Proton Transport in Catalyst Layers of a Polymer Electrolyte Water Electrolyzer: Effect of the Anode Catalyst Loading

The understanding of loss terms in polymer electrolyte water electrolysis (PEWE) cells is essential to maximize efﬁciency and minimize cost and enable the technology for energy applications, such as “power-to-X”. We adapt the use of the transmission-line model for porous electrodes in conjunction with electrochemical impedance spectroscopy measurements of the cell in the H 2 /N 2 mode known for fuel cells to PEWE cells to quantify proton transport resistance and double layer capacitance of the anode catalyst layer. Anode catalyst loading was varied between 0.05 and 3.2 mg IrO2 cm − 2 . A non-linear relationship was found between the anode IrO 2 loading and the proton transport resistance. Catalyst layers with very low IrO 2 loading (0.05–0.16 mg IrO2 cm − 2 ) had ∼ 4 times higher mass-speciﬁc activity and ∼ 2 times higher mass-speciﬁc capacitance, revealing an inhomogeneous utilization of the catalytic material. The kinetic and transport limitations associated with the anode catalyst layer have been correlated with its morphological features.©TheAuthor(s) 2019. Published by ECS. This an terms work in any medium, [DOI: 10.1149/2.0341904jes]

2][3][4][5][6] PEWE is especially attractive in power grids with a high share of fluctuating renewables, where the H 2 can be used for seasonal energy storage.H 2 as an energy vector is versatile since it can be re-electrified in a fuel cell for grid balancing, used in the mobility sector in fuel cell electric vehicles, or utilized in chemical industries to promote deep decarbonization of the energy sector. 1,5Furthermore, produced H 2 can be used in a downstream methanation process to form synthetic natural gas (SNG) using suitable, preferably renewable CO 2 and to feed it into the existing gas grid. 6Moreover, mixtures of H 2 and CO 2 can be used to produce syngas and subsequently liquid hydrocarbon energy carriers via a gas-to-liquid process.
The PEWE technology is currently plagued by issues associated with cost and component durability. 7,8The oxygen evolution reaction (OER) in acidic medium on the anode is currently catalyzed by precious materials from the platinum group metals (PGM).However, the cost of the catalyst materials has been estimated to contribute only 8% to the cost of the commercial stack. 9The final cost of the produced H 2 is not greatly affected by the cost contribution of the stack components. 10It is dominated by the cost of electricity (currently ∼70% of the hydrogen end cost), and is therefore heavily dependent on the voltage efficiency of the stack. 10The type of the OER catalyst and its loading on the other hand have a large impact the stack voltage efficiency through the activation overpotential (η act ). 11,12ridium oxide (IrO 2 ) is the state-of-the-art PGM to catalyze the OER in commercial PEWE stacks. 7,8The most active material toward the OER is ruthenium oxide, but the dissolution rates remain too high for commercial implementation in the aggressive PEWE anode environment, in spite of extensive efforts to stabilize it. 13Higher loadings are preferred to maintain high stack voltage efficiency and reduce the end cost of H 2 .Maintaining a high IrO 2 loading in the anode catalyst layer (CL a ) at high market penetration of PEWE in the energy sector might not be viable due to the low abundance and limited supply of iridium, especially considering it is a by-product of the Cu, Ni and Pt mining. 14The state-of-the art PEWE stacks (∼4 W cm −2 and 2 mg IrO2 cm −2 ) would require around 500 kg IrO2 /GW.This would allow for an annual rate of capacity increase of 2 GW/a, considering the annual production rate of iridium of 4 tons per annum and assuming a use of 25% of this for PEWE. 8Reduction of the IrO 2 loading using supported, [15][16][17] high surface area catalysts [18][19][20] or a nanostructured thin film catalyst 21 is necessary to enable higher penetration of PEWE.On the other hand, reduction of the IrO 2 loading is only justifiable while maintaining high voltage efficiency of the PEWE stack.
Understanding the factors that determine the catalyst utilization is necessary to direct the development of the next-generation catalyst layers for the electrochemical water splitting technology.A set of diagnostic tools is required to benchmark catalyst layer performance and make correlations between the morphology and the overpotential in the polarization characteristics of a PEWE cell.In this study we present a detailed methodology for obtaining the proton transport resistance (R H+ CLa ) and the double layer capacitance of the CL a from electrochemical impedance spectra collected in the H 2 /N 2 regime.The approach has been widely used in polymer electrolyte fuel cell (PEFC) technology 22 and is adapted here for a PEWE cell.The impedance is analyzed using the transmission-line model (TLM) [22][23][24][25][26] to establish a correlation between the physical characteristics of the CL a and the transport properties.The method is then tested on catalyst layers with loading varied between 0.05 and 3.2 mg IrO2 cm −2 .[29][30][31]

