Alkaline membrane water electrolysis with non-noble catalysts

Abstract

As renewable energy sources reach higher grid penetration, large scale energy storage solutions are becoming increasingly important. Hydrogen produced with renewable energy by water electrolysis is currently the only option to solve this challenge on a global scale, and green hydrogen is essential for the decarbonization of the transportation and industrial sector required to limit climate change. Electrolysis done with an alkaline electrolyte is a cheap, proven, and commercially available technology, but the systems suffer from inefficiency and limited operating flexibility. The work herein seek to address these issues by introducing alkaline polymeric membranes and efficient electrodes based on novel materials. Polymer electrolyte membranes with sufficient OH– -conductivity enable a drastic reduction of the electrode spacing, which lead to improved ohmic properties enabling operation at higher current density. This, combined with better gas separation properties and a higher operating flexibility, have the prospects of significantly reducing the capex and opex of electrolysis systems, and the cost of green hydrogen. Towards this goal, membranes based on poly(2,2’-(mphenylene)-5,5’-bibenzimidazole) (m-PBI) as well as poly(2,2’-(m-mesitylene)-5,5’-bibenzimidazole) (mes-PBI) were investigated as electrolyte for alkaline electrolysis cells. PBI membranes were equilibrated with aqueous KOH and applied as separator, and polarization data from cells at 20-25 wt% KOH using these membranes showed improved ohmic behaviour over cells with conventional porous separators. This was strikingly clear when combined with active electrodes with Raney-nickel-based coatings. With thin 40 μm m-PBI membranes, Raney-nickelmolybdenum cathodes and nickel anodes, cells operated at 80◦C with 24 wt% KOH (aq) achieved 1000 mA cm-2 at 1.7 V and 2800 mA cm-2 at 2.0 V. Electrochemical impedance spectroscopy data showed a 6-fold reduction in ohmic cell resistance compared to conventional materials. Albeit good performance, ex-situ characterization and durability tests showed that polymer backbone and membrane stability remained a problem under conventional operating conditions. To accompany novel membranes in alkaline electrolysis, electrodes can be employed in a zero-gap configuration. This enable different electrode concepts than used in commercial systems. Inspired by recent literature, nickel-iron based anodes, and nickel-tin as well as nickel-molybdenum cathodes were investigated in half cell tests. The materials were applied as coatings on nickel foam and showed improvements in the order of 150-300 mV over reference nickel materials at room temperature, depending on the specific electrode and electrolyte concentration used. In a secondary approach, electrodes were prepared using powder and polymeric binders. Using nickel powder with m-PBI binder in a nickel foam as cathode, a reduction in cell overpotential of more than 200 mV was achieved compared against a pristine nickel foam cathode.