Hydrogen + oxygen recombination and related heat generation in undivided electrolysis cells

Abstract

Abstract This paper presents a mathematical analysis that allows the rate of H 2 + O 2 recombination and related heat generation in single-compartment electrolysis cells to be calculated as a function of current density and temperature. The analysis employs electrochemical kinetics and gas evolution-enhanced mass transfer theory. Recent calorimetric results of others during the electrolysis of K 2 CO 3 + H 2 O solutions in the low current density range from 0.5 to 4 mA cm −2 are in good agreement with the theoretical predictions. The fraction of O 2 recombining with H 2 decreases significantly with increasing current density. As much as 27% of the O 2 recombines at 0.5 mA cm −2 , but only 4% at 100 mA cm −2 . It is shown that the heat generated by H 2 + O 2 recombination comprises a significant fraction of cell input energy only at low current densities. At 0.5 mA cm −2 , 0.8% at 40 mA cm −2 and only 0.03% at 400 mA cm −2 . The analysis also predicts quantitatively the observed enhancement of recombination by O 2 gas sparging and its elimination by N 2 sparging. On the basis of their results at low current densities, a group of researchers recently concluded that H 2 + O 2 recombination is the source for the ‘excess heat’ reported by other groups and attributed by some to ‘cold fusion’. However, reported excess heat values, ranging from a low of 23% at 14 mA cm −2 to a high of 3700% at 6 mA cm −2 , are much largern than can be explained by recombination. Whatever the explanation for the large amounts of excess heat reported by various groups, H 2 + O 2 recombination must be rejected as a tenable explanation.