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Cell Performance Determining Parameters in High Pressure Water Electrolysis
Electrochimica Acta 211 (2016) 989–997
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cell Performance Determining Parameters in High Pressure Water Electrolysis Michel Suermanna , Thomas J. Schmidta,b , Felix N. Büchia,* a b
Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland Laboratory of Physical Chemistry, ETH Zürich, CH-8093 Zürich, Switzerland
A R T I C L E I N F O
Article history: Received 16 March 2016 Received in revised form 22 June 2016 Accepted 23 June 2016 Available online 23 June 2016 Keywords: Polymer electrolyte water electrolysis overpotential analysis pressure dependence
A B S T R A C T
When hydrogen is used for energy applications, then often elevated pressure is required, i.e. up to 1000 bar for hydrogen refueling stations. When polymer electrolyte electrolyzers are operated at elevated pressures, to reduce external mechanical compression needs, the thermodynamically expected cell voltage increase with pressure is not generally observed. Consequently, beneficial processes have to compensate for the increased work performed. In this paper, the influence of the operating pressure up to 100 bar on the cell voltage behavior is investigated in detail. Data from galvanotstatic polarization curves up to 4 A/cm2 combined with high frequency resistance measurements are analyzed using a zerodimensional Tafel model. It shows that beneficial processes with increasing pressure are related to both the kinetic and mass transport overpotentials. At current densities above 1 A/cm2 about two thirds of the overpotential gain can be related to reaction kinetics. Based on the Tafel model approach an increase of the apparent exchange current density by a factor of 4 to 7 is observed for the pressure increase from 10 to 100 bar. In contrast, the Tafel slope is independent of pressure and only the expected increase with temperature (73 to 81 mV/dec) for 30 to 70 C is observed. Mass transport losses decrease with pressure. At 50 C and 2 A/cm2, increasing the pressure from 1 to 100 bar results in a decrease of the mass transport overpotential from 130 to 80 mV. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction A major challenge of renewable power technologies (solar and wind) is their intermittent electricity production. When production and consumption do not match, storage is required in particular to avoid curtailment [1]. Electricity storage for diurnal up to seasonal time frames can be performed by the power-to-gas concept converting electrical energy into hydrogen by using water electrolysis [2]. Polymer electrolyte electrolysis cell (PEEC) technology has potential advantages for this application in terms of dynamic behavior over other electrolysis technologies such as alkaline electrolysis (AEC) or high temperature solid oxide electrolysis (SOEC). PEEC offers dynamic operation with fast response times [3], a wide duty range (ideally 0 to 100%), good start/stop behavior and the possibility to run at ambient temperature. In comparison AEC usually needs auxiliary heating for start-up and its partial-load operation range is limited to 20 to 40% [4]. Approaches to run AEC
* Corresponding author. E-mail address: [email protected] (F.N. Büchi). http://dx.doi.org/10.1016/j.electacta.2016.06.120 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
at ambient temperature still produce a significant efficiency penalty [5]. SOEC might rather be interesting for continuous conversion due to the high operating temperature. Hydrogen produced in the power-to-gas process requires storage of the pressurized gas, where pressure depends on the final application. Pressures up to 80 bar are required for feeding into natural gas pipelines and up to 1000 bar for hydrogen refueling stations for mobility. Water electrolysis also produces oxygen in a stoichiometric ratio, which can be used for reelectrification and grid services using highly efficient hydrogen/ oxygen polymer electrolyte fuel cells (PEFC) systems [6]. Conventionally, electrolyzers produce hydrogen at 30 bar and the gases are compressed mechanically for the desired application. With PEEC direct electrochemical compression up to 345 bar has been demonstrated [7,8], which is considered to be more efficient than mechanical compression. Thus, pressurized PEEC can be used either as a stand-alone device or to reduce the number of mechanical compression stages, which results in lower investment and operating costs. In principle electrochemical hydrogen compression [9–11] could be used instead of mechanical compression.
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Interestingly, the cell voltage of PEEC operated at elevated balanced pressures shows an unexpected behavior, as the expected higher thermodynamic equilibrium potentials are not directly observed as increased cell voltage [12–14]. This is different from the behavior of PEFC where gas pressure has a prominent influence on the cell performance. The question arises what causes the difference between the observed cell voltage and the expected thermodynamic voltage due to pressurization? In literature it is discussed that higher pressure leads to smaller gas bubbles which may improve the two phase flow and therefore the water supply at the anode [15–20]. However, the fundamental causes have not been investigated in depth so far. An analysis of the influence of pressure up to 25 bar has been given 2001 by Grigoriev et al. [14]. The authors discovered that the anodic overpotential significantly decreases and the cathodic overpotential slightly increases with increasing pressure. It was also claimed that smaller bubbles facilitate the water transport, reduce the ohmic losses in the catalyst layer and improve the electric contact towards the porous transport layer. However there is still inconsistency, especially with respect to the contributions of the different overpotentials as a function of pressure and current density. Thus, a better insight into the contribution of the different overpotentials in PEEC operation is required, ultimately leading to better understanding of pressure effects on the electrochemical processes. This will be beneficial in the further development of PEEC such as catalyst and porous transport layer structures, respectively. Therefore, we characterized PEECs with commercial electrochemical components by galvanostatic polarization curves (up to 4 A/cm2) in a wide parameter domain of temperature (30 to 70 C) and pressure (up to 100 bar balanced). From the consistent data set the contributions of the different overpotentials on overall cell voltage is analyzed using a zero-dimensional Tafel model. Based on the findings from this combined set of data the possible beneficial processes with increasing pressure and temperature in PEEC are discussed. 2. Experimental 2.1. Electrolysis cell characteristics Experiments were made using a differential cell (i.e. negligible gradients along the water flow direction in the cell) with a square active area of 4 cm2. The flow field consists of 5 parallel channels of 2 1 mm (width x depth) separated by ribs of 2 mm with a length of 21 mm. All parts of the cell in contact with the fluids are gold coated. The cathode flow field is supported by an array of springs to have a reproducible contact pressure on the active area. Commercial catalyst coated membranes (CCMs) based on Nafion117 (N117) and N212 (GreenerityTM E400, SolviCore, D) combined with porous transport layers (PTL) made of sintered titanium powder (SIKA-T10, 2 mm thickness, GKN Sinter Metals Filters, D) were used. By means of ex-situ X-Ray tomographic microscopy the PTL was characterized as isotropic with a porosity of about 35% and a median pore size of 31 mm. Identifying the cell specific electronic resistance is important to ensure an accurate data analysis, particularly with respect to the contribution of the different overpotentials. From high frequency resistance (HFR) measurements of cells with the two membrane thicknesses, the ratio of the ionic and electronic ohmic resistance can be determined by extrapolating the measured HFR data for known membrane thicknesses to zero. An electronic area resistance of 23 mVcm2 was determined, irrespective of the current density and cell temperature as shown in Fig. 1. The cell
Fig. 1. Comparison of the HFR of N117 and N212 for different current densities and cell temperatures at 10 bar. The HFR-axis intercept represents the electronic area resistance of the cell.
voltages and HFR data presented in this work are corrected by the electronic resistance. 2.2. Test bench The test bench has a conventional set-up with two completely separated gas/water loops for anode and cathode with a gas/liquid separator each. Deionized water (<1 mS/cm) is fed by a HPLC pump to the anode. The water level in the cathode separator is kept constant by draining. All parts are from stainless steel and designed for a maximum operating pressure of 300 bar. To have a controlled and homogeneous temperature in the small cell the water is heated in the gas/liquid separators having a water volume at anode and cathode sides of about 100 mL. At the anode the water is recirculated with a volume flow of 30 mLmin1cm2 using a diaphragm pump (LEWA GmbH, D). At the cathode natural recirculation takes place. Thus the major heat fraction for cell heating/cooling is transferred through the anode side. The cell temperature is measured with a K-type thermocouple close to the active area inside the cell housing and equilibrated for a given cell temperature at a current density of 2 A/cm2. This current density was selected to avoid combustible mixtures in the electrolyzer even at high operating pressures. This operating point is used both as a reference steady state condition and as the starting point for the polarization curves. 2.3. Measurement set-up All electrochemical measurements were performed using a potentiostat (VSP-300, with two 5 V/10A booster boards, Z-board, Bio-Logic SAS, F). The parameters for the galvanostatic polarization curves were optimized (see chapter 4) and performed from low to high current densities with a holding time of 10 seconds at each measurement point. All polarization curves shown are the average of at least two measurements. Using a full spectrum from galvanostatic electrochemical impedance spectroscopy measurement, the high frequency intercept was found to be at around 25 kHz. Thus the high frequency resistance (HFR) was measured for every current density at 25 kHz for one second.
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3. Theory and calculations
by eq. (4):
The voltage of the electrolysis cell Ecell is considered to be the difference of the two pressure and temperature dependent electrode potentials of the anode and cathode E0A(p,T) and E0C(p, T) to which the three main overpotentials add:
hkin ¼ b log
j j0
991
ð4Þ
where hiR denotes the ohmic, hkin the kinetic and hmtx the mass transport overpotential, respectively. The difference of the two electrode potentials defines the thermodynamic cell voltage:
where the kinetic overpotential hkin is equal to the difference between the Tafel line and the thermodynamic cell voltage according to eq. (2), b is the Tafel slope, equal to 2.3RT/(aF), and a is the transfer coefficient, a constant related to the mechanism of the electrochemical reaction including the number of transferred electrons. In eq. (5) the apparent exchange current density j0 is defined as:
E0A ðp; TÞ E0C ðp; TÞ ¼ E0cell ðp; TÞ
ð2Þ
j0 ¼ j0;s rf
ð3Þ
where j0,S is the specific exchange current density (in terms of A/ cm2 of catalyst surface) and rf is the roughness factor. The latter is the ratio of the electrochemical surface area (ECSA) to the geometric area. According to the transition state theory, in equilibrium the specific exchange current density [26] can be expressed as:
Ecell ¼ E0A ðp; TÞ E0C ðp; TÞ þ hiR þ hkin þ hmtx
which can be calculated from the Nernst equation: pffiffiffiffiffiffiffiffiffiffi ! aðH2 Þ aðO2 Þ RT ln E0cell ðp; TÞ ¼ E0 ðTÞ þ 2F aðH2 OÞ
ð1Þ
with a the activity of the species and E0(T) the temperature dependent equilibrium potential [21]. Since the solubility of hydrogen and oxygen in water is constant up to 100 bar, ideal gas behavior for both H2 and O2 can be assumed, because at thermodynamic equilibrium conditions only the dissolved gas in the water is in contact with the electrodes [22– 24]. Consequently, the activity can be expressed by the partial pressure. The activity of water is assumed as unity. The ohmic overpotential hiR is obtained from the HFR measurement and represents the sum of the ionic and electronic ohmic losses. By subtracting the ohmic overpotential from the cell voltage, the iR-free cell voltage is obtained. The iR-free cell voltage shows a Tafel behavior at low current densities. This Tafel behavior is considered a property of the anode as the polarization of the cathode is minimal at the low current densities and the potential close to the one of a reversible hydrogen electrode (RHE) [25]. To the linear regions, the Tafel line can be fitted as described
j0;s ¼ F c kh
ð5Þ
ð6Þ
where c is the concentration of the reactant and kh is the heterogeneous electrochemical rate constant: ! DG06¼ kh ¼ k0 exp ð7Þ RT where k0 is the rate constant and DG06¼ is the standard Gibbs energy of activation. As long as only a single Tafel slope for the entire current density range is considered, the difference between the Tafel line and the iR-free cell voltage defines the mass transport overpotential hmtx. In this analysis only cell performance related overpotentials are considered. Additional system related losses such as gas crossover are not part of this work. Nevertheless they play an important role
Fig. 2. Tafel plots of the iR-free cell voltage for N117 at 50 C and 50 bar for current holding times of a) 1 s, b) 2 s, c) 10 s and d) 20 s in increasing and decreasing current measurement direction. Tafel line is fitted between 10 and 120 mA/cm2.
