Greig
Chisholm
,
Philip J.
Kitson
,
Niall D.
Kirkaldy
,
Leanne G.
Bloor
and
Leroy
Cronin
*
School of Chemistry, University of Glasgow, Glasgow, G12 8QQ, UK. Web: http://www.croninlab.com/E-mail: Lee.Cronin@Glasgow.ac.uk
First published on 24th June 2014
The electrolysis of water is considered a promising route to the production of hydrogen from renewable energy sources. Electrolysers based on proton exchange membranes (PEMs) have a number of advantages including high current density, high product gas purity and the ability to operate at high pressure. Despite these advantages the high cost of such devices is an impediment to their widespread deployment. A principal factor in this cost are the materials and machining of flow plates for distribution of the liquid reagents and gaseous products in the electrochemical cell. We demonstrate the production and operation of a PEM electrolyser constructed from silver coated 3D printed components fabricated from polypropylene. This approach allows construction of light weight, low cost electrolysers and the rapid prototyping of flow field design. Furthermore we provide data on the operation of this electrolyser wherein we show that performance is excellent for a first generation device in terms of overall efficiency, internal resistances and current–voltage response. This development opens the door to the fabrication of light weight and cheap electrolysers as well as related electrochemical devices such as flow batteries and fuel cells.
Broader contextUptake of renewable energy generation, especially wind and solar power is increasing at a dramatic rate and has the potential to contribute greatly to mankind's management of climate change due to accumulation of CO2. Matching supply and demand of these intermittent renewable energy sources is challenging and solutions offered often depend on the conversion of excess electrical power to chemical or potential energy for storage until required. Hydrogen is a potential storage medium in this regard and the use of polymer electrolyte membrane (PEM) electrolysers holds great promise for the generation of hydrogen from renewable sources. Successful realisation of this goal requires product development of all parts of the PEM electrolyser from the “active” components e.g. the catalysts, to the “passive” components, namely the structural parts of the electrolyser stack. 3D printing is an emerging technology which can be used in the fabrication of many different device types. Here we present the use of 3D printing to prepare components of an electrolyser cell and demonstrate its operation. This application of 3D printing enables rapid prototyping, cost reduction and a dramatic reduction in component weight. All of which may accelerate the development of new electrolyser technologies. |
PEM electrolysers follow a standard design illustrated in Fig. 1. The principal components of the electrolyser are (i) the membrane electrode assembly consisting of a proton permeable membrane, often Nafion®. Each side of this membrane is coated with a suitable electrocatalytic substance to accelerate the electrolysis process. (ii) Porous gas diffusion layers (GDL) – often made of titanium or carbon which transfer current from the flow plates and promote the release of the product gases from the electrolysis reaction. (iii) Flow plates which separate each cell in the electrolyser stack and which are machined with a flow path for circulation of the water.
PEM electrolysers are more expensive than their alkaline counterparts and this, in part has limited their application, with considerable research being undertaken to reduce the costs of these devices, principally by reduction of catalyst cost by substitution of the noble metal catalysts for cheaper and more readily available alternatives.6 Ayers et al. reported that the bipolar plate assembly is the highest cost component in the stack, representing nearly 40% of the overall cost.7 These plates also contribute to the overall resistance of the cell and thus to the required cell voltage. This effect is more pronounced at the high charge densities at which PEM electrolysers operate and is one of the dominating sources of cell efficiency. These plates are also essential in the mass transport of both the reacting liquid (water) and the extraction of the product gases away from the membrane and the current collectors. Currently flow plates are typically made from graphite, titanium or stainless steel.4 Each of these materials has its own advantages and disadvantages. Titanium is characterized by its high strength, high electrical and thermal conductivity and its low gas permeability, all of which are positive traits for a flow plate. The drawback of titanium is its relatively poor corrosion resistance, especially on the anodic side where an oxide layer can form, increasing the resistance and lowering the performance of the stack over time.8 Graphite, whilst having high conductivity, suffers from poor corrosion resistance, limited mechanical strength and high cost.8 Various grades of stainless steel may be employed.9 This is a cheaper alternative to both titanium and graphite, however the stainless steel can corrode quickly in acidic environments and can require the application of a protective coating to reduce this corrosion to acceptable levels, but this can increase the resistance of the flow plate.4 Considering all of the above, there is no ideal material for the construction of the PEM electrolyser flow plates.
