For NiMH cells, the process of moving or transporting hydrogen from the negative electrode to the positive electrode absorbs heat and is therefore endothermic. See Figure 3.2 Chemical Equations and Figure 3.3 Transport Diagram. This water then releases a hydrogen ion that is absorbed into the positive electrode to form nickel hydroxide. Hydrogen stored in the metal alloy of the negative electrode is released into the electrolyte to form water. When a NiMH cell is discharged, the chemical reactions are the reverse of what occurs when charged. Positive Electrode: Ni(OH)2 + OH - chargeĭischarge NiOOH + H2O + e - Negative Electrode: M + H2O + e- chargeĭischarge MH + OH - Overall Reaction: Ni(OH)2 + M chargeĭischarge NiOOH + MHFigure 3.3 Transport Diagram See Section 3.4 Overcharge and 3.5 Overdischarge.Figure 3.2 Chemical Equations Extreme elevated temperatures may be experienced if a cell is excessively overcharged. As a cell is charged, the generation of heat may not accumulate if it is effectively dissipated. See Figure 3.2 Chemical Equations and Figure 3.3 Transport Diagram.īecause heat is generated as a part of the overall chemical reaction during the charge of a NiMH cell, the charging reaction described above is exothermic. At the negative electrode the metal alloy (M) in the negative electrode, water (H20) from the electrolyte, and an electron (e¯) react to produce metal hydride (MH) in the negative electrode and hydroxide (OH¯) in the electrolyte. NICD BATTERY PULSE CHARGE SPECS FREEThis produces nickel oxyhydroxide (NiOOH) within the positive electrode, water (H20) in the electrolyte, and one free electron (e¯). The reaction begins when the nickel hydroxide (Ni(OH)2) in the positive electrode and hydroxide (OH¯) from the electrolyte combine. The hydrogen in turn is absorbed and stored in the negative electrode. When a NiMH cell is charged, the positive electrode releases hydrogen into the electrolyte. This is the primary reason for the higher capacity and longer service life of NiMH batteries over competing secondary batteries. These metal alloys contribute to the high energy density of the NiMH negative electrode that results in an increase in the volume available for the positive electrode. The success of the NiMH battery technology comes from the rare earth, hydrogen-absorbing alloys (commonly known as Misch metals) used in the negative electrode. The following sections will discuss the chemical reactions occurring within the cell when charged and discharged and the adverse effects of overcharge and overdischarge conditions. The principles in which NiMH cells operate are based on their ability to absorb, release, and transport (move) hydrogen between the electrodes within the cell. Nickel Metal Hydride 3.0 Nickel Metal Hydride (NiMH) 3.1 NiMH Principles of Operation
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