Membrane Interlayer with Pt Recombination Particles for Reduction of the Anodic Hydrogen Content in PEM Water Electrolysis

Single-sided catalyst coated membranes (CCMs) from HIAT gGmbH were used as base material for the electrolysis cells with an active area of 25 cm². On the anode side a Nafion N115 membrane with 2 mg cm^{− 2} iridium black (Ir) as catalyst layer (CL) and on the cathode side a Nafion NR-212 membrane with 1 mg cm^{− 2} Pt on advanced carbon as CL were used. As shown schematically in Fig. 1, the interlayers were fabricated by spray coating (Fig. 1 (1)) onto the membrane side of one of the single-sided CCMs and sandwiched between both membrane sides (Fig. 1 (2)).

Zoom In Zoom Out Reset image size

In order to prepare the interlayers, the Pt nanoparticles (Sigma-Aldrich, particle size < 50 nm), Nafion dispersion (D2020, Ion-power) and isopropanol were mixed in a weight ratio of 0.00114/1.43/10. A spray coater from Sono-Tek with a line spacing of 1.5 mm and a syringe pump rate of 0.1 mL min^{− 1} was used to apply the interlayers in 45 consecutive spraying cycles with a transversal speed of 30 mm s^{− 1}. The spray coater was equipped with a 48 kHz ultrasonic nozzle. To determine the Pt loading, the MEAs were weighed before and after spray coating and the Pt content was determined by the weight to weight ratio of Pt and Nafion in the dispersion. Two MEAs with Pt interlayers (Int 1 and Int 2) were made to test the interlayers with respect to repeatability and reproducibility.

As reference a conventional N117 membrane (Ref. 1) and the same half-sided-CCMs (N115 + NR-212) as for the Int 1 and Int 2 membrane electrode assembly (MEA) without depositing an interlayer (Ref. 2) with equal catalyst layers were used.

After testing, the Int 1 MEA was embedded in Araldite 502 epoxy resin (54.8 wt% Araldite 502, 42.7 wt% Dodecenylsuccinic anhydride, 2.5 wt% Dimethylbenzylamine) and cured for 12 hours at 60°C. The sample was prepared for imaging by wet ultra-thin sectioning using a RMC Boeckeler PowerTome ultramicrotome equipped with a Diatome ultra 45° diamond knife. Subsequently, the remaining blockface of the sample was imaged with a Scanning Electron Microscope (SEM, FEI 250 Quanta FEG with vCD back scatter detector) in "Low Vac" measurement mode at 111 Pa water pressure. ImageJ with the plug-in local thickness was used to determine the interlayer thickness.

For comparison with a pristine Pt interlayer the Pt nanoparticles/Nafion dispersion was sprayed onto a Nafion N115 membrane during the same spraying process as for the MEA preparation and prepared and imaged with the same procedure as described above.

Test station

Cell

Gas chromatograph

Power supply and impedance spectroscopy

Each MEA was investigated with the following test sequence of the single measurement techniques:

• (1)  
  begin of test (BOT): polarization curve + HFR, EIS and H₂ in O₂ content for different current densities
• (2)  
  long-term (10 days): cell voltage and H₂ in O₂ content at 1 A cm^{− 2}
• (3)  
  end of test (EOT): polarization curve + HFR, EIS and H₂ in O₂ content for different current densities

In the following the single measurement techniques are described in detail.

Polarization curve + HFR

Electrochemical impedance spectroscopy

Hydrogen in oxygen content

The final loading of solid material (Nafion and Pt) after spraying amounted to 4.67 mg cm^{− 2} which corresponds to a Pt loading of 0.02 mg cm^{− 2} and an additional interlayer thickness of about (26 ± 1) μm as can be seen in the cross-section of the Int 1 MEA shown in Fig. 2. The Pt interlayer was spray-coated onto the cathode half-sided CCM (Pt/C + NR-212) and sandwiched with the anode half-sided CCM (Ir + N115). No noticeable differences were observed in SEM images of cross-sections of pristine and used MEA.

Zoom In Zoom Out Reset image size

Fig. 3 shows the results of the two similar MEAs with interlayers (Int 1 and Int 2) and two reference MEAs without interlayers (Ref. 1 and Ref. 2) at the beginning of test (BOT).

Zoom In Zoom Out Reset image size

In Fig. 3a the H₂ in O₂ contents within the anodic product gas is plotted as a function of the current density. At 1 A cm^{− 2} the H₂ in O₂ content of the Ref. 1 MEA amounts 0.13 vol.%. Whereas the Ref. 2 MEA with approximately equal membrane thickness as the Ref. 1 but consisting of a sandwich of a Nafion N115 and a Nafion NR-212 membrane exhibits a higher H₂ in O₂ content for most of the measured current density range ( ≥ 0.25 A cm^{− 2}), e.g. is the H₂ in O₂ content at 1 A cm^{− 2} twice the amount of the Ref. 1 MEA. This can be caused by the membrane-membrane interface developing some cavities, which influence the gas permeation through the membrane or might arise from membrane defects occurring during cell assembly, since handling two half-CCMs is more likely to cause pinholes or electrical shorts.

