Power-to-methane via co-electrolysis of H2O and CO2: The effects of pressurized operation and internal methanation

This paper presents a model-based investigation to handle the fundamental issues for the design of co-electrolysis based power-to-methane at the levels of both the stack and system: the role of CO2 in co-electrolysis, the benefits of employing pressurized stack operation and the conditions of promoting internal methanation. Results show that the electrochemical reaction of co-electrolysis is dominated by H2O splitting while CO2 is converted via reverse water-gas shift reaction. Increasing CO2 feed fraction mainly enlarges the concentration and cathode-activation overpotentials. Internal methanation in the stack can be effectively promoted by pressurized operation under high reactant utilization with low current density and large stack cooling. For the operation of a single stack, methane fraction of dry gas at the cathode outlet can reach as high as 30 vol.% (at 30 bar and high flowrate of sweep gas), which is, unfortunately, not preferred for enhancing system efficiency due to the penalty from the pressurization of sweep gas. The number drops down to 15 vol.% (at 15 bar) to achieve the highest system efficiency (at 0.27 A/cm(2)). The internal methanation can serve as an effective internal heat source to maintain stack temperature (thus enhancing electrochemistry), particularly at a small current density. This enables the co-electrolysis based power-to-methane to.achieve higher efficiency than the steam-electrolysis based (90% vs 86% on higher heating value, or 83% vs 79% on lower heating value without heat and converter losses).

Optimal design of solid-oxide electrolyzer based power-to-methane systems: A comprehensive comparison between steam electrolysis and co-electrolysis

Stefan Diethelm, Hossein Madi, François Maréchal, Maria del Mar Perez Fortes, Jan Van Herle, Ligang Wang

Power-to-methane technologies have been regarded as a promising alternative to offer small or large-scale, long-timescale (daily/weekly/seasonal) energy storage as well as the opportunity of utilizing CO2. The performance of the core component, the electrolyzer, largely determines how well a power-to-methane system can perform, making high-temperature solid-oxide electrolysis attractive because of its inherent high electrical ef- ficiency. More importantly, solid oxide electrolysis uniquely allows co-electrolysis of steam and CO2 for producing syngas, the composition of which can be readily, flexibly adjusted to synthesize different hydrocarbon fuels. In this paper, for both steam and co-electrolysis, we comprehensively and comparatively investigate several critical design issues of a solid oxide electrolyzer based power-to-methane system with fixed bed methanation reactor and membrane-based methane upgrading: (1) system level heat integration, (2) the impacts of operating variables (e.g., operating voltage, reactant utilization, anode/cathode feed ratio, and operating pressure of the methanation reactor and membrane) on system performances, (3) the competitiveness of the electrolyzer operation with pure oxygen production, and (4) the possibility of avoiding electrical heating, which is necessary for thermoneutral operation to heat up the electrolyzer feeds to the required temperature. To achieve this target, a multi-objective optimization platform with integrated heat cascade calculation is employed with experimentally calibrated component models. The results show that, for both steam and co-electrolysis, there is a trade-off between system efficiency and methane yield: pursuing a higher efficiency generally reduces the methane yield, which is a consequence of electrochemistry, stack cooling and system-level heat integration. Instead of sweep air, pure oxygen production is preferred only at small current density, which delivers the highest system efficiency but the lowest methane yield. When the electrolyzer operates exothermically, methane production and the total power consumption can vary in much wider ranges than those with the electrolyzer operating under thermoneutral mode, which leads to potential enhancement of operation flexibility and reliability. The co-electrolysis coupling with strongly-exothermic syngas methanation, in general, offers better heat- integration opportunity with sweep air, but less with pure oxygen production. In addition, several design heuristics, e.g., the operating pressure of the electrolyzer and methanation reactor, are concluded to potentially guide practical applications.

2018

Préparation et caractérisation de nanoparticules bimétalliques (Pt-Au) sur un substrat en diamant dopé au bore

