- Email: [email protected]
Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator Arne Fallisch a,*, Leon Schellhase a, Jan Fresko a, Martin Zechmeister a, Mario Zedda a, Jens Ohlmann a, Lukas Zielke b, Nils Paust b, Tom Smolinka a a b
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg, Germany Department of Microsystem Engineering e IMTEK, Georg-K€ohler-Strasse 105, 79110 Freiburg, Germany
article info
abstract
Article history:
Hydrogen as a secondary energy carrier promises a large potential as a long term storage
Received 17 October 2016
for fluctuating renewable energies. In this sense a highly efficient solar hydrogen genera-
Received in revised form
tion is of great interest especially in southern countries having high solar irradiation. The
24 January 2017
patented Hydrogen Concentrator (HyCon) concept yields high efficiencies combining
Accepted 25 January 2017
multi-junction solar cells with proton exchange (PEM) membrane water electrolysis. In this
Available online xxx
work, a special PEM electrolysis cell for the HyCon concept was developed and investigated. It is shown that the purpose-made PEM cell shows a high performance using a ti-
Keywords:
tanium hybrid fiber sinter function both as a porous transport layer and flow field. The
Solar hydrogen production
electrolysis cell shows a high performance with 1.83 V at 1 A/cm2 and 24 C working under
PEM water electrolysis
natural convection with a commercially available catalyst coated membrane. A theoretical
IIIeV solar cells
examination predicts a total efficiency for the HyCon module from sunlight to hydrogen of
Porous transport layer
approximately 19.5% according to the higher heating value. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A secure and efficient energy supply is one of the key challenges in the 21st century. To decrease the dependency on fossil fuels like coal, oil and gas and also nuclear energy and to reduce the temperature rise due to global warming [1], renewable energy sources (RES) like solar, wind, biomass and water power are on the rise all over the world. In Germany the government has announced the “Energiewende” with the goal to increase the share of RES at least to 80% in 2050 [2]. In 2015 the market share of renewable energy in Germany is around
33% for the gross electricity consumption [3]. Due to the fluctuating nature of renewable energies, energy storage becomes a very important issue. Several solutions seem possible to overcome those challenges. E.g. battery storage systems for photovoltaics are already available for short term storage. To store energy from RES in the GWh or even TWh range chemical energy storage via hydrogen or further processed derivatives becomes a crucial necessity. Hydrogen can be used in mobile and stationary power applications. Therefore water electrolysis is considered a key component of future energy systems. The proton exchange membrane (PEM) water electrolysis is especially applicable for highly dynamic operation. It can
* Corresponding author. E-mail address: [email protected] (A. Fallisch). http://dx.doi.org/10.1016/j.ijhydene.2017.01.166 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
operate efficiently in the part-load regime. Furthermore using water is easier to handle compared to potassium hydroxide which is needed for alkaline electrolysis. Therefore PEM electrolysis is used for the patented Hydrogen Concentrator (HyCon) concept [4e7]. In contrast to conventional systems, which combine a photovoltaic system with a stand-alone electrolysis system [8,9], the HyCon concept links concentrating photovoltaics [10] with PEM electrolysis without any additional power electronics. Besides the advantage of missing power electronics, the electrical interconnection of the multi-junction solar cells (SC) is saved, as well as the interconnection of several sun tracking devices to the power electronics. Therefore a failure of a single SC leads only to a small reduction in hydrogen production and not to a breakdown of the whole hydrogen generator. Another advantage is the thermal management of the HyCon concept. The water flowing through the electrolysis cell (EC) is able to dissipate the heat of the SC which is beneficial for the voltage of the SC, as well as for the EC. A schematic view of the HyCon concept is shown in Fig. 1 and explained in the following. A Fresnel lens is used to focus the sunlight on a special tailored multi-junction SC. This SC is directly connected to the anode side of the PEM EC. Due to this direct connection, this concept yields a very high conversion efficiency from sunlight to hydrogen. The current generated by the SC flows via a conductor through a porous transport layer (PTL) to the anode electrode where the oxygen evolution reaction (OER) takes place. The PTL does not only transport the electric current but
Fig. 1 e Schematic view of the HyCon concept. A Fresnel lens focuses the sunlight on a multi-junction solar cell. This solar cell is directly connected to the anode of the electrolysis cell. The current is transported via the anode plate and porous transport layer to the electrode. The proton crosses the polymer electrolyte membrane to the cathode side. The electric circuit is closed by an electrical wire connecting the cathode side to the front side of the solar cell.
also the water and the gas. The remaining protons are transported through the proton exchange membrane to the cathode, where they are reduced to hydrogen, also called hydrogen evolution reaction (HER). The cathode side is connected to the front side of the solar cell using an electrical wire. In this work the focus lies on the PEM electrolysis cell design and the used PTLs. In particular PTLs made from titanium are one of the cost drivers in PEM electrolysis cells [11]. It is investigated how the cell behaves under similar conditions as they will occur in an outdoor environment. In the next section the EC design and indoor test equipment will be elaborately explained, followed by a detailed analysis of the performance, the flow behavior and the efficiency of the EC.
Experimental setup and used cell components PEM electrolysis cell design In this work three different electrolysis cell designs are used. These are the EC generation II, III and IV (Gen II, Gen III, Gen IV) which contain some specific design features which will be explained in the following. The main feature of the EC Gen II is an adjustable compression of the PTLs and the catalyst coated membrane (CCM) by a titanium (Ti) screw. These screws are inserted into a chlorinated polyvinyl chloride (CPVC) plate to contact the PTL. An exploded view of the design is shown in Fig. 2 on the left hand side. Both CPVC plates have inlets and outlets for the DI water and the gases. Although these cells can be sealed without any leakage, this design has several drawbacks, like an inhomogeneous compression and a fluidic bypass of the PTL due to the manufacturing process. In the HyCon module the solar cells are mounted on the Ti screw. This is another disadvantage of this design. Due to the length of the Ti screw of 19 mm the thermal coupling between the SC and the EC is very low [12]. Nevertheless an evaluation of different PTLs was done with this EC setup which will be discussed in the next section. The disadvantages are already overcome in the EC Gen III design. In this design, the Ti screw inserted into the CPVC plate in Gen II is replaced by a 4 mm Ti anode plate as it is depicted in Fig. 2 in the middle. The advantage of this design is that the SC is mounted directly on top of the anode plate, which increases the thermal coupling. In Gen III the PTL is placed in a milled deepening within the anode plate. On the cathode side a plastic plate holding a Ti pin is used. This plastic plate also contains all fluidic connections to the cell. The fluidic connections and the cathode plate material are the main differences between Gen III and Gen IV. For Gen IV the fluidic connections were moved from the bottom to the sides. Furthermore Polyphenylene sulfide (PPS) instead of CPVC material is used as this material has a higher mechanical, chemical and thermal stability. All ECs are sealed on the cathode side by an O-ring between the cathode plate and the CCM. The ECs are mounted with four screws, washers and nuts. All ECs are assembled with the same PTL on both sides. Certain authors claim that the combination of CPV with PEM electrolysis can achieve low investment costs [13,14]. However, in this work the EC design was optimized to give an optimum performance and is at this stage not cost-optimized.
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 2 e Figure shows on the left side an exploded view of the design of the HyCon EC Gen II. The main feature of the EC is an adjustable compression by Ti screws which are put into a CPVC plates. In the middle an exploded view of HyCon EC design Gen III is shown. On the anode side, the Ti screw is replaced by a Ti plate which contains a milled deepening for the PTL. On the cathode side a Ti pin which is put into a plastic plate is used to contact the PTL. In contrast to Gen IV on the right side, all fluidic connections are at the bottom. All ECs are assembled with four screws with nuts and washers. An O-ring on the cathode side is responsible for tightening.
Selected cell components Several PTLs were chosen for investigation within the HyCon EC. All tested PTLs consist of titanium. Basically these PTLs vary in thickness, porosity, the production process and in pricing. The thickness of the PTLs varies from 0.3 mm to 1.5 mm and their porosity ranges from 40% to 74%. Most PTLs are sintered Ti fibers. Furthermore one hybrid PTL and one sintered Ti powder is investigated. The hybrid PTL consists out of a 0.8 mm expanded mesh with a 0.2 mm sintered fiber on top. A complete list of all used PTLs is shown in Table 1. Their morphological and transport parameters were calculated and compared in an X-ray tomographic study [15]. All PTLs were delivered in big sheets. Thus, cylinders with a diameter of 12 mm had to be cut out these sheets to tightly fit into the HyCon cell. Water jet cutting has proven to be an adequate method, if the water inlet is located at the cross section of the PTL and no flow field structures are used as in EC Gen II. Laser cutting is also possible but leads to melting of Ti and thus reducing the open pores along the cut surface.
