Membrane based purification of hydrogen system (MEMPHYS)

A hydrogen purification system based on the technology of the electrochemical hydrogen compression and purification is introduced. This system is developed within the scope of the project MEMPHYS. Therefore, the project, its targets and the different work stages are presented. The technology of the electrochemical purification and the state of the art of hydrogen purification are described. Early measurements in the project have been carried out and the results are shown and discussed. The ability of the technology to recover hydrogen from a gas mixture can be recognized and an outlook into further optimizations shows the future potential. A big advantage is the simultaneous compression of the purified hydrogen up to 200 bar, therefore facilitating the transportation and storage.     

Optimization of electrochemical hydrogen compression through computational modeling

One of the main challenges in developing the hydrogen infrastructure is the distribution and storage of hydrogen. A common method to store hydrogen is as a compressed gas. Electrochemical compression (ECC) is a promising technology that can overcome some of the disadvantages of conventional mechanical compressors. ECC employs an externally powered electrochemical cell containing a polymer electrolyte membrane to compress the gas. This work presents a comprehensive 3D ECC model developed for a single cell using COMSOL Multiphysics 5.6 that incorporates all relevant physical and electrochemical processes, and examines the effect of key parameters on ECC performance. It also considers the important phenomenon of back diffusion resulting from the high-pressure differential between the cathode and anode during compression. Results from the current simulations were validated against experimental results obtained previously in our lab. Simulations were first conducted for the unpressurized cathode to understand the effect of membrane thickness, relative humidity of the anode hydrogen supply, temperature, and gas diffusion layer thickness on ECC performance. Next, simulations were conducted for the pressurized cathode, with and without considering back diffusion. In the absence of back diffusion, the pressure ratio reaches the value predicted by the Nernst equation. However, the presence of back diffusion greatly reduces the pressure ratio as was also observed in experiments. The study reveals that three parameters in particular viz. Membrane thickness, operating temperature, and voltage must be carefully selected to optimize ECC operation. These results also suggest that ECC is a viable alternative to conventional technologies for hydrogen compression. This work also provides a foundation for the modeling and analysis of full-scale ECC systems.     

Electrochemical hydrogen compressor: Recent progress and challenges

Hydrogen has higher specific energy than conventional fuels but compared per unit volume under normal conditions, its energy density is lower. This difference is compensated with compression. Theoretically, compression is possible with a proton exchange membrane electrolyzer (PEME), in the process of hydrogen production, but the hydrogen permeation to the oxygen side forms a potentially explosive mixture. An electrochemical hydrogen compressor (EHC) with an analogous working principle presents the most promising solution due to its noiseless and vibration-free operation, modularity, absence of moving parts, and higher efficiency compared to mechanical compressors. Hydrogen purification and its extraction from gaseous mixtures are additional benefits that give electrochemical compression further advantage. This paper discusses the working principle of electrochemical hydrogen compression technology and its design development. The focus is on research trends, recent advances, and transpired challenges. In addition, reviewed literature aspects not studied sufficiently are highlighted, and future research directions are proposed.     

Description and characterization of an electrochemical hydrogen compressor/concentrator based on solid polymer electrolyte technology

Proton-exchange membrane (PEM) technology is commonly used for manufacturing water electrolysers, H₂/O₂ fuel cells and unitized regenerative fuel cells. It can also be used to develop electrochemical compressors, for the purpose of concentrating and/or pressurizing gaseous hydrogen. The aim of the work reported here was to evaluate the main operating characteristics of a laboratory scale (≈10 N liter/h) monocell compressor. The role of various operating parameters (current density, temperature of electrochemical cell, water vapor partial pressure in the hydrogen feed gas, anodic gas composition, etc.) has been evaluated and is discussed. It is shown that the relative humidity of hydrogen oxidized at the anode of the compressor should be adapted to the current density during operation to avoid mass transfer limitations or electrode flooding. A cell voltage of 140 mV is required at 0.2 A cm⁻² to compress hydrogen in one step from atmospheric pressure up to 48 bar, corresponding to an energy consumption of ca. 0.3 kW h/Nm³. Experiments have been performed up to 130 bar. Series connection of several compressors is recommended to reach output pressures higher than 50 bar. To reduce gas cross-permeation effects which can negatively impact the efficiency of the compressor, additional experiments have been made using Nafion membrane modified by addition of zirconyl phosphate. Finally, data related to the extraction of hydrogen from H₂-N₂ gas mixtures are also reported and discussed.     

