Efficient Hydrogen Delivery for Microbial Electrosynthesis via 3D-Printed Cathodes

Abstract

The efficient delivery of electrochemically in situ produced H₂ can be a key advantage of microbial electrosynthesis over traditional gas fermentation. However, the technical details of how to supply large amounts of electric current per volume in a biocompatible manner remain unresolved. Here, we explored for the first time the flexibility of complex 3D-printed custom electrodes to fine tune H₂ delivery during microbial electrosynthesis. Using a model system for H₂-mediated electromethanogenesis comprised of 3D fabricated carbon aerogel cathodes plated with nickel-molybdenum and Methanococcus maripaludis, we showed that novel 3D-printed cathodes facilitated sustained and efficient electromethanogenesis from electricity and CO₂ at an unprecedented volumetric production rate of 2.2 L _CH4 /L _catholyte /day and at a coulombic efficiency of 99%. Importantly, our experiments revealed that the efficiency of this process strongly depends on the current density. At identical total current supplied, larger surface area cathodes enabled higher methane production and minimized escape of H₂. Specifically, low current density (<1 mA/cm²) enabled by high surface area cathodes was found to be critical for fast start-up times of the microbial culture, stable steady state performance, and high coulombic efficiencies. Our data demonstrate that 3D-printing of electrodes presents a promising design tool to mitigate effects of bubble formation and local pH gradients within the boundary layer and, thus, resolve key critical limitations for in situ electron delivery in microbial electrosynthesis.

Keywords: 3D-printing; additive manufacturing (3D printing); bioelectrochemical system; current density; gas fermentation; hydrogen mass transfer; microbial electrosynthesis.

FIGURE 1

Additive manufacturing process of the NiMo-plated carbon aerogel cathodes (A). Visualization of computational models of inversed ABS molds for 3D-printing (B) and photograph of one set of finished NiMo-plated carbon aerogel cathodes of varying surface areas (C). The geometric surface areas of the different cathodes are from left to right 39, 55, 86, and 111 cm².

FIGURE 2

Methane production rate, unused hydrogen in the reactor off gas and microbial growth during Exp1 (initial OD = 0.02, left column) and Exp2 (initial OD = 0.1, right column).

FIGURE 3

Coulombic efficiencies achieved with the 3D printed cathodes of different surface areas under low cell density (Exp1, left) and high cell density conditions (Exp2, right). The given values are averages of 4–12 individual measurements taken after reaching steady state, with error bars displaying the standard deviation between individual measurements.

FIGURE 4

Effect of increasing current density on electromethanogenesis performance in reactors with different cathodic surface areas. Methane production rate and unused hydrogen in the reactor off gas during Exp3. The total supplied current was increased stepwise as indicated per dashed vertical lines. Initial OD = 0.1.

FIGURE 5

Summary of process parameters during electromethanogenesis in dependency of the applied cathodic current density: (A) The rates of methane (diamonds, left axis) and H₂ (squares, right axis) produced per cathode area, (B) coulombic efficiencies for methane (circles) and (C) steady state biomass concentrations (triangles). Eight individual electrodes of four different surface areas 39, 55, 86, and 111 cm² were tested under constant current conditions of 50, 60, 70, and 80 mA. The given values are averages of 4–12 individual measurements under steady state conditions with error bars displaying the standard deviation between individual measurements.

FIGURE 6

Conceptual representation of the fate of H₂ in a (A) single-phase system at low current density and (B) gas-evolving system at high current density on a hydrogen producing cathode during electrosynthesis. The concentration of dissolved H₂ in the bulk aqueous phase is lower than in the boundary layer because of microbial consumption. If the cathodic current density produces dissolved H₂ at a rate that exceeds its diffusion out of the boundary layer and microbial consumption, bubble formation will occur (B). Hydrogen gas bubbles can passivate the active catalytic surface area of the cathode and, thus, reduce electric efficiencies. If mass transfer from gaseous H₂ in bubbles to dissolved H₂ in the bulk electrolyte is limited, there is a net loss of H₂ from the reaction space which reduces coulombic efficiency as well as volumetric production rates. When the electrocatalytic H₂ production exceeds the rate at which protons diffuse into the boundary layer, the local pH will rise to alkaline levels (indicated red, see B) potentially toxic to the microorganisms. With increasing current density, number and size of bubbles will increase and with that the intensity of all associated effects.