Introduction
Polymer electrolyte fuel cells (PEFCs) are one technology helping us to move away from fossil fuels and towards a hydrogen economy, powered by renewable energy. PEFCs fuelled by hydrogen have already been commercialized in stationary residential applications and as fuel cell vehicles (FCVs) in Japan (Sasaki et al.
2016). Other international vehicle manufacturers are now following suit, with plans to launch FCVs commercially in the near future. However, PEFCs currently are far more expensive than conventional combustion engines or batteries, partly due to the high cost of the fuel cell stack (as well as the hydrogen storage system) (Sun et al.
2010; Eberle et al.
2012). This high cost hampers their widespread distribution, thus limiting their potential impact on CO
2 and PM2.5 emissions.
One of the major factors contributing to the high cost of PEFCs is the ionomer membrane (Peighambardoust et al.
2010; Yee et al.
2012). This is typically Nafion, which is a sulfonated fluoropolymer and a registered trademark of DuPont. The commercial price of Nafion membranes currently ranges from e.g. 700–1400 $ m
− 2, depending on the supplier and scale of purchase (Peighambardoust et al.
2010; Yee et al.
2012). Due to this high price, PEFC membranes are estimated to account for 28% of the total cost of a PEFC stack (at a production scale of 1000 systems per year) (Marcinkoski et al.
2015). Clearly, substituting Nafion with a cheaper ionomer material could have a major impact on system cost, with a corresponding increase in market penetration. The United States Department of Energy (US DOE) has specific targets for PEFC membranes for FCVs including: a cost of < 20 $ m
− 2; a hydrogen crossover current of < 2 mA cm
− 2; an operation temperature > 120 °C; and a durability of 20,000 cycles, by 2020 (U.S. Department of Energy
2016). To satisfy all of these requirements will be extremely challenging. However, if selected targets (e.g. cost and hydrogen crossover) can be significantly exceeded, the other targets may be relaxed. In addition, stationary and portable fuel cell systems have less stringent targets, with cost playing a more heavily weighted role. Therefore, there is a potential gap-in-the market for low cost ionomer membranes with e.g. superior hydrogen barrier properties.
Cellulose is a biopolymer produced at great bulk in nature in the cell walls of plants and trees. As such, this polymer is not a petrochemical and is not synthesized from oil. It is processed at a scale of billions of tonnes per year as biofuel, food, cardboard, and paper. More than 400 million tonnes of paper and cardboard alone are produced annually (RISI
2015). There is a huge recycling industry for paper and cardboard, and therefore cellulose can be easily obtained from renewable sources. Recently, nanocellulose has emerged as a new variant on traditional cellulosic materials (Dufresne
2013; Kim et al.
2015). Nanocellulose is obtained by breaking down micron-scale cellulose fibers by mechanical, sonic, biological, or chemical treatments. The resulting cellulose nanofibers (CNFs) or cellulose nanocrystals (CNCs) can be readily dispersed in water to form gels or pastes. These dispersions can be filtered or printed to form strong, transparent nanocellulose membranes with temperature stability up to 150 °C (Henriksson et al.
2008; Nogi et al.
2009; Nogi et al.
2013). Applications of nanocellulose have so far included food packaging, textiles, lightweight polymer composites, intelligent inks, biomedical uses, adhesives, coatings, and even flexible organic solar cells (Dufresne et al.
2013; Lin and Dufresne
2014; Lee et al.
2014; Kim et al.
2015).
Nanocellulose is also a candidate material for use as an ionomer in P\EFC membranes. It has been reported to have excellent thermal stability of > 250 °C (Börjesson et al.
2018), well above the operating temperature of PEFCs and comparable to that of Nafion (280 °C) (Samms et al.