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Table 1 Influence of the holding time and the measurement direction on the Tafel slope b for increasing and decreasing current, the difference in the iR-free cell voltage between increasing and decreasing current in the 10 to 120 mA/cm2 interval (DEiR-free) and the corresponding volume exchange of the gas in the anodic PTL at 1 bar and 1 A/cm2. holding time
b increasing
b decreasing
DEiR-free
PTL volume exchange at 1 bar and 1 A/cm2
HFR at 0.4 A/cm2
HFR at 4 A/cm2
– 0.83 1.66 8.29 16.57
mV cm2 200.2 2.5 203.2 0.0 203.1 1.5 202.5 0.0
mV cm2 171.4 1.8 172.5 0.6 171.5 1.4 170.1 0.2
incr. vs. decr. s 1 2 10 20
mV/dec 75.5 0.6 75.7 0.6 77.9 1.4 82.8 0.4
mV/dec 75.3 0.1 77.0 0.5 77.5 0.7 79.4 0.2
mV 5.2 6.1 6.8 12.3
when it comes to system efficiencies, especially at high pressure operation. 4. Measurement protocol for polarization curves Ideally a single measurement protocol for polarization curves can be employed to characterize and compare the cell behavior at different operating conditions with respect to temperature and pressure. In particular when considering gas pressures of 100 bar the protocol needs to be carefully considered. Gas pressure needs to be considered not only for performance but also with respect to safety. Since the gas crossover through the membrane is governed by the partial pressure difference according to Fick‘s law, combustible atmospheres [27] can build up at low current densities. When no additional gas-recombination measure are taken (e.g. Pt coating of the anodic PTL or membrane surface, or catalytic reactors with Pd or Pt recombination catalysts [16]), there is a lower current density limit for continuous safe operation. But even for transient measurements, such as current voltage curves, the gas crossover severely limits the safe measurement time at low current densities. With polymer electrolyte fuel cells (PEFC), typically long holding times (up to 20 min) are required for accurate currentvoltage curve characterization. In PEFC this is due to the complex water management (time constants of water sorption and desorption of the membrane). In PEEC, where liquid water is always in contact with the membrane, this is not a limitation. On the other hand in electrolysis much higher specific power densities can be achieved than in PEFC, and therefore also significantly higher specific thermal loads have to be dealt with. While in PEFC maximum specific heat loads are limited to about 1 W/cm2, in the case of electrolysis, values in the order of 10 W/cm2 may be reached due to higher current densities and higher overpotentials. So in the case of PEEC at low current densities (and high pressures) the galvanostatic holding time to measure current-voltage curves is limited due to safety restrictions, while at high current densities thermal equilibrium may take a long time to reach due to the large specific heat produced. In order to overcome safety issues at low current densities as well as thermal issues at high current densities, short measurement times for the polarization curves become important. Therefore the characteristics of polarization curves for different short holding times of 1, 2, 10 and 20 seconds (plus one second each for HFR measurements) were investigated. Evaluation conditions of 50 bar and 50 C were chosen. After thermal equilibration of the cell at 2 A/cm2, polarization curves were conducted in forward (increasing current density starting from 1 mA/cm2) and backward (decreasing current density starting from 4 A/cm2) directions to check for hysteresis effects. Measurements were carried out up to 4 A/cm2. Current steps were chosen as 1 mA/cm2 below 10 mA/ cm2, 10 mA/cm2 below 50 mA/cm2, 50 mA/cm2 below 100 mA/cm2, 100 mA/cm2 below 200 mA/cm2 and 200 mA/cm2 above 200 mA/ cm2. As the region of low current densities is of central interest for the desired overpotential analysis, in Fig. 2 only current densities up to 1 A/cm2 are shown.
Only small differences are observed in the polarization curves with the different holding times. Comparing the iR-free cell voltage of increasing and decreasing current measurement direction in the fitted region, a difference of 12 mV at 20 s compared to 5 to 7 mV at 1 to 10 s holding time is observed. Since the membrane is well humidified due to the presence of liquid water, the difference must be due to minor thermal issues. For thermal analysis HFR data can be used. Above 2 A/cm2 (not shown here) the cell temperature increases more at higher holding times, while below 2 A/cm2 the low waste heat production compared to the equilibration current density of at 2 A/cm2 leads to a small cooling effect, slightly increasing with increasing holding time. This cooling effect is small in the Tafel region, as is shown below in Fig. 4b) where HFR variation below 0.5 A/cm2 is minimal. HFR data for 0.4 and 4 A/cm2 (measured at the end of the holding time) are shown in Table 1 for the different holding times. The independence of the HFR, respectively the temperature on holding time shows that the active components are close to thermal equilibrium at all holding times investigated. The quantitative correlation between HFR and temperature in the active area is discussed in in detail section 5.1 below. For fitting a representative Tafel line, the linear region at low current densities is of particular interest. However, especially at current densities below 10 mA/cm2 an unexpected non-linear behavior is noticed. Because its extent decreases with increasing holding time, it may be attributed to non-equilibrium conditions.1 The Tafel line is therefore fitted in the region between 10 and 120 mA/cm2. The resulting Tafel slopes as a function of the measurement direction and holding time are given in Table 1. Furthermore to estimate the (theoretical) gas volume exchange (neglecting any water flow) in the anodic PTL, during the holding time of the polarization curve, is calculated for ambient pressure and 1 A/cm2 is also listed in Table 1. The values in Table 1 show that the Tafel slope is slightly increasing with longer holding times. Because of the still good agreement of the Tafel slope for the forward and backward directions of less than 1 mV/dec and the relatively small difference of the iR-free cell voltage between increasing and decreasing measurement direction in the Tafel line fit region, and a reasonable gas volume exchange rate for the anode PTL, 10 s holding time and forward measurement direction are chosen for all measurements presented here. The relatively short holding time is a compromise between the safety and equilibrium requirements. Still, the thermal analysis shows that deviation from the thermal equilibrium is considered minor, as shown by the HFR data in Table 1 and Fig. 4a).
1 At very low current densities, the gas crossover equivalent current density (for N117 about 0.3 mAbar1cm2) can exceed the applied current density. Then the hydrogen oxidation reaction (HOR) of the permeated gas may have a dominant influence on the cell voltage, reducing it below the reversible potential of water electrolysis. When using platinum at the anode such a HOR mechanism is clearly noticed (not shown in this work).
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5. Results and discussion It is an interesting experimental observation that in most reports, increasing balanced gas pressure in PEEC does not lead to the thermodynamically expected increase of the cell voltage. Therefore the influence of operating pressure on the different overpotentials (kinetic, ohmic and mass transport) is analyzed based on the model presented in eq. (1) and the analysis outlined in section 3. For detailed understanding of the overpotentials also the parameter space for temperature (30 to 70 C) and current density up to 4 A/cm2 is explored. Measurements have been performed for two membrane thicknesses to underline the generic character of the results. 5.1. Ohmic overpotential The ohmic losses mainly consist of the proton transport in the membrane. The membrane independent part of the high frequency resistance, related to the cell set-up specific electronic resistance, was corrected as discussed in chapter 2. In Fig. 3 the cell voltage and the iR-free cell voltage for cells with N117 and N212 at 50 C and 10 bar are plotted vs. the current density. Above about 0.5 A/ cm2 small differences in the iR-free cell voltage between the two membranes of up to 40 mV are observed which can be explained by higher waste heat production (and thus higher temperature in the catalyst layer) for the thicker membrane reducing the kinetic overpotential. The temperature dependence of the high frequency resistance is shown in Fig. 4 a) from which a HFR decrease with increasing current density due to the increasing heat production can be deduced, even with the fast polarization curve protocol. The thermal influence of the waste heat is larger at lower cell temperatures, both due to a larger total overpotential and a higher ratio of the waste heat production compared to the heat imparted, respectively. For the current density of 2 A/cm2, where the cell temperature has been equilibrated before polarization curves, an Arrhenius plot is presented in Fig. 4 b). The calculated activation energy for proton conduction (Ea) of 0.12 0.01 eV corresponds well to the values reported in literature of about 0.12 to 0.15 eV [28,29]. From Fig. 4 b) a scaling factor of the HFR with membrane thickness of 3.1 to 3.2 can be deduced. This is somewhat less than the formal value of 3.5 expected from the dry membrane thickness, but well in line with values for swollen membranes [30,31] resulting in a swollen thickness ratio of 3.1 to 3.2. The thicker membranes swell less from dry to fully hydrated, as is the case during operation in PEEC. The data from the Arrhenius plot can also be used to predict the actual membrane temperature for a distinct current density. For instance, at 50 C and 10 bar, a decrease of the HFR at 4 A/cm2 of about 21.7 mVcm2 and 3.5 mVcm2 for N117 and N212, respectively, is observed, indicating an increase of the membrane temperature of about 9.8 C and 5.9 C at 4 A/cm2. This temperature increase is actually difficult to measure with a thermocouple close to the active area inside the cell housing (typically only +/ 0.5 C during I/V-curve), as there is a temperature drop between water in the channel and the CCM due to the heat transport in the PTL. When considering heat loss only to the anode, where water is recirculated, then the temperature gradient across the PTL can be calculated based on the heat conductivity of titanium of 21.9 W/ (mK) and the structure of the PTL described in the experimental section. Assuming a solid tortuosity of t = e0.5 = 1.24 and using Fourier‘s law a theoretical heat loss in the PTL of 10.3 C and 6.5 C for N117 and N212, respectively, is calculated, close to the values deduced from the HFR measurements. Considering the influence of pressure on the HFR, at 50 C only small differences close to the standard deviations are observed for
Fig. 3. Cell and iR-free cell voltage for N117 and N212 at 50 C and 10 bar balanced gas pressure.
