(Digital Presentation) Quantifying the Effect of Potential Cycling Conditions on the Resulting Performance of Stainless Steel as an Anode for Alkaline Water Electrolysis
Authors: Ferriday Thomas , Nuggehalli Sampathkumar Suhas , Middleton Peter Hugh , Van herle Jan 
Published: September 2023

Alkaline water electrolysis through the use of the anion exchange membrane (AEM) has great potential. The current industry standard for water electrolysis utilise diaphragm-based traditional alkaline water electrolysers (AWEs), however using AEM water electrolysers (AEMWEs) introduces benefits including greater current densities, superior hydrogen purity, simpler cell/stack designs and less corrosive electrolytes. These advantages are additions to the pre-existing benefits to AWE, namely the possibility of utilising inexpensive catalyst materials for both hydrogen and oxygen evolution.

Great attention has been payed to stainless steel (SS) as an anode for AEMWE due to its fair activity, low cost and good stability. Potential cycling (PC) is one method of electrochemically modifying the surface of various SS structures to increase electrochemical activity, however the PC conditions thus far reported in literature are rife with variation. As such, the full extent of PC conditions on SS remain unreported. Herein, we seek to fill this gap in literature by potential cycling a series of SS felt (SSF) electrodes under varied scan rates and ranges. Special attention is payed to surface conditions due to the intricate nuances affecting the electrocatalytic activity of the surface oxide layer.

Two redox couples are clearly visible in cyclic voltammetry (CV) in the range 0-1.90 VRHE, termed whole-range, namely the Fe+2/Fe+3 at 0-0.65 VRHE and Ni+2/Ni+3 at 1.50-1.75 VRHE. The SSF electrodes were PC over one of these ranges at either slow, intermediate or fast scan rates, producing six different SSF electrodes.

Initiating measurements were carried out to ascertain the baseline performance, including whole-range CV, electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). The pristine SSF surface was exceedingly sensitive to the whole-range CV sweeps during initial measurements. Thus, this work is divided between SSF electrodes with and without CV in the initial measurements, termed with and without pre-cycling.

Without pre-cycling, the decline in charge transfer resistance (Rct) before and after PC was more than tenfold for all scan speeds. Scan speed was relevant, where slow and fast scan speeds resulted in a mean Rct reduction of 84.15 and 90.59% for fast and slow scan speeds respectively. Post-experimental surface analysis with X-ray photoelectron spectroscopy revealed a decline in the average oxidation state of the principle elements in the SSF electrodes (Fe, Ni and Cr) and a surface depletion of iron. The relative increase in surface concentration of nickel and chromium was highly correlated with the reduction of Rct. Evaluation of affiliated Tafel slopes and LSV curves reveals similar trends, where the latter indicates a fair performance increment between 8-21% for fast and slow scan speeds.

With pre-cycling the decline in Rct was lower, in the range of 18-30% and 3-7% for the SSF electrodes cycled around the Fe+2/Fe+3 and Ni+2/Ni+3 redox couple respectively. The influence of scan rate was the same for these SSF electrodes as with those unexposed to pre-cycling. Tafel analysis reveals two dominant slopes, where PC elicits small changes in the low current density slope and greater changes in the high current density region for the Fe+2/Fe+3 cycled SSF electrodes. These trends are also seen in LSV curves, where the greatest improvement is clearly seen for the SSF electrode cycled with a slow scan rate.

Tafel analysis shows that the SSF electrodes cycled around the Ni+2/Ni+3 redox couple display a small decline in kinetics following PC, which corresponds to the rather meagre improvement in Rct affiliated with the PC conditions. The same trends are also seen by comparing the before and after LSV curves.

Additional measurements documenting the full cell performance of these anodes is necessary and will be featured in the unabridged version of this paper, in addition to a more thorough characterisation of the surface conditions.