Experimental
Cell and test-station.-Atest bench developed in-house, equipped with a SP-150 potentiostat and a VMP3B 80A booster from Biologic was used to perform experiments.MiliQ water was circulated at a flow rate of 400 mL min −1 from a heated gas-water separator through the anode PEWE cell compartment.A PEWE cell with an active area of 25 cm 2 with parallel flow fields was used to accommodate commercial T10 sintered-Ti porous transport layers (PTLs) from GKN.The electro-osmotically dragged water was discarded periodically from the cathodic gas-water separator.Cell housing, inlet and outlet temperatures were monitored during the experiments, and the average temperature was used to specify the operating temperature.
CCM preparation.-In-houseprepared CCMs were based on the Nafion 117 (DuPont) perfluorosulfonic acid membrane.Nafion 117 sheets of 10 × 10 cm were immersed in a ∼30 vol% HNO 3 (VWR Chemicals) solution at 80°C for 1 h to protonate the sulfonic acid groups and remove impurities.Boiling steps were repeated 4 times in MiliQ water to remove the acid from the membrane.Membranes were dried for 12 h before weighing, and afterwards mounted into a PEEK-based frame for spraying.The anode catalyst ink was prepared by mixing 0.85 mL isopropanol, 2.7 mL miliQ water and 2.54 mL Nafion Solution (Aldrich, 5 w% Nafion).The ink was sonicated for 15 minutes. 1 g of IrTiO 2 catalyst (Umicore, 66 mol% IrO 2 and 34 mol% TiO 2 ) was added to the solution and sonicated for additional 30 min.The ink was sprayed onto the membrane using an airbrush pencil (Conrad Electronics Airbrush Pistol HP-200).The CCMs were dried again for 12 h and subsequently weighed.The ink-spraying preparation method yielded anode loadings from 0.05 to 3.2 mg IrO2 cm −2 .A gas diffusion electrode (Johnson Matthey ELE-0244-0542 with HiSpec 9100 Pt/C 0.4 mg Pt /cm 2 ) was used as the cathode.The CCMs were assembled in the wet state after immersion in miliQ water for 12 h before testing.
Experimental conditions.-Cellswere conditioned at 60°C by cycling the current density between 1 and 2 A cm −2 until stable performance and uniform temperatures in the system were achieved.Polarization curves were recorded galvanostatically, while measuring the cell impedance at 10 kHz at every current step.The measurements were repeated at 50, 60 and 70°C.PEWE cells were operated in the H 2 /N 2 regime to measure the ionic resistance of the anode catalyst layer.A N 2 -stream of 500 Nml min −1 was injected into the working electrode (WE) water loop, while the reference/counter electrode (RE/CE) was supplied with humidified H 2 at 400 Nml min −1 .We ensured no faradaic current was measured before recording the electrochemical impedance spectrum (EIS).The impedance in H 2 /N 2 mode was recorded from 10 kHz to 300 mHz potentiostatically at 1.0, 1.2 and 1.4 V.No faradaic current was observed at these potentials during the collection of the impedance spectra.Cyclic voltammograms (CVs) were measured in the potential range between 0 and 1.4 V vs. RHE at a sweep rate of 50 mV/s.