the pressure range from 1 to 100 bar, with a tendency to lower resistance values at higher pressures in particular at low current densities. It may be speculated that higher pressure may lead to less waste heat production due to lower overpotentials according to a higher thermodynamic voltage (see eq. (2)). It should be taken into account, however, that for 50 C the difference in the thermodynamic voltage between 1 and 100 bar with 96 mV is rather small compared to the overall overpotential. 5.2. Mass transport overpotential The iR-free cell voltage for 1, 10 and 100 bar at 50 C is plotted semi-logarithmically vs. the current density in Fig. 5 a). At current densities up to about 200 mA/cm2, the expected, linear voltage behavior is observed indicating that no mass transport losses occur. Furthermore, at low current densities the cell voltage increases with increasing pressure as expected from thermodynamics until, with increasing current, the cell voltages coincide at around 1 A/cm2. At higher current densities, cell voltages are practically independent of pressure, but clearly an offset of the iR-free cell voltage from the extrapolated Tafel line is seen. This difference increases with current density to about 150 mV at 4 A/cm2. This loss is considered the mass transport overpotential. It increases monotonically with current, but flattens out at current densities above 2 A/cm2, which is attributed to an artifact of the nonequilibrium polarization curve measurement, i.e., the temperature of the CCM increases (Fig. 5 b) effectively reducing mass transport losses. The effect of pressure on the mass transport loss as deduced from Fig. 5 a) is summarized in Fig. 6 a). Above 3 A/cm2, the mass transport overpotential decreases slightly from around 170 to 140 mV for the pressure increase from 1 to 100 bar. The influence of temperature on the mass transport losses (Fig. 5 b) is analyzed in Fig. 6 b). A temperature increase from 30 to 70 C results in a decrease in transport overpotential from 200 to 100 mV above 2 A/cm2. This may even be biased by a thermal effect. The flattening of the iR-free cell voltage is stronger at lower cell temperatures. So the decrease of mass transport overpotential with temperature is probably even bigger under thermal equilibrium conditions.
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Fig. 4. a) HFR measurements of N117 and N212 for different cell temperatures and operating pressures as a function of current density; b) Arrhenius plot of the HFR data at 2 A/cm2 and 10 bar, for the data in sub-plot a).
Fig. 5. Tafel plots of N117 for different temperatures and pressures; a) for 1, 10 and 100 bar at 50 C and b) for 30 to 70 C at 10 bar.
When water transport in the gas phase contributes to the mass transport overpotential, the temperature effect may be due to the changing water partial pressure. In case liquid transport is critical and limiting, the decreasing viscosity and/or surface tension of liquid water may play a role. Furthermore the change of wettability of the hydrophilic pores of the PTL and/or CL [32] can have an influence. The effects of these potential causes cannot be separated with the present data. Summarizing, it can be stated that mass transport contributes to the cell voltage starting at current densities above 200 mA/cm2, reaching 100 to 200 mV at 2 A/cm2. Reasons of the mass transport losses are not clear, but it seems obvious that the two phase flow at the anode plays a critical role. To get a better understanding on the causes of the mass transport overpotential and its dependence on pressure and temperature, more data with different PTL/CL combinations and/or microscopic resolution is needed. Especially the interface between the PTL and the CL layer seems to be of interest. First steps using imaging have been made by Hoeh et al. [33,34] investigating the bubble formation at the PTL/channel interface as a function of current density and water flow rate using X-ray and neutron radiography. 5.3. Kinetic overpotential The kinetic overpotential is calculated from the difference between the fitted Tafel lines (as in Fig. 5) and the thermodynamic cell voltage calculated according to eq. (2). Typically, the anodic kinetic overpotential is significantly higher than the cathodic due
to the sluggish oxygen evolution reaction [35]. The value of the thermodynamic voltage is influenced by the activities of gases and water in eq. (2). It seems undisputed that the activity of the gases is represented by their partial pressures. However, how about activity of the liquid water? Is it really pressure independent and unity? With these assumptions, at 50 C, an increase of the thermodynamic voltage of 48 mV/dec is expected. However the measured iR-free cell voltage difference between 1 and 100 bar at 10 mA/cm2 is only 23 mV/dec (see Fig. 5 a), which indicates that either the assumptions for the activities are not correct or even at small current densities processes, not described by thermodynamics, take place. The different overpotentials are summarized in Fig. 6 a) as function of pressure at 50 C and in Fig. 6 b) as function of temperature at 10 bar. Increasing the pressure decreases the kinetic and transport overpotentials, while increasing cell temperature reduces all three loss mechanisms. In Fig. 6 it can be seen that for cells with N117 membranes, the kinetic overpotential is the dominant loss up to a current density of about 2 A/cm2. With thinner membranes this range is shifted to higher current densities. The mass transport overpotential plays a minor role at low current densities. However in the commonly used operation range of 1 to 3 A/cm2 it contributes 10 to 15% to the overall overpotential. For thinner membranes it will contribute a higher fraction in the same current density range. The fact that the kinetic and transport overpotentials drop with pressure is further illustrated by the values given in Table 2. Negative values indicate a reduction of the overpotentials with increase of the gas pressure from 1 to 100 bar.
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Fig. 6. Contributions of kinetic, ohmic and mass transport overpotentials for N117 as a function of current density; a) at 50 C for 1, 10 and 100 bar and b) at 10 bar for 30, 40, 50, 60 and 70 C.
The data in Table 2 show that only at low current densities a higher cell voltage is observed with the pressure increase over two decades. However, the raise in cell voltage is less than predicted by theory for an activity of liquid water equal to one. With increasing current densities the beneficial influences of mass transport and kinetics are more and more compensating the higher thermodynamic voltage. Finally at the highest current densities the cell voltage even slightly drops with increasing pressure. At low current densities the kinetics contribute most to cell voltage reduction. At current densities of 1 A/cm2 and larger the transport contributes about one third to the overall beneficial effects. The question arises why the reaction kinetics are improving with increasing pressure. Several parameters might be influenced according to the Tafel model. With respect to the reaction kinetics, pressure could influence (i) the Tafel slope or (ii) the apparent exchange current density caused by a change in the ECSA and/or specific exchange current density. Both hypotheses are discussed below: i) Tafel slope: a pressure dependent Tafel slope would mean that the transfer coefficient a (see eq. 3) is changing with pressure, meaning that a change in the reaction mechanism occurs, which cannot be completely excluded with the oxygen evolution reaction being a complex series of consecutive steps [36–38]. However in contrast to former work [39], no distinct trend with pressure could be observed here. The average Tafel slope at 50 C (obtained from 27 polarization curves with N117 in the pressure range from 1 to 110 bar) is found to be 76 5 mV/dec which corresponds to a transfer coefficient a of 0.84. For the change of the cell temperature from 30 to 70 C the Tafel slope increases as expected from 73 to 81 1 mV/dec, respectively, resulting in a transfer coefficient between 0.82 and 0.85. The obtained Tafel slopes are in good agreement with literature [40]. ii) Exchange current density: to the best of our knowledge there is only a single report in the literature by Gregoriev et al. [14] discussing the influence of pressure on the exchange current Table 2 Summary of the contributions of the kinetic and mass transport overpotentials at 100 bar (as compared to 1 bar operation) to the difference in the cell voltage for selected current densities. j
DE (p,T) Dhkinetic Dhmasstransport DEcell 0
A/cm2
0.001
0.01
0.1
0.4
1
4
mV mV mV mV
96 43 6 59
96 51 1 44
96 58 0 38
96 63 9 24
96 66 34 -4
96 70 36 -10
density. With an increase of gas pressure from 1 to 25 bar, the authors calculated an increase of the exchange current density by a factor of 3.5 to 8 for the anodic and of 20 to 45 for the cathodic reaction, depending on the temperature. For the present data, the apparent exchange current density (calculated from the extrapolation of the Tafel line to the equilibrium potential) is shown in Fig. 7 as function of pressure and temperature. At constant temperature (50 C) an increase of a factor of 4 to 7, is observed for the pressure increase from 10 to 100 bar. Below 10 bar the data is not conclusive as there is considerable noise due to the extrapolation over several orders of magnitude of current density. With temperature (from 30 to 70 C) at a constant pressure of 10 bar, the apparent exchange current density follows nicely an Arrhenius behavior (see Fig. 7 b) with an apparent kinetic activation energy of approx. 67 kJ/mol. An increase of the apparent exchange current density with pressure explains the trend of decreasing kinetic losses (detailed in Table 2). Consequently either the roughness factor and/or the specific exchange current density are pressure dependent factors. With respect to a pressure dependent increase of the roughness factor it may be speculated that pressure promotes the wetting of smaller pores of the catalyst or catalyst layer resulting in a higher effective electrochemical surface area. However, the wettability behavior of hydrophilic and/or hydrophobic micro-pores is not a trivial task to resolve unambiguously and would be a study on its own. Concerning the specific exchange current density either the concentration and/or the standard Gibbs Energy of Activation, according to eqs. (5) and (6), may be a function of pressure. However, since liquid water is almost incompressible, only a slight change of the concentration is expected and therefore a pressure dependence of DG0#, i.e., the standard Gibbs energy of activation of the rate determining step is most likely. 6. Conclusions By thermodynamic reasoning, the cell voltage of electrolysis cells should increase with the operating pressure. According to Nernst’s law, assuming ideal gas behavior and an activity of liquid water of unity, at 50 C an increase of 48 mV/dec of balanced gas pressure (hydrogen and oxygen pressurized) is expected. However, also the present experimental data up to 100 bar (balanced pressure), in agreement to previous work, shows that only at relatively small current densities below about 0.8 A/cm2 an increase in the cell and iR-free cell voltages is measured. In the commonly used operating range between 1 and 3 A/cm2 no cell
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Fig. 7. Apparent exchange current density for a) different balanced pressures (1 to 110 bar) at 50 C and for b) different temperatures (30 to 70 C) at 10 bar.
voltage increase is observed, the thermodynamic increase is completely compensated by beneficial effects. Polarization curves and HFR measurements confirmed that the ohmic drop is independent of the gas pressure. However, it is shown, based on a Tafel model analysis that the beneficial cell voltage effect upon pressurization is due to changes of the kinetic and mass transport overpotentials. The larger fraction of the effect can be attributed to improved kinetics. For instance at 50 C and 2 A/cm2, when increasing the pressure from 1 to 100 bar, the kinetic overpotential is reduced from around 500 to 430 mV and the mass transport overpotential from around 130 to 80 mV. The analysis of kinetic voltage gains with the Tafel model showed that no distinct trend of the Tafel slope is observed with pressure. On the contrary, the apparent exchange current density increases by about a factor of 4 to 7 with a pressure increase of 10 to 100 bar. Consequently either the electrochemically active surface area and/or the specific exchange current density are to change. On the one hand, an increase in the ECSA might be due to an improved penetration of the water into pores of the anodic catalyst layer or micro-pores of the catalyst. On the other hand, for an increase of the specific exchange current density, based on the transition state theory, either the concentration of liquid water and/or the standard Gibbs energy of activation of the rate determining step have to change, the latter being the most reasonable and likely scenario. The improvement of mass transport with increasing pressure might be due to an improved water transport to the anode catalyst layer as a result of a changed two phase flow in the porous structures. The reduction the mass transport losses from around 200 to 100 mV with a temperature increase from 30 to 70 C may be explained by an increased water vapor pressure and reduced viscosity and surface tension for the liquid water phase. A more detailed understanding of those effects will require more experiments with different PTL and/or CL structures. Also results from imaging techniques, giving insight into the gas and water flow such as operando X-Ray tomography or neutron radiography will be helpful. Acknowledgements Funding by the Swiss Federal Office of Energy (SFOE), Belenos Clean Power Holding Ltd. and the Energy System Integration (ESI) platform at PSI, technical support by Martin Ammann and Thomas Gloor (both PSI), as well as X-Ray tomographic microscopy work by Kazuhiro Takanohashi (visiting from Yamanashi University) are gratefully acknowledged. TJS thanks the Commission for Technology and Innovation Switzerland and the Swiss Competence Center for Energy Research Heat & Electricity Storage.
