| Summary: | Climate change and fossil fuel depletion are intertwined global challenges that necessitate urgent action and lead to the sustainability shift towards low-carbon biofuels, renewable energy, and clean hydrogen fuel, as well as green chemicals. High purity hydrogen can be generated from water electrolysis but is hindered by inefficient oxygen evolution reaction (OER) and the possible explosive mixture of oxygen and hydrogen under the condition of partial loading and membrane degradation. There is a widespread effort to replace OER with the more favorable electrooxidation of small organic molecules. Although value-added products can be generated, the production of these organics is process-intensive and costly. Most importantly, they lack the abundance necessary to meet the requirements of a hydrogen economy.
Abundant raw biomass with annual production of billions of tons in nature are promising alternatives to these small organics. Significantly, biomass reforming via electrooxidation (to replace OER) could close the carbon cycle and promote a circular economy. However, the complex structure of raw biomass poses challenges, limiting processability. Consequently, these biomass materials are conventionally used as fuel for electricity generation. Thermal process-based energy extraction, especially without proper carbon capture and storage, inevitably leads to CO2 emission and the underutilization of biomass. Therefore, from both resources recovery and carbon abatement perspectives, it is critical to develop advanced electro-refinery system for biomass valorisation. To overcome the low processability of raw biomass, highly efficient pretreatment methods were developed and thoroughly investigated.
In this thesis, fast-growing plant species were featured as promising biomass for reforming due to their rapid carbon fixation within short timeframe. Their fast growth rate also accompanies with a shorter lifespan; hence they release carbon back into the atmosphere only after a short period of storage, rendering advanced carbon storage and utilization crucial. Reforming of biomass from fast-growing plant species not only produces green commodity chemicals and hydrogen fuels but also holds promising potential for climate change mitigation. Sugarcane bagasse (food waste) and Paulownia (non-food waste), as two representative fast-growing plant species due to their high abundance in Asia.
In the first work, abundant sugarcane bagasse agricultural waste was investigated with a facile hydrothermal pretreatment to recover soluble mono/oligosaccharides feedstock for subsequent green hydrogen and valuable chemicals production by electroreforming. Having suppressed oxygen evolution and thus avoiding hydrogen-oxygen mixture formation, the electroreforming process was directly driven by photovoltaic electricity. Characterizations using SEM, TEM, XRD, and XPS revealed abundant active sites on the hierarchical porous nickel catalyst acts as a highly active and robust electrocatalyst for SCB electroreforming. Organic acid salts, primarily formate, comprised 63% of the total yield of valuable chemical products. The reaction pathways were thoroughly investigated with complete electrooxidation of the possible mono-/disaccharides and intermediates. Finally, life cycle assessment highlights that the electroreforming upcycling process driven by photovoltaic and waste heat contributes 8–20% less greenhouse gases compared to conventional waste management methods.
In the second work, wood waste from fast-growing Paulownia was processed using the energy-saving, microwave-assisted hydrothermal method. The optimal sugar yield (100% xylose and 17% glucose) with minimal acid usage and energy consumption was achieved by coupling mechanochemical size shrinking and microwave hydrothermal treatment. The sugars were further reformed to valuable chemicals via two approaches: 1) mild thermo-alkaline conversion to produce high-value lactate and 2) hybrid electrolysis to generate high-purity formate and green hydrogen. Remarkably high glucose yields were recovered, reaching 45% in one cycle and 90% in five cycles, rendering the cellulose-rich residue suitable for commercial bioethanol production Alternatively, the produced glucose can also be converted to formate with a yield of up to 90%. Finally, the feasibility of directly driving hybrid electrolysis of the pretreated wood biomass waste was confirmed, showcasing a sustainable way to cogenerate formate and green hydrogen.
Overall, this thesis has validated the high feasibility of valorization of raw lignocellulose biomass, enabling safe and low-carbon production of green hydrogen and green chemicals. This contributes to reducing reliance on fossil fuels and potentially paves the way for decarbonization efforts and sustainable development initiatives.
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