Catalyst Investigation
An anion exchange membrane water electrolyzer (AEMWE) was studied with three electrocatalysts (Co3O4,Mn2O3, MnO2) for the oxygen evolution reactions at 50 °C in 1 M K2CO3(aq). We employ an optimized robust high performance polymer based on a polyethylene mid-block copolymer, poly(vinylbenzyl-N‑methylpiperidinium carbonate)‑b‑polyethylene‑b‑poly(vinylbenzylN‑methylpiperidinium carbonate) as the AEM and the anode ionomer. The cathode utilized a high loading of Pt/C, 1 mg cm−2, to minimize contributions to the kinetics. We tested three catalyst loadings (0.5, 2.5, and 4.5 mg cm−2) with a fixed ionomer loading of 0.5 mg cm−2 to assess ionomer-catalyst interactions. The best-performing catalyst loadings were investigated in a 100 h durability test at 750 mA cm−2. The 2.5 mg cm−2 MnO2 catalyst displayed superior performance, with 2.40 ± 0.02 V at 1 A cm−2. In the 100 h durability test, the Mn2O3 catalyst showed a degradation rate of +269 ± 15 μVh−1, whereas Co3O4 and MnO2 showed −800 ± 157 μVh−1, −114 ± 15 μVh−1, respectively with no membrane thinning indicating a gradual improvement. The MnO2 electrode was further investigated in a 500 h test was conducted, revealing a voltage change rate of −21 μVh−1 for 24–375 h. Pre and post-test FTIR mapping revealed evolution of micrometer-sized morphology corresponding to templating by the Ni-foam electrode.
https://iopscience.iop.org/article/10.1149/1945-7111/ad2cbe/pdf
PE Based AEM
A challenge in anion exchange membrane (AEM) development is simultaneously optimizing alkaline chemical stability and mechanical integrity during thermal and humidity cycling and achieving high anionic conductivity. Here, we report on the hydrogenation of an ABA triblock copolymer polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene (PCMS-b-PCOE-b-PCMS) to yield a polyethylene-based triblock copolymer, polychloromethylstyrene-b-polyethylene-b-polychloromethylstyrene (PCMS-b-PE-b-PCMS). A polydisperse midblock was synthesized with narrowly disperse outer blocks to favor nanoscale phase separation and promote an interconnected morphology. Varying degrees of chemical cross-linking of the PCMS domains were achieved by using different processing temperatures to tune the water uptake and dimensional swelling. Quaternization with either trimethylamine or methylpiperidine resulted in AEMs with improved characteristics, including excellent Cl– and OH– conductivity (119 and 179 mS cm–1 at 80 °C, respectively) and moderate water uptake (33 wt %, λ = 12). Unexpectedly, extensional testing indicated that the mechanical strength of the film improved upon hydration. Wide-angle X-ray scattering revealed that in the presence of liquid water the PE backbone rearranges and forms larger crystalline domains, which led to the improved stress at break. These fundamental mechanistic insights are of critical importance in designing mechanically robust AEMs for aqueous applications such as electrolysis and reverse electrodialysis. This work demonstrates the applicability of tunable block copolymer systems for developing practical AEM materials for more modest pH liquid applications. Numerous tunable variables including chemical cross-linking and semicrystalline variability highlight how mechanical integrity, water management, and ionic conductivity can be simultaneously achieved.
Performance in Carbonate Electrolyzer
Electrolysis of water to produce hydrogen from renewable electricity is an extremely attractive strategy to reduce energy dependence on fossil fuels. Development of membrane and ionomer materials that maintain high performance with long lifetimes is needed. We developed and investigated the performance and durability of a series of polyethylene-based ABA triblock copolymer anion exchange ionomer and membrane (AEI, AEM) materials for anion exchange membrane water electrolysis. Poly(vinylbenzyl N-methylpiperidinium carbonate)-b-polyethylene-b-poly(vinylbenzyl N-methylpiperidinium carbonate) was synthesized with different hydrophobic/hydrophilic block ratios (1.02:1, 2.58:1, and 4.46:1) resulting in a range of ion exchange capacities (1.1–1.8 meq g–1) and water swelling (23–154%) characteristics. All AEMs showed full anionic dissociation, as evidenced by linear Arrhenius correlations, and excellent carbonate conductivity of 8–94 mS cm–1 at 50 °C. Hydrophilic phase separation may offer superior chemical durability by only wetting the ion-conducting region of the polymer and avoiding attack at the nonwetted backbone. We evaluated the performance and durability of these AEMs as a function of hydrophobic polyethylene (PE) content. The AEM containing the least amount of PE displayed the highest performance of 1 A cm–2 at 2.3 V but degraded at 40 mV h–1 before catastrophically failing after <3 h at 0.5 A cm–2. With greater PE-containing AEMs, the degradation rate was reduced by 3 orders of magnitude with only a 0.2 V increase in voltage. Constant current electrolysis for 50 h resulted in a voltage change of 0.3 mV h–1 in a single-cell water electrolyzer and 1 M potassium carbonate. In a 600 h test, the voltage change in the final 200 h was impressively low for an experimental film, 58 μV h–1. Postmortem analysis indicated that the membrane did not thin and that water becomes more tightly hydrogen-bound in the polymer after electrolysis.
Tuning for Electrodialysis
Development of improved durable, conductive, and selective anion exchange membranes (AEMs) are critical for electrodialysis. A series of three ABA triblock copolymers derived from polychloromethylstyrene-b-polyethylene-b-polychloromethylstyrene (PCMS-b-PE-b-PCMS), designated as large, 0.8:1, medium, 0.2:1, and small 0.1:1, block AEMs by their PCMS:PE block size were quaternized with methylpiperidine. The water uptake decreases from 100% for the large block AEM to 5% for the small block AEM. At this low water content in liquid water, the small block AEM shows complete ion dissociation, with similar diffusion (2.6 × 10−6 mol m−2 s−1) and Ea (24 kJ/mol) as the larger block AEMs. However, a 34% increase in apparent permselectivity was achieved with the small block AEM. In electrodialysis, the small block AEM improved salt removal by 300% compared with a commercial AEM. Under further investigation, migration was shown to be enhanced by 600% in the small block AEM. The small block AEM has a � = 5 in liquid water, which is too low to explain the chloride conductivity behavior. Furthermore, the permselectivity in the small block AEM is much higher than expected from its small-angle X-ray scattering morphology, indicating there must be a water-rich region of smaller dimensions where cation/anion pairs are fully solvated.
Silver Nanoparticle Interactions
Interactions between silver nanoparticles and a cationic triblock copolymer significantly alter bulk material properties and can be tuned by functionalizing the polymer with different quaternary ammonium cations (QACs). In this work, polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene (PCMS-b-PCOE-b-PCMS) was quaternized with either methylpiperidinium (MPRD) or trimethylammonium (TMA). The surface interacting groups were identified by using FT-IR to be the phenyl rings, the QACs from the outer blocks, and vinyl groups from the PCOE midblock. Changes in thermal characteristics and crystallinity were highly dependent on the QAC, uncovering differences in the nature of the interactions between silver and TMA or MPRD. Interactions induced by TMA greatly hinder crystalline domain formation and give rise to a higher water content, while MPRD promotes certain crystalline orientations and provides a greater degree of crystallinity with a lower water content. Our findings demonstrate that bulk polymer characteristics can be tuned, a highly desirable attribute for many nanocomposite materials.