References [1] C.J. Barnhart, M. Dale, A.R. Brandt, S.M. Benson, The energetic implications of curtailing versus storing solar- and wind-generated electricity, Energy & Environmental Science 6 (2013) 2804–2810. [2] A. Ursúa, L.M. Gandía, P. Sanchis, Hydrogen Production From Water Electrolysis: Current Status and Future Trends, Proceedings of the IEEE 100 (2012) 410–426. [3] P. Millet, S. Grigoriev, Water Electrolysis Technologies, in: L.M. Gandia, G. Arzamendi, P.M. Dieguez (Eds.), Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety, Elsevier B.V., Waltham, 2013, pp. 19–41. [4] T. Smolinka, M. Günther, J. Garche, Stand und Entwicklungspotenzial der Wasserelektrolyse zur Herstellung von Wasserstoff aus regenerativen Energien, NOW-Studie, 2011. [5] T.G. Douglas, A. Cruden, D. Infield, Development of an ambient temperature alkaline electrolyser for dynamic operation with renewable energy sources, International Journal of Hydrogen Energy 38 (2013) 723–739. [6] F.N. Büchi, M. Hofer, C. Peter, U.D. Cabalzar, J. Bernard, U. Hannesen, T.J. Schmidt, A. Closset, P. Dietrich, Towards re-electrification of hydrogen obtained from the power-to-gas process by highly efficient H2/O2 polymer electrolyte fuel cells, RSC Adv. 4 (2014) 56139–56146. [7] T. Norman, M. Hamdan, Unitized Design for Home Refueling Appliance for Hydrogen Generation to 5,000 psi, DOE Hydrogen and Fuel Cells Program (2012) 122–125. [8] K.E. Ayers, E.B. Anderson, C.B. Capuano, B.D. Carter, L.T. Dalton, G. Hanlon, J. Manco, M. Niedzwiecki, Research Advances towards Low Cost, High Efficiency PEM Electrolysis, ECS Transactions 33 (2010) 3–15. [9] R. Ströbel, M. Oszcipok, M. Fasil, B. Rohland, L. Jörissen, J. Garche, The compression of hydrogen in an electrochemical cell based on a PE fuel cell design, Journal of Power Sources 105 (2002) 208–215. [10] S.A. Grigoriev, I.G. Shtatniy, P. Millet, V.I. Porembsky, V.N. Fateev, Description and characterization of an electrochemical hydrogen compressor/ concentrator based on solid polymer electrolyte technology, International Journal of Hydrogen Energy 36 (2011) 4148–4155. [11] P.J. Bouwman, J. Konink, D. Semerel, L. Raymakers, M. Koeman, W. Dalhuijsen, E. Milacic, M. Mulder, Electrochemical Hydrogen Compression, ECS Transactions 64 (2014) 1009–1018. [12] P. Millet, R. Ngameni, S.A. Grigoriev, N. Mbemba, F. Brisset, A. Ranjbari, C. Etiévant, PEM water electrolyzers: From electrocatalysis to stack development, International Journal of Hydrogen Energy 35 (2010) 5043–5052. [13] S.A. Grigoriev, V.I. Porembsky, V.N. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy, International Journal of Hydrogen Energy 31 (2006) 171–175. [14] S.A. Grigoriev, M.M. Khaliullin, N.V. Kuleshov, V.N. Fateev, Electrolysis of Water in a System with a Solid Polymer Electrolyte at Elevated Pressure, Russian Journal of Electrochemistry 37 (2001) 819–822. [15] C. Rozain, P. Millet, Electrochemical characterization of Polymer Electrolyte Membrane Water Electrolysis Cells, Electrochimica Acta 131 (2014) 160–167. [16] S.A. Grigoriev, P. Millet, S.V. Korobtsev, V.I. Porembskiy, M. Pepic, C. Etievant, C. Puyenchet, V.N. Fateev, Hydrogen safety aspects related to high-pressure polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy 34 (2009) 5986–5991. [17] J. van der Merwe, K. Uren, G. van Schoor, D. Bessarabov, Characterisation tools development for PEM electrolysers, International Journal of Hydrogen Energy 39 (2014) 14212–14221. [18] F. Marangio, M. Santarelli, M. Cali, Theoretical model and experimental analysis of a high pressure PEM water electrolyser for hydrogen production, International Journal of Hydrogen Energy 34 (2009) 1143–1158. [19] H. Ito, T. Maeda, A. Nakano, A. Kato, T. Yoshida, Influence of pore structural properties of current collectors on the performance of proton exchange membrane electrolyzer, Electrochimica Acta 100 (2013) 242–248.
M. Suermann et al. / Electrochimica Acta 211 (2016) 989–997 [20] D.L. Fritz, J. Mergel, D. Stolten, PEM electrolysis simulation and validation, ECS Transactions 58 (2014) 1–9. [21] I. Barin, G. Platzki, Thermochemical Data of Pure Substances, VCH Verlagsgesellschaft mbH, Weinheim (1995) 795–796. [22] H. Ito, T. Maeda, A. Nakano, H. Takenaka, Properties of Nafion membranes under PEM water electrolysis conditions, International Journal of Hydrogen Energy 36 (2011) 10527–10540. [23] R. Wiebe, V.L. Gaddy, The solubility of Hydrogen in Water at 0, 50, 75 and 100 from 25 to 1000 Atmospheres, J. Am. Chem. Soc. 56 (1934) 76–79. [24] D. Tromans, Temperature and pressure dependent solubility of oxygen in water: a thermodynamic analysis, Hydrometallurgy 48 (1998) 327–342. [25] J. Durst, A. Siebel, C. Simon, F. Hasche, J. Herranz, H.A. Gasteiger, New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism, Energy & Environmental Science 7 (2014) 2255–2260. [26] J.O.M. Bockris, A.K.N. Reddy, M.E. Gamboa-Aldeco, Modern Electrochemistry 2A, Fundamentals of Electrodics, Springer, US, New York, 2000. [27] V. Schröder, B. Emonts, H. Janssen, H.-P. Schulze, Explosionsgrenzen von Wasserstoff-Sauerstoff-Gemischen bei Drücken bis 200 bar, Chemie Ingenieur Technik 75 (2003) 914–918. [28] G. Xu, Y.S. Pak, Dielectric Relaxation and Conductivity Study of Polytetrafluorene-Ethyl lonomers, Journal of The Electrochemical Society 139 (1992) 2871–2876. [29] A. Siu, J. Schmeisser, S. Holdcroft, Effect of Water on the Low Temperature Conductivity of Polymer Electrolytes, J. Phys. Chem. B 110 (2006) 6072–6080. [30] F.N. Büchi, G.G. Scherer, In-situ resistance measurements of Nafion 117 membranes in polymer electrolyte fuel cells, J. Electroanal. Chem. 404 (1996) 37–43. [31] L. Gubler, L. Bonorand, Radiation Grafted Membranes for Fuel Cells: Strategies to Compete with PFSA Membranes, ECS Transactions 58 (2013) 149–162.
