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The electrochemical reduction of carbon dioxide at an extremely low temperature (−30°C) was investigated with a copper electrode in methanol electrolyte. Products of the electrochemical reduction of carbon dioxide were methane, carbon monoxide and ethylene. Under optimal experimental conditions, the faradaic efficiency of methane was more than 42%. At the low temperature, the efficiency of hydrogen formation as the competitive reaction was depressed to less than 8%.

Carbon dioxide reduction to methane and acetic acid was investigated by an electro bioreactor. The electro bioreactor was constructed by using an immobilized bacteria electrode as the cathode. Dissolved carbon dioxide was fed to the electro bioreactor and a low voltage supplied, resulting in carbon dioxide reduction to methane and/or acetic acid. The reduction is driven by an electrical current, and results in high efficiency convension.

In non-catalytic direct conversion of methane to acetylene by using direct current pulse discharge under conditions of ambient temperature and atmospheric pressure, the selectivity of acetylene from methane was >95% at methane conversion ranging from 16 to 52%; coexisting oxygen was very effective in removing deposited carbon and stabilized the state of discharge.

High temperature electrolysis of water and steam may provide an efficient, cost effective and environmentally friendly production of H2 using electricity produced from sustainable, non-fossil energy sources. To achieve cost competitive electrolysis cells that are both high performing i.e. minimum internal resistance of the cell, and long-term stable, it is critical to develop electrode materials that are optimal for steam electrolysis. In this article electrolysis cells for electrolysis of water or steam at temperatures above 200 °C for production of H2 are reviewed. High temperature electrolysis is favourable from a thermodynamic point of view, because a part of the required energy can be supplied as thermal heat, and the activation barrier is lowered increasing the H2 production rate. Only two types of cells operating at high temperature (above 200 °C) have been described in the literature, namely alkaline electrolysis cells (AEC) and solid oxide electrolysis cells (SOEC). In the present review emphasis is on state-of-the art electrode materials and development of new materials for SOECs. Based on the state-of-the-art performance for SOECs H2 production by high temperature steam electrolysis using SOECs is competitive to H2 production from fossil fuels at electricity prices below 0.02–0.03 € per kWh. Though promising SOEC results on H2 production have been reported a substantial R&D is still required to obtain inexpensive, high performing and long-term stable electrolysis cells.

Electrolysis using molybdenum electrodes in a pH 4.2, 0.2 M Na 2 SO 4 carbon dioxide saturated solution produces methanol as the major carbon-containing product. The reduction proceeds at room temperature and −0.7 to −0.8 V vs. SCE with faradaic efficiencies of ⩾ 50% (efficiencies of near 100% have been measured in some cases). These potentials represent electrolysis at only 160 mV negative of the standard potential corrected for pH. Methanol can also be obtained with good efficiency in 0.05 M H 2 SO 4 at −0.57 to −0.67 V vs. SCE. Electrolysis also produces small amounts of carbon monoxide and traces of methane. No more than a trace of methane was produced even at 52°C. Extended electrolysis of molybdenum electrodes indicates that even over several days the production of methanol does not cease but the faradaic efficiency does increase. Molybdenum will reduce carbon dioxide to carbon monoxide at open circuit. Cyclic voltammetry implicates corrosion of molybdenum metal to molybdenum dioxide as the source of electrons for open circuit reduction of carbon dioxide. Analysis of the solution after electrolysis shows no molybdenum and cyclic voltammetry indicates no detectable molybdenum dioxide on the surface indicating that the electrodes are cathodically protected during electrolysis. If the electrode are cycled between −1.2 and +0.2 V vs. SCE before electrolysis, greater than 100% faradaic efficiencies are observed. The extent of cyclic is critical with yields reaching a maximum with cycling time and extended cycling leading to total passivation of the electrode. These efficiencies involve, at least in part, corrosion of the molybdenum metal. Cycled electrodes will also produce methanol at a low rate at open circuit.

The existing methods for ultrapurification of carbon dioxide and methane often suffer from a waste of time caused by the use of reagents requiring high temperatures and repeated regeneration.

