department of mechanical engineering, yuan ze university 1 1 water electrolysis (splitting water...

Department of Mechanical Engineering, Yuan Ze University

1Department of Mechanical Engineering, Yuan Ze University

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Water Electrolysis (Splitting Water Using Electricity)

Materials for Water Electrolysis Cells• Hydrogen generation can be accomplished via traditional DC electrolysis of aqueous

solutions at temperatures less than about 100 oC.

• However, electrolysis of stream can also be accomplished at higher temperatures at the cathode of electrolytic cells utilizing solid membranes. The solid membranes typically are electronic insulators and need to be gas-tight (hermetic), but have the special property of being able to conduct ions via fast diffusion through the solid.

• Hydrogen production via the conventional electrolysis largely depends upon the availability of cheap electricity (e.g. , from hydroelectric generators). Consequently, only about 5 % of the world hydrogen production is via electrolysis.

• The only complete hydrogen production process that is free of CO2 emissions is water

electrolysis (if the electricity is derived from nuclear or renewable fuels).

• However, 97% of the hydrogen currently produced is ultimately derived from fossil energy.



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Low-Temperature Electrolysis of Water Solutions

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OHHeOHCathode

eOHOOHAnode


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• The reversible electrical potential (ΔG/nF = Erev) to split the O-H bond in water is 1.229 V.

• In addition, heat is needed for the operation of an electrolysis cell. • If the heat energy is supplied in the form of electrical energy, then the thermal potential

is 0.252 V (at standard conditions), and this voltage must be added to Erev (i.e., add entropic term TΔS to ΔG).

• The (theoretical) decomposition potential for water at standard conditions (for ΔH ≈ ΔHo) is then 1.480 V.

• Anode and cathode reactions for electrolysis (see figure 2.1) are:

Department of Mechanical Engineering, Yuan Ze University3



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• For alkaline electrolysis, OH- ions must be able to move through the membrane (under influence of the electric field) from the cathode chamber into the anode chamber to supply OH- to participate in the reaction (equation 2.1) at the anode.

• Irreversible processes that occur at the anode and cathode and the electrical resistance of the cells cause the actual decomposition potential (voltage) to increase to about 1.85 to 2.05 V.

• This means that the electrolysis efficiency will be between 72 and 80 %. The total electrical resistance of the cell is dependent upon the conductivity of the electrolyte, the ionic permeability of the gas-tight diaphragm that separates the anodic region from the cathodic region, and the current density (normally in the fairly moderate range of 0.1 to 0.3 A ).

• Higher KOH concentrations (up up 47 %) yield higher conductivity, but this usually greatly increases the corrosion of various cell components.


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• Common aqueous electrolytes are better conductors at slightly elevated temperatures (70 to 90 oC), so the electrolysis cells are operated at these conditions.

• The original discovery of electrolytic water splitting used acidic (dilute H2SO4) water, but in industrial plants an alkaline

(e.g., 25 wt % KOH) medium is preferred because corrosion is more easily controlled and cheaper materials can be utilized.

• In order to reduce the actual cell voltage downward toward the 1.48 value (reduce energy consumption), many different catalytic materials have been examined for use as anodes or cathodes (or coatings on underlying electrodes).



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• Low-Temperature PEM-Type Electrolyzers

– Proton exchange membrane or PEM-type water electrolyzers utilize thin films (e.g., 0.25 mm) of a proton-conducting ion exchange material instead of a liquid electrolyte.

– When a reverse polarity is applied to a PEM fuel cell, the fuel cell reactions are reversed and become water electrolysis reactions (see equation 2.6 to 2.8).

– PEM fuel cells have been the subject of research and development for decades. In the 1960s NASA used PEM cells for their HOPE, Gemini, and Biosatellite missions.

– After a lull in the 1980s, a rush of development began in the early 1990s for transportation applications.

– This was initiated by improvements in bonded electrodes, which enabled much higher current densities.

