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Electrochemical carbon dioxide reduction represents a promising path to utilize CO2 as a feedstock for generating valuable products such as fuels and chemicals. Faradaic efficiencies near 100% have been achieved for certain CO2 reduction products such as CO, but the electrolyzer outlet streams usually contain large fractions of unreacted CO2, dropping the product concentrations below 1% in many cases. The system disclosed here recycles the unreacted CO2 together with the products and flows them back into the CO2 reduction reactor, enabling much higher CO2 conversion rates without dropping the gas flow rate. However, simple recirculation is shown to accumulate significant amounts of hydrogen, impeding effective CO2 reduction. In this looped system, an electrochemical H2 pump is placed in series with the CO2 reactor, which effectively removes all the H2 from the recycled gas stream, increasing the concentrations of carbon-containing products. The system was initially tested with a CO-generating catalyst and CO concentrations above 70% were achieved in the recycled gas stream, compared to a maximum CO concentration of 8% in single-pass configuration. Results with a CO2 reactor targeting ethylene as the main product show that ethylene concentrations of at least 10% can be achieved, which is roughly 20 times higher compared to a single-pass system.

Electrochemical CO2 reduction (CO2R) using heterogenized molecular catalysts usually yields 2-electron reduction products (CO, formate). Recently, it has been reported that certain preparations of immobilized cobalt phthalocyanine (CoPc) produce methanol (MeOH), a 6-electron reduction product. Here, we demonstrate the significant role of intermediate mass transport in CoPc selectivity to methanol. We first developed a simple, physically mixed, polymer (and polyfluoroalkyl, PFAS)-free preparation of CoPc on multiwalled carbon nanotubes (MWCNTs) which can be integrated onto Au electrodes using a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) adhesion layer. After optimization of catalyst preparation and loading, methanol Faradaic efficiencies and partial current densities of 36% (±3%) and 3.8 (±0.5) mA cm-2, respectively, are achieved in the CO2-saturated aqueous electrolyte. The electrolyte flow rate has a large effect. A linear flow velocity of 8.5 cm/min produces the highest MeOH selectivity, with higher flow rates increasing CO selectivity and lower flow rates increasing the hydrogen evolution reaction, suggesting that CO is an unbound intermediate. Using a continuum multiphysics model assuming CO is the intermediate, we show qualitative agreement with the optimal inlet flow rate. Polymer binders were not required to achieve a high Faradaic efficiency for methanol using CoPc and MWCNTs. We also investigated the role of formaldehyde as an intermediate and the role of strain, but definitive conclusions could not be established.

Intercalation is the process of inserting chemical species into the heterointerfaces of two-dimensional (2D) layered materials. While much research has focused on the intercalation of metals and small gas molecules into graphene, the intercalation of larger molecules through the basal plane of graphene remains challenging. In this work, we present a new mechanism for intercalating large molecules through monolayer graphene to form confined oxide materials at the graphene-substrate heterointerface. We investigate the intercalation of phosphorus pentoxide (P2O5) molecules directly from the vapor phase and confirm the formation of confined P2O5 at the graphene-substrate heterointerface using various techniques. Density functional theory (DFT) corroborates the experimental results and reveals the intercalation mechanism, whereby P2O5 dissociates into small fragments catalyzed by defects in the graphene that then permeates through lattice defects and reacts at the heterointerface to form P2O5. This process can also be used to form new confined metal phosphates (e.g., 2D InPO4). While the focus of this study is on P2O5 intercalation, the possibility of intercalation from predissociated molecules catalyzed by defects in graphene may exist for other types of molecules as well. This in-depth study advances our understanding of intercalation routes of large molecules via the basal plane of graphene as well as heterointerface chemical reactions leading to the formation of distinctive confined complex oxide compounds.

Electron transport layers (ETLs) used as components of photocathodes for light-driven CO2 reduction (CO2R) in aqueous media should have good electronic transport, be stable under CO2R conditions, and, ideally, be catalytically inert for the competing hydrogen evolution reaction (HER). Here, using planar p-Si (100) as the absorbing material, we show that TaOx satisfies all three of the above criteria. TaOx films were synthesized by both pulsed laser deposition (PLD) and radio-frequency (RF) sputtering. In both cases, careful control of the oxygen partial pressure during growth was required to produce ETLs with acceptable electron conductivity. p-Si/TaOx photocathodes were interfaced with ca. 10 nm of a CO2R catalyst: Cu or Au. Under front illumination with simulated AM 1.5G in CO2-saturated bicarbonate buffer, we observed, for both metals, faradaic efficiencies for CO2R products of ∼50% and ∼30% for PLD TaOx and RF sputtered TaOx, respectively, at photocurrent densities up to 8 mA cm−2. p-Si/TiO2/Cu photocathodes were also evaluated but produced mostly H2 (>97%) due to reduction of the TiO2 to Ti metal under CO2R conditions. In contrast, a dual ETL photocathode (p-Si/TiO2/TaOx/Cu) was selective for CO2R, which suggests a strategy for separately optimizing selective charge collection and the stability of the ETL/water interface. The maximum photovoltage obtained with p-Si/TaOx/Cu devices was 300 mV which was increased to 430-460 mV by employing ion implantation to make pn+-Si/TaOx/Cu structures. Photocathodes with RF sputtered TaOx ETLs are stable for CO2R for at least 300 min. Techno-economic analysis shows that the reported system, if scaled, could allow for an economically viable production of feedstocks for chemical synthesis under the adoption of specific CO2 credit schemes, thus becoming a significant component of carbon-neutral manufacturing.

Renewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar fuels generating devices. Specifically, the solar to hydrogen efficiency of photoelectrochemical cells has progressed steadily towards its fundamental limit, and the faradaic efficiency towards valuable products in CO2 reduction systems has increased dramatically. However, there are still numerous scientific and engineering challenges that must be overcame in order to turn solar fuels into a viable technology. At the electrode and device level, the conversion efficiency, stability and products selectivity must be increased significantly. Meanwhile, these performance metrics must be maintained when scaling up devices and systems while maintaining an acceptable cost and carbon footprint. This roadmap surveys different aspects of this endeavor: system benchmarking, device scaling, various approaches for photoelectrodes design, materials discovery, and catalysis. Each of the sections in the roadmap focuses on a single topic, discussing the state of the art, the key challenges and advancements required to meet them. The roadmap can be used as a guide for researchers and funding agencies highlighting the most pressing needs of the field.

Cascade photoelectrocatalysis (PEC) is a possible method to improve the selectivity of solar-driven CO2 reduction (CO2R). This concept can be realized by coupling different CO2R catalysts to different subcells in a multijunction photovoltaic (PV) stack. Efficient implementation will require careful tuning of the photocurrents and design of the photovoltages provided by the subcells to the CO2R catalysts in such a way as to facilitate the target reaction. Here, we outline the design principles of the tandem PEC approach using two-step conversion of CO2 to ethylene in aqueous electrolyte, via a CO intermediate, as a model system. To perform this reaction, the first coupled PV-catalyst component should provide 4 electrons to reduce 2 molecules of CO2 to CO; the second component should provide 8 electrons to reduce 2 CO molecules to C2H4. Based on known CO2R catalysts, the overpotential required to produce CO can be less than that required to reduce it to ethylene, creating the opportunity for improved efficiency. Cascade PEC can be realized in a three-terminal tandem (3TT) configuration using III-V-semiconductor based subcells coupled to Au (produces CO intermediate) and Cu (converts CO to ethylene). The current to each catalyst can be controlled by the area of the subcell exposed to the electrolyte, and the photovoltage is determined by the materials selected and device configuration. Operating conditions are found by simulating the coupled system using the open-source circuit simulator SPICE (Simulation Program with Integrated Circuits Emphasis). We identify conditions under which a 3TT configuration can have a higher solar to chemical conversion efficiency compared to a two-terminal two-junction tandem (2T 2J) with the same absorbers and a Cu catalyst only. We also show that 3TT PEC devices can be less sensitive to variations in catalyst activity compared to 2T devices. Finally, we discuss the applications of cascade PEC to CO2 reduction, using different intermediates, and to other chemical networks.

Electron transport layers (ETLs) used as components of photocathodes for light-driven CO2 reduction (CO2R) in aqueous media should have good electronic transport, be stable under CO2R conditions, and, ideally, be catalytically inert for the competing hydrogen evolution reaction (HER). Here, using planar p-Si (100) as the absorbing material, we show that TaOx satisfies all three of the above criteria. TaOx films were synthesized by both pulsed laser deposition (PLD) and radio-frequency (RF) sputtering. In both cases, careful control of the oxygen partial pressure during growth was required to produce ETLs with acceptable electron conductivity. p-Si/TaOx photocathodes were interfaced with ca. 10 nm of a CO2R catalyst: Cu or Au. Under front illumination with simulated AM 1.5G in CO2-saturated bicarbonate buffer, we observed, for both metals, faradaic efficiencies for CO2R products of ∼50% and ∼30% for PLD TaOx and RF sputtered TaOx, respectively, at photocurrent densities up to 8 mA cm−2. p-Si/TiO2/Cu photocathodes were also evaluated but produced mostly H2 (>97%) due to reduction of the TiO2 to Ti metal under CO2R conditions. In contrast, a dual ETL photocathode (p-Si/TiO2/TaOx/Cu) was selective for CO2R, which suggests a strategy for separately optimizing selective charge collection and the stability of the ETL/water interface. The maximum photovoltage obtained with p-Si/TaOx/Cu devices was 300 mV which was increased to 430–460 mV by employing ion implantation to make pn+-Si/TaOx/Cu structures. Photocathodes with RF sputtered TaOx ETLs are stable for CO2R for at least 300 min. Techno-economic analysis shows that the reported system, if scaled, could allow for an economically viable production of feedstocks for chemical synthesis under the adoption of specific CO2 credit schemes, thus becoming a significant component of carbon-neutral manufacturing. [ABSTRACT FROM AUTHOR]

Over the past decade there have been repeated calls for the greater taxation of wealth. These calls have had little impact on policy. There has been a global trend to reduce or abolish taxes on wealth. The contrast suggests that it may be better now to explore how taxes on wealth may be made a reality rather than designing new tax proposals. What are the barriers to tax wealth? This paper addresses this by conducting a case study of a high profile plan for introducing a one-off wealth tax in the UK. It identifies a tyranny of the status quo, framing and the policy process as key barriers to tax reform. It uses thematic analysis to study how the plans for a one-off wealth tax were discussed in the media and the UK Parliament. This paper argues that there were important shortfalls in both the way the case for a wealth tax was framed as well as the engagement with the policy process. It claims that a stronger framing would have discussed wealth inequality in greater depth and there was a need for a less equivocal case to Parliamentarians. [ABSTRACT FROM AUTHOR]