CO2 Gas Mixture Research Articles

Simultaneous collision-induced transitions in H2O+CO2 gas mixtures

A collision induced absorption (CIA) band has been measured near 6000 cm−1 in a spectrum of humidified CO2 recorded by cavity ring down spectroscopy (CRDS) at low pressure (<1 atm). This absorption adds a contribution to the H2O+CO2 continuum mostly originating from far wings of the CO2 and H2O resonance lines broadened by collisions with H2O and with CO2, respectively. The observed CIA corresponds to a simultaneous excitation of 12CO2 and H2O colliding molecules in the ν3 antisymmetric and ν1 symmetric stretching mode, respectively. CRDS spectra recorded near 5940 cm−1 with a highly enriched 13CO2 sample provide a confirmation of the assignment since the measured CIA isotopic spectral shift (of about −68 cm−1) coincides with that between the ν3 bands of 12CO2 and 13CO2. The integrated binary coefficient of the two CIA is evaluated and found to be on the order of 2.3 × 10−3 cm−2amagat−2. Classical molecular dynamics simulations (CMDS) of the considered CIA are also presented, based on the dominant dipole induction mechanism associated with the vibrational matrix elements of the dipole of CO2 (ν3) and isotropic polarizability of H2O (ν1). The results of the calculations are found in good agreement with the observations, thus further validating the attribution of the observed CIA structure to the above mentioned double transitions.

Improving the robustness of Micromegas detector with resistive DLC anode for the upgrade of the TPC readout chambers of the MPD experiment at the NICA collider

A prototype of the Micromegas detector with 30 × 30 mm2 active area and the resistive anode was developed and produced at DLNP JINR using a bulk Micromegas technological process and a diamond-like carbon resistive layer on a polyimide substrate. The prototype is considered as an option for the future upgrade of the readout chambers for the Time Projection Chamber of the Multi-Purpose Detector experiment at the NICA collider. The performance of the prototype of the Micromegas detector was studied using a 55Fe radioactive source and an Ar:CO2 (90:10) gas mixture. The dependencies of the gas gain and the energy resolution on applied high voltage are obtained. The detector works stably up to the gas gain of 30,000. For the gas gain of 10,000 the resolution for the line 5.9 keV of the 55Fe source is 22%. The robustness of the detector prototype to electric discharges was demonstrated by accumulating about 10,000 discharge events at a gas gain of 40,000 without any visible impact on the operation of the prototype.

Engineering Acetobacterium woodii for the production of isopropanol and acetone from carbon dioxide and hydrogen

The capability of four genetically modified Acetobacterium woodii strains for improved production of acetone from CO2 and hydrogen was tested. The acetone biosynthesis pathway was constructed by combining genes from Clostridium acetobutylicum and Clostridium aceticum. Expression of acetone production genes was demonstrated in all strains. In bioreactors with continuous gas supply, all produced acetic acid, acetone, and, surprisingly, isopropanol. The production of isopropanol was caused by an endogenous secondary alcohol dehydrogenase (SADH) activity at low gas-feeding rate. Although high amounts of the natural end product acetic acid of A. woodii were formed,14.5mM isopropanol and 7.6mM acetone were also detected, showing that this is a promising approach for the production of new solvents from C1 gases. The highest acetic acid, acetone, and isopropanol production was detected in the recombinant A. woodii [pJIR750_ac1t1] strain, with final concentrations of 438mM acetic acid, 7.6mM acetone, and 14.5mM isopropanol. The engineered strain A. woodii [pJIR750_ac1t1] was found to be the most promising strain for acetone production from a gas mixture of CO2 and H2 and the formation of isopropanol in A. woodii was shown for the first time.

Improving Catalytic Activity in the Electrochemical Separation of CO2 Using Membrane Electrode Assemblies

The direct electrochemically driven separation of CO2 from a humidified N2, O2, and CO2 gas mixture was conducted using an asymmetric membrane electrode assembly (MEA). The MEA was fabricated using a screen-printed ionomer bound Pt cathode, an anion exchange membrane (AEM), and ionomer bound IrO2 anode. Electrocatalyst materials were physically and chemically characterized prior to inclusion within the electrode. Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) measurements using a rotating disk electrode (RDE) were used to quantify the catalytic activity and determine the effects of the catalyst-to-ionomer ratio. Catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface analysis, and (dynamic light scattering) DLS to evaluate catalyst structure, active surface area, and determine the particle size and bulk particle size distribution (PSD). The electrocatalyst layer of the electrodes were fabricated by screen printing a uniformly dispersed mixture of catalyst, dissolved anionic ionomer, and a solvent system onto an electrode supporting gas diffusion layer (GDL). Pt-IrO2 MEAs were fabricated and current-voltage relationships were determined using constant-current measurements over a range of applied current densities and flow rates. Baseline reaction kinetics for CO2 separation were established with a standard set of Pt-IrO2 MEAs.

