Galvanic cell

(Redirected from Voltaic cell)
Galvanic cell with no cation flow

A galvanic cell, or voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell. It generally consists of two different metals connected by a salt bridge, or individual half-cells separated by a porous membrane.

Volta was the inventor of the voltaic pile, the first electrical battery. In common usage, the word "battery" has come to include a single galvanic cell, but a battery properly consists of multiple cells.[1]

History

In 1780, Luigi Galvani discovered that when two different metals (e.g., copper and zinc) are connected and then both touched at the same time to two different parts of a muscle of a frog leg, to close the circuit, the frog's leg contracts.[2] He called this "animal electricity". The frog's leg, as well as being a detector, was also the electrolyte (to use the language of modern chemistry).

A year after Galvani published his work (1790), using a force-based detector and brine-soaked paper (as electrolyte), Alessandro Volta showed that the frog was not necessary. (Earlier Volta had established the law of capacitance C=Q/V with force-based detectors). In 1799 Volta invented the voltaic pile, which consists of a pile of galvanic cells. He built it entirely out of non-biological material to challenge Galvani's (and the later experimenter Leopoldo Nobili) animal electricity theory in favor of his own metal-metal contact electricity theory.[3] Carlo Matteucci in his turn constructed a battery entirely out of biological material in answer to Volta.[4] Volta's contact electricity view characterized each electrode with a number that we would now call the work function of the electrode. This view ignored the chemical residue at the electrode-electrolyte interfaces.

Although Volta did not understand the operation of the battery or the galvanic cell, these discoveries paved the way for electrical batteries; Volta's cell was named an IEEE Milestone in 1999.[5]

Some forty years later Faraday (see Faraday's laws of electrolysis) showed that the galvanic cell -- now often called a voltaic cell -- was chemical in nature. Faraday introduced new terminology to the language of chemistry: electrode (cathode and anode), electrolyte, and ion (cation and anion). Thus Galvani incorrectly thought the source of electricity (or source of emf, or seat of emf) was in the animal, Volta incorrectly thought it was in the physical properties of the isolated electrodes, but Faraday correctly identified the source of emf as the chemical reactions at the two electrode-electrolyte interfaces. The authoritative work on the intellectual history of the voltaic cell remains that by Ostwald.[6]

It was suggested by Wilhelm König in 1940 that the object known as the Baghdad battery might represent galvanic cell technology from ancient Parthia. Replicas filled with citric acid or grape juice have been shown to produce a voltage. However, it is far from certain that this was its purpose—other scholars have pointed out that it is very similar to vessels known to have been used for storing parchment scrolls.[7]

Basic description

Schematic of Zn-Cu galvanic cell

In its simplest form, a half-cell consists of a solid metal (called an electrode) that is submerged in a solution; the solution contains cations (+) of the electrode metal and anions (-) to balance the charge of the cations. The full cell consists of two half-cells, usually separated by a semi-permeable membrane or by a salt bridge.

A specific example is the Daniell cell, where a salt bridge is used as separator (see figure). The zinc (Zn) half-cell has a solution of ZnSO4 (zinc sulfate) and the copper (Cu) half-cell has a solution of CuSO4 (copper sulfate).

Let an external electrical conductor connect the Cu and Zn electrodes. In the Zn half-cell, Zn from the Zn electrode dissolves into the solution as Zn2+ ions (oxidation), releasing electrons that enter the external conductor. In addition, via the salt bridge Zn ions leave and sulfate ions (SO4(2-)) enter the Zn half-cell.

In the Cu half-cell, the Cu ions plate onto the Cu electrode (reduction), taking up electrons that leave the external conductor. Since the Cu(2+) ions (cations) plate onto the Cu electrode, the latter is called the cathode. Correspondingly the Zn electrode is the anode. The electrochemical reaction is:

Zn + Cu2+
Zn2+
+ Cu

In addition, electrons flow through the external conductor, which is the primary application of the galvanic cell.

