Given how little carbon there is in any practically accessible volume of air, a major obstacle in any atmosphere scrubbing project, it probably doesn't matter. This is likely better understood as a power source that absorbs a bit of CO2 rather than a CO2 scrubber that produces a bit of electricity.
Probably true, which is another knock against the article's professionalism.
But even if the capture negligible, the negative-sign is still interesting - it's teased in the headline! And when a headline teases something, it owes us an explanation of where the carbon goes, a key part of the story to anyone with a middle-school-level understanding of 'carbon'.
Barring nuclear reactions or alchemical magic, it's impossible that "its only byproduct is water". The carbon has to collect somewhere.
Science journalism like this makes its readers dumber.
