High-Performance Alkaline Water Electrolysis Using Anion-Exchange Membrane-Electrode Assembly with Catalyst Coated Membrane and Platinum Free Catalysts

Possibility of operation a water electrolysis technology using cost-effective components for current collectors or porous transport layers and non-platinum group metal catalysts with fast response to intermittent loads from renewable energy sources makes membrane alkaline water electrolysis (MAWE) an attractive technology for producing low cost “green” hydrogen. MAWE represents a technology combining the advantages of the already established electrolyser technologies, i.e. proton exchange membrane water electrolysis (PEMWE) and alkaline water electrolysis (AWE). It combines the relatively low-cost materials of AWE with utilisation of the ion-selective polymer electrolyte as separator of the electrode compartments which is commonly used in PEMWE. In the case of the utilisation of the ion-selective polymer electrolyte the electrodes can be pressed directly to its surface thus creating the membrane-electrode assembly (MEA). Reduction of the gap between the electrodes and separator to zero lowers the ohmic drop on the cell. This positively affects the efficiency of the water electrolysis. In MAWE the anion-selective materials are still facing some limitations in terms of poorer chemical stability in high pH environments and at elevated temperatures due to degradation of the anion exchange groups either by Hoffman elimination or nucleophilic attack by OH- ions, or lower ionic conductivity. Primarily two different MEA fabrication methods are recognized in literature [1]: catalyst-coated substrate (CCS) and catalyst-coated membrane (CCM). In the CCS method, catalyst ink, consisting of a binder/ionomer, catalyst particles and solvent, is applied to one side of the porous transport layer whereas in CCM method the catalyst ink is applied to the ion-selective membrane. The CCM approach is established in the PEMWE technology, but in MAWE the above-mentioned problem with stability of anion-selective materials limits the research. From PEMWE we know that CCM approach provides several advantages, including close contact between the catalyst layer and membrane resulting in better utilization of the catalyst compared to CCS. This advantage allows the MEA to maintain the overall performance with lower catalyst loading. Also, the catalyst layer thickness is reduced, which lowers ohmic losses on the MEA. However, the CCM method can also cause poor electrical contact between the MEA and current collector or within the catalyst layer itself. In this work, we first compare the CCS and CCM approaches and study the possibility of catalyst loading reduction. Then, with CCM we further optimize the composition of the catalyst layer, study the effect of method of preparation and compare two different membranes – non-commercial with 2 different thicknesses and commercial one As anion-selective material we use chloromethylated block copolymer polystyrene-ethylene-butylene-styrene (PSEBS-CM) functionalized with 1,4-diazabicyclo[2.2.2]octane (DABCO) groups. The solution of PSEBS-CM is used also as a binder of the catalyst layer. This material showed good ionic conductivity, ion exchange capacity and stability at elevated temperature. For comparison reason, Fumapem® FAA-3-50 membrane is tested. NiCo2O4 and NiFe2O4 are used as anode and cathode catalysts, respectively. Air-brush or computer controlled ultrasonic dispersion of catalyst ink are used to prepare MEA. As many operating conditions have been reported to significantly affect MAWE performance, including membrane thickness, electrolyte type, and operating temperature we decided to test the performance of the prepared CCSs and CCMs by mean of the load curves in in the range 1.5 – 2.0 V in different concentrations of KOH (1 – 15 wt.%) at 50 °C. To identify the influence of membrane thickness, we use membrane with thickness of 250 or 60 µm. To have deeper insight on the system, electrochemical impedance spectroscopy is used under the MAWE conditions at voltage of 0, 1.5 and 1.8 V in frequency range 100 kHz – 0.1 Hz with amplitude of 10 mV. Scanning electron microscopy is used to observe the morphology of the layers. The chosen CCMs undergo the stability test measured at 50 °C in 10 wt.% KOH at current density of 250 mA cm-2 for 160 hours. The obtained results show the possibility of: 1) replacing the CCS with CCM, 2) reduction of the catalyst load by 75%, 3) improvement of performance with optimized catalyst/binder ratio and 4) improvement of performance with reduction of the membranes thickness. Acknowledgments: This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 875118. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe research. Financial support by the Technology agency of the Czech Republic within the framework of the project No. TK 02030001 is gratefully acknowledged. Huah, L.B., et al., Chinese Journal of Chemical Engineering, 2020.