Increasing the efficiency and current density of water electrolyzers is of great importance for further lowering the cost of hydrogen (H
2) production and becoming cost competitive with H
2 production from steam methane reforming. Conventional proton exchange membrane (PEM) electrolyzer stacks operate at relatively high current densities (1.5-2.5 A cm
-2) and efficiencies (47-66 kWh/kg H
2) using Nafion membranes, but significant improvements are needed to come close to meeting the US Department of Energy’s
Hydrogen Shot Initiative goal of producing hydrogen at a cost of 1 $ kg
-1 H
2.
[1] Currently, a major barrier to operating PEM electrolyzers at higher efficiencies is the large ohmic drop across the Nafion membrane, which becomes the dominant loss mechanism at high current densities ( > 2 A cm
-2). Additionally, there is strong interest in developing alternatives to Nafion, which is classified as a per- and polyfluoroalkyl substance (PFAS) material that raises environmental and health concerns. To reduce membrane resistance and enable efficient operation at high current densities, our team has developed zero-gap electrolyzers that operate at room temperature using PFAS-free proton-conducting silicon oxide membranes made using atomic layer deposition (ALD). Although the proton conductivity of these oxide membranes is lower than Nafion, we show that their total ionic resistance can be lower than that of Nafion membranes by decreasing their thickness to the nanoscale.
[2] However, safe operation of sub-micron thick membranes requires very low H
2 permeability. In this work, we show that the H
2 crossover rates of 250 nm thick SiO
2 membranes can be made an order of magnitude lower than Nafion-117 thanks to the high density of SiO
2 and the application of nanoscopic plugs that are selectively deposited in pinhole defects in the membrane without adversely affecting membrane resistance. Investigation of the transport properties of SiO
2 membranes and systematic analysis of the design rules for PEM electrolyzers based on these materials shows that the area specific membrane resistance of nanoscale SiO
2 membranes can be reduced to less than 20% that of Nafion-117 membranes while still maintaining desirable H
2 blocking capabilities. Overall, this study highlights the potential of PFAS-free proton conducting oxide membranes to serve as efficient and cost effective replacements of Nafion membranes in PEM electrolyzers.
References
[1] US DOE Hydrogen Shot Initiative: https://www.energy.gov/eere/fuelcells/hydrogen-shot
[2] L.A. Cohen, M.S. Weimer, K. Yim, J. Jin, D.V. Fraga Alvarez, A.A. Dameron, C.B. Capuano, R.J. Ouimet, S. Fortiner, D.V. Esposito. “How Low can you Go? Nanoscale Membranes for Efficient Water Electrolysis”, ACS Energy Letters, 2024, 9 (4), 1624.
