(Invited) Shunt-currents in Alkaline Water-Electrolyzers and Renewable Energy

Abstract

Shunt-currents in bipolar cell stack design with a common electrolyte-feed and -outlet is an inevitable physical phenomenon governed by Ohms law that causes some extra challenges when alkaline water-electrolysers shall operate on renewable energy that is both dynamic and intermittent. Shunt-currents are also referred to as bypass-current or creep-current. The shunt-currents are to much degree governed by the electrolyte inside the cell stack manifold system that transports lye in and out of the cells. The electrolyte inside the manifold system also electrically connects the individual cells together and enables transport of ions/electricity in parallel to the cell stack. Thus, a small portion of the rectifier current is being shunted outside the cells and does not contribute to any production. Previous work on shunt-current modelling brought new insight on how to design the manifolds and raised the awareness on shunt-current and the use of metallic manifolds which both reduced the ohmic resistivity of the manifold system and was a source of secondary electrolysis.Classic alkaline water-electrolysers are typically using an internal manifold system where the inlet ports are located at the bottom of the cell stack and the outlet ports are located at the top, and where the ports connecting the cell-interior and the common manifold channel are short and straight. Such design has in the past worked satisfactory for alkaline water-electrolysers that have been working on a high nominal load and only being shut-down for maintenance a few times over the stack-lifetime, mainly causing a modest reduction in the current efficiency. Membrane-chlorine electrolysers on the other hand, are designed for very small shunt-currents by using an external manifold system, which enables a current protection system that protects electrodes from corrosion under shutdown conditions.Shunt-currents bypassing the cell stack does not contribute to any product and therefore constitutes a loss in current efficiency and, hence, an accordingly loss in the energy efficiency (the shunt-currents still adds to the electricity bill). The atmospheric alkaline electrolyser has a modest loss in current efficiency at nominal load where the high gas volume blocks much of the current path in the rather open outlet ports. The high-pressure alkaline electrolysers on the other hand, where the gas volumes are much smaller, the current efficiency can be as low as 89% at nominal load due to shunt-currents when using simple internal manifold system. For an electrolyser with already low current efficiency at the nominal load, the current efficiency will drop dramatically as the electrolyser is taken to lower load, severely compromising the energy efficiency. The impact on shunt-currents also dramatically increases for increased number of cells in a cell stack, and eventually limits the number of cells that can be assembled in one single cell stack operating on the same common lye system.Shutdown and discharge of the electrodes may further lead to corrosion and degradation of the electrodes, strongly influenced by the shunt-currents and the manifold system. Large cell stacks will discharge faster and deeper, eventually causing corrosion of the electrodes. As the cell stack is discharged the current in the cell stack is reversed where the hydrogen electrode is being polarized to anodic potentials, and the oxygen electrode is polarized to low cathodic potentials which eventually may challenging the material stability. Thus, evaluation of electrode potentials must be an integral part of development of industrial electrodes [LeRoy] and especially in intermittent operation where frequent shutdown will occur. A good integration of the manifold system into the cell stack can potentially mitigate both the loss of current efficiency under dynamic operation and the electrode corrosion under intermittent operation.