Electrolysis of Water

"Personally, I think that 400 years hence the power question in England may be solved somewhat as follows. The country will be covered with rows of metallic windmills working electric motors, which in their turn supply current at a very high voltage to great electric mains. At suitable distances there will be great power stations where during windy weather the surplus power will be used for the electrolytic decomposition of water into hydrogen and oxygen In times of calm, the gases will be recombined in explosion motors working dynamos which produce electrical energy once more or more probably in oxidation cells."

Haldane, in his talk entitled, Daedalus or Science and the Future, Cambridge University, 1923.13

Hydrogen as an energy carrier and potentially widely-used fuel is attractive because it can be produced easily without emissions by splitting water. In addition, the readily available electrolyzer can be used in a home or business where off-peak or surplus electricity could be used to make the environmentally preferred gas. Electrolysis was first demonstrated in 1800 by William Nicholson and Sir Anthony Carlisle and has found a variety of niche markets ever since. Two electrolyzer technologies, alkaline and proton exchange membrane (PEM), exist at the commercial level with solid oxide electrolysis in the research phase.

Electrolysis is defined as splitting apart with an electric current. Decomposition of the water occurs when a direct current (DC) is passed between two electrodes immersed in water separated by a non-electrical conducting aqueous or solid electrolyte to transport ions and completing the circuit. The voltage applied to the cell must be greater than the free energy of formation of water plus the corresponding activation and ohmic losses before decomposition will proceed. Ion transport through the electrolyte is critical as the purest of water would only contain small amounts of ions making it a poor conductor.

Ideally, 39 kWh of electricity and 8.9 liters of water are required to produce 1 kg of hydrogen at 25 °C and 1 atmosphere pressure. Typical commercial electrolyzer system efficiencies are 56%-73% and this corresponds to 70.1-53.4 kWh/kg.14 The U.S. consumes somewhere between 140-150 billion gallons of gasoline per year equating to the same number of kilograms if we were to use only hydrogen for transportation. This would result in needing 330 billion gallons of water to make that much hydrogen. If the hydrogen were used in a fuel cell that is two times as efficient as an internal combustion engine in a car the amount of water required would be half. For comparison, gasoline production uses 300 billion gallons per year, domestic water use tops 4800 billion gallons per year and thermal electric power generation 70 trillion gallons per year.15

When comparing literature from fuel cell (FC) models with water electrolysis work it is important to remember the differences between the system anode and cathode. This basic understanding may be trivial to most but is many times confused when switching between the two processes. The anode is always the electrode at which oxidation occurs, where electrons are lost. The cathode is defined as the electrode at which electrons enter, where reduction takes place. In electrolysis the cathode is the electrode where H2 gas is created, in FC systems the anode is the electrode where H2 is introduced.

2.1 Alkaline

The alkaline electrolyzer is a well-established technology that typically employs an aqueous solution of water and 25-30 wt.% potassium hydroxide (KOH). However, sodium hydroxide (NaOH), sodium chloride (NaCl) and other electrolytes have also been used. The liquid electrolyte enables the conduction of ions between the elec trodes and is not consumed in the reaction but does need to be replenished periodically due other system losses. Typically commercial alkaline electrolyzers are run with current densities in the range of 100-400 mA cm-2. The reactions for the alkaline anode and cathode are shown in Eqs. 1 and 2 respectively, showing the hydroxyl (OH-) ion transport.

The first water electrolyzers used the tank design and an alkaline electrolyte.20 These electrolyzers can be configured as unipolar (tank) or bipolar (filter press) designs. In the unipolar design (see Figure 1), electrodes, anodes, and cathodes are alternatively suspended in a tank. In this design, each cell is connected in parallel and the entire system operated at 1.9-2.5 Vdc.

The advantage to the unipolar design is that it requires relatively few parts, is extremely simple to manufacture and repair because individual cells can be taken offline while the remaining cells remain productive. The disadvantage is that it usually operates at lower current densities and lower temperatures.16 More recent unipolar designs include operation at high hydrogen pressure outputs (up to 6,000 psig). The bipolar design (Fig. 2), often called the filter-press, has alternating layers of electrodes and separation diaphragms that are clamped together. The cells are connected in series and result in higher stack voltages. Since the cells are relatively thin, the overall stack can be considerably smaller than the unipolar design. The advantages to the bipolar design are the reduced stack footprints, higher current densities, and its ability to produce higher pressure gas. The disadvantage is that it cannot be repaired without servicing the entire stack.16,17 Fortunately, it rarely needs servicing. Previously asbestos was used as a separation diaphragm, but manufacturers have replaced or are planning to replace this with new polymer materials such as Ry-ton®.18

2.2 Proton Exchange Membrane

A second commercially available electrolyzer technology is the solid polymer electrolyte membrane (PEM). PEM electrolysis (PEME) is also referred to as solid polymer electrolyte (SPE) or polymer electrolyte membrane (also, PEM), but all represent a system that incorporates a solid proton-conducting membrane which is not electrically conductive. The membrane serves a dual purpose, as the gas separation device and ion (proton) conductor. High-purity deionized (DI) water is required in PEM-based electrolysis, and PEM electrolyzer manufacturer regularly recommend a minimum of 1 MQ-cm resistive water to extend stack life.