Results and Discussion
Morphological analysis.-Post-testCCM samples were cryogenically fractured in liquid nitrogen and analyzed using a Zeiss Supra VP55 scanning electron microscope, with an acceleration voltage of 5 kV.CCM cross-sections were previously sputter-coated with a ∼7 nm Cr-layer for better electrical conductivity.CCM cross-sections were investigated using the scanning electron microscope (SEM) to determine the thickness of the CL a with different loadings (Table I), which was found to increase with 2.15 ± 0.13 μm mg IrO2 −1 cm 2 .An airbrush sprayed CL a consists of catalytic agglomerates 100-400 nm in size, connected by an ionomer film (Figure 1).The structure contains pores (voids) with a wide size distribution (0.1-1.5 μm in diameter).The cross-section of the 0.05 mg IrO2 cm −2 anode shows only a few catalytic agglomerates.The anode surface images reveal how the catalytic agglomerates are interconnected laterally.Lower loaded anodes are less homogeneous in-plane, and the ionic network is more frequently dis-rupted.The CL a of the 0.05 mg IrO2 cm −2 anode is very thin, and the agglomerates are often disconnected.The surface catalytic agglomerates of the thick CL a are ionically well interconnected through the ionomer covering the agglomerates underneath.
Transmission line model for porous electrodes.-The3][24][25][26] According to the TLM, the CL consists of differential elements describing the proton transport resistance, double layer capacitance (C DL ) and charge transfer resistance (R CT ) (Figure 2).The TLM is often simplified by using an inert gas fed working electrode (WE), and a H 2 -fed reference/counter electrode (RE/CE).When the potential is applied, the permeating H 2 from the RE/CE cell compartment is the reactant for the permeation-rate limited hydrogen oxidation reaction (HOR) on the WE.The measured DC current corresponds to the hydrogen crossover current, which varies with the membrane thickness,  temperature, pressure and relative humidity conditions in the cell.The crossover current does not contribute to the total impedance, and the low charge-transfer resistance of the HOR and the HER reactions allows for the simplification of the CL a equivalent circuit (Figure 2).The electrode-electrolyte interface in this case behaves as a simple capacitor, which simplifies the analysis of the impedance spectra, as the charge-transfer arc following the high-frequency 45°region toward low frequencies vanishes (Figure 3). 22,23Deviation from the capacitive behavior at low frequency could result from structural inhomogeneity of the electrode. 32Impedance spectra are recorded at 1.0, 1.2 and 1.4 V representing a potential region beyond the OER regime. 33he absence of a faradaic reaction on the WE greatly simplifies the impedance analysis and the estimation of the TLM equivalent circuit elements describing R H+ CLa and C DL .The equation describing the impedance response of the CL 22 can therefore be simplified according to where j is the imaginary unit (the symbol 'i' is used for current density).
In the special case when the frequency ( f ) tends to zero, the WE impedance becomes The C DL can be obtained from the limiting impedance at low f. 22Instead of assuming constant Im(Z) and averaging it at low f, 24 we have estimated the C DL according to Equation 3 using Im(Z) at the minimum phase angle (ϕ min ), when the transition to the capacitive behavior sets in.The ϕ min is obtain at lower frequencies for thicker CL a , as the AC current needs to penetrate longer pores.When the pores are penetrated, the ϕ goes through a minimum value and increases asymptotically to 90 o .The uncertainty of averaging is in this way mitigated, since the Im(Z) in Equation 2often does not exhibit a plateau at low f, especially with low catalyst loadings. 34Another way to calculate C DL is by dividing the average CV current in the potential range of 1.0-1.4V by the sweep rate.C DL obtained from the CV appears to be higher compared to the C DL calculated from the EIS in the H 2 /N 2 regime.(Figure 4a).Higher C DL measured in the CV can be related to the pseudocapacitance resulting from the fast electrosorption processes while cycling the electrode potential.For both methods C DL increases with the anode loading due to the increase of the absolute catalyst mass and therefore the surface area.
The mass-specific capacitance (c m DL ) normalized to the amount of IrO 2 in the active area shows little deviation between the anodes with different IrO 2 loading, except for the samples with 0.05 and 0.16 mg IrO2 cm −2 where it is approximately 2 times higher.Deviation from the constant c m DL could be related to the inhomogeneous utilization of the catalyst in the case of thicker CL a with higher loading.A trend similar to the one of C DL and c m DL vs. the IrO 2 loading is observed (cf.below) when estimating the mass-specific activity from the iR-free polarization curves.It is assumed that the TiO 2 in the catalyst layer is an insulator and that the electronic conductivity of the catalyst is ensured through the IrO 2 percolation network. 35The capacitances are normalized only to the mass of IrO 2 , since the contribution of TiO 2 would in this case be negligible.At high f , the 45 o region appears in the Nyquist plot (Figure 3).Here, the impedance can be described by followed by the capacitive region at low frequencies, represented by the 90 o line in the Nyquist plot and the HFR-corrected impedance given by R H+ CLa /3 can be extracted from the Nyquist plots of H 2 /N 2 operated cells by projecting the low f impedance vertically to the real axis (Figure 3). 