A holistic green system coupling hydrogen production with wastewater valorisation

Abstract

Green hydrogen represents a critical underpinning technology for achieving carbon neutrality. Although researchers often fixate on its energy inputs, a truly ‘green’ hydrogen production process would also be sustainable in terms of water and materials inputs. To address this holistic challenge, we demonstrate a new green hydrogen production system which can utilize secondary wastewater as the input (preserving scarce fresh water supplies for drinking and sanitation). The enabling feature of the proposed system is a self‐grown bifunctional CoNi electrode which consists of ultrathin, spontaneously deposited CoNi nanosheets on a three‐dimensional nickel foam. As such, a green synthesis process was developed using an immersion procedure at room‐temperature with zero net energy input. Testing revealed that the synthesized CoNi electrodes can reach a current density of 10 mA cm−2 at a small overpotential of 197 mV for the hydrogen evolution reaction and 315 mV for the oxygen evolution reaction in alkalified wastewater. The values are ~16.5% and ~6.5% smaller than that from precious catalysts (20 wt% Pt/C and RuO2, respectively). Importantly, this CoNi catalyst offers outstanding durability for overall wastewater splitting. A prototype solar‐energy‐powered rooftop wastewater splitting system was constructed and can produce more than 100 L hydrogen on a sunny day in Sydney, Australia. Taken together, these results indicate that it is promising to unlock holistically green routes for hydrogen production by wastewater uplifting with regards to water, energy, and materials synthesis.

A ternary system exploiting the full solar spectrum to generate renewable hydrogen from a waste biomass feedstock

“…Although the diagram illustrates direct coupling between the PV and FEC, if the I-V characteristics of PV cells is matched with the electrolysers, indirect coupling can also be realized by using a DC-DC power controller. 6 To enhance 5-HMF electrooxidation at the anode and simultaneously produce hydrogen efficiently at the cathode, a bifunctional electrode (NiMo alloy deposited on Ni foam) was developed. This transition metalbased alloy film facilitates biomass electrolysis (i.e., 5-HMF electro-oxidation at the anode and hydrogen evolution at the cathode) at a lower potential than Ni foam and can outperform overall pure water splitting.…”

“…4 In contrast, water consumption (15 L kg −1 H 2 ) 5 contributes to ∼1–6% of the cost, however it imposes a high social impact as it potentially competes with domestic water needs. 6 Competing water uses could limit deployment at scale for renewable electrolysis installations in arid, sunny climates. 7 Consequently, to realize a more sustainable hydrogen economy, innovative strategies are needed to significantly decrease the energy consumption and the water footprint, one of which is to use a waste biomass stream in place of clean water as a feedstock for the electrolyser.…”