Alkaline Water Electrolysis at 20 a cm-2 with a Microfibrous, Flow-through Electrolyzer

Abstract

Improving both the efficiency and productivity of water electrolysis is necessary for making the electrochemical generation of hydrogen economically competitive. Flow-through electrodes are widely used to improve the efficiency and productivity of electrochemical processes, but studies of their use in alkaline water electrolysis are relatively rare. This work aimed to (1) test the hypothesis that the use of a flow-through electrode could enhance the rate of bubble removal from the electrode surface, thereby improving productivity, and (2) determine what length scale of electrode would maximize the surface area available for electrolysis without hindering bubble removal. To determine how the efficiency and productivity of the flow-through electrode depends on the electrode length scale, the performance of the oxygen evolution reaction was compared with flow-through electrodes consisting of a Ni foam, Ni microfibers (Ni MFs, d = 1.2 mm), and Ni-coated Cu nanowires (NiCu NWs, d = 350 nm). Although the turnover frequency (TOF) for the Ni foam was 2.4 times higher than the Ni MF electrode and 42 times higher than the NiCu NW electrode, its surface area was more than 90 times lower than the other electrodes, resulting in the lowest overall current density. Although the NiCu NW electrode had a higher surface area than the Ni MF electrode, it had a lower overall current density because of its low TOF. We attribute the high performance of the Ni MF electrode to the fact that the mean pore size of the Ni MF electrode (5-10 mm) is on the order of the mean bubble size (20 mm) that was measured by examining the bubbles generated from a mesh electrode. The performance of the Ni MF electrode was further improved by coating it with a Ni-Fe layered double hydroxide (LDH) catalyst, resulting in an OER overpotential of 247 mV at 0.1 A cm-2 and a Tafel slope of 26 mV dec-1. These values are among the lowest reported for Ni-Fe LDH catalysts. A Ni MF anode was paired with a Pt-coated Ni MF cathode to measure the overall productivity for water splitting. The maximum current density for water splitting was 20 A cm-2 at 3.9 V in 30% KOH, which is 20 times greater than the highest current densities previously reported for alkaline water electrolysis. This current density and voltage could be sustained for at least 8 hours with no signs of degradation. This high current density was attributed to the ability of the flowing fluid to promote bubble removal and keep the hot electrode surface from boiling water. This work demonstrates that flow-through microfibrous electrodes can improve the efficiency and productivity of alkaline water electrolysis. Figure 1