Manufacturing of Electrocatalytic Material for Large Scale Hydrogen Production by the Combination of Electroplating, PVD, Thermochemical Diffusion and Leaching Processes
This paper is a fine example of the multitude of ways that surface finishing can be brought to bear in process development. In this case, a combination of processes is applied to large scale hydrogen production.
by Cecilía Kjartansdóttir,a Michael Caspersen,b Sune Egelund,b Lars Pleth Nielsenc and Per Møllera
aTechnical University of Denmark, Lyngby, Denmark
bSiemens A/S Denmark
cDanish Technological Institute, Aarhus, Denmark
Editor’s Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2014 in Cleveland, Ohio on June 10, 2014. A printable PDF version is available by clicking HERE.
Novel high performance electrodes for industrial alkaline water electrolysis have been manufactured by the physical vapor deposition of aluminum onto a nickel-plated substrate. Nanoporous high surface area nickel is obtained by thermochemical diffusion followed by selective leaching of aluminum. The developed porous nickel surface is uniform and has good interlayer adhesion due to its chemical bonding properties. Electrochemical measurements reveal that 400 mV less potential is needed to decompose water into hydrogen and oxygen with the developed electrodes as compared to a solid nickel electrode. High resolution scanning electron microscope (HR‐SEM) photographs reveal nickel-electrode surfaces characterized by a large surface area with pores down to a few nanometers in size. Durability testing (>9,000 hr) was carried out in a commercially produced electrolyzer stack, revealing no serious deactivation or decomposition during the test period.
Keywords: Electrode production, electrolysis of water, hydrogen production, multi-step surface finishing, electroplating, physical vapor deposition, thermochemical diffusion, leaching
Energy demand worldwide will, according to the World Energy Outlook,1 increase by one-third from 2011 to 2035. Accordingly, in order to meet this ever increasing energy demand and diminish the risk of dramatic climate change in the coming years, a rapid shift in the global energy trend away from coal and fossil fuel is necessary.
Renewable energy is commonly recognized to be the answer for a more secure, reliable and sustainable future. Combining the production of hydrogen via water electrolysis with renewable energy sources offers the possibility of increased production capacity and load management with no greenhouse emissions. Hydrogen as an energy carrier comprises the advantage of flexibility, being able to contribute to various kinds of energy sources. The most obvious route is the production of hydrogen with excessive power from fluctuating renewables such as wind turbines and solar cells followed by storage and later production of electricity via fuel cells or gas turbines, when needed. The hydrogen can also be used as a raw material for production of synthetic fuels, such as methane by the Sabatier process2 and liquid fuels by the Fischer-Tropsch synthesis.3
Among the commercially available water electrolysis systems, alkaline water electrolysis (AWE) comprises by far the highest production rate and the lowest production cost, making the technique the current standard for large‐scale water electrolysis systems.4,5 Commercial alkaline electrolyzers are typically operated in a liquid electrolyte containing 25‐40% potassium hydroxide at a temperature ranging from 60 to 90°C.6,7,8
The highly alkaline electrolyte, the intermediate temperature and the oxygen and hydrogen gas evolution all contribute in making the electrode media extremely corrosive. Thus, finding low cost electrode materials that are both efficient and have long‐term stability is one of the remaining challenges within the field of AWE.
There are two ways to increase the efficiency of an electrode: (1) increasing the electrocatalytic properties of the electrode surface, by selecting materials that are good catalysts for the reactions involved, or (2) increasing the specific surface area of the electrode. Both of these result in a reduction of the necessary voltage to draw the current applied.
Among non‐noble metals, nickel is one of the most stable metals in strong alkaline solutions. Nickel also has relatively good electrocatalytic properties for hydrogen and oxygen formation. Nickel or nickel plated substrates are therefore typically the core material used in electrodes for AWE systems.5,8,9 The electrochemical efficiency of the nickel electrodes can be improved even further by increasing the number of active sites, by increasing the actual surface area and by introducing additional surface defects.10-18
A common method for preparing high surface area nickel catalysts is by selectively alkaline leaching aluminum or zinc from a Ni‐Al or a Ni‐Zn alloy, respectively. This method of increasing the surface area of nickel by alkaline leaching was first introduced by Murray Raney in 1925 for the purpose of producing a catalyst for the hydrogenation of vegetable oils.19 However, the production of a durable electrocatalyst for AWE is much more challenging than for non‐electrochemical processes such as hydrogenation. The reason for this is the risk of gas‐erosion of the electrocatalytic surfaces during AWE because of the hydrogen and oxygen evolution on the electrodes. Hence, when choosing the right process technique for electrode development, the interlayer adhesion between the porous structure and the substrate, as well as the purity, i.e., elimination of oxides, of the final product must be taken into consideration.
In the present work, a state‐of‐the‐art electrode has been produced by the physical vapor deposition of aluminum onto an electroplated nickel substrate, followed by thermochemical diffusion and selective alkaline leaching. The efficiencies of the electrodes were investigated by steady state polarization measurements and measurements in a commercial electrolysis stack.
