Membrane electrode assemblies for anion exchange membrane water electrolysers

Abstract

Membrane electrode assemblies for anion exchange membrane water electrolysers Developing technologies that may reduce our climate impact is always beneficial, though our current climate issues render the advancement of such technologies invaluable. While the hydrogen society imagined by great minds of the past may never come to pass, its inclusion in our current and near-future society is indubitable. The continued use of hydrogen as both an energy vector and a direct means to drive a fuel cell is con-tingent on a steady supply of hydrogen, the origin of which is highly relevant if society is to consider hydrogen more seriously than the bells and whistles of campaign slogans and news headlines. The vast majority of hydrogen is currently produced through CO2-intensive methane- and coal reforming, though for a change to make sense, the associate economy of creating hydrogen through more environmentally friendly means has to be feasible. Thus, sustainable hydrogen production methods such as water electrolysis demand a significant amount of R&D to lower the associated cost of hydrogen produc-tion. Traditional alkaline water electrolysis is an example of an established production method which already operates at a cost e˙ective scale, though novel technologies such as the anion exchange membrane can significantly improve the rate of production. This will further lower the cost of sustainable hydrogen, thus decreasing the impact of the CO2-intensive technologies that have previously been necessary to produce hydrogen at an industrial scale. This PhD has been a study of interfaces, wherein the importance of the electro-chemically active surface area has repeatedly been illustrated by utilising inexpensive, abundant materials. The usage of such materials is necessary for the imperative of low-cost hydrogen, however it also creates additional engineering challenges, as the most active/stable materials carry hefty price tags in the form of material scarcity and financial cost. Here, the alkaline water electrolyser environment o˙ers benefits over its acidic counterpart, as inexpensive, abundant materials are stable under high-pH conditions. A proverb on the topic says "It is easier to increase the activity of a sta-ble material, than to increase the stability of something active.", where this has been executed several times throughout the duration of this PhD thesis. Stainless steel is a good example of a stable abundant material, whose initial performance leaves a lot to be desired. True to the proverb, a tenfold increment in kinetic activity was noted after activation through brief potential cycling. Additional potential cycling with specific conditions such as range and rate were complicit in tailoring the surface oxide, endow-ing the stainless steel with kinetic enhancements owing to changes in oxide structure, composition and oxidation state. Microscopically augmenting a surface area through controlled pitting corrosion is another way of increasing the geometric current density, as acid treatments increase the micro-scale surface roughness. Provided the surface is catalytically active, it will directly increase the number of catalytically active sites, in turn improving the electrode performance. These benefits remain even after coating the augmented surface, as a fine coating will retain the changes made to the underlying surface. Thus, simple, controlled surface treatments can increase the electrochemical activity of any electrode. The activity of a surface may also be enhanced by annealing, as shown for both a NiFe2O4 anodes and a Pt/C cathodes, where this heat-treatment had a fair e˙ect upon the catalytic activity. While the anode improved its performance over time, the Pt/C cathode indicated instability. Pt/C is a classic material for creating benchmark performances, however its stability in alkaline electrolytes has recently come under scrutiny. A slightly di˙erent degradation pathway is proposed, wherein the interstitial layers of the carbon support particles expand after exposure to an alkaline electrolyte. As a consequence, the Pt-C bonds are strained and ultimately severed, resulting in dissolution and agglomeration of the platinum nanoparticles. This material is and will be frequently employed, which makes the understanding of its stability highly important for current and future R&D within alkaline water electrolysis.