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Electrochemical conversion of CO2 to useful products at temperatures below 100 °C is nearing the commercial scale. Pilot units for CO2 conversion to CO are already being tested. Units to convert CO2 to formic acid are projected to reach pilot scale in the next year. Further, several investigators are starting to observe industrially relevant rates of the electrochemical conversion of CO2 to ethanol and ethylene, with the hydrogen needed coming from water. In each case, Faradaic efficiencies of 80% or more and current densities above 200 mA cm−2 can be reproducibly achieved. Here we describe the key advances in nanocatalysts that lead to the impressive performance, indicate where additional work is needed and provide benchmarks that others can use to compare their results. This Perspective describes the key advances in nanocatalysts that have led to the impressive electrochemical conversion of CO2 to useful products and provides benchmarks that others can use to compare their results.

Electrochemical conversion of CO

Water electrolyzers are being developed as a way of storing renewable energy, as a way to produce hydrogen for fuel cell automobiles, and as a route to renewable fuels and chemicals. In this paper the performance of an alkaline water electrolyzers at 60 °C with 1 M KOH and iron/nickel/cobalt catalysts with several different membranes: Sustainion® 37–50, Fumasep FAS-50, Fumasep FAPQ, Neosepta ACM, AMI 7001, Nafion® 115, and Celazole® PBI. Measured area specific resistances (ASR) at 60 °C with 1 M KOH varied from 0.045 Ω-cm2 with Sustainion® 37–60 to over 50 Ω-cm2 with Neosepta ACM. The current at a cell potential of 1.9 V varied from 1 A/cm2 with Sustainion® 37, 0.5 A/cm2 with Fumasep FAS-50, 0.17 A/cm2 with Fumasep FAPQ and less than 0.1 A/cm2 for Neosepta ACM, AMI 7001, Nafion 115 and Celazole® PBI. Constant current runs at 1 A/cm2 were done with the Sustainion® 37 membranes and the Fumasep FAS-50. The cell with the Sustainion® 37 membrane was very stable. The voltage to maintain 1 A/cm2 rose only 3–5 μV/h over a 2000 hr run. In contrast, the voltage to maintain 1 A/cm2 in the cell with the FAS-50 membrane showed over 200 μV/h increase and failed after 200 h. One Sentence Summary: The paper shows that one can double the current output of an alkaline water electrolyzer by using Sustainion® 37 membranes.

Formic acid generated from CO2 has been proposed both as a key intermediate renewable chemical feedstock as well as a potential energy storage chemical media for hydrogen. In this paper, we describe a novel three compartment electrochemical cell configuration with the capability of directly producing a pure formic acid product in the concentration range of 5 – 20 wt% at high current densities and Faradaic yields. The electrochemical cell employs a Dioxide Materials Sustainion™ anion membrane, allowing for the improved CO2 electrochemical reduction performance. Stable electrochemical cell performance for more than 500 hours has been experimentally demonstrated.

Formic acid generated from CO2 has been proposed both as a key intermediate renewable chemical feedstock as well as a potential chemical-based energy storage media for hydrogen. In this paper, we describe a novel three-compartment electrochemical cell configuration with the capability of directly producing a pure formic acid product in the concentration range of 5–20 wt% at high current densities and Faradaic yields. The electrochemical cell employs a Dioxide Materials Sustainion™ anion exchange membrane and a nanoparticle Sn GDE cathode containing an imidazole ionomer, allowing for improved CO2 electrochemical reduction performance. Stable electrochemical cell performance for more than 500 h was experimentally demonstrated at a current density of 140 mA cm−2 at a cell voltage of only 3.5 V. Future work will include cell scale-up and increasing cell Faradaic performance using selected electrocatalysts and membranes.

