An electro-galvanic fuel cell is an electrchemical device which consumes a fuel to produce an electrical output by a chemical reaction.
One form of electro-galvanic fuel cell based on the oxidation of lead is commonly used to measure the concentration of oxygen gas in underwater diving and medical breathing gases.
Video Electro-galvanic fuel cell
Operation
Oxygen sensors
A chemical reaction occurs in the fuel cell when the potassium hydroxide in the cell comes into contact with oxygen. This creates an electric current between the lead anode and the gold-plated cathode through a load resistance. The current produced is proportional to the concentration (partial pressure) of oxygen present.
They are used in oxygen analysers in technical diving to display the proportion of oxygen in a nitrox or trimix breathing gas before a dive. They are also used in electronic, closed-circuit rebreathers to monitor the oxygen partial pressure during the dive.
The partial pressure of oxygen in diving chambers and surface supplied breathing gas mixtures can also be monitored using these cells. This can either be done by placing the cell directly in the hyperbaric environment, wired through the hull to the monitor, or indirectly, by bleeding off gas from the hyperbaric environment or diver gas supply and analysing at atmospheric pressure, then calculating the partial pressure in the hyperbaric environment. This is frequently required in saturation diving and surface oriented surface supplied mixed gas commercial diving.
Electro-galvanic fuel cells have a limited lifetime which is reduced by exposure to high concentrations of oxygen. The reaction between oxygen and lead at the anode consumes lead, which eventually results in the cell failing to sense high concentrations of oxygen. Typically, a cell used for diving applications will function correctly for 3 years if stored in a sealed bag of air but only for four months if stored in pure oxygen.
Maps Electro-galvanic fuel cell
Cell limitations
Oxygen cells behave in a similar way to electrical batteries in that they have a finite lifespan which is dependent upon use. The chemical reaction described above causes the cell to create an electrical output that has a predicted voltage which is dependent on the materials used. In theory they should give that voltage from the day they are made until they are exhausted, except that one component of the planned chemical reaction has been left out of the assembly: oxygen.
Oxygen is one of the fuels of the cell so the more oxygen there is, the more electricity is generated. The chemistry sets the voltage and the fuel, the oxygen, sets how much electric current it can give. If you put an electric circuit on the cell that draws current you can draw up to this current but ask for more and the voltage from the cell fades.
Failures in cells can be life-threatening for technical divers and in particular, rebreather divers. The failure modes common to these cells are: failing with a higher than expected output due to electrolyte leaks, current limitation due to exhausted cell life and non linear output across its range. These failures are usually attributable to physical damage, contamination during manufacture or defects in manufacture.
Failing high is invariably a result of a manufacturing fault or mechanical damage. In rebreathers, failing high will result in the rebreather assuming that there is more oxygen in the loop than there actually is which results in hypoxia.
Current limited cells do not give a high enough output in high concentrations of oxygen. The rebreather assumes there is insufficient oxygen in the loop and injects to reach a setpoint the cell will never achieve resulting in hyperoxia.
Non-linear cells do not perform in an expected manner across its range of oxygen partial pressures. Calibration will not pick up this fault which results in inaccurate loop contents of a rebreather. This gives the potential for decompression illness.
Preventing accidents in rebreathers from cell failures is possible in most cases by accurately testing the cells before use. Some divers carry out in-water checks by pushing the oxygen content in the loop to a pressure that is above that of pure oxygen at sea level to indicate if the cell is capable of high outputs. This test is only a spot check and does not accurately assess the quality of prediction of failure of that cell. The only way to accurately test a cell is with a calibrated test chamber which can hold a static pressure without deviation and the ability to log the results and graph them.