Recent advances in membrane technologies for hydrogen purification

Planet Earth is facing accelerated global warming due to greenhouse gas emissions from human activities. The United Nations agreement at the Paris Climate Conference in 2015 highlighted the importance of reducing CO₂ emissions from fossil fuel combustion. Hydrogen is a clean and efficient energy carrier and a hydrogen-based economy is now widely regarded as a potential solution for the future of energy security and sustainability. Although hydrogen can be produced from water electrolysis, economic reasons dictate that most of the H₂ produced worldwide, currently comes from the steam reforming of natural gas and this situation is set to continue in the foreseeable future. This production process delivers a H₂-rich mixture of gases from which H₂ needs to be purified up to the ultra-high purity levels required by fuel cells (99.97%). This driving force pushes for the development of newer H₂ purification technologies that can be highly selective and more energy efficient than the traditional energy intensive processes of pressure swing adsorption and cryogenic distillation. Membrane technology appears as an obvious energy efficient alternative for producing the ultra-pure H₂ required for fuel cells. However, membrane technology for H₂ purification has still not reached the maturity level required for its ubiquitous industrial application. This review article covers the major aspects of the current research in membrane separation technology for H₂ purification, focusing on four major types of emerging membrane technologies (carbon molecular sieve membranes; ionic-liquid based membranes; palladium-based membranes and electrochemical hydrogen pumping membranes) and establishes a comparison between them in terms of advantages and limitations.     

Effect of back-diffusion on the performance of an electrochemical hydrogen compressor

Hydrogen offers the potential to decarbonize the automotive and stationary power sectors and is therefore expected to play an increasingly significant role in meeting global energy demand. However, due to its low volumetric and gravimetric energy densities, it is important to find methods to efficiently store hydrogen in order to grow the hydrogen economy. Storing hydrogen as a compressed gas could be achieved by electrochemical compression (ECC), which is a membrane-based alternative to conventional mechanical compressors. ECC can be superior to mechanical compressors because of its higher efficiency, lack of moving parts and noiseless operation. Here, we report on the ECC of hydrogen using a Nafion 115 membrane at room temperature. Pressure vs. time curves have been collected at various operating voltages, and a compression ratio of 150 has been achieved with a single cell at an operating voltage of 0.1 V. This work focuses on the loss in electrochemical compression efficiency due to back-diffusion. A theoretical formulation for the ECC process incorporating back-diffusion is proposed and validated by experiments. A robust definition for ECC efficiency that properly accounts for back diffusion is also proposed.     

Performance analysis and exergoeconomic assessment of a proton exchange membrane compressor for electrochemical hydrogen storage

Electrochemical hydrogen compression (EHC) is a promising alternative to conventional compressors for hydrogen storage at high pressure, because it has a simple structure, low cost of hydrogen delivery, and high efficiency. In this study, the performance of an EHC is evaluated using a three-dimensional numerical model and finite volume method. The results of numerical analysis for a single cell of EHC are extended to a full stack of EHC. In addition, exergy and exergoeconomic analyses are carried out based on the numerical data. The effects of operating temperature, pressure, and gas diffusion layer (GDL) thickness on the energy and exergy efficiencies and the exergy cost of hydrogen are examined. The motivation of this study is to examine the performance of the EHC at different working conditions and also to determine the exergy cost of hydrogen. The results reveal that the energy and exergy efficiency of EHC stack improve by almost 3.1% when operating temperature increases from 363 K to 393 K and the exergy cost of hydrogen decreases by 0.5% at current density of 5000 A m⁻². It is concluded that energy and exergy efficiency of EHC stack decrease by 25% and 5.4% when the cathode pressure increases from 1 bar to 30 bar, respectively. Moreover, it is realized that the GDL thickness has a considerable effect on the EHC performance. The exergy cost of hydrogen decreases by 53% when the GDL thickness decreases from 0.5 mm to 0.2 mm at current density of 5000 A m⁻².     

