Talk:Fuel cell/Battery?
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My rewrite of the first section of Fuel Cell includes two kinds of changes:
- Corrections of misunderstandings on the part of editors who accepted their schools' compromised notions of what a liberal education requires in the way of hard sciences, and
- Specifically asserting that a fuel cell is not simply *like* a battery, but *is* a battery that has special additional characteristics.
The first take-away idea i am asserting here is that even if i'm wrong about the battery part, reversion should be selective: look at what i changed that isn't explained by the battery issue, and don't put back the ignorant errors i fixed.
Note i am not calling for a change in usage, but rather asserting that everything that is true of all batteries other than fuel cells, is also true of fuel cells. I assert that the connection between fuel cells and batteries is not an analogy (subject to fallacious conclusions "at the edges" of the analogy's relevance), but a matter of the laws of other batteries being laws of fuel cells as well, and of reliable statements about all batteries being safely extendible to fuel cells, by those not inclined to understand all the ins and outs of batteries that lead to the laws of batteries.
My second take-away idea is that if i'm wrong about the battery part, reversion of that part of my edit is only part of the solution: if i'm wrong, then the information in the article is inadequate. If a fuel cell is merely a battery analog (and not an unconventional, but nevertheless true, battery), the article needs at least enuf evidence of that to guide a reader with a solid (tho pre-fuel-cell) physical-science background to the convincing details of the difference. -- Jerzy 14:11, 2003 Dec 10 (UTC)
- According to battery (electricity) and every other definition I've seen, a battery is purely an energy storage device. A fuel cell is an open-ended energy conversion device. Mkweise 01:31, 18 Feb 2004 (UTC)
Mk, even tho we don't have a Primary cell article to go with Secondary cell, battery (electricity)#Common battery types, in its 2nd 'graph, discusses them, and i think it'd be interesting to know if you consider them "purely" ESDs.
I suspect that you have not worked thru the implications of the two terms ECD and ESD. IMO, nearly all ESDs are ECDs. For example, the most obvious forms of ESD are, for me,
- secondary cells, also called storage batteries, and
- pumped-storage power plants.
The old standby storage battery, the lead-acid automotive battery, is still in use after the most-of-a-century since people stopped cranking their engines by hand. It receives energy in the form of current at (these days) approximately 12 volts DC, from the alternator (nowadays) and converts it into chemical energy. (IIRC, it converts lead sulfate in one electrode of a cell into lead metal in the other cell and sulfuric acid in the "battery acid" electrolyte, which i think is why you test the specific gravity of the electrolyte with hydrometer to see how thoroughly charged it is. But i am not confident about such details.) Later, when the engine is idling or stopped, and you're listening to the radio and making out, chemical energy is converted back to electrical (running the battery in a figurative opposite direction, and a literal opposite direction of current, to run the ignition system, lights, and/or radio.
A pumped storage plant works away at night consuming the electricity, that the nearest nuclear plant is capable of producing at the level of X megawatts 24 hours a day (at an insignificantly greater cost than letting it sit idle), but can't sell to people who are sleeping. The PS plant runs its generators (figuratively) "backwards" as motors, driving pumps to move water up to a high altitude reservoir. Come daylight, at least, and better yet, evening peak-demand time, it lets the water run out of the reservoir, down thru its turbines, which turn the generators and generate power. It has converted electrical energy into gravitational potential energy, and then done the opposite conversion to generate electricity again.
Note that in both cases energy is converted from one form to another, and back again, and that one of the forms of energy is practical to store. Aren't both these systems ECDs? The reservoir is the ESD component of the pumped storage plant, and the electrodes and/or electrolyte of the batter are the ESD component(s) of the storage battery. (On the other hand, the wiring between cells of the battery, and the cases of the cells and the battery as a whole are not energy storage components; the wiring is involved only in the conversion, and the cases merely physically confine the electrolyte; the energy does not actually reside in the material of the cases, but (as with a bottle of gasoline) in the material that is mechanically confined by the cases.)
I will agree with you that the fuel cell cannot be reasonably construed as an ESD: yes, it holds fuel (i presume) while waiting for a demand for electricity, but IMO that is more in transit than stored; the energy storage is basically in the fuel tank that supplies the fuel cell. We could discuss hypothetical "secondary" fuel cells: instead of charging a battery when you step on the regenerataive braking system of your electric car, let the current generated run right back into the secondary fuel cell, which would, say, electrolyze the water that ran out of it a few minutes ago, and hold the resulting hydrogen for later consumption. If the fuel cell could run in "both directions", it would function as an ESD.
