Sulfonic acid-crosslinked nanocellulose as a novel polymer electrolyte membrane for hydrogen fuel cells SELYANCHYN Olena Graduate School of Integrated Frontier Sciences, Department of Automotive Sciences, Kyushu University HYDROGEN FUEL CELL anode cathode electrolyte (PEM) gas diffusion layer bipolar plate H 2 H 2 O 2 O 2 Cell n Cell n-1 Cell n+1 Fuel cell - core technologi`cal element of the sustainable “Hydrogen society” Barriers of wide deploymet: > Cost of hydrogen fuel > Lack of infrastructure (e.g. fuelling stations) > Cost of fuel cells (Pt in electrocatalyst, bipolar plates and proton exchange membrane) Benchmark materials for PEM - perfluorinated sulfonic acid ionomers: Nafion®, Aquivion®, 3M® Proton conductivity ~ 100 mS/cm IEC ~ 0.9 mmol [H + ]/g Cost ~ US$600 to 1200 per m 2 C C F2 F2 F2 F2 F2 F2 C F C O C F C 6.6 n C SO 3 H O CF3 C Nafion® Disadvantages: high-cost, degradation, non-recyclable Development of low-cost and efficient PEM based on nanocellulose Purpose of this work: Proton exchange membrane fuel cell (PEMFC) RESEARCH MATERIAL: NANOCELLULOSE Main types of NC: - cellulose nanocrystals (CNC) - cellulose nanofibers (CNF) Characteristic properties: - high mechanical strength - low density & high surface area - non toxicity & biodegradability - flexibility Membranes features - uniform thickness in casted membranes (aqueous solution) - natural drying (no extra energy) - suitable for mass production - flat and stable after hot-pressing Nanocellulose can be obtained from various types of plants by mechanochemical processing or directly in bacteria: - strong acid treatment - mechanical shearing - grown in microorganisms Nanocellulose Molecular structure nanocrystals nanofibers strong acid treatment mechanical shearing O HO OH HO O O HO OH OH O O OH HO O O HO OH OH OH OH OH n Cellobiose Glucose Ordered (crystalline) domains Disordered (amorphous) regions Conventional cellulose (microfibers) Lignin Cellulose Hemicellulose Plant cell wall mechanical, chemical tretment Higher plants, bacteria, simple animals (e.g. tunicates) “Eco-friendly, low-cost material for fuel cell applications” crosslinked cellulose MODIFICATION APPROACH: SULFONIC ACID CROSSLINKING O OH HO O O HO OH OH O O OH HO O O HO OH OH OH OH ∗ ∗ O OH HO O O HO OH OH O O OH HO O O HO OH OH OH OH ∗ ∗ HO O S O OH O O OH O OH HO O O HO OH O O OH HO O O HO OH OH OH ∗ ∗ O HO O O HO O H OH O O HO O O HO OH OH OH OH ∗ ∗ O O S O O O O OH O O S O O O O HO + ester bonds hydrogen-bonds 130 ºC hot-press (oven) Sulfosuccinic acid sulfonic group hydroxyl group One-step approach Crosslinking: one-step reac- tion between mixed acid and nanocellulose results in a formation of multiple ester bonds disrupting the natural hydrogen-bonding network of cellulose. Hypothesis: surface of nano- cellulose covered with suffi- cient amount of sulfonic acid group (strong proton con- ducting moiety) will make a good proton conductor. nanofiber-based nanocrystal-based MACROSCOPIC & MICROSCOPIC MORPHOLOGY [hot-pressed] CNF paper Conventional paper 3%-SSA@CNF Membranes of 3-30 microns in thickness are distinctively differ- ent from conventional cellulosic membranes (e.g. paper), free- standing and self supporting. Maximum concentration of SSA in CNF ~ 10 wt%, up to 50 wt% can be blended with CNC 5 x 5 cm O.Selyanchyn, R.Selyanchyn & S.M.Lyth* Front.Energy.Res. 2020, doi.org/10.3389/fenrg.2020.596164 