Optimization of anode performance in the ethanol electrochemical reforming for clean hydrogen production

Carbon-based fuel electrochemical reforming is considered as a promising hydrogen production method. Ethanol is one of the most appropriate carbon-based fuels. In this work, anode performance, especially the flow, ethanol electro-oxidization and energy consumption in the ethanol electrochemical reforming is numerically studied and experimental verified. Take the straight serpentine channel with square cross-section as a base structure in the electrochemical cell (EC), the effects of channel geometry and operating parameters are analyzed. Another five different configurations of flow channels, as well as another three different cross-sections are designed and explored. Results indicate that at the same cross-section area, the wider channel provides the higher effective area for proton transfer, and thereby improves the electrode reactions. The appropriate decrease of inlet velocity or increase of input voltage promotes the anode reaction and reduces the pressure drop in channel, while the operating temperature has the opposite effects on ethanol conversion and pressure drop. The arc channel is found optimal considering its highest ethanol conversion, although its pressure drop is a bit higher. The sector cross-section with uniform flow field distribution is found most favorable for the straight serpentine channel considering the ethanol electro-oxidization. These findings will favor the improvement of EC.     

A fundamental study on electrochemical hydrogen generation from borohydrides

Borohydrides (LiBH₄, NaBH₄, KBH₄, etc.) are the most attractive materials for hydrogen storage due to their high-volumetric and -gravimetric hydrogen density as well as safety issues. Although H₂ for fuel cells is generated by the hydrolysis of borohydrides, it is very difficult to control the rate of H₂ generation due to the nature of the catalytic reaction. In addition, the change in enthalpy (ΔH) of the reaction is directly wasted as heat generation. We propose a method for the electrochemical generation of hydrogen, in which a borohydride in an alkaline solution is oxidized at the anode while water is reduced at the cathode to generate H₂ gas. The cell has a cation exchange polymer electrolyte membrane between a precious metal anode and a Pt cathode to inhibit the crossover of BH₄⁻ anion. The open circuit voltage of the cell is positive, which raises the possibility of spontaneous operation with electrical generation as an alternative to the heat generation in hydrolysis. At the cathode, the rate of H₂ generation coincides well with the current density, indicating that H₂ generation from borohydrides can be electrochemically controlled by means of this hydrogen generator.     

Impact of porous transport layer compression on hydrogen permeation in PEM water electrolysis

Gas permeation through a membrane electrode assembly (MEA) is an important issue in the development of polymer electrolyte membrane (PEM) water electrolyzers, because it can cause explosions and efficiency losses. The influence of operating pressure, temperature and MEA modifications on the permeation was already investigated. However, most of the studies pay no attention to the compression of the porous transport layer (PTL) of the MEA when assembling it in a test cell to carry out the experiments.This paper deals with the impact of the PTL compression on hydrogen permeation and cell voltage. Polarization, impedance and permeation measurements are used to demonstrate that the compression significantly affects the MEA's properties. Measurements show either a linear or nonlinear correlation between current density and hydrogen permeation, depending on the compression.The results indicate that the compression of the PTL must be taken into account for developing MEAs and comparing different permeation measurements.     

Application of hydrides in hydrogen storage and compression: Achievements,outlook and perspectives

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications.With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus.In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized.In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles.In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage”, different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications.     

Pressurized hydrogen from charged liquid organic hydrogen carrier systems by electrochemical hydrogen compression

We demonstrate that the combination of hydrogen release from a Liquid Organic Hydrogen Carrier (LOHC) system with electrochemical hydrogen compression (EHC) provides three decisive advantages over the state-of-the-art hydrogen provision from such storage system: a) The EHC device produces reduced hydrogen pressure on its suction side connected to the LOHC dehydrogenation unit, thus shifting the thermodynamic equilibrium towards dehydrogenation and accelerating the hydrogen release; b) the EHC device compresses the hydrogen released from the carrier system thus producing high value compressed hydrogen; c) the EHC process is selective for proton transport and thus the process purifies hydrogen from impurities, such as traces of methane. We demonstrate this combination for the production of compressed hydrogen (absolute pressure of 6 bar) from perhydro dibenzyltoluene at dehydrogenation temperatures down to 240 °C in a quality suitable for fuel cell operation, e.g. in a fuel cell vehicle. The presented technology may be highly attractive for providing compressed hydrogen at future hydrogen filling stations that receive and store hydrogen in a LOHC-bound manner.     