The electrochemical reduction of CO2 to formate and formic acid has attracted a great amount of academic and commercial interest over the past five years. A number of experimental studies have generated data on the Faradaic performance and stability of various candidate catalyst materials in producing formate or formic acid. However, most of the data has been conducted at low current densities and over short time periods, typically hours to days. There is a critical need in providing long-term catalyst stability as well as electrolyzer-based operating data, which are needed for the commercial scale-up and operation of this technology, especially at high current densities. In this paper, the electrochemical CO2 conversion to pure formic acid was conducted using a three-compartment design electrolyzer, demonstrating electrolyzer catalyst and performance stability for over 1000 h at current densities up to 200 mA cm−2. Depending on the operation conditions, the electrolyzer directly produced a 6.03–12.92 wt% (1.3–2.8 M) formic acid product at Faradaic efficiencies ranging between 73.0–91.3%. Data on electrolyzer performance, including formic acid product generation rate, energy efficiency, and energy consumption are reported at three different current densities, 100, 200, and 250 mA cm−2. Lastly, a long term 1000 h electrolyzer stability run at 200 mA cm−2 is presented, providing information on the operating conditions required in obtaining stable electrolyzer performance. All the data will be extremely useful in the commercial scale-up of this technology.

The efficient conversion of formate into oxalate could enable the industrial-scale synthesis of multi-carbon compounds from CO2 by C−C bond formation. We found conditions for the highly selective catalytic conversion of molten alkali formates into pure solid oxalate salts. Nearly quantitative conversion was accomplished by calcination of sodium formates with sodium hydride. A catalytic mechanism proceeding through a carbonite intermediate, generated upon H2 evolution, was supported by density functional theory calculations, Raman spectroscopy, and the observed changes in the catalytic performance upon changing the nature of the base or the reaction conditions. Whereas the conversion of formate into oxalate by using a hydroxide ion catalyst was previously studied, hydride ion catalysis and the chain reaction mechanism for the conversion involving a carbonite ion intermediate are reported herein for the first time.

This paper is a review on the application of imidazolium-based ionic liquids tethered to polymer backbones in the electrochemical conversion of CO2 to carbon monoxide and formic acid. These tethered ionic liquids have been incorporated into novel anion ion exchange membranes for CO2 electrolysis, as well as for ionomers that have been incorporated into the cathode catalyst layer, providing a co-catalyst for the reduction reaction. In using these tethered ionic liquids in the cathode catalyst composition, the cell operating current increased by a factor of two or more. The Faradaic efficiencies also increased by 20–30%. This paper provides a review of the literature, in addition to providing some new experimental results from Dioxide Materials, in the electrochemical conversion of CO2 to CO and formic acid.

The storage of solar energy as formic acid generated electrochemically from carbon dioxide has been identified as a viable solar fuel pathway. We report that this transformation can be accomplished by separating light absorption and CO 2 reduction through the use of a commercial solar panel illuminated with natural AM1.5 sunlight to power a custom closed-loop electrochemical flow cell stack. Faradaic yields for formate of up to 67% have been demonstrated in this system, yielding a solar energy to fuel thermionic conversion efficiency above 1.8%.

Water electrolyzers are being developed as a way of storing renewable energy, as a way to produce hydrogen for fuel cell automobiles, and as a route to renewable fuels and chemicals. In this paper the effect of changes in the cathode catalyst on the performance of an anion exchange membrane water electrolyzer with Sustainion® membranes running at 60 °C in 1 M KOH. We choose commercial catalysts for study: Raney nickel from W.R. Grace, NiFeCo and Ni8Fe2 from US Nano, NiMoB, FeCoBSi, Ni80Mo20 from Fraunhofer Institute for Manufacturing Technology (IFAM), and NiMo/C from Pajarito Powder(PP). They are all supported on nickel fiber paper. Surprisingly, Raney nickel showed the best performance. At a fixed cell voltage of 2.0 volts in 1 M KOH at 60 °C, we observe 2.9 A/cm² cell current with the Raney Nickel catalyst, compared to 2.3 and 2.1 A/cm² with the US Nano catalysts, 1.7, 1.65 and 1.1 A/cm² with the IFAM catalysts, 1.5 A/cm² with the Pajarito Powder catalyst, and 0.95 A/cm² with the nickel fiber paper alone. These results demonstrate that catalysts that catalysts that show superior performance in a conventional alkaline water electrolyzer, may not show performance superior to Raney nickel in an anion exchange membrane water electrolyzer. Figure 1