Kinetic Analysis on Reduction Rate of Fe1-yO Sintered Pellet with H2-CO gas mixture at 1093 and 1273 K

Kinetic Analysis on Reduction Rate of Fe_1-yO Sintered Pellet with H₂-CO gas mixture at 1093 and 1273 K

Development of catalysts for the synthesis of higher hydrocarbons from carbon dioxide

The paper is devoted to the development of a catalyst for the synthesis of the higher hydrocarbons from carbon dioxide and hydrogen. The conversion of greenhouse gas CO2 into value-added chemicals contributes not only to sol-ving environmental problems, but also to obtaining valuable fuel. Catalysts that are active in the hydrogenation reaction of CO are also active in the process of CO2 hydrogenation. The most active catalysts in the hydrogenation of carbon dioxide are based on transition metals, in the presence of which mainly methane and a small amount of carbon monoxide are formed. Investigation of the kinetics of hydrocarbons synthesis by hydrogenation of carbon dioxide in the presence of transition metals allowed establishing the mechanism of the process and no perspective of such catalytic systems for hydrocarbons synthesis via intermediate formation of carbon monoxide. It is established that the optimal method of synthesis of higher hydrocarbons is the indirect path, through the intermediate formation of methanol with its further dehydration. The synthesis of methanol from gas mixtures of H2, CO, and CO2 with different composition in the presence of industrial catalysts takes place along the same macroscopic route, by reducing CO2, and is accompanied by a reverse water-gas shift reaction. Therefore, gas mixtures of different composition can be used as raw material for methanol production. Based on our own and literature data, the general peculiarities of the carbon dioxide hydrogenation reaction were obtained, which allowed developing a complex multifunctional catalyst for the synthesis of higher hydrocarbons from carbon dioxide and hydrogen. The proposed catalyst combines the properties of ones for the synthesis of oxygenates from carbon dioxide, and catalysts for the processes of hydrocarbons synthesis from me-thanol and other alcohols. The synthesis of higher hydrocarbons with a predominant content of branched hydrocarbons on the developed bifunctional catalyst was carried out and the optimal conditions for the process were determined.

Nickel-based alumina supported catalysts for dry reforming of biogas in the absence and the presence of H2S: Effect of manganese incorporation

In this study, the effects of manganese incorporation on the activity of nickel-based sol-gel alumina-supported catalysts in the dry reforming of biogas were investigated in the absence and the presence of H2S. This article presents new information on reactions using gas mixtures of H2S, CO2, and CH4. Characterization studies were performed to explain catalyst performance during activity tests using XRD, N2 adsorption/desorption, XPS, DRIFTS, TGA/DTA, and SEM/EDX techniques. N2 adsorption/desorption and XRD studies showed that synthesized catalysts have a mesoporous structure with metallic nickel and γ-Al2O3 phases. Activity tests of the catalysts were performed in a tubular reactor at 750 °C and 1 atm in the absence and the presence of H2S (2 ppm and 50 ppm). Alumina-supported Ni, and Ni-Mn catalysts showed stable activity during dry reforming of biogas reaction in the absence of H2S. Mn incorporation decreased the activity of Ni-based catalysts in terms of CH4 conversion; however, it increased its activity in terms of CO2 conversion due to enhancement of reverse water gas shift reaction. In the catalytic activity tests performed in the presence of 2 ppm H2S, the activity of mono and bimetallic catalysts decreased slightly with reaction time. In order to explain the effect of H2S in the feed stream on the catalytic activity for dry reforming, reaction tests were performed with different gas mixtures, namely: H2S + He, H2S + CO2 + He, and H2S + CO2 + CH4 + He. In that case, the reactor exit stream was analyzed by an online FTIR spectrometer. In reaction tests, different sulfur components, such as H2S, COS, S, SO2, and H2O were observed by FTIR analyses. In the case of the presence of H2S in the feed stream, the formation of COS and H2O were predominantly observed. Mn incorporation did not enhance the sulfur resistance of Ni catalyst due to the formation of sulfate on the catalyst surface.