997
[32] N. Pérez-Hernández, T.Q. Luong, M. Febles, C. Marco, H.-H. Limbach, M. Havenith, C. Pérez, M.V. Roux, R. Pérez, J.D. Martín, The Mobility of Water Molecules through Hydrated Pores, The Journal of Physical Chemistry C 116 (2012) 9616–9630. [33] M.A. Hoeh, T. Arlt, N. Kardjilov, I. Manke, J. Barnhart, D.L. Fritz, J. Ehlert, W. Lüke, W. Lehnert, In-operando Neutron Radiography Studies of Polymer Electrolyte Membrane Water Electrolyzers, ECS Transactions 69 (2015) 1135–1140. [34] M.A. Hoeh, T. Arlt, I. Manke, J. Banhart, D.L. Fritz, W. Maier, W. Lehnert, In operando synchrotron X-ray radiography studies of polymer electrolyte membrane water electrolyzers, Electrochemistry Communications 55 (2015) 55–59. [35] E. Brightman, J. Dodwell, N. van Dijk, G. Hinds, In situ characterisation of PEM water electrolysers using a novel reference electrode, Electrochemistry Communications 52 (2015) 1–4. [36] E. Antolini, Iridium As Catalyst and Cocatalyst for Oxygen Evolution/Reduction in Acidic Polymer Electrolyte Membrane Electrolyzers and Fuel Cells, ACS Catalysis 4 (2014) 1426–1440. [37] L.A. De Faria, J.F.C. Boodts, S. Trasatti, Electrocatalytic properties of ternary oxide mixtures of composition Ru0.3Ti(0.7-x)CexO2: Oxygen evolution from acidic solution, Journal of Applied Electrochemistry 26 (1996) 1195–1199. [38] E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt, Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction, Catal. Sci. Technol. 4 (2014) 3800–3821. [39] M. Suermann, T.J. Schmidt, F.N. Büchi, Investigation of Mass Transport Losses in Polymer Electrolyte Electrolysis Cells, ECS Transactions 69 (2015) 1141–1148. [40] S. Siracusano, V. Baglio, A. Di Blasi, N. Briguglio, A. Stassi, R. Ornelas, E. Trifoni, V. Antonucci, A.S. Aricò, Electrochemical characterization of single cell and short stack PEM electrolyzers based on a nanosized IrO2 anode electrocatalyst, International Journal of Hydrogen Energy 35 (2010) 5558–5568.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cell Performance Determining Parameters in High Pressure Water Electrolysis Michel Suermanna , Thomas J. Schmidta,b , Felix N. Büchia,* a b
Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland Laboratory of Physical Chemistry, ETH Zürich, CH-8093 Zürich, Switzerland
A R T I C L E I N F O
Article history: Received 16 March 2016 Received in revised form 22 June 2016 Accepted 23 June 2016 Available online 23 June 2016 Keywords: Polymer electrolyte water electrolysis overpotential analysis pressure dependence
A B S T R A C T
When hydrogen is used for energy applications, then often elevated pressure is required, i.e. up to 1000 bar for hydrogen refueling stations. When polymer electrolyte electrolyzers are operated at elevated pressures, to reduce external mechanical compression needs, the thermodynamically expected cell voltage increase with pressure is not generally observed. Consequently, beneficial processes have to compensate for the increased work performed. In this paper, the influence of the operating pressure up to 100 bar on the cell voltage behavior is investigated in detail. Data from galvanotstatic polarization curves up to 4 A/cm2 combined with high frequency resistance measurements are analyzed using a zerodimensional Tafel model. It shows that beneficial processes with increasing pressure are related to both the kinetic and mass transport overpotentials. At current densities above 1 A/cm2 about two thirds of the overpotential gain can be related to reaction kinetics. Based on the Tafel model approach an increase of the apparent exchange current density by a factor of 4 to 7 is observed for the pressure increase from 10 to 100 bar. In contrast, the Tafel slope is independent of pressure and only the expected increase with temperature (73 to 81 mV/dec) for 30 to 70 C is observed. Mass transport losses decrease with pressure. At 50 C and 2 A/cm2, increasing the pressure from 1 to 100 bar results in a decrease of the mass transport overpotential from 130 to 80 mV. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction A major challenge of renewable power technologies (solar and wind) is their intermittent electricity production. When production and consumption do not match, storage is required in particular to avoid curtailment [1]. Electricity storage for diurnal up to seasonal time frames can be performed by the power-to-gas concept converting electrical energy into hydrogen by using water electrolysis [2]. Polymer electrolyte electrolysis cell (PEEC) technology has potential advantages for this application in terms of dynamic behavior over other electrolysis technologies such as alkaline electrolysis (AEC) or high temperature solid oxide electrolysis (SOEC). PEEC offers dynamic operation with fast response times [3], a wide duty range (ideally 0 to 100%), good start/stop behavior and the possibility to run at ambient temperature. In comparison AEC usually needs auxiliary heating for start-up and its partial-load operation range is limited to 20 to 40% [4]. Approaches to run AEC
* Corresponding author. E-mail address: [email protected] (F.N. Büchi). http://dx.doi.org/10.1016/j.electacta.2016.06.120 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
at ambient temperature still produce a significant efficiency penalty [5]. SOEC might rather be interesting for continuous conversion due to the high operating temperature. Hydrogen produced in the power-to-gas process requires storage of the pressurized gas, where pressure depends on the final application. Pressures up to 80 bar are required for feeding into natural gas pipelines and up to 1000 bar for hydrogen refueling stations for mobility. Water electrolysis also produces oxygen in a stoichiometric ratio, which can be used for reelectrification and grid services using highly efficient hydrogen/ oxygen polymer electrolyte fuel cells (PEFC) systems [6]. Conventionally, electrolyzers produce hydrogen at 30 bar and the gases are compressed mechanically for the desired application. With PEEC direct electrochemical compression up to 345 bar has been demonstrated [7,8], which is considered to be more efficient than mechanical compression. Thus, pressurized PEEC can be used either as a stand-alone device or to reduce the number of mechanical compression stages, which results in lower investment and operating costs. In principle electrochemical hydrogen compression [9–11] could be used instead of mechanical compression.
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Interestingly, the cell voltage of PEEC operated at elevated balanced pressures shows an unexpected behavior, as the expected higher thermodynamic equilibrium potentials are not directly observed as increased cell voltage [12–14]. This is different from the behavior of PEFC where gas pressure has a prominent influence on the cell performance. The question arises what causes the difference between the observed cell voltage and the expected thermodynamic voltage due to pressurization? In literature it is discussed that higher pressure leads to smaller gas bubbles which may improve the two phase flow and therefore the water supply at the anode [15–20]. However, the fundamental causes have not been investigated in depth so far. An analysis of the influence of pressure up to 25 bar has been given 2001 by Grigoriev et al. [14]. The authors discovered that the anodic overpotential significantly decreases and the cathodic overpotential slightly increases with increasing pressure. It was also claimed that smaller bubbles facilitate the water transport, reduce the ohmic losses in the catalyst layer and improve the electric contact towards the porous transport layer. However there is still inconsistency, especially with respect to the contributions of the different overpotentials as a function of pressure and current density. Thus, a better insight into the contribution of the different overpotentials in PEEC operation is required, ultimately leading to better understanding of pressure effects on the electrochemical processes. This will be beneficial in the further development of PEEC such as catalyst and porous transport layer structures, respectively. Therefore, we characterized PEECs with commercial electrochemical components by galvanostatic polarization curves (up to 4 A/cm2) in a wide parameter domain of temperature (30 to 70 C) and pressure (up to 100 bar balanced). From the consistent data set the contributions of the different overpotentials on overall cell voltage is analyzed using a zero-dimensional Tafel model. Based on the findings from this combined set of data the possible beneficial processes with increasing pressure and temperature in PEEC are discussed. 2. Experimental 2.1. Electrolysis cell characteristics Experiments were made using a differential cell (i.e. negligible gradients along the water flow direction in the cell) with a square active area of 4 cm2. The flow field consists of 5 parallel channels of 2 1 mm (width x depth) separated by ribs of 2 mm with a length of 21 mm. All parts of the cell in contact with the fluids are gold coated. The cathode flow field is supported by an array of springs to have a reproducible contact pressure on the active area. Commercial catalyst coated membranes (CCMs) based on Nafion117 (N117) and N212 (GreenerityTM E400, SolviCore, D) combined with porous transport layers (PTL) made of sintered titanium powder (SIKA-T10, 2 mm thickness, GKN Sinter Metals Filters, D) were used. By means of ex-situ X-Ray tomographic microscopy the PTL was characterized as isotropic with a porosity of about 35% and a median pore size of 31 mm. Identifying the cell specific electronic resistance is important to ensure an accurate data analysis, particularly with respect to the contribution of the different overpotentials. From high frequency resistance (HFR) measurements of cells with the two membrane thicknesses, the ratio of the ionic and electronic ohmic resistance can be determined by extrapolating the measured HFR data for known membrane thicknesses to zero. An electronic area resistance of 23 mVcm2 was determined, irrespective of the current density and cell temperature as shown in Fig. 1. The cell
Fig. 1. Comparison of the HFR of N117 and N212 for different current densities and cell temperatures at 10 bar. The HFR-axis intercept represents the electronic area resistance of the cell.
voltages and HFR data presented in this work are corrected by the electronic resistance. 2.2. Test bench The test bench has a conventional set-up with two completely separated gas/water loops for anode and cathode with a gas/liquid separator each. Deionized water (<1 mS/cm) is fed by a HPLC pump to the anode. The water level in the cathode separator is kept constant by draining. All parts are from stainless steel and designed for a maximum operating pressure of 300 bar. To have a controlled and homogeneous temperature in the small cell the water is heated in the gas/liquid separators having a water volume at anode and cathode sides of about 100 mL. At the anode the water is recirculated with a volume flow of 30 mLmin1cm2 using a diaphragm pump (LEWA GmbH, D). At the cathode natural recirculation takes place. Thus the major heat fraction for cell heating/cooling is transferred through the anode side. The cell temperature is measured with a K-type thermocouple close to the active area inside the cell housing and equilibrated for a given cell temperature at a current density of 2 A/cm2. This current density was selected to avoid combustible mixtures in the electrolyzer even at high operating pressures. This operating point is used both as a reference steady state condition and as the starting point for the polarization curves. 2.3. Measurement set-up All electrochemical measurements were performed using a potentiostat (VSP-300, with two 5 V/10A booster boards, Z-board, Bio-Logic SAS, F). The parameters for the galvanostatic polarization curves were optimized (see chapter 4) and performed from low to high current densities with a holding time of 10 seconds at each measurement point. All polarization curves shown are the average of at least two measurements. Using a full spectrum from galvanostatic electrochemical impedance spectroscopy measurement, the high frequency intercept was found to be at around 25 kHz. Thus the high frequency resistance (HFR) was measured for every current density at 25 kHz for one second.
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3. Theory and calculations
by eq. (4):
The voltage of the electrolysis cell Ecell is considered to be the difference of the two pressure and temperature dependent electrode potentials of the anode and cathode E0A(p,T) and E0C(p, T) to which the three main overpotentials add:
hkin ¼ b log
j j0
991
ð4Þ
where hiR denotes the ohmic, hkin the kinetic and hmtx the mass transport overpotential, respectively. The difference of the two electrode potentials defines the thermodynamic cell voltage:
where the kinetic overpotential hkin is equal to the difference between the Tafel line and the thermodynamic cell voltage according to eq. (2), b is the Tafel slope, equal to 2.3RT/(aF), and a is the transfer coefficient, a constant related to the mechanism of the electrochemical reaction including the number of transferred electrons. In eq. (5) the apparent exchange current density j0 is defined as:
E0A ðp; TÞ E0C ðp; TÞ ¼ E0cell ðp; TÞ
ð2Þ
j0 ¼ j0;s rf
ð3Þ
where j0,S is the specific exchange current density (in terms of A/ cm2 of catalyst surface) and rf is the roughness factor. The latter is the ratio of the electrochemical surface area (ECSA) to the geometric area. According to the transition state theory, in equilibrium the specific exchange current density [26] can be expressed as:
Ecell ¼ E0A ðp; TÞ E0C ðp; TÞ þ hiR þ hkin þ hmtx
which can be calculated from the Nernst equation: pffiffiffiffiffiffiffiffiffiffi ! aðH2 Þ aðO2 Þ RT ln E0cell ðp; TÞ ¼ E0 ðTÞ þ 2F aðH2 OÞ
ð1Þ
with a the activity of the species and E0(T) the temperature dependent equilibrium potential [21]. Since the solubility of hydrogen and oxygen in water is constant up to 100 bar, ideal gas behavior for both H2 and O2 can be assumed, because at thermodynamic equilibrium conditions only the dissolved gas in the water is in contact with the electrodes [22– 24]. Consequently, the activity can be expressed by the partial pressure. The activity of water is assumed as unity. The ohmic overpotential hiR is obtained from the HFR measurement and represents the sum of the ionic and electronic ohmic losses. By subtracting the ohmic overpotential from the cell voltage, the iR-free cell voltage is obtained. The iR-free cell voltage shows a Tafel behavior at low current densities. This Tafel behavior is considered a property of the anode as the polarization of the cathode is minimal at the low current densities and the potential close to the one of a reversible hydrogen electrode (RHE) [25]. To the linear regions, the Tafel line can be fitted as described
j0;s ¼ F c kh
ð5Þ
ð6Þ
where c is the concentration of the reactant and kh is the heterogeneous electrochemical rate constant: ! DG06¼ kh ¼ k0 exp ð7Þ RT where k0 is the rate constant and DG06¼ is the standard Gibbs energy of activation. As long as only a single Tafel slope for the entire current density range is considered, the difference between the Tafel line and the iR-free cell voltage defines the mass transport overpotential hmtx. In this analysis only cell performance related overpotentials are considered. Additional system related losses such as gas crossover are not part of this work. Nevertheless they play an important role
Fig. 2. Tafel plots of the iR-free cell voltage for N117 at 50 C and 50 bar for current holding times of a) 1 s, b) 2 s, c) 10 s and d) 20 s in increasing and decreasing current measurement direction. Tafel line is fitted between 10 and 120 mA/cm2.
