Comparison of Alternative Molten Electrolytes for Water Splitting to Generate Hydrogen Fuel

Abstract

This study explores a new high temperature molten hydroxide domain to electrochemically split water into hydrogen fuel. Hydrogen fuel, if produced without greenhouse gas emissions, is a promising fuel for transportation. This study opens a pathway to low energy water splitting and the electrolytic production of hydrogen fuel without carbon dioxide emission. A wide range of pure and mixed alkali and alkali earth hydroxide electrolytes are explored at temperatures ranging from 200 to 700 • C. Higher temperature leads to improved (lower voltage) water splitting and improved rates of charge transfer, but carries challenges (increased rates of parasitic side reactions as the molten electrolytes dehydrate with increasing temperature). This study extends the range of hydrogen formation in alkaline electrolysis water splitting to over 600 • C, and demonstrates that lithium and/or barium hydroxide electrolytes remain hydrated at high temperatures, and in the high temperature domain are advantageous over sodium or potassium hydroxide electrolytes. In pure LiOH, the coulombic efficiency for hydrogen generation decreases with temperature and is measured respectively at η H 2 = 88%, 21%, 4% and 0%, respectively at 500, 600, 700 and 800 • Hydrogen has advantageous features compared to fossil fuels. It has a higher specific energy, is not polluting, has a combustion product of water, and in particular it does not emit the greenhouse gas carbon dioxide. Challenges of hydrogen storage are being addressed and hydrogen powered vehicles have been introduced to the commercial marketplace.1-4 Unfortunately, the predominant fraction of hydrogen generated today is produced by steam reformation of fossil fuel by reacting the fossil fuel with steam to release H 2 and CO 2 . Steam reformation is a substantial source of CO 2 emissions, negating the climate mitigation effect of using H 2 as a fuel. Thus, a process to efficiently generate hydrogen without carbon dioxide emissions is needed.The development of electrolytic splitting of water dates back to 1802. 5 The electrolysis of water produces hydrogen and oxygen directly without CO 2 emission (if the energy source used to generate the electricity did not release CO 2 ). Utilization of renewable or nuclear energy to generate the electricity for this electrolysis can drive this electrolytic water splitting to produce hydrogen as a fuel without carbon dioxide emissions. The most abundant of renewable energy resource for electrical generation is solar energy, and as early as 2001 we were driving stable solar electrolytic water splitting to hydrogen fuels at over 18% solar to chemical energy conversion by using efficient multiple bandgap solar cells.6 One of the challenges to the use of illuminated junctions to drive electrochemical or photoelectrochemical water splitting is that the bandgap of efficient semiconductors lies in the visible spectrum which generate a photo-potential less than the minimum needed rest potential of 1.23 volt required to split water to hydrogen and oxygen at room te...

Design and optimization of electrochemical cell potential for hydrogen gas production

“…This transfer of hydrogen and steam molecules to and from the nickel cathode surface during electrolysis process can be listed as a possible reason for the noise noticed in the current-time plot. It is sig-nificant to mention here that the nickel cathode is highly stable for molten hydroxide water splitting as stated by literature [11] , that corroborates with these research findings.…”

“…The reason for being the current efficiency more than 100% is an experimental error at this specific operating temperature. The reason for decrease in current efficiency for hydrogen generation with an increased temperature of eutectic molten hydroxide is because the formation of superoxide O 2 − reduction increases and subsequently competes with the hydrogen formation reaction [11] as seen in Eqs. (4) and 5:…”

Electrochemical investigation of novel reference electrode Ni/Ni(OH)₂ in comparison with silver and platinum inert quasi-reference electrodes for electrolysis in eutectic molten hydroxide

“…The cyclic voltammetry of the platinum electrode was performed with a supply of argon gas or ammonia at 200 o C and Ag was used as a reference electrode throughout the study. Litcht et al [1] found in their study that increasing the temperature of molten NaOH or KOH beyond the maximum of 400 o C decreased the quantity of water available for hydrogen generation, instead of resulting in the formation of oxides. This increase in temperature also decreased the coulombic efficiency of hydrogen gas.…”

“…Hydrogen is an efficient energy carrier and emission free candidate to supersede the continuous use of fossil fuels in future because of high mass energy density, fast kinetic rate of electrochemical reaction and only water containing emission gas [1]. Hydrogen gas (H2) can be produced from biomass gasification, steam reformation of fossil fuel, coal gasification, partial oxidation of hydrocarbons [2] and biomass fermentation [3].…”

Efficient Electrocatalytic Synthesis of Ammonia from Water and Air in a Membrane‐Free Cell: Confining the Iron Oxide Catalyst to the Cathode

“…In the supporting information, we explore under various pressures, temperatures, and electrolytes the co-production of ammonia and iron oxide catalysis. These experiments, lead us to believe that high temperatures can deactivate the ammonia electrochemical synthesis process by dehydrating the electrolyte to inhibit the requisite hydrogen source [70][71] This, and opening the pathway to utilize polymer membranes, which would degrade at higher temperature in the electrosynthesis, is why we have moved to more ambient conditions.…”