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At least 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

The Value of Reversible Carbon Storage in a Zero-Emissions World

Atmospheric carbon dioxide removal (CDR) is required to stabilize global temperature. CDR can be achieved via ecosystem-based approaches that are cost-effective but reversible (e.g., soil and forest management) or by more durable but expensive approaches (e.g., direct air capture coupled with geologic storage). Here, we examine trade-offs between these approaches, focusing on timing, climate impacts, and cost. We simulated reversible carbon accrual for a range of CDR contract structures using a general minimalist model of ecosystem carbon cycling, and parameterized it to simulate US agricultural soil management─specifically cover cropping─as a case study. We then quantified the resulting impact on atmospheric carbon and global temperature using a climate model emulator. We find that maintaining a patchwork of reversible CDR projects by replacing lapsed projects with new projects can reduce warming by 22–195 μ°C in 2100 and that the magnitude of this cooling effect depends on how effectively the patchwork is maintained. Long-term maintenance of reversible CDR projects requires institutional stability that cannot be guaranteed over multiple decades. Consequently, effective CDR ultimately requires replacing reversible projects with durable projects. To address this problem, we modeled the cost of replacing reversible agricultural soil CDR with geologic CDR. We found that using reversible CDR as a bridge to durable CDR is potentially more cost-effective as a global cooling strategy (0.20–0.81 billion USD per μ°C avoided) than perpetual maintenance of reversible CDR (0.32–1.31 billion USD per μ°C avoided) or an immediate transition to durable CDR (1.37–2.19 billion USD per μ°C avoided). However, we emphasize that institutional commitments to maintain reversible CDR projects cannot be guaranteed. Reliance on reversible CDR as a bridge to durable CDR therefore carries an unknown amount of risk and will only function if efforts to maintain reversible CDR are robust.

carbon capture and storage↗
DOE OSTI2021

CO 2 Hydrogenation on ZrO 2 /Cu(111) Surfaces: Production of Methane and Methanol

The conversion and utilization of carbon dioxide is a critical challenge for the control of greenhouse gas pollution and in the production of high value chemicals in C1 chemistry. ZrO 2 /Cu(111) is an inverse oxide/metal catalyst that displays high activity and stability for the hydrogenation of CO 2 into methanol at 500-600 K. At elevated temperatures, ZrO 2 grows on a CuO x /Cu(111) substrate forming islands of 10-12 nm in size and an average height of ~ 3 Å. Reaction with H 2 leads to the removal of the copper oxide producing ZrO 2 /Cu(111) surfaces which are very active for the binding and dissociation of CO 2 into CO and C. After exposing ZrO 2 /Cu(111) to moderate or elevated pressures of a CO 2 /H 2 mixture at 300 K, atomic C and minor amounts of CH x O and CO x are deposited on the catalyst surface. The adsorbed CH x O and CO x disappear upon heating above 400 K. The catalytic tests for CO 2 hydrogenation give CO as the main reaction product and CH 4 and CH 3 OH as secondary products. The relative yields of methane and methanol change with time and track the amount of atomic C deposited on the active ZrO 2 /Cu(111) surface. The formation of methane stops once the catalyst surface is saturated with C. Under steady-state conditions, ZrO 2 /Cu(111) is a much better catalyst for methanol synthesis than ZnO/Cu(111). This trend reflects variations in the size of the oxide islands and in the strength of oxide-metal interactions. The use of an inverse oxide/metal configuration is an important synthetic tool when preparing active, selective, and stable catalysts for CO 2 hydrogenation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Understanding the Surface Structure and Catalytic Activity of SnO x /Au(111) Inverse Catalysts for CO 2 and H 2 Activation