Commercially available CCMs were used for all examinations in this work. In the first experiments CCMs based on Nafion 117 membranes were used. For later experiments CCMs based on Nafion 115 were used. For the oxidation reaction of oxygen at the electrode on the anode side typically iridium is used as catalyst [16]. On the cathode side mostly carbon-supported platinum is used as a catalyst [16]. However, the exact composition of the electrodes is not known. All other conducting EC parts consist of Ti grade 2, like the Ti screws or pins and the anode plate in EC Gen III and Gen IV (compare Fig. 2). The screws used for mounting as well as the screw fittings consist of stainless steel.
Test bench for PEM electrolysis cells All experiments were carried out on a purpose made test bench for HyCon PEM ECs operating under atmospheric conditions. A piping and instrumentation diagram (P&ID) is depicted in Fig. 3. The water can be circulated on the anode and cathode side. Therefore a MCP Process hose pump with
Table 1 e List of all PTLs that were characterized in this work. The PTLs vary in thickness, porosity, the production process and the price. The thickness reaches from 0.3 to 1.5 mm and their porosity ranges from 40 to 74%. Most PTLs are sintered Ti fibers. One PTL is a sintered Ti powder and one is a hybrid consisting out of an expanded Ti mesh and a sintered Ti fiber. No 1 2 3 4 5 6 7 8
Name Ti-FS e 1.5-51 Ti-FS e 1-50 TiPt-FS e 1-57 Ti-PS e 1-40 Ti-FS e 0.95-74 Ti-FS e 0.5-73 Ti-FS e 0.3-56 Ti-HFS e 1-50
PTL material Ti Ti Ti Ti Ti Ti Ti Ti
fiber sinter fiber sinter Pt fiber sinter powder sinter fiber sinter fiber sinter fiber sinter hybrid fiber sinter (expanded mesh and fiber)
Thickness 1.50 1.00 1.00 1.00 0.95 0.50 0.30 1.00
mm mm mm mm mm mm mm mm
Porosity 51% 50% 57% 40% 74% 73% 56% 50% (only fiber)
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 3 e Simplified piping and instrumentation diagram of the test bench, which has been used for investigation. The electrolysis cell can either be operated under natural convection or under forced convection with a constant water flow rate which is adjusted at the pumps. All inlet and outlet temperatures are measured, as well as the pressure drop across the electrolysis cell. Furthermore the amount of produced hydrogen is measured with a gas counter.
a 4 channel pump head from Ismatec is used. The pump can be bypassed, if the electrolysis cell should run under natural convection. The water volume flow is measured directly after the pump using Bronckhorst coriolis flowmeters type M14-ABD-33-O-S. To maintain a high purity of the water, ion exchange resins are used. The tubular heat exchangers, which were designed and built at Fraunhofer ISE, are placed as near as possible to the electrolysis cell to maintain the required temperature at cell inlet. The heat exchangers are connected on the secondary side to a cryostat type CF31 from Julabo to control the temperature. The temperature is measured at all inlets and outlets of the EC with PT100 sensors. Furthermore another PT100 sensor can be used to measure the temperature near the anode, if the cell design is adapted. The temperature control can be adjusted in the software to any of those sensors. Temperatures between 10
and 80 C can be preset when the circulation pumps are used. Furthermore the differential pressure between inlet and outlet of the anode is measured using a Keller differential pressure sensor type PD-23. Downstream the gas water separators, which are at the same time the water reservoir, pressure sensors from Huba Control are installed. Both gas water separators are interconnected with a pipe. With a ball valve in the middle of this connection, the interconnection can be disrupted. This is needed for the measurement of the amount of hydrogen. After the gas pressure sensors, PT100 temperature sensors are used to measure the gas temperature. Afterwards the gas is led through a gas meter from Ritter type MGC-1. Additionally the ambient temperature and pressure is measured. The EC is positioned on a support that can be tilted to simulate different positions of the tracker.
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
To simulate the current generated by the solar cell, a Delta Elektronika power supply from the ES 150 series is used. The power supply can operate in a voltage range between 0 and 30 V and currents from 0 to 5 A. Thus more than one EC can be electrically interconnected in series. The current is measured using 3 shunts in parallel each with a resistivity of 0.1 U. The voltage is measured directly at the EC. All generated data are recorded by an Agilent 34790A data logger. The data logger, as well as all active components, are connected to a PC were a custom-programmed LabView software records all data in 3 s time steps. The software allows fully automated measurements, e.g. polarization curves at different temperatures.
Results and discussion Screening of different porous transport layers All eight PTLs as presented in Table 1 were investigated with the EC Gen II design. The Ti screws were adjusted on both sides in such a way that the remaining pocket depth for the PTL is 0.05 mm less than the thickness of the PTL. Thereby a suitable compression was achieved as the measured high frequency resistance below 300 mU cm2 shows. The ECs were always mounted with the same PTL on both sides and a Nafion 117 based CCM in between. Polarization curves were logged in galvanostatic mode with the test bench described in the previous section. The current density i was varied from 8 mA/cm2 to 3 A/cm2. The water volume flow Q on anode and cathode side was kept constant to 15 ml/min. The temperature T was controlled to the anode inlet to 30 C. The measured polarization curves of the ECs using all eight PTLs are depicted in Fig. 4. The overall performance varies depending on the used PTL. The EC with PTL Ti-HFS e 1-50 shows the lowest voltage for all current densities, followed by the EC with PTL Ti-FS e 0.5-73 and TiPtFS e 1-57. All other PTLs show significant higher voltages. At the desired operation point at a current density of 1 A/cm2 the voltage varies from 1.85 to 1.92 V. The difference increases at
5
higher current densities reaching up to 260 mV at 3 A/cm2. The differences in the polarization curves for different PTLs cannot be explained finally. The PTLs Ti-FS e 0.95-74, Ti-FS e 0.5-73, TiPt-HFS e 1-57 and Ti-HFS e 1-50 show the highest performance according to Fig. 4. Generally, it seems that a higher porosity leads to a higher performance. PTLs with a lower porosity tend to perform worse, which is partly confirmed in Ref. [15] where all PTLs except the Ti-HFS e 1-50 have been investigated in an X-ray tomography study. It could be shown (refer to second table in Ref. [15]), that some PTLs have a much higher permeability in in-plane direction. These correspond to the PTLs with a high porosity. For PTLs with a lower porosity Ti-HFS e 1.5-51, Ti-FS e 0.356, Ti-FS e 1-50 and Ti-PS e 1-40 a step of the voltage around 2 V is visible. Several reasons might be responsible for such an increase but a final explanation cannot be provided with the help of the measured polarization curves. One possible reason could be mass transport limitation due to drying of the membrane above a given current density and thus established two phase flow regime in the PTL or at the interface towards the electrode. Either not enough water is reaching the membrane or an effective blockage of the active area at the electrode surface by bubbles takes place [17]. However, this effect should result in further increasing voltages for increasing currents but instead, at higher voltages the slope starts to decrease again to the initial value at lower current densities. Another possible reason could be transient behavior of the interfacial contact resistance between the electrode and the PTL due to changing morphology. Above 2.1 V and a pH-value of 3, the Ti and its oxide layer on the surface may alter according to the Pourbaix diagram [18]. This will have an impact on the interfacial contact resistance as well as the hydrophilicity. A change in the hydrophilicity also influences the fluidic behavior of water within the PTL. Thus an increase in the hydrophilicity of the PTL could compensate the effect of mass transport limitation due to a poor water supply of the active area. Some of these PTLs are currently under investigation in a 25 cm2 large EC under pressure and with additional ex-situ characterization. These results will be published in the future. As the PTLs Ti-HFS e 1-50 and Ti-FS e 0.5-73 result in the lowest EC voltages and are also the cheapest PTLs, these PTLs were used in all following experiments.
Investigation on porous transport layers with EC Gen III
Fig. 4 e Figure shows polarization curves using different PTLs. The EC with Ti-HFS e 1-50 shows the lowest voltage of all PTLs followed by Ti-FS e 0.5-73 and TiPt-FS e 1-57.
