffusion coefficients of V 2+ and V 3+ through Nafion ® 117 are determined as D V 2 + , N 117 = 9.05 · 10 − 6 cm 2 min −1 and D V 3 + , N 117 = 4.35 · 10 − 6 cm 2 min −1 and are used to calculate vanadium crossover due to diffusion which allows differentiation between vanadium crossover due to diffusion and migration/electroosmotic convection. In order to optimize coulombic efficiency of VARFB, membranes need to be designed with reduced oxygen permeation and vanadium crossover. [ABSTRACT FROM AUTHOR]