Discharge and when they get to the electrode they

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discharge, and when they get to the electrode they might be able to give up and electron, or take an electron through a chemical reaction, depending on whether the unit is charging or discharging. In more detail, when the cell discharges, it is connected to an external load, and electrons flow from the negative terminal through the external load to the positive terminal. Simultaneously within the cell, negatively charged anions flow toward the negative electrode and positively charged cations flow toward the positive electrode to complete the circuit. This is shown visually in the left side of Figure 2.1. Note that since the negative electrode is losing electrons, oxidation occurs at the negative electrode, and since the positive electrode is accepting electrons, reduction is occurring at the positive electrode [1]. As the positively charge cations flow toward the positive electrode, it is becoming more negatively charged since it is accepting electrons traveling from the negative electrode through the load. Since the positive electrode is becoming less positively charged, then the cell is losing energy (as would be expected during discharge). When the cell recharges these processes are reversed electrons flow from the positive terminal, through the DC power source, to the negative terminal. Within the cell negatively charged particles flow, this time, toward the positive electrode (the anode since oxidation—loss of electrons—occurs here), and cations flow toward the negative electrode (the cathode since reduction—gaining electrons—occurs here). This is shown visually in the right side of Figure 2.1. From the perspective of the circuit the anode is where electrons originate and the cathode is where the head to. Since electric current is defined as the direction opposite to that of electron flow, current always flows from the cathode to the anode. As an example of these processes, consider the reactions in a nickel-cadmium cell. In this case, the negative electrode is composed of cadmium metal, Cd, the positive electrode is nickel hydroxide, Ni(OH) 2 , and the electrolyte is typically aqueous potassium hydroxide, KOH(H 2 O) [2]. The reactions that occur at the negative electrode and positive electrode during charge and discharge are shown in Equation 2.1 and Equation 2.2 [1], respectively. Notice that at the positive electrode during discharge electrons are gained through reduction (so that electrode acts as the cathode), and at the negative electrode during discharge electrons are lost through oxidation (so that electrode acts as the anode). Reactions at the negative electrode: Cd + 2 OH - discharge ------ ------ charge Cd(OH) 2 + 2 e (2.1) Reactions at the positive electrode: NiOOH + H 2 O + e discharge ------ ------ charge Ni(OH) 2 + OH - (2.2) 4
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Storage Technology Basics A Brief Introduction to Batteries Anode (oxidation occus) Cathode (reduction occurs) Negative Electrode Positive Electrode Aqueous Electrolyte Cation Flow Anion Flow Load + _ Flow of Electrons Flow of Electric Current (by convention) Electrochemical Cell
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  • Energy storage

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