22,32,335][26] Graphical extrapolation of R H+ CLa from the Nyquist plot relies on the presence of a vertical, capacitive impedance at low f, which is not always observed in the measured data. 22,33,36eviations from the low f capacitive behavior can be attributed to inhomogeneity of the catalyst layer, stemming from the preparation process or caused by the coarse PTL surface. 32,37The morphologic properties of the CL together with the CL/PTL interface properties might result in electrically insulated regions in the CL with high resistance, accessible only at low f. 37Furthermore, the wetting of the CL pores could be non-uniform in PEWE, 11,33 resulting in distributed capacitance through the CL thickness.This can lead to a large variance in the extrapolation of the R H+ CLa (Figure 5).On the other hand, approximation of C DL from Equation 3 requires that -Im(Z) plateaus at low f. 24If this is not the case, the uncertainty in the calculation of C DL , and subsequently of R H+ CLa , increases.To circumvent the possible issues and errors in the estimation of C DL , we propose a method of finding the C DL values at the frequency at which a minimum in the phase angle ϕ is observed.This point marks the impedance transition from the 45 o region to the capacitive behavior.HFR-corrected impedance spectra in H 2 /N 2 mode are used to calculate the ϕ values (Figure 3), and the value of f ϕmin is obtained at ϕ min .The method results in a defined kneepoint in the Nyquist plot based on the pore length, from which R H+ CLa and C DL can be calculated.The f ϕmin is lower for thicker catalyst layers since the current has to penetrate longer distances, which is evident in Comparison of R H+ CLa values calculated using the f ϕmin approach (cf. Figure 3) and from the graphical extrapolation of the vertical line at low f.The minimum R H+ CLa is observed for the sample with an IrO 2 loading in the range from 0.16 to 1.2 mg IrO2 cm −2 .
Figure 3 for samples of 3.2 vs. 0.8 mg IrO2 cm −2 .In this study we have used both methods and compared the R H+ CLa values obtained for PEWE anodes with catalyst loading ranging from 0.05 to 3.2 mg IrO2 cm −2 .The values of R H+ CLa obtained from the graphical extrapolation from the Nyquist plot and from the C DL at f ϕmin are shown in Figure 5. Graphical extrapolation resulted in higher values of R H+ CLa in the case of very low and very high loadings, while both methods yield similar R H+ CLa for the cells with 1.8 and 2.6 mg IrO2 cm −2 .
The measured R H+ CLa decreases with higher cell temperatures due to the increased ionic conductivity of the ionomer in the CL a (13.0/10.3/9.1 mΩ cm 2 at 50/60/70 o C for 0.8 mg IrO2 cm −2 , respectively, based on data from Figure 3).The contribution of the ionic resistance of the CL a appears as a loss term of magnitude R H+ CLa /3 in the polarization curve, as part of mass transport losses (η mtx ) 24,33,38 Improved proton transport in the CL a would partially account for the temperature dependent η mtx previously reported by Suermann et al. 28 The change of η mtx at 1 A cm −2 between 50 and 70 o C was ∼7 mV for the CL with 0.8 mg IrO2 cm −2 in this study.The change in the area resistance of a ∼3 μm thick Nafion ionomer sheet between 50 and 70°C is in the range of ∼1 mΩ cm 2 (measured value in the H 2 /N 2 regime ∼4 mΩ cm 2 ), and can only account for a fraction of η mtx .Stronger sensitivity of R H+ CLa to temperature variations during H 2 /N 2 measurements compared to what is reported for Nafion 39 could stem from thin film limitations and the water distribution in the CL as a factor of the PTL/CL interface, and will be discussed further in the overpotential breakdown section below.
Considering CL a as a sheet electrode, R H+ CLa would decrease with the thickness, hence lower IrO 2 loading. 25Two trends of R H+ CLa have been observed with varying loading in the H 2 /N 2 regime.R H+ CLa decreases with decreasing loading in the range from 3.2 to 1.2 mg IrO2 cm −2 on the account of lower CL a thickness, since the proton transport path is longer for thicker catalyst layers.The lowest R H+ CLa values were observed for the IrO 2 loading range from 1.2 to 0.16 mg IrO2 cm −2 .Interestingly, R H+ CLa was highest for the sample with 0.05 mg IrO2 cm −2 .Proton transport from the catalytically active sites relies both on throughplane as well as in-plane transport when the structure is composed of catalytic agglomerates connected by the ionomer.At very low IrO 2 loading applied using the airbrush method the CL a structure becomes less homogeneous, as observed in the SEM images.This results in a lower degree of connectivity between the catalyst agglomerates by the ionomer network and subsequently higher in-plane resistance contribution to the R H+ CLa .Alternative catalyst structures, such as the nanostructured thin film catalyst (NSTF) developed by 3M, 21 and different application methods 40 are necessary to achieve low-loaded anodes with uniform structure.The deviation of the measured impedance from the characteristic 45 o slope at high f is more pronounced for the CL a with low IrO 2 loading (<1.2 mg IrO2 cm −2 ).This behavior of the impedance has previously been ascribed to electrodes with open, wedge-shaped pores, 41,42 which is the case for low-loaded anodes with inhomogeneous distribution and packing of the catalyst agglomerates.Other possible causes of non-linear behavior of R H+ CLa with varying IrO 2 loading are discussed in more detail in the section on overpotential analysis, in the context of the mass transport losses.