Within the design of the flow plate, another aspect to consider is the effect of different configurations of the channels distributing the water and conducting the product gases away from the reaction sites. Possible configurations include pin-type flow fields,10a straight flow fields,10b serpentine flow fields,10c integrated flow fields,10d and interdigitated flow fields.11 Considering the costs and time required to machine flow plates of a suitable material, detailed studies of the effect of the flow plate configuration on the performance of the electrolyser are lacking and performance testing is often carried out on a single configuration due to cost and time constraints.12 An additional consideration in the manufacture of PEM electrolysers is the weight of the system which is largely influenced by the heavily engineered flow plates, current collectors and any further engineering required to render the electrolyser safe and stable particularly during high pressure operation.13
3D printing is an emerging technology which promises to revolutionize many areas of manufacturing processes, transforming the relationships between the design, manufacture and operation of functional devices.14 In recent years there has been considerable interest in 3D-printing technologies for large-scale industrial prototyping,15 and the manufacture of bespoke electronic16 and pneumatic devices.17–19 Considerable progress has been made in demonstrating the utility of 3D printing in the chemical sciences in the “reactionware” series of publications.20–22 Herein we explore the revolutionary device fabrication potential of 3D printing applied to the aforementioned challenges of the design and manufacture of electrolyser components, resulting in a new manufacturing paradigm wherein 3D printed components are, for the first time, incorporated into an electrolyser.
The electrical resistance of the flow plates decreased with each stage of the coating process. Average resistances measured during the fabrication of 3 plates are given in Table 1. We believe that the large standard deviations at each stage of the coating process are due to variations in the thickness of the layers applied by painting. Alternative methods of applying these initial silver layers that may reduce this variation will be the subject of future work. For comparison, a variation of 0.002 Ω in the ohmic region of the polarisation curve would equate to an additional 26 mV or 1% of the total voltage required to drive the electrolysis at the maximum charge density. As shown in Table 1, after electrocoating with silver, flow plates with extremely low resistance can be obtained, however the performance of these plates in the electrolyser quickly deteriorated. This was due to oxidation of the silver on the anode during water electrolysis which resulted in silver ions that were able to migrate through the Nafion® membrane to the reductive side of the cell where they were reduced back to silver resulting in both an increase in the resistance of the anodic flow plate and irreversible damage to the Nafion® membrane. This production of silver within a Nafion® membrane is known and is exploited in the preparation of silver nanoparticles.24 In order to prevent this oxidation of the anodic flow plate, a layer of gold was sputter coated onto the silver surface. This rendered the anodic flow plate inert to oxidation. The resistance of the plates was unchanged after sputter coating.
Coat 1 | Coat 1 – cured | Coat 2 | Coat 2 – cured | Electro-dep. | |
---|---|---|---|---|---|
Av. R (Ω) | 7.187 | 1.924 | 0.849 | 0.519 | 0.002 |
Std. dev. | 5.441 | 1.271 | 0.311 | 0.144 | 0.002 |
Photographic and SEM images of the flow plates at different stages of their preparation are given in Fig. 2. All SEM images were recorded at ×1000 magnification and the scale bar corresponds to 50 μm. Fig. 2a shows the polypropylene plate immediately after printing was complete and prior to application of any coating. Fig. 2b shows the flow plate after application of both layers of silver paint. The overlapping flakes of silver can be clearly observed. Contrast this with the appearance after electrodeposition of the silver (Fig. 2c) where layers of large crystallites of silver can be observed. This dense crystallite structure gives rise to the significant reduction in resistance observed between the cured 2nd coating of silver paint and the electrodeposited layer. The weight of the coated 3D printed flow plate was compared with the weight of a flow plate fabricated from titanium using an identical design. Both flow plates were 3 mm thick. The 3D printed flow plate was substantially lighter, weighing only 13.9 g whereas the titanium flow plate weighed 59.2 g, more than 4 times as much. This is extremely significant when one considers the number of flow plates that would be assembled in a stack.
The costs of the components were compared using the following market values: titanium $13.53 per kg;25 polypropylene $1.54 per kg 26 and silver $620 per kg.27 Using the weights above this gives a materials cost for the titanium plate of $0.80 per plate, compared with $0.17 per plate for the 3D printed component, a cost which includes the deposition of 0.25 g of silver. This amount of silver is typical of that deposited at the current used during electrocoating.
Polarization curves were collected for the 3D printed cell at temperatures of 30, 50 and 70 °C and are illustrated in Fig. 3.