Carbon dioxide hydrogenation is a promising approach for the reduction of greenhouse gas pollution via the production of fuels and high-value chemicals utilizing C1 chemistry. In this process, the activation of nonpolar molecules, CO 2 and H 2 , at mild conditions is challenging. In this study, we report a well-defined inverse SnO x /Au(111) catalyst that shows the ability to activate both CO 2 and H 2 at room temperature. Scanning tunneling microscopy (STM) and ambient pressure X-ray photoemission spectroscopy (AP-XPS) are combined to understand the surface structure, growth mode, chemical state, and activity of SnO x /Au(111) surfaces. Nanostructures of SnO x at the sub-monolayer level were prepared by depositing Sn on Au(111) followed by O 2 oxidation. For the as-prepared SnO x /Au(111), two-dimensionally formed SnO x thin films on a Au(111) substrate were observed with STM of two different moieties, discernible based on their height: clusters (~0.4 Å) and nanoparticles (NPs, 1–2.5 Å), which are assigned to Sn–Au alloys and SnO x , respectively, in corroboration with XPS analysis. Furthermore, SnO x /Au(111) was annealed under UHV to test its thermal stability. Upon annealing at 400–600 K, a disappearance of SnO x NPs and reappearance of highly dispersed Sn clusters were clearly noticeable from the STM and XPS results, identifying the thermal decomposition of SnO x and subsequent formation of Sn–Au alloys on the surface due to the recombination of Sn clusters with Au. We investigated the reactivity of the SnO x /Au(111) surfaces toward CH 4 , CO 2 , and H 2 . The SnO x /Au(111) surfaces have excellent CO 2 and H 2 activation abilities even at room temperature with negligible reactivity for methane activation. Our AP-XPS results show that H 2 can be activated on the SnO x NPs by the reduction to Sn. For CO 2 , the activation and further dissociation are identified by a reoxidation of Sn with newly formed Sn–O bonds and the formation of surface carbon. Therefore, we propose that SnO x is a potential catalyst or additive to achieve CO 2 hydrogenation under mild conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2024

MgO Nanostructures on Cu(111): Understanding Size- and Morphology-Dependent CO 2 Binding and Hydrogenation

To design and optimize cost-effective technologies for the capture, utilization, and storage of carbon dioxide (CO 2 ), we need a fundamental knowledge and control of chemical interactions associated with the capture and conversion of the molecule into high-value chemicals, minerals, and all kinds of materials. Bulk magnesium oxide (MgO) is frequently used for the trapping and storage of CO 2 by generation of magnesium carbonates. In this study, the growth and reactivity of MgO nanostructures on a Cu 2 O/Cu(111) substrate were investigated using scanning tunneling microscopy (STM) and synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). For extremely small concentrations of Mg (~ 0.01 monolayer (ML)), a well-ordered film of copper oxide with small clusters (0.2-0.5 nm in width, 0.4-0.6 Å in height) of embedded MgO was seen. At a coverage of 0.1 ML, MgO nanoparticles with a width of 0.4 to 1 nm and a height of ~ 1.5 Å were randomly distributed on the copper oxide. Further, random distribution was also observed when the MgO coverage was raised to 0.25 ML, with the width of the MgO particles increasing to 2-2.5 nm and the height reaching 2 Å. These oxide nanostructures displayed a high reactivity towards CO 2 and H 2 that is not seen for bulk MgO. Dissociation of H 2 was observed at room temperature with reaction of the H adatoms with CuO x and C-containing groups. On the small MgO nanostructures (< 1 nm in width), instead of plain carbonate formation, there was dissociation of CO 2 into CO and C species, opening reaction channels for the conversion of this harmful molecule into oxygenates and light alkanes.

03 NATURAL GAS↗
DOE OSTI2026

The Effects of Iron and Manganese Doping on the Carbonation of Brucite [Mg(OH) 2 ]

