Self-discharge processes

Self-Discharge Processes. The shelf life of the lead—acid battery is limited by self-discharge reactions, first reported in 1882 (46), which proceed slowly at room temperature. High temperatures reduce shelf life significantly. The reactions which can occur are well defined (47) and self-discharge rates in lead—acid batteries having immobilized electrolyte (48) and limited acid volumes (49) have been measured. 

The self-discharge process has made experimental determination of the reversible potential of the Ni(OH)2/NiOOH couple very difficult. A major advance was the realization by Bourgault and Conway... 

One practical and one fundamental question are of interest here (a) How much charge can be stored in carbons (b) How does the amount of charge stored depend on the nature of the carbon and thus on its surface chemistry Their answer(s) should lead to the resolution of an apparent contradiction that is implicit in the following statements from recent authoritative reviews [T]he preferred carbon materials [for electrochemical capacitors] should be free from... surface quinonoid structures that can set up self-discharge processes that must be minimized [68] [s]ubstitutional heteroatoms in the carbon network (nitrogen, oxygen) are a promising way to enhance the capacitance [95],... 

The self-discharge reactions cause hydrogen and oxygen evolution, which may end up causing thermal runaway in the battery. The rates of the self-discharge processes (Eqs 71 and 72) depend on temperature, electrolyte volume and concentration, and impurity content (specially antimony content). More details about the self-discharge process in these batteries is given in the literature [7]. 

Mechanism of the self-discharge processes on the negative plate at open circuit. 

CSM grids are not suitable for positive plates because the corrosion wiU quickly eat up the lead layer covering the grid and copper ions will diffuse to the solution and thus reduce the charge efficiency and accelerate greatly the self-discharge processes. 

Inhibitors of hydrogen evolution slow down the self-discharge processes and improve the charge acceptance of the negative plates. 

Influence of Sulfuric Acid Concentration on the Self-discharge Processes of the Positive Plates of Wet-charged Lead-acid Batteries... 

It has also been established that the hydrogen evolved by the self-discharge processes on the negative plates can be oxidized on the positive plates via the following reaction [15-18] ... 

That is E depends on the sulfuric acid concentration. The rate of the cathodic reaction (13.12) is very slow, because of the very high overpotential of hydrogen evolution on lead electrode of high purity grade. And it is the reaction of hydrogen evolution that exerts the strongest influence on the rate of the self-discharge process, i.e. it practically determines this rate. 

The second reaction that accelerates the self-discharge processes of the negative plates in wet-charged batteries is the oxidation of lead by the oxygen evolved by the self-discharge processes at the positive plates. This oxidation of the lead active mass can be represented by the following equation ... 

The third major factor that affects the self-discharge processes in wet-charged batteries is the storage temperature. It should not exceed 25—30 °C. 

Reduced electrolyte stratification. In flooded batteries, gravitation causes the formation of vertical layers of different electrolyte concentration (i.e. electrolyte stratification). This creates potential differences between the top and bottom parts of vertical cells [19] and leads eventually to slow self-discharge processes. In VRLA batteries, the electrolyte is immobilized, which allows the cells to be positioned (mounted) horizontally as well. Thus, the degree of electrolyte stratification is reduced substantially and hence the service life of the cells is extended. 

Thus, one can state that the mechanisms of self-discharge processes of ECSC carbon electrodes are quite diverse and complicated. 

Noticeably, Chen et al. [923] reported that ESs using the HQ-based redox-active electrolyte suffered from a much faster self-discharge process, which was due to the transfer of redox-active species in the electrolytes between positive and negative electrodes. The authors also proposed two strategies to address this issue (a) the selection of a redox-active electrolyte that could be reversibly converted into insoluble species during the charging/discharging process and (b) the use of a suitable separator membrane such as Nafion. 

Zhang, Q., J. Rong, D. Ma, and B. Wei. 2011. The governing self-discharge processes in activated carbon fabric-based supercapacitors with different organic electrolytes. Energy Environmental Science 4 2152-2159. 

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