PEM technology was originally developed as part of the Gemini space program.16 In a PEM electrolyzer, the electrolyte is contained in a thin, solid ion-conducting membrane rather than the aqueous solution in the alkaline electrolyzers. This allows the H+ ion (proton) or hydrated water molecule (H3O+) to transfer from the anode side of the membrane to the cathode side, and separates the hydrogen and oxygen

Water Electrolyzer Stack
Fig. 1. Unipolar (tank) electrolyzer design.
Alkaline Electrolysis Stack
Fig. 2. Bipolar (filter-press) electrolyzer design.
Electrolysis Nafion Stack Anode Cathode

Fig. 3. PEM cell components and reaction showing the positive anode and negative cathode electrodes.

Fig. 3. PEM cell components and reaction showing the positive anode and negative cathode electrodes.

gases. Oxygen is produced at the anode side and hydrogen is produced on the cathode side. The most commonly used membrane material is Nafion® from DuPont. PEM electrolyzers use the bipolar design and can be made to operate at a high differential pressure across the membrane.

DI water is introduced at the anode of the cells, and a potential is applied across the cell to dissociate the water. The protons (H+) are pulled through the membrane under the influence of an electric field and rejoin with electrons being supplied by the power source at the cathode to form hydrogen, H2, gas. PEM electrolyzers are operated at higher current densities (> 1600 mA cm-2) almost an order of magnitude higher than their alkaline counterparts. Stack efficiency decreases as current density increases but is necessary to increase hydrogen production to offset the higher capital costs of PEM cells. PEM advantages over alkaline include the ability to maintain a significant differential pressure across the anode and cathode avoiding the risk of high pressure oxygen. In addition, PEM electrolysis requires DI water but avoids the hazards surrounding KOH. The PEM anode and cathode reactions are described in Eqs. 3 and 4, respectively, and shown in Figure 3,

Figure 4 shows the major water and hydrogen components inside Proton Energy Systems HOGEN 40RE® including the heart of the system: the PEM stack in front

Proton Energy Systems
Fig. 4. Internal components of HOGEN 40RE.

center. The compartment behind these systems (not viewable) contains the AC/DC power converter, ventilation fan, 24 Vdc power supply, system controller, radiator, and control relays. The RE version contains a DC/DC power converter and DC disconnects used to interconnect to a PV array. The combustible gas detector monitors hydrogen levels in this compartment and the oxygen phase separator.

Figure 5 shows the step currents from the power supplies and the resulting hydrogen flow in standard cubic feet per hour (scfh) from the system. Hydrogen production ripple is caused by the internal hydrogen phase separator pumping down the

Electrolyzer Flow
Fig. 5. Sample current step waveform from external power supplies and resulting hydrogen mass flow from HOGEN 40RE.

accumulated water and desiccant drying tube crossover. These system functions cause a drop in system pressure resulting in varying hydrogen production output.

The system efficiency (Eq. 5) is calculated using both the ancillary losses plus the stack energy. The system efficiency uses the higher heating value of hydrogen (39 kWh kg-1), the energy consumed by the stack (kWh), efficiency of the DC power supplies, and the balance of plant ancillary loads like pumps, valves, sensors and controller (kWh). Stack efficiency (Eq. 6) is determined by calculating the ideal cell potential at the operating temperature and pressure multiplied by the number of cells in the stack and then divided by the measured stack voltage,

HHVI kWh

Stack Input Energy (kWh)) + Ancillary Losses (kWh) v Power Supply Efficiency J

HydrogenProduced (kg)

Ideal Stack Potential

ActualStack Potential

The HHV of hydrogen is 39 kWh kg-1 and the ideal stack potential is a function of temperature and pressure. All efficiencies are referenced to the HHV of hydrogen. The minimum amount of energy that must be consumed to split water into hydrogen

Table 1. Constants for heat capacities of gases in ideal state and liquid water.

A

B

C

D

H2

3.249

0.422x10-3

0

0.083x105

O2

3.639

0.506x10-3

0

-0.227x105

H2O

8.712

1.25 x10-3

-0.18x10-6

0

and oxygen is known as the heat of formation (enthalpy) and corresponds to the HHV of hydrogen.

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Responses

  • Eulalia
    Which is positive anode or cathode?
    8 years ago
  • lori
    What purpose does the electrolyzer serve?
    8 years ago
  • manuel
    How much current required to produce 1 kg hydrogen through water electrolysis?
    8 years ago
  • EERO
    How much h2 can be produced from home electrolysis systems?
    6 years ago
  • Murray
    Why is type II DI water required for electrolysis?
    5 years ago
  • temshe
    How is hydrogen for fc seprrated and pressurized?
    1 year ago
  • LAURA
    Why high purity water is required for electrolysis hydrogen generation?
    5 months ago

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