Innovative Electrochemical Strategies for Hydrogen Production: From Electricity Input to Electricity Output.

Renewable H2 production by water electrolysis has attracted much attention due to its numerous advantages. However, the energy consumption of conventional water electrolysis is high and mainly driven by the kinetically inert anodic oxygen evolution reaction. An alternative approach is the coupling of different half-cell reactions and the use of redox mediators. In this review, we, therefore, summarize the latest findings on innovative electrochemical strategies for H2 production. First, we address redox mediators utilized in water splitting, including soluble and insoluble species, and the corresponding cell concepts. Second, we discuss alternative anodic reactions involving organic and inorganic chemical transformations. Then, electrochemical H2 production at both the cathode and anode, or even H2 production together with electricity generation, is presented. Finally, the remaining challenges and prospects for the future development of this research field are highlighted.

Thermodynamic Analysis of CO2 Hydrogenation to Higher Alcohols (C2-4OH): Effects of Isomers and Methane.

Synthesis of higher alcohols (C2-4OH) by CO2 hydrogenation presents a promising way to convert CO2 into value-added fuels and chemicals. Understanding the thermodynamics of CO2 hydrogenation is of great importance to tailor the reaction network toward synthesis of higher alcohols; however, the thermodynamic effects of various alcohol isomers and methane in the reaction system have not yet been fully understood. Thus, we used Aspen Plus to perform thermodynamic analysis of CO2 hydrogenation to higher alcohols, studying the effects of alcohol isomers and methane. Thermodynamically, methane is the most favorable product in a reaction system containing CO, CO2, and H2, as well as C1-4 alkanes, alkenes, and alcohols. The thermodynamic favorability of alcohol isomers varies significantly. The presence of methane generally deteriorates the formation of higher alcohols. However, low temperature, high pressure, high H2/CO2 ratio, and formation of alcohols with a longer carbon chain can reduce the effects of methane. Our current study, therefore, provides new insights for enhancing the synthesis of higher alcohols by CO2 hydrogenation.

Integrated Co-Electrolysis and Syngas Methanation for the Direct Production of Synthetic Natural Gas from CO2 and H2 O.

The concept of an integrated power-to-gas (P2G) process was demonstrated for renewable energy storage by converting renewable electrical energy to synthetic fuels. Such a dynamically integrated process enables direct production of synthetic natural gas (SNG) from CO2 and H2 O. The produced SNG can be stored or directly injected into the existing natural gas network. To study process integration, operating parameters of the high-temperature solid oxide electrolysis cell (SOEC) producing syngas (H2 +CO) mixtures through co-electrolysis and a fixed bed reactor for syngas methanation of such gas mixtures were first optimized individually. Reactor design, operating conditions, and enhanced SNG selectivity were the main targets of the study. SOEC experiments were performed on state-of-the-art button cells. Varying operating conditions (temperature, flow rate, gas mixture and current density) emphasized the capability of the system to produce tailor-made syngas mixtures for downstream methanation. Catalytic syngas methanation was performed using hydrotalcite-derived 20 %Ni-2 %Fe/(Mg,Al)Ox catalyst and commercial methanation catalyst (Ni/Al2 O3 ) as reference. Despite water in the feed mixture, SNG with high selectivity (≥90 %) was produced at 300 °C and atmospheric pressure. An adequate rate of syngas conversion was obtained with H2 O contents up to 30 %, decreasing significantly for 50 % H2 O in the feed. Compared to the commercial catalyst, 20 %Ni-2 %Fe/(Mg,Al)Ox enabled a higher rate of COx conversion.

Elucidating the mechanism of the CO2 methanation reaction over Ni-Fe hydrotalcite-derived catalysts via surface-sensitive in situ XPS and NEXAFS.

Hydrotalcite-derived Ni and Fe-promoted hydrotalcite-derived Ni catalysts were found to outperform industrial catalysts in the CO2 methanation reaction, however the origin of the improved activity and selectivity of these catalysts is not clear. Here, we report a study of these systems by means of in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy elucidating the chemical nature of the catalysts surface under reaction conditions and revealing the mechanism by which Fe promotes activity and selectivity towards methane. We show that the increase of the conversion leads to hydroxylation of the Ni surface following the formation of water during the reaction. This excessive Ni surface hydroxylation has however a detrimental effect as shown by a controlled study. A dominant metallic Ni surface exists in conditions of higher selectivity towards methane whereas if an increase of the Ni surface hydroxylation occurs, a higher selectivity towards carbon monoxide is observed. The electronic structure analysis of the Fe species under reaction conditions reveals the existence of predominantly Fe(iii) species at the surface, whereas a mixture of Fe(ii)/Fe(iii) species is present underneath the surface when selectivity to methane is high. Our results highlight that Fe(ii) exerts a beneficial effect on maintaining Ni in a metallic state, whereas the extension of the Fe oxidation is accompanied by a more extended Ni surface hydroxylation with a negative impact on the selectivity towards methane.

Electrocatalytic Synthesis of Ammonia at Room Temperature and Atmospheric Pressure from Water and Nitrogen on a Carbon-Nanotube-Based Electrocatalyst.

Ammonia is synthesized directly from water and N2 at room temperature and atmospheric pressure in a flow electrochemical cell operating in gas phase (half-cell for the NH3 synthesis). Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this half-cell. A rate of ammonia formation of 2.2×10-3  gNH3 m-2 h-1 was obtained at room temperature and atmospheric pressure in a flow of N2 , with stable behavior for at least 60 h of reaction, under an applied potential of -2.0 V. This value is higher than the rate of ammonia formation obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with a total Faraday efficiency as high as 95.1 % was obtained. Data also indicate that the active sites in NH3 electrocatalytic synthesis may be associated to specific carbon sites formed at the interface between iron particles and CNT and able to activate N2 , making it more reactive towards hydrogenation.