| Summary: | Electrochemical and photoelectrochemical water splitting have been demonstrated to be potential routes to address the energy and environmental issues. In these processes such as photovoltaic (PV)-driven water splitting, intermittent renewable energy can be stored in hydrogen chemical bonds. Green hydrogen from water splitting plays an indispensable role in decarbonization. However, water electrolysis only contributes about 4% of global hydrogen production. To promote an even wider adoption of water electrolysis, a lower cost and safer operation are needed. The much higher cost of green hydrogen (compared to grey/blue hydrogen from steam methane reforming) is due to the thermodynamic and kinetic unfavourable redox reactions involved, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The safety concern comes from the hydrogen gas crossover at partial load condition or upon the degradation of gas-impermeable membrane. To address both cost and safety issues, improvement of the catalyst efficiency and replacement of OER half-reaction by other more favourable electro-oxidation reactions that do not generate oxygen, have been extensively studied.
In the first half of the project, efficient, reliable, and cost-effective electrocatalysts have been developed. Briefly, a high pressure solid-vapor method was employed to introduce cyanide (CN) groups to the crystal lattice of 3D hierarchically porous Ni substrate to optimize the electronic structure of Ni surface for efficient water dissociation and subsequent hydrogen reduction, which was corroborated by the DFT theoretical calculation. Furthermore, electrochemical testing results showed that the synthesized catalysts exhibited ultrahigh HER activity and stability under both alkaline and neutral conditions. Detailed material characterization proved the critical roles of the rich interfaces and defects formed, which not only minimized the energy barriers, but also accelerated the kinetics. Moreover, the CN framework endowed the catalyst with excellent stability.
In the second half of the project, the replacement of OER by biomass oxidation reaction, which could address the aforementioned safety concerns arising from OER half-reaction, was investigated. Electro-oxidation of biomass-derived small organic molecules has been extensively studied. However, the annual production of these small organics is low, which cannot meet the demand of hydrogen production globally (about 70 million tons per year). Such a poor scalability of biomass derivatives can be addressed by substituting small organics with raw biomass feedstock that are sufficiently abundant. Chitin is the most abundant amino biopolymer with an annual production of 100 billion tons in nature. The feasibility of chitin electro-oxidation was thus tested in this project. Different methods have been explored to dissolve chitin in alkaline electrolyte including freeze-thawing and mechanochemical pretreatment. With freeze-thawing method, up to 16.7 mg L-1 solubility of chitin in 1.0 M potassium hydroxide (KOH) solution was achieved. The detailed characterization and analysis proved the kinetic favorability of chitin oxidation reaction (COR) over OER, as well as high selectivity of chitin to acetic acid with up to 90 TOC%. To further increase the solubility of chitin, mechanochemical pretreatment was adopted. High solubility of over 10 g L-1 was achieved with mechanochemical pretreatment, which led to 170 mV negative potential shift (from OER), suggesting the thermodynamic favorability of COR over OER. Detailed MALDI-TOF-MS and NMR analyses verified the greatly reduced molecular weight during mechanochemical pretreatment. Finally, the feasibility of safe hydrogen production driven by intermittent solar energy via PV panels was assessed. A solar energy driven membrane-free single-compartment reactor was demonstrated to produce hydrogen gas at rate of 73 mL per min with greatly suppressed OER.
Lastly, to further reduce the cost of chitin feedstock, the hybrid electrolysis technology (HER coupled to biomass oxidation) was extended to shrimp shell waste (SSW), which is a major component of seafood waste, as the anodic reactant. Briefly, SSW was firstly fractionated via KOH-catalysed mechanochemical method. Thereafter, the solute of SSW was electrooxidized with simultaneous production of hydrogen. During this hybrid electrolysis, chitin was converted to acetic acid, while most of peptides in SSW remained intact. Finally, a biosynthesis process was adopted to further upgrade the effluent of the hybrid electrolysis to single-cell protein. As such, a three-step integrated process to upgrade SSW to protein with simultaneous production of green hydrogen has been developed, which poses significant value to close the carbon loop of food waste.
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