The average proton resistivity of the anode catalyst layers with >0.05 mg IrO2 cm −2 (ρ H+ CLa ) was found to be 130 ± 33 Ω cm, using the R H+ CLa and the CL a thickness (d CLa ) values from the SEM crosssectional images (Table I).The calculated value is in the range of values typically obtained for a fuel cell catalyst layer under fully humidified conditions at low ionomer to carbon ratio. 26The reason for a large variation in ρ H+ CLa in the case of 0.05 mg IrO2 cm −2 is the strong deviation of the calculated R H+ CLa from the decreasing trend with the IrO 2 loading.ρ H+ CLa is normally found to be a property of the catalyst layer that is independent of its thickness. 26Our CL a structure is laterally very inhomogeneous at lower loadings, and the corresponding in-plane R H+ CLa contribution results in an increase of the apparent ρ H+ CLa .Taking the ionomer resistivity (ρ H+ i ) from the literature and using the ionomer volume fraction in the electrode (ɛ i ), the tortuosity (τ) can be calculated: We obtained ɛ i = 0.314, following the equations given in detail by Liu et al., 26 and assumed ρ H+ i from 8 to 11 Ω cm 26,43 to find that τ goes through a minimum close to 1 for the cell with a loading of 1.2 mg IrO2 cm −2 (Table I).The findings from the H 2 /N 2 measurements suggest that the ionomer does not form a well-connected network between the catalytic agglomerates especially in the case of 0.05 mg IrO2 cm −2 , which is reflected in the R H+ CLa and the η mtx values in the overpotential analysis.
Overpotential breakdown.-Theoverpotential breakdown of the PEWE polarization curve was conducted according to the procedure described in the Appendix.Polarization curves of PEWE cells with differently loaded CL a have been iR-corrected using the HFR to eliminate the influence of the variations in ohmic overpotential (η ) that might stem from small variations in the cell assembly, membrane ionic resistance and contact resistances with different PTL samples.The HFR was found to be 257 ± 16 mOhm cm 2 for all samples at 60°C, without any correlation to the IrO 2 loading.Increasing the CL a loading results in lower E iR−free (Figure 6), which is lowest for the highest loading (3.2 mg IrO2 cm −2 ) and highest for the lowest loading (0.05 mg IrO2 cm −2 ), as may be expected.The E iR−free values of the cells with 0.16-3.2mg IrO2 cm −2 have a constant Tafel slope b of 55 ± 1 mV/dec, and are offset by a factor depending on the CL a loading.The Tafel slope is higher only in the case of the 0.05 mg IrO2 cm −2 sample with 70 mV/dec.A similar effect of the CL a loading on the Tafel slope was observed by Bernt et al., 11 albeit already at 0.2 mg IrO2 cm −2 .The catalyst activity i a , expressed as the current density at a fixed E iR−free of 1.47 V, increased linearly with the CL a loading, as expected.The calculated mass activity i m a is in the range of 10.2 ± 1.3 A g −1 for the cells with CL a loading between 0.8 and 3.2 mg IrO2 cm −2 .However, the i m a for the cell with 0.16 mg IrO2 cm −2 is found to be approximately three times higher (Figure 7).Decreasing the loading even further to 0.05 mg IrO2 cm −2 results in i m a of 54.8 ± 4.2 A g −1 .The deviation of i m a in the case of CL a with a very low IrO 2 loading (<0.2 mg IrO2 cm −2 ) indicates that the catalyst utilization of higher loaded CLs is low. 445][46][47][48] Assuming negligible differences in the in-and through-plane electronic conductivity of the CL a , the catalyst material  closer to the PEM would not be fully utilized.Although more catalyst contributes to the OER in the case of anodes with higher IrO 2 loading, the fraction of active material would be lower.This effect is illustrated in Figure 8.
The variation of the IrO 2 loading affected η mtx in a similar fashion as the R H+ CLa obtained in H 2 /N 2 configuration, i.e. in the absence of faradaic current (Figure 9).Since the R H+ CLa contribution appears as a part of the η mtx , 33,38 a good correlation in the trend of these two independently determined quantities can be taken as a validation of the transmission line model method to determine R H+ CLa in PEWE cells.Structural inhomogeneity of low IrO 2 -loaded CL a related to the preparation method could be the cause of the non-linear behavior of R H+ CLa , and consequently η mtx .Low IrO 2 loadings were found to have a similar effect on the electronic conductivity of the catalyst layer. 115][26] Bernt et al. 11  which would affect R H+ CLa , and ultimately η mtx .As the OER in PEWE cells occurs below the PTL particles, [45][46][47][48] the water has to diffuse both laterally and perpendicularly to the membrane surface (Figure 8) from the PTL pore to the active sites below the PTL particle. 33Since the water diffusion through Nafion thin films (< 60 nm) is orders of magnitude slower compared to the bulk, 49,50 thicker CLs would result in longer diffusion distances for the reactant, and thus yield an inhomogeneous water distribution in the CL.Development of an anodic PTL with a microporous layer is necessary to alleviate problems which diminish the electronic and ionic connectivity of anode CLs with low IrO 2 loadings. 11

