Communication—Contribution of Catalyst Layer Proton Transport Resistance to Voltage Loss in Polymer Electrolyte Water Electrolyzers

Mass transport losses ( η mtx ) play a signiﬁcant role at high current densities in polymer electrolyte water electrolyzers (PEWEs). Previous work has shown that η mtx depends on the porous transport layer (PTL) structure, although a clear correlation between the material morphology and η mtx has not been established. In this work, we experimentally determine the overpotential η H + CLa associated with the proton transport in the anodic catalyst layer by measuring the ionic resistance in the catalyst layer using electrochemical impedance spectroscopy (EIS) and the transmission-line model. We found that overpotentials, including η H + CLa , depend on the PTL morphologic surface properties.

It is necessary to identify voltage losses in PEWE on the cell level in order to optimize the design for higher efficiency. The ohmic overpotential is associated with the charge transport (ionic, electronic) in cell components and contact resistances. [1][2][3] Kinetic losses (η act ) are related to the electrochemical reactions, mainly the sluggish oxygen evolution reaction (OER). 4 The mass-transport overpotential (η mt x ) is still not well understood. Previous work has shown that η mt x occurs on the anode side, and was hypothesized to stem from the two-phase flow through the anodic PTL (PTL a ), 5 and the removal of O 2 bubbles that hinder the water supply to the electrode. 6 In this work we experimentally determine the overpotential η H + C La related to the proton transport resistance in the anode catalyst layer (R H + C La ). This overpotential is often unaccounted for in PEWE and has so far been lumped together with general 'mass transport' losses. [5][6][7] Recently, Bernt and Gasteiger 12 calculated the effective proton transport resistance in the CL a from the electrode sheet resistance for proton conduction to be in a range from 6 to 171 m cm 2 , depending on CL a ionomer content. The study presented here is the first to report experimentally determined R H + C La values and its dependence on the PTL a /CL a interface.

Experimental
Electrolysis cell and test-bench.-Experiments were performed using an in-house developed test-bench equipped with a SP-150 potentiostat and a HCP-803 80A booster from Biologic. Water was circulated at 400 mL min −1 only in the anode cell compartment, while the electro-osmotically dragged water was discarded periodically from the cathodic gas-water separator. The PEWE cell with an active area of 25 cm 2 was assembled using a Nafion 117 based E400 catalyst coated membrane (CCM) from Greenerity, with an Ir-based catalyst for the anode and the Pt-based catalyst for the cathode. Sintered-Ti PTLs T5, T10 and T20 from GKN were varied on the anode and cathode side. Data describing the cells with different PTLs will be designated with T5/T10/T20. The previous study from Suermann et al. 5  Operating conditions -electrolysis regime.-CCMs were conditioned for 12 hours prior to measurements by cycling the current density between 1 and 2 A cm −2 in 5 min intervals at 60 • C. The polarization curves were recorded galvanostatically at 60 • C, and the high frequency resistance (HFR) was measured at 10 kHz for the iR-correction.
Operating conditions -H 2 /N 2 regime.-For the determination of R H + C La , 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 . At the applied DC-bias of the cell, hydrogen evolution takes place at the RE/CE, and oxidation of hydrogen that has permeated through the membrane from the H 2 to the N 2 side at the WE. The DC current in this case is the limiting hydrogen crossover current. The PEWE cell was operated potentiostatically at 1.0, 1.2 and 1.4 V, and the impedance was measured from 10 kHz to 100 mHz. The measurements were performed in the potential region with higher capacitance to eliminate the effects of the inductive elements. 8 No contribution of the faradaic current was observed at these potentials in the H 2 /N 2 regime (cf. Supplementary Material, Figure S3). In this study, we analyze the WE AC response using a one-dimensional transmission-line model consisting of differential elements describing charge transfer and proton transport in the CL. The general model is widely used in fuel cell CL characterization [8][9][10] and the governing equations are given in detail in the Supplementary Material. The proton transport losses in the CL a appear as R H + C La /3 in the polarization curve, under the assumption of uniform potential distribution in the CL. [8][9][10] Due to a good agreement between the measured and the modeled impedance response (cf. Supplementary Material, Figure S2), we extrapolated the low frequency, capacitive response to the real axis obtaining the R H + C La /3 projections. This procedure was repeated for different PTLs to determine the impact of the CL/PTL interface on R H + C La . The R H +

C Lc
contribution is assumed to be negligible as a result of the low charge transfer resistance of the hydrogen evolution reaction (HER). 11 Furthermore, the fast HER takes place at the catalyst/membrane interface, resulting in a negligible effective R H + C Lc .