As discussed under Cell Voltage, the emf of the cell is the difference of the half-cell potentials, a measure of the relative ease of dissolution of the two electrodes into the electrolyte. The emf depends on both the electrodes and on the electrolyte, an indication that the emf is chemical in nature.

Detailed description

In essence, a half-cell contains a metal in two oxidation states; inside an isolated half-cell, there is an oxidation-reduction (redox) reaction that is in chemical equilibrium, a condition written symbolically as follows (here, "M" represents a metal cation, an atom that has a charge imbalance due to the loss of "n" electrons):

Mn+ (oxidized species) + ne ⇌ M (reduced species)

A galvanic cell consists of two half-cells, such that the electrode of one half-cell is composed of metal A, and the electrode of the other half-cell is composed of metal B; the redox reactions for the two separate half-cells are thus:

An+ + ne ⇌ A
Bm+ + me ⇌ B

In general, then, these two metals can react with each other:

m A + n Bm+n B + m An+

In other words, the metal atoms of one half-cell are able to induce reduction of the metal cations of the other half-cell; conversely stated, the metal cations of one half-cell are able to oxidize the metal atoms of the other half-cell. When metal B has a greater electronegativity than metal A, then metal B tends to steal electrons from metal A (that is, metal B tends to oxidize metal A), thus favoring one direction of the reaction:

m A + n Bm+ ${\displaystyle \rightarrow }$ n B + m An+

This reaction between the metals can be controlled in a way that allows for doing useful work:

• The electrodes are connected with a metal wire in order to conduct the electrons that participate in the reaction.
In one half-cell, dissolved metal-B cations combine with the free electrons that are available at the interface between the solution and the metal-B electrode; these cations are thereby neutralized, causing them to precipitate from solution as deposits on the metal-B electrode, a process known as plating.
This reduction reaction causes the free electrons throughout the metal-B electrode, the wire, and the metal-A electrode to be pulled into the metal-B electrode. Consequently, electrons are wrestled away from some of the atoms of the metal-A electrode, as though the metal-B cations were reacting directly with them; those metal-A atoms become cations that dissolve into the surrounding solution.
As this reaction continues, the half-cell with the metal-A electrode develops a positively charged solution (because the metal-A cations dissolve into it), while the other half-cell develops a negatively charged solution (because the metal-B cations precipitate out of it, leaving behind the anions); unabated, this imbalance in charge would stop the reaction.
• The solutions are connected by a salt bridge or a porous plate that allows metal ions to pass from one solution to the other, which balances the charges of the solutions and allows the reaction to continue.

By definition:

• The anode is the electrode where oxidation (loss of electrons) takes place; in a galvanic cell, it is the negative electrode, as when oxidation occurs, electrons are left behind on the electrode.[8] These electrons then migrate to the cathode (positive electrode). However, in electrolysis, an electric current stimulates electron flow in the opposite direction. Thus, the anode is positive, and the statement anode attracts anions is true (negatively charged ions flow to the anode, while electrons are expelled through the wire). The metal-A electrode is the anode.
• The cathode is the electrode where reduction (gain of electrons) takes place; in a galvanic cell, it is the positive electrode, as less oxidation occurs, fewer ions go into solution, and fewer electrons are left on the electrode.[8] Instead, there is a greater tendency for aqueous ions to be reduced by the incoming electrons from the anode. However, in electrolysis, the cathode is the negative terminal, and attracting positive ions from the solution. In this situation, the statement the cathode attracts cations is true (positively charged, oxidized metal ions flow toward cathode as electrons travel through the wire). The metal-B electrode is the cathode.

Galvanic cells are typically used as a source of electrical power. By their nature, they produce direct current. The Weston cell has an anode composed of cadmium mercury amalgam, and a cathode composed of pure mercury. The electrolyte is a (saturated) solution of cadmium sulfate. The depolarizer is a paste of mercurous sulfate. When the electrolyte solution is saturated, the voltage of the cell is very reproducible; hence, in 1911, it was adopted as an international standard for voltage.

