On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities

international journal of hydrogen energy xxx (xxxx) xxx

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On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities Thomas Lickert*, Maximilian L. Kiermaier, Kolja Bromberger, Jagdishkumar Ghinaiya, Sebastian Metz, Arne Fallisch, Tom Smolinka Fraunhofer Institute for Solar Energy System ISE, Heidenhofstrasse 2, D-79110, Freiburg, Germany

highlights
Performances of electrolysers with and without flow field can differ significantly.
Also temperature (T), pressure (p) and flow rate (Q) influence the performance.
Performance of electrolysers without flow field benefit from increased T, p and Q.
Porosities and particle/fiber diameters are not enough to characterise PTLs.
Also in-plane and through-plane permeability are needed for proper characterisation.

article info

abstract

Article history:

In this study, the influence of the anodic porous transport layer (PTL) on the performance

Received 6 September 2019

of a proton exchange membrane (PEM) electrolysis laboratory test cell was investigated up

Received in revised form

to a current density of 5 A*cm2. Operation parameters such as water volume flow rate (0.2

14 December 2019

e0.8 l*min1), temperature (40e80  C) and pressure (1e30 barg) have been varied to study

Accepted 28 December 2019

their influence on the polarisation curve. Special attention has been paid to the appearance

Available online xxx

of mass transport losses (MTL) and their dependency on the operation parameters. Two stack designs that are commercially in use - one with and one without flow channels

Keywords:

underneath the PTL - were tested and evaluated. Fundamental differences in performance

PEM water electrolysis

have been observed between the two cell designs. Operation parameters only show impact

Porous transport layer (PTL)

on performance for the configuration without flow channels. Here, MTL were observed in

High current density

several cases already for current densities around and above 1.0 A*cm2. An increase in

Mass transport losses (MTL)

pressure, temperature or water flow rate reduces MTL for these configurations.

Cell design

© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Flow field (FF)

* Corresponding author. Department of Chemical Energy Storage, Hydrogen Technologies Division, Fraunhofer-Institut fu¨r Solare Energiesysteme ISE, Heidenhofstr. 2, 79110, Freiburg, Germany. E-mail address: [email protected] (T. Lickert). URL: http://www.ise.fraunhofer.de https://doi.org/10.1016/j.ijhydene.2019.12.204 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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Introduction For the transition of a fossil fuel based energy system towards the usage of renewable energy sources, hydrogen has gained an increasing amount of attention as a “green” energy carrier [1e3]. As response times of proton exchange membrane (PEM) electrolysers are fast [4e6] they are particularly well suited to intermittent power profiles of wind and solar power plants. To compensate for the high capital/investment costs of this technology, the production rate of hydrogen needs to be maximised which can be achieved by increasing the operation temperature, which in turn increases the efficiency of a cell. In addition, a higher current density leads to higher hydrogen production rates and therefore to lower specific hydrogen generation costs. Good performance at high current densities, however, needs to be reflected in the setup of the entire system (stack and periphery) and hence has to be taken into account already at the design stage. In the high current density regime, mass transport losses become dominant and can drastically decrease efficiency [7e10]. One of the most important components within an electrolysis cell/stack is the anodic porous transport layer (PTL) [7,11e14]. It is responsible for the transport of the incoming water to the catalyst layer and the removal of the product gases. As it also contributes significantly to the stack costs, avoiding expensive materials and production processes, like machining of flow channel structures in titanium, has shown great cost reduction potential [13,15,16]. Therefore, planar bipolar plates (BPP) are preferable, but their usage strongly influences the requirements for the PTL structure. For this design, water and gases flow through the PTL in in-plane direction and therefore sample dimensions and parameters, like porosity, pore distribution and permeability become important. This holds particularly true as PEM electrolysis technology develops towards large active cell areas and high current densities to reduce the costs. Choosing an appropriate anodic PTL hence becomes crucially important for the overall performance of the cell. Different strategies have been followed on how PTL structures have to be specified. Mostly, porosity and pore size distribution are discussed in this respect [17]. Grigoriev et al. [18] obtain optimum values for porosities of 30e50% and pore sizes of 12e13 mm, for a PTL consisting of spherically shaped titanium particles. The value of the optimal pore size was derived from theoretical calculations on the cell voltage and the pore structure. Ito et al. [19,20] investigated titanium fiber felts with different porosities (46e75%) and fiber diameters of 20e80 mm. Based on in-situ polarisation measurements it was experimentally established that the cell performance is more influenced by the fiber diameter than by the porosity [21]. Besides porosity and pore size, the gas flow characteristic through the water saturated PTL is of crucial importance. Here, the capillary pressure and in particular the wettability of the PTL have a large impact on the cell performance at higher current densities. Using capillary flow porometry and developed ex-situ methods, Bromberger et al. [21] were able to characterise the capillarity and contact angle of several porous PTLs. Both parameters are found to have a significant

influence on the performance, especially at high current densities. Only few publications investigated further parameters of porous transport layers [21e24]. Siracusano et al. [23] achieved significant improvement in performance in a short stack when the thickness of the anodic titanium felt was changed from 260 to 500 mm. The authors concluded that this effect originated in a reduction of the interface resistances, shown by a shift in the Nyquist plot. The authors also reported “an increase of mass transport constraints” by changing the PTL, even at rather low gas production rates (1.5 V DC-bias). Mechanical characteristics can also be of great importance as fluidic behavior of the PTL may change when the porous matrix is compressed. Due to a reduction of the interfacial contact resistances (ICR) between the cell components [24], electrical resistance is typically reduced with increased compression.