– These improvements can be advantageous to PEM cells used as electrolyzers. Department of Mechanical Engineering,

Yuan Ze University7


• The PEM cells typically use sulfonated polymer (e.g., NafionTM) electrolytes that conduct the protons away from the anode to the cathode (in electrolysis mode).

• For smaller generators, the solid polymer can be more attractive than a dangerous, caustic electrolyte.

• A complicating factor is that the solid-state conduction of the protons is

accompanied by multiple water molecules (H2O)nH+.

• Also, the membrane must be kept hydrated to sustain the conduction mechanism.

• Therefore, water recycling becomes a large consideration since water is constantly removed from the anode and reappears at the cathode (mixed with the hydrogen).



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• At temperatures less than 100 oC, gaseous hydrogen is easily removed from liquid water, but the hydrogen still contains water vapor that most likely requires dehumidification (e.g., pressure swing absorption dryer).

• Electrodes generally have utilized finely divided platinum black or, more recently, IrO2 or RuO2 (for increased electronic

conductivity) as catalysts.

• Research is currently being conducted into PEM-type membranes that have better kinetics, yet are chemically stable at elevated temperatures such that they could operate in steam.



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• PEM water electrolysis cells have a potential advantage over traditional low-temperature electrolysis cells (e.g., KOH in water electrolytes with palladium, titanium, or alternative metal or ceramic electrodes) because PEM devices have been shown to be reversible.

• They can ”load level” by generating electricity from hydrogen (and oxygen) operating as a fuel cell when needed (peak) and reverse to operate as an electrolyer by consuming electricity to produce hydrogen (and oxygen).

• This is convenient if excess electrolysis is available during low periods of consumption (off-paek).

• PEM electrolysis cells could also be used in hybrid systems utilizing solar energy.



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eHHOOHOHAnode

HHeCathode

222:

22:

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2


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• Anode and Tanaka have recently used a Nafion electrolyte in electrolysis mode to decompose two water molecules to simultaneously generate one molecule of hydrogen and one of hydrogen peroxide (used in paper/pulp and chemical industries).

• They do this by using a high applied voltage (1.77 to 2.00 V) in a two-electron transfer process:

and a NaOH anolyte collection solution. No oxygen is generated.


• Low-Temperature Inorganic Membrane Electrolyzers– Electrolyzers operated at low temperatures do not take full advantage of

thermodynamic efficiency advantages.

– The required cell voltage drops considerably (to Eoo = 0.9 V at 927 oC) because of

the positive entropy value (ΔGo = ΔHo - TΔSo) when operating at high temperatures.

– However, sealing bipolar plate devices should be easier at low temperatures since thermal cycling would not result in high stresses due to thermal expansion mismatches between cell components and sealing material.

– Also, inorganic membranes will be more chemically stable in the 200 to 300 oC temperature range than most organic proton-conducting membranes.

– A typical pressurized-water nuclear reactor heats water from 285 to 306 oC (at 2150 psia) in its core and might be a heat source (heat-exchanged steam at temperatures significantly lower than the core temperature) for a low-temperature electrolysis device.



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• Solid inorganic materials exhibiting fast proton conduction at low temperatures seem to be more prevalent than fast oxygen ion conductors.

• Some proton-conducting glasses achieve high proton mobility due to incorporation of water (bonded to POH groups).

• These glasses can be fabricated by sol-gel techniques at low temperatures.

• However, the gels are deliquescent and also are easily fractured into pieces when heated.

• This limits the practical application of these glasses to very low temperatures, and therefore limits the flux values of hydrogen that can be achieved.



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• Fabrication of proton-exchanged ”-alumina compositions is difficult because waters of hydration are lost during firing, and therefore the crystal structure is irreversibly destroyed.

• One approach used to solve this problem, for ”-alumina, has been to fabricate a potassium ion crystal structure by firing to high temperatures. Then, at room temperature, protons can be electrochemically ion exchanged into the crystals from a mineral acid.

