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Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO2

机译：系统实现太阳能驱动的可持续减少大气CO2的运行限制和策略

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The operational constraints for a 6-electron/6-proton CO2 reduction system that operates at the concentration of CO2 in the current atmosphere (p(CO2) = 400 ppm) have been evaluated on a variety of scale lengths that span from laboratory scale to global scale. Due to the low concentration of CO2 in the atmosphere, limitations due to mass transport of CO2 from the tropopause have been evaluated through five different regions, each with different characteristic length scales: the troposphere; the atmospheric boundary layer (ABL); the canopy layer; a membrane layer; and an aqueous electrolyte layer. The resulting CO2 conductances, and associated physical transport limitations, will set the ultimate limit on the efficiency and areal requirements of a sustainable solar-driven CO2 reduction system regardless of the activity or selectivity of catalysts for reduction of CO2 at the molecular level. At the electrolyte/electrode interface, the steady-state limiting current density and the concomitant voltage loss associated with the CO2 concentration overpotential in a one-dimensional solar-driven CO2 reduction cell have been assessed quantitatively using a mathematical model that accounts for diffusion, migration and convective transport, as well as for bulk electrochemical reactions in the electrolyte. At p(CO2) = 400 ppm, the low diffusion coefficient combined with the low solubility of CO2 in aqueous solutions constrains the steady-state limiting current density to <0.1 mA cm(-2) in a typical electrochemical cell with natural convection and employing electrolytes with a range of pH values. Hence, in such a system, the CO2 capture area must be 100- to 1000-fold larger than the solar photon collection area to enable a >10% efficient solar-driven CO2 reduction system (based on the solar collection area). This flux limitation is consistent with estimates of oceanic CO2 uptake fluxes that have been developed in conjunction with carbon-cycle analyses for use in coupled atmosphere/ocean general circulation models. Two strategies to improve the feasibility of obtaining efficient and sustainable CO2 transport to a cathode surface at p(CO2) = 400 ppm are described and modeled quantitatively. The first strategy employs yet unknown catalysts, analogous to carbonic anhydrases, that dramatically accelerate the chemically enhanced CO2 transport in the aqueous electrolyte layer by enhancing the acid-base reactions in a bicarbonate buffer system. The rapid interconversion from bicarbonate to CO2 in the presence of such catalysts near the cathode surface would in principle yield significant increases in the steady-state limiting current density and allow for >10% solar-fuel operation at the cell level. The second strategy employs a thin-layer cell architecture to improve the diffusive transport of CO2 by use of an ultrathin polymeric membrane electrolyte. Rapid equilibration of CO2 at the gas/electrolyte interface, and significantly enhanced diffusive fluxes of CO2 in electrolytes, are required to increase the steady-state limiting current density of such a system. This latter approach however only is feasible for gaseous products, because liquid products would coat the electrode and therefore thicken the hydrodynamic boundary layer and accordingly reduce the diffusive CO2 flux to the electrode surface.
机译：在从实验室规模到实验室规模到世界规模。由于大气中CO2的浓度低，已通过五个不同区域评估了对流层顶CO2的质量迁移所产生的局限性，每个区域的特征长度尺度各不相同。大气边界层（ABL）；冠层膜层；和水电解质层。由此产生的CO2电导率和相关的物理传输限制，将对可持续的太阳能驱动的CO2还原系统的效率和面积要求设置最终极限，而与在分子水平上还原CO2的催化剂的活性或选择性无关。在电解质/电极界面处，已使用数学模型对一维太阳能驱动的CO2还原电池中的稳态极限电流密度和与CO2浓度超电势相关的伴随电压损失进行了定量评估，该数学模型考虑了扩散，迁移对流传输，以及电解质中的大量电化学反应。在p（CO2）= 400 ppm的情况下，低扩散系数与CO2在水溶液中的低溶解度相结合，将典型的自然对流电化学电池的稳态极限电流密度限制为<0.1 mA cm（-2） pH值范围内的电解质。因此，在这样的系统中，CO2捕获面积必须比太阳光子收集面积大100到1000倍，以实现效率> 10％的太阳能驱动的CO2还原系统（基于太阳收集面积）。这种通量限制与海洋二氧化碳吸收通量的估计值一致，该估计值是与碳循环分析结合起来开发的，用于耦合的大气/海洋总循环模型。描述并定量建模了两种策略，以提高在p（CO2）= 400 ppm处获得有效且可持续的CO2传输至阴极表面的可行性。第一种策略采用类似于碳酸酐酶的未知催化剂，该催化剂通过增强碳酸氢盐缓冲系统中的酸碱反应来显着加速化学增强的CO2在水性电解质层中的传输。原则上，在阴极表面附近存在此类催化剂的情况下，从碳酸氢盐向CO2的快速相互转化原则上将使稳态极限电流密度显着提高，并允许在电池级运行> 10％的太阳能。第二种策略采用薄层电池架构，通过使用超薄聚合物膜电解质来改善CO2的扩散传输。为了增加这种系统的稳态极限电流密度，需要在气体/电解质界面处迅速平衡CO 2，并显着提高电解质中CO 2的扩散通量。然而，后一种方法仅对于气态产物是可行的，因为液态产物将覆盖电极并因此使流体力学边界层变厚并因此减小了扩散至电极表面的CO 2通量。

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