Let's do some sums.
A quick google showed:-
7g is the typical acceleration at the ankle when running in conventional running shoes,
A PVDF transducer generates ~ maximum 16uW at 0.92g acceleration.
So, with 23 PVDFs at 7g you could get a max 23 x 16uW x 7/0.92 = ~2800uW = ~2.8mW.
That's the
peak output during the impact of shoe on ground. Let's assume the average is half the peak = 1.4mW.
Let's assume, too, that when running or walking each shoe is on the ground for half the time. If each shoe is fitted with 23 PDFs then the total energy harvested is 1.4 x 2 / 2 = 1.4mW.
So, for a 3 Volt 2000mAHr battery, you would have to walk/run for 3 x 2000/1.4 hours = 4285 hours = 178 days to charge it !!