Effect of airbrush type on sprayed platinum and platinum-cobalt catalyst inks: Benchmarking as PEMFC and performance in an electrochemical hydrogen pump

Proton-exchange membrane fuel cells and electrochemical hydrogen pumps are based on membrane electrode assemblies containing Pt catalysts. During their preparation, a catalyst ink is usually applied to the gas diffusion layer. Among the available methods, ink spray using an airbrush can be advantageous for making electrodes at universities and companies. This work compares the suitability of gravity-feed and suction-feed airbrushes during the evaluation of in-house developed Pt and Pt–Co catalysts. The surface morphology of the catalyst layers was analysed by SEM and EDS and related to the preparation technique. The catalysts were compared against an off-the-shelf commercial catalyst. The in-house Pt catalyst afforded similar polarisation curves as the commercial catalyst whereas the Pt–Co catalyst exhibited a slightly lower performance. The suction-feed airbrush was deemed preferable because it produced cracked mud-like, homogeneous and smoother catalyst layers in contrast to the gravity-feed method, which resulted in poor-quality deposits with loose particles, lower Pt utilisation and higher resistance.     

Effect of substituting Mn for Ni on the hydrogen storage and electrochemical properties of ReNi2.6−xMnxCo0.9 alloys

ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys were prepared by induction melting. The effects of partially substituting Mn for Ni on the phase structure and electrochemical properties of the alloys were investigated systematically. In the alloys, (La, Ce)₂Ni₇ phase with a Ce₂Ni₇-type structure, (Pr, Ce)Co₃ phase with a PuNi₃-type structure, and (La, Pr)Ni₅ phase with a CaCu₅-type structure were the main phases. The (La,Pr)Ni phase appeared when x increased to 0.45, and the (La, Pr)Ni₅ phase disappeared with further increasing x (x > 0.45). The hydrogen-storage capacity of the ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys initially increased and reached a maximum when Mn content was x = 0.45, and then decreased with further increasing Mn content. The ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloy exhibited a hydrogen-storage capacity of 0.81, 0.98, 1.04, 0.83 and 0.53 wt.%, respectively. Electrochemical studies showed that the maximum discharge capacity of the alloy electrodes initially increased from 205 mAh/g (x = 0.0) to 352 mAh/g (x = 0.45) and then decreased to 307 mAh/g (x = 90). The hydrogen absorption rate first increased and then decreased with addition of Mn element. The ReNi_2.15Mn_0.45Co_0.9 alloy showed faster hydrogen absorption kinetics than that of the other alloys. The presence of Mn element slowed hydrogen desorption kinetics.     

Sonochemical synthesis of CeVO4 nanoparticles for electrochemical hydrogen storage

In this study, the synthesis of cerium vanadate (CeVO₄) nanoparticles using ammonium metavanadate, cerium (III) nitrate hexahydrate as the primary reactant and hydrazine as the source of OH^? was presented in the absence and presence of ultrasonic waves. Reaction control was performed using OH^? and ethylenediamine sources. Other parameters such as solvent, surfactant, power, and time were also examined. Nanostructures were analyzed by XRD, FESEM, FTIR, DLS, BET, and EDS. FESEM results showed that using ultrasonic irradiation, relatively fine spherical nanoparticles were formed in one step while uniform spherical nanostructures were formed in a two-step path. The obtained product was used for electrochemical storage of hydrogen. The discharge capacity of spherical nanoparticles of CeVO₄ with high uniformity was recorded at about 4299 mAh/g.     