• Since the potassium ion is larger than the sodium ion, using the potassium composition lessens lattice strain during the proton exchange process. In these oxide ceramics, two protonic species can exist. The first type is a H2O molecule associated with a proton as hydronium ion (H3O+). The

second type is proton bound to an oxygen ion of the crystal lattice (=OH+).


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• Moderate-Temperature Inorganic Membrane

Electrolyzers

– Steam electrolysis is feasible at moderate temperatures

using cells constructed with solid inorganic (ceramic)

membranes.

– These temperatures could range from approximately 500

to 800 oC using ceramic membranes that are either oxygen

ion or proton conductors.



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• Moderate-Temperature Oxygen Ion Conductors

– The electrolysis reactions to produce hydrogen using oxygen ion conductors are:

– During the electrolysis reaction, oxygen is removed from the reaction site via the membrane (oxygen ion conductor), leaving hydrogen gas and any unreacted steam on the cathode side. In order to obtain pure hydrogen gas, the hydrogen must be separated from the steam by using one of a number of methods. Methods could include condensation of the steam (followed by drying) or the use of a hydrogen-conducting membrane (likely used at elevated temperature and perhaps elevated pressure).

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• Moderate-Temperature Proton Conductors

– Using proton-conducting ceramics as an electrolyte for a steam electrolyzer involves the same reactions as for a low-temperature proton-conducting polymer membrane:

– For fuel cell operations, the proton-conducting cells have a thermodynamic advantage over oxygen ion-conducting cells (due to product water being swept from the cathode by excess air required for cell cooling).

– Applications that are driven by maximizing efficiency at the expense of power density favor proton cells.

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• Proton conductors like the cerates (BaCeO3 and SrCeO3) have been studied for number of years, while doped barium zirconate (BeZrO3) has been advancing strongly in the last couple of years due to reports of high conductivity and good chemical resistance to CO2 (not relevant for steam electrolysis).

• The aliovalent doping creates oxygen vacancies; an incorporation example is given by equation 2.9:

• Water vapor in the cell can react with the oxygen vacancies to from protons per equation 2.10:

• The OHoo species is a proton bound to an oxygen ion in the lattice. However, the proton can

hop from one oxygen ion to another, giving rise to proton conductivity.

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• Moderate-Temperature Bipolar Plates (Interconnects)– At low to moderate temperatures new possibilities arise for using various metals

as bipolar plate (for series connected cells in a bipolar stack arrangement).

– Most metals have too high (e.g., 15 E-6 oC-1) of thermal expansion to match that of zirconia (10.5 E-6 oC-1).

– In order to get a lower thermal expansion metal (to match zirconia), SOFC developers originally tried to use special high-chromium alloys like 95 Cr4- 5 Fe

(Plansee alloy) or 94 Cr - 5 Fe -1 Y2O3.

– However, they ran into the problem of high temperature Cr oxidation.

– The reaction is Cr2O3 + ½ O2 2CrO3 (high vapor pressure gas).

– The presence of alloying element in the interconnect tends to minimize the tendency for the Cr oxidation to take place (especially after oxide scale formation).

– Alloy elements like Y, Ce, Hf, Zr, and Al are reported to slow scale growth. However, these elements tend to form scales with low electronic conductivity, whereas Cr2O3 scales are semiconductors.



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• High-Temperature Inorganic Membrane Electrolyzers– The most common high-temperature cells being investigated are

solid-oxide fuel cells (SOFCs) using yttria-or scandia-stabilized zirconia (cubic phase) electrolytes that are rapid oxygen conductors.

– Over many years, yttrium and scandium have been used to substitute on the zirconium lattice site to stabilize the cubic structure and increase oxygen ion diffusion by creating oxygen vacancies to compensate for their aliovalent (Y 3+ or Sc 3+ on Zr 4+ site) charges.

– Yttria provides excellent structural stabilization and good ionic conductivity, but at significant additional material cost.


