Catalytic effects of Carbon: Dynamic Charge-Acceptance versus Water Loss
After many years of investigating the effect of different types of carbon on the electrochemical behaviour of lead electrodes many battery companies started the commercialization of Enhanced Flooded Batteries (EFB) batteries. In this technology, the classic lead electrode is partially replaced by carbon electrodes, either by applying a carbon coating on the lead surface or directly mixing the carbon with lead oxide and sulfuric acid to produce a mixed lead–carbon electrode. Although certain varieties of high surface-area carbon are known to produce very high charge-acceptance (but also water loss), low surface-area carbon or graphite is preferred for high-temperature, maintenance-free EFB applications. Therefore, in such service, the beneficial effects of high surface-area carbons on charge-acceptance cannot be fully exploited given the need to reduce gassing.
The electrochemical effects of certain impurities coming from different chemical components of the active materials has been the subject of many investigations but a systematic approach to the mechanisms of water loss at high temperature is still missing. Furthermore, the possibility of interactions between impurities in the electrolyte (or leached from the plates) with lead-carbon electrodes should not be overlooked. This presentation will describe several examples of catalytic effects on the hydrogen evolution and gassing behaviour of EFB batteries under high-temperature conditions. The well-known effects of certain impurities (e.g., Fe, Ni, Co, Cr) on hydrogen evolution may be amplified by the high surface-area carbons in a similar way that electrocatalysts (combinations of carbon substrates and noble metals) function in electrolysers.
The combined effects of the physical parameters of carbon (e.g., surface area and pore size microstructure) and the catalytic action of certain impurities on hydrogen evolution will be discussed. The favoured mechanism considers the rate-determining step to be the adsorption of hydrogen on the metal surface. The catalytic efficiency of metals is described in terms of a so-called ‘Volcano plot’ which shows why certain elements produce electrocatalytic effects on hydrogen evolution (mainly noble metals) whereas others, although theoretically very electroactive (due to the hydrogen adsorption energy), are in fact passivated by oxide films. Highly corrosive environments, such as hot sulfuric acid electrolyte inside the vehicle engine compartment, may dissolve the oxide layers and thereby convert these ‘non-harmful’ elements into catalysts for hydrogen evolution. The consequence is uncontrolled water loss and corrosion of current collectors to produce early battery failure. Analytical and electrochemical methods to investigate and prevent his deleterious behaviour will also be described.
Francisco Trinidad holds a MSc and PhD. From the University of Madrid. In 1977, he joined Tudor Spain in 1977. Following Exide’s acquisition of the company, he became its the Research Director in Paris, then the Development Director of Transportation Europe, and more recently the Director of Battery Technology in Exide Europe. During more than 41 years of experience with several electrochemical systems, he has been the author of more than 60 papers and 20 patents.