Metal organic frameworks for hydrogen purification

High purity hydrogen is one of the key factors in determining the lifetime of proton exchange membrane (PEM) fuel cells. However, the current industrial processes for producing high purity hydrogen are not only expensive, but also come with low energy efficiencies and productivity. Finding more cost-effective methods of purifying hydrogen is essential for ensuring wider scale deployment of PEM fuel cells. Among various hydrogen purification methods, adsorption in porous materials and membrane technologies are seen as two of the most promising candidates for the current industrial hydrogen purification methods, with metal organic frameworks (MOF) being particularly popular in research over the last decade. Despite many available reviews on MOFs, most focus on synthesis and production, with few reports focused on performance for hydrogen purification. This review describes the working principle and performance parameters of adsorptive separations and membrane materials and identifies MOFs that have been reported for hydrogen purification. The MOFs are summarised and their performance in separating hydrogen from common impurities (CO₂, N₂, CH₄, CO) is compared systematically. The challenges of commercial application of MOFs for hydrogen purification are discussed.     

Developing a mathematical tool for hydrogen production,compression and storage

Among the few lessons learned presented in the literature, authors put in evidence the on-going need to investigate on station components and their integration. The specific power consumption of station units with on-site hydrogen generation is often subject to uncertainty, and it would have been desirable to find more details about the energy contribution of each component. To address this gap, this paper focuses on the development of a mathematical modeling as a dynamic and multi-physical design tool to predict the energy performance of hydrogen production systems. Particularly, the model aims to describe and analyze the energy performance of two different electrolyzer technologies (PEM and Alkaline), integrated with a compressor system and gaseous buffer storage. Multiple tank options and a switching strategy are investigated, as well as a control system to simulate a real infrastructure operation. Auxiliaries and components related to the thermal management system have been also included. A carbon-footprint analysis follows the energy one, focusing on the CO₂ emission reduction. Comparisons between literature data and model show that the hydrogen system proposed model is suitable to evaluate systems with respect to energy efficiency and system performance. The model could be a powerful tool for exploring control strategies and understanding the contributions to the overall energy consumption from the various internal components as a guide to researchers aiming for improved performance.     

Vehicle refueling with liquid hydrogen thermal compression

We have modeled an approach for dispensing pressurized hydrogen to 350 and/or 700 bar vehicle vessels. Instead of relying on compressors, this concept stores liquid hydrogen in cryogenic pressure vessels where pressurization occurs through heat transfer, reducing the station energy footprint from 12 kW h/kgH₂ of energy from the US grid mix to 1.5–2 kW h/kgH₂ of heating. This thermal compression station presents capital cost and reliability advantages by avoiding the expense and maintenance of high-pressure hydrogen compressors, at the detriment of some evaporative losses. The total installed capital cost for a 475 kg/day thermal compression hydrogen refueling station is estimated at about $611,500, an almost 60% cost reduction over today's refueling station cost. The cost for 700 bar dispensing is $5.23/kg H₂ for a conventional station vs. $5.45/kg H₂ for a thermal compression station. If there is a demand for 350 bar H₂ in addition to 700 bar dispensing, the cost of dispensing from a thermal compression station drops to $4.81/kg H₂, which is similar to the cost of a conventional station that dispenses 350 bar H₂ only. Thermal compression also offers capacity flexibility (wide range of pressure, temperature, and station demand) that makes it appealing for early market applications.     

Parametric study on electrochemical reforming of glycerol for hydrogen production

Hydrogen production by electrochemical reforming of glycerol was investigated in this study. Within this scope, the performance of the system under different operating conditions was evaluated by parametric studies and optimum operating conditions were determined. The effects of membrane type, membrane pre-treatment procedure and temperature were investigated. System performance was examined also with long-term tests. The formation of hydrogen at the cathode was determined by analyzing the product gases by gas chromatography. Optimum condition for maximum hydrogen production was obtained with the Zn/Zn electrode pair in the presence of 0.4 M glycerol and 0.04 M H₂SO₄ at the anode side, 0.04 M H₂SO₄ at the cathode side and with pre-treated Nafion XL membrane. As the result of performance tests, room temperature and 2 V potential were found to be the most suitable operating conditions.