Brucite [Mg(OH) 2 ] is a promising sorbent for carbon dioxide removal (CDR) due to its availability and low calcination temperatures. However, natural and synthetic brucites tend to contain metal impurities, such as iron or manganese, and how these impurities affect the interfacial chemical reactivity is uncertain. Here, in this study, the impact of low concentrations of iron and manganese impurities on the carbonation efficiency of Mg(OH) 2 was examined. Mg(OH) 2 with small amounts (1–5 mol %) of Fe and Mn was synthesized. The increasing substitution of Fe into Mg(OH) 2 was accompanied by the oxidation of Fe. The phase transformation sequence during the carbonation was found to be brucite [Mg(OH) 2 ] → amorphous magnesium carbonate (MgCO 3 ·nH 2 O) → nesquehonite (MgCO 3 ·3H 2 O), regardless of impurity concentration. Both the Fe- and Mn-doped Mg(OH) 2 samples were more reactive than endmember Mg(OH) 2 , possibly due to their higher surface areas and lower stabilities. During carbonation, 3 mol % Fe- and Mn-doped Mg(OH) 2 showed the highest reactivity. The variance in reactivity for Mn-doped Mg(OH) 2 was less than that of Fe-doped Mg(OH) 2 . These results suggest that natural or industrial waste Mg(OH) 2 with less than 5 mol % Fe and Mn impurities may be targeted as more effective CDR sorbents than endmember Mg(OH) 2 .

Carbon dioxide removal↗
DOE OSTI2023

Comparisons of bpy and phen Ligand Backbones in Cr-Mediated (Co-)Electrocatalytic CO 2 Reduction

Due to the rise in atmospheric carbon dioxide (CO 2 ) concentrations, there is a need for the development of new strategies to enhance the selectivity and activity of the electrocatalytic conversion of CO 2 to value-added products. The incorporation of redox mediators (RMs) as co-catalysts to enhance the transfer of redox equivalents during catalysis has been gaining more attention in recent years across a variety of small molecule transformations. We have shown that using Cr-centered complexes with sulfone-based RMs leads to an enhancement of CO 2 reduction electrocatalysis under protic conditions via an inner-sphere mechanism. In these co-catalytic systems, an oxygen atom of the reduced RM binds to the Cr center to form a key intermediate stabilized by pancake bonding between the reduced aromatic components of the catalyst ligand backbone and the RM. This interaction facilitates the transfer of an electron and accesses a more kinetically favorable reaction pathway. Here, we show that expanding the aromatic character of the ligand backbone of the catalyst as well as the RM can cause a greater enhancement of co-electrocatalytic activity. These results suggest that further activity improvements can be achieved by focusing on the kinetic and thermodynamic parameters which control association between the catalyst and RM.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Investigating the Influence of Hexanuclear Clusters in Isostructural Metal–Organic Frameworks on Toxic Gas Adsorption

The efficient capture of toxic gases, such as ammonia (NH 3 ) and sulfur dioxide (SO 2 ), can protect the general population and mitigate widespread air pollution. Metal–organic frameworks (MOFs) comprise a tunable class of adsorbents with high surface areas that can meet this challenge by selectively capturing these gases at low concentrations. In this work, we explored how modifying the metal ions in the node of an isostructural MOF series from a transition metal to a lanthanide or actinide influences the electronic environment of the node-based active site. Next, we investigated the adsorption properties of each MOF toward the relatively basic NH 3 and relatively acidic SO 2 gases. Within the NU-907 family of MOFs, we found that Zr 6 -NU-907 exhibits the best uptake toward NH 3 at low pressures, while Th 6 -NU-907 demonstrates the best low-pressure performance for SO 2 adsorption. Here, tracking the infrared (IR) stretching frequency of the node-based μ 3 -OH groups provides insights into the electronegativity of the metal ion and suggests that the most electronegative metal ion (Zr) affords the node with the best NH 3 uptake at low pressures. In contrast, the Th 6 node contains additional coordinated water groups relative to the other M 6 nodes, which appears to yield the MOF with the greatest affinity for SO 2 uptake that occurs predominately through reversible physisorption interactions. Finally, in situ NH3 IR spectroscopic studies indicate that both NH 4 + and Lewis-bound NH 3 species form during adsorption. Combined, these results suggest that tuning the electronic properties and structure of the node-based active site in an MOF presents a viable strategy to change the affinity of an MOF toward toxic gases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Light-Induced Transformation of Virus-Like Particles on TiO 2

