Abstract: The electrochemical CO₂ reduction reaction (CO₂RR) has been proposed as a sustainable way of closing the carbon cycle while synthesizing useful commodity chemicals. One of the possible routes to scale up the process is the elevated pressure CO₂RR, as this increases the concentration of the poorly soluble CO₂ in aqueous systems. Yet, there are not many studies that focus on this route owing to the inherent challenges with high pressure systems. In this study, a novel high pressure flow cell setup has been designed and validated. The modular design uses a clamp system, which facilitates simple stacking of multiple cell parts while being capable of handling pressures up to 50 bar. The effects of CO₂ pressure on the reaction were investigated on a gold (Au) foil cathode in a 0.1 M KHCO₃ electrolyte. We successfully measured gaseous products produced during high pressure operation using an inline gas chromatograph. We find that the selectivity toward CO₂ reduction products is enhanced while that of H₂ evolution is suppressed as the pressure is increased from 2 to 30 bar. The reported setup provides a robust means to conduct high pressure electrolysis experiments in an easy and safe manner, making this technology more accessible to the electrochemical CO₂RR community. Graphical abstract: [Figure not available: see fulltext.].

The electrochemical CO2 reduction reaction (CO2RR) is important for a sustainable future. Key insights into the reaction pathways have been obtained by density functional theory (DFT) analysis, but so far, DFT has been unable to give an overall understanding of selectivity trends without important caveats. We show that an unconsidered parameter in DFT models of electrocatalysts-the surface coverage of reacting species-is crucial for understanding the CO2RR selectivities for different surfaces. Surface coverage is a parameter that must be assumed in most DFT studies of CO2RR electrocatalysts, but so far, only the coverage of nonreacting adsorbates has been treated. Explicitly treating the surface coverage of reacting adsorbates allows for an investigation that can more closely mimic operating conditions. Furthermore, and of more immediate importance, the use of surface coverage-dependent adsorption energies allows for the extraction of ratios of adsorption energies of CO2RR intermediates (COOHads and HCOOads) that are shown to be predictive of selectivity and are not susceptible to systematic errors. This approach allows for categorization of the selectivity of several monometallic catalysts (Pt, Pd, Au, Ag, Zn, Cu, Rh, W, Pb, Sn, In, Cd, and Tl), even problematic ones such as Ag or Zn, and does so by only considering the adsorption energies of known intermediates. The selectivity of the further reduction of COOHads can now be explained by a preference for Tafel or Heyrovsky reactions, recontextualizing the nature of selectivity of some catalysts. In summary, this work resolves differences between DFT and experimental studies of the CO2RR and underlines the importance of surface coverage.

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.

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.

In this work a model of an elevated pressure CO₂ electrolyzer producing primarily formate or formic acid is presented. It consists of three parts: A model of the bulk electrolyte, the diffusion layer, and the electrode surface. Data from the literature was used to validate both the bulk portion of the model, as well as the overall model. Results from the literature were further explored and explained by reference to the model and faradaic efficiency is predicted very well (R-Square of 0.99 for the fitted data, and 0.98 for the non-fitted data). The primary effect of increasing the pressure on a CO₂ electrolyzer is seen to be increasing the maximum attainable partial current density, while the faradaic efficiency and specific energy of formation plateau at pressures above 10-20 bar, at 95% and of 3.7 kWh/kg, respectively. Unlike the efficiencies, the profitability of running a reactor increases with pressure, following a similar trend as partial current density, showing the importance of this quantity as a performance metric of a CO₂ electrolyzer. In general this work shows the utility of a model of this sort in the design, evaluation and operation of CO₂ electrolyzers.

We use a high-pressure semicontinuous batch electrochemical reactor with a tin-based cathode to demonstrate that it is possible to efficiently convert CO₂ to formic acid (FA) in low-pH (i.e., pH < pK_a) electrolyte solutions. The effects of CO₂ pressure (up to 50 bar), bipolar membranes, and electrolyte (K₂SO₄) concentration on the current density (CD) and the Faraday efficiency (FE) of formic acid were investigated. The highest FE (?80%) of FA was achieved at a pressure of around 50 bar at a cell potential of 3.5 V and a CD of ?30 mA/cm². To suppress the hydrogen evolution reaction (HER), the electrochemical reduction of CO₂ in aqueous media is typically performed at alkaline conditions. The consequence of this is that products like formic acid, which has a pK_a of 3.75, will almost completely dissociate into the formate form. The pH of the electrolyte solution has a strong influence not only on the electrochemical reduction process of CO₂ but also on the downstream separation of (dilute) acid products like formic acid. The selection of separation processes depends on the dissociation state of the acids. A review of separation technologies for formic acid/formate removal from aqueous dilute streams is provided. By applying common separation heuristics, we have selected liquid-liquid extraction and electrodialysis for formic acid and formate separation, respectively. An economic evaluation of both separation processes shows that the formic acid route is more attractive than the formate one. These results urge for a better design of (1) CO₂ electrocatalysts that can operate at low pH without affecting the selectivity of the desired products and (2) technologies for efficient separation of dilute products from (photo)electrochemical reactors.