Hydrogen Depolarized Anode Assisted pH Shift Electrolysis: An Experimental Analysis
Authors: Kiefel Robert , Jupke Andreas 
Published: February 2024

Prominent purification challenges such as carbon capture, lithium-ion recovery from aqueous solutions, and isolation of carboxylic acids from fermentation broths are often tackled by downstream process concepts that rely on pH gradients as the driving force for separation. For this purpose, the sole use of acids and bases is only tolerable if the jointly produced co-salts are in demand on the market or are disposable at acceptable costs. However, neither of those alternatives recycles the salt for reuse as acid and base despite several regeneration methods suggested in the literature [1]. Next to thermal salt splitting, the pH shift electrolysis is intensively researched to this end [2,3]. The latter is a particularly attractive technique due to the growing availability of renewable electricity from wind and photovoltaic power plants. Conventional water electrolysis produces oxygen and protons at the anode and hydrogen and hydroxide ions at the cathode. The migration of either ion across the membrane keeps the charge balanced and the pH constant in both chambers. In pH shift electrolysis, a salt is added as an electrolyte providing excess ions for the charge transfer between both chambers. Thus, protons at the anode and hydroxide ions at the cathode are largely retained in their respective chambers generating the intended pH difference.

If the added salt behaves electrochemically inert, the minimum voltage for the cell to deliver a faradaic current by splitting water is 1.229 V. A less energy-intensive alternative is offered by incorporating a hydrogen depolarized anode (HDA). It suppresses the oxygen evolution reaction by providing hydrogen as an oxidation reagent. The cell’s open circuit voltage (OCV) can thus be reduced to 0 V. The minimum potential difference in both setups diverges when the anolyte turns acidic and the catholyte alkaline. A pH of 1 in the anolyte and 14 in the catholyte increases the OCV in a conventional pH shift cell to 1.996 V and in a HDA-incorporating cell to 0.767 V. Nevertheless, the energetic advantage of the latter remains apparent.

Recently, several journal articles have been published applying the HDA technology to various use cases [4-6]. However, a sole investigation of the cell with an electrochemically inert salt, e.g., sodium sulfate, has not been fully addressed. The presentation demonstrates successful pH shifts with a HDA-incorporating two-chamber cell over a range of stationary operating points. The suppression of the oxygen evolution reaction at the anode is highlighted. Moreover, the overall cell voltage constituents are analyzed using Luggin-Haber capillaries, conductivity probes, and electrochemical impedance spectroscopy.

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