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– Loss of conductivity for Scandia-stabilized zirconia has been reported due to phase changes upon aging at high temperatures (i.e., 1,000 oC).

– This instability certainly would be less of a problem for cells operated at lower temperatures (e.g., 800 oC ).

– For long-life operation at high temperatures, it is very important to use suitable electrodes that do not interact (e.g., interdiffuse) unduly with the electrolyte or lose their activity (e.g., sintering).

– Fuel cells using zirconia electrolytes have traditionally used Ni-ZrO2 and doped LaMnO3 electrodes. These combinations have proven to be structurally and chemically stable at high temperatures for long periods with fuel cells operating for up to 25,000 h with performance degradation of less than 0.1 % per 1,000 h.

– Some interdiffusion and formation of nonconductive compounds (e.g., La2Zr2O7) has been reported. These interactions are more severe at high temperatures and long times.



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High-Temperature Electrolysis• The electrolysis reaction can be expressed as: Cathode reaction (3.1)

Anode reaction (3.2) Overall reaction (3.3)

• The enthalpy of the overall reaction is ∆H = 242 kJ/mole at 298 K and 248 kJ/mole at 1,000 K.

• A schematic of an electrolysis cell using an oxygen ion conductor is shown in figure 3.1.


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222 2

1OHOH Energy


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• The benefit of high-temperature electrolysis (HTE) stems from the fact that a portion of endothermic heat of reaction can be supplied by thermal energy instead of electric energy.

• Figure 3.2 shows the energy input required for electrolysis of steam.

– It can be seen that at higher temperatures substantial energy is provided as thermal energy, resulting in considerable reduction of primary (electrical) energy.

• The high temperature also allows high current density operation as both ohmic resistance losses from the electrolyte and electrode materials, and non-ohmic resistance losses from the electrode reaction processes are thermally activated.

• Hydrogen production via room temperature electrolysis of liquid water has the disadvantage of much lower overall thermal-to -hydrogen efficiencies of 24 to 32 % (including power generation), while at higher temperatures practical efficiency can be as high as 50 to 60 %.




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• Materials and Design– The high operating temperature that is necessary for an efficient electrolysis

process requires the use of materials that are stable at those temperatures. – In general, the materials and fabrication technology that are used for high-

temperatures solid-oxide fuel cells (SOFCs) are directly applicable to high-temperature electrolysis devices.

– The high-temperature electrolysis cell is commonly referred to as the solid-oxide electrolysis cell (SOEC).

1. Series-Connected Tubes


The cell used traditional SOFC materials such as 9 mol % yttria-doped zirconia (YSZ) as the electrolyte, a cermet mixture of 50: 50 wt. % nickel and YSZ as the hydrogen electrode, and Ca-doped LaMnO3 perovskite (La0.5Ca0.5MnO3) as the air electrode.

The electrolyte was about 300 m, and the electrodes were about 250 m.

The typical cell diameter was 14 mm, with an active cell length of 10 mm.


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2. Tubular Stack Design The cell materials consist of YSZ electrolyte, 10 mol % yttria, Sr-doped

LaMnO3 air electrode, and a nickel-zirconia cermet hydrogen electrode.

The individual tubes are electrically connected using a Mg-doped LacrO3 interconnection layer.

The primary difference in the material of construction is the interconnect material. Unlike the series-connected tube design, a strip of interconnect is used.

The Mg-doped LaCrO3 perovskite is known for its stability over the range of oxygen partial pressures the interconnect must face.

While Mg-doped LaCrO3 exhibits a low electrical conductivity, it shows excellent stability and low loss of oxygen, minimizing ionic short circuit in the interconnect as well as change in lattice dimensions.

Thus, in the tubular design where a small cross-section of the interconnect is used to connect the cells, a low-conductivity material is favored for its stability.



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3. Planar Stack Design The advantage of the planar design stems from the fact that the

current path of the device has a much larger area and shorter lengths favoring low electrical resistance.




