Solid polymer electrolyte electrochemical oxygen control system with integral reactor feedback sensing

Abstract

Disclosed is a novel proton-exchange membrane (PEM) based solid polymer electrolyte electrochemical oxygen control (EOC) system that can deplete and control the oxygen from a closed container to levels sufficient for both disinfestation and preservation. With the use of this electrochemical process, many insects that infest raw agricultural products and other produce can be exterminated, without detriment to the quality of the produce and without deposition of harmful residue, by reducing the ambient oxygen to a controlled low level for several days. The electrochemical process features the use of a bipolar stack comprised of a selected number of PEM cells connected electrically in series and separated by an electrically conductive bipolar plate. Each cell contains a membrane and electrode assembly, consisting of an anode structure and cathode structure in intimate contact with a PEM. When DC power is applied to the cell stack, electrons are supplied to the cathode, supporting reduction of oxygen at the cathode with the formation of water, and electrons removed from the anode, supporting oxygen evolution by the decomposition of water at the anode. The anode and cathode compartments are separated by the solid ionomer PEM, which transports protons generated at the anode through the PEM to the cathode to complete the electrical circuit internally. Oxygen is depleted by recirculating the gas in the dosed container over the cathode, and expelling the oxygen evolved at the anode by separating the oxygen from the recirculating anode water stream and venting it to the outside of the closed container. Nitrogen or other inert gas is added as makeup gas to avoid creating a negative pressure in the container. A unique feature of this process is that at a low oxygen concentration, the cell and stack cathode current becomes rate-limited in direct proportion to the oxygen level in the recirculating gas and can therefore be used as a measure of the container oxygen level. In the sensing/control scheme developed as part of this invention, the current is periodically allowed to rise to the diffusion limit in a “measure mode” and then reset according to the desired oxygen level, determined from a slope-intercept “measure mode” calibration curve, for a longer “control mode” period.