Posted on by christophepochari
Dramatically reducing the cost of alkaline water electrolyzers using high surface area mesh electrodes, commercial off the shelf components and non-Zirfon diaphragm separators.
Christophe Pochari, Pochari Technologies, Bodega Bay California
Alkaline electrolyzer technology is ripe for dramatic cost reduction. Current alkaline electrolyzer technology is excessively expensive beyond what material costs would predict, mainly due to very small production volumes, a noncompetitive market with a small number of big players, and relatively little use of COTS (commercial off the shelf) methodology of cost reduction. Pochari Technologies’s researchers have thus applied this methodology to finally bring to market affordable hydrogen generators fabricated from readily available high-quality components, raw materials, and equipment procured on Alibaba.com ready to be assembled as kits to reduce labor costs. An alkaline cell is a relatively simple system, consisting of four major components: The electrode (a woven wire mesh), a gasket (made of cheap synthetic rubbers, EPDM etc), and a material for fabricating the diaphragm membrane for separating and oxygen and hydrogen while permitting sufficient ionic conductivity (usually composites of potassium titanate (K2TiO3) fibers and polytetrafluoroethylene (PTFE) (as felt and as woven), polyphenylene sulfide coated with zirconium oxide, (Zirfon), or polysulfone, and asbestos coated with polysulfone. Many polymers are suitable for constructing separators, such as Teflon® and polypropylene”. “A commercially available polyethersulfone ultrafiltration membrane (marketed as Pall Corporation, Supor®-200) with a pore size of 0.2 um and a thickness of 140 um was employed as the separator between the electrodes”. Nylon monofilament mesh with a size of over 600 mesh/inch, or a pore size of 5 micron can also be used. Polyethersulfone is ideal due to small more size, retaining high H2/O2 selectivity at elevated pressures. It can handle temperatures up to 130 C. If polyethersulfone is not satisfactory (excessive degredation rate if temperature is above 50 C), Zirfon-clones are available to purchase on B2B marketplaces https://b2b.baidu.com for $30/m2 from Shenzhen Maibri Technology Co., Ltd.
The fourth component are the “end plates” which consist of heavy-duty metallic or composite flat sheets which house a series of rods tightly pressing the stacks to maintaining sufficient pressure within the stack sandwich. For higher pressure systems, such as up to 30 bar, the endplates encounter significant force. Unlike PEM technology, noble mineral intensity in alkaline technology is relatively small, if nickel is to be considered a “noble” metal, than alkaline technology is intermediate. Nickel is not abundnant but not rare either, it’s approximately the 23rd most abundant element. For an alkaline electrolyzer using a high surface area electrode, a nickel mesh electrode loading of under 500 grams/m2 of active electrode surface area is needed to achieve anode life of 5 or more years assuming a corrosion rate of below 0.25 MPY. With current densities of 500 miliamp/cm2 at 1.7-2 volts being achievable at 25-30% KOH concentration, power densities of nearly 10 kW/m2 are realizable. This means a one megawatt electrolyzer at an efficiency of 75% (45 kWh/kg-H2 LHV) would use 118 square meters of active electrode surface area. Assuming a surface/density ratio of a standard 80×80 mesh, 400 grams of nickel is used per square meter of total exposed area of the mesh wires. Thus, a total of 2.25 kg of nickel is needed to produce 1 kg of hydrogen per hour. For a 1 megawatt cell, the nickel would cost only $1000 assuming $20/kg. This number is simply doubled if the TBO of the cell is desired to increase to 10 years, or if the power density of the cell is halved. Pochari Technologies is planning on using carbon-steel electrodes to replace nickel in the future to further redux CAPEX below $30/kW, our long term goal is $15/kW, compared to $500 for today’s legacy system from Western manufacturers. Carbon steel exhibited a corrosion rate of 0.66 MPY, while this is significantly above nickel, the cost of iron is $200 per ton (carbon steel is $700/ton), while nickel is $18,000, so despite a corrosion rate of at least 3x higher, the cost is 25x lower, yielding of 8.5x lower for carbon steel. The disadvantage of carbon steel despite the lower capex is decreased MTBO (mean time before overhaul). Pochari Technologies has designed the cell to be easier to disassemble to replace the corroded electrodes, we are also actively studying low-corrosion ionic liquids to replace potassium hydroxide. We are actively testing a 65Mn (0.65% C) carbon steel electrode under 20% KOH at up to 50 C and experiencing low corrosion rates confirming previous studies. Pochari Technologies is testing these carbon steel electrodes for 8000 hours to ascertain an exact mass loss estimate.
What kind of current density can be achieved by smooth plates?
Current densities of 200mA/cm2 at 1.7 volts (3.4 kW/m2) generates an efficiency of 91% even with non-activated nickel electrodes.
