External power is not needed, but, to use this particular flash drive with an iPhone, you'll need an adaptor. The adaptor will have a USB female connector on one end and a lightning connector on the other. And, it will not require external power. Another option is to get a flash drive with the lightning connector built in,, like the recommendations that I've listed.
SanDisk - iXpand 32GB USB 3.0/Lightning Flash Drive - Black / Silver
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SanDisk - iXpand 128GB USB 3.0/Lightning Flash Drive - Black / Silver
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