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• Modes of Operation– Unlike the SOFC mode where the reaction is exothermic, the electrolysis mode of operation is endothermic. – In both modes of operation heat is released from the ohmic loss due to the resistance to current flow. – In the SOFC mode, as the stack voltage is decreased, the current increases, causing the stack to release heat. – In fact, heat removal is one of the challenging design and operational issues that limits materials selection, operating

point (i.e., current density), and stack footprint. – In contrast, the endothermic electrolysis reaction and the exotherm of ohmic loss move in opposite directions. – At a certain cell operating voltage, the two balance, resulting in no net heat release. This voltage is referred to as the

thermal neutral voltage, Etn, defined as

– When an SOEC stack is operated at the thermal neutral voltage, the stack operation is isothermal, whereas it is exothermic above and endothermic below that voltage.

– In general, operating the stack near Etn, which is approximately 1.3 V, has certain benefits, in particular the reduced need for cooling air for heat removal, or the need to supply the heat for the reaction.


Fn

HEtn


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• Advanced concepts for high-temperature electrolysis

1.Natural Gas – Assisted Mode Higher operating temperature allows for a reduction in the electricity needed for

electrolysis. However, materials constraints such as oxidation of metal interconnect or other

metallic manifold components and continued sintering of porous electrodes may result in performance degradation at high temperatures.

Pham et have proposed a method for reducing the voltage necessary for steam electrolysis, thereby reducing the electric power consumption. The process, known as natural gas-assisted steam electrolysis (NGASE), uses natural gas as the anode reactant in place of commonly used air or steam as the sweep gas for removing the oxygen evolved in the anode compartment. Thus, the oxygen transported through the electrolyte membrane partially or fully oxidizes the natural gas, which in effect provides a significant portion of the driving force for the oxygen transport through the membrane.



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2. Hybrid SOFC-SEOC stacks The SOFC mode of operation is exothermic while the SOEC mode can be

endothermic, thermal neutral, or exothermic depending on the operating voltage. The hydrogen production efficiency, defined as the ratio of heating value of

generated hydrogen to electric power input, is 100 % at the thermal neutral voltage, higher in the endothermic mode as the operating voltage moves closer to the open-circuit voltage, and lower when the voltage is higher than thermal neutral. The efficiency can be as high as 140 % near the open-circuit voltage.

Operating near the thermal neutral voltage is generally considered favorable from both the operational and hydrogen production perspectives. As the SOEC can be operated with minimal requirement for heat supply or removal, it can potentially be scaled up to large-footprint devices, unlike SOFC, where the heat removal requirement constrains the overall footprint. Thus, in a reversible fuel cell, one that operates in SOFC and SOEC modes, the cell area is constrained by the cooling requirements in the SOFC mode.

In order to overcome the heat removal constraints, a hybrid stack concept has been proposed. By integrating both SOFC and SOEC cells in a single stack, the exothermic SOFC and endothermic SOEC operations can be used to reduce the cooling air requirement, and thus allow for larger-footprint devices.



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3. Integration of primary energy sources with high-temperature electrolysis process

The attraction of the high-temperature steam electrolysis process comes from the fact that a portion of the required energy for the process is supplied as thermal energy, thereby reducing the electrical need.

The concept of using electricity to produce hydrogen, which in turn will be used to produce electricity, makes sense only if the electric power for electrolysis is inexpensive or from excess capacity, and thus the hydrogen becomes an energy carrier.

Additionally, the compression of hydrogen for transport typically consumes 10 % of the energy content. Considering the overall environmental effect, combining high-temperature electrolysis with a renewable energy source is a good option － in particular when the electricity generation is intermittent or the demand is low.



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• Materials ChallengesHigh-temperature operation physical and chemical changes to

the cell materials performance degradation (resistance increases with time)

Interconnect that joins the individual cells to form a stack.

Corrosion scale near the seal area

Evaporation and condensation of chromium vapor from the

interconnect onto the air electrode.


department of mechanical engineering, yuan ze university 1 1 water electrolysis (splitting water...

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