From Faraday's Laws to Current Efficiency【Part1】: What is Current Efficiency?

From Faraday's Law to Current Efficiency

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1.What is Current Efficiency?

1.1 Faraday's Law

1.2 Electrochemical Equivalent

1.3 Current Efficiency

2.Factors Affecting Electrode Current Efficiency

2.1 Anodic Current Efficiency of Chlor-Alkali Electrolyzers

2.2 Anodic Current Efficiency of Sodium Hypochlorite Production by Electrolysis

2.3 Cathodic Current Efficiency of Metal Electrodeposition from Aqueous Solutions

3.Factors Affecting Current Efficiency of Electrochemical Reactors

3.1 Counter Electrode Side Reactions and Electrolyte Side Reactions

3.2 Design Factors of Electrochemical Reactors

We will start with Faraday's Law, elaborating on the definition and quantification method of current efficiency; we will also specifically analyze the factors affecting electrolytic current efficiency from two aspects: electrodes and electrochemical reactors. The article will be divided into three parts (Part 1, Part 2, and Part 3) for sequential discussion: 1. What is Current Efficiency? 2. Factors Affecting Electrode Current Efficiency 3. Factors Affecting Current Efficiency of Electrochemical Reactors, which will be continuously updated as a series of tweets. This is the first installment of the series, aiming to explain what current efficiency is.

Part1: What is Current Efficiency?

As the name suggests, current efficiency refers to the efficiency of current utilization for generating target products through electrochemical reactions. But how is current efficiency quantified? We need to start with Faraday's Law.

Faraday's Law

In 1833, the British scientist Michael Faraday first proposed the basic concept of Faraday's Law. Its accurate statement is: In an electrochemical reaction, the amount of electricity passing through the interface between two types of conductors is proportional to the quantity of substances generated at the interface. Electrochemically, it can be expressed more concisely as follows: if the number of electrons gained or lost in the electrochemical reaction is n, then for each Faraday of electricity (1F) passing through the interface, 1/n moles of the substance should be generated.1F represents the charge of 1 mole of electrons, i.e., the charge carried by a number of electrons equal to the Avogadro constant (6.0221367*10²³). The charge of a single electron is 1.6021892*10^-19C; therefore, 1Fis specifically equal to:

1F=6.0221367*1023*1.6021892*10-19=96486C/mol

The above value is known as the Faraday constant. When expressed in ampere-hours (Ah) or ampere-minutes (Amin) — units commonly used in electrochemical engineering——1 F is equivalent to 26.8 Ah or 1608 Amin. For example, in the chlorine evolution reaction 2Cl^-=Cl₂+2e^-，n=2; that is, the production of 1 mole of chlorine requires the consumption of 2 moles of electrons. Therefore, the passage of 1 Faraday of electricity should yield 1/2 mole of chlorine, which is 35.5 grams of chlorine.

Electrochemical Equivalent

In addressing practical problems, direct calculation using Faraday's Law is inconvenient; instead, the concept of electrochemical equivalent (denoted by the symbol K) is generally adopted. The definition of electrochemical equivalent is: the quantity of a substance generated at the interface when a unit amount of electricity passes through it. When different units are used, the electrochemical equivalent of the same substance takes different numerical values. In electrolytic engineering, especially when calculating electrical energy consumption, it is necessary to know the amount of electricity required to produce a unit mass of the substance — this is referred to as the theoretical power consumption (k), which is the reciprocal of the electrochemical equivalent, i.e.:

k=1/K

Taking chlorine as an example again, the calculation method for converting its electrochemical equivalent to g/Ah is as follows:

1 Faraday of electricity = 26.8 Ah (96486/3600)1 Faraday generates 35.5 g of chlorineElectrochemical equivalent of chlorine = 35.5 / 26.8 g/Ah = 1.325 g/Ah (1.323 is commonly used)

Thus, the theoretical power consumption required to produce 1 ton of chlorine is:

k=1000000/K=1000000/1.323=755860 Ah=755.86 kAh

Obviously, for industrial electrolysis, the larger the electrochemical equivalent of a substance, the lower its theoretical power consumption for synthesis — which is more favorable for the chemical process, as it reduces electricity consumption. However, for chemical power sources, a smaller electrochemical equivalent is more advantageous, because this reduces the amount of active material required to generate a unit of electricity.

Faraday's Law is one of the most rigorous laws in nature; it does not change with variations in material type, properties, or reaction conditions. However, apparent deviations are often observed in electrochemical research and production — specifically, the products of electrochemical reactions are less than (and sometimes greater than) the theoretically calculated values. For this reason, the basic definition of current efficiency has been established.

Current Efficiency

Current efficiency is defined in two ways:

a. For a fixed amount of electricity:Current efficiency = Actual quantity of substance generated by the electrode reaction / Theoretical quantity of substance calculated in accordance with Faraday's Law

b. For a fixed quantity of substance:Current efficiency = Electricity required as calculated by Faraday's Law / Actually consumed electricity

Both methods are used in practical production and research and can be freely selected as needed. However, they characterize the same content — the effective utilization rate of electric current (or electricity, to be precise).

In chemical power sources, a similar concept is often expressed by the active material utilization rate. Specifically, it refers to the ratio of the actual capacity of an electrode (the amount of electricity that can be released) to its theoretical capacity (calculated in accordance with Faraday's Law), and is termed the active material utilization rate.

Typically, it is difficult for current efficiency to reach 100%. Below, we take electrolytic engineering as an example to list the factors affecting current efficiency.

Sneak Peek: Part 2 | Factors Affecting Electrode Current Efficiency

"Electrode current efficiency is crucial to the current efficiency of target product yield. Here, we mainly refer to the potential side reactions that may occur on the electrode where the target electrochemical reaction takes place."

After understanding what current efficiency is and how to quantify it in Part 1, we will take chlor-alkali electrolysis, sodium hypochlorite production by electrolysis, and metal electrodeposition from aqueous solutions as examples in Part 2 to illustrate the factors that may affect electrode current efficiency.

Stay tuned for more—exciting content is coming soon!