Integrating high content carbon into the negative electrodes of advanced lead–acid batteries effectively eliminates the sulfation and improves the cycle life, but brings the problem of hydrogen evolution, which increases inner pressure and accelerates the water loss. In this review, the mechanism of hydrogen evolution reaction in advanced lead–acid batteries, including lead–carbon battery and ultrabattery, is briefly reviewed. The strategie. Integrating high content carbon into the negative electrodes of advanced lead–acid batteries effectively eliminates the sulfation and improves the cycle life, but brings the problem of hydrogen evolution, which increases inner pressure and accelerates the water loss. In this review, the mechanism of hydrogen evolution reaction in advanced lead–acid batteries, including lead–carbon battery and ultrabattery, is briefly reviewed. The strategies on suppression hydrogen evolution via structure modifications of carbon materials and adding hydrogen evolution inhibitors are summarized as well. The review points out effective ways to inhibit hydrogen evolution and prolong the cycling life of advanced lead–acid battery, especially in high-rate partial-state-of-charge applications.••Lead–carbon batteryUltrabatteryHydrogen evolution reactionHydrogen inhibitionLead–acid battery has been commercially used as an electric power supply or storage system for more than 100 years and is still the most widely used rechargeable electrochemical device 1., 2., 3., 4. Most of the traditional valve-regulated Lead–acid (VRLA) batteries are automotive starting, lighting and ignition (SLI) batteries, which are usually operated in shallow charge/discharge cycles. Recently, Lead–acid battery has attracted considerable attentions for hybrid electric vehicles (HEVs) and energy storage applications because of low initial cost, simplicity of design, reliability and relative safety, and high recycling efficiency 5., 6., 7., 8. However, the cycle life of VRLA batteries under such applications has been found to be much shorter than the design life 9., 10. The major failure mode has been identified to be the negative progressive sulfation at high discharge rate or under high-rate partial-state-of-charge (HRPSoC) cycling. It can be attributed to the following aspects. First, the formation of a thin-layer lead sulfate under HRPSoC hinders further discharge, which will grow progressively and thus lead to irreversible hard sulfation in the negative plates. Second, high rate charge increases the mass transport overpotential of the primary reactions, resulting in evolution of hydrogen and oxygen at negative electrode and positive electrode, respectively 9., 12., 13.To improve the cycle life of Lead–acid batteries, considerable effort has been. Hydrogen evolution is a secondary and side reaction in Lead–acid batteries, which influences the volume, composition and concentration of the electrolyte, and thus the battery performance. Generally accepted hydrogen evolution reaction (HER) mechanisms in acid solutions are as follows:Electrochemical hydrogen adsorption (Volmer reaction)(1)H++M+e−↔M−H*.Followed by electrochemical desorption (Heyrovsky reaction)(2)M−H*+H++e−↔M+H2,or chemical desorption (Tafel reaction)(3)2M−H*↔2M+H2,where H* is the hydrogen atom chemically adsorbed on an active site of the electrode surface (M). These pathways are strongly dependent on the electrochemical, chemical and physical properties of the electrode surface. The possible rate controlling steps (1, 2 or 3) can be simply determined by evaluating the Tafel slope from the HER polarization curve, which has been carefully explained by Conway and Tilak.The equilibrium potentials of the positive and negative electrodes in a Lead–acid battery and the evolution of hydrogen and oxygen gas are illustrated in Fig. 4. When the cell voltage is higher than the water decomposition vol. The main requirements of carbon additives to negative plate of lead–acid battery have been summarized by Lam and co-workers : (1) similar working potential to that of the lead–acid negative plate; (2) low hydrogen gassing rate; (3) higher capacity to share the current with the lead–acid negative plate; (4) long cycle life; (5) sufficient mechanical strength and ability to produce in the existing Lead–acid factory; and (6) low cost.The hydrogen gassing rate is mainly influenced by two properties of an electrode: surface area and surface activity. In general, the capacitance of carbon increases with surface area. However, higher surface area may increase hydrogen reaction exchange current density and promote hydrogen gassing. To suppress the hydrogen evolution, the effort must be to lower the specific area without sacrificing too much capacitance. Shi investigated the relationship between specific capacitance and specific area for activated microbead carbon and AC fiber and found that the specific double-layer capacitance of AC did not have a linear relation with their total surface area. Instead, the specific capacitance could be described as the addition of two different parts, as shown in Eqs. (5), (6):(5)C=CdlmiSmi+CdlextSext,or(6)C/Sext=CdlmiSmi/Sext+Cdlext,where Cdlext,Cdlmi are capacitance per unit area for external pores and micro pores.