Titanium dioxide (TiO 2 ) shows significant potential as a self-cleaning material to inactivate severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and prevent virus transmission. This study provides insights into the impact of UV-A light on the photocatalytic inactivation of adsorbed SARS-CoV-2 virus-like particles (VLPs) on a TiO 2 surface at the molecular and atomic levels. X-ray photoelectron spectroscopy, combined with density functional theory calculations, reveals that spike proteins can adsorb on TiO 2 predominantly via their amine and amide functional groups in their amino acids blocks. We employ atomic force microscopy and grazing-incidence small-angle X-ray scattering (GISAXS) to investigate the molecular-scale morphological changes during the inactivation of VLPs on TiO 2 under light irradiation. Notably, in situ measurements reveal photoinduced morphological changes of VLPs, resulting in increased particle diameters. These results suggest that the denaturation of structural proteins induced by UV irradiation and oxidation of the virus structure through photocatalytic reactions can take place on the TiO 2 surface. The in situ GISAXS measurements under an N 2 atmosphere reveal that the virus morphology remains intact under UV light. This provides evidence that the presence of both oxygen and UV light is necessary to initiate photocatalytic reactions on the surface and subsequently inactivate the adsorbed viruses. The chemical insights into the virus inactivation process obtained in this study contribute significantly to the development of solid materials for the inactivation of enveloped viruses.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ultrathin VO 2 Films on Functional Substrates

The metal–insulator transition (MIT) in vanadium dioxide (VO 2 ) thin films is strongly affected by grain size, thickness, and interfacial properties. Typically, a minimum thickness around 50 nm is required for VO 2 to exhibit a significant MIT when functional substrates like sapphire and silicon are used. Several works have shown that thin films below 20 nm, with up to 2–3 decades of change in the resistance across the MIT, can be achieved but require complex pre- or postprocessing of the samples. We show that predeposition substrate condition control facilitates the direct growth of VO 2 ultrathin 15 nm films, exhibiting a resistance change between 3 and 4 decades across the MIT. Our findings indicate that the interface between the film and the substrate is crucial in determining the initial growth layers and the structural evolution. With appropriate substrate surface treatment, the desired VO 2 MIT can be enhanced regardless of the substrate crystallographic orientation. Moreover, we propose a novel approach to obtain large resistance changes across the MIT in ultrathin VO 2 films by incorporating a predeposited 1.5 nm vanadium oxide buffer layer, thereby eliminating the need to use different materials or complex pre- or postprocessing of the samples. Here we also demonstrate that this method improves the transition of 25–50 nm VO 2 thin films on silicon substrates. Our study reveals a simple approach for direct growth of ultrathin VO 2 films exhibiting a significant MIT, which is commonly accepted unattainable over substrates of technological importance, such as sapphire and silicon.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

CO 2 Adsorption and Hydrogenation on Inverse InO x /Cu(111) Catalysts: Active Role of the Oxide–Metal Interface

The direct conversion of carbon dioxide (CO 2 ) into methanol via hydrogenation is essential for industrial applications. Recent studies on catalysts that contain an inverse oxide/metal configuration have shown very good catalytic performance for the CO 2 hydrogenation to methanol process. Here, in this study, we investigated the behavior of indium oxide-Cu(111) interfaces under pure CO 2 and CO 2 /H 2 mixtures using synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). Initially, a single layer of copper oxide (Cu x O) was grown on the Cu(111) surface by controlled oxidation. On this surface, indium was deposited at room temperature. Oxygen atoms transferred from Cu x O/Cu(111) to the indium metal upon deposition, forming In-O-Cu bonds and active interfaces. Although Cu(111) is not very active for the binding and activation of CO 2 , the formed InO x -Cu(111) interfaces had no problem adsorbing and dissociating the molecule at room temperature. Reaction of CO 2 with H 2 on InO x -Cu(111) yielded surface-bound H 3 CO, CO 2 δ− , CO 3 , and CH x species that are typical intermediates in the production of methanol and other oxygenates. The InO x -Cu(111) interface underwent dynamic chemical changes under reaction conditions, forming In-Cu alloys at low indium coverages (< 0.05 monolayer), while at higher indium coverages a mixture of an In-Cu and InO x was detected in XPS. These findings indicate that InO x /In-Cu interfaces can play a key role in processes aimed at the trapping and valorization of CO 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2026