1996). It has been used as an additive in conventional ionomers such as Nafion. For example, Gadim et al. (
2016) impregnated a bacterial nanocellulose membrane with Nafion and obtained a PEFC power density of 16 mW cm
− 2 at room temperature. Jiang et al. (
2015) prepared a bacterial nanocellulose / Nafion composite (1:7 wt%) by solvent casting, obtaining a PEFC power density of 106 mW cm
− 2 at room temperature. In previous work, we showed for the first time that pure nanocellulose paper is a proton conductor and can function as an ionomer membrane. Membranes around 30 µm in thickness were fabricated from CNFs by vacuum filtration. They were weak proton conductors, but with impressive strength, and hydrogen permeability three orders of magnitude lower than Nafion (Bayer et al.
2016a). The proton conductivity was much higher in membranes fabricated from CNCs, due to the presence of sulfonic acid groups. However, CNC membranes were mechanically weak and unstable in water, and the maximum PEFC power density obtained was just 17 mW cm
− 2. Whilst this work was an important proof-of-concept, the performance was clearly far from sufficient for practical fuel cell applications. Increasing the proton conductivity of nanocellulose without compromising the mechanical stability and water tolerance, as well as reducing the membrane thickness is imperative.
Here, we synthesize sulfonated cellulose nanofibers (S-CNFs) to improve the ionic conductivity, whilst hopefully maintaining high mechanical strength and good water tolerance. Instead of vacuum filtration, which usually yields rather thick free-standing membranes (e.g. several tens of microns), spray-deposition is utilized here to directly coat thin S-CNF membranes onto an electrode-supported electrocatalyst layer, followed by spray-deposition of a top electrode electrocatalyst layer, in a process akin to additive manufacturing or 3D printing. The reduced thickness of the membrane results in significantly reduced membrane resistance, leading to enhanced PEFC performance. Whilst difficult to directly compete with established (but more expensive) ionomers such as Nafion, our S-CNF PEFCs already have power density comparable to other “low cost” technologies such as direct methanol fuel cells (DMFCs), or PEFCs using non-platinum group metal (non-PGM) catalysts. As such, these innovations bring commercialization of cheap, Nafion-free PEFCs a step closer.
Methods
Cellulose nanofiber (CNF) slurry (extracted from wood pulp via ultra-fine friction grinding, 3.0 wt% solids) was purchased from the University of Maine. A schematic of the nanocellulose sulfonation procedure is shown in Fig.
1a (Rajalaxmi et al.
2010). 100 g of CNF slurry was dispersed into 100 ml distilled water. 0.6 g of sodium periodate (Sigma Aldrich, Japan) was mixed into the dispersion and the flask was covered with aluminium foil to protect it from light. The dispersion was then stirred for 72 hours at room temperature. The product (2,3-dialdehyde nanocellulose) was then washed with water 5 times by centrifugation (High-speed Micro Centrifuge CF16RN, Hitachi, Japan, 5500×
g, 1 h), discarding the supernatant after each run. The 2,3-dialdehyde nanocellulose was then dispersed into 50 ml distilled water and reacted with 3 g sodium bisulfite (Sigma Aldrich, Japan). This dispersion was stirred for three days and the resulting product was washed five times at higher centrifugation speed (14,000×
g, 1 h). Finally, ion exchange was performed by adding 1 mL concentrated hydrochloric acid to the dispersion, followed by centrifugation washing (14,000×
g, 1 h). This ion exchange procedure was repeated twice to ensure the reaction went to completion.
The dispersions of sulfonated cellulose nanofibers (S-CNFs) were filtered and dried (hydrophilic PTFE Millipore filters, with a pore size of 0.1 µm). The resultant S-CNF membranes were hot-pressed between Teflon sheets for 20 min at 110 °C and 1.1 MPa, before being peeled from the filter. A schematic of this preparation process is in the supplementary information of our previous publication (Bayer et al.
2016a).
Fuel cell related properties of our nanocellulose membranes e.g. tensile strength, gas barrier and proton conductivity were compared with Nafion® PFSA NR-212 (Sigma Aldrich, Japan, 50.8 µm thickness), hereafter referred to as “Nafion”.