Losses in a PEM electrolysis cell The cell voltage Vcell is usually described as the sum of the reversible voltage Vrev and all irreversible losses due to kinetics, ohmic effects and mass transport losses (Equation (1)). Vcell ¼ Vrev þ

X

hkinetics þ

X

hohm þ

X

hMTL

(1)

Depending on the current density region, different losses dominate the cell behavior. At low current densities (<< 0.1 A*cm2) kinetic losses (Equation (2)) of the oxygen and hydrogen evolution reactions (OER on anode (hact;anode ) and HER  on cathode (jhact;cathode )) dominate the cell performance as other losses are low. X

  hkinetics ¼ hact;cathode  þ hact;anode

(2)

At medium current densities (0.1e1.5 A*cm2), ohmic losses dominate the cell performance. These losses are mainly related to the ionic resistivity of the polymer electrolyte membrane (PEM; hPEM ), the bulk electronic resistance (hbulk ) and the electronic resistance at the interfaces of the cell components (hICR ) (Equation (3)). X

hohm ¼ hPEM þ

X

hbulk þ

X

hICR

(3)

The boundaries of the regions can only be roughly specified as several cell design parameters, material properties and operating conditions have an impact on the cell performance. At high current densities mass transport losses (MTL) start to become relevant and might dominate the performance. Usually, MTL are described as an additional loss term, see Equation (1). Here we want to emphasise that MTL can eventually have an influence on the kinetic and ohmic loss terms. Various processes must occur in order to be able to assume this. The following hypothesis might explain such a behavior of the cell. A slow removal of dissolved oxygen in the liquid phase of water from the catalyst surface to the bulk (water) leads to a diffusion driven overpotential that can be described by an increase in Nernstian potential. Due to the drastic change in porosity (nm-range in the catalyst and mm-range in the PTL) oxygen bubbles are formed at the interface within the PTL

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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pore volume [25]. The bubbles inside the PTL, immobile or not, represent a hindrance for water getting towards the electrode. Water is further consumed by the electrochemical reaction and transported away from the electrode layer by electroosmotic drag through the membrane. If, at the same time, an inhibition of the reactant water flow occurs, the galvanostatic mode can only be sustained if the driving force for the electrochemical reaction is increased. This is the overpotential, hact;anode . With this hypothesis the impact of mass transport losses is located at the electrode. Even though, mass transport losses could therefore be seen as a part of the anodic activation overpotential, hact;anode , an additional term hMTL will be used for easier comparison to existing literature. Further effects, like a reduction of the water content l in the membrane (local drying) are conceivable but would be speculation at this point. As the PTL structure and its impact on the cell performance at high current densities is subject of the investigations, the mutual inhibition of reactant water from the channel to the electrode and the inhibition of gaseous oxygen from the electrode to the channel are the major effects to keep in mind for analysis of the results presented herein. With neglecting the cathodic activation overpotential and the voltage drop originating from the bulk electronic resistance of the cell components, Equation (1) can be simplified to: Vcell ¼ VREV þ hact;anode þ hPEM þ

X

hICR þ

X

hMTL

(4)

Investigations on mass transport phenomena have gained an increasing amount of attention in recent literature as resulting losses have been identified as dominant losses at high current density operation [7e10]. Lee et al. [7] reviewed different computational studies as well as experimental publications and concluded that MTL and their underlying effects are not fully understood but have presented three hypotheses of their origin. (1) Limitations by incomplete removal of product gas or poor water supply, (2) reduction of the electrochemically active surface area (ECSA) and (3) insufficient thermal management. A fourth effect that could be significant when operating the cell at high current densities was identified and further investigated by Sun et al. [10]. The investigations on segmented cells within a stack using impedance spectroscopy showed that both ohmic and charge transfer resistances gradually increase when the current density is increased while water stoichiometry is reduced. A local drying of the membrane could explain the observed effect in cases when the transport properties of the PTL were chosen for different current densities. Fritz et al. [26] published a PEM electrolysis model and the corresponding experimental data, capturing mass transport effects and varying pore sizes of the anodic PTL. The results showed that mass transport losses could be reduced by increasing the pore size from 9 mm to 12 mm. But a comprehensive conclusion on the origin of MTL and the correlation to the PTL structure is still an open issue in literature. More insight could be brought by ex-situ characterisation of the PTLs with which specific transport parameters can be extracted or e.g. interface issues as done by Heoh et al. [27].

