how to read only time in a data frame

I have a data frame of date, time. I read it with as.POSIXct, but i don't need the dates, i only need the time. then i want to make box plot with the times that i have. how i can only read the time data?

C1
   13/07/2017 15:15
   13/07/2017 15:02
   13/07/2017 14:40
   13/07/2017 15:15
   13/07/2017 15:10
   13/07/2017 15:23
   13/07/2017 14:56
   13/07/2017 15:15
   13/07/2017 15:15
   13/07/2017 14:56
   13/07/2017 14:56
   13/07/2017 14:56
   13/07/2017 15:15
   13/07/2017 15:02
   13/07/2017 15:10

df <-  read.csv2(file="DF.csv")


df <- as.POSIXct(df$C1, format="%H:%M:%S", tz="GMT")