Reaction Mechanism of Electrodeposited ε-MnO 2 : A Proton-Centered Pathway in Aqueous Zn-Ion Systems

Aqueous Zn/MnO 2 batteries have garnered significant interests owing to their abundance, high theoretical specific capacity, safety, and low cost. However, large-scale application of these systems is limited by the incomplete understanding of the MnO 2 reaction chemistry. The different crystal lattice structures among MnO 2 polymorphs contribute to the variations in reported reaction mechanisms. Among them, ε-MnO 2 polymorph, the dominant phase in electrolytic manganese dioxide (EMD), is notably observed during the charge cycles of aqueous Zn/MnO 2 batteries. Here, in this work, we investigate the electrochemical behavior of an ε-MnO 2 cathode synthesized via electrodeposition from a ZnSO 4 and MnSO 4 electrolyte, onto a 3-dimensional carbon cloth substrate. Proton intercalation emerges as the dominant charge storage mechanism, critically enabling the reversibility of ε-MnO 2 during cycling, as revealed by operando synchrotron X-ray diffraction and X-ray absorption spectroscopy. Additionally, a proton-coupled dissolution/redeposition pathway operates alongside minor Zn 2+ intercalation, as quantified by Rietveld refinement. Morphological and chemical heterogeneities are studied by transmission X-ray microscopy further validates this reaction mechanism. These mechanistic insights provide the foundation for rationally designing Zn/MnO 2 batteries with optimized proton dynamics and charge transfer, advancing these systems as a viable solution for safe, cost-effective grid-scale energy storage.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Proton Conducting Silicon Oxide Membranes as a Fluorine Free Alternative to Nafion for Low Temperature Water Electrolysis

Driven by environmental and health concerns related to per- and polyfluoroalkyl substances (PFAS), there has been growing interest in developing fluorine-free proton (H + ) exchange membrane (PEM) materials for fuel cells and water electrolyzers. In this study, we present a side-by-side comparison of the key transport properties of submicron thick, PFAS-free amorphous silicon dioxide (SiO 2 ) membranes to Nafion, a fluorinated polymer electrolyte membrane that represents the industry standard for PEM fuel cells and electrolyzers. Here, measurements of proton (H + ) conductivity (σ H + ), hydrogen (H 2 ) permeability (P H 2 ), and electrical resistivity (ρ e – ) were conducted using model thin films comprised of SiO 2 membranes deposited by atomic layer deposition (ALD). Although the H + conductivity of the SiO 2 membranes is 2–3 orders of magnitude lower than Nafion, the addition of phosphorus dopants (PO x ) improves H + conductivity such that the area specific membrane resistance of thin (<50 nm) PO x -doped SiO 2 membranes is more than an order of magnitude lower than Nafion-117. Importantly, the safe operation of such nanoscale membranes within a PEM electrolyzer is feasible thanks to the low H 2 permeability of dense SiO 2 -based membranes, which are predicted to limit H 2 crossover rates to acceptable levels for pressures up to ≈ 100 bar. As a proof-of-principle demonstration, a chip-scale water electrolyzer based on 100 nm thick PO x -SiO 2 membrane is shown to achieve a current density of 2 A cm –2 at a potential of 2.5 V. If this technology can be successfully scaled up, H + conducting oxide membranes offer an attractive PFAS-free alternative to Nafion for efficient and durable water electrolysis and fuel cell technologies.

