Hydrogen production by alkaline water electrolysis

104

Strategies are presented to enhance operating potential and cycle life of AC/AC capacitors using salt aqueous electrolytes. Li 2 SO 4 (pH = 6.5) allows 99% efficiency to be exhibited at 1.6 V cell potential with low self-discharge, while in BeSO 4 (pH = 2.1) efficiency is low (81%). Li 2 SO 4 performs better due to high di-hydrogen over-potential at the negative electrode and related pH increase in AC porosity. When stainless steel current collectors are used in Li 2 SO 4 , the cell resistance suddenly increases after 12 hours floating at 1.6 V, due to corrosion of the positive collector. With nickel negative and stainless steel positive collectors, the electrode potentials are shifted by −105 mV at cell potential of 1.6 V, allowing stable cell parameters (capacitance, resistance) and reduction of corrosion products formation on positive steel collector after 120 hours floating. Phenanthrenequinone was grafted on activated carbon to get an additional faradaic contribution in buffer solutions (pH = 4.0 or 7.2). The three-electrode cell CVs show that the redox peaks of the phenanthrenequinone graft shift toward negative values when pH increases from 4 to 7. Electrical double-layer capacitors (EDLCs) based on activated carbons (AC) and traditional aqueous electrolytes, such as H 2 SO 4 and KOH, operate at low cell potential (up to ∼0.8 V), limiting their capability at industrial level. [1][2][3][4] In order to improve the energy (E = 1 2 CU 2 ), both capacitance (C) and cell potential (U) determined by the stability window of the electrolyte have to be optimized. In this context, it has been recently demonstrated that symmetric capacitors based on AC electrodes and salt aqueous electrolyte (0.5 mol L −1 Na 2 SO 4 ) exhibit excellent cyclability under galvanostatic charge/discharge up to 1.6 V. 5,6 . While using 1 mol L −1 Li 2 SO 4 and gold current collectors, excellent cycle life has been shown up to ∼1.9 V using galvanostatic charge/discharge. 7,8 Such high cell potential value is caused by a high over-potential for di-hydrogen evolution as a consequence of water reduction and OH − ions generation in the porosity of the negative AC electrode. 5,7 According to the Nernst equation (E red = −0.059 pH), the pH increase associated to OH − ions causes locally a shift of redox potential to lower values. Based on this fact, it has been recently demonstrated that the di-hydrogen evolution over-potential is higher in almost neutral electrolyte solutions (pH = 4-8) than in acidic ones, resulting in larger operating cell potential of asymmetric capacitors in the former media.9 From the foregoing, it makes sense to study in more details the influence of aqueous electrolyte pH on the electrochemical performance of AC/AC capacitors beyond only the cell potential.Besides, when shifting from gold to stainless steel current collectors in order to develop low cost AC/AC capacitors in 1 mol L −1 Li 2 SO 4 , constant capacitance and low cell resistance have been observed during potentiostatic floating at cell potential of 1.5 V, whil...

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confidence: 77%

Strategies to Improve the Performance of Carbon/Carbon Capacitors in Salt Aqueous Electrolytes

Abbas

Ratajczak

Babuchowska

et al. 2015

104

“…1, provides a new low energy conditions to drive electrolytic water splitting. Solar thermal energy is readily absorbed at conversion efficiency exceeding 65%, and provides an efficient energy mechanism to maintain a high reactant pressure that substantially decreases the energy needed to split water by sunlight.Sodium and potassium hydroxide electrolytes at temperatures up to 200 degrees C and operating at moderate pressures have been used to decrease the energy required and increase the sustainable hydrogen production rate of commercial alkaline hydrogen generators.14,15 Subsequent to an initial study indicating high energy efficiencies for water electrolysis in molten hydroxides operating up to 350• C, 16 there have been surprisingly few studies of water splitting in molten hydroxide electrolytes at higher temperatures (up to ∼400• C), [17][18][19] and there have been only two studies of water sodium hydroxide mixtures at supercritical conditions. 20,21 This study provides an initial exploration of water splitting in molten hydroxide electrolytes at higher temperatures ranging up to 700…”

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confidence: 99%

“…• C), [17][18][19] and there have been only two studies of water sodium hydroxide mixtures at supercritical conditions. 20,21 This study provides an initial exploration of water splitting in molten hydroxide electrolytes at higher temperatures ranging up to 700…”

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confidence: 99%

Comparison of Alternative Molten Electrolytes for Water Splitting to Generate Hydrogen Fuel

