Emerging Electrolytic Technologies

For electrolytic hydrogen production, the increase in production effi­ciency reduces the amount (and thus the cost) of electricity consumed, whereas an increase in production rate reduces the size and cost of the electrolyzer stack. Nanochemistry research efforts carried out on a global scale, aimed at manufacturing low-cost electrodes made of metal nanoparticles, featuring an enhanced surface area available for the catalytic reaction that generates hydrogen, thus increasing efficiency and production rates, have lately been successful, at least on the laboratory scale.

In the USA, for example, a number of small hi-tech companies have developed new Ni-based catalysts. QuantumSphere manufactures a Nano NiFe coating of nickel and iron nanoparticles capable of increasing the efficiency of an alkaline electrolyzer using coated cathodes by more than 10%.35 The company’s experimental electrolyzer stack produces 2.8 Nm3 of hydrogen per day at 68% efficiency in normal operation, close to the 2012 target of 69% efficiency for advanced electrolyzers set by the Department of Energy.

Another small company, GridShift, Inc., uses a coating technique that coats all surfaces of a three-dimensional shape (like reticulate nickel foam) with a nano catalyst exposed to the electrolyte’s boundary layer, to generate hydrogen in an electrolyzer running at 80% energy efficiency with a current density of 1000 mA cm-2 overall, delivering compressed hydrogen at around $2.51 kg-1 H2.36

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Figure 2.19 The cobalt oxygen-evolving catalyst deposited on the ITO-passivated p – side of an np-silicon junction enables the majority of the voltage gener­ated by the solar cell to be utilized for driving the water-splitting. In the laboratory, the system worked continuously for three days. (Reproduced from Ref. 41, with kind permission.)

Similarly, Sun Catalytix37 is currently trying to commercialize a nickel borate catalyst38 for the oxygen evolution reaction. The cobalt oxygen – evolving catalyst39 (Figure 2.19) originally reported gave a current density of 1 mAcm~2 with an overpotential of 410 mV, which is worse than the typical performance (e. g. 1mA cm~2 at <200 mV over­potential) for nickel anodes.40

These and other companies are seeking financial support to scale up their processes. In any case, the trend towards reducing the cost by enhancing the efficiency of electrolysis is clear, and a number of new companies are emerging that have commercialized ever more efficient electrolyzers.

New technology recently developed in Italy combines the advantages of the cheap, nickel-based electrode materials used in alkaline electrolysis with the production of hydrogen at the high pressures typical of solid polymer membrane PEM electrolyzer technology (Figure 2.20). With no liquid electrolyte on the H2 side, hydrogen is produced at higher purity.

The resulting home generator produces hydrogen safely on demand from water, directly compressed, dry and pure (Table 2.4), providing the

Table 2.4 Requirements of the H2 produced by Acta’s EL100 electrolyzer. (Reproduced from Actaenerrgy. it, with kind permission.)

Production

100 Lh-1

Pressure, bar

15 (30)

Purity

99.95%

Power consumption

550 W

Water consumption

0.085 Lh"

Width

25 cm

Height

46 cm

Depth

50 cm

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ideal refill for fuel cell applications on the market that require com­pressed hydrogen for reasons of energy density.

At the end of 2011, the company shipped its first hydrogen generator stack that can produce 500 L of hydrogen per hour to an Italian engi­neering firm that specializes in industrial heating systems.42

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Figure 2.21 ITM Power’s stack, featuring proprietary membrane materials, for the HFuel generator.

(Reproduced from hfcletter. com, with kind permission.)

Similarly, in the UK, the company ITM Power has developed an innovative alkaline solid polymer membrane platform for its electrolyzer (Figure 2.21), which achieved 57% cost savings compared with their PEM-based stack, due to the removal of Pt catalysts and the simplifi­cation of the system.43

The company commercializes an on-site HFuel generator for refueling hydrogen-powered vehicles that produces hydrogen by electrolysis, compresses it, stores it and dispenses the gas on demand at high pressure (350 bar). The new ITM electrolyzer membrane platform transports OH~ rather than H+ ions, enabling smaller stack sizes as a result of the higher ionic conductivity that makes high current densities achievable, while the high water permeability allows considerable simplification of the water management system.

