Abstract
Over the past few years, significant progress has been made in the performance of AEMFCs. Now, it is almost routine to find AEMFCs that report peak power densities that exceed 1 W/cm
2
, and there are regular reports of AEMFCs operating near 2 W/cm
2
[1–3] and one recent study that achieved 3.4 W/cm
2
. [4] Much of this progress has been made through electrolyte development, electrode understanding and a new understanding of AEMFC water dynamics. However, one big challenge that remains in AEMFC is cell stability [5], and very few studies have been reported that can achieve more than 100h durability at relevant current densities (³ 600 mA/cm
2
). The DOE target for AEMFC durability is 2000 h operation at ³ 600 mA/cm
2
with less than 10% voltage degradation and a final cell voltage > 0.6 V. The challenges in achieving this target come from multiple aspects, such as development of a robust ionomer and membrane with high ion exchange capacity and ionic conductivity, the design of electrodes that are specifically designed for water management, and proper operating conditions. Our group has consistently shown that the performance of AEMFCs utilizing extremely hydrophilic components (particularly the ionomer) can be very high, but that performance can be sensitive to operating conditions (current density and reacting gas dew points). The conditions for cell operation can drastically change the water distribution in the catalyst layers, membrane, and gas diffusion layers [6,3]. Hence, electrode design is a critical piece to managing water and achieving good performance. [3,7]
In this work, our team uses
operando
neutron imaging coupled with
operando
micro X-ray computed tomography to further study the water dynamics in AEMFCs under different operating conditions. The results showed the effects of cell operating conditions – from flooding in the anode at high current density which led to severe ionomer swelling to very low water content in the cathode during low current, low RH operation. The high-resolution neutron images indicated the existence of water back diffusion from anode to cathode, which helps to balance the cell water, but it was found that the conditions that lead to the highest possible power density are also conditions where the cell may not be stable from a water perspective for 100’s of hours. In short, to enable very long-term operation, the AEMFCs need to be operated at high reacting gas dew point. With our previous electrodes, optimized for peak power density at lower dew points, extensive cell flooding was observed at higher dew points during cell operation. Therefore, our team developed new electrode structures based on this feedback – manipulating the catalyst layer and gas diffusion layer hydrophobicity. The result was AEMFCs that are able to achieve over 1000h continuous operation at 600 mA cm
-2
under H
2
/Air (CO
2
-free) at 65 °C. During this test, the cell voltage lost only 6% of its initial value at an average rate of 0.019 mV/h, which far exceeds the literature state-of-the-art for AEMFC durability. These electrodes were also capable of high peak power. The AEMFC power density and durability are shown in the figure below. The purpose of this presentation is to detail the cell-level phenomena during different operating conditions and the electrode designs that allowed for the best performance and durability.
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Figure 1