A battery is a set of galvanic cells that are connected together to form a single source of voltage. For instance, a typical 12V lead–acid battery has six galvanic cells connected in series with the anodes composed of lead and cathodes composed of lead dioxide. Large battery rooms, for instance in a telephone exchange providing central office power to user's telephones, may have cells connected in both series and parallel.

Cell voltage

The standard electrical potential of a cell can be determined by the use of a standard potential table for the two half cells involved. The first step is to identify the two metals reacting in the cell. Then one looks up the standard electrode potential, E0, in volts, for each of the two half reactions. The standard potential for the cell is equal to the more positive E0 value minus the more negative E0 value.

For example, in the figure above the solutions are CuSO4 and ZnSO4. Each solution has a corresponding metal strip in it, and a salt bridge or porous disk connecting the two solutions and allowing SO42− ions to flow freely between the copper and zinc solutions. To calculate the standard potential one looks up copper and zinc's half reactions and finds:

Cu2+ + 2
e
⇌ Cu: E0 = +0.34 V
Zn2+ + 2
e
⇌ Zn: E0 = −0.76 V

Thus the overall reaction is:

Cu2+ + Zn ⇌ Cu + Zn2+

The standard potential for the reaction is then +0.34 V − (−0.76 V) = 1.10 V. The polarity of the cell is determined as follows. Zinc metal is more strongly reducing than copper metal; equivalently, the standard (reduction) potential for zinc is more negative than that of copper. Thus, zinc metal will lose electrons to copper ions and develop a positive electrical charge. The equilibrium constant, K, for the cell is given by

${\displaystyle \ln K={\frac {nFE^{0}}{RT}}}$

where F is the Faraday constant, R is the gas constant and T is the temperature in kelvins. For the Daniell cell K is approximately equal to 1.5×1037. Thus, at equilibrium, a few electrons are transferred, enough to cause the electrodes to be charged.[9]

Actual half-cell potentials must be calculated by using the Nernst equation as the solutes are unlikely to be in their standard states,

${\displaystyle E_{\text{half-cell}}=E^{0}-{\frac {RT}{nF}}\ln _{e}Q}$

where Q is the reaction quotient. This simplifies to

${\displaystyle E_{\text{half-cell}}=E^{0}-2.303{\frac {RT}{nF}}\log _{10}\{M^{n+}\}}$

where {Mn+} is the activity of the metal ion in solution. The metal electrode is in its standard state so by definition has unit activity. In practice concentration is used in place of activity. The potential of the whole cell is obtained by combining the potentials for the two half-cells, so it depends on the concentrations of both dissolved metal ions.

The value of 2.303R/F is 0.19845×10−3 V/K, so at 25 °C (298.15 K) the half-cell potential will change by ${\displaystyle {0.05918\ {\text{V}}}/{n}}$ if the concentration of a metal ion is increased or decreased by a factor of 10.

${\displaystyle E_{\text{half-cell}}=E^{0}-{\frac {0.05918\ {\text{V}}}{n}}\log _{10}[M^{n+}]}$

These calculations are based on the assumption that all chemical reactions are in equilibrium. When a current flows in the circuit, equilibrium conditions are not achieved and the cell potential will usually be reduced by various mechanisms, such as the development of overpotentials.[10] Also, since chemical reactions occur when the cell is producing power, the electrolyte concentrations change and the cell voltage is reduced. A consequence of the temperature dependency of standard potentials is that the voltage produced by a galvanic cell is also temperature dependent.

Galvanic corrosion

Galvanic corrosion is a process that degrades metals electrochemically. This corrosion occurs when two dissimilar metals are located in contact with each other in the presence of an electrolyte, such as salt water, forming a galvanic cell. A cell can also be formed if the same metal is exposed to two different concentrations of electrolyte. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material.

Cell types

6. ^ Ostwald, Wilhelm (1896, Veit & Co., Leipzig). Electrochemistry: History and Theory. translated from German, New Delhi: Amerind Publishing Co.; Springfield, Va, 1980, 2 vols. Check date values in: |year= (help)