When introducing a Pt interlayer in between the N115 and NR-212 membranes (Int 1 and Int 2), the H₂ in O₂ content decreases significantly for all current densities. For current densities > 0.5 A cm^{− 2} it is below the detection limit of 0.005 vol.%. The increase of the membrane thickness by (26 ± 1) μm due to the recombination interlayer is not sufficient to explain the strong reduction of the H₂ in O₂ content. Theoretically, the H₂ in O₂ content for a (26 ± 1) μm thicker membrane should be reduced by a factor of 180 μm/206 μm. Consequently, the H₂ in O₂ reduction is mainly caused by the recombination of permeating H₂ and O₂ to water within the interlayer. Thus enough O₂ (at least the half of the H₂ flux) is present at the interlayer for the recombination of the permeating H₂.

Fig. 3b shows the cell voltage over the current density for the examined MEAs. As can be seen when comparing the Int 1 and Int 2 MEAs to the sandwiched MEA without interlayer (Ref. 2), despite thicker membrane, the Pt interlayer does not highly affect the performance. However, the best performance is measured for the Ref. 1 MEA. Despite the same membrane thickness, the cell voltage of the Ref. 2 MEA at 1 A cm^{− 2} is 34 mV higher than for the Ref. 1 MEA (1.679 V). This increase in cell voltage correlates to an increase in high frequency resistance (HFR) as shown over the current density in Fig. 3c. At 1 A cm^{− 2} the HFR of the Ref. 2 MEA is 27 mΩ cm² higher than of the Ref. 1 MEA (0.179 Ω cm²), causing an increase in cell voltage of 27 mV, which is in good agreement with the measured voltage increase. The higher HFR might be caused by the additional membrane-membrane interface adding an additional resistance, which is much more pronounced for higher current densities as can be seen in Fig. 3c. For small current densities the HFR of the Ref. 2 and the Ref. 1 MEA is almost equal ( ≈ 0.175 Ω cm²) as expected for MEAs with the same membrane thickness.

The Int 1 and Int 2 MEAs show a slight increase of cell voltage compared to the Ref. 2 MEA, here as well related to an increase in the HFR, which is shown in Fig. 3c. The higher HFR of Int 1 and Int 2 compared to the Ref. 1 and Ref. 2 MEAs can be explained to some extent by the thicker membrane, caused by the spray coated interlayer (+ (26 ± 1) μm). With a proton conductivity calculated by the equation of Springer et al.²⁶ at a temperature of 80°C and assuming a water content of 22 water molecules per sulfonic acid group²⁷ this yields an increase in the area normed membrane resistance of (13 ± 1) mΩ cm². For Int 2 this is in the range of the measured increase of 14 mΩ cm². However, for Int 1 the measured increase is much higher (33 mΩ cm²).

Watanabe et. al²³ and Shichun et al.¹⁷ came to the result that introducing Pt nanoparticles into their Nafion membranes in fuel cells with comparable or even higher loading does not affect the HFR. Therefore, some additional lamination issues or also the nanoparticle size and distribution across the membrane play an important role and should be subject of optimization.

As can be seen from the similar trend of the HFR curve versus current density of the Ref. 2 MEA and the HFR curves of the MEAs with Pt interlayer (Int 1 and Int 2), the increase in HFR with current density can be related to sandwiching of the MEAs. Consequently, this increasing HFR with current density might be solved by an improved setup, e.g. higher clamping pressures, hot pressing of the MEAs with interlayer or different fabrication processes. In comparison to the sandwiched MEAs, the Ref. 1 MEA shows an almost constant HFR.

The two insets in Fig. 3c show impedance spectra at a current density of 0.5 A cm^{− 2} (left) and 1 A cm^{− 2} (right). While the HFRs differ, the kinetic semicircles are comparable for all MEAs, especially for small current densities. Thus the performance of the electrodes is not affected by the Pt interlayers.

Fig. 4a shows the time evolution of the H₂ in O₂ content when holding the MEAs at 1 A cm^{− 2} for 10 days. The H₂ in O₂ contents at 1 A cm^{− 2} at the beginning of test (BOT) are 0.14 vol.%, 0.24 vol.% and 0.00 vol.% for the Ref. 1, Ref. 2 and both Int MEAs, respectively. Over the duration of 245 h, the H₂ in O₂ content of the Ref. 1 and Ref. 2 rise by a factor of 3.1 (0.43 vol.%) and 2.6 (0.62 vol.%). The Int 1 and Int 2 samples do not show any measurable H₂ in O₂ content for the first 90 and 77 hours, respectively. Afterwards also for those samples an increase in H₂ in O₂ content to 0.01 vol.% and 0.05 vol.% can be observed.