The morphology of the nanocellulose was observed using atomic force microscopy (AFM, Seiko Instruments, Japan, SPA300HV; SPI 3800 N probe station; SN-AF01 cantilever), after being dropping from dispersion onto silicon substrates. The average roughness was calculated from a total of 20 horizontal and vertical line profiles. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition (PHI 5000 Versa Probe II), and the data was charge-corrected. Fourier transform infrared spectroscopy (FT-IR) was collected in attenuated total reflection (ATR) mode using an infrared imaging microscope (Nicolet iN10 MX, Thermo Fisher Scientific, Japan).
The ion exchange capacity (IEC) was obtained by immersing nanocellulose membranes in 1 M NaCl solution for 72 h, followed by titration to neutral pH with 0.001 M NaOH solution.
Tensile strength, elongation until rupture, and elastic modulus were measured at ambient temperature and relative humidity (~ 65%). Samples were cut into dumbbell shaped specimens (total length 35 mm, 2 mm sampling width) using a bespoke punch. A hydraulic testing machine with 5N force gauge (FGO-C-TV, NIDEC-SHIMPO, Japan) was used to apply tensile load, until rupture, with an elongation speed of 10 mm per minute. The tensile stress was calculated from the applied force and the cross-sectional area of the individual specimen (thickness x width). In five samples each of CNF paper and Nafion, and three samples of S-CNF were used for tensile strength testing.
Water uptake and swelling of the membranes were investigated by immersion in deionized water. Samples (10 × 10 mm square) were first vacuum dried at 80 °C for two hours. The mass and thickness were measured using an analytical balance (Mettler Toledo, USA, ± 0.1 mg) and a micrometer (Mitutoyo, Japan ± 1 µm), respectively. The specimens were then submerged in a water bath at ambient temperature, for 60 minutes. The samples were then removed, and any excess surface water was carefully removed using tissue paper. Finally, the mass and thickness were measured again.
Water uptake was calculated as an average from 5 separate measurements, where m
wet and m
dry are the wet and dry masses, respectively:
$$Water\,Uptake\,(\%)=\frac{{m}_{wet}-{m}_{dry}}{{m}_{dry}}\cdot 100\,\%$$
(1)
Swelling was calculated as follows, where t
wet is thickness measured when saturated with water, and t
dry is the initial thickness.
$$Swelling \,(\%)=\frac{{\text{t}}_{wet}-{t}_{dry}}{{t}_{dry}}\cdot 100\,\%$$
(2)
The chemical stability of nanocellulose membranes and Nafion were investigated by using Fenton’s test (Fenton
1894). The samples were dried in vacuum at 80 °C for two hours, and then their dry weight was measured. The samples were then immersed into Fenton’s reagent (200 ml H
2O
2, 20 ppm Fe
2+) at 80 °C for one hour. Finally, the samples were vacuum-dried again and their weight loss was determined.
Measurement of gas permeance was performed by masking the membrane area with Kapton® and aluminum tape to obtain the desired area of interest (
S = 0.2–1.3 cm
2). A porous polycarbonate support filter (Isopore™, RTTP, 1.2 µm pore size) was used to prevent membrane deformation during measurements. A gas barrier testing system (GTR-11A/31A, GTR Tec Corp., Japan) was used to measure the dry hydrogen permeability between room temperature (ca. 25 °C) and 80 °C. Gas permeation was induced by applying pressure at the feed side, and a vacuum on the permeate side, with a total transmembrane pressure differential of 200 kPa (Bayer et al.
2016b). The sample collection time (
t) after vacuuming the permeate side of the membrane was 30 minutes. The collected gas was transferred to a gas chromatograph combined with a thermal conductivity detector (G3700T, Yanaco, Japan) and the volume (
V) was measured. Gas permeability is defined as
\(P=V\times l/S\times t\times \varDelta p\); with volume of the permeated gas at standard temperature and pressure (V), membrane thickness (l), membrane area (S), time of gas sample collection (t) and transmembrane pressure
\((\varDelta p)\).