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For the results presented herein, the appearance of MTL and the dependency of MTL on operating parameters is a crucial observation, which is used, when PTLs are compared. Based on literature and experience by previous measurements, we believe that incomplete gas removal through immobile oxygen gas bubbles inside the PTL and the associated hindrance of water coming to the electrode is the main cause of MTL. This can trigger an entire cascade of effects that are still not fully understood in the community.

Experimental Investigated porous transport layers Five different titanium based PTL structures were tested (see Table 1): One powder based Ti-sinter (PTL 3) and three fibrous Ti-sinters (PTL 1, PTL 2 and PTL 4). PTL 5 is a hybrid material consisting of a Ti-expanded mesh (thickness: 800 mm) with a fibrous structure on top (thickness: 200 mm). The powder based material (PTL 3) served already as internal reference material in previous works [28]. The fibrous structures were selected as they have the potential to be a cost-effective alternative to powder based materials. The hybrid structure (PTL 5) is a promising candidate for commercial stack platforms as the expanded mesh as bottom layer could replace the flow channels and therefore reduce stack costs. It has already been successfully used in a solar hydrogen generator [17]. The characteristic pore size and the corresponding in-plane gas permeabilities are given in Table 1. These values were generated using the procedure described in [21] and will be used herein for better understanding of the effects. The parameters for PTL 5 are left out as the hybrid character of the PTL leads to non-interpretable values as the in-plane characteristic is dominated by the mesh and the trough-plane characteristic is dominated by the fiber sinter and therefore no useful information is generated for the sandwich of both.

Laboratory test cell and test bench All electrochemical measurements were performed using inhouse developed test hardware. The laboratory test cell was designed for the investigation of different PTLs and cell configuration with and without flow fields (FF). A 3D illustration of the cell and a schematic view for each setup is depicted in Fig. 1. The cell consists of identical half-cell bodies on both anodic and cathodic side (1). Electrical connectors are realised by 3 mm thick titanium plates (2). Sense connections for the voltage measurement are connected to this plate as well. To mount the two half-cells, 20 mm thick titanium plates with 12 bolts (M8) were used (3). The cell was tightened with a torque of 20 Nm on each bolt. The cell temperature was controlled by the process water that flows through the cell and controlled the temperature at the outlet on anodic side. Two different cell configurations were used to perform measurements one with and one without flow channels. These setups are depicted as schematics in Fig. 1. For the test setup with flow channels, a gold coated titanium flow field (5) with longitudinal channels was inserted in the

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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Table 1 e Specification of evaluated PTLs with type, initial thickness, porosity, characteristic pore size and in-plane gas permeability. Porosity data originate from Hg-porosimetry. In-plane permeabilities were measured in-house [21]. Sample No. 1 2 3 4 5 6

Type of PTL

Initial thickness [mm]

Fibrous Ti-Sinter Fibrous Ti-Sinter Ti powder sinter 1 Fibrous Ti-Sinter Fibrous Ti-Sinter þ expanded Ti- mesh Ti powder sinter 2 (cathodic PTL for all tests)

1 1 1 1 1 1

Porosity Characteristic pore size/modal In-plane gas [%] pore diameter [mm] permeability [m2] 51.19 52.68 60.02 67.21 40.42

27.48 18.93 49.16 41.57 e 14.36

5.31E-12 2.25E-12 1.30E-11 2.25E-11 e 1.00E-12

Fig. 1 e CAD drawing of the used test cell (left) and schematic drawings of the two cell configurations; with flow channels on both sides (top right) and without flow channels on anodic side (bottom right). Ti cell body (1), Ti current connection plate (2), pressure plate (3), Au coated Ti plate for adjustment of the PTL pocket (4) and Au coated Ti flow field (5) are the relevant components.

anodic half-cell. Water flows through the channel structure with a low fluidic resistance and is transported through the PTL mainly in through-plane direction towards the electrode. The produced gas is transported towards the opposite direction. The water transport from the channel to the electrode is driven by capillary forces and the water pressure generated by the circulation pumps. For measurements without flow channels (Fig. 1), the plate with the flow channels was replaced by a massive, gold coated titanium plate of the same thickness (4) to ensure similar compression and contact resistances. Here, water has to flow in inplane direction through the porous structure of the PTL and therefore has to overcome a larger fluidic resistance of the PTL, which can be described by the in-plane permeability (see Table 1). But more importantly, water and the produced

gases flow by forced convection in in-plane direction. For both cell configurations all porous transport layers PTL 1 to PTL 5 were tested consecutively on the anode side, resulting in a series of 10 measurements. The cathodic cell configuration remained unchanged for all measurements using an identical flow field plate with channels (5) and a titanium powder sinter as PTL (No. 6 in Table 1). The tests were performed using an in-house developed test bench. A simplified flow chart is given in Fig. 2. Water supply to the cell is realised by water circulation loops on anodic (blue) and cathodic side (red) of the cell. Reactant water is heated to the desired temperature by an additional circulation loop (green) and controlled by the temperature at the outlet of the anode (see Fig. 2). The temperature of the cathode flow is not controlled but has the same cell

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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Fig. 2 e Reduced piping and instrumentation diagram (P&ID) of the test bench with three H2O circulation loops, anodic (blue) and cathodic (red) and a heat circulation (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

inlet temperature as the anode. To ensure a low water conductivity (<0.1 mS/cm), an ion exchanger, equipped with a mixed bed ion exchanger resin (Dowex™ Monosphere™ MR-450 UPW, DoW chemical company®), is used in both water circulation loops. The water flow is controlled by gear circulation pumps via fan wheel sensors. Pressure is built up by the product gas itself and controlled by electro pneumatic back pressure valves in the oxygen and hydrogen gas chain. For test bench automatisation, an inhouse developed LabView™ interface was used.