Conclusions
Understanding the relationship between cell performance and the structural features of the anode catalyst layer in PEWE is necessary for tailoring next-generation catalyst layers for optimal transport properties and activity.We provide a methodology to determine the proton transport resistance in the anode catalyst layer, R H+ CLa , based on impedance measurements in the H 2 /N 2 regime by identifying the knee point in the Nyquist plot corresponding to the minimum phase angle observed in the Bode phase plot.The presented diagnostic method is a universal tool for characterizing catalyst layers for water electrolyzers.Application of the method to anodes with varying IrO 2 loading from 0.05 to 3.2 mg IrO2 cm −2 has revealed that the loading affects R H+ CLa in a non-linear fashion.Overpotential analysis showed that there is a correlation between the trends observed in R H+ CLa and the mass transport overpotential η mtx versus the IrO 2 loading.Low loaded anodes (<1.2 mg IrO2 cm −2 ) exhibit a high degree of in-plane inhomogeneity, which could explain the non-linear relation between the IrO 2 loading and R H+ CLa .Other potential causes related to the water management in the catalyst layer are discussed in the context of η mtx values measured with different IrO 2 loadings.
We have demonstrated that more IrO 2 catalyst yields lower activation losses, as expected, yet a lower effective utilization of the material.Moreover, a systematic variation of the IrO 2 loading has revealed a relationship between the activation overpotential η act and η mtx that can to a certain extent compensate the apparent loss of voltage efficiency in going toward lower IrO 2 loading.Optimizing the morphology of the catalyst layer by adapting the preparation method could open a pathway to low loaded catalyst layers with high volumetric utilization of the active material.oxygen evolution reaction (OER). 52The cathode reaction is assumed not to be kinetically limiting. 38η act is obtained by subtracting E rev from the Tafel line, which is calculated from the iR-free cell voltage (E iR−free ) at low current densities (0.01-0.1 A cm −2 ).η mtx appears at higher current densities and is obtained from the difference of E iR−free and the fitted Tafel line.Part of η mtx is related to the proton transport resistance in the CL a , 11,33,38 and can be obtained from the Nyquist plot of the EIS during H 2 /N 2 operation. 33

