Quantifying the local pH of a gas diffusion electrode undergoing CO2 reduction is a complicated problem owing to a multitude of competing processes, both electrochemical- and transport-related, possibly affecting the pH at the surface. Here, we present surface-enhanced Raman spectroscopy (SERS) and electrochemical data evaluating the local pH of Cu in an alkaline flow electrolyzer for CO2 reduction. The local pH is evaluated by using the ratio of the SERS signals for HCO3 – and CO3 2–. We find that the local pH is both substantially lower than expected from the bulk electrolyte pH and exhibits dependence on applied potential. Analysis of SERS data reveals that the decrease in pH is associated with the formation of malachite [Cu2(OH)2CO3, malachite] due to the presence of soluble Cu­(II) species from the initially oxidized electrode surface. After this initial layer of malachite is depleted, the local pH maintains a value >11 even at currents exceeding −20 mA/cm2.

Target properties of CO 2 capture adsorbents that would ensure economic viability of bioenergy with carbon capture and storage (BECCS) are defined. The key role of sorbent lifetime in the process cost is demonstrated, and an optimal heat of adsorption for BECCS is postulated through a balance of adsorbent–adsorbate affinity and regeneration energy demand. Using an exponential decay model of sorbent capacity increases the process cost and results in an optimum sorbent lifetime. To ensure a levelized cost of carbon below $100/tonne-CO 2 , adsorbents should be designed to have working capacities above 0.75 mol/kg, lifetimes over 2 years, heats of adsorption of approximately −40 kJ/mol, and exponential degradation decay constants below 5 × 10 –6 cycle –1 (equivalent to a half-life of 1.3 years). Our model predicts a BECCS process cost of $65/t-CO 2 can be achieved with a degradation-resistant adsorbent, $40/kg sorbent cost, 2.0 mol/kg working capacity, −40 kJ/mol heat of adsorption, and at least a 2 year lifetime.

We describe a straightforward and scalable fabrication of diamine-appended metal–organic framework (MOF)/polymer composite hollow fiber sorbent modules for CO 2 capture from dilute streams, such as flue gas from natural gas combined cycle (NGCC) power plants. A specific Mg-MOF, Mg 2 (dobpdc) (dobpdc 4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), incorporated into poly(ether sulfone) (PES) is directly spun through a conventional “dry-jet, wet-quench” method. After phase separation, a cyclic diamine 2-(aminomethyl)piperidine (2-ampd) is infused into the MOF within the polymer matrix during postspinning solvent exchange. The MOF hollow fibers from direct spinning contain as high as 70% MOF in the total fibers with 98% of the pure MOF uptake. The resulting fibers exhibit a step isotherm and a “shock-wave-shock” breakthrough profile consistent with pure 2-ampd-Mg 2 (dobpdc). This work demonstrates a practical method for fabricating 2-ampd-Mg 2 (dobpdc) fiber sorbents that display the MOF’s high CO 2 adsorption capacity while lowering the pressure drop during operation.