Sample preparation and electrochemical characterisation Sample preparation: All PTLs were cleaned using an ultrasonic bath with different solvents for 10 min each in the following order: First Deconex OP153 ® (Borer Chemie, Schweiz) and acetone was used to remove organic contaminations. 2-Propanol and ethyl alcohol were used for removal of residual Deconex®, acetone and anorganic compounds. Finally, the PTLs were sonicated in deionised water (conductivity s z 0.9 mS) to remove all residual solvents. During sonication the temperature of the bath was increased to almost 60  C by the ultrasonic waves. Pre-tests: After cleaning, the respective PTL was assembled in the laboratory test cell. To check if the internal cell resistance had changed during previous tests, each PTL was installed in the cell without a membrane electrode assembly (MEA) but with a 0.1 mm thick, gold plated titanium sheet in the size of the active area (5  5 cm2) instead. High frequency resistance (HFR) at a fixed frequency of f ¼ 1 kHz and a perturbation current of i ¼ 10 mA (type: 4338B, Agilent Technologies®, USA) was recorded. When showing the expected conductivity, the cell was assembled with a swollen MEA (stored in deionised water at room temperature for at least

12 h). Again, the HFR was measured at room temperature to check the cell assembly with the MEA. If the ohmic resistance showed values < 20 mU, the full characterisation protocol was started. In-situ tests: In-situ analysis was mainly done using Vicurves. As a first step the cells were conditioned by a rectangular current profile with current densities of 1 A*cm2 and 2 A*cm2 that were kept constant for 30 min each (with an initial current ramp from 0 A*cm2 to 1 A*cm2 within 15 min). The periodic profile was repeated 12 times resulting in a total conditioning time of 12 h with a constant cell temperature of T ¼ 80  C, a pressure of p ¼ 1 barg and a water flow rate of Q ¼ 0.5 l*min1. After conditioning, the influence of the operating parameters (T, p, and Q) on cell performance was investigated by Vi curves in the following chronological sequence: (1) temperature variation: a. T ¼ 40, 60 and 80  C b. Q ¼ 0.5 l*min1 c. p ¼ 1 barg (anode and cathode) (2) water flow variation: a. T ¼ 60  C b. Q ¼ 0.2; 0.3; 0.4; 0.5; 0.6 and 0.8 l*min-1 c. p ¼ 1 barg (anode and cathode) (3) pressure variation: a. T ¼ 60  C b. Q ¼ 0.5 l*min-1 c. p ¼ 1; 2; 5; 10 and 30 barg (anode and cathode) The values were chosen to cover the optimal operating conditions predicted for similar cells [29]. All polarisation curves were acquired in galvanostatic mode in 0.05 A*cm2 steps from 0.05 A*cm2 up to 0.25 A*cm2 (low current density

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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region) and from 0.25 A*cm2 up to 2.00 A*cm2 in 0.25 A*cm2 steps (mid current density region). For higher current densities (from 2.00 A*cm2 to 5.00 A*cm2), 0.10 A*cm2 steps were used. Each step was kept constant for 3 min. The corresponding voltage data was acquired with a sampling rate of 5 points per second. Each point of the presented Vi curves is a calculated mean value of the last 10 data points at each current step. Hereby, the statistical error is smaller than the measurement resolution of 10 mV. As the error bars would not be visible in the graphics, they have been omitted. While the statistical error hence is very small, the systematic error has to be treated with some caution, especially as it has been shown, that results can change significantly when measured with different cell assemblies [30]. However, even when assuming errors of the same magnitude as provided by Bender et al., all conclusions made in this article hold true.

Physical characterisation Scanning electron microscope (SEM) measurements were performed using a FEI Quanta 400 MK2 scanning electron microscope, equipped with a BSE (back scattered electrons) detector, at an electron acceleration voltage of 15e30 kV to obtain an impression of the structure and the surface of the individual PTL’s. In-plane gas permeability measurements were performed according to Bromberger et al. [21] to investigate the fluidic behavior of the PTLs when no flow channels were present underneath the PTL (Fig. 1, bottom right). This was done using an in-house developed test apparatus in which the pressure drops (Dp) of nitrogen at different flow rates were measured. The setup was equipped with a moveable piston which allows

the characterisation of PTLs with various thicknesses. Using Darcy’s law, the in-plane gas permeability Κip can be calculated (Equation (5)). Kip ¼

QN2 ,mN2 ,d Dp,As

(5)

where Kip is the absolute in plane gas permeability, QN2 the given nitrogen flow rate, mN2 is the dynamic viscosity of the gas, d is the thickness of the PTL, As is the cross-sectional area of the PTL sample and Dp is the pressure drop over the PTL sample length.