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Table 1 Influence of the holding time and the measurement direction on the Tafel slope b for increasing and decreasing current, the difference in the iR-free cell voltage between increasing and decreasing current in the 10 to 120 mA/cm2 interval (DEiR-free) and the corresponding volume exchange of the gas in the anodic PTL at 1 bar and 1 A/cm2. holding time
b increasing
b decreasing
DEiR-free
PTL volume exchange at 1 bar and 1 A/cm2
HFR at 0.4 A/cm2
HFR at 4 A/cm2
– 0.83 1.66 8.29 16.57
mV cm2 200.2 2.5 203.2 0.0 203.1 1.5 202.5 0.0
mV cm2 171.4 1.8 172.5 0.6 171.5 1.4 170.1 0.2
incr. vs. decr. s 1 2 10 20
mV/dec 75.5 0.6 75.7 0.6 77.9 1.4 82.8 0.4
mV/dec 75.3 0.1 77.0 0.5 77.5 0.7 79.4 0.2
mV 5.2 6.1 6.8 12.3
when it comes to system efficiencies, especially at high pressure operation. 4. Measurement protocol for polarization curves Ideally a single measurement protocol for polarization curves can be employed to characterize and compare the cell behavior at different operating conditions with respect to temperature and pressure. In particular when considering gas pressures of 100 bar the protocol needs to be carefully considered. Gas pressure needs to be considered not only for performance but also with respect to safety. Since the gas crossover through the membrane is governed by the partial pressure difference according to Fick‘s law, combustible atmospheres [27] can build up at low current densities. When no additional gas-recombination measure are taken (e.g. Pt coating of the anodic PTL or membrane surface, or catalytic reactors with Pd or Pt recombination catalysts [16]), there is a lower current density limit for continuous safe operation. But even for transient measurements, such as current voltage curves, the gas crossover severely limits the safe measurement time at low current densities. With polymer electrolyte fuel cells (PEFC), typically long holding times (up to 20 min) are required for accurate currentvoltage curve characterization. In PEFC this is due to the complex water management (time constants of water sorption and desorption of the membrane). In PEEC, where liquid water is always in contact with the membrane, this is not a limitation. On the other hand in electrolysis much higher specific power densities can be achieved than in PEFC, and therefore also significantly higher specific thermal loads have to be dealt with. While in PEFC maximum specific heat loads are limited to about 1 W/cm2, in the case of electrolysis, values in the order of 10 W/cm2 may be reached due to higher current densities and higher overpotentials. So in the case of PEEC at low current densities (and high pressures) the galvanostatic holding time to measure current-voltage curves is limited due to safety restrictions, while at high current densities thermal equilibrium may take a long time to reach due to the large specific heat produced. In order to overcome safety issues at low current densities as well as thermal issues at high current densities, short measurement times for the polarization curves become important. Therefore the characteristics of polarization curves for different short holding times of 1, 2, 10 and 20 seconds (plus one second each for HFR measurements) were investigated. Evaluation conditions of 50 bar and 50 C were chosen. After thermal equilibration of the cell at 2 A/cm2, polarization curves were conducted in forward (increasing current density starting from 1 mA/cm2) and backward (decreasing current density starting from 4 A/cm2) directions to check for hysteresis effects. Measurements were carried out up to 4 A/cm2. Current steps were chosen as 1 mA/cm2 below 10 mA/ cm2, 10 mA/cm2 below 50 mA/cm2, 50 mA/cm2 below 100 mA/cm2, 100 mA/cm2 below 200 mA/cm2 and 200 mA/cm2 above 200 mA/ cm2. As the region of low current densities is of central interest for the desired overpotential analysis, in Fig. 2 only current densities up to 1 A/cm2 are shown.
Only small differences are observed in the polarization curves with the different holding times. Comparing the iR-free cell voltage of increasing and decreasing current measurement direction in the fitted region, a difference of 12 mV at 20 s compared to 5 to 7 mV at 1 to 10 s holding time is observed. Since the membrane is well humidified due to the presence of liquid water, the difference must be due to minor thermal issues. For thermal analysis HFR data can be used. Above 2 A/cm2 (not shown here) the cell temperature increases more at higher holding times, while below 2 A/cm2 the low waste heat production compared to the equilibration current density of at 2 A/cm2 leads to a small cooling effect, slightly increasing with increasing holding time. This cooling effect is small in the Tafel region, as is shown below in Fig. 4b) where HFR variation below 0.5 A/cm2 is minimal. HFR data for 0.4 and 4 A/cm2 (measured at the end of the holding time) are shown in Table 1 for the different holding times. The independence of the HFR, respectively the temperature on holding time shows that the active components are close to thermal equilibrium at all holding times investigated. The quantitative correlation between HFR and temperature in the active area is discussed in in detail section 5.1 below. For fitting a representative Tafel line, the linear region at low current densities is of particular interest. However, especially at current densities below 10 mA/cm2 an unexpected non-linear behavior is noticed. Because its extent decreases with increasing holding time, it may be attributed to non-equilibrium conditions.1 The Tafel line is therefore fitted in the region between 10 and 120 mA/cm2. The resulting Tafel slopes as a function of the measurement direction and holding time are given in Table 1. Furthermore to estimate the (theoretical) gas volume exchange (neglecting any water flow) in the anodic PTL, during the holding time of the polarization curve, is calculated for ambient pressure and 1 A/cm2 is also listed in Table 1. The values in Table 1 show that the Tafel slope is slightly increasing with longer holding times. Because of the still good agreement of the Tafel slope for the forward and backward directions of less than 1 mV/dec and the relatively small difference of the iR-free cell voltage between increasing and decreasing measurement direction in the Tafel line fit region, and a reasonable gas volume exchange rate for the anode PTL, 10 s holding time and forward measurement direction are chosen for all measurements presented here. The relatively short holding time is a compromise between the safety and equilibrium requirements. Still, the thermal analysis shows that deviation from the thermal equilibrium is considered minor, as shown by the HFR data in Table 1 and Fig. 4a).
1 At very low current densities, the gas crossover equivalent current density (for N117 about 0.3 mAbar1cm2) can exceed the applied current density. Then the hydrogen oxidation reaction (HOR) of the permeated gas may have a dominant influence on the cell voltage, reducing it below the reversible potential of water electrolysis. When using platinum at the anode such a HOR mechanism is clearly noticed (not shown in this work).
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5. Results and discussion It is an interesting experimental observation that in most reports, increasing balanced gas pressure in PEEC does not lead to the thermodynamically expected increase of the cell voltage. Therefore the influence of operating pressure on the different overpotentials (kinetic, ohmic and mass transport) is analyzed based on the model presented in eq. (1) and the analysis outlined in section 3. For detailed understanding of the overpotentials also the parameter space for temperature (30 to 70 C) and current density up to 4 A/cm2 is explored. Measurements have been performed for two membrane thicknesses to underline the generic character of the results. 5.1. Ohmic overpotential The ohmic losses mainly consist of the proton transport in the membrane. The membrane independent part of the high frequency resistance, related to the cell set-up specific electronic resistance, was corrected as discussed in chapter 2. In Fig. 3 the cell voltage and the iR-free cell voltage for cells with N117 and N212 at 50 C and 10 bar are plotted vs. the current density. Above about 0.5 A/ cm2 small differences in the iR-free cell voltage between the two membranes of up to 40 mV are observed which can be explained by higher waste heat production (and thus higher temperature in the catalyst layer) for the thicker membrane reducing the kinetic overpotential. The temperature dependence of the high frequency resistance is shown in Fig. 4 a) from which a HFR decrease with increasing current density due to the increasing heat production can be deduced, even with the fast polarization curve protocol. The thermal influence of the waste heat is larger at lower cell temperatures, both due to a larger total overpotential and a higher ratio of the waste heat production compared to the heat imparted, respectively. For the current density of 2 A/cm2, where the cell temperature has been equilibrated before polarization curves, an Arrhenius plot is presented in Fig. 4 b). The calculated activation energy for proton conduction (Ea) of 0.12 0.01 eV corresponds well to the values reported in literature of about 0.12 to 0.15 eV [28,29]. From Fig. 4 b) a scaling factor of the HFR with membrane thickness of 3.1 to 3.2 can be deduced. This is somewhat less than the formal value of 3.5 expected from the dry membrane thickness, but well in line with values for swollen membranes [30,31] resulting in a swollen thickness ratio of 3.1 to 3.2. The thicker membranes swell less from dry to fully hydrated, as is the case during operation in PEEC. The data from the Arrhenius plot can also be used to predict the actual membrane temperature for a distinct current density. For instance, at 50 C and 10 bar, a decrease of the HFR at 4 A/cm2 of about 21.7 mVcm2 and 3.5 mVcm2 for N117 and N212, respectively, is observed, indicating an increase of the membrane temperature of about 9.8 C and 5.9 C at 4 A/cm2. This temperature increase is actually difficult to measure with a thermocouple close to the active area inside the cell housing (typically only +/ 0.5 C during I/V-curve), as there is a temperature drop between water in the channel and the CCM due to the heat transport in the PTL. When considering heat loss only to the anode, where water is recirculated, then the temperature gradient across the PTL can be calculated based on the heat conductivity of titanium of 21.9 W/ (mK) and the structure of the PTL described in the experimental section. Assuming a solid tortuosity of t = e0.5 = 1.24 and using Fourier‘s law a theoretical heat loss in the PTL of 10.3 C and 6.5 C for N117 and N212, respectively, is calculated, close to the values deduced from the HFR measurements. Considering the influence of pressure on the HFR, at 50 C only small differences close to the standard deviations are observed for
Fig. 3. Cell and iR-free cell voltage for N117 and N212 at 50 C and 10 bar balanced gas pressure.