Preparation of electrodes
Electrodeposited sulfamate nickel coatings are known to have low stress and high ductility. These properties could benefit the durability of the nickel electrodes. Thus, here the sulfamate nickel coating was selected over other nickel coatings. The sulfamate nickel coating was electroplated onto a 50×50 mm2 304 stainless steel substrate in an electrolyte containing 300 g/L Ni(SO3NH2), 40 g/L H3BO3 and 10 g/L NiCl2•6H2O. The deposition current density was 4.5 A/dm2 and the plating duration was 2 hr at 45°C. The pretreatment of the steel substrate was as follows: cathodic degreaser for 2 min, pickling for 1 min, etching in 20% HCl and 5% H2O2 for 1 min (to remove chromium oxides from the surface), followed by cathodic degreasing for 2 min and pickling for 1 min. In order to assure good adhesion, a thin layer of Woods nickel strike was electroplated onto the steel substrate according to the following sequence: anodic at 7 A/dm2 for 2 min (to remove the last oxides on the surface) and cathodic deposition at 14 A/dm2 for 2 min.
The nickel plated steel substrates were coated with approximately 20 μm of aluminum by DC magnetron sputtering using a CC800/9 SinOx coating unit from CemeCon AG. The aluminum source consisted of four aluminum 1050 targets, run at 750 W. The bias on the substrate was set to 800 W and the start pressure in the chamber was 1 mPa. The nickel surfaces were cathodically degreased for 2 min prior to the PVD process. The substrates were heated and etched in situ by argon sputtering, prior to the sputtering process, to remove any nickel oxide (NiO) from the surface. The aluminum PVD specimens were subsequently heat treated in an atmospheric furnace for 4 hr at 610°C, followed by selective aluminum leaching in 30 wt% KOH and 10 wt% KNaC4H4O6•4H2O at 80°C with stirring for 24 hr. The reason for adding the sodium potassium tartrate tetrahydrate to the alkaline leaching solution is that it works as a complexing agent for the leached aluminum according to:20
Al(OH)4- + n(C4H4O6)-2 ⇒ Al(OH)3(C4H4O6)n-2 + OH-,
thereby preventing the aluminum hydroxide precipitates from redepositing into the nickel pores:21
Al(OH)4- ⇒ Al(OH)3 + OH-
Structural characterization and composition
For cross-section characterization and composition analyses the coated specimens were hot‐mounted in PolyFast from Struers, ground down to 4000 grit and subsequently polished with 3 and then 1 μm diamonds. The cross-section studies were made with a TM 3000 tabletop scanning electron microscope (SEM) from Hitachi. An integrated energy‐dispersive x‐ray spectroscopy (EDS) was used for elemental analyses. The surface structure and morphology of the electrode coating was prepared in the same manner as described before, except that the substrate was made of solid nickel, and was characterized by means of a FEI Quanta 200 ESEM FEG‐SEM.
Electrochemical and durability measurements
The electrocatalytic activity of the electrodes was evaluated by means of anodic and cathodic potentiodynamic polarization. The measurements were carried out using a Gamry Reference 3000 potentiostat. A typical three-electrode electrochemical cell made of Teflon was used for the electrochemical measurements. The cell contained a Hg/HgO reference electrode from Radiometer Analytical and a counter electrode made of pure nickel. The electrolyte consisted of 1M KOH and the measurements were carried out at 25°C. The potentiodynamic and cyclic voltammetry measurements were IR‐compensated by means of a current interruption technique. All following half‐cell potentials are reported vs. Hg/HgO. A durability test was carried out in a 17-cell bipolar, non‐zero gap alkaline water electrolysis stack manufactured by GreenHydrogen.dk. The electrodes in the stack measured 270 cm2 in area. The stack contained 30% KOH and was operated under altered conditions, at a maximum temperature of 80°C and a pressure of 22 bar for more than 9,000 hr. As the electrodes were treated with the alkaline-leach etched PVD Al/Ni on both sides (Here the nickel substrate was of solid nickel plates.), the developed surface functioned both as anode and cathode in the electrolysis stack.
Results and discussion
Structural characterization and composition
When heat‐treated at 610°C, a thermochemical diffusion process takes place at the interface between the aluminum and the nickel. The red horizontal line in the phase diagram of Fig. 1 indicates which Ni‐Al diffusion couples are thermodynamically stable at 610°C and atmospheric pressure. The thickness of each intermetallic phase formed depends on the diffusion kinetics, the amount of nickel and aluminum available in the diffusion system and the heat treatment parameters.
Among Al/Ni alloys, aluminum can only be alkaline leached from the Al3Ni2 and Al3Ni phases.23 Obviously, the Al3Ni2 phase contains more nickel and is therefore more mechanically stable compared to its Al3Ni counterpart. Hence, the aim during the alloy formation procedure, i.e., the thermochemical diffusion process, was to obtain a relatively thick layer of the Al3Ni2 phase.
Figures 2 and 3 show SEM micrographs of the PVD aluminum nickel structure after 4 hr of heat treatment and after heat treatment followed by selective aluminum leaching. The results from the EDS analyses performed on the cross‐sections are listed in Table 1.