Graphene supercapacitors with a high electrode surface area have great potential in efficient energy storage. For practical applications, it is critical to develop highly-manufacturable methods to synthesize graphene nanosheets with high ion accessibility and uniform pore-sizes. Here we demonstrate a nontoxic approach for graphene synthesis which involves a low vacuum annealing at an easy-to-reach temperature. The supercapacitors based on the low-temperature exfoliated graphene materials show excellent electrochemical performance superior to those synthesized by high-temperature methods. Our investigation presents an efficient, yet industry-compatible and low cost way that can be scaled up for mass production of graphene for electrochemical energy storage application.