the pressure range from 1 to 100 bar, with a tendency to lower resistance values at higher pressures in particular at low current densities. It may be speculated that higher pressure may lead to less waste heat production due to lower overpotentials according to a higher thermodynamic voltage (see eq. (2)). It should be taken into account, however, that for 50 C the difference in the thermodynamic voltage between 1 and 100 bar with 96 mV is rather small compared to the overall overpotential. 5.2. Mass transport overpotential The iR-free cell voltage for 1, 10 and 100 bar at 50 C is plotted semi-logarithmically vs. the current density in Fig. 5 a). At current densities up to about 200 mA/cm2, the expected, linear voltage behavior is observed indicating that no mass transport losses occur. Furthermore, at low current densities the cell voltage increases with increasing pressure as expected from thermodynamics until, with increasing current, the cell voltages coincide at around 1 A/cm2. At higher current densities, cell voltages are practically independent of pressure, but clearly an offset of the iR-free cell voltage from the extrapolated Tafel line is seen. This difference increases with current density to about 150 mV at 4 A/cm2. This loss is considered the mass transport overpotential. It increases monotonically with current, but flattens out at current densities above 2 A/cm2, which is attributed to an artifact of the nonequilibrium polarization curve measurement, i.e., the temperature of the CCM increases (Fig. 5 b) effectively reducing mass transport losses. The effect of pressure on the mass transport loss as deduced from Fig. 5 a) is summarized in Fig. 6 a). Above 3 A/cm2, the mass transport overpotential decreases slightly from around 170 to 140 mV for the pressure increase from 1 to 100 bar. The influence of temperature on the mass transport losses (Fig. 5 b) is analyzed in Fig. 6 b). A temperature increase from 30 to 70 C results in a decrease in transport overpotential from 200 to 100 mV above 2 A/cm2. This may even be biased by a thermal effect. The flattening of the iR-free cell voltage is stronger at lower cell temperatures. So the decrease of mass transport overpotential with temperature is probably even bigger under thermal equilibrium conditions.
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Fig. 4. a) HFR measurements of N117 and N212 for different cell temperatures and operating pressures as a function of current density; b) Arrhenius plot of the HFR data at 2 A/cm2 and 10 bar, for the data in sub-plot a).
Fig. 5. Tafel plots of N117 for different temperatures and pressures; a) for 1, 10 and 100 bar at 50 C and b) for 30 to 70 C at 10 bar.
When water transport in the gas phase contributes to the mass transport overpotential, the temperature effect may be due to the changing water partial pressure. In case liquid transport is critical and limiting, the decreasing viscosity and/or surface tension of liquid water may play a role. Furthermore the change of wettability of the hydrophilic pores of the PTL and/or CL [32] can have an influence. The effects of these potential causes cannot be separated with the present data. Summarizing, it can be stated that mass transport contributes to the cell voltage starting at current densities above 200 mA/cm2, reaching 100 to 200 mV at 2 A/cm2. Reasons of the mass transport losses are not clear, but it seems obvious that the two phase flow at the anode plays a critical role. To get a better understanding on the causes of the mass transport overpotential and its dependence on pressure and temperature, more data with different PTL/CL combinations and/or microscopic resolution is needed. Especially the interface between the PTL and the CL layer seems to be of interest. First steps using imaging have been made by Hoeh et al. [33,34] investigating the bubble formation at the PTL/channel interface as a function of current density and water flow rate using X-ray and neutron radiography. 5.3. Kinetic overpotential The kinetic overpotential is calculated from the difference between the fitted Tafel lines (as in Fig. 5) and the thermodynamic cell voltage calculated according to eq. (2). Typically, the anodic kinetic overpotential is significantly higher than the cathodic due
to the sluggish oxygen evolution reaction [35]. The value of the thermodynamic voltage is influenced by the activities of gases and water in eq. (2). It seems undisputed that the activity of the gases is represented by their partial pressures. However, how about activity of the liquid water? Is it really pressure independent and unity? With these assumptions, at 50 C, an increase of the thermodynamic voltage of 48 mV/dec is expected. However the measured iR-free cell voltage difference between 1 and 100 bar at 10 mA/cm2 is only 23 mV/dec (see Fig. 5 a), which indicates that either the assumptions for the activities are not correct or even at small current densities processes, not described by thermodynamics, take place. The different overpotentials are summarized in Fig. 6 a) as function of pressure at 50 C and in Fig. 6 b) as function of temperature at 10 bar. Increasing the pressure decreases the kinetic and transport overpotentials, while increasing cell temperature reduces all three loss mechanisms. In Fig. 6 it can be seen that for cells with N117 membranes, the kinetic overpotential is the dominant loss up to a current density of about 2 A/cm2. With thinner membranes this range is shifted to higher current densities. The mass transport overpotential plays a minor role at low current densities. However in the commonly used operation range of 1 to 3 A/cm2 it contributes 10 to 15% to the overall overpotential. For thinner membranes it will contribute a higher fraction in the same current density range. The fact that the kinetic and transport overpotentials drop with pressure is further illustrated by the values given in Table 2. Negative values indicate a reduction of the overpotentials with increase of the gas pressure from 1 to 100 bar.
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Fig. 6. Contributions of kinetic, ohmic and mass transport overpotentials for N117 as a function of current density; a) at 50 C for 1, 10 and 100 bar and b) at 10 bar for 30, 40, 50, 60 and 70 C.
The data in Table 2 show that only at low current densities a higher cell voltage is observed with the pressure increase over two decades. However, the raise in cell voltage is less than predicted by theory for an activity of liquid water equal to one. With increasing current densities the beneficial influences of mass transport and kinetics are more and more compensating the higher thermodynamic voltage. Finally at the highest current densities the cell voltage even slightly drops with increasing pressure. At low current densities the kinetics contribute most to cell voltage reduction. At current densities of 1 A/cm2 and larger the transport contributes about one third to the overall beneficial effects. The question arises why the reaction kinetics are improving with increasing pressure. Several parameters might be influenced according to the Tafel model. With respect to the reaction kinetics, pressure could influence (i) the Tafel slope or (ii) the apparent exchange current density caused by a change in the ECSA and/or specific exchange current density. Both hypotheses are discussed below: i) Tafel slope: a pressure dependent Tafel slope would mean that the transfer coefficient a (see eq. 3) is changing with pressure, meaning that a change in the reaction mechanism occurs, which cannot be completely excluded with the oxygen evolution reaction being a complex series of consecutive steps [36–38]. However in contrast to former work [39], no distinct trend with pressure could be observed here. The average Tafel slope at 50 C (obtained from 27 polarization curves with N117 in the pressure range from 1 to 110 bar) is found to be 76 5 mV/dec which corresponds to a transfer coefficient a of 0.84. For the change of the cell temperature from 30 to 70 C the Tafel slope increases as expected from 73 to 81 1 mV/dec, respectively, resulting in a transfer coefficient between 0.82 and 0.85. The obtained Tafel slopes are in good agreement with literature [40]. ii) Exchange current density: to the best of our knowledge there is only a single report in the literature by Gregoriev et al. [14] discussing the influence of pressure on the exchange current Table 2 Summary of the contributions of the kinetic and mass transport overpotentials at 100 bar (as compared to 1 bar operation) to the difference in the cell voltage for selected current densities. j
DE (p,T) Dhkinetic Dhmasstransport DEcell 0
A/cm2
0.001
0.01
0.1
0.4
1
4
mV mV mV mV
96 43 6 59
96 51 1 44
96 58 0 38
96 63 9 24
96 66 34 -4
96 70 36 -10
density. With an increase of gas pressure from 1 to 25 bar, the authors calculated an increase of the exchange current density by a factor of 3.5 to 8 for the anodic and of 20 to 45 for the cathodic reaction, depending on the temperature. For the present data, the apparent exchange current density (calculated from the extrapolation of the Tafel line to the equilibrium potential) is shown in Fig. 7 as function of pressure and temperature. At constant temperature (50 C) an increase of a factor of 4 to 7, is observed for the pressure increase from 10 to 100 bar. Below 10 bar the data is not conclusive as there is considerable noise due to the extrapolation over several orders of magnitude of current density. With temperature (from 30 to 70 C) at a constant pressure of 10 bar, the apparent exchange current density follows nicely an Arrhenius behavior (see Fig. 7 b) with an apparent kinetic activation energy of approx. 67 kJ/mol. An increase of the apparent exchange current density with pressure explains the trend of decreasing kinetic losses (detailed in Table 2). Consequently either the roughness factor and/or the specific exchange current density are pressure dependent factors. With respect to a pressure dependent increase of the roughness factor it may be speculated that pressure promotes the wetting of smaller pores of the catalyst or catalyst layer resulting in a higher effective electrochemical surface area. However, the wettability behavior of hydrophilic and/or hydrophobic micro-pores is not a trivial task to resolve unambiguously and would be a study on its own. Concerning the specific exchange current density either the concentration and/or the standard Gibbs Energy of Activation, according to eqs. (5) and (6), may be a function of pressure. However, since liquid water is almost incompressible, only a slight change of the concentration is expected and therefore a pressure dependence of DG0#, i.e., the standard Gibbs energy of activation of the rate determining step is most likely. 6. Conclusions By thermodynamic reasoning, the cell voltage of electrolysis cells should increase with the operating pressure. According to Nernst’s law, assuming ideal gas behavior and an activity of liquid water of unity, at 50 C an increase of 48 mV/dec of balanced gas pressure (hydrogen and oxygen pressurized) is expected. However, also the present experimental data up to 100 bar (balanced pressure), in agreement to previous work, shows that only at relatively small current densities below about 0.8 A/cm2 an increase in the cell and iR-free cell voltages is measured. In the commonly used operating range between 1 and 3 A/cm2 no cell
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Fig. 7. Apparent exchange current density for a) different balanced pressures (1 to 110 bar) at 50 C and for b) different temperatures (30 to 70 C) at 10 bar.
voltage increase is observed, the thermodynamic increase is completely compensated by beneficial effects. Polarization curves and HFR measurements confirmed that the ohmic drop is independent of the gas pressure. However, it is shown, based on a Tafel model analysis that the beneficial cell voltage effect upon pressurization is due to changes of the kinetic and mass transport overpotentials. The larger fraction of the effect can be attributed to improved kinetics. For instance at 50 C and 2 A/cm2, when increasing the pressure from 1 to 100 bar, the kinetic overpotential is reduced from around 500 to 430 mV and the mass transport overpotential from around 130 to 80 mV. The analysis of kinetic voltage gains with the Tafel model showed that no distinct trend of the Tafel slope is observed with pressure. On the contrary, the apparent exchange current density increases by about a factor of 4 to 7 with a pressure increase of 10 to 100 bar. Consequently either the electrochemically active surface area and/or the specific exchange current density are to change. On the one hand, an increase in the ECSA might be due to an improved penetration of the water into pores of the anodic catalyst layer or micro-pores of the catalyst. On the other hand, for an increase of the specific exchange current density, based on the transition state theory, either the concentration of liquid water and/or the standard Gibbs energy of activation of the rate determining step have to change, the latter being the most reasonable and likely scenario. The improvement of mass transport with increasing pressure might be due to an improved water transport to the anode catalyst layer as a result of a changed two phase flow in the porous structures. The reduction the mass transport losses from around 200 to 100 mV with a temperature increase from 30 to 70 C may be explained by an increased water vapor pressure and reduced viscosity and surface tension for the liquid water phase. A more detailed understanding of those effects will require more experiments with different PTL and/or CL structures. Also results from imaging techniques, giving insight into the gas and water flow such as operando X-Ray tomography or neutron radiography will be helpful. Acknowledgements Funding by the Swiss Federal Office of Energy (SFOE), Belenos Clean Power Holding Ltd. and the Energy System Integration (ESI) platform at PSI, technical support by Martin Ammann and Thomas Gloor (both PSI), as well as X-Ray tomographic microscopy work by Kazuhiro Takanohashi (visiting from Yamanashi University) are gratefully acknowledged. TJS thanks the Commission for Technology and Innovation Switzerland and the Swiss Competence Center for Energy Research Heat & Electricity Storage.
