A Holistic Green System Coupling Hydrogen Production with Wastewater Valorisation

Green hydrogen represents a critical underpinning technology for achievingcarbon neutrality. Although researchers often fixate on its energy inputs, atruly ‘green’ hydrogen production process would also be sustainable in termsof water and materials inputs. To address this holistic challenge, we demon-strate a new green hydrogen production system which can utilize secondarywastewater as the input (preserving scarce fresh water supplies for drinkingand sanitation). The enabling feature of the proposed system is a self-grownbifunctional CoNi electrode which consists of ultrathin, spontaneously depos-ited CoNi nanosheets on a three-dimensional nickel foam. As such, a greensynthesis process was developed using an immersion procedure at room-temperature with zero net energy input. Testing revealed that the synthesizedCoNi electrodes can reach a current density of 10 mA cm2 at a small overpo-tential of 197 mV for the hydrogen evolution reaction and 315 mV for the oxy-gen evolution reaction in alkalified wastewater. The values are 16.5%and 6.5% smaller than that from precious catalysts (20 wt% Pt/C and RuO 2 ,respectively). Importantly, this CoNi catalyst offers outstanding durability foroverall wastewater splitting. A prototype solar-energy-powered rooftop waste-water splitting system was constructed and can produce more than 100 Lhydrogen on a sunny day in Sydney, Australia. Taken together, these resultsindicate that it is promising to unlock holistically green routes for hydrogenproduction by wastewater uplifting with regards to water, energy, and mate-rials synthesis.