Energy_Storage_Technologies.pdf

Electrochemical cell discharge anode oxidation occus

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Electrochemical Cell Discharge Anode (oxidation occus) Cathode (reduction occurs) Cation Flow Anion Flow + _ Flow of Electrons Flow of Electric Current (by convention) Electrochemical Cell Electrochemical Cell Charge + _ DC Power Supply Negative Electrode Positive Electrode oxidation (lose electrons) reduction (gain electrons) oxidation (lose electrons) reduction (gain electrons) _ _ Aqueous Electrolyte _ _ _ _ _ _ _ _ Figure 2.1: Charge and discharge of an electrochemical cell. Adapted from [1] The net reaction during the charge and discharge process are shown in Equation 2.3. Cd + 2 NiOOH + 2 H 2 O discharge ------ ------ charge Cd(OH) 2 + 2 Ni(OH) 2 (2.3) Chemically, the result of charge and discharge is to change the composition of the cell from one set of chemical species to another, then back again, and ideally the reversibility of this reaction could go on indefinitely, however, the equations presented here are a simplification of the actual reactions that take place, and those are unique for each type of electrochemical cell. The specific degradation modes of each device will be described in later sections for each technology. Why Do These Reactions Occur? When an electrochemical cell sits disconnected from a load in an open circuit state (no connection between the termi- nals to complete the circuit) there is an electromagnetic potential energy difference (voltage) that exists between the electrode with fewer electrons (the positive terminal) and the electrode with more electrons (the negative terminal). Prompting the discharge process is the presence of a conductive path (a load) connected externally between these positive and negative terminals. By completing the circuit, the voltage results in a force applied to the electrons prompting them to flow from the negative electrode to the positive electrode (a flow of electrons being known as current, measured in Amperes), once the electron flow has begun, there is a charge imbalance that leaves the positive and negative electrodes slightly less positively and negatively charged respectively, and this chemical charge imbal- ance results in the flow of anions to the anode and cations to the cathode to bring the system back toward a state of chemical equilibrium. During this process the potential difference between the two electrodes decreases reflecting the loss of chemical energy stored within the cells, so during discharge energy is delivered from the cells to the load. During charge a voltage potential is placed across the terminals such that the directions of electron flow and ion flow are reversed, and chemical energy lost during discharge may restored from the input electrical energy which reverses the chemical reaction and is able to restore the available chemical energy. 5
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Storage Technology Basics A Brief Introduction to Batteries Pump Pump Electrolyte Electrolyte Stack Electrode Source/Load + _ Ion Selective Membrane Cells Figure 2.2: Design of a generic flow battery system Increasing Energy and Power Capacity
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