Nanotechnology for photolytic hydrogen production: Colloidal anodic oxidation

It is widely accepted that the nanomaterials with the most promise for industrial photo-catalytic hydrogen production are prone to disabling photo-corrosion through anodic oxidation upon illumination in aqueous environments. Steps must be taken to either arrest the photo-oxidation of these particular materials, or to develop suitable alternatives which would provide sufficient photo-catalytic activity for industrial purposes. This review article addresses the background of photolytic hydrogen production from water, and examines the theory and current level of research aimed at overcoming semiconductor photo-corrosion. It should be noted that this review does not seek to present technical knowledge outlining synthesis methods or photo-catalytic libraries, rather to outline the recent efforts in semiconductor based nano-systems, being inert metal coatings, Z-schemes, doping of UV-absorbing metal-oxides, and dimensional nanorod structures.     

Technological aspects of sulfur dioxide depolarized electrolysis for hydrogen production

Significant advancements in the SO₂-depolarized electrolysis technology have been obtained through a series of technical improvements in the electrode structure and cell configurations. Using carbon-supported platinum catalyst of loading 1 mg/cm², the currently achievable cell potentials are 680 and 910 mV at 200 and 400 mA/cm², respectively, in 50 w/o aqueous H₂SO₄ solution at 75°C and atmospheric pressure. To reduce further the electrical energy input for an SO₂-depolarized cell, major experimental efforts will be made to investigate the performance characteristics of pressurized electrolyzers. An advanced concept, using a solid oxide electrolyte, has been proposed for the electrochemical oxidation of gas-phase SO₂. Due to the elimination of aqueous electrolyte, this novel sulfur-based cycle is expected to result in a higher cycle efficiency and a lower corrosion environment than the conventional sulfur cycle hydrogen product process.     

The anodic oxidation of sulfur dioxide in the sulfuric acid hybrid cycle

The anodic oxidation of sulfur dioxide as a function of sulfuric acid concentration, temperature and pressure was studied. Measurements demonstrate that the sulfuric acid concentration should be lower than about 60 weight % in order to obtain a reasonable catalysis of anodic sulfur dioxide oxidation. The current density increases in the potential region of the slope of the current/potential curve with increasing temperature and pressure. The system of graphite (carbon) plus very small amounts of hydrogen iodide proved to be a good catalyst of anodic sulfur dioxide oxidation. These very small amounts of hydrogen iodide can be separated from the sulfuric acid by vaporization before entering the high temperature step of the sulfuric acid hybrid cycle.For the separation of anolyte and catholyte in order to prevent sulfur deposition on the cathode from sulfur dioxide out of the anode compartment, a three compartment electrolytic cell with an intermediate electrolyte flow is being suggested.     

A techno-economic analysis of decentralized electrolytic hydrogen production for fuel cell vehicles

Hydrogen from decentralized water electrolysis is one of the main fuelling options considered for future fuel cell vehicles. In this study, a model is developed to determine the key technical and economic parameters influencing the competitive position of decentralized electrolytic hydrogen. This model incorporates the capital, maintenance and energy costs of water electrolysis, as well as a monetary valuation of the associated greenhouse gas (GHG) emissions. It is used to analyze the competitive position of electrolytic hydrogen in three specific locations with distinct electricity mix: Vancouver, Los Angeles and Paris. Using local electricity prices and fuel taxes, electrolytic hydrogen is found to be commercially viable in Vancouver and Paris. Hydrogen storage comes out as the most important technical issue. But more than any technical issue, electricity prices and fuel taxes emerge as the two dominant issues affecting the competitive position of electrolytic hydrogen. The monetary valuation of GHG emissions, based on a price of $20/ton of CO₂, is found to be generally insufficient to tilt the balance in favor of electrolytic hydrogen.     

Rare-earth modified platinum-based electrocatalysts incorporating anodic glycerol oxidation reactions while promoting cathodic hydrogen evolution reactions

Glycerol electrooxidation (GOR) is a safe and clean method for conversion of glycerol. In the course of this work, a series of Pt_xCe/C catalysts have been synthesized by successive polyol reduction methods as well as etching and calcination. Various physicochemical characterrisation and electrochemical measurements have demonstrated the excellent performance of Pt₃Ce/C composites in glycerol oxidation (GOR) and hydrogen evolution reactions (HER). The Pt₃Ce/C catalyst showed better performance in GOR and HER compared to commercially available Pt/C electrocatalysts. This paper demonstrates an energy efficient electrocatalyst for the simultaneous precipitation of hydrogen from glycerol oxidation.     

Improved cathodes for industrial electrolytic hydrogen production

Reduced overpotentials for the generation of hydrogen by alkaline water electrolysis can be achieved with a.c. activation of porous Ni electrodes. Reductions of 50–60 mV were attained which would contribute towards reducing costs in commercial pure hydrogen production by electrolysis.     

