Kinetic Reference Potential, pH-Effect, and Energy Recovery in Electrolysis of Water

Abstract

The electrolysis of water will likely become of superior importance for a sustainable energy economy. However, the electrocatalysis of electrochemical water splitting is complicated and the origin of significant energy losses. Among the heavily discussed open questions in this field at present is the origin of experimentally observed differences between electrolysis kinetics in acidic vs. alkaline electrolyte, and the effect of high-pressure operation on electrolyser performance. Our thermodynamic analysis reveals answers and fundamental connections between these questions by the definition of balanced reactive conditions and the kinetic reference voltage of the electrolysis reaction. Unlike the reversible cell voltage, the kinetic reference voltage <i>U</i><sub>kin</sub> is not biased by product H<sub>2</sub> and O<sub>2</sub> concentrations, and it represents a reliable intrinsic reference point for electrolysis kinetics. At standard temperature <i>T</i> = 25<sup>◦</sup>C, its value is <i>U</i><sub>kin</sub> = 1.441 V, which is in remarkable agreement with commonly observed onset voltages for macroscopic electrolysis rates. We define the reactive excess overvoltage <i>η</i><sub>rxs</sub> = <i>U</i><sub>kin</sub> − <i>U</i><sub>rev</sub> as the difference between the kinetic reference voltage and the reversible cell voltage. Comparing the hydrogen evolution (HER) and oxygen evolution (OER) half-cell reactions in acidic vs. alkaline electrolyte, we find an asymmetric and pH-dependent distribution of <i>η</i><sub>rxs</sub> among HER and OER. Increasing the electrolysis gas pressure results in a reduction of <i>η</i><sub>rxs</sub> due to an increased free energy content of the evolved gases. Our analysis provides a new perspective on activation losses in water electrolysis, on pH-effects in hydrogen and oxygen evolution electrocatalysis, and on high-pressure electrolysis as a means for energy recovery.<br>

Highly Ionic Conductive, Stretchable, and Tough Ionogel for Flexible Solid‐State Supercapacitor

“…These ionic hydrogels have been developed in recent years with superior mechanical properties (stretchability of > 10 times its original length, strength over 1.0 MPa, and fracture toughness > 10 4 J m −2 ) and sufficient ionic conductivity (as high as > 1.0 S m −1 ). [6][7][8] However, a large amount of water solvent (> 60 wt%) in these hydrogels limits their electrochemical stability windows (ESWs) to ≈1.0 V, if no special functionalization or modification of polymer chains is introduced, [9] together with poor physical stability due to water evaporation [10] and reduced low-temperature performance because of water freezing. [11] By far, ionogel replaces water with an ionic liquid as its solvent, leading to enhanced ESW (above 1.0 V) while maintaining a satisfying ionic conductivity (as high as 2.5 S m −1 ).…”

Impact of Water Properties on the Performance of PEM Electrolyzer

“…Another important factor is conductivity which also lowers the overall potential consequently decreasing the energy required. There is also an asymmetric and pH-dependent distribution of reactive excess overvoltage among hydrogen and oxygen evolution reactions [9]. High conductivity can also degrade the membrane, so, there is a need for an optimized value of pH, TDS as well as conductivity to ensure the enhanced performance of PEM electrolyzers.…”