Bayer, 2016 (TP) Seo, 2009 (IP) Lin, 2013 (TP) Jiang, 2015 (IP) Gadim, 2016 (IP) Vilela, 2016 (TP) Vilela, 2017 (TP) Wang, 2019 (TP) Ni, 2018 (IP) Ni, 2018 (IP) Cai, 2018 (IP) Zhao, 2019 (IP) Etuk, 2020 (TP) Vilela, 2020 (TP) Zhao, 2019 (IP) Guccini, 2019 (TP) Di, 2019 (IP) Tritt-Goc, 2020 (?) Tritt-Goc, 2019 (?) Sriruangrungkamol, 2020 (TP) Rogalsky, 2018 (TP) Gadim, 2014 (TP) 0 10 20 30 40 50 60 70 80 90 100 0.1 1 10 100 Proton conductivity (mS⋅cm -1 ) Cellulosic material content (%) 20 85 30 30 90 90 60 94 80 Tri 9% Tri 13% Tri 16% Tri 20% S-CNC/Tri/PVA CNC(75%)/Im MFC@SSA CNC with residual sulfonic groups Carboxylated CNF Unmodified BC, CNF (hydrated) BG 60% BG 80% BG 80% PANI 6.4% BC/PMOEP BC/PMOEP CNF with residual sulfonic groups CNF@SSA AMPS-g-BC Composites with low content of cellulose BC/Nafion MFC/RDP BC/Fucoidan/Tannic acid CNC(78%)/Im PSS/BC BC/Nafion MFC/Nafion BC/PMACC BG 95% 25%SSA@CNC 9%SSA@CNF 3 x 3 cm CNC 3%SSA@CNC 7%SSA@CNC 10%SSA@CNC 20%SSA@CNC 2 μm 20 μm 2 μm μ 10 μm 10 μm 9%SSA@CNF bottom surface 9%SSA@CNF top surface SSA@CNF [hot-press] CNF paper [hot-press] Conventional paper 1 μm 1 μm 2 μm CNC surface 7%SSA@CNC surface 0 10k 20k 30k 40k 50k 60k 0 -10k -20k -30k Z'' / Ω Z' / Ω 20% RH 40% RH 60% RH 80% RH 100% RH 0 10k 20k 30k 40k 50k 0 -10k -20k -30k -40k -50k Z'' / Ω Z' / Ω 40°C 50°C 60°C 70°C 80°C 90°C 0.04 0.05 0.13 1.03 1.73 Conditions: 90 ºC, 95% RH 1 0.1 Proton conductivity (mS/cm) CNF 3%-SSA 5% 7% 9% Lower thickness of nanocellulose PEMs is possible due to material properties + high gas barrier - Thickness of the CNF and SSA@CNF membranes is below 10 μm - State-of-art Nafion in Toyota Mirai: 2008 (~50 μm); 2017 (14 μm); goal for 2020 - 10 μm PROTON CONDUCTIVITY OF CROSSLINKED MEMBRANES Strong dependence of σ on relative humidity, weaker dependence on temperature (9%-SSA@CNF). Crosslinking of the CNF with SSA resulted in ca. 40 times increased proton conductivity compared to unmodified CNF sample. (PRELIMINARY RESULTS ): COMPARISON WITH LITERATURE - Results of this work compared to literature shows that utilization of asid crosslinked nocellulose allows substantial increase in the proton conductivity and fabrication of thinner membranes. - Considering high gas barrier of nanocellulose membranes PEMs with competitive properies (specific resistance, chemical & mechanical stability) can be fabricated, that are environmentally friendly and have substantially lower cost compared to benchmarks (e.g. Nafion). CONCLUSIONS & FUTURE WORK 1. Nanocellulose is a promising biopolymer platform for the development of novel PEM for fuel cell applications. 2. Structural integrity of the organic acid crosslinked cellulose nanofiber and nanocrystal membranes was proven in the region of sub-10 micron thicknesses. 3. Morphological features (SEM), chemical structure (FTIR) and swelling behaviour in water suggest a promising material with competitive proton conductivity. Future experiments: mechanical properties, proton conductivity at high temperatures, chemical stability in hot water, O 2 and H 2 permeability, fuel cell performance. MACROSCOPIC & MICROSCOPIC MORPHOLOGY ACKHOWLEDGEMENT: Kyushu University Platform of Inter/Transdisciplinary Energy Research Support Program for Doctoral Students E-mail: [email protected] / Corresponding author: Prof. S.M. Lyth [email protected] / The Lyth Lab: https://sites.google.com/view/lythlab/