A new cathodic electrode deposit with palladium nanoparticles for cost-effective hydrogen production in a microbial electrolysis cell

Microbial electrolysis cell (MEC) provides a sustainable way for hydrogen production from organic matters, but it still suffers from the lack of efficient and cost-effective cathode catalyst. In this work carbon paper coated with Pd nanoparticles was prepared using electrochemical deposition method and used as the cathodic catalyst in an MEC to facilitate hydrogen production. The electrode coated with Pd nanoparticles showed a lower overpotential than the carbon paper cathode coated with Pt black. The coulombic efficiency, cathodic and hydrogen recoveries of the MEC with the Pd nanoparticles as catalyst were slightly higher than those with a Pt cathode, while the Pd loading was one order of magnitude less than Pt. Thus, the catalytic efficiency normalized by mass of the Pd nanoparticles was about fifty times higher than that of the Pt black catalyst. These results demonstrate that utilization of the cathode with Pd nanoparticles could greatly reduce the costs of the cathodic catalysts when maintaining the MEC system performance.     

Advantages of integration with industry for electrolytic hydrogen production

This paper evaluates possible synergies with industry, such as heat and oxygen recovery from the hydrogen production. The hydrogen production technology used in this paper is electrolysis and the calculations include the cost and energy savings for integrated hydrogen production. Electrolysis with heat recovery leads to both cost reduction and higher total energy efficiencies of the hydrogen production. Today about 15–30% of the energy supplied for the production is lost and most of it can be recovered as heat. Utilization of the oxygen produced in electrolysis gives further advantages. The integration potential has been evaluated for a pulp and paper industry and the Swedish energy system, focusing on hydrogen for the transportation sector. The calculated example shows that the use of the by-product oxygen and heat greatly affects the possibility to sell hydrogen produced from electrolysis in Sweden. Most of the energy losses are recovered in the example; even gains in energy for not having to produce oxygen with cryogenic air separation are shown. When considering cost, the oxygen income is the most beneficial but when considering energy efficiency, the heat recovery stands for the greater part.     

Optimization of electrolytic input power for the production of hydrogen

In this paper, effect of input voltage to the electrodes on hydrogen production rate (HPR) and efficiency (η)$(\eta )$ of hydrogen production was studied. The input DC voltage to the cell was maintained at 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 3.0, 6.0, 9.0 and 12.0 V. The optimum input voltage was found between 2.0 and 2.4 V for the best efficiency. The extra-over potential voltage to the electrodes would be the cause of energy loss through heat. Sensitivity analysis of the system for voltage with respect to HPR has been carried out and the results are also presented in the paper.     

Recent advancements in the cathodic catalyst for the hydrogen evolution reaction in microbial electrolytic cells

The microbial electrochemical technology is a foremost viable technology for hydrogen production from organic matter or wastewater catalyzed by electroactive microorganisms. Developing a high-efficient and low-cost cathode for hydrogen production is crucial for the practical applications of MEC. In the present article, cathode materials and catalysts for hydrogen evolution reaction (HER) in MECs are reviewed. There is an essential requirement of cost-effective HER catalysts for improving MEC performance and as the practical findings fell short of the ideal catalyst's expectations, the density functional theory (DFT) can give essential molecular knowledge and anticipate viable catalysts. Additionally, this article provides an overview of the development of density functional theory (DFT), as well as computer simulations for HER processes using DFT, and also computational designs and virtual screens of novel HER catalysts. The development of catalysts combined with DFT simulations offers significant advancements in the near future on the path to an ideal electrocatalyst in MEC.     

Potential of carbon dioxide radiolysis for hydrogen production

Carbon dioxide radiolysis was investigated theoretically and experimentally, and its potential for use in hydrogen production was examined. Elementary processes in the physical, physicochemical and chemical stages of carbon dioxide radiolysis were examined from a viewpoint of energy conversion efficiency from radiation to chemical energy. The energy conversion efficiency in the physical stage was ca 70%, and in the physicochemical stage ca 40%. The back reaction in carbon dioxide radiolysis reduces product yields and lowers the energy conversion efficiency, and should be suppressed. Effects of additives, high absorbed doses and fission fragment irradiation were studied experimentally.     

Energy and exergy analyses of electrolytic hydrogen production with molybdenum-oxo catalysts

This paper analyzes a new low-temperature electrolysis hydrogen production system using molybdenum-oxo catalysts in the cathode and a platinum based anode. A thermodynamic model is developed for the electrolysis process in order to predict and analyze the energy and exergy efficiencies. The new electrolysis system with molybdenum-oxo catalysts consists of two half cells of PEM (proton exchange membrane) and alkaline electrolysis. The effects of temperature and membrane thickness are reported at varying current densities. The results are presented and compared with previous studies to demonstrate the promising performance of the system.     

