The Scanning Vibrating Electrode Technique for the Study of Hydrogen Evolution from Iron Electrodes
Authors: Glover Carol Frances , Mabbett Ian , Subramanian Raman , Barnes Alistair , Williams Geraint 
Published: February 2020

Key drivers for rechargeable battery technologies, that are compatible with photovoltaics (PV) and practical for residential applications, are long cycle-life and safety. As such, ‘eco-friendly’ nickel-iron (NiFe) batteries have been highlighted as a potential contender for incorporation in large-scale electrical energy storage. Typical manufacturer data suggests a cycle life of well over the reported 5,000 cycles required to fulfill the needs for intermittent resources [1]. However, the overall performance of iron-based electrodes is limited by poor charging efficiency due to hydrogen evolution, and poor discharge rate capability due to passivation by electrically non-conductive iron (II) hydroxide [2]. Typical charging efficiencies of no more than 60% are reported; this compares poorly with alternative Li-ion and lead-acid technologies where efficiencies of 75% and 95% are quoted respectively [3].
An in situ scanning vibrating electrode technique (SVET) has been used extensively in the corrosion field [4-5]. The technique can be used to scan an area of exposed metal immersed in electrolyte and measure the area-averaged current densities, and mass lost, over a desirable time period. The size and location of corrosion activity can be determined from a current density surface map produced for each scan. Previously, SVET has been used to measure the amount of H2 evolved from corroding magnesium substrates [5].
In the present work a three-electrode electrochemical cell, with a platinum gauze counter electrode and a mercury/mercury oxide (MMO) reference electrode, is employed. Iron samples are potentiostatically polarised to a value of -1.3 V vs. MMO in a solution of potassium hydroxide (30% w/v%). The principal aim is to compare the volume of H2 evolved when measured using both SVET and a volumetric measurement technique [6] - preliminary experiments show a good correlation. For SVET experiments, the volume of H2 evolved is calculated by determining area-averaged cathodic current density (Jct) by numerical integration, using the trapezium rule, of the time-dependent total current density (jz) distribution produced for each scan. The total equivalent quantity of H2 evolved at the exposed iron surface in each scan is then calculated by applying the cathodic current density data to Faraday’s Law.
An improved charging efficiency to 96% by the incorporation of high purity carbonyl iron electrodes has been reported [2]. Additives such as bismuth sulfide have been shown to suppress the H2 evolution reaction. A further aim of the current study is to use the SVET to measure the H2 evolution on iron samples through a range of purities. In a separate experiment, additions of bismuth sulfide are made to the electrolyte at varying concentrations to determine the effect on H2 evolution.
The current work aims to validate in situ SVET as a tool for the study of electrodes. In doing so there will be great scope for quantifying levels of H2 evolution and for determining any surface discontinuities, via the study of current density distributions given by the surface maps. This will facilitate the investigation of electrodes manufactured on site in the future.

References

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