Project description:Carbon dioxide capture underpins an important range of technologies that can help to mitigate climate change. Improved carbon capture technologies that are driven by electrochemistry are under active development, and it was recently found that supercapacitor energy storage devices can reversibly capture and release carbon dioxide. So-called supercapacitive swing adsorption (SSA) has several advantages over traditional carbon dioxide capture technologies such as lower energy consumption and the use of nontoxic materials. However, the mechanism for the capture of CO2 in these devices is poorly understood, making it challenging to design improved systems. Here, the mechanism of SSA is investigated via finite-element modeling with COMSOL of aqueous continuum transport equations, coupled to the CO2 to bicarbonate reaction. This simple computational model reproduces the key experimental observations and shows that charging leads to bicarbonate depletion (or accumulation) in the electrodes, driving CO2 capture (or release) at the gas-exposed electrode. This suggests that relevant aspects of the mechanism are captured without excluding other mechanisms that might be at play in parallel as well. At very low charging currents, both experiments and modeling reveal a decrease in the amount of carbon dioxide captured, suggesting the presence of competing processes at the two electrodes, and that SSA is an inherently kinetic phenomenon. This study highlights the importance of the operating conditions of these devices and may aid their development in the future.
Project description:A novel luminescent azo-linked polymer (ALP) has been constructed from 1,3,6,8-tetra(4-aminophenyl)pyrene using a copper(I)-catalyzed oxidative homocoupling reaction. The polymer displays high porosity with a Brunauer-Emmett-Teller surface area of 1259 m2 g-1 and narrow pore size distribution (1.06 nm) and is able to take up a significant amount of CO2 (2.89 mmol g-1) at 298 K and 1.00 bar with a high isosteric heat of adsorption of 27.5 kJ mol-1. Selectivity studies applying the ideal adsorbed solution theory revealed that the novel polymer has moderately good selectivities for CO2/N2 (55.1) and CO2/CH4 (10.9). Furthermore, the ALP shows fluorescence quenching in the presence of Hg2+, Pb2+, Tl+, and Al3+ ions. Compared with these ions, the ALP showed no sensitivity to light metal ions such as Na+, K+, and Ca2+ in ethanol-water solution, clearly indicating the high selectivity of the ALP toward heavy metal ions. The exceptional physiochemical stability, high porosity, and strong luminescence make this polymer an excellent candidate as a fluorescent chemical sensor for the detection of heavy metal ions.
Project description:Functional nanoporous materials are widely explored for CO2 separation, in particular, small-pore aluminosilicate zeolites having a "trapdoor" effect. Such an effect allows the specific adsorbate to push away the sited cations inside the window followed by exclusive admission to the zeolite pores, which is more advantageous for highly selective CO2 separation. Herein, we demonstrated that the protonated organic structure-directing agent in the small-pore silicoaluminophosphate (SAPO) RHO zeolite can be directly exchanged with Na+, K+, or Cs+ and that the Na+ form of SAPO-RHO exhibited unprecedented separation for CO2/CH4, superior to all of the nanoporous materials reported to date. Rietveld refinement revealed that Na+ is sited in the center of the single eight-membered ring (s8r), while K+ and Cs+ are sited in the center of the double 8-rings (d8rs). Theoretical calculations showed that the interaction between Na+ and the s8r in SAPO-RHO was stronger than that in aluminosilicate RHO, giving an enhanced "trapdoor" effect and record high selectivity for CO2 with the separation factor of 2196 for CO2/CH4 (0.02/0.98 bar). The separation factor of Na-SAPO-RHO for CO2/N2 was 196, which was the top level among zeolitic materials. This work opens a new avenue for gas separation by using diverse silicoaluminophosphate zeolites in terms of the cation-tailored "trapdoor" effect.
Project description:Direct electrochemical reduction of CO2 to C2 products such as ethylene is more efficient in alkaline media, but it suffers from parasitic loss of reactants due to (bi)carbonate formation. A two-step process where the CO2 is first electrochemically reduced to CO and subsequently converted to desired C2 products has the potential to overcome the limitations posed by direct CO2 electroreduction. In this study, we investigated the technical and economic feasibility of the direct and indirect CO2 conversion routes to C2 products. For the indirect route, CO2 to CO conversion in a high temperature solid oxide electrolysis cell (SOEC) or a low temperature electrolyzer has been considered. The product distribution, conversion, selectivities, current densities, and cell potentials are different for both CO2 conversion routes, which affects the downstream processing and the economics. A detailed process design and techno-economic analysis of both CO2 conversion pathways are presented, which includes CO2 capture, CO2 (and CO) conversion, CO2 (and CO) recycling, and product separation. Our economic analysis shows that both conversion routes are not profitable under the base case scenario, but the economics can be improved significantly by reducing the cell voltage, the capital cost of the electrolyzers, and the electricity price. For both routes, a cell voltage of 2.5 V, a capital cost of $10,000/m2, and an electricity price of <$20/MWh will yield a positive net present value and payback times of less than 15 years. Overall, the high temperature (SOEC-based) two-step conversion process has a greater potential for scale-up than the direct electrochemical conversion route. Strategies for integrating the electrochemical CO2/CO conversion process into the existing gas and oil infrastructure are outlined. Current barriers for industrialization of CO2 electrolyzers and possible solutions are discussed as well.
