If you're away from civilization or just want a renewable way to charge your phone, you could go solar. Portable solar chargers range in price from $20 to $80. Typically, these have a built-in battery that you can charge during the day and use when you want.
The model shown here is the Revolve XEMini Plus. It falls on the high end of the price spectrum, but its eco-friendly construction and broad range of features are a rarity.