Using a Peltier element to power an Arduino
The question behind your question is &quot;can a TEC collect enough energy in half an hour to power an ESP8266 for a few seconds?&quot;
Before you do anything else, you need to get straight in your head just what it is you are working with.  In one of your comments, you mention &quot;0.4 volts&quot; and call it a &quot;small amount of energy.&quot;  This says that you don't understand energy.
Voltage is not energy.  Voltage is one of three elements that make up electrical energy.
To calculate energy, you need three things:

Voltage (V)
Current (I)
Time (t)


Power is the product of voltage and current (\$P=VI\$.)
Energy is the product of voltage, current, and time (\$E=VIt\$.)

Where V is in volts, I is in amperes and t is in seconds.
You have some amount of power that your TEC can deliver.  You have some amount of power that your ESP8266 needs to run.  You have half an hour to collect energy, and your ESP8266 needs to run for several seconds (call it 5 to have a number.)
You need to find out several things:

The output voltage of your TEC (\$V_{TEC}\$)
The output current of your TEC (\$I_{TEC}\$)
The input voltage of your ESP8266 (\$V_{ESP}\$)
The current consumed by the ESP8266 (\$I_{ESP}\$)

We already have time:

Collection time = 60 seconds * 30 minutes = 1800 seconds = \$t_{TEC}\$
Run time = 5 seconds = \$t_{ESP}\$

The energy collected is \$E_{TEC} = V_{TEC}I_{TEC}t_{TEC}\$.
The energy consumed by the ESP8266 is \$E_{ESP} = V_{ESP}I_{ESP}t_{ESP}\$.
You do your measurements, then do the math.

If \$E_{TEC} &gt;&gt; E_{ESP}\$ you're good. Your TEC collects enough energy in half an hour to run your ESP8266, and you shouldn't have to worry about losses in conversion.
If \$E_{TEC} &gt; E_{ESP}\$ then things are close, and you'll have to worry about being really efficient in conversion to get maximum use of the collected energy.
If \$E_{TEC} = E_{ESP}\$ or \$E_{TEC} &lt; E_{ESP}\$ then you can't get enough energy out of the TEC in half an hour to run you ESP8266.

In cases 2. and 3. you might be able to extend the collection period to get enough energy together.  Say, from half an hour to an hour.
Whatever else you do, you need to get (measure or estimate) the power (\$ P_{TEC} = V_{TEC}I_{TEC}\$) your TEC can deliver, and the power your ESP8266 will consume (\$ P_{ESP} = V_{ESP}I_{ESP}\$.)  From there you get the energy.  Your TEC may have a rated power output in watts.  That's already the product of voltage and current.
Once you get approximate numbers for \$E_{TEC}\$ and \$E_{ESP}\$ you can begin thinking about how to power the ESP8266 from the TEC.

Depending on how much excess energy your TEC collects, you may use a regular boost converter or one of the crazy efficient energy collection ICs that are available (which are &quot;just&quot; really efficient boost converters that operate on low voltage.)
A Joule thief is a DIY implementation of a low voltage input boost converter.
One example of a commercial low voltage boost converter is the AD3106.  It operates from a low voltage input and provides a regulated 3.3V which you can use to charge a battery.  There are other ICs available, such as ones that include MPPT and battery charging circuits.
What solution you take depends on how much energy your TEC can deliver and how much work you want to put into developing your solution.