But a better approach is to think about a system with a primary battery that is is not storing energy, but only converting it from chemical to electrical form. It's tempting to say that just disconnecting the charging system doesn't change the function of the battery, but instead of that, let's do the more thorough thought experiment of a lead-acid primary battery running your electrical car. You've got two batteries with one active and the other disconnected at any given moment, and periodically they change roles. You, or your robot battery restorer, is at work on the disconnected battery: the depleted electrolyte gets dumped in the road and replaced from your sulphuric acid tank, while the sulphated electrodes get pulled out and stuffed in the trunk (for trade in), and fresh lead-metal electrodes, delivered like belted machine-gun ammunition, get installed in their place, so that battery is rebuilt and ready to take its turn as active battery. The tank and electrode magazine are storing energy as the fuel-cell tank does, but the electrolyte and electrodes in the lead-acid ECD have an energy storage function no more than is does the fuel in the space between the electrodes of the fuel cell; this kind of storage is merely incidental to the ECD process. Thus i think we have an "open ended", ESD-free, lead-acid battery ECD.
(I haven't checked the battery (electricity) article to be sure whether you've been misinformed by it or misinterpreted it; if you want to point out specifics, i'd be willing to express an opinion as to which applies.)
Does that help? --Jerzy 08:22, 2004 Feb 18 (UTC)
Glancing again at how you edited me, i think what is implicit in the article but not touched on in what i say above is what i consider the hallmark of batteries: not energy storage, but electrochemical reactions that produce free electrons at one site, and capture (consume) them at another, thereby forcing migration of positive ions in one direction and negative in the other to limit the buildup of charge, reaction-product concentrations, or both. The free electrons are the current produced, and the internal ionic currents complete the circuit thru the circuit's electrolyte portion (inside the battery).
Note that i am far from being up to speed on fuel cells, but i am quite solid on the physical laws that will not have changed. (We're not, for instance, talking about cold fusion here; if new principles in chemistry had been involved, i'm confident i'd have heard about that.) My impression is that fuel cell reactions are probably more complex than the battery reactions that get taught in undergrad electrochemistry (e.g. probably multistage); IMO that is implied by the covalently bonded fuels i hear mentioned, in contrast to the ionic reactants of typical batteries. That diffence may justify thinking of fuel cells as different critters from batteries, just as blast furnaces are (in practice) different from blacksmith's forges. But i am confident that fuel cells are essentially batteries and subject to the same laws, just as blast furnaces are essentially forced-air oxidation systems subject to the same laws as forges. You don't have to worry about the covalent reactions in mere batteries, and you don't have to worry about approaching equilibrium in mere forges, but you still have to worry, with fuel cells and blast furnaces, about the same things you do with batteries and forges. --Jerzy 08:55, 2004 Feb 18 (UTC)
- From the viewpoint of pure physics (which you seem to be taking), batteries and fuel cells would both be referred to as electrochemical cells. They are fundamentally the same, just as motors and generators are - but from a functional viewpoint they are completely different: One is a closed system that holds an exhaustible supply of energy, while the other depends on a continuous external fuel supply. Think of a syringe vs. an IV line. Or, to use your own analogy: you wouldn't think of saying that a blast furnace is a type of forge (or that a forge is a type of blast furnace)—would you?
- And yes, all ESDs except capacitors internally employ two-way energy conversion but that, again, is beside the point as these are also functionally (as opposed to fundamentally) defined terms. Mkweise 10:00, 18 Feb 2004 (UTC)
Here's my opinion, on the fuel cell =? battery issue on special request by Jerzy. :)
The discussion sounds a bit like mathematical discussions I had in high school. Should a 3-dimensional axis frame be with the x left or right? What is sqrt(-1)? Some people had heard about imaginary numbers. Then is sqrt(-1) plus or minus i? In these arguments, it always sounded as if there were one absolute truth, while in fact the issue was that the definition was incomplete. Or what about biological species? Whatever way you choose to categorize things, there will always be boundary cases.
