My creature is capable of obtaining energy from infrared radiation, especially the heat emitted directly by fire (essentially, the closest thing we have to a true "fire-eating" organism). Using the thermoelectric effect, this energy is then used to further power the body and works in tandem with the biological electromagnetic processes .It evolved on planet with only infrared radiation as light source and my creature can also strike so fast. What feasible chemical it need to allow obtain energy from infrared spectrum? and what materials is it made of? what reaction would it need to metabolize infrared into chemical energy?
Your main problem is that in a large organism, "photosynthesis" and "fast" tend to be mutually exclusive terms. Your creature would suffer the same issue that's been long-noted for suggesting about photosynthesizing animals; photosynthesis simply doesn't produce enough energy for high levels of activity that an animal would require.
To give a concrete example, an average adult human at rest requires about 85 watts of energy. Walking adds 280 watts, so if you are walking you need at least 365 watts of power to do so. If you walked, or did equivalent work, for four hours a day, your total energy need for the day is 3.1 kw/h.
The problem is that the entire amount of energy received from the sun at the equator on a clear day at noon is 1 kw per square meter. An adult male has a total surface area of about 1.9 square meters, with obviously at least half not facing the sun. To use nice, round, figures, say there's the equivalent of 1 square meter of photosynthesizing surface able to gather energy. Simple math, a human would have to vegetate (rimshot) for 3.1 hours to collect sufficient energy to make it through a day with that includes doing nothing for 20 hours and doing very low-intensity work for four hours.
At this point, of course, someone should have noted the problem: that's assuming 100% conversion of solar energy into some kind of energy humans can use. You're actually looking at well under 10% efficiency for real photosynthesis. Which means a human would need to collect solar energy at maximum efficiency for 31 hours to make it through a 24 hour day.
You see the problem.
Limiting it to infrared, or some other specific wavelengths, actually makes it worse because you're reducing the the total amount of potential energy available to be harnessed. Infrared has the further problem that each photon has a lower amount of energy and thus less energy able to be transferred and used, which even further reduces the potential amount of energy available.
The thermocouple is the tricky thing.
You mandate the thermoelectric effect which requires a thermocouple. https://en.wikipedia.org/wiki/Thermocouple
That is tricky with biology. There exist none that I know of. The ones on the list are are made with two metals which is not a biology thing. Maybe the thermocouples can be crystals that are found by the parent creature with fragments passed to the young. Or they are grown with fictional biology.
Once you have a thermocouple the rest can be biology. ATP is generated by pumping ions with a current and then letting them fall back in a way that the energy is captured as ATP. Pumping ions with an electrical current is not that edgy.
Using a thermocouple and the thermoelectric effect has the potential to be more energy rich than photosynthesis.