Batteries and Fuel Cells with Convective Mobile Electrolyte
Authors: Fischel Halbert , Lubin Philip , Clark Brian 
Published: February 2020

Ultralow resistance electrode architecture, previously described by the presenters in (MS #JESP-17-0147R), is here configured for use with mobile convective electrolyte using unique porous electrode structure in a novel oxygen breathing battery and direct-hydrocarbon oxidation AFC. Apollo Mission or F. Bacon mobile KOH-electrolyte H2/O2 fuel cells were competitive power producers by today’s standards, but have been replaced by PEMFC due to historic BOP issues. Porous electrode permeation described within was not an option and inter-electrode electrolyte flow was replaced by static electrolyte as multiple cell series connection caused inter-cell electrolyte pathway ionic short circuits. PEM replacement uses noble metal catalyst, expensive membranes, high electrode area ohmic resistance, low current and power density at low temperature associated with high overvoltage among other issues.
Historic fuel cells used Ni anode and NiO cathode catalyst at 230 oC. Apollo limited pressure to 0.34 MPa in 75% KOH. Bacon used 4.5 MPa and 30% KOH to produce 1 A/cm2 at 0.8 volt; indicating an efficacy of non-noble catalyst for AFC at temperatures too high for PEM. Raney-Ni raised catalytic activity with greater surface density; here greatly improved as strongly coupled coatings of CNT in low resistance electrode architecture. 80% KOH, 300 oC and 0.7 MPa is useful for H2/air catalysis due to high solubility and low vapor pressure of KOH. The electrodes are also stable at 700 oC with molten carbonate electrolyte.
Here, mobile electrolyte permeates both anode and cathode, preferably in that order, to move convectively between and through electrodes in a single pathway circulation loop. The concept is first described as a convection battery where CNT fibers are coated by or lodge and immobilize entrained nanoscale faradaic particles within nonwoven web-like nanoporous CNT structures attached to metal as described within. It is then expanded to include fuel/gas injection to create highly concentrated foam menisci that coat catalyst surfaces while preserving combined gas/electrolyte mobility in a fuel cell configuration. Sources demonstrate that catalyzed redox activity occurs mainly within thin menisci. The concept radically departs from all fuel cell architectural paradigms that use a separating membrane to stop cross-electrode fuel contamination. Virtually complete fuel oxidation and 1-way convection obviates the need for an ion selective semipermeable membrane.
Electrode volumetric redox activity in mobile electrolyte is several orders of magnitude higher due, in the example, to higher active catalytic/faradaic surface density exposed to convective electrolyte. Exchange current density, io is concomitantly significantly increased. The purpose is high area specific electrode current density, i operating close to io with less voltage decrement. In batteries that translates into faster recharge, less heat and in fuel cells, higher power. LIB charge current must be less than 10 mA/cm2 where high ohmic resistance, e.g. i2R is a real hazard and iR lowers voltage. PEMFC operates at 0.7 volt and 0.5 A/cm2 or peak power of 3.5 W/cm2. If the popular LIB form could produce 0.5 A/cm2 current density it would charge 50 times faster and as previously shown by the authors, produce negligible ohmic heat. It is a common mistake to characterize Nernstian Tafel resistance, r = ∂V/∂i, influenced by ion diffusion in electrolyte as ohmic resistance due to electron conduction in metal. When either r or R is increased, i is reduced but only R produces heat. Electrolyte cannot be an electrical conduction path or it would short circuit any cell in which it is used. Ultralow resistance electrodes refer to ohmic resistance at less than 10-4 Ω-cm2 to permit 0.5 and 5 A/cm2 unprecedented area specific current density respectively, for batteries and fuel cells; uniquely possible using electrolyte convection, e.g. 1 cm/sec or 103times faster than ion diffusion in electrolyte.
A stack of paired nested concentric annular bi-polar convection electrodes also comprise a stack of unit-cells separated by insulation and connection in series of for high voltage. Electrolyte flows from the central lumen of an inner annular anode, through a gap separating anode and outer annular cathode and into a chamber from which electrolyte exits the stack. The described electrode layered structure forces normal vector convective flow through electrolyte permeable membranes and transversely between layers. The stack uses a single electrolyte circulation loop that simplifies electrolyte processing, restoration and stack drainage for long shelf-life storage; pump hardware is also minimal. In spite of simplified electrolyte circulation the classic problem of back emf through one or more electrolyte conduits is completely eliminated. Anodes and cathodes comprise nonwoven CNT membranes compressed between square weave metal screens brazed to one another so that all screens lie in window registry.

Figure 1