Which rear wheel axle does my “Citizen Gotham 7” folding bike use?
That looks like standard 10mm bolt on axle.
Citizens' web site photos show a standard quick release on the Citizen 7. The three speed internally geared models use a bolt on axle.
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Newer photos show a single-speed with a solid rear axle and traditional wheel nuts. Perhaps OP's bike had a different wheel fitted at some point? – Criggie♦ Sep 12 '19 at 23:13