The electrolysis cell EC design Gen III was tested with the PTLs Ti-HFS e 1-50 and Ti-FS e 0.5-73 which showed the highest performance in the EC Gen II. In this examination a commercially available CCM with a Nafion 117 membrane was used. The pocket depth of the anode and cathode compartments was 0.05 mm less than the total thickness of the PTL. This leads to a compression of 5% of the total thickness in case of the Ti-HFS e 1-50 and to a compression of 10% in case of the Ti-FS e 0.5-73. Before the polarization curves were recorded, the total cell impedance was measured at a frequency of 1 kHz at room temperature. For the EC with a Ti-HFS e 1-50 a high frequency resistance (HFR) of 258 mU cm2 is found. The HFR of 266 mU cm2 of the EC with Ti-FS e 0.5-73 is in the same range. As the main resistance of the EC is attributed to the
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
membrane, the electrical conductivity and interfacial contact resistances of the PTL are of minor importance. Higher compression rates of 10% for the Ti-HFS e 1-50 were tested, which did not lead to a lower resistance and lead to the same performance. This proved that the cell assembly of the electrolysis cell was successful. Polarization curves were logged in galvanostatic mode at different temperatures between 10 and 60 C. The current density i was increased stepwise from 8 mA/cm2 to 3 A/cm2. The volume flow Q on anode and cathode side was kept constant to 10 ml/min. Fig. 5 shows the polarization curves of both EC setups with different PTLs. The typical temperature dependence e decreasing voltage for increasing temperature e mainly due to improved kinetics at the anode and an increased proton conductivity of the membrane is clearly visible. The performance of both EC setups is very similar. At the desired operation point at iop ¼ 1 A/cm2 [7] the voltage is below 2 V for temperatures above 20 C. Based on these results it is of minor concern which PTL is used to achieve the highest performance. Therefore another consideration is of importance which will be addressed in the following section.
Differential pressure drop across the EC As operation of the HyCon ECs under natural convection would be beneficial, the pressure drop across the EC on the anode side was investigated for different water flow rates at zero current. The results are depicted in Fig. 6. The increase of the differential pressure is constant with increasing volume flow (this cannot be seen in Fig. 6 due to logarithmic scale). Moreover a fluidic bypass in Gen II ECs due to manufacturing processes is present. It is found that the theoretical pressure drop for the Ti-FS e 0.5-73 should be more than two orders of magnitude larger than the measured pressure drop (see the simulated solid curve in Fig. 6). The pressure drop via the EC is mainly determined by the fluidical resistance of the PTL. Here, the theoretical pressure drop was calculated using Darcy's law with permeabilities acquired by simulating water flow
Fig. 6 e Figure shows the pressure drop across the EC for Gen II and III with different PTLs depending on the volume flow. The EC Gen II has a bypass of the PTL which leads to a 100 times lower pressure drop. For both EC generations the Ti-HFS e 1-50 PTL shows a lower pressure drop. through a reconstructed volume of the PTL using the finite difference solver GeoDict. For the reconstruction, X-ray tomography was used [15]. As the theoretical permeability of the PTL are known from the finite difference solver results, bypasses can artificially be calculated using the free and porous flow module in COMSOL. Such a study yielded that the bypass is between 400 mm and 600 mm large (see dotted curve in Fig. 6). This bypass is reduced in generation III ECs resulting in a differential pressure drop via the EC, which is about a factor of 100 higher. The fluidic resistance of Gen III EC is in excellent accordance with the simulation of the fluidic resistance of the Ti-FS e 0.5-73 PTL (exp.: 1.01 1011 kg/m4s, sim.: 1.03 1011 kg/m4s), which demonstrates that the fluidic bypass was successfully eliminated. As this approach (1. X-ray reconstruction 2. permeability calculation using GeoDict and 3. fluidic resistance calculation using Darcy's law) is able to validate experimental pressure drop curves, it is suited for virtually designing PTLs. This means that morphological features can be introduced or altered and the influence of the introduction/altering can be reliably estimated without conducting any experiments. In the experiments the Ti-HFS e 1-50 PTL shows a lower differential pressure for both ECs Gen II and III. Therefore TiHFS e 1-50 was chosen as a PTL for all following investigation.
Impact of natural convection on cell performance
Fig. 5 e Figure shows polarization curves with two different PTLs in EC Gen IV. At the desired operation point at i ¼ 1 A/cm2 the voltage is below 2 V for temperature above 20 C.
To investigate how the EC will perform under more realistic condition in the outdoor module, the EC design Gen IV was analyzed working under natural convection with bypassed circulation pumps. The test bench support, carrying the EC, was tilted to simulate the position of the tracker in the outdoor module. Furthermore the cathode inlet was closed. The polarization curves were recorded starting from 1 A/cm2 going down to 0.1 A/cm2 at room temperature with no active heating or cooling. The higher limit of 1 A/cm2 was chosen as this is also the desired operation point for the HyCon outdoor
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
module. Each current density step was kept constant for 30 min. In this experiment a CCM with a Nafion 115 membrane was used. In Fig. 7 several parameters are depicted with the operation time on the x-axis. At the top of Fig. 7 the inlet and outlet temperature as well as the temperature measured within the Ti anode plate is shown. The inlet temperature of the anode corresponds to the room temperature as the total amount of water in the whole system is much larger than the circulated water. The outlet temperatures of the anode and cathode are slightly higher. The temperature near the anode plate is much higher for high current densities due to the Joule heating. The volume flow is fluctuating between 0.1 and 0.6 ml/min. This volume flow is not sufficient to keep a constant temperature of the EC at its active area, leading to the higher temperature of the anode and the water outlets. The differential pressure behaves the same way as the volume flow. This repetitive fluctuation of the water flow rate is also visible in the transparent hose lines. In the lower section of Fig. 7 the current densities and the cell voltages are shown. It is visible that the natural convection has no negative effect on the cell voltage despite the fluctuating water flow. This means that no mass transport limitations occur as this would lead to an increased voltage. If the polarization curve under natural convection is compared to a polarization curve obtained with a constant water flow, a slightly lower cell voltage, especially for high current densities, can be seen for an EC working under natural convection. These polarization curves are depicted in Fig. 8. Here a membrane based on Nafion 115 was used. The slightly lower voltages at high current densities around 1 A/cm2 are due to a higher anode plate temperature. The temperature of the EC working at 1 A/cm2 under natural convection is approximately 24 C. For the fixed volume flow of approximately 5 ml/min the anode temperature was controlled to 20 C. For low current densities the anode temperature drops down to 15 C for current densities below 0.2 A/cm2 due to the
7
Fig. 8 e Figure shows polarization curves measured under natural and forced convection. The EC working under natural convection shows a slightly lower voltage for current densities above 0.7 A/cm2.
thermal inertia of the temperature control. For high current densities above 0.8 A/cm2 it is between 20 and 21 C. In this prospect, working under natural convection is beneficial to reduce the operation voltage.
Faraday efficiency As stated in the beginning, high efficiencies from sunlight to hydrogen can be achieved using the HyCon concept. There are several loss mechanisms inside an EC which decrease the efficiency. As ohmic and activation overpotentials can easily be taken into account by simply measuring the EC voltage, Faraday efficiency is not that easy to determine. The amount of hydrogen produced by a single cell with an area of 1.13 cm2 is rather low. It takes about 32 min to generate 1 L of hydrogen at a current density of 1 A/cm2. To measure this amount
Fig. 7 e Figure shows the voltage, current density and temperatures over time of an EC working under natural convection. It is visible that the anode temperature Tanode is much higher for high current densities as the heat is not fully transferred by the water flow. Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
leads to certain uncertainties some values do not always follow the trend, especially for current densities between 0.3 and 0.5 A/cm2. For high current densities above 0.8 A/cm2 the Faraday efficiency is above 98.5%. This value corresponds well to results from Grigoriev at 2 bar [20]. For the lower current densities regime the Faraday efficiency is always above 94%. According to Grigoriev approximately 0.5% of H2 in O2 were detected in an EC with a Nafion 117 membrane at 1 bar pressure and 1 A/cm2. As in this work the membrane is a Nafion 115 membrane, a little bit more than 0.25% of H2 is expected to cross the membrane. The remaining losses are attributed to leakage of the EC and the test bench. For the estimation of the maximum efficiency from sunlight to H2 in the next section, a Faraday efficiency of 98.5% is taken into account.
Prediction of the operation point of HyCon cells Fig. 9 e Faraday efficiency of an EC with Nafion 115 membrane. The trend shows an increasing Faraday efficiency for increasing current densities. Above current densities of 0.8 A/cm2 the Faraday efficiency is above 98.5%.