Licht

Liu

Cui

et al. 2016

This study explores a new high temperature molten hydroxide domain to electrochemically split water into hydrogen fuel. Hydrogen fuel, if produced without greenhouse gas emissions, is a promising fuel for transportation. This study opens a pathway to low energy water splitting and the electrolytic production of hydrogen fuel without carbon dioxide emission. A wide range of pure and mixed alkali and alkali earth hydroxide electrolytes are explored at temperatures ranging from 200 to 700 • C. Higher temperature leads to improved (lower voltage) water splitting and improved rates of charge transfer, but carries challenges (increased rates of parasitic side reactions as the molten electrolytes dehydrate with increasing temperature). This study extends the range of hydrogen formation in alkaline electrolysis water splitting to over 600 • C, and demonstrates that lithium and/or barium hydroxide electrolytes remain hydrated at high temperatures, and in the high temperature domain are advantageous over sodium or potassium hydroxide electrolytes. In pure LiOH, the coulombic efficiency for hydrogen generation decreases with temperature and is measured respectively at η H 2 = 88%, 21%, 4% and 0%, respectively at 500, 600, 700 and 800 • Hydrogen has advantageous features compared to fossil fuels. It has a higher specific energy, is not polluting, has a combustion product of water, and in particular it does not emit the greenhouse gas carbon dioxide. Challenges of hydrogen storage are being addressed and hydrogen powered vehicles have been introduced to the commercial marketplace.1-4 Unfortunately, the predominant fraction of hydrogen generated today is produced by steam reformation of fossil fuel by reacting the fossil fuel with steam to release H 2 and CO 2 . Steam reformation is a substantial source of CO 2 emissions, negating the climate mitigation effect of using H 2 as a fuel. Thus, a process to efficiently generate hydrogen without carbon dioxide emissions is needed.The development of electrolytic splitting of water dates back to 1802. 5 The electrolysis of water produces hydrogen and oxygen directly without CO 2 emission (if the energy source used to generate the electricity did not release CO 2 ). Utilization of renewable or nuclear energy to generate the electricity for this electrolysis can drive this electrolytic water splitting to produce hydrogen as a fuel without carbon dioxide emissions. The most abundant of renewable energy resource for electrical generation is solar energy, and as early as 2001 we were driving stable solar electrolytic water splitting to hydrogen fuels at over 18% solar to chemical energy conversion by using efficient multiple bandgap solar cells.6 One of the challenges to the use of illuminated junctions to drive electrochemical or photoelectrochemical water splitting is that the bandgap of efficient semiconductors lies in the visible spectrum which generate a photo-potential less than the minimum needed rest potential of 1.23 volt required to split water to hydrogen and oxygen at room te...

“…10,11 Despite their importance to device operation, the membranes of these electrolyzers can be costly, prone to degradation or fouling, 12-16 increase cell resistance, and entail the use of a membrane electrode assembly (MEA)-based design that requires at least 10 components. 14,[17][18][19] The high cost of * Electrochemical Society Member.…”

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confidence: 99%

“…5 Within these devices, the membrane serves two key purposes, which are facilitating ion transport between the anode and cathode, and physically separating the product species produced at the anode and cathode to prevent crossover. 10,11 Despite their importance to device operation, the membranes of these electrolyzers can be costly, prone to degradation or fouling, [12][13][14][15][16] increase cell resistance, and entail the use of a membrane electrode assembly (MEA)-based design that requires at least 10 components. 14,[17][18][19] The high cost of * Electrochemical Society Member.…”

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confidence: 99%

Hydrogen Production with a Simple and Scalable Membraneless Electrolyzer

O’Neil

Christian

Brown

et al. 2016

Ion-conducting membranes are essential components in many electrochemical devices, but they often add substantial cost, limit performance, and are susceptible to degradation. This work investigates membraneless electrochemical flow cells for hydrogen production from water electrolysis that are based on angled mesh flow-through electrodes. These devices can be fabricated with as few as three parts (anode, cathode, and cell body), reflecting their simplicity and potential for low-cost manufacture. 3D printing was used to fabricate prototype electrolyzers that were demonstrated to be electrolyte agnostic, modular, and capable of operating with minimal product crossover. Prototype electrolyzers operating in acidic and alkaline solutions achieved electrolysis efficiencies of 61.9% and 72.5%, respectively, (based on the higher heating value of H 2 ) when operated at 100 mA cm −2 . Product crossover was investigated using in situ electrochemical sensors, in situ imaging, and by gas chromatography (GC). GC analysis found that 2.8% of the H 2 crossed over from the cathode to the anode stream under electrolysis at 100 mA cm −2 and fluid velocity of 26.5 cm s −1 . Additionally, modularity was demonstrated with a three-cell stack, and high-speed video measurements tracking bubble evolution from electrode surfaces provide valuable insight for the further optimization of electrolyzer design and performance. Solar and wind energy have the potential to power the planet without the environmental impact of fossil fuels, but encounter significant challenges to widespread adoption due to their low capacity factors and inherent intermittency.1 In order to overcome this challenge, affordable grid-scale energy storage technology is needed that can make electricity generation from these technologies more widespread.2 One solution to this issue is to convert excess renewable electricity into stored chemical energy in the form of hydrogen gas (H 2 ), 3 which represents a promising candidate for grid scale energy storage and as a carbon-free replacement of fossil fuels in the transportation and industry sectors. 4 Electrolyzers, which use electricity and water to produce hydrogen and oxygen, are well-established commercially available technologies, 5 but the cost of producing H 2 by water electrolysis is currently too expensive.6-8 Presently, much of the cost of producing H 2 by water electrolysis comes from the price of electricity, 6,8 but as the price of electricity from wind and solar continues to decrease and time-of-use pricing schemes become more prevalent, decreasing the cost of electrolyzer technology will be of great importance to making a renewable hydrogen future a reality.The majority of electrolyzers are based on a design in which the cathode and anode are separated by an ion-conducting membrane or diaphragm. 9 The two most common types of electrolyzers are alkaline and polymer electrolyte membrane (PEM) electrolyzers, which are able to electrolyze alkaline and ultra-pure water, respectively. These electrolyzers are mature t...