Project description:Aprotic organic solvents such as acetonitrile offer a potential solution to promote electrochemical CO2 reduction over the competing hydrogen evolution reaction. Tetraalkylammonium cations (TAA+) are widely used as supporting electrolytes in organic media due to their high solubility and conductivity. The alkyl chain length of TAA+ cations is known to influence electron transfer processes in electrochemical systems by the adsorption of TAA+, causing modifications of the double layer. In this work, we elucidate the influence of the cation chain length on the mechanism and selectivity of the CO2RR reaction under controlled dry and wet acetonitrile conditions on copper cathodes. We find that the hydrophobic hydration character of the cation, which can be tuned by the chain length, has an effect on product distribution, altering the reaction pathway. Under dry conditions, smaller cations (TEA+) preferentially promote oxalate production via dimerization of the CO2 ·- intermediate, whereas formate is favored in the presence of water via protonation reaction. Larger cations (TBA+ > TPA+ > TEA+) favor the generation of CO regardless of water content. In situ FTIR analysis showed that TBA+ cations are able to stabilize adsorbed CO more effectively than TEA+, explaining why larger cations generate a higher proportion of CO. Our findings also suggest that higher cation concentrations suppress hydrogen evolution, particularly with larger cations, highlighting the role of cation chain length size and hydrophobic hydration shell.
Project description:We investigate a continuous electrochemical pH-swing method to capture CO2 from a gas phase. The electrochemical cell consists of a single cation-exchange membrane (CEM) and a recirculation of a mixture of salt and phenazine-based redox-active molecules. In the absorption compartment, this solution is saturated by CO2 from a mixed gas phase at high pH. In the electrochemical cell, pH is reduced, and CO2 is selectively released in a desorption step. We investigate the influence of redox molecule concentration on the charge storage capacity of the solution, as well as the impact of current density and solution recirculation rate on process performance. A theoretical framework, based on a minimal set of assumptions, is established. This framework describes the data very accurately and can be used for system design and optimization. We evaluate the trade-off between energy consumption and CO2 capture rate and compare with published reports. We report a low energy consumption of 32 kJ/mol of CO2 at a capture rate of 39 mmol/m2/min.
Project description:Electrochemical reduction of CO2 (CO2RR) to value-added liquid fuels is a highly appealing solution for carbon-neutral recycling, especially to syngas (CO/H2). Current strategies suffer from poor faradaic efficiency (FE), selectivity, and controllability to the ratio of products. In this work, we have synthesized a series of single and dual atomic catalysts on the carbon nitride nanosheets. Adjusting the ratio of La and Zn atomic sites produces syngas with a wide range of CO/H2 ratios. Moreover, the ZnLa-1/CN electrocatalyst generates the syngas with a ratio of CO/H2 = 0.5 at a wide potential range, and the total FE of CO2RR reaches 80% with good stability. Density functional theory calculations have confirmed that the Zn and La affect electronic structures and determine the formation of CO and H2, respectively. This work indicates a promising strategy in the development of atomic catalysts for more controllable CO2RR.
Project description:The electrochemical reduction of CO2 offers an elegant solution to the current energy crisis and carbon emission issues, but the catalytic efficiency for CO2 reduction is seriously restricted by the inherent scaling relations between the adsorption energies of intermediates. Herein, by combining the concept of single-atom catalysts and multiple active sites, we design heteronuclear dual-atom catalysts to break through the stubborn restriction of scaling relations on catalytic activity. Twenty-one kinds of heteronuclear transition-metal dimers are embedded in monolayer C2N as potential dual-atom catalysts. First-principles calculations reveal that by adjusting the components of dimers, the two metal atoms play the role of carbon adsorption sites and oxygen adsorption sites respectively, which results in the decoupling of adsorption energies of key intermediates. Free energy profiles demonstrate that CO2 can be efficiently reduced to CH4 on CuCr/C2N and CuMn/C2N with low limiting potentials of -0.37 V and -0.32 V, respectively. This study suggests that the introduction of multiple active sites into porous two-dimensional materials would provide a great possibility for breaking scaling relations to achieve efficient multi-intermediate electrocatalytic reactions.
Project description:Ni-rich layered transition metal oxides show great energy density but suffer poor thermal stability and inferior cycling performance, which limit their practical application. In this work, a minor content of Co and B were co-doped into the crystal of a Ni-rich cathode (LiNi0.8Co0.1Mn0.1O2) using cobalt acetate and boric acid as dopants. The results analyzed by XRD, TEM, XPS and SEM reveal that the modified sample shows a reduced energy barrier for Li+ insertion/extraction and alleviated Li+/Ni2+ cation mixing. With the doping of B and Co, corresponding enhanced cycle stability was achieved with a high capacity retention of 86.1% at 1.0C after 300 cycles in the range of 2.7 and 4.3 V at 25 °C, which obviously outperformed the pristine cathode (52.9%). When cycled after 300 cycles at 5C, the material exhibits significantly enhanced cycle stability with a capacity retention of 81.9%. This strategy for the enhancement of the electrochemical performance may provide some guiding significance for the practical application of high nickel content cathodes.
Project description:A high pressure semicontinuous batch electrolyzer is used to convert CO2 to formic acid/formate on a tin-based cathode using bipolar membranes (BPMs) and cation exchange membranes (CEMs). The effects of CO2 pressure up to 50 bar, electrolyte concentration, flow rate, cell potential, and the two types of membranes on the current density (CD) and Faraday efficiency (FE) for formic acid/formate are investigated. Increasing the CO2 pressure yields a high FE up to 90% at a cell potential of 3.5 V and a CD of ∼30 mA/cm2. The FE decreases significantly at higher cell potentials and current densities, and lower pressures. Up to 2 wt % formate was produced at a cell potential of 4 V, a CD of ∼100 mA/cm2, and a FE of 65%. The advantages and disadvantages of using BPMs and CEMs in electrochemical cells for CO2 conversion to formic acid/formate are discussed.