precisely, a special gas counter from Ritter was used with a remaining error less than 1% from the total flow range. The measured gas volume was corrected to norm volume. Therefore the gas is assumed to be 100% saturated with water and partial pressure of the water vapor depending on the gas temperature is taken into account [19]. An EC with Nafion 115 based membrane was used to determine the Faraday efficiency. The current density was varied between 0.1 and 1 A/cm2. Each current density step was kept until a theoretical H2 volume of 0.5 l was produced. The water volume flow was adjusted to 5 ml/min at room temperature. As the amount of O2 in H2 is in the ppm regime [20] it is neglected. This was also known from other experiments carried out at Fraunhofer ISE. In Fig. 9 the measured Faraday efficiency in dependence on the current density is shown. In principle, it can be seen that the Faraday efficiency is increasing with increasing current density. Due to the complexity of the measurement which
As shown in the previous section the HyCon EC Gen IV with a Ti-HFS e 1-50 as PTL shows promising results on an EC level. Therefore this design was taken to manufacture 10 ECs at the mechanical workshop of Fraunhofer ISE. These ECs were assembled with a commercial available CCM with a Nafion 115 membrane and Ti-HFS e 1-50 as PTL on both sides. Afterwards all ECs were conditioned at 80 C with half-hourly alternating current densities of 1 and 2 A/cm2 for 12 h. To speed up the process two ECs were connected fluidically in parallel at the test bench. A fixed volume flow of Q ¼ 10 ml/min was used for the conditioning and the following measurement of polarization curves at temperatures between 10 and 60 C. In Fig. 10a) the polarization curves of all 10 ECs at 30 C are depicted. The manufacturing and assembling process results in a high reproducibility. This is visible regarding the voltage at the desired operation point of 1 A/cm2, which lies between 1.79 and 1.83 V for all ECs. To estimate the operation point which can be achieved in the outdoor module, the iV-curve of the solar cell has to be taken into account. In Ref. [12] a metamorphic GaInP/GaInAs solar cell was built into an electrical mono module and characterized in an outdoor measurement. The used lens size was 90.7 cm2 with a solar cell size of 0.36 cm2 leading to a geometrical concentration of 252. The iV-curve at an irradiation density of 870 W/m2 is shown in
Fig. 10 e a) Polarization curves of 10 equally fabricated ECs are shown. Two ECs were fluidically connected in parallel. b) Prediction of the operation point in a HyCon module is shown. The operational current density lies above 0.9 A/cm2 at a voltage below 2 V. This yields a maximum efficiency of 19.5% taking into account a Faraday efficiency of 98.5%. Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 10b). The cross-section with the polarization curves of the EC measured at 30 C and 60 C marks the predicted operation point of the module. In this case the operation current density is iop ¼ 0.93 A/cm2 for 30 C and 60 C. The operation voltage changes from Vop ¼ 1.84 V-1.66 V if the temperature is increased to 60 C. In the HyCon module a measurement resistor, a relay and cables will be implemented in between the connection from the SC to the EC. Therefore an ohmic resistance of Rs ¼ 100 mU is assumed in series with the EC. This resistance increases the operation voltage of the SC to Vop ¼ 1.89 V for an EC operation temperature of 30 C. This leads to small reduction of the operational current density to iop ¼ 0.92 A/cm2. A reduction of the current density has also an effect on the efficiency of the HyCon cell. The total efficiency from sunlight to the amount of stored energy within hydrogen according to the higher heating value is calculated by εHyCon ¼ εF
Iop Vth PDNI
(1)
where Iop is the current generated at the operation point, Vth is the thermoneutral voltage of 1.48 V and PDNI is the irradiation. Calculating the efficiency according to equation (1) leads to maxima of 19.5% at a temperature of 60 C and 19.4% at 30 C taking into account the Faraday efficiency of εF ¼ 98.5%. If the resistances are included the efficiency is reduced to 19.4% and 19.2%, respectively. As the iV-curve of the solar cell and the polarization curves are highly temperature dependent, it is hard to predict the real behavior in the module. However, temperatures above 30 C seem reasonable for summer in an outdoor environment.
Conclusion and future work Several PTLs have been investigated in different EC design. It has been shown that the Ti hybrid fiber sinter PTL Ti-HFS e 1-50 shows the highest performance in all electrolysis cell design from Gen II to Gen IV. As this PTL is relatively cheap and causes only a low pressure drop, it was used in all EC that will be integrated into a real HyCon module for outdoor measurements. The bypass between PTL and cell compartment, which has led to a very low pressure drop across the EC Gen II, was characterized by virtual flow experiments and finally successfully avoided by adapting the cell design. The final cell design shows very low voltages below 1.83 V at i ¼ 1 A/cm2 at 24 C when working under natural convection. Due to voltage behavior it is concluded that mass transportation limitation is not an issue with this cell configuration at such an operation current density. Furthermore a small positive effect on the cell voltage is expected for EC working under natural convection as a higher anode temperature is visible. This effect will probably be enhanced in an outdoor environment by the heat coming from the solar cell. The Faraday efficiency is above 98.5% for current densities above 0.8 A/cm2. 10 EC in this design configuration were manufactured performing equally. The voltage pof all 10 cells at the desired operation point of 1 A/cm2 is below 1.85 V at 30 C working under forced convection. Taking into account an outdoor measurement from an electrical mono module of a SC a total
9
efficiency from sunlight to hydrogen of a HyCon cell above 19% is predicted. In future work the manufactured ECs will be used to build a HyCon module consisting out of 8 HyCon cells. This HyCon module will be put up on a solar tracker and characterized in an outdoor measurement. Especially the influence of the temperature and natural convection on the system performance will be investigated.
Acknowledgements The authors would like to thank the department of chemical energy storage and the department of III-V epitaxy and solar cells. A. This project has been funded by the German Federal Ministry of Education and Research (BMBF) under the HyCon project (contract number 03SF0432A).
references
[1] Core Writing Team, Pachauri RK, Meyer LA, editors. “Climate change 2014 synthesis report: contribution of working groups I, II and III to the fifth assessment report of the intergovernmental panel on climate change” Geneva, Switzerland; 2015. [2] Bundesministerium fu¨r Wirtschaft und Energie, “Erneuerbare Energien in Zahlen e Nationale und internationale Entwicklung im Jahr 2014”. [3] B. Burger, Energy Charts. [Online] Available: https://www. energy-charts.de/power_de.htm. [4] Peharz G, Dimroth F, Wittstadt U. Solar hydrogen production by water splitting with a conversion efficiency of 18%. Int J Hydrogen Energy 2007;32(no. 15):3248e52. [5] Dimroth F, Peharz G, Wittstadt U, Hacker B, Bett AW. Hydrogen Production in a PV Concentrator Using III-V MultiJunction Solar Cells. Fraunhofer ISE; 2006. [6] F. Dimroth, “Vorrichtung und Verfahren zur photovoltaischen Erzeugung von Wasserstoff”. [7] Rau S, Vierrath S, Ohlmann J, Fallisch A, Lackner D, Dimroth F, et al. Highly efficient solar hydrogen generation e an integrated concept joining III-V solar cells with PEM electrolysis cells. Energy Technol 2014;2(no. 1):43e53. [8] Ahmad GE, El Shenawy ET. Optimized photovoltiac system for hydrogen production. Renew Energy 2006;31(no. 7):1043e54. [9] Gibson TL, Kelly NA. Optimization of solar powered hydrogen production using photovoltaic electrolysis devices. Int J Hydrogen Energy 2008;33(no. 21):5931e40. [10] High-concentration PV using III-V solar cells. In: Bett AW, Burger B, Dimroth F, Siefer G, Lerchenmuller H, editors. 2006 IEEE 4th world conference on photovoltaic energy conference; 2006. € tter, and A. Fallisch, “Cost Break [11] T. Smolinka, C. Voglsta Down and Analysis of PEM Electrolysis System for Different Industrial and Power to Gas Applications”. [12] J. Ohlmann et al., “Recent development in direct generation of hydrogen using multi-junction solar cells,” in 12th International Conference On Concentrator, p. 80004. [13] Rodriguez CA, Modestino MA, Psaltis D, Moser C. “Design and cost considerations for practical solar-hydrogen generators. Energy Environ Sci 2014;7(no. 12):3828e35. [14] Rau S. Hocheffiziente solare Wasserstoffproduktion mittels direkter Verschaltung von III-V-Mehrfachsolarzellen mit
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
PEM-Elektrolysezellen. Dissertation. Stuttgart, Germany: €t Fraunhofer Institute for Solar Energy Systems, Universita Stuttgart; 2014. [15] Zielke L, Fallisch A, Paust N, Zengerle R, Thiele S. Tomography based screening of flow field/current collector combinations for PEM water electrolysis. RSC Adv 2014;4(no. 102):58888e94. [16] Carmo M, Fritz D, Margel J, Stolten D. A comprehensive review on PEM water electrolysis. Int J Hydrogen Energy 2013;38:4901e34. [17] Lee CH, Banerjee R, Arbabi F, Hinebaugh J, Bazylak A. Porous transport layer related mass transport losses in polymer electrolyte membrane electrolysis: a review. In: ASME 2016
14th international conference on nanochannels, microchannels, and minichannels collocated with the ASME 2016 heat transfer summer conference and the ASME. Fluids Engineering Division Summer Meeting; 2016. V001T07A003. [18] Pourbaix Marcel, editor. Atlas of electrochemical equilibria in aqueous solutions: Part 1. Houston: National Association of Corrosion Engineers; 1966. [19] Buck AL. New equations for computing vapor pressure and enhancement factor. 1981. [20] Grigorieva SA, Milletb P, Korobtseva SV, Porembskiya VI, Pepicc M, Etievant C, et al. Hydrogen safety aspects related to high-pressure polymer electrolyte membrane water electrolysis. Int J Hydrogen Energy 2009;34:5986e91.