Also in the UK, RE Hydrogen has developed a low cost, ambient pressure alkaline electrolyzer (Figure 2.22) based on a 5kW stack module, using plastic materials and a proprietary regenerative carbon aerogels-based catalyst, whose retail price is estimated to be 70% lower than the current market price for conventional alkaline electrolyzers.44 In detail, the low cost cathode consists of resorcinol-formaldehyde (RF) carbon aerogels of high surface area (>700m2g-1) and nano-pore sizes (4 nm) thermally deposited on molybdenum metal.45

The scanning electron microscope (SEM) images of the Mo-RF electrode (Figure 2.23), where the hydrogen evolution reaction takes place at 298 K, indicate formation of a highly porous carbon

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nanostructure whose effectiveness in alkaline water electrolysis is clearly shown by the data in Table 2.5, wherein the charge-transfer resistance of the Mo-RF electrode is reduced by about 70% when compared with pure molybdenum metal.

Electrode

Voltage (V vs. SHE)

Rs (O cm 2)

Rct (O cm 2)

Rdl (O cm2)

Mo

1.5

0.36

59.96

0.51

Mo-RF

1.5

4.51

11.98

0.31

Table 2.5 Solution resistance, Rs, charge-transfer resistance, Rct, and double­layer capacity, Cdl, derived from analysis of impedance spectra recorded at E = 1.5 V in 30% by vol. KOH solution at 298 K. (Reproduced from Ref. 43, with kind permission.)

Table 2.6 Cost of hydrogen using RE Hydrogen electrolyzer subject to dif­ferent electricity costs.

Electricity price RE Hydrogen cost of H2

Green grid electricity at £0.12 kWh-1 £!.! kg-1 green hydrogen

Oxygen produced free

Commercial grid electricity at £0.077 kWh-1 £5.5 kg-1 hydrogen

Direct solar PV electricity at £0.04kWh-1 £3.6 kg-1 hydrogen

High gas purity

Molybdenum is about 3 to 6 times more expensive than nickel but far less expensive than platinum, offering a clear economic benefit in reduced capital cost investment compared with other electrodes (such as Pt-C) previously used in electrolyzers.

Unlike conventional electrolyzers, the RE Hydrogen electrode can now operate under variable and intermittent mode for unlimited on-off switching cycles, owing to its in-situ regenerative capability of the elec­trode-catalyst, which bestows a long life. As a result, the company claims that its electrolyzer can produce ‘‘green’’ hydrogen at a price in the range of £3-1.1 kg-1 (subject to the electricity cost), far lower than the current cost of commercial hydrogen, stored in cylinders, which is sold at £12-110 kg-1 (Table 2.6).

Thanks to the innovations mentioned above, the company claims to have reduced the capital cost by a remarkable 90%. Under these con­ditions, then, the cost of hydrogen is mainly influenced by the price of electricity.

The electricity contributes 60% of the cost of hydrogen for RE Hydrogen’s system, while for conventional pressurized electrolyzers only 22% of the cost of hydrogen can be ascribed to the cost of the electricity (Figure 2.24). Therefore cheaper or free electricity will bring a greater reduction in hydrogen price via these innovative electrolyzers.

The company’s current offering includes 5kW, 25 kW and 100 kW systems, while larger electrolyzers can be made available on request by adding modular 5 kW electrolyzer stacks.

25kWRE Hydrogen Elecirolyser
Total cost=£3.61/kg hydrogen

Green electrioty – £D.04/kWh
10 years slack life,20 years plant life,
Financing of capita I cost owerS v[1]sis at8% APR

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image088

Capital,

jd9sic9,264

 

Figure 2.24 Hydrogen cost breakdown for RE Hydrogen (left) and conventional electrolyzer technologies.

(Reproduced from Rehydrogen. com, with kind permission.)

 

There are very few manufacturers of hydrogen compressors in the world, and the cost is often almost higher than that of an electrolyzer. Hence, to increase the marketability of its atmospheric electrolyzers, RE Hydrogen is currently working to develop a low cost hydrogen com­pressor that will also be used as a hydrogen dispenser for refueling vehicles with H2 compressed at 350 and 700 bar. Thus far, the company has built a working prototype (Figure 2.25) suitable for a 30 kW elec­trolyzer for 6 Nm3 h-1 gas flow rate, which is claimed to cost up to 70% less and to be 50% more efficient than a conventional compressor.46

Aiming to establish the UK’s first complete solar hydrogen energy supply chain, RE Hydrogen partnered with fuel-cell manufacturer Arcola Energy and with Linde’s BOC in a collaborative project known as rabh2.47 Using RE Hydrogen’s 5 kW electrolyzer to reduce the capital cost of the stack by 90%, while retaining its capability for unlimited on – off switching cycles (vital for intermittent and variable operation of renewable energy power), the team will be able to afford to power the electrolyzer with solar and wind electricity. The high purity hydrogen thereby obtained will be used to power a wide range of fuel cells built by Arcola Energy at their factory in East London.

 

Updated: August 16, 2015 — 5:10 am