Bahaa El Roustom

Using petrol as a main source of energy, during the 20th century, has caused considerable amounts of damage among which are atmospheric pollution and global warming. At the same time, many geologists and economists expect that the discovery rate of new petrol sources will not follow the market's demand causing a serious shortage of petrol in the near future. These alarming perspectives have motivated scientists around the world to develop new clean and renewable energy sources. It is in that objective that electrochemists intensified their research in order to develop the fuel cell technology. This latter consists in directly converting chemical energy into electricity. The main advantage of this technology over traditional energy production is that the fuel cell energy efficiency is Carnot cycle independent. The theoretical efficiency of such an energy source is nearly 90%. Direct Alcohol Fuel Cell (DAFC), fed by ethanol on one hand and by air (oxygen) in the other hand, is a very interesting option for such kind of energy production. Ethanol is a renewable energy source of vegetal origin whose CO2 production is consumed by plants during the cycle. Even though Pt has long been known to be a perfect catalyst for DAFC, it has some principal limitations, the most important ones being poisoning by intermediate products (OH on the cathodic side and CO on the anodic side) and a relatively high cost compared to other materials. These problems have pushed the experts in the field to focalize their research on the development of more suitable and better performing materials. Gold nanoparticles have shown a surprising catalytic activity, very different from the bulk, making them a preferential candidate to replace Pt. A system based on using two or more metals as catalysts (example Au-Pt) dispersed on an appropriate substrate seems to be also an interesting candidate to enhance the cell's efficiency. The substrate should be chemically inert, non-oxydable and should not present any interference with the electrochemical and electrocatalytic behaviour of the nanoparticles. In the first part, the employed substrate, which is boron doped diamond (BDD) is characterized. The substrate's main characteristics are its chemical inertness, its weak residual current and its resistance to corrosion. These characteristics contribute to making the substrate an ideal support for nanoparticles. In this work, the substrate is a diamond film, but for fuel cell applications diamond should be used as powder or dispersed particles. In chapters III and IV, we studied the effect of boron incorporation on the morphology and the behaviour of the diamond film. For this purpose, samples with different ratios of boron/carbon (B/C) in the vapour phase during preparation were characterized (B/C = 300, 1000, 2500 and 6000 ppm). Results showed that the grain size decreases with increasing boron content in the film. This is accompanied by a transition towards a quasi-metallic character. It was also found that graphitic impurities in the diamond film increase when the B/C ratio increases. We also established some graphitic impurities increase in the diamond film when the ratio of B/C increases. The identity of these impurities was established in chapter III as being surface redox couples (semi-quinone/semi-hydroquinone). Using cyclic voltametry we were able to observe a total elimination (B/C=300 ppm) or partial elimination (B/C=6000 ppm) of these surface redox couples. We also found in chapter IV that these surface redox couples are responsible of the electrochemical activity of the BDD electrode. BDD electrodes with different doping level B/C=300, 1000 and 6000 ppm) in the vapour phase are modified with IrO2 nanoparticles using the method of thermal decomposition. Our results showed that, at low potentials, the behaviour of the composite electrode IrO2/BDD with respect to the reaction of oxygen evolution is independent of the doping level. On the other hand, at high potentials, the weakly doped electrode (B/C=300 ppm) presents a supplementary overpotential due to the interface BDD-IrO2. An approach based on the double junction is developed in this work describing two distinct behaviours depending on the boron concentration in the diamond film. A new method for Au nanoparticles deposition on highly doped BDD electrodes (B/C= 2500 ppm) is presented in chapter VI. This method involves two steps: the first step consists on sputtering the equivalent of few nanolayers of Au on the diamond surface; the second step is a heat treatment in air at high temperatures. Several parameters that could influence the dispersion, the stability as well as the diameter of the Au nanoparticles are studied. In chapter VII, the behaviour of the composite electrode Au/BDD is studied in a solution containing redox couples in acid media. This electrode was simulated as an arrangement of active Au microelectrodes dispersed on a less active BDD substrate. The overall behaviour of the composite electrode (Au/BDD) has been related to the electrochemical rate constants on each material. In chapter VIII, another new synthesis method for a bimetallic system formed by Au and Pt nanoparticles dispersed on a BDD substrate is developed. This method consists in electrodepositing a small amount of Pt on a composite electrode Au/BDD. Our results showed that Pt was preferentially deposited on Au covering its whole surface. In fact, Au nanoparticles serve as germ of nucleation for Pt. In the following step, the bimetallic PtbAu/BDD is heated to 400°C in air to enhance the interaction between Au and Pt. The experiments showed that Au reappeared on the surface. The electrochemical behaviour of the bi-metallic electrode is characterized by a negative shift in the peak potential of reduction of Pt oxides. This is not the case of heterogeneous alloys obtained by the fusion of two metals at high temperatures. The composite electrode Au/BDD as well as the bi-metallic electrode Pt-Au/BDD are used for electrocatalytic applications. The studied reaction in chapter IX is the oxygen reduction reaction (ORR) in an acid medium. We found that the number of exchanged electrons increases with increasing the amount of Pt at the surface. Oxygen is reduced into H2O2 on the Au/BDD electrode and into H2O on the Pt-Au/BDD electrode. In chapter X we studied the reaction of oxidation of oxalic acid. The behaviour of both Au and Pt electrodes seemed very similar for this reaction. The bi-metallic electrode containing both Pt and Au on its surface has shown an intermediate behaviour between the behaviour of the two metals.

EPFL2006

Photo‐Electrochemical Conversion of CO 2 Under Concentrated Sunlight Enables Combination of High Reaction Rate and Efficiency

Etienne Boutin, Sophia Haussener, Mahendra Patel, Silvan Suter

Photo-electrochemical production of solar fuels from carbon dioxide, water, and sunlight is an appealing approach. Nevertheless, it remains challenging to scale despite encouraging demonstrations at low power input. Higher current densities require notable voltage input as ohmic losses and activation overpotentials become more significant, resulting in lower solar-to-CO conversion efficiencies. A concentrated photovoltaic cell is integrated into a custom-made heat managed photo-electrochemical device. The heat is transferred from the photovoltaic module to the zero-gap electrolyzer cell by the stream of anodic reactant and produce synergetic effects on both sides. With solar concentrations up to 450 suns (i.e., 450 kW m−2) applied for the first time to photo-electrochemical reduction of CO2, a partial current for CO production of 4 A is achieved. At optimal conditions, the solar-to-CO conversion efficiency reaches 17% while maintaining a current density of 150 mA cm−2 in the electrolyzer and a CO selectivity above 90%, representing an overall 19% solar-to-fuel conversion efficiency. This study represents a first demonstration of photo-electrochemical CO2 reduction under highly concentrated light, paving the way for resource efficient solar fuel production at high power input.