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator Arne Fallisch a,*, Leon Schellhase a, Jan Fresko a, Martin Zechmeister a, Mario Zedda a, Jens Ohlmann a, Lukas Zielke b, Nils Paust b, Tom Smolinka a a b
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg, Germany Department of Microsystem Engineering e IMTEK, Georg-K€ohler-Strasse 105, 79110 Freiburg, Germany
article info
abstract
Article history:
Hydrogen as a secondary energy carrier promises a large potential as a long term storage
Received 17 October 2016
for fluctuating renewable energies. In this sense a highly efficient solar hydrogen genera-
Received in revised form
tion is of great interest especially in southern countries having high solar irradiation. The
24 January 2017
patented Hydrogen Concentrator (HyCon) concept yields high efficiencies combining
Accepted 25 January 2017
multi-junction solar cells with proton exchange (PEM) membrane water electrolysis. In this
Available online xxx
work, a special PEM electrolysis cell for the HyCon concept was developed and investigated. It is shown that the purpose-made PEM cell shows a high performance using a ti-
Keywords:
tanium hybrid fiber sinter function both as a porous transport layer and flow field. The
Solar hydrogen production
electrolysis cell shows a high performance with 1.83 V at 1 A/cm2 and 24 C working under
PEM water electrolysis
natural convection with a commercially available catalyst coated membrane. A theoretical
IIIeV solar cells
examination predicts a total efficiency for the HyCon module from sunlight to hydrogen of
Porous transport layer
approximately 19.5% according to the higher heating value. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A secure and efficient energy supply is one of the key challenges in the 21st century. To decrease the dependency on fossil fuels like coal, oil and gas and also nuclear energy and to reduce the temperature rise due to global warming [1], renewable energy sources (RES) like solar, wind, biomass and water power are on the rise all over the world. In Germany the government has announced the “Energiewende” with the goal to increase the share of RES at least to 80% in 2050 [2]. In 2015 the market share of renewable energy in Germany is around
33% for the gross electricity consumption [3]. Due to the fluctuating nature of renewable energies, energy storage becomes a very important issue. Several solutions seem possible to overcome those challenges. E.g. battery storage systems for photovoltaics are already available for short term storage. To store energy from RES in the GWh or even TWh range chemical energy storage via hydrogen or further processed derivatives becomes a crucial necessity. Hydrogen can be used in mobile and stationary power applications. Therefore water electrolysis is considered a key component of future energy systems. The proton exchange membrane (PEM) water electrolysis is especially applicable for highly dynamic operation. It can
* Corresponding author. E-mail address: [email protected] (A. Fallisch). http://dx.doi.org/10.1016/j.ijhydene.2017.01.166 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
operate efficiently in the part-load regime. Furthermore using water is easier to handle compared to potassium hydroxide which is needed for alkaline electrolysis. Therefore PEM electrolysis is used for the patented Hydrogen Concentrator (HyCon) concept [4e7]. In contrast to conventional systems, which combine a photovoltaic system with a stand-alone electrolysis system [8,9], the HyCon concept links concentrating photovoltaics [10] with PEM electrolysis without any additional power electronics. Besides the advantage of missing power electronics, the electrical interconnection of the multi-junction solar cells (SC) is saved, as well as the interconnection of several sun tracking devices to the power electronics. Therefore a failure of a single SC leads only to a small reduction in hydrogen production and not to a breakdown of the whole hydrogen generator. Another advantage is the thermal management of the HyCon concept. The water flowing through the electrolysis cell (EC) is able to dissipate the heat of the SC which is beneficial for the voltage of the SC, as well as for the EC. A schematic view of the HyCon concept is shown in Fig. 1 and explained in the following. A Fresnel lens is used to focus the sunlight on a special tailored multi-junction SC. This SC is directly connected to the anode side of the PEM EC. Due to this direct connection, this concept yields a very high conversion efficiency from sunlight to hydrogen. The current generated by the SC flows via a conductor through a porous transport layer (PTL) to the anode electrode where the oxygen evolution reaction (OER) takes place. The PTL does not only transport the electric current but
Fig. 1 e Schematic view of the HyCon concept. A Fresnel lens focuses the sunlight on a multi-junction solar cell. This solar cell is directly connected to the anode of the electrolysis cell. The current is transported via the anode plate and porous transport layer to the electrode. The proton crosses the polymer electrolyte membrane to the cathode side. The electric circuit is closed by an electrical wire connecting the cathode side to the front side of the solar cell.
also the water and the gas. The remaining protons are transported through the proton exchange membrane to the cathode, where they are reduced to hydrogen, also called hydrogen evolution reaction (HER). The cathode side is connected to the front side of the solar cell using an electrical wire. In this work the focus lies on the PEM electrolysis cell design and the used PTLs. In particular PTLs made from titanium are one of the cost drivers in PEM electrolysis cells [11]. It is investigated how the cell behaves under similar conditions as they will occur in an outdoor environment. In the next section the EC design and indoor test equipment will be elaborately explained, followed by a detailed analysis of the performance, the flow behavior and the efficiency of the EC.
Experimental setup and used cell components PEM electrolysis cell design In this work three different electrolysis cell designs are used. These are the EC generation II, III and IV (Gen II, Gen III, Gen IV) which contain some specific design features which will be explained in the following. The main feature of the EC Gen II is an adjustable compression of the PTLs and the catalyst coated membrane (CCM) by a titanium (Ti) screw. These screws are inserted into a chlorinated polyvinyl chloride (CPVC) plate to contact the PTL. An exploded view of the design is shown in Fig. 2 on the left hand side. Both CPVC plates have inlets and outlets for the DI water and the gases. Although these cells can be sealed without any leakage, this design has several drawbacks, like an inhomogeneous compression and a fluidic bypass of the PTL due to the manufacturing process. In the HyCon module the solar cells are mounted on the Ti screw. This is another disadvantage of this design. Due to the length of the Ti screw of 19 mm the thermal coupling between the SC and the EC is very low [12]. Nevertheless an evaluation of different PTLs was done with this EC setup which will be discussed in the next section. The disadvantages are already overcome in the EC Gen III design. In this design, the Ti screw inserted into the CPVC plate in Gen II is replaced by a 4 mm Ti anode plate as it is depicted in Fig. 2 in the middle. The advantage of this design is that the SC is mounted directly on top of the anode plate, which increases the thermal coupling. In Gen III the PTL is placed in a milled deepening within the anode plate. On the cathode side a plastic plate holding a Ti pin is used. This plastic plate also contains all fluidic connections to the cell. The fluidic connections and the cathode plate material are the main differences between Gen III and Gen IV. For Gen IV the fluidic connections were moved from the bottom to the sides. Furthermore Polyphenylene sulfide (PPS) instead of CPVC material is used as this material has a higher mechanical, chemical and thermal stability. All ECs are sealed on the cathode side by an O-ring between the cathode plate and the CCM. The ECs are mounted with four screws, washers and nuts. All ECs are assembled with the same PTL on both sides. Certain authors claim that the combination of CPV with PEM electrolysis can achieve low investment costs [13,14]. However, in this work the EC design was optimized to give an optimum performance and is at this stage not cost-optimized.
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 2 e Figure shows on the left side an exploded view of the design of the HyCon EC Gen II. The main feature of the EC is an adjustable compression by Ti screws which are put into a CPVC plates. In the middle an exploded view of HyCon EC design Gen III is shown. On the anode side, the Ti screw is replaced by a Ti plate which contains a milled deepening for the PTL. On the cathode side a Ti pin which is put into a plastic plate is used to contact the PTL. In contrast to Gen IV on the right side, all fluidic connections are at the bottom. All ECs are assembled with four screws with nuts and washers. An O-ring on the cathode side is responsible for tightening.
Selected cell components Several PTLs were chosen for investigation within the HyCon EC. All tested PTLs consist of titanium. Basically these PTLs vary in thickness, porosity, the production process and in pricing. The thickness of the PTLs varies from 0.3 mm to 1.5 mm and their porosity ranges from 40% to 74%. Most PTLs are sintered Ti fibers. Furthermore one hybrid PTL and one sintered Ti powder is investigated. The hybrid PTL consists out of a 0.8 mm expanded mesh with a 0.2 mm sintered fiber on top. A complete list of all used PTLs is shown in Table 1. Their morphological and transport parameters were calculated and compared in an X-ray tomographic study [15]. All PTLs were delivered in big sheets. Thus, cylinders with a diameter of 12 mm had to be cut out these sheets to tightly fit into the HyCon cell. Water jet cutting has proven to be an adequate method, if the water inlet is located at the cross section of the PTL and no flow field structures are used as in EC Gen II. Laser cutting is also possible but leads to melting of Ti and thus reducing the open pores along the cut surface.