Carbon dioxide (CO2) electrolysis provides a pathway to turn waste CO2 into valuable fuels and chemicals but most previous efforts demonstrate low selectivity, high overpotential and low rates for the targeted products. In recent work Dioxide Materials has been using Sustainion™ imidazolium functionalized membranes and ionomers to improve the performance. Runs at a fixed current density of 200 mA/cm2 showed that one can maintain 98% selectivity for six months at about 3 V applied potential, with a voltage increase of only 5 µV/hour. Other runs showed stable performance at 400 and 600 mA/cm². Present areas for improvement of the technology include a better understanding of the processes that are occurring on the cathode catalysts, improving response to process upsets, developing cell designs that can be scaled to the m2 size and alternates to iridium as an anode catalyst. Initial work on these issues will be discussed.

Water electrolyzers are being developed as a way of storing renewable energy, as a way to produce hydrogen for fuel cell automobiles, and as a route to renewable fuels and chemicals. In this paper, we demonstrate alkaline water electrolyzers at 60 °C with iron/nickel/cobalt catalysts that show 1 A/cm² at a 1.9 V cell potential The performance is stable for 2000 hours, showing only 3-5 µV/hr degradation. Key to the findings is the development of new Sustainion™ alkaline exchange membranes that show high conductivity and stability in 1 M KOH. The paper will discuss new developments that improve the response of the electrolyzers to process upsets, and to improve the on-off performance of the devices.

This paper will consider the effects of catalyst composition on alkaline membrane electrolyzers with Sustainion™ membranes. In previous work1, 2we have demonstrated that Sustainion™ membranes are stable at 60 °C for at least 1500 hours in 1 M KOH. Here we compare a number of base metal catalysts on the performance of an alkaline water electrolyzer. Figure 1 shows the voltage needed to maintain 1 A/cm2 in an alkaline water electrolyzer running at 60 °C in 1 M KOH. The blue circles in Figure 1 were taken with nickel nanoparticles on carbon paper on both anode and cathode. Initially we observed good performance, with a cell voltage of 2.1 V but the voltage increased by about 110 µV/hr over the 2000 hour run. The voltage needed to maintain 1 A/cm2 decreased to 2.05 V when NiFeCo was substituted for Ni on the cathode, but we still observed a voltage rise of 44 µV/hr. We stopped the runs after 2000 hours and found that the membrane was still fine, but the carbon paper on the anode was degrading. So we replaced the carbon paper with metal cloth. We also substituted NiFeOx for Ni on the anode. The voltage needed to maintain 1 A/cm2 dropped to 1.9 V, and was stable for 500 hours. The run is continuing. It is useful to compare our results to those from the recent literature. The best previous alkaline water electrolyzers with base metal catalysts ran at currents below 0.25 A/cm² at 2 V3, 4 and even precious metal catalysts have had trouble maintaining 1 A/cm2 at under 2 V5, 6. These results show that alkaline water electrolyzers with Sustainion™ membranes and base metal catalysts show near precious metal performance. [1]. Kutz, R., Q. Chen, H. Yang, S.D. Sajjad, Z. Liu, and R.I. Masel, Sustainion™ Imidazolium Functionalized Polymers for CO2 Electrolysis.Energy Technology, 2017to appear. [2]. Masel, R.I., Z. Liu, and S. Sajjad, Anion Exchange Membrane Electrolyzers Showing 1 A/cm2 at Less Than 2 V. ECS Transactions, 2016. 75: p. 1143-1146. [3]. Zeng, K. and D. Zhang, Recent progress in alkaline water electrolysis for hydrogen production and applications. Progress in Energy and Combustion Science, 2010. 36: p. 307-326. [4]. Bodner, M., A. Hofer, and V. Hacker, H2 generation from alkaline electrolyzer. Wiley Interdisciplinary Reviews: Energy and Environment, 2015. 4: p. 365-381. [5]. Ahn, S.H., S.J. Yoo, H.-J. Kim, D. Henkensmeier, S.W. Nam, S.-K. Kim, and J.H. Jang, Anion exchange membrane water electrolyzer with an ultra-low loading of Pt-decorated Ni electrocatalyst. Applied Catalysis B: Environmental, 2016. 180: p. 674-679. [6]. Leng, Y., G. Chen, A.J. Mendoza, T.B. Tighe, M.A. Hickner, and C.-Y. Wang, Solid-State Water Electrolysis with an Alkaline Membrane. Journal of the American Chemical Society, 2012. 134: p. 9054-9057. Figure 1