Ampakines Stimulate Diaphragm Activity after Spinal Cord Injury.

Respiratory compromise after cervical spinal cord injury (SCI) is a leading cause of mortality and morbidity. Most SCIs are incomplete, and spinal respiratory motoneurons as well as proprio- and bulbospinal synaptic pathways provide a neurological substrate to enhance respiratory output. Ampakines are allosteric modulators of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, which are prevalent on respiratory neurons. We hypothesized that low dose ampakine treatment could safely and effectively increase diaphragm electromyography (EMG) activity that has been impaired as a result of acute- or sub-acute cervical SCI. Diaphragm EMG was recorded using chronic indwelling electrodes in unanesthetized, freely moving rats. A spinal hemi-lesion was induced at C2 (C2Hx), and rats were studied at 4 and 14 days post-injury during room air breathing and acute respiratory challenge accomplished by inspiring a 10% O2, 7% CO2 gas mixture. Once a stable baseline recording was established, one of two different ampakines (CX717 or CX1739, 5 mg/kg, intravenous) or a vehicle (2-hydroxypropyl-beta-cyclodextrin [HPCD]) was delivered. At 4 days post-injury, both ampakines increased diaphragm EMG output ipsilateral to C2Hx during both baseline breathing and acute respiratory challenge. Only CX1739 treatment also led to a sustained (15 min) increase in ipsilateral EMG output. At 14 days post-injury, both ampakines produced sustained increases in ipsilateral diaphragm EMG output and enabled increased output during the respiratory challenge. We conclude that low dose ampakine treatment can increase diaphragm EMG activity after cervical SCI, and therefore may provide a pharmacological strategy that could be useful in the context of respiratory rehabilitation.

New WSGG model for gas mixtures of H2O, CO2, and CO in typical coal gasifier conditions

This study develops a new weighted-sum-of-gray-gases (WSGG) model for the radiative characteristics of CO, H2O, and CO2 mixtures in pulverized coal gasification plants under a typical high pressure (45 atm). This is significant for prediction of radiative heat transfer in computational fluid dynamic (CFD) calculation. The new high-pressure WSGG model and its parameters are based on the statistical narrow band (SNB) model of EM2C Laboratory. The new model applies to the temperature range of 500–3000 K and the path length range of 0.1–20 m. Moreover, the model can calculate the radiation characteristics of gas mixtures under different partial pressures of three gases. Therefore, the new model can be applied to the radiation transfer in pulverized coal gasification under specific high pressure. The results of one-dimensional cases show that the new model can well predict the radiative heat transfer process of gas mixtures. The error of radiation heat flux is less than 5.38%, and the radiation source term error is less than 0.67%, while the accuracy of the traditional WSGG model is very low under gasification conditions, and the error is more than 25% compared with the benchmark. When the new model is embedded into the CFD simulation of a high-pressure gasifier, the maximum temperature difference between the new and traditional models is up to 300 K, while the calculated absorption coefficient and radiation source term of gas mixtures are noticeably different. In conclusion, compared with the traditional model, the new WSGG model has higher accuracy for calculating gas radiation heat transfer under the condition of coal gasification. This paper also provides the parameters of the new model, which can be used in the CFD engineering simulation of coal gasification plants.

Screening of Gas Substrate and Medium Effects on 2,3-Butanediol Production with C. ljungdahlii and C. autoethanogenum Aided by Improved Autotrophic Cultivation Technique