Results and discussion Pre-test and physical characterisation None of the cell assembly pre-tests as described in section Sample preparation and electrochemical characterisation did indicate any wear or degradation of cell components during the measurement series. HFR values with the Au coated Ti foil range from 0.4 mU*cm2 to 0.8 mU*cm2. As typical cell resistances are in the region of 200 mU*cm2, the changes in the electrical resistances are negligible. SEM images are depicted in Fig. 3 to provide an initial impression of the structures of all PTLs. PTL 1 and the fiber side of PTL 5 which is orientated towards the MEA consist of the same kind of fiber structure and have the same porosities of 51.19% (see Table 1). The structure of the mesh side of PTL 5 was orientated towards the channels. PTL 2 and PTL 4 also consist of the same kind of fibers (thinner in diameter than PTL 1 and PTL 5) but have other porosities. The only powder sinter that was tested within this contribution is PTL 3.

Fig. 3 e SEM images of all investigated PTLs (PTL 1 e PTL 5). As the hybrid material is made of two different structures, both sides are depicted (bottom middle and bottom right). Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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Fig. 4 e Polarisation curves at 40  C (black, filled symbols), 60  C (blue, open symbols) and 80  C (red, half-filled symbols) for all five PTLs in a cell configuration with flow channels underneath the porous transport layers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Cell characterisation with flow channels Influence of the PTL on the polarisation for different temperatures Fig. 4 shows temperature dependent polarisation curves for all tested PTLs. As is known from literature, lower voltage is needed with higher cell temperature originating from a better protonic conductivity of the PEM and faster reaction kinetics. Due to a nearly linear behavior of all polarisation curves at high current densities (>3 A*cm2) it is assumed that MTL does not affect the cell performance for any of the PTLs under the applied operating conditions. The non-linear behavior observed for the 40  C measurement (filled black symbols) is likely to originate from insufficient temperature management within the cell. As the cell

Fig. 5 e Temperature dependency of current density measured during acquisition of a polarisation curve for PTL 2 at inlet and outlet temperatures of the test cell.

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temperature is controlled by the outlet flow at the anode side and both inlet flows are regulated by the same heating circulation, the temperature of the cathodic outlet flow was not controlled. Increasing temperatures at the outlet on the cathode is observed (see Fig. 5) with increasing current density. As production of Joule heat on the cathode side is low (due to low overvoltages), the increase in temperature must be caused by the heat released at the anode and transported through the PEM. Due to the significant voltage drop across the PEM, the PEM itself is a significant heat source as well and therefore contributes to the heating of the cathode side. The effect is depicted in Fig. 5 by the progression of the cell temperatures (inlet and outlet of anode and cathode) with increasing current density (40  C curves as an example). When current density is increased, the cryostat cools down the heating circulation loop to compensate the heat release generated by the cell itself. Hence, the inlet temperatures (black and red line in Fig. 5) therefore decrease. The anodic outlet temperature (blue dotted line) is fairly stable at 40 ± 0.06  C as the cell temperature is regulated to this sensor. The cathodic outlet temperature (green) starts to increase at a current density close to 2 A*cm2 and rises beyond 40  C even with inlet temperatures that decrease to less than 38  C. This effect is also observable at other temperatures but less significant. The effect of heating up the cathode outlet is observable for the PTL 1 e PTL 4, but not for PTL 5 (hybrid material). As the main difference between PTL 5 and the other samples is the in-plane transport characteristic. It is assumed that PTL 5, due to its different construction, is able to remove heat more effectively than PTL 1 e PTL 4.

Influence of the water flow using a cell with flow channels In order to investigate the influence of water flow on the cell performance at high current densities, polarisation curves up to 5 A*cm2 at water flow rates of 0.2e0.8 l*min1 were performed in 0.2 l*min1 steps. The lowest flow rate of 0.2 l*min1 was given by the lower detection limit of the flow rate sensors. The highest flow rate of 0.8 l*min1 was chosen based on

Fig. 6 e H2O volume flow dependend polarisation curves (with flow channels) for two flow rates (0.2 l*min-1 (squares) and 0.8 l*min-1 (circles) and three PTLs (PTL 2, PTL 4 and PTL 5).

