Durability of Electrocatalysts for Hydrogen Evolution in Alkaline Water Electrolysis: Bridging Academic and Industrial Research

Alkaline water electrolysis is a mature technology with an extensive industrial history. Modern use cases are focused on consuming renewable electricity when prices are low, which causes an intermittent operation pattern that is different from the continuous operation pattern of the old. In this thesis, the additional stressors generated by intermittent operation are examined.

Most modern electrolyzers are bipolar, which means each cathode is electrically connected to an anode through a bipolar plate. These bipolar plates are connected in series via the electrolyte. The hydrogen (oxygen) electrolyte is shared between all hydrogen (oxygen) evolving electrodes. During electrolyzer off-periods, each bipolar plate behaves like a shorted galvanic cell, discharging the electromotive force between the hydrogen-evolving and oxygen-evolving electrodes through the electrolyte loop, causing the phenomenon reverse currents. Reverse currents stress electrodes. In this thesis, a simple laboratory setup replicates this discharge behavior without the use of complicated miniature electrolyzer setups. I show how the potential reached as the electrode’s discharge is controlled by the surface area ratio of the negative and positive electrodes and that the resistance in the system controls the discharge current. The results indicate that electrocatalyst design for industrial cell assemblies should consider the interplay between the hydrogen and oxygen electrodes to extend the total lifetime of the cell assembly. The thesis also presents a patent application for passive gas traps designed to open the ion connection, thus stopping the reverse currents.

As reverse currents only affect electrodes in bipolar electrolyzers, their relevance for fundamentally minded researchers is limited. Still, the intermittency of operation is a concern to anyone researching technology with industrial application in mind. Durability towards intermittent operation is a prerequisite for electrocatalysts to reach industrial applications, but clear targets have been lacking. To achieve a more precise language about the durability of electrodes, I propose conceptualizing stability as a hierarchy. Lowest in the hierarchy is the electrode’s ability to sustain continuous water electrolysis. Then, the electrode must prove durability towards cycling to open circuit potential (OCP), necessitating knowledge of the OCP. Finally, stability towards further oxidation for hydrogen electrodes (reduction for oxygen electrodes) caused by reverse currents can be quantified. Here, I developed an electrochemical protocol where hydrogen electrodes are cycled from high-current hydrogen evolution to incrementally more positive potentials. This protocol leads to knowledge of the durability of the geometric activity when cycled to increasingly harsh potentials, obtaining what is dubbed the practical stability window (PSW). One can assess two main electrode properties by combining the PSW with the OCP. If cycling to OCP leads to degradation, the PSW can show if a protective bias can halt the degradation. If the PSW exceeds the OCP the electrode can sustain harsh reverse currents.

The main body of the thesis concerns the high surface area NiMo electrocatalysts synthesized directly onto a Ni mesh substrate through hydrothermal synthesis. The electrocatalysts show durability towards continuous operation while cycling to open circuit rapidly degrades the electrode. Comparing short-cycled experiments between hydrogen evolution and OCP with longer cycles reveals that degradation strongly correlates with cycle count rather than experiment duration. The correlation is evident in the geometric activity as well as the dissolution of the Mo from the catalyst layer. The electrochemically active surface area (ECSA) is followed operando using electrochemical impedance spectroscopy (EIS), showing that the ECSA correlates poorly with both cycle count and experiment duration. If cycling is limited to 70 mV_RHE, the degradation of the geometric activity is slowed, but it accelerates when cycling to 80 mV_RHE and above. As the electrode quickly oxidizes spontaneously to 150 mV_RHE, the industrial application of NiMo electrodes in modern use cases is deemed very unlikely.

High-surface-area Ni produced by leaching co-deposited NiZn₃ precatalysts on Ni mesh showed high durability towards cycling between operation and OCP. The geometric activity of the electrode is very dependent on the potentials it is cycled to, and through careful leaching, one may be able to optimize the catalyst layer further. During harsh cycling to potentials of 700 mVRHE, 800 mV_RHE and 900 mV_RHE the catalyst degrade both through a loss of ECSA, and by detachment of the catalyst layer. The promising stability towards harsh electrochemical cycling shows high-surface-area Ni may be promising for low-cost alternatives for alkaline hydrogen evolution reaction (HER) electrodes. To compare the NiMo and NiZn3 results with industrial state-of-the-art electrodes, non-disclosed non-platinum-group metal (PGM) and PGM state-of-the-art electrodes were tested following the same protocol. The two electrodes show different open circuit behaviors, underlining the importance of measuring the OCP of electrodes. The PGM electrode’s degradation is correlated with cycle count when cycled between HER and 830 mV_RHE (near OCP), while the non-PGM electrode’s degradation correlate with experiment time when cycled to its OCP at 148 mV_RHE. Limiting electrolyzer off-period duration is suggested to extend the PGM electrode’s lifetime in electrolyzer operation. Pt on Ni for HER was tested in pilot experiments showing both high activity and durability towards cycling up to 900 mV_RHE, well above the highest potential reached during 24 h of open circuit monitoring. Despite the benefits of operating alkaline water electrolyzers without platinum-group metals, a better understanding of the behavior of Pt in industrially relevant operations may lead to an expanded electrocatalyst catalog.

Finally, the thesis describes the next steps, both for industrial and academically centered experiments, to precisely quantify the transferability to industrial electrolyzers and elucidate the degradation mechanisms of the catalysts. The research in the thesis serves as an attempt to transfer industrial knowledge into an academic setting while using academic methods to optimize the knowledge gained from experiments in industrially relevant conditions in the search for the next generation of electrocatalysts for large-scale alkaline water electrolysis.