Hydrogen Production Via the Electrolysis of Municipal Effluent Using Carbon Fibre and Nickel-Foam Anode

Abstract

This study aims to explore hydrogen production via alkaline water electrolysis using municipal effluent. The experiments were conducted in a flow cell electrolyzer using carbon fibre and Ni-foam as the anode, with another Ni-foam electrode employed as the cathode for all experiments. Synthetically prepared effluent and real municipal effluent obtained from the Harnaschpolder water treatment plant effluent was used with potassium hydroxide (KOH) as the supporting electrolyte. The composition of the synthetic effluent was prepared with 23.0 ± 2.1 mg O2/L humic acid (Sigma Aldrich) as the primary organic pollutant. Experiments were conducted to investigate the effect of electrolyte concentration ranging from 0.01M -1M KOH and the effect of applied current density ranging from 25 A/m2, 50 A/m2, 100 A/m2 and 150 A/m2. Further experiments were conducted to assess the effect of humic acid concentration ranging from 23.0 ± 2.1 mg O2/L to 90.0 ± 0.7 mg O2/L. This was subsequently investigated under potentiostatic conditions at an applied cell voltage of 1.5V using the carbon fibre anode and under galvanostatic conditions for the Ni-foam anode. Regarding the Ni-foam anode, the current density was calculated based on the concentration using the relationship for limiting current density of humic acid oxidation. The investigation focused on assessing the performance of the electrolyzer in terms of volumetric hydrogen production rate and energy efficiency of hydrogen production. The performance data was also compared with conventional alkaline water electrolysis using Ni-foam as the anode and cathode in 1M KOH solution. In addition, the extent of humic acid oxidation was also assessed in terms of COD removal efficiency, TOC removal efficiency and changes in the spectral scans obtained via UV-Vis spectrophotometry.
The performance data revealed that the energy efficiency of hydrogen production was lower for both synthetic and municipal effluent compared to alkaline water electrolysis; this was evident for both carbon fibre and Ni-foam anode. Further, the energy efficiency was also higher when the Ni-foam anode was used compared to the carbon fibre anode. For the electrolysis of municipal effluent, a maximum energy efficiency of 75 ± 2.7% was obtained for the carbon fibre electrode, whereas for the Ni-foam anode, the maximum energy efficiency obtained was 83 ± 3.0% at an applied current density of 12.55 A/m2 with 1M KOH as the electrolyte. Despite this observation, the volumetric hydrogen production rates were not significantly affected because the rates converged closely to their theoretical values; this was also evidenced by the coulombic efficiency of hydrogen evolution reaction (HER), which exceeded 90% in all the experiments.
Concerning the extent of humic acid degradation, the Ni-foam anode was able to oxidize some of the humic acid molecules during electrolysis, and a maximum COD removal of 24.6 ± 8 % was observed after electrolysis of synthetic effluent at an applied current density of 100 A/m2 with 1M KOH as the supporting electrolyte. In contrast, the carbon fibre anode was not able to do so. Instead, the humic acid molecules tended to adsorb on the surface of the carbon fibre anode, evidenced by the increase in COD and TOC after electrolysis. Similar behaviour was also observed for the carbon fibre anode when the effect of humic acid was investigated under potentiostatic conditions, whereas under galvanostatic conditions for the Ni-foam anode, oxidation of humic acid was evident. The extent of oxidation was dependent on the duration of electrolysis, i.e., humic acid concentrations of 49.8 ± 0.98 mg O2/L and 90.0 ± 0.7 mg O2/L required a duration of 6 hrs and 8 hrs to achieve a COD removal efficiency of 43.5 ± 5.0 % and 60.4 ± 4.2 %, respectively. Additionally, the possibility to integrate an electrolyzer with an aerobic treatment plant to supply high purity oxygen was investigated. The comparison was done using BioWin simulations with a standard aerobic bioreactor for a flow of 10,000 m3/day under two conditions: 1) with atmospheric O2(baseline) 2) with high purity O2(95% purity). The investigation revealed that combining a 1 MW electrolyzer with the aerobic wastewater treatment plant would provide sufficient amount of oxygen for the proper functioning of the high purity O2 plant. In addition, the flow requirement to produce the required amount of oxygen was only 0.03% of the discharged effluent (i.e., only 3.1 m3/day of the discharged 9708 m3/day).
Further, the use of high purity oxygen in the aerobic wastewater treatment plant was accompanied by energy savings of 1.2 kWh (4320 kJ) due to reduced load on the air pumps