References [1] C.J. Barnhart, M. Dale, A.R. Brandt, S.M. Benson, The energetic implications of curtailing versus storing solar- and wind-generated electricity, Energy & Environmental Science 6 (2013) 2804–2810. [2] A. Ursúa, L.M. Gandía, P. Sanchis, Hydrogen Production From Water Electrolysis: Current Status and Future Trends, Proceedings of the IEEE 100 (2012) 410–426. [3] P. Millet, S. Grigoriev, Water Electrolysis Technologies, in: L.M. Gandia, G. Arzamendi, P.M. Dieguez (Eds.), Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety, Elsevier B.V., Waltham, 2013, pp. 19–41. [4] T. Smolinka, M. Günther, J. Garche, Stand und Entwicklungspotenzial der Wasserelektrolyse zur Herstellung von Wasserstoff aus regenerativen Energien, NOW-Studie, 2011. [5] T.G. Douglas, A. Cruden, D. Infield, Development of an ambient temperature alkaline electrolyser for dynamic operation with renewable energy sources, International Journal of Hydrogen Energy 38 (2013) 723–739. [6] F.N. Büchi, M. Hofer, C. Peter, U.D. Cabalzar, J. Bernard, U. Hannesen, T.J. Schmidt, A. Closset, P. Dietrich, Towards re-electrification of hydrogen obtained from the power-to-gas process by highly efficient H2/O2 polymer electrolyte fuel cells, RSC Adv. 4 (2014) 56139–56146. [7] T. Norman, M. Hamdan, Unitized Design for Home Refueling Appliance for Hydrogen Generation to 5,000 psi, DOE Hydrogen and Fuel Cells Program (2012) 122–125. [8] K.E. Ayers, E.B. Anderson, C.B. Capuano, B.D. Carter, L.T. Dalton, G. Hanlon, J. Manco, M. Niedzwiecki, Research Advances towards Low Cost, High Efficiency PEM Electrolysis, ECS Transactions 33 (2010) 3–15. [9] R. Ströbel, M. Oszcipok, M. Fasil, B. Rohland, L. Jörissen, J. Garche, The compression of hydrogen in an electrochemical cell based on a PE fuel cell design, Journal of Power Sources 105 (2002) 208–215. [10] S.A. Grigoriev, I.G. Shtatniy, P. Millet, V.I. Porembsky, V.N. Fateev, Description and characterization of an electrochemical hydrogen compressor/ concentrator based on solid polymer electrolyte technology, International Journal of Hydrogen Energy 36 (2011) 4148–4155. [11] P.J. Bouwman, J. Konink, D. Semerel, L. Raymakers, M. Koeman, W. Dalhuijsen, E. Milacic, M. Mulder, Electrochemical Hydrogen Compression, ECS Transactions 64 (2014) 1009–1018. [12] P. Millet, R. Ngameni, S.A. Grigoriev, N. Mbemba, F. Brisset, A. Ranjbari, C. Etiévant, PEM water electrolyzers: From electrocatalysis to stack development, International Journal of Hydrogen Energy 35 (2010) 5043–5052. [13] S.A. Grigoriev, V.I. Porembsky, V.N. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy, International Journal of Hydrogen Energy 31 (2006) 171–175. [14] S.A. Grigoriev, M.M. Khaliullin, N.V. Kuleshov, V.N. Fateev, Electrolysis of Water in a System with a Solid Polymer Electrolyte at Elevated Pressure, Russian Journal of Electrochemistry 37 (2001) 819–822. [15] C. Rozain, P. Millet, Electrochemical characterization of Polymer Electrolyte Membrane Water Electrolysis Cells, Electrochimica Acta 131 (2014) 160–167. [16] S.A. Grigoriev, P. Millet, S.V. Korobtsev, V.I. Porembskiy, M. Pepic, C. Etievant, C. Puyenchet, V.N. Fateev, Hydrogen safety aspects related to high-pressure polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy 34 (2009) 5986–5991. [17] J. van der Merwe, K. Uren, G. van Schoor, D. Bessarabov, Characterisation tools development for PEM electrolysers, International Journal of Hydrogen Energy 39 (2014) 14212–14221. [18] F. Marangio, M. Santarelli, M. Cali, Theoretical model and experimental analysis of a high pressure PEM water electrolyser for hydrogen production, International Journal of Hydrogen Energy 34 (2009) 1143–1158. [19] H. Ito, T. Maeda, A. Nakano, A. Kato, T. Yoshida, Influence of pore structural properties of current collectors on the performance of proton exchange membrane electrolyzer, Electrochimica Acta 100 (2013) 242–248.
M. Suermann et al. / Electrochimica Acta 211 (2016) 989–997 [20] D.L. Fritz, J. Mergel, D. Stolten, PEM electrolysis simulation and validation, ECS Transactions 58 (2014) 1–9. [21] I. Barin, G. Platzki, Thermochemical Data of Pure Substances, VCH Verlagsgesellschaft mbH, Weinheim (1995) 795–796. [22] H. Ito, T. Maeda, A. Nakano, H. Takenaka, Properties of Nafion membranes under PEM water electrolysis conditions, International Journal of Hydrogen Energy 36 (2011) 10527–10540. [23] R. Wiebe, V.L. Gaddy, The solubility of Hydrogen in Water at 0, 50, 75 and 100 from 25 to 1000 Atmospheres, J. Am. Chem. Soc. 56 (1934) 76–79. [24] D. Tromans, Temperature and pressure dependent solubility of oxygen in water: a thermodynamic analysis, Hydrometallurgy 48 (1998) 327–342. [25] J. Durst, A. Siebel, C. Simon, F. Hasche, J. Herranz, H.A. Gasteiger, New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism, Energy & Environmental Science 7 (2014) 2255–2260. [26] J.O.M. Bockris, A.K.N. Reddy, M.E. Gamboa-Aldeco, Modern Electrochemistry 2A, Fundamentals of Electrodics, Springer, US, New York, 2000. [27] V. Schröder, B. Emonts, H. Janssen, H.-P. Schulze, Explosionsgrenzen von Wasserstoff-Sauerstoff-Gemischen bei Drücken bis 200 bar, Chemie Ingenieur Technik 75 (2003) 914–918. [28] G. Xu, Y.S. Pak, Dielectric Relaxation and Conductivity Study of Polytetrafluorene-Ethyl lonomers, Journal of The Electrochemical Society 139 (1992) 2871–2876. [29] A. Siu, J. Schmeisser, S. Holdcroft, Effect of Water on the Low Temperature Conductivity of Polymer Electrolytes, J. Phys. Chem. B 110 (2006) 6072–6080. [30] F.N. Büchi, G.G. Scherer, In-situ resistance measurements of Nafion 117 membranes in polymer electrolyte fuel cells, J. Electroanal. Chem. 404 (1996) 37–43. [31] L. Gubler, L. Bonorand, Radiation Grafted Membranes for Fuel Cells: Strategies to Compete with PFSA Membranes, ECS Transactions 58 (2013) 149–162.
997
[32] N. Pérez-Hernández, T.Q. Luong, M. Febles, C. Marco, H.-H. Limbach, M. Havenith, C. Pérez, M.V. Roux, R. Pérez, J.D. Martín, The Mobility of Water Molecules through Hydrated Pores, The Journal of Physical Chemistry C 116 (2012) 9616–9630. [33] M.A. Hoeh, T. Arlt, N. Kardjilov, I. Manke, J. Barnhart, D.L. Fritz, J. Ehlert, W. Lüke, W. Lehnert, In-operando Neutron Radiography Studies of Polymer Electrolyte Membrane Water Electrolyzers, ECS Transactions 69 (2015) 1135–1140. [34] M.A. Hoeh, T. Arlt, I. Manke, J. Banhart, D.L. Fritz, W. Maier, W. Lehnert, In operando synchrotron X-ray radiography studies of polymer electrolyte membrane water electrolyzers, Electrochemistry Communications 55 (2015) 55–59. [35] E. Brightman, J. Dodwell, N. van Dijk, G. Hinds, In situ characterisation of PEM water electrolysers using a novel reference electrode, Electrochemistry Communications 52 (2015) 1–4. [36] E. Antolini, Iridium As Catalyst and Cocatalyst for Oxygen Evolution/Reduction in Acidic Polymer Electrolyte Membrane Electrolyzers and Fuel Cells, ACS Catalysis 4 (2014) 1426–1440. [37] L.A. De Faria, J.F.C. Boodts, S. Trasatti, Electrocatalytic properties of ternary oxide mixtures of composition Ru0.3Ti(0.7-x)CexO2: Oxygen evolution from acidic solution, Journal of Applied Electrochemistry 26 (1996) 1195–1199. [38] E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt, Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction, Catal. Sci. Technol. 4 (2014) 3800–3821. [39] M. Suermann, T.J. Schmidt, F.N. Büchi, Investigation of Mass Transport Losses in Polymer Electrolyte Electrolysis Cells, ECS Transactions 69 (2015) 1141–1148. [40] S. Siracusano, V. Baglio, A. Di Blasi, N. Briguglio, A. Stassi, R. Ornelas, E. Trifoni, V. Antonucci, A.S. Aricò, Electrochemical characterization of single cell and short stack PEM electrolyzers based on a nanosized IrO2 anode electrocatalyst, International Journal of Hydrogen Energy 35 (2010) 5558–5568.