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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preliminary experiments with current densities up to 3 A*cm2 and flow rates of 0.5 l*min1 which did not show any MTL. Fig. 6 presents Vi curves with varying water flow rates. Only the data for the minimum flow rate of 0.2 l*min1 and the maximum flow rate of 0.8 l*min1, as well as only three out of five measured samples, are depicted here. Further measured data of the remaining samples and flow rates which are not depicted here were in accordance with the presented results in Fig. 6 (representative samples). All Vi curves show nearly linear behavior indicating that hardly any MTL occurs within the operating windows for the used PTLs. At high current densities (>4 A*cm2), slight differences (10e20 mV) are observable but no clear tendency towards higher or lower voltages and no correlation with the porosity or permeability can be concluded from the results when looking at the dependency of the flow rate. PTL 5 shows the lowest voltages over the entire current density range. This behavior is already observable in the low current density region (inset in the figure) which leads us to conclude that there is a lower anodic overpotential for this PTL. This difference in voltages is assigned to the different interfaces for different PTLs and the associated catalyst utilisation. Also visible in Fig. 6 is the difference in ohmic behavior of the three depicted PTL’s. As described in Section Introduction the ohmic loss is the sum of the ionic resistance of protons within the membrane (hPEM), the bulk electronic resistances from the cell components hbulk and the interface resistances between the cell components hICR. Differences in hPEM and hbulk for the different cell assemblies can be excluded here for two reasons: (1) the same MEA and therefore the same PEM is used for all measurements. (2) bulk electronic resistances Rbulk are assumed to be constant. But different contact resistances RICR could explain the difference in the ohmic behavior as different PTLs lead to different contact areas and therefore to different contact resistances. On the one hand, pretests, to check the cell assembly, indicate that the hICR originated from the PTL and the underlying flow channels do not change significantly with the PTL structure (resistance variation > 500 mU). On the other hand, PTL 2 and PTL 5 have a similar fiber structure and therefore should have a similar contact resistance. But PTL 2

Fig. 7 e Pressure dependend polarisation curves (with flow channels) for two pressure levels (1 barg and 5 barg) and three PTLs (PTL 1, PTL 2 and PTL 4).

and PTL 4 show a higher similarity in performance. It means differences between PTLs are higher than dependency on the flow rate for the used setup.

Influence of the operating pressure using a cell with flow channels At low current densities the Vi curve is dominated by the Nernst potential and the reaction kinetics. Nernstian potential changes to higher values when the concentration, meaning the gas pressure, of the reaction species increases. The beneficial compensation of an increased Nernst voltage was also shown by other others [9,31]. Therefore, the Vi curve will be affected negatively in the low current density region. At high current densities the behavior is different and the main effect originates from the compressibility of the produced gas. For a given current density, the evolving gas bubbles are smaller which affects the flow regime positively and leading to an easier removal of gases and consequently lower hindrance of water getting to the electrode.

Dependency up to 5 barg. As described in Section Laboratory test cell and test bench, all PTLs were characterised at five different pressures (1; 2; 5; 10 and 30 barg). Vi curves for three representative PTLs (PTL 1, PTL 2 and PTL 4) at two different pressures (1 barg and 5 barg) are depicted in Fig. 7. All polarisation curves followed the expected pressure behavior originating from higher Nernst potential in the low current density region (slightly higher voltages at higher pressure), see inset in Fig. 7. But a clear tendency towards higher or lower voltages could not be given for the high current density region as the differences are too small. Over all, Vi curves are more or less unaffected by an increase in pressure when the test cell has flow channels. Dependency up to 30 barg. A further increase in pressure up to 30 barg is not expected to have a major impact on the cell performance. This holds true for three out of the tested five PTLs (PTL 1, PTL 2 and PTL 5), as can be seen in Fig. 8. The curves show the typical negative impact originating from Nernst potential for low current densities smaller than 0.5

Fig. 8 e Pressure dependent polarisation curves for PTL 1, PTL 2 and PTL 5 (cell set-up with flow channels) at two pressure levels (1 barg and 30 barg).

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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Fig. 9 e Pressure dependent polarisation curves for PTL 3 and PTL 4 (cell set-up with flow channels) at two pressure levels (1 barg and 30 barg).

Fig. 10 e Temperature dependent polarisation curves (without flow channels) for three temperature levels (40  C, 60  C and 80  C) and two PTLs (PTL 2 and PTL 4).

A*cm2, see inset. At higher current densities only small deviation of the cell voltage are visible but no clear dependency to higher or lower voltages can be given. In general, Vi curves do not change much in this region. Surprisingly, PTL 3 and PTL 4 showed a non-linear behavior and bend up towards higher cell voltages for higher current densities and increasing pressures higher than 5 barg, indicating MTL within these pressure regime (see Fig. 9). The effect increased when current density increased, resulting in a nonlinear Vi curve above 2 A*cm2. The behavior in the low current density region remains the same for all 5 PTLs. The origin of this non-linear behavior of the cell performance for certain PTLs due to the pressures is not understood yet. It is assumed that the compression behavior of the PTLs has an impact on the cell performance and therefore the cell setup itself. A change in contact resistance due to plastic deformation of the PTL is imaginable in a cell that is not able to react on such effects, like it was used herein. PTL 3 and PTL 4 exhibit a higher compressibility then the other PTLs. A deeper “imprint” of the flow channel structure into the PTL was observed for these PTL’s after disassembling the cells. A better knowledge of the mechanical properties of PTLs would be beneficial and is object to further investigations. Also a change in the pore structure and thus a change in the transport behavior of the fluids cannot be excluded. But as the troughplane behavior of the PTL is more important when a FF is used, the hypothesis of higher ICR is considered more probable.