Nafion↗
DOE OSTI2026

Charged Defects in UO 2 Bulk and Surface: A First-Principles Study

Uranium dioxide (UO 2 ) is the primary fuel used in nuclear reactors. Under the extreme heat and radiation inside a reactor, this material inevitably develops defects in its crystal structure. To investigate the nature and behavior of these defects, DFT+U calculations were employed to investigate charged point defects in both bulk UO 2 and its most stable surface, the (111) plane. The formation of defects and their impact on the electronic structure were systematically examined. The results reveal that these defects introduce localized electronic states, alter magnetic behavior, and modify the structural properties. In general, such defects act as deep traps capable of capturing and retaining charge carriers. The stability of these defects depends strongly on the chemical environment and the position of the Fermi level. Surface defect calculations reveal that oxygen vacancies form more readily at the surface than in the bulk over a wide range of electron chemical potential, with subsurface oxygen vacancies being more stable than those in the top layer. Overall, the findings demonstrate how charged defects influence magnetism, transport, and stability in UO 2 , providing insights that may guide improvements in the safety and efficiency of nuclear fuel.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Operando Mechanistic Studies of CO 2 Hydrogenation by Ruthenium Complexes Using High-Pressure NMR Spectroscopy

As a result of increased energy demands, the ability to both reduce carbon dioxide with dihydrogen to formate and conduct the reverse reaction with the same catalyst is of interest as a method for potential fuel generation and use. Ruthenium bis(diphosphine) complexes with and without pendant amines were reacted with mixtures of CO 2 /H 2 gases in the presence of added base to catalytically yield formate; when the base was triethylamine, the reaction was found to be reversible. The reactions were monitored using high-pressure operando 1 H and 31 P{ 1 H} NMR spectroscopy at 18 °C in THF under 40 atm of a 1:1 mixture of H 2 and CO 2 . The rate of production of formate was correlated with the observation of specific organometallic species by NMR spectroscopy under catalytic conditions, including a hydrido–dihydrogen complex. From this operando study, a mechanism is proposed with two competing catalytic cycles for which the predominant cycle is dependent on which base and catalyst are used. Finally, the role of the base is shown to be vital for both catalytic rate and reversibility of the chemical transformation, indicating base selection should be carefully considered.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Insights into the Surface Electronic Structure and Catalytic Activity of InO x /Au(111) Inverse Catalysts for CO 2 Hydrogenation to Methanol

In this article, the direct conversion of carbon dioxide (CO 2 ) into methanol (CH 3 OH) via low-temperature hydrogenation is crucial for recycling anthropogenic CO 2 emissions and producing fuels or high value chemicals. Nevertheless, it continues to be a great challenge due to the trade-off between selectivity and catalytic activity. For CO 2 hydrogenation, In 2 O 3 catalysts are known for their high CH 3 OH selectivity. Subsequent studies explored depositing metals on In 2 O 3 to enhance CO 2 conversion. Despite extensive research on metal (M) supported In 2 O 3 catalysts, the role of In-M alloys and M/In 2 O 3 interfaces in CO 2 activation and CH 3 OH selectivity remains unclear. In this work, we have examined the behavior of In/Au(111) alloys and InO x /Au(111) inverse systems during CO 2 hydrogenation using synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and catalytic tests in a batch reactor. Indium forms alloys with Au(111) after deposition. The In-Au(111) alloys display high reactivity towards CO 2 and can dissociate the molecule at room temperature to generate InO x nanostructures. At very low coverages of In (≤ 0.05 ML), the InO x nanostructures are not stable under CO 2 hydrogenation conditions and the active In-Au(111) alloys produces mainly CO and little methanol. An increase in indium coverage to 0.3 ML led to stable InOx nanostructures under CO 2 hydrogenation conditions. These InO x /Au(111) catalysts displayed a high selectivity (~ 80 %) towards CH 3 OH production and an activity for CO 2 conversion that was at least 10 times larger than that of plain In 2 O 3 or Cu(111) and Cu/ZnO(000$\overline{1)}$ benchmark catalysts. The results of AP-XPS show that InO x /Au(111) produces methanol via methoxy intermediates. Inverse oxide/metal catalysts containing InOx open up a possibility for improving CO 2 → CH 3 OH conversion in processes associated with the control of environmental pollution and the production of high value chemicals.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Comparing Tandem Cell Designs for Electrochemical CO 2 Reduction to Ethylene

