Date of Award

1-1-2023

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Environmental and Sustainable Engineering

Content Description

1 online resource (xiii, 103 pages) : illustrations (some color)

Dissertation/Thesis Chair

Kyoung-Yeol Kim

Committee Members

Rixiang Huang, Michael T Yeung, Harry Efstathiadis

Keywords

Flow cathode, Hydrogen production, Microbial electrolysis cells, Nickel activated carbon, Nickel-based catalyst, Wastewater treatment, Sewage

Subject Categories

Environmental Engineering

Abstract

Microbial electrolysis cells (MECs) can electrochemically produce green hydrogen from waste streams. Although MECs are excellent options for implementing an energy recovery process while treating wastewater, the cathode side still hinders practical applications. Platinum (Pt) has been the top reference for hydrogen evolution reaction (HER) in MECs, however, Pt is not practical due to the high capital cost. Inexpensive nickel-loaded activated carbon (Ni/AC) cathodes were recently developed for replacing Pt in MECs and have shown comparable performance to Pt. This dissertation aims to breakthrough the current cathode challenges regarding the catalytic activity, manufacturing, and operational concept toward a cost-competitive scaling-up process of MECs. First, the electroactivity of the Ni/AC cathode was improved by increasing the oxygen (16.9%) and nitrogen (124%) containing species on the AC surface using nitric acid oxidation. The acid-treated AC (t-AC) showed 21% enhanced wettability which reduced the charge transfer resistance (33%) and ohmic resistance (6.7%) of the cathode. Ni/t-AC achieved 84% higher hydrogen production rates (0.35±0.02 L-H2/L-d) than the control (pristine AC). Second, increases in polyvinylidene fluoride binder loading (60%), during the manufacturing process, demonstrated to increase by 47% the hydrogen production rates in MECs, yet all cathodes showed a decline in electroactivities (≤9.2%) after MEC cycles. It is unclear how the binder content will impact the performance on a large scale. Third, to overcome this drawback, we have examined a novel binder-free flowable cathode. Ni/AC powders were suspended in a buffering solution as a cathode with no electrode fabrication processes. The Ni/AC flow cathode with higher Ni content and minimum Ni/AC loading (4 Ni-atom% and 0.125 wt-AC%, noted as Nix/ACY, X is nickel content, and Y is powder loading. Hereafter: Ni4AC0.125) demonstrated the highest catalytic activities (─0.86 V vs. Ag/AgCl at ─10 A/m2) and Faradaic response (1.6 F/g) among Ni/AC flow cathodes tested. This result indicates that pseudo-capacitive behavior toward Faradaic reactions can be promoted by increasing Ni loadings on AC particles. The MEC with Ni4AC0.125 flow cathode produced comparable hydrogen production rates (1.62±0.15 L-H2/Lreactor-d) to the Pt control (1.64±0.09 L-H2/L-d), and 40% higher than the blank (without Ni/AC, 1.29±0.02 L-H2/L-d). There was a 10% increase in hydrogen production rates with the lowest carbon black (CB) blending (CB: 0.06 wt.%) in a Ni2/AC0.125 flow cathode but hydrogen production rates were not further improved as CB content increased. The new Ni/AC flow cathodes showed ~5 times higher hydrogen production rates than previous stationary Ni/AC cathodes. To understand the underlying factors in the flow cathode system, Ni4/AC0.125 flow cathodes were further examined by coupling different current collectors (Ni foam, nickel mesh, stainless steel mesh, and Ti sheet). Flowable powders promoted more positive HER potentials at the cathode with the different current collectors, with the Ni foam flow cathode displaying the most positive values (─0.82 V vs. Ag/AgCl). In addition, intrinsically electroactive collector materials produced a higher accumulated charge toward HER than Ti (electrochemically inactive). These results suggest a synergistic effect between collector electroactivity and catalysts powders. There was a voltage drop across the flow cathode volume (≥0.49 V vs. Ag/AgCl, 2 cm distant from the current collector), indicating that suspended particles are likely active closer to the current collector rather than in the bulk catholyte. Finally, the nickel foam flow cathode produced 48% higher hydrogen production rates (2.33±0.25 L-H2/L-d) in MECs than the Ti flow cathode (1.57±0.14 L-H2/L-d) under the same conditions, demonstrating that the intrinsic electroactivity toward HER and the projected area, of the current collector, play a crucial role in the overall performance of flow cathodes.

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