Chloric acid, HClO{sub 3}, is a new oxidizer which has recently been shown to be an effective agent in the simultaneous removal of NOx and/or SOx from combustion flue gases and various chemical processes, including nitrations and metal pickling. Aqueous chloric acid readily reacts with NO and SO{sub 2} even in dilute solutions at ambient temperatures. Chlorine dioxide, ClO{sub 2}, is formed as a chemical intermediate in the solution phase oxidation reactions. The oxidation by-products of NO include NO{sub 2} and nitric acid. The ClO{sub 2} generated from the solution phase reactions also participates in gas phase oxidation reactions with NO and NO{sub 2}. The combined solution phase and fast gas phase reaction chemistries provide the means for creating a new type of high performance NOx/SOx removal process. Wet scrubber based pilot plant tests have demonstrated up to 99% removal of NO. Additional recent research work has shown that chloric acid is an effective reagent for the removal of air toxic metals, such as elemental mercury, which are present in the waste gas output streams from incinerators, hydrogen from mercury cell chlor-alkali plants, and flue gases of coal-fired power plants. Work in this area is being conducted by Argonne Nationalmore » Laboratories and Olin. This paper discusses the oxidation chemistry of chloric acid and its unique solution and gas phase reactions with NO, SO{sub 2}, and air toxics in wet scrubber type process equipment. 32 refs., 16 figs., 5 tabs.« less

Previous investigations have shown ferrous iron application to be an effective and economically feasible method of removing residual chlorine dioxide and chlorite ion from drinking water. This treatment, however, was not effective in reducing concentrations of chlorate ion. To address this issue, combined methodologies (alternative chlorine dioxide generation and application together with iron reduction) have been developed that significantly reduce finished-water oxychlorine residuals

Chlorine dioxide is finding increasing use as an oxidizing bleaching agent in the pulp and paper industry, replacing chlorine gas and aqueous sodium hypochlorite. Chlorine dioxide significantly reduces the quantity of adsorbable organically bound halide (AOX) effluents produced in the paper pulp bleaching process while producing high brightness paper with excellent mechanical strength. Increasing chlorine dioxide bleaching substitution levels in the pulp mills as well as the need for a chlorine-free gas for bleaching are the driving forces for design/process changes in the commercial chlorine dioxide generators that use sodium chlorate. Chlorine dioxide is also increasing in usage as a disinfectant or biocide in municipal and industrial water treatment as well as an oxidizer in oil field and pollution abatement processes. Sodium chlorite is the choice precursor chemical for generating chlorine dioxide gas in quantities of less than about 2000 kg/d. Chlorine dioxide has been shown to be an effective biocide as well as an effective oxidizer for the control of total trihalomethanes, in drinking water treatment to meet USEPA drinking water regulations. Methods for the removal of the disinfection by-products produced from chlorine dioxide treatment, such as chlorite and chlorate, are being developed to alleviate toxicological concerns about their presence in drinking water. Keywords: Chlorine dioxide; Reductants; Pulp bleaching; Biocide; Sulfur dioxide; Sodium chlorite; Disinfection; Water treatment