Enhanced wettability improves catalytic activity of nickel-functionalized activated carbon cathode for hydrogen production in microbial electrolysis cells.

A nickel-functionalized activated carbon (AC/Ni) was recently developed for microbial electrolysis cells (MECs) and showed a great potential for large-scale applications. In this study, the electroactivity of the AC/Ni cathode was significantly improved by increasing the oxygen (16.9%) and nitrogen (124%) containing species on the AC using nitric acid oxidation. The acid-treated AC (t-AC) showed 21% enhanced wettability that consequently reduced the ohmic resistance (6.7%) and the charge transfer resistance (33.3%). As a result, t-AC/Ni achieved peak values of hydrogen production rate (0.35 ± 0.02 L-H2/L-d), energy yield (129 ± 8%), and cathodic hydrogen recovery (93 ± 6%) in MECs. The hydrogen production rate was 84% higher using t-AC/Ni cathode than the control, likely due to the enhanced wettability and a higher fraction of N on the t-AC. Also, the increases in polyvinylidene fluoride (PVDF) binder loadings (from 4.6 mg-PVDF/cm2 to 7.3 mg-PVDF/cm2) demonstrated 47% higher hydrogen productions rates in MECs.

Electrochemical Ammonia Recovery from Anaerobic Centrate Using a Nickel-Functionalized Activated Carbon Membrane Electrode.

Ammonia (NH3) recovery from used water (previously wastewater) is highly desirable to depart from fossil fuel-dependent NH3 production and curb nitrogen emission to the environment. Electrochemical NH3 recovery is promising since it can simply convert aqueous NH4+ to gaseous NH3 using cathodic reactions (OH- generation). However, the use of a separated electrode and membrane imposes high resistances to the cathodic reaction and NH3 transfer. This study examined an activated carbon (AC)-based membrane electrode functionalized with nickel to electrochemically recover NH3 from synthetic anaerobic centrate. The membrane electrode was fabricated using nickel-adsorbed AC powder and a polyvinylidene fluoride (PVDF) binder, and the PVDF membrane layer was formed at the electrode surface by phase inversion. The NH3-N recovery flux of 50.3 ± 0.4 gNH3-N/m2/d was produced at 17.1 A/m2 with a recovery solution at pH 7, and NH3-N fluxes and energy consumptions were improved as the recovery solution became acidic (62.2 ± 2.1 gNH3-N/m2/d with 16.0 ± 1.6 kWh/kgNH3-N at pH 2). Increasing PVDF loadings did not impact the electrochemical performances of the Ni/AC-PVDF electrode, but slightly lower (7%) NH3-N fluxes were obtained with higher PVDF loadings. Ni dissolution (3.7-6.0% loss) was affected by the recovery solution pH, but it did not impact the performances over the cycles.

Application of phase-pure nickel phosphide nanoparticles as cathode catalysts for hydrogen production in microbial electrolysis cells.

Transition metal phosphide catalysts such as nickel phosphide (Ni2P) have shown excellent activities for the hydrogen evolution reaction, but they have primarily been studied in strongly acidic or alkaline electrolytes. In microbial electrolysis cells (MECs), however, the electrolyte is usually a neutral pH to support the bacteria. Carbon-supported phase-pure Ni2P nanoparticle catalysts (Ni2P/C) were synthesized using solution-phase methods and their performance was compared to Pt/C and Ni/C catalysts in MECs. The Ni2P/C produced a similar quantity of hydrogen over a 24 h cycle (0.29 ±â€¯0.04 L-H2/L-reactor) as that obtained using Pt/C (0.32 ±â€¯0.03 L-H2/L) or Ni/C (0.29 ±â€¯0.02 L-H2/L). The mass normalized current density of the Ni2P/C was 14 times higher than that of the Ni/C, and the Ni2P/C exhibited stable performance over 11 days. Ni2P/C may therefore be a useful alternative to Pt/C or other Ni-based catalysts in MECs due to its chemical stability over time.

Regenerable Nickel-Functionalized Activated Carbon Cathodes Enhanced by Metal Adsorption to Improve Hydrogen Production in Microbial Electrolysis Cells.

While nickel is a good alternative to platinum as a catalyst for the hydrogen evolution reaction, it is desirable to reduce the amount of nickel needed for cathodes in microbial electrolysis cells (MECs). Activated carbon (AC) was investigated as a cathode base structure for Ni as it is inexpensive and an excellent adsorbent for Ni, and it has a high specific surface area. AC nickel-functionalized electrodes (AC-Ni) were prepared by incorporating Ni salts into AC by adsorption, followed by cathode fabrication using a phase inversion process using a poly(vinylidene fluoride) (PVDF) binder. The AC-Ni cathodes had significantly higher (∼50%) hydrogen production rates than controls (plain AC) in smaller MECs (static flow conditions) over 30 days of operation, with no performance decrease over time. In larger MECs with catholyte recirculation, the AC-Ni cathode produced a slightly higher hydrogen production rate (1.1 ± 0.1 L-H2/Lreactor/day) than MECs with Ni foam (1.0 ± 0.1 L-H2/Lreactor/day). Ni dissolution tests showed that negligible amounts of Ni were lost into the electrolyte at pHs of 7 or 12, and the catalytic activity was restored by simple readsorption using a Ni salt solution when Ni was partially removed by an acid wash.