Recent progress in alkaline water electrolysis for hydrogen production and applications

To improve the sustainability of transportation, a major goal is the replacement of conventional petroleum-based fuels with more sustainable fuels that can be used in the existing infrastructure (fuel distribution and vehicles). While fossil-derived synthetic fuels (e.g. coal derived liquid fuels) and biofuels have received the most attention, similar hydrocarbons can be produced without using fossil fuels or biomass. Using renewable and/or nuclear energy, carbon dioxide and water can be recycled into liquid hydrocarbon fuels in non-biological processes which remove oxygen from CO2 and H2O (the reverse of fuel combustion). Capture of CO2 from the atmosphere would enable a closed-loop carbon-neutral fuel cycle. This article critically reviews the many possible technological pathways for recycling CO2 into fuels using renewable or nuclear energy, considering three stages—CO2 capture, H2O and CO2 dissociation, and fuel synthesis. Dissociation methods include thermolysis, thermochemical cycles, electrolysis, and photoelectrolysis of CO2 and/or H2O. High temperature co-electrolysis of H2O and CO2 makes very efficient use of electricity and heat (near-100% electricity-to-syngas efficiency), provides high reaction rates, and directly produces syngas (CO/H2 mixture) for use in conventional catalytic fuel synthesis reactors. Capturing CO2 from the atmosphere using a solid sorbent, electrolyzing H2O and CO2 in solid oxide electrolysis cells to yield syngas, and converting the syngas to gasoline or diesel by Fischer–Tropsch synthesis is identified as one of the most promising, feasible routes. An analysis of the energy balance and economics of this CO2 recycling process is presented. We estimate that the full system can feasibly operate at 70% electricity-to-liquid fuel efficiency (higher heating value basis) and the price of electricity needed to produce synthetic gasoline at U.S.D$ 2/gal ($ 0.53/L) is 2–3 U.S. cents/kWh. For $ 3/gal ($ 0.78/L) gasoline, electricity at 4–5 cents/kWh is needed. In some regions that have inexpensive renewable electricity, such as Iceland, fuel production may already be economical. The dominant costs of the process are the electricity cost and the capital cost of the electrolyzer, and this capital cost is significantly increased when operating intermittently (on renewable power sources such as solar and wind). The potential of this CO2 recycling process is assessed, in terms of what technological progress is needed to achieve large-scale, economically competitive production of sustainable fuels by this method.

Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy

The processes of passive oxidation, deposit-induced corrosion, active oxidation, scale/substance interactions, and scale volatility are presently studied in the case of high-purity SiC and Si3N4 in pure oxygen, giving attention to such secondary elements in the ceramics as water and CO2 oxidants, combustion environment impurities, and thermal cycling. Deposit-induced corrosion is discussed for the cases of NaSO4 as well as vanadate and oxide-slag deposits; issues associated with the active-to-passive oxidation transition are noted.

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Corrosion of Silicon-Based Ceramics in Combustion Environments

The oxidation behaviour of metals and alloys at high temperatures in atmospheres containing water vapour: A review

Accident tolerant fuel cladding development: Promise, status, and challenges

The oxidation of Cr and Fe‐Cr alloys which form scales involves at least two processes: (i) the solid‐state, diffusive transport of ionic species, with parabolic kinetics, and (ii) the oxidation of to a volatile species, probably , with linear kinetics. The effects of this second reaction cannot be neglected, either in interpretation of short‐term oxidation results, or in predicting long‐term oxidation behavior. In this paper equations are derived which describe the oxidation behavior of a system in which both diffusive and gas/oxide interface processes occur simultaneously. According to the model, the oxide scale grows to a limiting thickness which is determined by the parabolic rate constant and the surface‐reaction rate constant; as the limiting thickness is approached, a transition from parabolic to linear kinetics takes place. The model is applied to the oxidation of Cr and Fe‐Cr alloys, and good agreement with experimental data is found.

The Effect of Oxide Volatilization on the Oxidation Kinetics of Cr and Fe‐Cr Alloys

This paper describes the dissociation of water vapor in high‐temperature, solid oxide electrolyte cells. The characteristics of these cells are related to a composition‐dependent open‐circuit voltage, which can be obtained from thermodynamic considerations, to ohmic resistance, and to mass transport overvoltages. The problem of maintaining a heat balance in a system consisting of such cells is considered in terms of the heat loss through the leads connecting the cells with the surroundings. A conclusion is that the electrical energy requirement for dissociation can be reduced substantially by series connection of individual cells within the high‐temperature region of the system, but that ohmic resistance limits the optimized current density through the electrolyte to values well below the maximum values that are feasible.

C. S. Tedmon

Papers

The Effect of Oxide Volatilization on the Oxidation Kinetics of Cr and Fe‐Cr Alloys

Electrochemical Dissociation of Water Vapor in Solid Oxide Electrolyte Cells: I . Thermodynamics and Cell Characteristics