The membrane conductivity was investigated at different temperature and relative humidity using a membrane testing device (MTS-740, Scribner Associates, USA) in tandem with an impedance analyzer (
Solartron SI1260) (Cooper
2019). An S-CNF membrane with a thickness of 23 µm was measured at an AC amplitude of 10 mV in a frequency range from 30 to 10 Hz, between 30 and 120 °C at 100% RH. At > 100 °C the measurement chamber was pressurized. An equivalent circuit was fitted using ZPlot (
Scribner), and the resistance (R) determined from the high frequency intercept. The conductivity (σ) was calculated from Eq.
3, where L is the membrane thickness in cm, R is the membrane resistance in, and A is the cross-sectional area (0.5 cm
2) (Bayer et al.
2014).
$$\sigma =\frac{L}{R\cdot A}$$
(3)
S-CNF paper with a thickness of 28 µm was used to fabricate a membrane electrode assembly (MEA). The catalyst ink was a mixture of Pt/C (Tanaka Kikinzoku Kogyo K.K., Japan, 46.2 wt% Pt) and 5 wt% Nafion (Sigma Aldrich, Japan), ethanol (Sigma Aldrich, Japan), and deionized water. Whilst nanocellulose will be explored in the future as an electrode ionomer in its own right, at this stage it is important to investigate these new membranes using state-of-the-art electrocatalyst layers. The catalyst ink was stirred overnight, and sonicated for 30 min before use (SMT Ultra Sonic Homogenizer UH-600). Catalyst ink was sprayed onto the membranes (Nordson K.K. Spraying Device, C-3J) with an electrode size of 0.5 cm
2, and a loading of 0.3 mg
Pt cm
− 2 at both the anodeand cathode. The MEA was then sandwiched between hydrophobic carbon paper gas diffusion layers (GDLs, EC-TP1-060T) at 0.2 kN for 190 s at 132 °C.
In order to increase the performance, an electrode-supported MEA was fabricated in the same process as reported by Breitwieser et al. previously (Klingele et al.
2015; Breitwieser et al.
2017). First, catalyst ink was sprayed onto two pieces of carbon paper (H23C8, Freudenberg, Germany, 1 × 1 cm) with a total loading of 0.3 mg
Pt cm
− 2. This carbon paper was then hot-pressed for 3 minutes at 132 °C and 0.3 kN (Digital Press CZPT-10, Sinto, Japan). Then, S-CNF/water/ethanol dispersion (1:18.5:43.8 mass ratio) was sprayed onto the electrocatalyst layers, with a thickness of ~ 4 microns (measured by micrometer). Finally, these sprayed components were assembled into a MEA with a 0.5 cm
2 active area, by using a PTFE sub-gasket.
The MEAs were installed in a NEDO single cell holder (1 cm
2 flow field), and their performance was measured in a fuel cell test rig (AutoPem-CVZ01, Toyo Corporation, Japan). The cells were preconditioned for two hours at 80 °C and a nitrogen gas flow of 100 ml min
− 1 (95% RH). The gas flow was then swapped to hydrogen and air (100 ml min
− 1, 95% RH), and the performance was measured after waiting for ten minutes. Polarization curves, power density plots, and durability were investigated by potentiostat (VersaSTAT 4, Amtek, USA and Fuel Cell Test System 890e, Scribner Associates, USA
) (Bayer et al.
2014; Bayer et al.
2016a).
The in-situ hydrogen crossover was determined after the IV-curve measurement by linear sweep voltammetry (LSV). Hydrogen and nitrogen were fed to the anode and cathode at a constant flow rate of 100 ml min
− 1. The potential of the cell was swept in 20 mV steps from the rest potential to 0.6 V using a Solartron SI1280 potentiometer (Solartron Analytical, England). The hydrogen crossover current density was evaluated at 300 mV (Inaba et al.
2006).
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