For a lower corrosion rate of 1 um/yr, a total mass loss of 7% per year will occur with a surface/mass ratio of 140 grams/m2-exposed area, the nickel requirement is only $350 or 17.5 kg for one megawatt! Although this number is achievable, higher corrosion rates will likely be encountered. To ensure sufficient electrode reserve, a nickel loading of around 400-500 grams/m2 is chosen. Pure nickel experiences an excessively high corrosion rate when it is “active”, it becomes “passive” when a sufficient concentration of iron (NiFe2O4), or silicate is found in the oxide layer. For Incoloy alloy 800 with 30% Ni, 20% Cr and 50% Fe experiences a corrosion rate of 1 um/yr at 120 C in 38% KOH, pure nickel is over 200 um. “The “active” corrosion of nickel corresponds to the intrinsic behavior of this metal in oxygenated caustic solutions; the oxide layer is predominantly constituted of NiO at 180°C and of Ni(OH) 2 at 120°C. The nickel corrosion is inhibited when the oxide layer contains a sufficient amount of iron or silicon is present”. The results drawn from this study indicates the ideal alloy contains around 34% Ni, 21% Cr, and 45% Fe. The cost breakdown for the three elements are $18/kg, $9/kg and $0.2/kg, giving an average of $8.1/kg. For a passive corrosion rate of 1 um/yr, a 10% annual material loss corresponds to an electrode mesh loading of 90-100 grams/m2, or $0.11/kW. That is 11 cents per kW! This does not include mesh weaving costs. A 600 mesh weaving machine costs $13,000. The conclusion is meshing costs are very minimal, less than a few cents per square meter.
For the diaphragm separators using a 200 um thick sheet of polyethersulfone (PES), around 20 grams is used per kilowatt, at a typical cost of PES of $25/kg assuming density of 1.37 g/cm2, the cost would be around $0.50/kilowatt assuming an electrode power density of 6.8 kW/m2 (400 miliamps at 1.7 volts). Since Pochari Technologies’ always adheres to COTS methodology, the expensive and specialized Zirfon membrane is dispensed with in favor of a more ubiquitous material, this saves considerable cost and eases manufacturability as the need to purchase a specialized hard to access material is eliminated. Gasket costs are virtually negligible, with only 4.8 grams of rubber needed per kilowatt, EPDM rubber price are typically in the range of $2-4/kg. For 30% NaOH at 117 C, a corrosion rate of 0.0063 millimeter per year (0.248 MPY) is observed for an optimal nickel concentration of 80%. This means 55 grams of Ni is lost for one square meter, if we choose 10% per year as an acceptable weight loss, we return to 550 grams per square meter as the most realistic target nickel loading, with much lower loading achievable with reduced corrosion rates. A lower concentration of KOH/NaOH and lower operating temperature can be utilized as a trade-off between corrosion and power density. The total selling price of these units cost including labor and installation is $30/kW. In 2006, GE estimated alkaline electrolyzers could be produced for $100/kW, clearly, must lower prices are possible today. At an efficiency of 6.5 MMW (47.5 kWh/kg-H2), the price is $1430/kg-hour. After the cell stack costs, which we demonstrated can be made very minimal with COTS design philosophy, the second major cost contributor is the power supply. For a DC 12 volt power supply, $50 is a typical price of 1000 watt DC power module. This, in compendium, alkaline stack costs are effectively miniscule, and the cost structure is dominated by the power supplies and unique requirements of low voltage direct current high amperage power. High efficiency DC power supplies cost as little as $30/kW and last over 100,000 hours.
It should be noted the activity of the nickel electrode depends heavily on its morphology. A smooth sheet has very little activity and is thus not suitable for industrial scales, although for small electrolyzers, a smooth catalyst can be sufficient if power density is not an exigency. Catalysts activity depends not on the total surface area available exposed to the reactant material, rather, catalyst activity depends almost exclusively on the so called “active sites” or “absorption sites” comprised of kink sites, ledges and steps, adatoms and holes. These sites, characterized by local geometric perturbation, account for effectively all the activity of a catalyst. It can be said that the vast majority of the catalyst area is not active. By achieving a high fraction of active sites, current density holding voltage constant can be increased 10 fold.
The most challenging aspect of manufacturing a high performance AWE is catalyst preparation. Plasma spraying is the most common method to achieve a highly denticulate surface. Raney nickel, an alloy comprised of aluminum and nickel, is sprayed on the bare nickel surface, the high velocity and temperature of the metal particle cause them to mechanically adhere to the nickel surface. This process is called sputtering deposition. After the material has cooled and solidified, the aluminum is then leached (extracted) from the surface using a caustic solution. Pochari Technologies is developing a low cost plasma spraying machine using ubiquitous microwave components to perform catalyst preparation. On catalyst surface preparation is complete, the electrolyzer is ready to assemble.
180 C at 38% wt KOH at 4 MPa Oxygen
150 C at 38% wt KOH at 4 MPa Oxygen
120 C at 38% wt KOH at 4 MPa Oxygen
Typical alkaline electrolyzer degradation rate. The degradation rate varies from as little as 0.25% per year to nearly 3%. This number is almost directly a function of the electrocatalyst deactivation due to corrosion.
Diaphragm membrane rated for up to 100 C in 70% KOH for $124/m2: $8.8/kW
*Note: Sandvik materials has published data on corrosion rates of various alloys under aeriated sodium hydroxide solutions (the exact conditions found in water electrolyzers), and found that carbon steel with up to 30% sodium hydroxide provided temperatures are kept below 80 Celsius.