Commercially available CCMs were used for all examinations in this work. In the first experiments CCMs based on Nafion 117 membranes were used. For later experiments CCMs based on Nafion 115 were used. For the oxidation reaction of oxygen at the electrode on the anode side typically iridium is used as catalyst [16]. On the cathode side mostly carbon-supported platinum is used as a catalyst [16]. However, the exact composition of the electrodes is not known. All other conducting EC parts consist of Ti grade 2, like the Ti screws or pins and the anode plate in EC Gen III and Gen IV (compare Fig. 2). The screws used for mounting as well as the screw fittings consist of stainless steel.
Test bench for PEM electrolysis cells All experiments were carried out on a purpose made test bench for HyCon PEM ECs operating under atmospheric conditions. A piping and instrumentation diagram (P&ID) is depicted in Fig. 3. The water can be circulated on the anode and cathode side. Therefore a MCP Process hose pump with
Table 1 e List of all PTLs that were characterized in this work. The PTLs vary in thickness, porosity, the production process and the price. The thickness reaches from 0.3 to 1.5 mm and their porosity ranges from 40 to 74%. Most PTLs are sintered Ti fibers. One PTL is a sintered Ti powder and one is a hybrid consisting out of an expanded Ti mesh and a sintered Ti fiber. No 1 2 3 4 5 6 7 8
Name Ti-FS e 1.5-51 Ti-FS e 1-50 TiPt-FS e 1-57 Ti-PS e 1-40 Ti-FS e 0.95-74 Ti-FS e 0.5-73 Ti-FS e 0.3-56 Ti-HFS e 1-50
PTL material Ti Ti Ti Ti Ti Ti Ti Ti
fiber sinter fiber sinter Pt fiber sinter powder sinter fiber sinter fiber sinter fiber sinter hybrid fiber sinter (expanded mesh and fiber)
Thickness 1.50 1.00 1.00 1.00 0.95 0.50 0.30 1.00
mm mm mm mm mm mm mm mm
Porosity 51% 50% 57% 40% 74% 73% 56% 50% (only fiber)
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 3 e Simplified piping and instrumentation diagram of the test bench, which has been used for investigation. The electrolysis cell can either be operated under natural convection or under forced convection with a constant water flow rate which is adjusted at the pumps. All inlet and outlet temperatures are measured, as well as the pressure drop across the electrolysis cell. Furthermore the amount of produced hydrogen is measured with a gas counter.
a 4 channel pump head from Ismatec is used. The pump can be bypassed, if the electrolysis cell should run under natural convection. The water volume flow is measured directly after the pump using Bronckhorst coriolis flowmeters type M14-ABD-33-O-S. To maintain a high purity of the water, ion exchange resins are used. The tubular heat exchangers, which were designed and built at Fraunhofer ISE, are placed as near as possible to the electrolysis cell to maintain the required temperature at cell inlet. The heat exchangers are connected on the secondary side to a cryostat type CF31 from Julabo to control the temperature. The temperature is measured at all inlets and outlets of the EC with PT100 sensors. Furthermore another PT100 sensor can be used to measure the temperature near the anode, if the cell design is adapted. The temperature control can be adjusted in the software to any of those sensors. Temperatures between 10
and 80 C can be preset when the circulation pumps are used. Furthermore the differential pressure between inlet and outlet of the anode is measured using a Keller differential pressure sensor type PD-23. Downstream the gas water separators, which are at the same time the water reservoir, pressure sensors from Huba Control are installed. Both gas water separators are interconnected with a pipe. With a ball valve in the middle of this connection, the interconnection can be disrupted. This is needed for the measurement of the amount of hydrogen. After the gas pressure sensors, PT100 temperature sensors are used to measure the gas temperature. Afterwards the gas is led through a gas meter from Ritter type MGC-1. Additionally the ambient temperature and pressure is measured. The EC is positioned on a support that can be tilted to simulate different positions of the tracker.
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
To simulate the current generated by the solar cell, a Delta Elektronika power supply from the ES 150 series is used. The power supply can operate in a voltage range between 0 and 30 V and currents from 0 to 5 A. Thus more than one EC can be electrically interconnected in series. The current is measured using 3 shunts in parallel each with a resistivity of 0.1 U. The voltage is measured directly at the EC. All generated data are recorded by an Agilent 34790A data logger. The data logger, as well as all active components, are connected to a PC were a custom-programmed LabView software records all data in 3 s time steps. The software allows fully automated measurements, e.g. polarization curves at different temperatures.
Results and discussion Screening of different porous transport layers All eight PTLs as presented in Table 1 were investigated with the EC Gen II design. The Ti screws were adjusted on both sides in such a way that the remaining pocket depth for the PTL is 0.05 mm less than the thickness of the PTL. Thereby a suitable compression was achieved as the measured high frequency resistance below 300 mU cm2 shows. The ECs were always mounted with the same PTL on both sides and a Nafion 117 based CCM in between. Polarization curves were logged in galvanostatic mode with the test bench described in the previous section. The current density i was varied from 8 mA/cm2 to 3 A/cm2. The water volume flow Q on anode and cathode side was kept constant to 15 ml/min. The temperature T was controlled to the anode inlet to 30 C. The measured polarization curves of the ECs using all eight PTLs are depicted in Fig. 4. The overall performance varies depending on the used PTL. The EC with PTL Ti-HFS e 1-50 shows the lowest voltage for all current densities, followed by the EC with PTL Ti-FS e 0.5-73 and TiPtFS e 1-57. All other PTLs show significant higher voltages. At the desired operation point at a current density of 1 A/cm2 the voltage varies from 1.85 to 1.92 V. The difference increases at
5
higher current densities reaching up to 260 mV at 3 A/cm2. The differences in the polarization curves for different PTLs cannot be explained finally. The PTLs Ti-FS e 0.95-74, Ti-FS e 0.5-73, TiPt-HFS e 1-57 and Ti-HFS e 1-50 show the highest performance according to Fig. 4. Generally, it seems that a higher porosity leads to a higher performance. PTLs with a lower porosity tend to perform worse, which is partly confirmed in Ref. [15] where all PTLs except the Ti-HFS e 1-50 have been investigated in an X-ray tomography study. It could be shown (refer to second table in Ref. [15]), that some PTLs have a much higher permeability in in-plane direction. These correspond to the PTLs with a high porosity. For PTLs with a lower porosity Ti-HFS e 1.5-51, Ti-FS e 0.356, Ti-FS e 1-50 and Ti-PS e 1-40 a step of the voltage around 2 V is visible. Several reasons might be responsible for such an increase but a final explanation cannot be provided with the help of the measured polarization curves. One possible reason could be mass transport limitation due to drying of the membrane above a given current density and thus established two phase flow regime in the PTL or at the interface towards the electrode. Either not enough water is reaching the membrane or an effective blockage of the active area at the electrode surface by bubbles takes place [17]. However, this effect should result in further increasing voltages for increasing currents but instead, at higher voltages the slope starts to decrease again to the initial value at lower current densities. Another possible reason could be transient behavior of the interfacial contact resistance between the electrode and the PTL due to changing morphology. Above 2.1 V and a pH-value of 3, the Ti and its oxide layer on the surface may alter according to the Pourbaix diagram [18]. This will have an impact on the interfacial contact resistance as well as the hydrophilicity. A change in the hydrophilicity also influences the fluidic behavior of water within the PTL. Thus an increase in the hydrophilicity of the PTL could compensate the effect of mass transport limitation due to a poor water supply of the active area. Some of these PTLs are currently under investigation in a 25 cm2 large EC under pressure and with additional ex-situ characterization. These results will be published in the future. As the PTLs Ti-HFS e 1-50 and Ti-FS e 0.5-73 result in the lowest EC voltages and are also the cheapest PTLs, these PTLs were used in all following experiments.
Investigation on porous transport layers with EC Gen III
Fig. 4 e Figure shows polarization curves using different PTLs. The EC with Ti-HFS e 1-50 shows the lowest voltage of all PTLs followed by Ti-FS e 0.5-73 and TiPt-FS e 1-57.