Influence of the temperature using a cell without flow channels

Cell characterisation without flow channels The investigation of PTLs without flow channels was done by replacing the channels on anodic side of the cell by a solid material plate with the same thickness (see Fig. 1). In this case, the cell performance depends on the in-plane characteristic of the PTLs [23]. Temperature, pressure and H2O flow rate have been varied in the same way as described in the previous section.

When varying the temperature from 40  C to 80  C (at a H2O flow rate of Q ¼ 0.5 l*min1 and a pressure of p ¼ 1 barg), three of the tested PTLs (PTL 1, PTL 2 and PTL 5) showed nonlinear behavior in the medium and high current density region independent of the temperatures, indicating MTL. PTL 3 and PTL 4, the ones with high porosities, show Vi curves with relatively linear behavior over the entire current density range indicating that no MTL occurs. The cell resistance (calculated from the slope of the Vi curve) even flattens off when approaching high current densities. In the first approximation, a drop of DR ¼ 104 mU*cm2 was calculated when comparing the resistance in the region of 2e2.5 A*cm2 with the region of 4e4.5 A*cm2. Fig. 10 shows representative Vi curves of the above mentioned observations of the two groups of Vi curves, one with and one without MTL. PTL 2 is depicted in blue and represents the PTLs where MTL (hMTL ) is assumed to bend the Vi curves up in cell voltage for higher current densities. PTL 4, depicted in black, represents the PTLs where no MTL is observed. The dependency on the temperature, towards lower cell voltage at higher temperature is the same as described in section Cell characterisation without flow channels. The same holds true for the measurements where no flow channels are used. The differences in slope for medium current densities (between 0.5 A*cm2 and 1 A*cm2) indicate changes in the ohmic behavior or beginning MTL. The behavior of the Vi curves below 0.5 A*cm2 can again be explained with increased reaction kinetics at higher T (see inset in Fig. 10). But most interestingly, when discussing the temperature dependency, the nonlinear behavior of PTL 2 does not seem to change much with temperature. The slope is almost the same for all temperatures, meaning that the effect of MTL is either almost independent of temperature or more effects, leading to MTL, counteract each other. As mentioned in the introduction, we believe that incomplete gas removal and the linked hindrance of water supply to the electrode is one of the main causes of MTL. With this and the observation of unchanged

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

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3.50

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Current density i / A*cm-2 Fig. 11 e H2O flow rate dependent polarisation curves (without flow channels) for two flow rates (0.2/min and 0.8 l*min-1) and three PTLs (PTL 1, PTL 2 and PTL 5).

Fig. 12 e Pressure dependent polarisation curves (without flow channels) for two pressure levels (1 barg and 30 barg) and three PTLs (PTL 2, PTL 4 and PTL 5).

MTL with higher temperature, we conclude that temperature does not help significantly removing the oxygen bubbles from the PTL. The key difference of the PTLs when measured without flow channels, is the transport characteristic in inplane direction, the respective values are given in Table 1. PTL 4 shows the highest value (2.25*1011 m2) and PTL 2 the lowest value (2.25*1012 m2) of in-plane gas permeability. The result of this difference is well observable in Fig. 10, where PTL 2 shows MTL and PTL 4 does not. The order of the inplane gas permeabilities correlate with the extend of the observed MTL, for all PTLs where permeability data is available.

more passively in through-plane direction, like a PTL does in the tests with flow channels. PTL 5 was depicted anyway to show that such effects can occur with these kinds of structures as well, meaning that a mesh cannot replace the flow channels completely or at least has to be chosen according to the operating condition.

Influence of volume flow in cells without flow channels In Fig. 11, the Vi curves of three PTLs (PTL 1, PTL 2 and PTL 5) are shown for the minimum (0.2 l*min1) and the maximum (0.8 l*min1) water flow rates as they show the full range of cell voltages that were measured (better readability). As can be seen, MTL occurs for all three PTLs at the minimum flow rate of 0.2 l*min1. For the PTL, where MTL is most pronounced (PTL 2, crossed black symbols), the effect is still visible at the maximum flow rate of 0.8 l*min1. For PTL1 and PTL 5, the Vi curves show almost linear behavior at 0.8 l*min1. Apparently, increasing the water flow rate can suppress MTL, most probably by removing the immobile oxygen bubbles. The flow rate needed to efficiently do so, however, will depend on the PTLs used. While for PTL 1 and PTL 5, the flow rate of 0.8 l*min1 is sufficient, this is no longer the case for PTL 2. Based on these observed results, one could define critical in-plane permeabilities, but the values would only be significant for the considered cell geometries and operating parameters. A universal critical in-plane permeability does not exist. It is important to note here, that the circumstances for PTL 5, do not fit to the general idea of having entirely forced transport inside the PTL when measured without flow channels. The mesh acts like channels (H2O distributor) and the fiber structure above, controls the transport of water and gas

