Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

Abstract

The proton transport property of Nafion nanothin films (4-300 nm) has been investigated by electrochemical impedance spectroscopy (EIS) using interdigitated array (IDA) of gold electrodes on SiO 2 substrate. The proton conductivity has been investigated as a function of film drying/heating protocol, relative humidity, temperature and film thickness. It is found that the film treatment protocol makes a difference in film transport property. Ultimately, the proton conductivity and capacitance of the Nafion nanothin film is thicknessdependent, where the former decreased and the latter increased exponentially with decreasing film thickness. Moreover, the activation energy increased exponentially with decreasing film thickness. The proton conductivity of the thin films of Nafion is significantly lower than that of the membrane counterpart regardless of the thickness, which is consistent with the high activation energy found in the thin films. These differences are likely related to the polymer confinement and morphological differences between the thin films and the freestanding membrane. Proton transport is a key functional property of the ionomer used in polymer electrolyte based energy conversion and storage devices including polymer fuel cells (PEFC), 1 polymer electrolyte membrane (PEM) water electrolyzers, 2 photo electrochemical cells 3,4 and artificial photosynthesis device.5 For PEFCs and PEM electrolyzers, Nafion is the most extensively employed. As a freestanding film of thickness (25-100 μm thick), Nafion serves the function of the electrolyte membrane separating the anode and the cathode. The ionomer is also present in the electrochemically active component of these energy conversion devices as a thin film coating the electrocatalyst. For example, the conventional PEFC catalyst layer (CL) is a porous composite layer (10-25 μm thick) of Platinized carbon (Pt/C) and ionomer. In the CL, the ionomer forms a semi-continuous, web-like nanothin film (4-10 nm) covering the aggregates of Pt/C. 1,6 The thickness of such nanothin Nafion films in the CL is comparable to the ionic domain size (∼4 nm) in the bulk Nafion membrane. 7,8 It is obvious that the ionomer structure and property will affect both the proton and mass transport, which in turn will impact the local electrochemical reaction rate and, thereby, the overall PEFC performance. In fact, researchers of the electrochemical energy research laboratory at General Motors (GM) 9 examined the electrode performance as a function of catalyst loading and found that low-Pt loading catalyst layers exhibited higher transport losses, which they attributed to the transport resistance of ionomer thin film covering the catalyst. The proton conductivity of Nafion membrane is widely investigated and discussed in the literature. [10][11][12][13] In contrast, the proton conductivity of thin Nafion film has remained relatively unexplored, especially for ionomer films of thickness comparable to ionic domain size (∼4-5 nm). The ionic conduction property of materials is highly reg...

Sources of Current Density Distribution in the Land-Channel Direction of a PEMFC

“…Moreover, with increasing current density, HFR distribution becomes less nonuniform and overall HFR does not vary much between 0.2 V and 0.1 V. Along with the dehydration of membrane the ionomer dehydration in catalyst layer also accounts for the significant ohmic voltage loss because proton conductivity of few nanometer thick ionomer film is much lower than the conductivity of bulk membrane, especially in dry condition. 33 In Figure 6c, at 60% RH the local current generation is uniform for all the segments at cell voltages above 0.4 V, corresponding to overall current density lower than 1.3 A/cm 2 . However there is variation in HFR to a certain extent.…”

“…• C and relative humidity below 95% RH, Paul et al 33 measured conductivity of ionomer film of 4 nm thickness (typical ionomer film thickness in catalyst layer) and found that the conductivity much less than bulk membrane. In our work the cell is operated in the similar conditions as that of Neyerlin et al 32 so that the equations are useful to determine local oxygen reduction overpotential.…”

High Temperature Proton Conduction in Nanocellulose Membranes: Paper Fuel Cells

“…60 Despite the small difference in activation energy, the conduction mechanism in the case of nanocellulose is likely to be dominated by a Grotthuss-like, water mediated mechanism. [61][62][63] At lower humidity levels the vehicle mechanism dominates proton conduction. Here, protons diffuse as H 3 O + hydronium ions whilst a counter diffusion of unprotonated water occurs.…”

Water‐Stable Nanoporous Polymer Films with Excellent Proton Conductivity

“…The value of 0.36 S cm −1 is impressively high and exceeds the proton conductivity of a commercial Nafion@211 membrane (0.13 S cm −1 at 353 K and 95% RH) by a factor of almost 3. It should be noted that the reported NAFION nanofilm with a thickness of 300 nm yields comparable conductivities to that of Nafion membrane . We attribute the very high proton conductivity of the SURGELs to the regular packing of proton hopping sites.…”