The electrolysis cell EC design Gen III was tested with the PTLs Ti-HFS e 1-50 and Ti-FS e 0.5-73 which showed the highest performance in the EC Gen II. In this examination a commercially available CCM with a Nafion 117 membrane was used. The pocket depth of the anode and cathode compartments was 0.05 mm less than the total thickness of the PTL. This leads to a compression of 5% of the total thickness in case of the Ti-HFS e 1-50 and to a compression of 10% in case of the Ti-FS e 0.5-73. Before the polarization curves were recorded, the total cell impedance was measured at a frequency of 1 kHz at room temperature. For the EC with a Ti-HFS e 1-50 a high frequency resistance (HFR) of 258 mU cm2 is found. The HFR of 266 mU cm2 of the EC with Ti-FS e 0.5-73 is in the same range. As the main resistance of the EC is attributed to the
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
membrane, the electrical conductivity and interfacial contact resistances of the PTL are of minor importance. Higher compression rates of 10% for the Ti-HFS e 1-50 were tested, which did not lead to a lower resistance and lead to the same performance. This proved that the cell assembly of the electrolysis cell was successful. Polarization curves were logged in galvanostatic mode at different temperatures between 10 and 60 C. The current density i was increased stepwise from 8 mA/cm2 to 3 A/cm2. The volume flow Q on anode and cathode side was kept constant to 10 ml/min. Fig. 5 shows the polarization curves of both EC setups with different PTLs. The typical temperature dependence e decreasing voltage for increasing temperature e mainly due to improved kinetics at the anode and an increased proton conductivity of the membrane is clearly visible. The performance of both EC setups is very similar. At the desired operation point at iop ¼ 1 A/cm2 [7] the voltage is below 2 V for temperatures above 20 C. Based on these results it is of minor concern which PTL is used to achieve the highest performance. Therefore another consideration is of importance which will be addressed in the following section.
Differential pressure drop across the EC As operation of the HyCon ECs under natural convection would be beneficial, the pressure drop across the EC on the anode side was investigated for different water flow rates at zero current. The results are depicted in Fig. 6. The increase of the differential pressure is constant with increasing volume flow (this cannot be seen in Fig. 6 due to logarithmic scale). Moreover a fluidic bypass in Gen II ECs due to manufacturing processes is present. It is found that the theoretical pressure drop for the Ti-FS e 0.5-73 should be more than two orders of magnitude larger than the measured pressure drop (see the simulated solid curve in Fig. 6). The pressure drop via the EC is mainly determined by the fluidical resistance of the PTL. Here, the theoretical pressure drop was calculated using Darcy's law with permeabilities acquired by simulating water flow
Fig. 6 e Figure shows the pressure drop across the EC for Gen II and III with different PTLs depending on the volume flow. The EC Gen II has a bypass of the PTL which leads to a 100 times lower pressure drop. For both EC generations the Ti-HFS e 1-50 PTL shows a lower pressure drop. through a reconstructed volume of the PTL using the finite difference solver GeoDict. For the reconstruction, X-ray tomography was used [15]. As the theoretical permeability of the PTL are known from the finite difference solver results, bypasses can artificially be calculated using the free and porous flow module in COMSOL. Such a study yielded that the bypass is between 400 mm and 600 mm large (see dotted curve in Fig. 6). This bypass is reduced in generation III ECs resulting in a differential pressure drop via the EC, which is about a factor of 100 higher. The fluidic resistance of Gen III EC is in excellent accordance with the simulation of the fluidic resistance of the Ti-FS e 0.5-73 PTL (exp.: 1.01 1011 kg/m4s, sim.: 1.03 1011 kg/m4s), which demonstrates that the fluidic bypass was successfully eliminated. As this approach (1. X-ray reconstruction 2. permeability calculation using GeoDict and 3. fluidic resistance calculation using Darcy's law) is able to validate experimental pressure drop curves, it is suited for virtually designing PTLs. This means that morphological features can be introduced or altered and the influence of the introduction/altering can be reliably estimated without conducting any experiments. In the experiments the Ti-HFS e 1-50 PTL shows a lower differential pressure for both ECs Gen II and III. Therefore TiHFS e 1-50 was chosen as a PTL for all following investigation.
Impact of natural convection on cell performance
Fig. 5 e Figure shows polarization curves with two different PTLs in EC Gen IV. At the desired operation point at i ¼ 1 A/cm2 the voltage is below 2 V for temperature above 20 C.
To investigate how the EC will perform under more realistic condition in the outdoor module, the EC design Gen IV was analyzed working under natural convection with bypassed circulation pumps. The test bench support, carrying the EC, was tilted to simulate the position of the tracker in the outdoor module. Furthermore the cathode inlet was closed. The polarization curves were recorded starting from 1 A/cm2 going down to 0.1 A/cm2 at room temperature with no active heating or cooling. The higher limit of 1 A/cm2 was chosen as this is also the desired operation point for the HyCon outdoor
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
module. Each current density step was kept constant for 30 min. In this experiment a CCM with a Nafion 115 membrane was used. In Fig. 7 several parameters are depicted with the operation time on the x-axis. At the top of Fig. 7 the inlet and outlet temperature as well as the temperature measured within the Ti anode plate is shown. The inlet temperature of the anode corresponds to the room temperature as the total amount of water in the whole system is much larger than the circulated water. The outlet temperatures of the anode and cathode are slightly higher. The temperature near the anode plate is much higher for high current densities due to the Joule heating. The volume flow is fluctuating between 0.1 and 0.6 ml/min. This volume flow is not sufficient to keep a constant temperature of the EC at its active area, leading to the higher temperature of the anode and the water outlets. The differential pressure behaves the same way as the volume flow. This repetitive fluctuation of the water flow rate is also visible in the transparent hose lines. In the lower section of Fig. 7 the current densities and the cell voltages are shown. It is visible that the natural convection has no negative effect on the cell voltage despite the fluctuating water flow. This means that no mass transport limitations occur as this would lead to an increased voltage. If the polarization curve under natural convection is compared to a polarization curve obtained with a constant water flow, a slightly lower cell voltage, especially for high current densities, can be seen for an EC working under natural convection. These polarization curves are depicted in Fig. 8. Here a membrane based on Nafion 115 was used. The slightly lower voltages at high current densities around 1 A/cm2 are due to a higher anode plate temperature. The temperature of the EC working at 1 A/cm2 under natural convection is approximately 24 C. For the fixed volume flow of approximately 5 ml/min the anode temperature was controlled to 20 C. For low current densities the anode temperature drops down to 15 C for current densities below 0.2 A/cm2 due to the
7
Fig. 8 e Figure shows polarization curves measured under natural and forced convection. The EC working under natural convection shows a slightly lower voltage for current densities above 0.7 A/cm2.
thermal inertia of the temperature control. For high current densities above 0.8 A/cm2 it is between 20 and 21 C. In this prospect, working under natural convection is beneficial to reduce the operation voltage.
Faraday efficiency As stated in the beginning, high efficiencies from sunlight to hydrogen can be achieved using the HyCon concept. There are several loss mechanisms inside an EC which decrease the efficiency. As ohmic and activation overpotentials can easily be taken into account by simply measuring the EC voltage, Faraday efficiency is not that easy to determine. The amount of hydrogen produced by a single cell with an area of 1.13 cm2 is rather low. It takes about 32 min to generate 1 L of hydrogen at a current density of 1 A/cm2. To measure this amount
Fig. 7 e Figure shows the voltage, current density and temperatures over time of an EC working under natural convection. It is visible that the anode temperature Tanode is much higher for high current densities as the heat is not fully transferred by the water flow. Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
leads to certain uncertainties some values do not always follow the trend, especially for current densities between 0.3 and 0.5 A/cm2. For high current densities above 0.8 A/cm2 the Faraday efficiency is above 98.5%. This value corresponds well to results from Grigoriev at 2 bar [20]. For the lower current densities regime the Faraday efficiency is always above 94%. According to Grigoriev approximately 0.5% of H2 in O2 were detected in an EC with a Nafion 117 membrane at 1 bar pressure and 1 A/cm2. As in this work the membrane is a Nafion 115 membrane, a little bit more than 0.25% of H2 is expected to cross the membrane. The remaining losses are attributed to leakage of the EC and the test bench. For the estimation of the maximum efficiency from sunlight to H2 in the next section, a Faraday efficiency of 98.5% is taken into account.
Prediction of the operation point of HyCon cells Fig. 9 e Faraday efficiency of an EC with Nafion 115 membrane. The trend shows an increasing Faraday efficiency for increasing current densities. Above current densities of 0.8 A/cm2 the Faraday efficiency is above 98.5%.