Biological electromethanogenesis is a promising technology which can be applicable to convert geologically-stored CO2 into CH4 by electrochemical active microorganisms as biocatalysts. To gain biocatalysts applicable within CO2-storage reservoirs, a Microbial Fuel Cell (MFC) was used to enrich electrochemical active microorganisms in reservoir brine sample from a petroleum reservoir. The MFC began to produce electricity after 25 hours of inoculation and reached the first maximum current at 50 hours post inoculation. The maximum power density of the thermophilic MFC was ca. 1003 mW/m2, higher than that of most thermophilic MFCs reported previously, suggesting that subsurface reservoir harbors highly functional biocatalysts.

An integrated system for oxygen recovery from carbon dioxide was investigated as a breadboard laboratory model of nominal 1/2-man capacity. System design for carbon dioxide reduction was based on alternate operation of two Bosch reactors with periodic cool down for removal of carbon and replenishment of catalyst. Experimental studies demonstrated attainment of the design objectives of a carbon-to-catalyst ratio above 20 and an overall carbon- packing density of 0.45 g/cu.cm. in the catalyst chamber. Degradation of Bosch reactor materials during extended operation was a problem that was not completely resolved. Experimental studies indicated that a regenerable solid- adsorbent based on combinations of silica gel and molecular sieve operating on alternate cycles of absorption and desorption can be used for efficient transfer of water vapor from the Bosch recycle gas to a water-vapor electrolysis cell. The original matrix-type water-vapor electrolysis unit with Pd-25Ag hydrogen diffusion cathodes did not perform satisfactorily and was replaced by a water- vapor electrolysis unit with phosphoric acid electrolyte for satisfactory evaluation of water-vapor transfer for the integrated system.

The performance and biomass enrichment of the biocathode of a pair of lab-scale two-chambered microbial electrolysis cells (MEC) were assessed for 95 days as a technology for upgrading the biogas produced in anaerobic digesters, converting CO2 into CH4 through the electromethanogenic process. Two different inocula were compared: (i) a mixture of biomass from the anode of a MEC and anaerobic granular sludge (BC1); (ii) biomass enriched in a methanol-fed upflow anaerobic sludge blanket reactor (UASB) (BC2). Quantitative and qualitative microbial community assessment of the enrichment process on the biocathodes was performed by means of high-throughput sequencing of 16S rDNA- and 16S rRNA-based massive libraries as well as RT-qPCR of 16S rRNA and mcrA genes. Although BC2 had a faster increase in current density than BC1, there were no significant differences neither in the average CH4 production (0.23 ± 0.01 and 0.22 ± 0.05 L m–3 day–1 for BC1 and BC2, respectively) nor in the cathodic methane recovery efficiency (65 ± 8% and 79 ± 17%, respectively). Independently from the origin of the inoculum, total and active archaeal microbial community in both biocathodes was dominated by hydrogenotrophic methanogenic archaea, especially belonging to Methanobacteriaceae family (mainly Methanobrevibacter genus) (84–98% of both 16S rDNA and 16S rRNA relative abundance).

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

In this paper, a theoretical model is presented for organic pollutants mineralization at high current efficiency (close to 100%) and low energy consumption on boron-doped diamond electrodes. The model is formulated for a perfect mixed electrochemical reactor operated as a batch recirculation system under multiple current steps, in which the applied current is adjusted during the electrolysis to be close to the limiting value. An experimental validation with the anodic oxidation of 3,4,5-trihydroxybenzoic acid is also provided. The results have shown that multiple current steps electrolysis and continuous current control allowed obtaining high oxidation rate and current efficiency.

We present a study on a microbial electrolysis cell with methanogenic microorganisms adapted to reduce CO2 to CH4 with the direct injection of electrons and without the artificial addition of H2 or an additional carbon source except gaseous CO2. This is a new approach in comparison to previous work in which both bicarbonate and gaseous CO2 served as the carbon source. The methanogens used are known to perform well in anaerobic reactors and metabolize H2 and CO2 to CH4 and water. This study shows the biofilm formation of those microorganisms on a carbon felt electrode and the long-term performance for CO2 reduction to CH4 using direct electrochemical reduction. CO2 reduction is performed simply by electron uptake with gaseous CO2 as the sole carbon source in a defined medium. This “electrometabolism” in such microbial electrolysis cells depends strongly on the potential applied as well as on the environmental conditions. We investigated the performance using different adaption mechanisms and a constant potential of −700 mV vs. Ag/AgCl for CH4 generation at 30–35 °C. The experiments were performed by using two-compartment electrochemical cells. Production rates with Faradaic efficiencies of around 22 % were observed.