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The integration of electrochemical and biological CO2 reduction in artificial photosynthetic processes holds great promise to alleviate the current environmental stress of carbon-intensive industri...

Increasingly abundant and low-cost renewable electricity is driving the fast development of electrolysis for energy storage and CO2 valorization. However, current electrolyzers rely on the oxygen evolution reaction (OER), which has been expensive, location limited, high-risk, and generates low value (O2) recovery. In this perspective review, we analyzed the state-of-the-art in electrolysis processes that use alternative anode reactions to improve the economic viability and scalability of water or CO2 electrolysis. We quantitatively compared a wide range of inorganic and organic electron donors in the anode that can lower energy costs and/or produce value-added products, and then assessed the use of different biotic and abiotic catalysts and the feasibility of using low-grade water sources as electrolytes. Through this wide-ranging assessment, we developed an example study for large-scale electrolysis in California, USA, provided long-term perspectives on OER substitutes for anode co-valorization, and delivered insight on future research directions.

The hydrogen evolution reaction (HER) is one of the most effective and sustainable ways to produce hydrogen gas as an alternative clean fuel. The rate of this electrocatalytic reaction is highly dependent on the properties (dispersity and stability) of electrocatalysts. Herein, we developed well-dispersed and highly stable platinum nanoparticles (PtNPs) supported on a covalent organic framework (COF-bpyTPP), which exhibit excellent catalytic activities toward HER as well as the hydride reduction reaction. The nanoparticles have an average size of 2.95 nm and show superior catalytic performance compared to the commercially available Pt/C under the same alkaline conditions, producing 13 times more hydrogen with a far more positive onset potential (−0.13 V vs. −0.63 V) and ca. 100% faradaic efficiency. The reaction rate of the hydride reduction of 4-nitrophenol was also 10 times faster in the case of PtNPs@COF compared to the commercial Pt/C under the same loading and conditions. More importantly, the PtNPs@COF are highly stable under the aqueous reactions conditions and can be reused without showing noticeable aggregation and activity degradation.

This study presents a new bottom-up biofabrication method to produce highly porous magnetic biochar from waste-derived fungal biomass. Neurospora crassa was grown in iron containing coagulation backwash (BW) diluted with primary effluent (PE) wastewater in two ratios of 1:4 (PE-BW 1:4) and 3:4 (PE-BW 3:4). The fungi encapsulated iron directly into biomass hyphae and carbonization resulted in one-step biochar preparation and maghemite (Fe2O3) formation. The morphology and structure of the materials were investigated using a suite of characterization tools. Results indicated that the physiochemical properties of each char were dependent on the blend used for fungal cultivation. PE-BW 1:4 had much larger average pore diameters (13.2 nm vs. 6.1 nm), less elemental surface carbon (2.1% vs. 23.7%), and more expansive Fe2O3 formation. Batch phosphorus adsorption experiments were conducted in the range of 0–90 mg-P/L, and a maximum adsorption density of 23.9 mg/g was achieved. Langmuir, Freundlich and Temkin isotherms were used to describe the interactions of the phosphate on the absorbents and an in-depth error analysis was conducted. Further characterization of the P-loaded chars indicated adsorption primarily via P-OH bonding on the surface of the materials. This new biofabrication method showed great potential to magnetic biochar production with excellent phosphorus adsorption, which can be effectively used in wastewater resource recovery.

Clostridium ljungdahlii was grown on syngas of varying composition to investigate how syngas blends used in fermentation can be tailored toward desired product formation. Experiments were conducted with liquid batch cultures exposed to the same volume of gas with varying ratios of hydrogen to carbon monoxide (H2/CO). Formation of acetate was favored by higher concentrations of hydrogen in the headspace. The maximum acetate concentration of 35.21 ± 0.84 mM was obtained using an H2/CO ratio of 2.0. Generation of more reduced metabolites was favored by greater carbon monoxide concentrations in the headspace. Maximum ethanol (7.53 ± 0.58 mM) and 2,3-Butanediol (5.20 ± 0.79 mM) concentrations were achieved under an H2/CO ratio of 0.5. Linear relationships were obtained in that describe the effect of the syngas H2/CO ratios on acetate, ethanol, and 2,3-BD formation, respectively.