Influence of operating pressure using cells without flow field When recorded under increasing pressure, the Vi curves show lower cell voltage at a given current density (see Fig. 12). At low current density, the expected negative influence, originating from the pressure dependency of the Nernst Voltage, is observed for all PTLs (see inset of Fig. 12). Three representative PTLs are depicted at minimum (p ¼ 1 barg) and maximum (p ¼ 30 barg) pressure. Two of them show significant MTL (PTL 2 and PTL 5) and one (PTL 4) show almost linear behavior up to 5 A*cm2. PTL 4 has the highest porosity and the highest in plane gas permeability (see section Influence of the temperature using a cell without flow channels). Even though voltage is already relatively low at low pressure, a positive effect due to a pressure increase was observed. PTL 2 and PTL 5 have a lower porosity and, at least PTL 2, a lower in-plane gas permeability. As mentioned above, permeability could not be measured for PTL 5 due to its particular structure with mesh and fibers. The contacting structure to the MEA is the same for bath PTL 2 and PTL 5. Both samples show MTL for low pressure. For PTL 5, the effect almost disappears at 30 barg and the Vi curve shows almost linear behavior up to 5 A*cm2 (small steps in the Vi curve can be explained by the mean value calculation). MTL cannot be suppressed completely for PTL 2. In general one can see from Fig. 12 that the stronger the MTL is at lower pressure, the stronger is also the benefit of increasing pressure, with the strongest effect for PTL 2. This means, to certain extend, MTL losses can be decreased by increasing the pressure, when the PTL structure was not chosen properly. Almost all Vi curves (except PTL 2) show linear behavior at 30 barg.

Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204

international journal of hydrogen energy xxx (xxxx) xxx

Conclusion In this study, the performance and the influence of temperature, pressure and flow rate of two different cell designs (one with and one without flow channels on the anodic side) is presented. Five different PTL structures were investigated by analyzing the polarisation curves. We observed significant differences in the performance between the two cell configurations. Measurements with flow channels: For the investigation of PTLs combined with flow channels underneath, in general no strong mass transport losses were observed up to the maximum current density of 5 A*cm2 and the minimum temperature of 40  C. In addition, the cell performance does not change significantly in dependency on the operating parameters and differences between the tested PTLs are rather small: The increase of the flow rate from 0.2 l*min1 to 0.8 l*min1 changes the voltage by ~0.03 V at the highest measured current density of 5 A*cm2. Equally, the increase of pressure from 1 to 30 barg in three cases only reduces the voltage by ~0.06 V. For two of the tested cases, however, increasing the pressure from 1 to 30 barg has shown a strong negative effect with an increase of voltage for PTL 4 by ~0.37 V (and would be even higher for PTL 3) at current densities of 5 A*cm2. In general, the cell performance is defined by the balance of how well water as reactant is transported to the electrode layer and how well product gases are transported away into the flow channel. Small pores are beneficial for water transport (due to their micro capillary behavior) and large pores are beneficial to remove the gases. Measurements without flow channels: When the PTL does not have flow channels underneath, dependencies on the operating parameters become significant. Increasing the water flow rates from 0.2 l*min1 to 0.8 l*min1 has a significant effect of ~0.62 V at current densities of 3 A*cm2. Also, increasing the pressure from 1 barg to 30 barg improves the cell performance (e.g. voltage reduction by ~0.55 V for PTL 2). Both results demonstrate the beneficial effect of faster gas removal from the cell. Due to the higher gas fraction, the effect becomes more important for high current densities, but is PTL specific, such that no general limit can be provided. Increasing the temperature from 40  C to 80  C does not seem to change MTL significantly. For the cell design without flow channels, the in-plane gas permeability is an important parameter to look at. This can play a major role when innovative stack designs are developed where milled flow channels and thick PTLs are typically too expensive, especially when stacks become larger in active area and current densities are increased. The permeability of the PTL, or the PTL-support combination, must therefore be chosen according to the stack size and operating conditions. Based on these observations, we conclude that an insufficient removal of oxygen gas originating from the anodic PTL seems to be one of the key sources for significant losses that lowered performances. When transport of the fluids within the PTL is adequate, no MTL is observable up to 5 A*cm2. While traditionally, only porosities and particle/fiber

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diameters are published as PTL characterisation, we found that the in-plane and through-plane permeabilities (depending on water saturation, capillarity, tortuosity, wettability, compressibility, thickness and accessibility of the PTL for water) provide key insights on the transport properties and should be measured in order to properly characterise PTL structures for PEM electrolysis.

Acknowledgements The authors acknowledge the financial support of the Federal Ministry of Transport and Digital Infrastructure/Bundesministerium fu¨r Verkehr und digitale Infrastruktur (BMVI) under grant number 03BI110. We would like to thank the company Baekart and Ruben de Bruycker for the fruitful collaboration and the permission to publish some of the results obtained with their PTL.

references

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Please cite this article as: Lickert T et al., On the influence of the anodic porous transport layer on PEM electrolysis performance at high current densities, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.204