Testing
The first certified cell checking device that was commercially available was launched in 2005 by Narked at 90 but did not achieve commercial success. A much revised model was released in 2007 and won the "Gordon Smith Award" for Innovation at the Diving Equipment Manufacturers Exhibition in Florida. Narked at 90 Ltd won the Award for Innovation for the Development of Advanced Diving products at Eurotek 2010 for the Cell Checker and its continuing Development. Now used throughout the world by organisations such as Teledyne/Vandegraph National Oceanic and Atmospheric Administration, NURC (NATO Underwater Research Centre) and Diving Diseases Research Centre.
Oxygen sensors
Electronically monitored diving rebreather systems and many medical life-support systems use galvanic O2 sensors, which are metal/air galvanic cells where oxygen molecules are dissociated and reduced to hydroxyl ions at the cathode. These diffuse through the electrolyte and oxidize the metal anode. A current proportional to the rate of oxygen consumption is generated when the cathode and anode are electrically connected through a resistor
The cell reaction for a lead/oxygen cell is: 2Pb+O2=2PbO, made up of the cathode reaction: O2+2H2O+4e-4OH, and anode reaction: 2Pb+4OH-2PbO+2H2O + 4e.
The cell current is proportional to the rate of oxygen reduction at the cathode, but this is not linearly dependent on the partial pressure of oxygen in the gas to which the cell is exposed: This is achieved by placing a diffusion barrier between the gas and the cathode, which limits the amount of gas reaching the cathode to an amount that can be fully reduced without significant delay, making the partial pressure in the immediate vicinity of the electrode close to zero. As a result of this the amount of oxygen reaching the electrode follows Fick's law of diffusion and is proportional to the partial pressure in the gas beyond the membrane. This makes the current proportional to PO2. The load resistor over the cell allows the electronics to measure a voltage rather than a current. This voltage depends on the construction and age of the sensor, and typically varies between 7 and 28 mV for a PO2 of 0.21 bar
Diffusion is linearly dependent on the partial pressure gradient, but is also temperature dependent, and the current rises about two to three percent per kelvin rise in temperature. A negative temperature coefficient resistor is used to compensate, and for this to be effective it must be at the same temperature as the cell. Oxygen cells which may be exposed to relatively large or rapid temperature changes, like rebreathers, generally use thermal conductive paste between the temperature compensating circuit and the cell to speed up the balancing of temperature.
The sensor should be placed in the rebreather where a temperature gradient between the gas and the electronics in the back of the cell will not occur. Temperature also affects the signal response time, which is generally between 6 and 15 seconds at room temperature for a 90% response to a step change in partial pressure. Cold cells react much slower and hot cells much faster. As the anode material is oxidised the output current drops and eventually will cease altogether. The oxidation rate depends on the oxygen reaching the anode from the sensor membrane. Lifetime is measured in oxygen-hours, and also depends on temperature and humidity
Failure modes
When new, a sensor can produce a linear output for over 4 bar partial pressure of oxygen, and as the anode is consumed this drops, eventually to below the range of partial pressures which may be expected in service, at which stage it is no longer fit to control the system. This state is called current-limited. When a current limited sensor can no longer reliably activate the control system at the upper set-point in a life support system, there is a severe risk of an excessive oxygen partial pressure occurring which will not be noticed, which can be life-threatening. Other failure modes include mechanical damage, such as broken conductors, corroded contacts and loss of electrolyte due to damaged membranes
Lifetime
There are two commonly used ways to specify expected sensor life span: The time in months at room temperature in air, or volume percentage oxygen hours (Vol%O2h). Storage at low oxygen partial pressure when not in use would seem an effective way to extend cell life, but when stored in anoxic conditions the sensor current will cease and the surface of the electrode may be passivated, which can lead to sensor failure. High ambient temperatures will increase sensor current, and reduce cell life In diving service a cell typically lasts for 12 to 18 months, with perhaps 150 hours service in the diving loop at an oxygen partial pressure of about 1.2 bar and the rest of the time in storage in air at room temperature.
See also
- Glossary of fuel cell terms
References
Source of the article : Wikipedia
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