Gas fermentation by acetogens of the genus Clostridium is an attractive technology since it affords the production of biochemicals and biofuels from industrial waste gases while contributing to mitigate the carbon cycle alterations. The acetogenic model organisms C. ljungdahlii and C. autoethanogenum have already been used in large scale industrial fermentations. Among the natural products, ethanol production has already attained industrial scale. However, some acetogens are also natural producers of 2,3-butanediol (2,3-BDO), a platform chemical of relevant industrial interest. Here, we have developed a lab-scale screening campaign with the aim of enhancing 2,3-BDO production. Our study generated comparable data on growth and 2,3-BDO production of several batch gas fermentations using C. ljungdahlii and C. autoethanogenum grown on different gas substrates of primary applicative interest (CO2 · H2, CO · CO2, syngas) and on different media featuring different compositions as regards trace metals, mineral elements and vitamins. CO · CO2 fermentation was found to be preferable for the production of 2,3-BDO, and a fair comparison of the strains cultivated in comparable conditions revealed that C. ljungdahlii produced 3.43-fold higher titer of 2,3-BDO compared to C. autoethanogenum. Screening of different medium compositions revealed that mineral elements, Zinc and Iron exert a major positive influence on 2,3-BDO titer and productivity. Moreover, the CO2 influence on CO fermentation was explored by characterizing C. ljungdahlii response with respect to different gas ratios in the CO · CO2 gas mixtures. The screening strategies undertaken in this study led to the production of 2.03 ± 0.05 g/L of 2,3-BDO, which is unprecedented in serum bottle experiments.

Role of hydrogen to promote hydrate formation of flue gas mixture of CO2 and N2 in silica nanofluid of single-step origin

This study explores the role of hydrogen as doping agent to enhance CO2 hydrate formation in water and polymer (polyacrylamide, PAM) solution in the presence of varying composition of N2 (0–80 mol%). CO2 storage capacity in clathrate hydrate was explored in the presence of varying quantity of nitrogen (N2) to mimic a flue gas mixture, where N2 composition was gradually increased to simulate effluent gas composition. The silica nanofluids of varying size (35–39 nm, 0.1–1 wt%) were used to accelerate the rate of gas solubility and hydrate formation. All experiments were performed with an initial pressure of 50 bar and ambient temperature while an rpm of 350 was maintained to ensure agitation in the pot. No gas-hydrate was observed in flue gas mixture containing N2 > 45 mol% in both water and PAM solution. To induce hydrate formation in high quantity of N2, the role of hydrogen as a promoter was explored in low dosage volume (1 mol%). The inclusion of hydrogen helped to form gas hydrate for a mixture containing 50 mol% N2. Further, inclusion of silica nanoparticle (1 wt%) increased the rate of gas consumption and hydrate formation for flue gas mixture of 50 mol% N2, indicating positive effect of silica nanoparticle on hydrate formation. From the experimental data, it could be established that silica nanofluid had a positive impact on formation of CO2 hydrate. Gas consumption increased by atleast 12% during dissolution and over 25% during crystallization thus, increasing nanoparticle concentration was beneficial for stimulating gas consumption. The results indicate favorable role of SiO2 nanoparticle and hydrogen on carbon storage of impure gas mixtures exhibiting high amount of N2 (50 mol%).

Effect of Gas Exchange Interval on CH4 Recovery Efficiency and Study of Mechanism of CH4 Hydrate Replacement by CO2 Mixture

As an environment-friendly natural gas hydrate exploitation method, CO2 replacement method can not only achieve the purpose of mining natural gas hydrate, but also store the current greenhouse gas CO2 in the form of hydrate on the seabed, and maintain the stratum stability of hydrate deposit area. In order to improve the rate and efficiency of CH4-CO2 replacement reaction, researchers proposed to use CO2 contained gas mixture instead of pure CO2 to replace CH4 in natural gas hydrate. Based our previous work about CH4 hydrate recovery with 40% CO2 + 60% H2, in this study, the effect of gas concentration in gas phase on final CH4 recovery are investigated by implying different time interval of gas exchange operation. Experimental results show that The CH4 recovery efficiency is 10.41 when the gas exchange is continues through the whole replacement process, and CH4 recovery efficiency changes to 12.25, 32.24 and 28.86 when gas exchange operation is carried out every 12, 24, 36 h. Indicating that replaced CH4 needs to be discharged in time to avoid CH4 molecules being replaced to form hydrates again, and it is necessary to accurately control the time interval of gas exchange operation to avoid insufficient contact time between CO2 and H2 molecules and CH4 hydrate, which affects the final replacement efficiency. In addition, the mechanism of CO2 gas mixture containing small gas molecule such as H2, N2 are studied. The results indicate that when CO2 containing small molecules such as H2 and N2 displace CH4 hydrate, the existence of small molecules (H2, N2) can give rise to decompose the hydrate lattice and release CH4 gas. If the gas molecules (CO2, N2, H2, CH4) in the gas phase have enough driving force to enter the hydrate lattice and remain stability, CH4 hydrate will not decompose completely; If not, CH4 hydrate will be completely decomposed.

