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Metal Air Fuel Cell
The Metal Air Fuel Cell Principle is one step closer to a battery than hydrogen-air fuel cells. The metal fuel acts as a fuel, anode and current collector. The electrolyte used is typically Potassium Hydroxide (KOH) which is strongly alkali and hence they could be classified as ALKALI fuel cells. Fuel can be loaded continuously in pellet form or intermittently in preformed porous blocks. Metals used are (in order of electrochemical energy equivalent) lithium, aluminium, magnesium, calcium, zinc, iron). Zinc and aluminium are the favoured metals due to their abundance, high yield and relative ease of use. Aluminium has the benefit of releasing 3 electrons per molecule (compared with 2 from hydrogen or zinc), which increases the specific energy (both volumetric and gravimetric). Typically the oxidant is atmospheric oxygen.
The technology is less developed than other fuel cell technologies and less well known. The reason for this is unclear, but may be due to the near universal abandonment of hydrogen-air alkali fuel cells, which is perhaps why those with an interest in them definie them by fuel type rather than by electrolyte.
A variant on typical planar metal air fuel cells is:
The
Centrifugal Metal-Air Fuel Cell Principle
This is a technical innovation rather than a scientific innovation.
It is not a new type of fuel cell but a new way of building fuel cells.
The benefits of the centrifugal principle are primarily hydrodynamic.
A centrifugal configuration also greatly reduces some of the balance of plant
issues which hamper alkali fuel cells.
Advantages
1. Simplification of Balance of Plant – Pumps, piping, etc for
the corrosive electrolyte are eliminated, circulation achieved instead by
‘natural’ buoyancy driven flow.
2. Improved Power Control – Variation of centrifuge spin speed allows power output to be controlled by controlling electrolyte flow.
3. Increased Power Density – Use of porous carbon air electrodes (3-D rather than usual 2-D) gives much greater surface area per unit volume, which coupled with increased oxygen flow due to local acceleration and bubbling of oxygen at the three phase boundary (improving electrochemical reaction rates) eliminates this rate limiting step (?).
4. Reduced Polarisation Losses – Better ion transfer from electrode surfaces and with flow of electrolyte results in reduced polarisation and hence higher terminal voltage
5. Regenerative – Like all metal air fuel cells products can be electrolyticaly regenerated. Little information available on cycle efficiency but for non-centrifugal zinc-air 40-60% is specified by Metalic Power Inc.
Issues
1. Aluminium and zinc are energy intensive and environmentally nasty to extract.
2. Cycle efficiency is not too impressive, though it is probably comparable
with other technologies on the market
3. Carbon Dioxide poisoning is problematic and hence must be removed by formation
of CaO3 from CaO.
4. Strong Alkali Liquid Electrolyte is always going to be problematic.
Applications
Everything I’ve seen has been automotive and/or small scale generation
oriented. Metal air batteries are already widely used for the likes
of hearing aids and the fuel cell implementation of the principle simply allows
for separation of fuel from generator hence increasing life and allowing easy
regeneration. In terms of specific energy they seem to be about as close
as you can get to hydrocarbon fuels in conjunction with combustion engines.
References:
Centrifugal Fuel Cells
Brian Worth, 'The Centrifugal Fuel Cell Concept – Prospects for the
Future of Vehicle Propulsion’, Presented at Eurotech 99.
Paper available free from European Commission Public Relations and Publications
Unit, ISPRA (Italy), Phone +39 332 789370 / 789889
Paper is also available from the IMechE Library.
Brian Worth, European Economic Community Patent, ‘Fuel Cell with Means of Rotating Electrolyte’. This was submitted as an application named ‘Centrifugal Fuel Cell’. Check out Eurpoean Patent (EP) 0911896 (see Patents Links for help)
Letter from the Joint Research Council of The European Commision: http://www.jrc.cec.eu.int/
Zinc Air Fuel Cells
Evonyx make automotive systems: http://www.evonyx.com
Metallic Power: http://www.metallicpower.com
Aluminium Air Fuel Cells
Aluminium Power: http://www.aluminium-power.com