Results and Discussion
Proton transport resistance in the anode catalyst layer.-So far, there have not been any attempts to experimentally determine the R H + C La contribution to η mt x . The values of R H + C La /3 are extrapolated from the impedance spectra from the H 2 /N 2 operation ( Figure 1) (8.1 ± 0.9/11.2 ± 1.8/20.6 ± 2.1 m cm 2 for T5/T10/T20, respectively). The R H + C La appears to strongly depend on the interface between the PTL and the CL a , as the PTL with the coarsest and finest surfaces result in highest and lowest R H + C La , respectively. To better comprehend the effect of the PTL/CL a interface on the cell characteristics, we have conducted an overpotential analysis on the polarization curves.
Overpotential analysis.-The PEWE polarization curves were analyzed according to the Tafel model, with the procedure and equations given in detail in References 5-7. PEWE cell voltage is a sum of the thermodynamic, reversible cell voltage E rev ( p, T ), given by the Nernst equation, and the overpotentials, which can be broken down to kinetic η act , ohmic η ohm , and mass-transport η mt x contributions. 7 Correcting the cell voltage, E cell , using the HFR yields the iR-free cell voltage (E I R− f ree ) and η ohm . The Tafel slope was fitted in the low current density region (0.01-0.08 A cm −2 ) and subsequent subtraction of E rev ( p, T ) yielded η act . The difference between E I R− f ree and the extrapolated Tafel line yields residual losses, considered to be the results of mass transport losses. [5][6][7] E cell = E rev ( p, T ) + η act + η ohm + i · R H + C La + η mt x−rest [1] Polarization curves and the impedance spectra ( Figure 2) reveal significant differences in cell performance with different PTLs. Although cells with T5 showed the lowest HFR, they are outperformed by the cells with T10 PTL. The cells with T20 exhibited both the highest HFR and different impedance response at low frequencies (Figure 2b), indicating higher mass transport losses. The difference in HFR most likely stems from a higher ohmic interfacial contact resistance in the cell when using coarser T20 compared to T5. Interestingly, Tafel slopes are somewhat different for the three cells; η H+ CLa (V) Figure 3. Activation (η act ) , CL a proton transport (η H + C La ) and the rest of the mass transport overpotential (η mt x−rest ) for T5/T10/T20 cell configurations. 71 ± 1, 68 ± 1, 78 ± 2 mV/dec for T5, T10, T20, respectively. PTL-induced structural inhomogeneity in the CL a might result in the different apparent Tafel slopes. Overall, calculated overpotentials with different commercial PTLs are in line with the previous study from Suermann et al. 5 Since R H + C La /3 appear as part of η mt x , η mt x was corrected for the R H + C La contribution from the H 2 /N 2 measurements to extract η mt x−rest . R H + C La contributes 11, 18 and 21% to η mt x at 3 A cm −2 for the cells with T5, T10 and T20 PTLs, respectively. The η mt x−rest still accounts for the major contribution to η mt x at high current densities and differs between the three configurations, being highest for T20 and lowest for T10 (Figure 3). A coarse PTL a surface appears to be detrimental to the cell performance indicators. We offer the following tentative explanation: as the CCM deforms between the rigid Ti-PTLs, the CL a area under the solid PTL particles is compressed and its porosity is thereby reduced. The surface of T20 consists of larger particles, leading to more pronounced local reductions in porosity. We assume that proton transport takes place in the compacted CL a structure, as it is the area under the PTL particle in contact with the CL a that contributes to the OER. 12,13 The water reaches the CL a through the voids on the PTL surface and relies on lateral diffusion through the ionomer binder to reach the active sites under the PTL solid particles (Figure 4). The T20 PTL surface consists of larger particles and pores, 5 Figure 4. Schematic of the CL a /PTL a interface. Macroporous sintered-Ti PTLs are thought to result in local porosity variations in the CL a , causing increased R H + C La . Water needs to diffuse laterally through the ionomer to reach active sites under the Ti-particles. PTL pore and particle size are obtained from Reference 5. in larger diffusion distances for the water in the CL a ionomer, and thus higher η mt x−rest . The different η mt x−rest for the T5 and T10 might stem from the more constrained T5 structure, with a higher fraction of small, potentially bottlenecking pores. 5 The compaction of the CL a under the PTL particle leads to higher degree of ionomer binder confinement and inhomogeneous wetting of the CL a active sites, resulting in increased R H + C La . 14,15 SEM images of the CL a surface and CCM cross-sections are presented in the Supplementary Material to give more insight into the impact of the interface morphology on the CCM physical properties.

Conclusions
We have analyzed the impact of the PTL properties on the cell characteristics in a PEWE cell by conducting a detailed overpotential analysis. Experimentally determined R H + C La vary based on the PTL surface morphology, with the highest and lowest values measured in the case of cells with coarsest and finest PTL surfaces, respectively. The variations in R H + C La are most likely caused by confinement effects in the CL a , resulting in inhomogeneous water distribution along the CL a . Overall the R H + C La accounts for 11, 18 and 21% of η mt x at 3 A cm −2 for the cells with T5, T10, and T20 sinters, respectively. The residual mass transport loss, η mt x−rest , varies between the cells with different PTLs, and is assumed to be influenced the PTL/CL interface. The coarse PTL/CL a interface results in longer water diffusion paths in the CL a .