Electrochemical carbon dioxide reduction (CO 2 R) is a promising approach for the decentralized production of fuels such as ethylene (C 2 H 4 ). However, the use of Cu, the most efficient metal CO 2 R catalyst for the generation of C 2 H 4 known to date, generally yields a product stream with poor selectivity. In an effort to increase selectivity, the reaction from CO 2 to C 2 H 4 can be broken down into two steps using tandem CO 2 R electrolyzers: formation of CO from CO 2 and subsequent reduction of CO to C 2 H 4 . Here, in this study, we present two novel tandem electrolyzer architectures that closely integrate two cathodes, one for CO generation and one for conversion to C 2 H 4 , while still enabling independent electrical control of the cathodic surfaces. Cathode segmentation in each of these designs also permits the controlled sequencing of mass flow of chemical intermediates in the order of Au to Cu cathode catalysts, in contrast to earlier work relying on uncontrolled, passive diffusion to facilitate the flow of chemical intermediates between catalysts. When comparing the performance of the newly developed electrolyzer cell designs with a dual electrolyzer system, we found that the dual electrolyzer system yields the highest C 2 H 4 faradaic efficiencies (FEs) of 31% and C 2 H 4 concentrations (∼8 mol %). However, a single Cu-containing electrolyzer outperformed all three tandem systems in terms of C 2 H 4 FE (34%). Our findings, enabled by independent control of the two tandem cathode surfaces, indicate that tandem CO 2 R systems need to be evaluated carefully by testing them at various relevant current densities.

C2H4↗
DOE OSTI2026

How to Minimize Faradaic Efficiency Error in Electrochemical CO 2 Reduction for Gas Products

Faradaic efficiency (FE) is an important metric for evaluating electrochemical processes, such as carbon dioxide reduction, that is often used for performance comparisons. Although the equation to calculate FE is well-known, details needed to yield accurate values are often overlooked, potentially leading to errors exceeding 100%. Avoiding any errors is crucial for drawing correct conclusions, especially during a catalysis optimization process. Factors with high potential for FE errors include incorrect mass flow rates and temperature values, imprecise gas chromatography calibrations, and uncorrected gas viscosities, while factors with less potential are also identified. In conclusion, this manuscript presents FE calculation guidelines intended for both newcomers and experts in the field of electrochemical fuel production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Removal of Calcium and Silica from Simulated Blowdown Water Using CO 2 -Assisted Magnesium Electrocoagulation

Blowdown water from various industries and cooling facilities contains high concentrations of calcium (Ca), magnesium (Mg), silica, and corrosion inhibitors, which render the water unsuitable for discharge into surface water and wastewater treatment plants. Electrocoagulation (EC) is effective at removing Ca and silica. Here, in this study, Mg-alloy electrodes EC resulted in ∼91% silica removal at a 5400 C/L charge loading. Ca removal, however, was only 31%. To enhance Ca removal, carbon dioxide (CO 2 ) containing gas was introduced. CO 2 addition formed carbonate ions, which reacted with Ca/Mg ions to generate carbonate precipitates. Introducing 5% and 14% CO 2 , simulating natural gas-fired and coal-fired flue gas, enhanced Ca removal to 38% and 47%, respectively. Pure CO 2 facilitated 70% Ca removal, more than two times higher than that without CO 2 , while maintaining high silica removal. FTIR, XRD, and SEM-EDS analyses confirmed enhanced Ca/Mg carbonate formation upon CO 2 . In situ Raman spectroscopy was employed to monitor carbonate-ions formation. The results demonstrated carbonate-ions removal through Ca-carbonate formation after CO 2 addition. A techno-economic assessment showed that recycled electrode materials and flue gas lead to ∼3 times lower cost compared with conventional silica/Ca removal through chemical coagulation, softening, or membrane filtration. The study demonstrated that CO 2 addition removes high concentrations of Ca and silica ions from wastewaters at a competitive cost.

blowdown water↗