Zoom In Zoom Out Reset image size

By dividing the difference of H₂ in O₂ contents at BOT and EOT by the time duration, the increase of H₂ in O₂ can be calculated to 1250 · 10^{− 6} vol.% h^{− 1} (Ref. 1), 1600 · 10^{− 6} vol.% h^{− 1} (Ref. 2), 40 · 10^{− 6} vol.% h^{− 1} (Int 1) and 140 · 10^{− 6} vol.% h^{− 1} (Int 2). This clearly shows the beneficial effect of introducing a recombination catalyst into the membrane.

However, the increase of H₂ in O₂ by time, observed for the Int 1 and Int 2 shows that with time not all permeating H₂ molecules are recombined with O₂ molecules to water. A reason for this might be that the activity of the recombination catalyst decrease with time, due to e.g. dissolution, washing out or agglomeration. However, all this is unlikely for the duration of 245 h, and is also supported by Fig. 2, which shows Int 1 after testing. Another possibility is that the H₂ crossover increases in a higher extent than O₂ does (as it can be seen for the reference MEAs), so that at the position of the interlayer (closer to cathode than anode) there is not enough O₂ present to enable a full recombination of all permeated H₂.

In Fig. 4b the cell voltage is depicted for the same duration of 245 hours. After the first strong increase in cell voltage, which is characteristic for such long-term experiments ( ≈ 10 hours),²⁸ the cell voltage of the Ref. 1 MEA increases by 260 μV h^{− 1}. Compared to values found in literature measured in similar tests this increase is high (e.g. 180 μV h^{− 129} and even 0 μV h^{− 130} for a constant current hold @ 1 A cm^{− 2}). The cell voltage of the Ref. 2 MEA increases by 190 μV h^{− 1} during the same time frame, which is in good agreement with some values found in literature (e.g.)²⁹ but still very high, compared to others (e.g.).³⁰ However, the observed degradation rates match the specifications given by the CCM manufacturer.³¹ For Int 1 the voltage increases only half as fast (90 μV h^{− 1}), whereas Int 2 shows a 2.6 times higher rate of 240 μV h^{− 1}, which is comparable to the Ref. 1 MEA. Thus observing the cell performance over ten days does not suggest a negative impact of the Pt recombination interlayer on long-term stability.

After the long-term experiments the MEAs were investigated again with respect to performance and H₂ in O₂ content. In Fig. 5 the Ref. 1 and the Int 1 MEA are compared at BOT and EOT.

Zoom In Zoom Out Reset image size

Particularly noticeable in Fig. 5a is that the H₂ in O₂ content is higher for the EOT measurements at higher current densities. For the lower current densities the H₂ content is nearly the same as before. For this reason, thinning or increases in permeability of the membrane cannot explain this behavior, since then the H₂ content would increase for all current densities.

The increase in H₂ in O₂ content for the Ref. 1 MEA at EOT and current densities higher than 0.5 A cm^{− 2} can only be explained by a disproportionate, strong increase of H₂ crossover at higher current densities. It is assumed that due to the degradation during the long-term experiments the mass transfer within the cathode was affected. Higher mass transfer limitations can lead to higher dissolved H₂ concentration (supersaturation),¹¹ which leads to a higher H₂ crossover. This assumption is supported by the large semicircles at low frequencies of the impedance spectra that evolve especially at higher current densities, e.g. at 1 A cm^{− 2} for the EOT measurements (Fig. 5c), right inset). These low frequencies impedance data are often related to mass transport phenomena e.g.^32,33 The disproportionate, strong increase of H₂ crossover can also be seen for the EOT measurement of the Int 1 MEA. At higher current densities ( > 1 A cm^{− 2}) small H₂ in O₂ contents can be measured, whereas at low current densities still no H₂ reaches the anode. Thus, at low current densities enough O₂ species for the recombination reaction exist at the interlayer and for higher current densities the H₂ crossover increases in higher extent as the O₂ crossover. Consequently, not all of the H₂ crossover can be recombined because of missing O₂.

Fig. 5b shows the polarization curves before and after the long-term runs for the Ref. 1 and Int 1 MEAs. The performance of the Ref. 1 MEA is drastically reduced. Particularly, this is caused by an increased HFR (Fig. 5c). In addition to the increase in HFR, the remaining losses e.g. kinetic and transport losses also increase, which can be seen by the larger semicircles of the Nyquist plots (insets of Fig. 5c) at 0.5 and 1 A cm^{− 2}. Furthermore, the polarization curves of the Int 1 MEA are close together. The EOT curve is for current densities >  1 A cm^{− 2} even slightly better than the initial BOT curve and is mainly a result of the decrease in HFR (s. Fig. 5c). This might be explained by a reduction of the contact resistance between interlayer and membrane due to laminating. However, very similar to the Ref. 1 MEA the remaining losses (kinetic, transport) are also increased after the long-term experiment.