Gas diffusion electrodes for the oxidation of sulfur dioxide with reduced gas permeability

The effect of the thickness and structure of the active layer of gas diffusion electrodes for the oxidation of sulfur dioxide on the gas permeability and on their current-voltage characteristics were investigated. As a result of this operation electrodes ensuring 75% utilization of sulfur dioxide at i = 50 mA cm^?2 and ? = +600 mV (HE) have been developed.     

Development of the once-through hybrid sulfur process for nuclear hydrogen production

The Once-through Hybrid Sulfur (Ot-HyS) process, proposed in this work, produces hydrogen using the same Sulfur dioxide Depolarized water Electrolysis (SDE) process found in the original Hybrid Sulfur cycle (HyS). In the process proposed here, the Sulfuric Acid Decomposition (SAD) process in the HyS procedure is replaced with the well-established sulfur combustion process. First, a flow sheet for the Ot-HyS process was developed by referring to existing facilities and to the work done by the Savannah River National Laboratory (SRNL) under their reasonable assumptions. The process was then simulated using Aspen Plus with appropriate thermodynamic models. It was demonstrated that the Ot-HyS process has higher net thermal efficiency, as well as other advantages, over competing benchmark processes. The net thermal efficiency of the Ot-HyS process is 47.1% (based on LHV) and 55.7% (based on HHV) assuming 33.3% thermal-to-electric conversion efficiency of a nuclear power plant with no consideration given to the work for the air separation. Hydrogen produced through the Ot-HyS process would be used as off-peak electricity storage, to relieve the burden of load-following and could help to expand applications of nuclear energy, which is regarded as a ’sustainable development’ technology.     

Status of advanced electrolytic hydrogen production in the United States and abroad

For the past five years the Department of Energy has sponsored a program aimed at the development of advanced technology for the electrolytic production of hydrogen. The original goal was to create a large-scale hydrogen production capability in the United States which would be based upon non-fossil energy resources and would be suitable for commercialization by industry as markets would be realized. Brookhaven National Laboratory (BNL) has been assigned technical responsibility for a program which addresses hardware development at three industrial contractors as well as supporting fundamental research at BNL and several university laboratories. Recently, revised federal guidelines and budgetary reductions have resulted in a reversal of priorities which favor longer-range research activities and which virtually eliminate the support of large-scale development engineering and test projects. Programs at BNL have been redirected reflecting these revised guidelines and emphasizing longer-term research activities. In view of the change in programmatic direction it is appropriate to provide an overview of the current status of electrolytic hydrogen production. A survey of international pursuits in advanced water electrolysis technology is provided based upon progress reported via International Energy Agency Agreements.     

Sulfur dioxide depolarized electrolysis for hydrogen production: Development status

Four promising approaches were pursued to improve electrolysis technology: (a) electrochemical study of catalysts, (b) development of electrode fabrication processes, (c) optimization of cell configuration and (d) selection and investigation of separator materials. Palladium and palladium oxide have been shown to be better catalysts than platinum for SO₂ oxidation. The activation energy for SO₂ oxidation on a palladium electrode is ～25 kcal mol ^?1. Fabrication processes for producing high-surface-area electrodes have been successfully developed. By use of Pt/C catalysts, stable cell voltages of 0.77 and 1.05 V (including ohmic losses) were achieved at 200 and 400 mA cm^?2, respectively, while operating an SO₂-depolarized electrolyser at 50°C and atmospheric pressure using 50 w/o sulfuric acid. As verified in an endurance test of 173 hr duration, a stabilized performance of an electrolyser was demonstrated with a cell voltage ～675 mV at 100 mA cm^?2. The purity of hydrogen gas evolved in the electrolyser was approximately 98.7 v/o. From the viewpoint of cell voltage, the optimum acid concentration for operating an SO₂-depolarized electrolyser has been identified to be ca 30 w/o.     

Hybrid sulfur cycle flowsheets for hydrogen production using high-temperature gas-cooled reactors

Two hybrid sulfur (HyS) cycle process flowsheets intended for use with high-temperature gas-cooled reactors (HTGRs) are presented. The flowsheets were developed for the Next Generation Nuclear Plant (NGNP) program, and couple a proton exchange membrane (PEM) electrolyzer for the SO₂-depolarized electrolysis step with a silicon carbide bayonet reactor for the high-temperature decomposition step. One presumes an HTGR reactor outlet temperature (ROT) of 950 °C, the other 750 °C. Performance was improved (over earlier flowsheets) by assuming that use of a more acid-tolerant PEM, like acid-doped poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI), instead of Nafion^®, would allow higher anolyte acid concentrations. Lower ROT was accommodated by adding a direct contact exchange/quench column upstream from the bayonet reactor and dropping the decomposition pressure. Aspen Plus was used to develop material and energy balances. A net thermal efficiency of 44.0-47.6%, higher heating value basis is projected for the 950 °C case, dropping to 39.9% for the 750 °C case.