I believe that the definition of what constitutes a battery is too vague to resolve this particular argument. So, choose either "a kind of battery" or "similar to a battery" in the description of a fuel cell. In both cases, describe the important features that distinguish a fuel cell from what is commonly called a battery, mainly the fact that the latter normally is a chemically closed system. But then, a zinc-air battery for hearing aids isn't closed either.
Hankwang 17:04, 23 Feb 2004 (UTC)
Just wanted to add my "two cents" to the battery versus something else discussion. First it has always been helpful to me to consider a traditional secondary battery from the standpoint of simple first year college biology. If you consider the ATP cycle, you start to get a very good view of the battery function. If the secondary battery were simply a storage device such as a capacitor then efficiency would be a simple equation. In fact efficiency lost would translate into heat. In other words if one measured the heat change in a battery during operation one could determine the overall storage efficiency. The problem is this is not what actually happens. Like the human body the battery stores energy by converting chemicals from low potential energy(stable) forms to higher potential energy (less stable forms) and like the human body is subject to all manner of competing reactions. The competition is very much subject to internal and environmental conditions. For example: As a battery approaches full state of charge and the voltage raises additional reactions (which typically require more energy) become possible. This can be seen through gassing and corrosion reactions. Hence, much like the human body, the battery efficiency is very difficult to model (trust me I and many brighter than I have tried). Another point to support this view is the capability of the secondary battery to hold charge (resist self-discharge). Again if the battery were simply a storage device it would be subject to a predictable loss rate. This rate would be linked, as in the case of the Nickel Metal Hydride Anode (a hydrogen storage media)to physical phenomina such as available surface area or molecular bond strength. These are easy to model characteristics and therefore it would be practical to model this type of behavior over a variety of conditions. Again, in the case of a battery this proves untrue. The main factors that affect the stability during storage of a traditional battery are: 1) the stability of the high potential energy chemical state, the degree of seperation of the electrodes and the conductivity of the electrolye. Again if one considers the human body, some forms of energy are easily consumed (such as carbohydrates) with physical activity, and others (such as protein) are more suited to slow use. Also the human body consumes more energy when it is warm due to internal activities. While these may not be the best analogies they do serve to provide some basis to understand the effect of very unstable chemicals (such as some of the early lithium batteries) which led to poor shelf life and low density electrolytes (such as early Nickel Metal Hydride batteries which led to the same conclusion. But what about primary batteries? Are they merely storage devices and therefore in a unique class? My answer is emphatically no. This is for all the reasons that I listed above. The only real difference between the primary and secondary battery is the reversibility of the coversion. This is clearly evidenced by the alkaline rechargable cells produced by Ray-o-Vac which, used traditional primary chemistry in a secondary cell. In fact given enough energy and the proper conditions any primary battery could be "recharged". In most cases the real problem is the energy balance is skewed in a way that would make this costly and impractical. In my opinion, the problem that most have when viewing the fuel cell is truely linked to poor terminology. I view the fuel cell has closer to a primary battery than any thing else. This can be seen when one applies the proper terminology to the technology. First it has become traditional to call the membrane and catalyst the cathode and anode and this is not correct from a battery view. To be technically correct one should view Hydrogen as the anode and Oxygen as the cathode. I understand that this is a somewhat simplified view that does offer some problems in practice but would ask the reader to consider the Nickel Hydrogen battery in which gaseous Hygrogen is truely considered the Anode. In this battery gaseous Hydrogen is the electron donor and the reaction is facilitated by a platinum substrate. In more traditional batteries (such as the ever popular lead acid battery) the anode and cathode are defined as the substrate in combination with the electron donor and receptors. In the case of the fuel cell the membrane and catalyst play a role that is much more similar to that of a "smart" battery seperator. This type of separator is not unique to the fuel cell. One need only to look at the advanced Lithium Ion batteries to see similar separators which serve dual functions of limiting and allowing chemical transport. If you do then accept this view then the fuel cell can be seen as a primary cell with a non-reversable conversion reaction and a constant replacement of the anode and cathode through refueling and and constant expulsion of the expended materials. Under this view the major difference between a fuel cell and a traditional battery is the fact that both the anode and cathode are gases where in traditional battery technology at least one of the participants is in the form of a solid. bblakemo@ford.com 8/10/2004
