Dynamic and interconnected influence of dissolved iron on the performance of alkaline water electrolysis

Dissolved iron (Fe) species is an intriguing player in the overall alkaline water electrolysis (AWE) system, considered both as a poison that needs to be avoided and as a precursor for enhancing the water splitting activity. Here, we unveil the intricate mechanisms governing the Fe influence on practical AWE systems, by measuring the dynamic changes in cell voltage and overpotential of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The dissolved Fe will deposit on the cathode, which significantly enhances the HER activity of bare Ni mesh (BN) while showing negligible impact on the porous RANEY® Ni mesh (RN). The dissolved Fe will also improve the OER activity of the BN by a mechanism based on an equilibrium between leaching and incorporation of Fe onto the oxide layer of the anode. The continuous deposition of Fe on the cathode will gradually deplete the electrolyte of dissolved Fe, which will in turn push the anode surface equilibrium towards low density of active Fe sites thus to a decrease of OER activity. Inspired by the above results, by optimizing the addition of Fe(iii) salt into the system, an impressively low cell voltage of 1.95 V for a water splitting current density of 0.4 A cm^-2 was achieved for a simple, cheap and robust BN cathode//BN anode zero-gap assembly. This performance is equivalent to a power consumption around 19.3% lower compared to the system without Fe(iii) addition.