Impacts of Syngas Composition on Anaerobic Fermentation

Energy consumption places growing demands on modern lifestyles, which have direct impacts on the world’s natural environment. To attain the levels of sustainability required to avoid further consequences of changes in the climate, alternatives for sustainable production not only of energy but also materials and chemicals must be pursued. In this respect, syngas fermentation has recently attracted much attention, particularly from industries responsible for high levels of greenhouse gas emissions. Syngas can be obtained by thermochemical conversion of biomass, animal waste, coal, municipal solid wastes and other carbonaceous materials, and its composition depends on biomass properties and gasification conditions. It is defined as a gaseous mixture of CO and H2 but, depending on those parameters, it can also contain CO2, CH4 and secondary components, such as tar, oxygen and nitrogenous compounds. Even so, raw syngas can be used by anaerobic bacteria to produce biofuels (ethanol, butanol, etc.) and biochemicals (acetic acid, butyric acid, etc.). This review updates recent work on the influence of biomass properties and gasification parameters on syngas composition and details the influence of these secondary components and CO/H2 molar ratio on microbial metabolism and product formation. Moreover, the main challenges, opportunities and current developments in syngas fermentation are highlighted in this review.

Controlled syngas production by electrocatalytic CO2 reduction on formulated Au25(SR)18 and PtAu24(SR)18 nanoclusters.

Syngas, a gaseous mixture of CO and H2, is a critical industrial feedstock for producing bulk chemicals and synthetic fuels, and its production via direct CO2 electroreduction in aqueous media constitutes an important step toward carbon-negative technologies. Herein, we report controlled syngas production with various H2/CO ratios via the electrochemical CO2 reduction reaction (CO2RR) on specifically formulated Au25 and PtAu24 nanoclusters (NCs) with core-atom-controlled selectivities. While CO was predominantly produced from the CO2RR on the Au NCs, H2 production was favored on the PtAu24 NCs. Density functional theory calculations of the free energy profiles for the CO2RR and hydrogen evolution reaction (HER) indicated that the reaction energy for the conversion of CO2 to CO was much lower than that for the HER on the Au25 NC. In contrast, the energy profiles calculated for the HER indicated that the PtAu24 NCs have nearly thermoneutral binding properties; thus, H2 production is favored over CO formation. Based on the distinctly different catalytic selectivities of Au25 and PtAu24 NCs, controlled syngas production with H2/CO ratios of 1 to 4 was demonstrated at a constant applied potential by simply mixing the Au25 and PtAu24 NCs based on their intrinsic catalytic activities for the production of CO and H2.

Spectroscopic Identification on CO2 Separation from CH4 + CO2 Gas Mixtures Using Hydroquinone Clathrate Formation

The formation of hydroquinone (HQ) clathrate and the guest behaviors of binary (CH4 + CO2) gas mixtures were investigated by focusing on an application to separate CO2 from landfill gases. Spectroscopic measurements show that at two experimental pressures of 20 and 40 bar, CO2 molecules are preferentially captured in HQ clathrates regardless of the gas composition. In addition, preferential occupation by CO2 is observed more significantly when the formation pressure and the CH4 concentration are lower. Because the preferential occupation of CO2 is found with binary (CH4 + CO2) gas mixtures regardless of the composition of the feed gas, a clathrate-based process can be applied to CO2 separation or concentration from landfill gases or (CH4 + CO2) mixed gases.

Development of a selective ammonia YSZ-based sensor and modeling of its response

In the present study, Au-V2O5 sensing material was tested as a sensing electrode for developing mixed-potential ammonia gas sensors. The results of gas sensing measurements indicated selective responses to NH3 while this selectivity was highly dependent on the temperature. Different V2O5 contents were tested in the sensing electrode. The results showed that by increasing V2O5 content from 15 wt.% to 50 wt.%, selective ammonia sensors could be achieved at 550 °C. The selectivity of Au-50 %V2O5 sensor was also confirmed in gas mixtures of CO, NH3, NO, and NO2 gases. Modeling of the sensor responses in the ammonia concentration range 2–40 ppm at four oxygen concentrations was performed based on mixed-potential theory. Nernst and Butler-Volmer equations with an electron transfer assumption were used for data modeling.

