Durability of Electrodeposited Ni-Fe-Co-C Anodes for Alkaline Water Electrolysis

Abstract

IntroductionWe have been studying more than a quarter of century for supply of renewable energy in the form of methane via electrolytic hydrogen generation using carbon dioxide as the feedstock. Although we created cathodes and anodes for direct seawater electrolysis, the energy efficiency was not sufficient. For immediate industrialization, we are concentrating our current effort on alkaline water electrolysis, creating efficient cathodes and anodes. The present work aimed to determine the composition of active Ni-Fe-Co-C alloy anodes prepared by electrodeposition and to examine the durability for continuous electrolysis at a high current density of 6000 Am-2 in 4.5 M KOH at 90°C. Experimental Procedures The substrate for electrodeposition was a chemically etched Ni plate. Electrodeposition was carried out at a current density of 300 Am-2 for 10 min in 1.14 M NiSO4-0.189 M NiCl2-0.49 M H3BO3-0.104 mM CH3(CH2)11OSO3Na solutions containing various concentrations of FeSO4, CoSO4 and lysine, the pH of which was adjusted at 1.5 by the addition of 18 M H2SO4. Lysine was added as the carbon source. The composition of metallic elements in the electrodes was analyzed by electron probe microanalysis, and carbon was chemically analyzed by combustion infrared absorption method. The solution used for oxygen evolution was 4.5 M KOH at 90oC. The activity of the anodes was evaluated from galvanostatic polarization curves measured by the current density rise every 1 min. The potential rise due to the solution resistance at high current densities was corrected by the current interruption technique. Potentiodynamic polarization curves were also measured at a potential sweep rate of 0.52mV-1 for comparison. Results and Discussion As-deposited alloys were used for examination of oxygen evolution activity. Ni was a stable anode but less active for industrial electrolysis. The addition of Co ion and lysine to the deposition electrolyte containing Ni ion enhanced the oxygen evolution activity, but the activity of Ni-Co-C alloys was not yet sufficiently high. Ni-Fe-Co-C alloys deposited showed significantly high activity for oxygen evolution. An increase in Fe content in the alloy was more effective than the increase in Co content for enhancement of oxygen evolution activity. Thus, an increase in Co concentration in deposition electrolytes did not largely change the activity because the increase in Co content in the alloy resulted in the decrease in the content of Fe. The addition of lysine to the deposition electrolytes containing Ni, Fe and Co ions enhanced the oxygen evolution activity. The highest activity was attained at 0.2 M lysine in the deposition solution, but further increase in lysine concentration was not effective. An increase in lysine concentration led to asymptotic increase in the C content in the Ni-Fe-Co-C alloys up to about 6 at%. The deposition electrolyte to form the most active Ni-Fe-Co-C alloy for oxygen evolution was 1.33M Ni-0.072M Fe-0.018M Co-0.2M lysine solution. When the Ni-Fe-Co-C alloy prepared from this electrolyte was used for oxygen evolution at a current density of 6000 Am-2 for 800 h, the mass loss was about 5 % of the original mass. The galvanostatic polarization potential of as-deposited Ni-Fe-Co-C alloy measured by the current density rise every 1 min was in the active region at lower current densities, but the potential shifed to the oxygen evolution region at higher current densities. Once the specimen was polarized in the oxygen evolution region, the potential was stayed in the oxygen evolution region even when the current density was lowered. After oxygen evolution at 6000 Am-2 for long period of time, the potential stayed in the oxygen evolution region regardless of current densities. In fact, the dissolution current density estimated from the mass loss by polarization at 6000 Am-2 for 800 h even on the assumption of Fe dissolution in the form of Fe6+ as FeO4 2- was only 4.5 x 10-3 Am-2 that is less than one-millionth of oxygen evolution current. Consequently, the active Ni-Fe-Co-C alloy anode thus prepared is sufficiently stable maintaining the passive state in the severe environment for oxygen evolution at 6000 Am-2 in a concentrated KOH solution at 90°C for long period of time. Conclusion The active oxygen evolution anode in alkaline water electrolysis for hydrogen production was created. The addition of Fe, Co and C to Ni enhanced the oxygen evolution efficiency in 4.5 M KOH solution at 90°C. The Ni-Fe-Co-C alloy thus prepared kept the passive state during electrolytic oxygen evolution at 6000 Am-2 for a long period of time.