Onsets for water splitting are determined by extrapolation of the ohmic (linear) region of the polarisation curve. The onset of water electrolysis varied slightly with temperature: at 30 °C onset was 1.61 V, at 50 °C onset was at 1.57 V, and at 70 °C onset was at 1.54 V. The maximum current density achieved at 2.5 V and 30 °C was 1.04 A cm−2. This increased when the temperature was increased to 50 °C, where a current density of 1.09 A cm−2 was reached. There was no further improvement in the performance of the cell when the temperature was increased to 70 °C. Temperatures greater than 70 °C were not investigated to avoid dehydration of the Nafion® membrane.
The limiting of the performance with increasing temperature is surprising as a standard electrolyser will achieve progressively higher current densities at a given voltage as the temperature increases. We believe that this limitation in performance can be explained by comparing the linear thermal expansion coefficients of the polypropylene substrate and the topmost silver layer. The linear thermal expansion coefficient of polypropylene varies with temperature from 1 × 10−4 to 2 × 10−4 K−1 depending on temperature.28 This is 5–10 times the value for silver (1.9 × 10−5 K−1).29 Given this expansion of the polypropylene layer it is likely that this induces a distortion in the silver coating, increasing the distance between the particles and/or decreasing the extent of overlap. This will have the effect of increasing the resistance of the flow plate and effectively cancelling out the expected improvement in performance at a higher temperature.
The Faradaic and energy efficiencies of the cell were recorded at 30 °C, 2 V and 0.39 A cm−2. The Faradaic efficiency was 94% and the energy efficiency was 70%. The 6% Faradaic loss may be due to parasitic electrochemical processes occurring within the flow plates, especially within the binder used in the silver paint.
Vcell = Vrev + Vser + Vpol + Vcon | (1) |
This equation describes the overall cell voltage, Vcell as the sum of four components. Vrev is the reversible voltage and is the minimum voltage required for electrolysis to take place. This is a function of the enthalpy of water and assumes no overpotentials or resistances and all reagents and products being in the gas phase. The remaining voltages can be considered as inefficiencies which the cell operation and design attempt to address. Vser corresponds to the voltage drop associated with the resistance of the various cell components e.g. the membrane, the electrodes and the quality of the connection between these components. This connection is most often optimised by varying the torque on the bolts that maintain the integrity of the cell. Vpol is related to the voltage drop associated with the activation energy of the oxygen and hydrogen evolving reactions. Vcon is due to mass transfer effects, namely the production of product gases at the electrodes and their efficient removal such that sufficient reactant can continue to be supplied to maintain a given charge density. Electrochemical impedance spectroscopy allows us to probe the resistances associated with these overpotentials in particular Vser and Vpol. The Nyquist plots for the 3D printed cell are given in Fig. 4.
The series resistance (Rs) was largely constant at all temperatures, ranging only from 0.73–0.75 Ω cm2. The polarization resistance (Rp) spanned the range 1.87 Ω cm2 at 30 °C to 1.38 Ω cm2 at 70 °C. The minimum value was reached at 50 °C, where Rp was 1.30 Ω cm2. The EIS results agree well with those obtained from the polarisation curves. The fact that the EIS curve is largely unchanged between 50 and 70 °C is reflected in the similarities between the polarisation curves at these temperatures. Although typically a lesser effect versus the reduction in polarisation resistance with increasing temperature, the series resistance is also typically observed to drop. This does not occur in our cell and is possibly due to the thermal expansion of the polypropylene layer as previously discussed.
During the 96 h duration of the experiment, the performance of cell decreases by an average of 2.1 mV h−1, however this does not fully describe the behaviour or the stability of the cell over this time. The durability of the cell is characterised by 3 distinct phases. During the first hour of operation there was a marked decline in performance of 161.2 mV. After this initial degradation, the cell enters a period of comparative stability where the performance degrades at 1.0 mV h−1. This phase lasts for approximately 26 hours. Thereafter the cell becomes even more stable degrading at only 0.14 mV h−1 for the remaining 70 hours of the experiment. The reason for these distinct rates of degradation are not known, however they are likely to be associated with electrochemical degradation of the binder components of the silver paint and oxidation of any exposed silver crystallites and concomitant degradation of both conductivity and membrane performance.
Efficiency measurements were carried out by setting the cell to 2 V. The hydrogen produced was collected in a 50 ml gas burette equipped with a levelling bulb. Gas collection was carried out over 60 seconds. Data was averaged over 4 repetitions and gas volumes were corrected for temperature and barometric pressure. Energy efficiencies were calculated on the basis of the higher heating value of hydrogen.30
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