You should specify what sort of environment you're talking about. Most of the methods I've heard of also use batteries to store the energy, but you could also use an array of supercapacitors. However, that's getting expensive as well.
Ideas I've seen used successfully to some degree:
Solar is the first one, but it does require light. Any light will do of course, even fluorescents. Will it keep your circuit powered for a significant amount of time? Depends on where it's put.
Vibration/Piezoelectric generation has been used successfully to power sensors placed under busy stairs. The key is busy stairs - imagine the New York subway or the main stairs at a university between classes.
EM capturing - If there's a lot of ambient EM then you can put out an antenna, rectify the results and regulate it - boom, power. However, it's usually illegal to do this with significant sources. For example, you can power a light bulb if you're with a quarter mile of a large radio antenna, but harvesting that power is illegal in most countries. It WILL be noticed too.
As Endolith said, you'll probably want a less power-hungry module. Consider Zigbee instead of WiFi - it was designed to be used in low-power devices that transmit for short bursts then stop.
[EDIT] Ok, office/home/domestic is the application. If you control it significantly, maybe you can alter it. Say by adding inductive chargers? I don't think there's that many source of power in a standard office other than solar.
Best Answer
The contacts are missing because they're not used. The A-key variant of the m.2 spec has provisions for display port signals while the E-key variant drops the display port and adds SDIO, UART and PCM (aka I2S), I'd bet those are the ones missing, especially as this is a dual use card (as it has both the A and E key slots present, any pins that differ between the two slots have to be removed). It's probably not using the USB or the second PCIe lane either.