precisely, a special gas counter from Ritter was used with a remaining error less than 1% from the total flow range. The measured gas volume was corrected to norm volume. Therefore the gas is assumed to be 100% saturated with water and partial pressure of the water vapor depending on the gas temperature is taken into account [19]. An EC with Nafion 115 based membrane was used to determine the Faraday efficiency. The current density was varied between 0.1 and 1 A/cm2. Each current density step was kept until a theoretical H2 volume of 0.5 l was produced. The water volume flow was adjusted to 5 ml/min at room temperature. As the amount of O2 in H2 is in the ppm regime [20] it is neglected. This was also known from other experiments carried out at Fraunhofer ISE. In Fig. 9 the measured Faraday efficiency in dependence on the current density is shown. In principle, it can be seen that the Faraday efficiency is increasing with increasing current density. Due to the complexity of the measurement which
As shown in the previous section the HyCon EC Gen IV with a Ti-HFS e 1-50 as PTL shows promising results on an EC level. Therefore this design was taken to manufacture 10 ECs at the mechanical workshop of Fraunhofer ISE. These ECs were assembled with a commercial available CCM with a Nafion 115 membrane and Ti-HFS e 1-50 as PTL on both sides. Afterwards all ECs were conditioned at 80 C with half-hourly alternating current densities of 1 and 2 A/cm2 for 12 h. To speed up the process two ECs were connected fluidically in parallel at the test bench. A fixed volume flow of Q ¼ 10 ml/min was used for the conditioning and the following measurement of polarization curves at temperatures between 10 and 60 C. In Fig. 10a) the polarization curves of all 10 ECs at 30 C are depicted. The manufacturing and assembling process results in a high reproducibility. This is visible regarding the voltage at the desired operation point of 1 A/cm2, which lies between 1.79 and 1.83 V for all ECs. To estimate the operation point which can be achieved in the outdoor module, the iV-curve of the solar cell has to be taken into account. In Ref. [12] a metamorphic GaInP/GaInAs solar cell was built into an electrical mono module and characterized in an outdoor measurement. The used lens size was 90.7 cm2 with a solar cell size of 0.36 cm2 leading to a geometrical concentration of 252. The iV-curve at an irradiation density of 870 W/m2 is shown in
Fig. 10 e a) Polarization curves of 10 equally fabricated ECs are shown. Two ECs were fluidically connected in parallel. b) Prediction of the operation point in a HyCon module is shown. The operational current density lies above 0.9 A/cm2 at a voltage below 2 V. This yields a maximum efficiency of 19.5% taking into account a Faraday efficiency of 98.5%. Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 10b). The cross-section with the polarization curves of the EC measured at 30 C and 60 C marks the predicted operation point of the module. In this case the operation current density is iop ¼ 0.93 A/cm2 for 30 C and 60 C. The operation voltage changes from Vop ¼ 1.84 V-1.66 V if the temperature is increased to 60 C. In the HyCon module a measurement resistor, a relay and cables will be implemented in between the connection from the SC to the EC. Therefore an ohmic resistance of Rs ¼ 100 mU is assumed in series with the EC. This resistance increases the operation voltage of the SC to Vop ¼ 1.89 V for an EC operation temperature of 30 C. This leads to small reduction of the operational current density to iop ¼ 0.92 A/cm2. A reduction of the current density has also an effect on the efficiency of the HyCon cell. The total efficiency from sunlight to the amount of stored energy within hydrogen according to the higher heating value is calculated by εHyCon ¼ εF
Iop Vth PDNI
(1)
where Iop is the current generated at the operation point, Vth is the thermoneutral voltage of 1.48 V and PDNI is the irradiation. Calculating the efficiency according to equation (1) leads to maxima of 19.5% at a temperature of 60 C and 19.4% at 30 C taking into account the Faraday efficiency of εF ¼ 98.5%. If the resistances are included the efficiency is reduced to 19.4% and 19.2%, respectively. As the iV-curve of the solar cell and the polarization curves are highly temperature dependent, it is hard to predict the real behavior in the module. However, temperatures above 30 C seem reasonable for summer in an outdoor environment.
Conclusion and future work Several PTLs have been investigated in different EC design. It has been shown that the Ti hybrid fiber sinter PTL Ti-HFS e 1-50 shows the highest performance in all electrolysis cell design from Gen II to Gen IV. As this PTL is relatively cheap and causes only a low pressure drop, it was used in all EC that will be integrated into a real HyCon module for outdoor measurements. The bypass between PTL and cell compartment, which has led to a very low pressure drop across the EC Gen II, was characterized by virtual flow experiments and finally successfully avoided by adapting the cell design. The final cell design shows very low voltages below 1.83 V at i ¼ 1 A/cm2 at 24 C when working under natural convection. Due to voltage behavior it is concluded that mass transportation limitation is not an issue with this cell configuration at such an operation current density. Furthermore a small positive effect on the cell voltage is expected for EC working under natural convection as a higher anode temperature is visible. This effect will probably be enhanced in an outdoor environment by the heat coming from the solar cell. The Faraday efficiency is above 98.5% for current densities above 0.8 A/cm2. 10 EC in this design configuration were manufactured performing equally. The voltage pof all 10 cells at the desired operation point of 1 A/cm2 is below 1.85 V at 30 C working under forced convection. Taking into account an outdoor measurement from an electrical mono module of a SC a total
9
efficiency from sunlight to hydrogen of a HyCon cell above 19% is predicted. In future work the manufactured ECs will be used to build a HyCon module consisting out of 8 HyCon cells. This HyCon module will be put up on a solar tracker and characterized in an outdoor measurement. Especially the influence of the temperature and natural convection on the system performance will be investigated.
Acknowledgements The authors would like to thank the department of chemical energy storage and the department of III-V epitaxy and solar cells. A. This project has been funded by the German Federal Ministry of Education and Research (BMBF) under the HyCon project (contract number 03SF0432A).
references
[1] Core Writing Team, Pachauri RK, Meyer LA, editors. “Climate change 2014 synthesis report: contribution of working groups I, II and III to the fifth assessment report of the intergovernmental panel on climate change” Geneva, Switzerland; 2015. [2] Bundesministerium fu¨r Wirtschaft und Energie, “Erneuerbare Energien in Zahlen e Nationale und internationale Entwicklung im Jahr 2014”. [3] B. Burger, Energy Charts. [Online] Available: https://www. energy-charts.de/power_de.htm. [4] Peharz G, Dimroth F, Wittstadt U. Solar hydrogen production by water splitting with a conversion efficiency of 18%. Int J Hydrogen Energy 2007;32(no. 15):3248e52. [5] Dimroth F, Peharz G, Wittstadt U, Hacker B, Bett AW. Hydrogen Production in a PV Concentrator Using III-V MultiJunction Solar Cells. Fraunhofer ISE; 2006. [6] F. Dimroth, “Vorrichtung und Verfahren zur photovoltaischen Erzeugung von Wasserstoff”. [7] Rau S, Vierrath S, Ohlmann J, Fallisch A, Lackner D, Dimroth F, et al. Highly efficient solar hydrogen generation e an integrated concept joining III-V solar cells with PEM electrolysis cells. Energy Technol 2014;2(no. 1):43e53. [8] Ahmad GE, El Shenawy ET. Optimized photovoltiac system for hydrogen production. Renew Energy 2006;31(no. 7):1043e54. [9] Gibson TL, Kelly NA. Optimization of solar powered hydrogen production using photovoltaic electrolysis devices. Int J Hydrogen Energy 2008;33(no. 21):5931e40. [10] High-concentration PV using III-V solar cells. In: Bett AW, Burger B, Dimroth F, Siefer G, Lerchenmuller H, editors. 2006 IEEE 4th world conference on photovoltaic energy conference; 2006. € tter, and A. Fallisch, “Cost Break [11] T. Smolinka, C. Voglsta Down and Analysis of PEM Electrolysis System for Different Industrial and Power to Gas Applications”. [12] J. Ohlmann et al., “Recent development in direct generation of hydrogen using multi-junction solar cells,” in 12th International Conference On Concentrator, p. 80004. [13] Rodriguez CA, Modestino MA, Psaltis D, Moser C. “Design and cost considerations for practical solar-hydrogen generators. Energy Environ Sci 2014;7(no. 12):3828e35. [14] Rau S. Hocheffiziente solare Wasserstoffproduktion mittels direkter Verschaltung von III-V-Mehrfachsolarzellen mit
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
PEM-Elektrolysezellen. Dissertation. Stuttgart, Germany: €t Fraunhofer Institute for Solar Energy Systems, Universita Stuttgart; 2014. [15] Zielke L, Fallisch A, Paust N, Zengerle R, Thiele S. Tomography based screening of flow field/current collector combinations for PEM water electrolysis. RSC Adv 2014;4(no. 102):58888e94. [16] Carmo M, Fritz D, Margel J, Stolten D. A comprehensive review on PEM water electrolysis. Int J Hydrogen Energy 2013;38:4901e34. [17] Lee CH, Banerjee R, Arbabi F, Hinebaugh J, Bazylak A. Porous transport layer related mass transport losses in polymer electrolyte membrane electrolysis: a review. In: ASME 2016
14th international conference on nanochannels, microchannels, and minichannels collocated with the ASME 2016 heat transfer summer conference and the ASME. Fluids Engineering Division Summer Meeting; 2016. V001T07A003. [18] Pourbaix Marcel, editor. Atlas of electrochemical equilibria in aqueous solutions: Part 1. Houston: National Association of Corrosion Engineers; 1966. [19] Buck AL. New equations for computing vapor pressure and enhancement factor. 1981. [20] Grigorieva SA, Milletb P, Korobtseva SV, Porembskiya VI, Pepicc M, Etievant C, et al. Hydrogen safety aspects related to high-pressure polymer electrolyte membrane water electrolysis. Int J Hydrogen Energy 2009;34:5986e91.
Please cite this article in press as: Fallisch A, et al., Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.166