Salt water lamp, a sustainable alternative to lighting, has attracted wide attention at the APEC summit. It uses a "metal air" battery as its power supply.
The battery that powers the lamp is technically called "metal air cell" or "metal air" fuel cell. It is similar to a fuel cell in that it takes oxygen from the air, but uses metal as an anode rather than hydrogen or other fuel.
'Metal-air' battery as the power source
The lamp is not powered by salt dissociation in the water because it does not produce the electrons needed for lighting. The flow of electrons is caused by reactions inside the battery located inside the lamp housing. Therefore，It is irresponsible to call it a salt water lamp, as it exacerbates confusion and misleading claims.
Metal air battery is an attractive battery because of its high energy density (it can store and generate more energy in the battery) and its design is simple. Compared with other batteries (such as Ni MH batteries and Li ion batteries), this is due to the unique function of metal air batteries, so there is no need to store an electroactive material (such as oxygen) in the battery. Due to the sufficient supply of oxygen in the air, the theoretical energy density of metal air battery is much higher than that of traditional water-based and lithium-ion batteries.
Saltwater is 'only' an electrolyte
Salts dissolved in water (now in the form of ions) close the circuit and allow electrons and ions to flow through the cell compartment. When you want to turn on a light, this is very similar to turning on a switch. Soon, salt water acts as an electrolyte, promoting the flow of electric current through the battery. Any electrolyte with enough ions can do it. In fact, the best electrolytes for metal air batteries are "alkaline" solutions, such as potassium hydroxide and sodium hydroxide.
The 'invisible' metal electrodes
During lamp operation, the metal anode is continuously dissolved in the electrolyte until the battery is exhausted and the lamp needs to supply the metal anode again. When an element metal produces electrons in the oxidation process, it will change from the metal form to the positively charged ion form. By running the current, it is easy to recover metal from the solution, but this requires much more energy than the energy generated during battery discharge.
When the metal electrode contacts with salt water, it will be corroded, which will devour the metal and produce hydrogen. If the corrosion rate is high, even if the lamp is not used, the lamp owner must always dispose of the salt water electrolyte, otherwise the metal will be continuously dissolved in the electrolyte. It is difficult to avoid this situation because pure metals are not really thermodynamic stable relative to their oxidation forms, which leads to finding natural metal ores with high pure metal content not easy.
The 'crucial' air electrodes
Although metal electrodes have their challenges, the real challenge is to use them as catalysts for air electrodes. In the cathode, electrons generated in the metal anode are absorbed and used to reduce oxygen in the air. This reaction is equally important because the two reactions have to be equal in speed to each other. Unfortunately, the reduction of oxygen is very slow, which requires the help of a catalyst to accelerate it.
In order to get the best power performance from the battery and get a brighter light from the lamp, excellent catalysts must be used. Platinum is the best catalyst for this, but it is extremely expensive; therefore, it is highly likely that cheaper metals can be used in the cathode,such as manganese or cobalt.And expanding cathode production is another serious challenge.