Carbon dioxide (CO2) valorization to high energy density fuels presents an economically sound approach to directly couple waste mitigation with sustainable energy production, yielding tremendous environmental and societal benefits. Here, a new two-stage hybrid system was developed that pairs highly efficient electrochemical CO2 reduction (CO2-R) to carbon monoxide (CO) with acetogenic bacteria for the rapid production of green chemicals. This original design seeks to solve issues in mediator production and utilization evident in related studies. A progressive silver-based gas diffusion electrode with anion exchange membrane sustained current densities of 200 mA/cm2 for over 240 h at a time and produced CO at excellent Faradaic efficiencies of over 80% at a cell voltage ca. 3.2 V. The effluent gas from the CO2 electrolyzer was connected to a bioreactor containing Clostridium ljungdahlii where the blend of CO/CO2 was converted into a mixture of mainly acetate and ethanol. This demonstrates the first time this organism has been tested in a two-stage hybrid CO2-R system. Competitive acetate and ethanol production rates of 17.87 ± 7.1 and 3.23 ± 1.4 mg/L/h were achieved under autotrophic conditions. In addition, mercapto-modified silica nanoparticles were developed to increase gas mass transfer and resulted in 217% and 224% increases in acetate and ethanol production rates, respectively. This system presents a viable approach to produce sustainable fuels and chemicals in a process that exploits commercially scalable C1 electron carries.

Biological electrochemical systems (BESs) have the potential for decentralized treatment in developing countries. A 46 L, two-chamber, hydraulically partitioned microbial fuel cell (MFC) was designed to replicate low-flow scenarios leaving a composting toilet. The co-evolution of electricity and methane in this MFC was evaluated by testing two distinct waste streams: synthetic feces (Case F) and municipal primary effluent (Case W). Oxidation of organic matter was 76 ± 24% during Case F and 67 ± 21% during Case W. Methanogenesis was dominant in the anode, yielding potential power of 3.3 ± 0.64 W/m3 during Case F and 0.40 ± 0.07 W/m3 during Case W. Electrical power production was marginal, Case F = 4.7 ± 0.46 and Case W = 10.6 ± 0.39 μW/m3, although potentially useful in energy-limited areas. Complimentary batch cultivations with anode inocula yielded greater methane production in the presence of graphite. 74 ± 11% more methane was produced with graphite than suspended growth enrichments and 58 ± 10% more than enrichments with non-conductive plastic beads. The co-production of methane and electricity in an MFC may have utility in decentralized treatment. Further work is needed to optimize power from both electricity and methane.

Catalytic hydrogenation is an attractive approach to produce green fuels and chemicals. The building blocks for these processes may be effectively produced from renewable power via direct electrochemical reduction of carbon dioxide in an aqueous media. For the first time, the impact of increasing the local proton concentration of cobalt porphyrin was examined by synthesizing new cobalt porphyrins 2, Co(o-OCH3)TPP and cobalt porphyrin 3, Co(o-OH)TPP. Cobalt porphyrins coated on carbon paper converted carbon dioxide and water into a mixture of hydrogen and carbon monoxide in an aqueous electrolyte at near neutral pH. Increasing the local proton availability of the commercial cobalt porphyrin 1, accelerates hydrogen generation under heterogeneous conditions across the range of potentials tested (−0.85 to −1.5 V vs. Ag/AgCl) and demonstrates high Faradaic efficiencies (ca. 90%) at low over-potentials (ca. 540 mV). The culmination of this work can help identify key parameters that facilitate generation of sustainable reagents for catalytic hydrogenation under practical and scalable conditions.