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Natural Science Forum / Chemistry / Electrochemistry / July 2008Some thoughts on the Firefly cell
Probably naive and ignorant thoughts, but how else does one learn ... 1) The surface area to volume ratio is no greater for a convoluted surface than a flat plate. So how is the sponge electrode an advantage? over a flat plate of the same mass, that is. 2) Burnt cellulose is not very sturdy and would not last long in Caterpillar tractor use. 3) Why do you need a collector (or solid metallic anode) when you have an electrolyte? Aren't electrons practically everywhere in an electrolyte? Stick a carbon rod in one side as your collector. 3a) which, if true, eliminates the main disadvantage of the old cuprous oxide photovoltaic cell: the cost of the flat copper cathode. 4) Nickel dioxide has a higher potential than lead dioxide. Has anybody tried a lead-nickel cell? About fifty years ago, in the village of Sparksville, Indiana near where I live, one garage industry was melting anodes from exhausted lead-acid car batteries and casting the lead into ingots, to be recycled. I suspect they buried the cathodes, polluting the soil, which might explain the level of intelligence in Sparksville today. I live two miles up the road and my water doesn't come from there. So I don't have that excuse. Ed, I would not be so pessimistic about lead-acid. See more specific comments: > Probably naive and ignorant thoughts, but how else does one learn ... > 1) The surface area to volume ratio is no greater for a convoluted surface > than a flat plate. So how is the sponge electrode an advantage? over a > flat plate of the same mass, that is. You are assuming that diffusion from plate to liquid is what matters. But diffusion in liquid has negligible effect on battery impedance as it is already very low. What is limiting is the reaction rate on the surface of material and diffusion inside material particles. Both these factors are benefited by having highly porous electrodes and small particle size. > 2) Burnt cellulose is not very sturdy and would not last long in > Caterpillar tractor use. True. Apparently interesting part of Firefly technology is that they reinforce the glass-carbon to make it less brittle. > 3) Why do you need a collector (or solid metallic anode) when you have an > electrolyte? Aren't electrons practically everywhere in an electrolyte? > Stick a carbon rod in one side as your collector. No, there is no electrons in electrolyte, only ions. Electrochemical reaction happens on metal/electrolyte surface, that transfers and electron to a molecule in electrolyte, that makes an ion. This reaction is slow (electrically = high impedance), that is why large surface of metal reduces battery impedance. > 3a) which, if true, eliminates the main disadvantage of the old cuprous > oxide photovoltaic cell: the cost of the flat copper cathode. Well this battery has other issues, like very low efficiency and rate capability.. > 4) Nickel dioxide has a higher potential than lead dioxide. Has anybody > tried a lead-nickel cell? Actually PbO2 is one of the strongest oxidizers known in aqueous chemistry. That is why Pb/PbO2 battery has 2.4V potential vs only 1.2 of nickel- cd battery. Which means that PbO2 is definitely stronger oxidizer than NiO2. What would be interesting is to couple PbO2 with Zn anode, this would give a battery with voltage close to 3V. Would beat the hell out of silver-zinc in terms of energy and price. Too bad PbO2 does not work very well in base environment needed for zink, but this could be fixed with better electrolyte choice. > About fifty years ago, in the village of Sparksville, Indiana near where I > live, one garage industry was melting anodes from exhausted lead-acid car > batteries and casting the lead into ingots, to be recycled. I suspect they > buried the cathodes, polluting the soil, which might explain the level of > intelligence in Sparksville today. Lead recycling from lead-acid battery is the one and only perfect recycling program in the world. It recovers more than 90% of all lead used in lead-acid batteries. http://www.leadacidbatteryinfo.org/lead-acid-battery-recycling.htm The beauty of Lead-acid battery that makes it so suitable for recycling is that it has only one metal (both cathode and anode) so both can be recycled in similar manner (after reduction of oxide with suitable reducing agent, such as carbon or hydrogen). Basically lead- acid battery is a highly enriched lead oar. > I live two miles up the road and my water doesn't come from there. So I > don't have that excuse. Regards, Yevgen > No, there is no electrons in electrolyte, only ions. Electrochemical > reaction [quoted text clipped - 3 lines] > impedance), that is why large surface of metal reduces battery > impedance. Thanks for the comments. I still don't understand the above. When you pass a current through an electrolyte, say aqueous sodium chloride, the electrons travel from the anode to the cathode. You don't reduce Na+ to Na and make Cl2 from Cl- (at low voltage). If in an electromotive cell you have a solid metal anode, or one plated onto a collector, I can see how the electrons will travel through it instead of jumping off into the electrolyte. However, in a flow cell or the cathode half-cell of a Daniell, Grove, or Bichromate Cell, the active material is in solution. Does the reduction and oxidation take place only on the surface of the electrode? The Daniell cell in its original form was progressively de-activated by the copper plating out on the porous separator. That would indicate that the cathode reaction takes place there, not on the electrode. > > No, there is no electrons in electrolyte, only ions. Electrochemical > > reaction [quoted text clipped - 8 lines] > chloride, the electrons travel from the anode to the cathode. You don't > reduce Na+ to Na and make Cl2 from Cl- (at low voltage). No, electrons absolutely can NOT pass through electrolyte (unless we are talking high-frequencies in MHz range where we only recharging capacitor on the surface. The way it works in your example is: + electrode: Cl- ---> Cl2 + e- - electrode: Na+ + e- --> Na; 2Na + 2H2O -- > 2NaOH + O2 Indeed this will not happen at very low voltage (below 0.5V), that is why you will have NO current through NaCl below activation threshold of this reaction. Voltages we deal in electronics are way above this threshold, so that is why we think NaCl is conductive. >If in an > electromotive cell you have a solid metal anode, or one plated onto a > collector, I can see how the electrons will travel through it instead of > jumping off into the electrolyte. Nope, electrolytes are perfect "electron" insulators below lowest voltages of any electrochemical reaction that can take place. In many cases lowest activation voltage will correspond to impurities, sometimes organic once or dissolved CO2. >However, in a flow cell or the cathode > half-cell of a Daniell, Grove, or Bichromate Cell, the active material is > in solution. Does the reduction and oxidation take place only on the > surface of the electrode? ...as it is in all cases. That is why electrode surface is always limiting for electrochemical reaction (unless ionic conductivity of electrolte is very bad, or distance between electrodes is so large that IR drop along it becomes >> than IR drop due to electrochemical reaction) >The Daniell cell in its original form was > progressively de-activated by the copper plating out on the porous > separator. That would indicate that the cathode reaction takes place > there, not on the electrode. No it takes place still on the electrode, but active material has to diffuse from the bulk of electrolyte to the surface of electrode to react. That is main reason why such batteries have relatively bad kinetics - even if surface of electrodes is high, reaction rate is limited by delivery of active materials from the bulk. Often they use actively pumped flow or active mixing to accelerate this delivery. Regards, Yevgen |
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