Solvation force and adsorption isotherm of a fluid mixture in nanopores of complex geometry based on fundamental measure theory

A novel method based on the fundamental measure theory is developed to calculate the solvation force and adsorption isotherm of a Lennard-Jones fluid mixture in complex geometries. Fast Fourier transform and 3D-voxel discretization are used for accurately computing the confined fluid densities in a closed pore of arbitrary geometry. Given the fluid densities, the solvation force distribution at the solid surface can be calculated using a new formulation from either mechanical or thermodynamic approach. Understanding the solvation force behavior, which depends on many factors such as pore geometry, confined density distribution, molecule size, is very important to analyze the pore deformation from a poromechanical point of view. Special attention in the numerical simulations is given to the adsorption problem of CH4 and CO2 gas mixture in ellipsoidal pore.

Benchmark C2 H2 /CO2 Separation in an Ultra-Microporous Metal-Organic Framework via Copper(I)-Alkynyl Chemistry.

Separation of acetylene from carbon dioxide remains a daunting challenge because of their very similar molecular sizes and physical properties. We herein report the first example of using copper(I)-alkynyl chemistry within an ultra-microporous MOF (CuI @UiO-66-(COOH)2 ) to achieve ultrahigh C2 H2 /CO2 separation selectivity. The anchored CuI ions on the pore surfaces can specifically and strongly interact with C2 H2 molecule through copper(I)-alkynyl π-complexation and thus rapidly adsorb large amount of C2 H2 at low-pressure region, while effectively reduce CO2 uptake due to the small pore sizes. This material thus exhibits the record high C2 H2 /CO2 selectivity of 185 at ambient conditions, significantly higher than the previous benchmark ZJU-74a (36.5) and ATC-Cu (53.6). Theoretical calculations reveal that the unique π-complexation between CuI and C2 H2 mainly contributes to the ultra-strong C2 H2 binding affinity and record selectivity. The exceptional separation performance was evidenced by breakthrough experiments for C2 H2 /CO2 gas mixtures. This work suggests a new perspective to functionalizing MOFs with copper(I)-alkynyl chemistry for highly selective separation of C2 H2 over CO2 .

Gas decomposition and electrode degradation characteristics of a 20% C 3 F 7 CN and 80% CO 2 gas mixture for high voltage accelerators

Sulphur hexafluoride (SF6) is a potent greenhouse gas used in high voltage accelerators. As a promising alternative to SF6, the C3F7CN/CO2 gas mixture and its by-products are of great interest to ensure the safe operation of accelerators that will adopt any SF6-free solution. This work experimentally examines the electrical ageing characteristics of a 20% C3F7CN/80% CO2 gas mixture tested using spark gaps under a pressure of 7.2 bar (abs.). Gas samples were collected after 1000 DC breakdowns and analysed using gas chromatography mass spectrometry (GC-MS) with an estimated toxicity value of 54,459 ppmv, which indicates the aged mixture to be non-toxic. Subsequent investigation was conducted on the gas-solid interface after 500 breakdowns for both SF6 and the 20% C3F7CN/80% CO2 gas mixture. Aged electrodes were analysed using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques. Electrode surface analysis revealed the formation of metal fluorides on the electrode surface tested using the 20% C3F7CN/80% CO2 mixture, whereas metal fluorides and sulphides were detected for electrodes tested with SF6. The findings provide a reference on the toxicity and gas-solid interaction of the electrically aged 20% C3F7CN/80% CO2 gas mixture for potential retro-fill application in high voltage accelerators.

Negative DC Breakdown Characteristics of C3F7CN / CO2 Gas Mixture for Application in High Voltage Accelerators

This study investigates the technical feasibility of retro-filling a C3F7CN/CO2 gas mixture for existing SF6-insulated accelerators. Negative DC breakdown characteristics of 20% C3F7CN / 80% CO2 gas mixture and SF6 for varying pressure, gap distance and field uniformity are analyzed experimentally using four distinctly different electrode configurations. The C3F7CN/CO2 gas mixture shows a higher breakdown strength than SF6 for configurations with high field uniformity, whereas SF6 outperforms C3F7CN/CO2 gas mixture under the non-uniform field. Importantly, 20% C3F7CN / 80% CO2 gas mixture exhibits comparable insulation capability to SF6 in a 10/30 mm coaxial geometry with similar field uniformity as found in the practical gas-insulated equipment. The results of this work contribute to the development of a potential retro-fill solution using 20% C3F7CN / 80% CO2 gas mixture for accelerators.