Figure 1 .
Figure 1.Surface of the anode catalyst layers with a loading of a) 0.05 mg IrO2 cm −2 and c) 1.2 mg IrO2 cm −2 and their respective cross-sections b) and d), observed under the scanning electron microscope.

Figure 2 .
Figure 2. Equivalent circuit of the transmission line model for porous electrodes, consisting of differential elements describing the proton transport resistance in the catalyst layer R H+CLa , double layer capacitance C DL and the kinetic resistance related to the electrochemical reaction R CT .In the H 2 /N 2 regime, the impact of the kinetic resistances becomes negligible (R CT → ∞) due to the near-absence of the faradaic reaction on the electrode.

Figure 3 .Figure 4 .
Figure 3. a) HFR-corrected impedance response of the PEWE cell with 0.8 mg IrO 2 cm −2 in the H 2 /N 2 regime measured at 50, 60 and 70°C, showing the temperature dependence of R H+ CLa .Impedance of the cell with 3.2 mg IrO 2 cm −2 is included to demonstrate the increase of R H+ CLa with the thickness of the anode catalyst layer.b) Bode phase plot of the cell with 0.8 mg IrO 2 cm −2 in the H 2 /N 2 regime measured at 50, 60 and 70°C, and of the cell with 3.2 mg IrO 2 cm −2 at 60°C.The R H+ CLa /3 projection is obtained by first finding f min at which ϕ min appears in b), and extracting the impedance at f min from the Nyquist plot in a).

Figure 5 .
Figure 5.Comparison of R H+CLa values calculated using the f ϕmin approach (cf.Figure3) and from the graphical extrapolation of the vertical line at low f.The minimum R H+ CLa is observed for the sample with an IrO 2 loading in the range from 0.16 to 1.2 mg IrO2 cm −2 .

Figure 6 .
Figure 6.E IR-free , η act and η mtx for the cells with different IrO 2 loadings.Data were obtained by iR-correcting the polarization curves using the HFR at 10 kHz measured at 60°C and atmospheric pressure.

Figure 7 .
Figure 7. Activity in the form of the current measured at a fixed iR-free potential of 1.47 V (black), and the activity normalized to the IrO 2 loading (mass activity, red).

Figure 8 .
Figure 8. Schematic representation of the PEWE anode catalyst layer.The ionomer phase conducts protons from the catalytically active surface sites.The metallic phase provides the electronic conductivity of the layer.

Table I . Properties of the IrO 2 /TiO 2 based catalyst layers for PEWE. The catalyst layer thickness (d CLa ) is obtained from the CCM cross-sections observed with SEM. The anode catalyst layer proton transport resistance (R H+ CLa ) and the double layer capacitance (C DL ) values are extracted from the cell impedance measured in the H 2 /N 2 regime at 60°C. Proton resistivity of the anode catalyst layer (ρ H + CLa ) was calculated from R H+ CLa and d CLa values. The tortuosity (τ) of the electrode was calculated using Equation 5. The Tafel slope (b) and the anode catalyst activity (i a ) are extrapolated from the iR-free polarization curves at 60°C. The mass specific quantities c m DL and i m a are obtained by normalizing the calculated values to the IrO 2 loading.
Calculated from the EIS in the H 2 /N 2 regime.Calculated from the iR-free polarization curve at 1.47 V.