Wang L, Magliocca E, Cunningham E, Mustain W, Poynton S, Escudero-Cid R, Nasef M, Ponce-Gonzalez J, Bance-Souahli R, Slade R, Whelligan D, Varcoe J (2016) An optimised synthesis of high performance radiation-grafted anion-exchange membranes, Green Chemistry 19 pp. 831-843
Royal Society of Cemistry
High performance benzyltrimethylammonium-type alkaline anion-exchange membranes (AEM), for application in electrochemical devices such as anion-exchange membrane fuel cells (AEMFC), were prepared by the radiation grafting (RG) of vinylbenzyl chloride (VBC) onto 25 ¼m thick poly(ethylene-co-tetrafluoroethylene) (ETFE) films followed by amination with trimethylamine. Reductions in electron-beam absorbed dose and amount of expensive, potentially hazardous VBC were achieved by using water as a diluent (reduced to 30 ? 40 kGy absorbed dose and 5%vol VBC) instead of the prior-art method that used organic propan-2-ol diluent (required 70 kGy dose and 20%vol VBC monomer). Furthermore, the water from the aqueous grafting mixture was easily separated from residual monomer (after cooling) and was reused for a further grafting reaction: the resulting AEM exhibited an ion-exchange capacity of 2.1 mmol g-1 (cf. 2.1 mmol g-1 for the AEM made using fresh grafting mixture). The lower irradiation doses resulted in mechanically stronger RG-AEMs compared to the reference RG-AEM synthesised using the prior-art method. A further positive off-shoot of the optimisation process was the discovery that using water as a diluent resulted in an enhanced (i.e. more uniform) distribution of VBC grafts as proven by Raman microscopy and corroborated using EDX analysis: this led to enhancement in the Cl- anion-conductivities (up to 68 mS cm-1 at 80°C for the optimised fully hydrated RG-AEMs vs. 48 mS cm-1 for the prior-art RG-AEM reference). A down-selected RG-AEM of ion-exchange capacity = 2.0 mmol g-1, that was synthesised using the new greener protocol with 30 kGy electron-beam absorbed dose, led to an exceptional beginning-of-life H2/O2 AEMFC peak power density of 1.16 W cm?2 at 60°C in a benchmark test using industrial standard Pt-based electrocatalysts and unpressurised gas supplies: this was higher than the 0.91 W cm-1 obtained with the reference RG-AEM (IEC = 1.8 mmol g-1) synthesised using the prior-art protocol.
Ponce-González J, Whelligan DK, Wang L, Bance-Soualhi R, Wang Y, Peng Y, Peng H, Apperley DC, Sarode HN, Pandey TP, Divekar AG, Seifert S, Herring AM, Zhuang L, Varcoe J (2016) High performance aliphatic-heterocyclic benzyl-quaternary ammonium radiation-grafted anion-exchange membranes, Energy and Environmental Science 9 (12) pp. 3724-3735
Anion-exchange membranes (AEM) containing saturated-heterocyclic benzyl-quaternary ammonium (QA) groups
synthesised by radiation-grafting onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films are reported. The relative
properties of these AEMs are compared with the benchmark radiation-grafted ETFE-g-poly(vinylbenzyltrimethylammonium)
AEM. Two AEMs containing heterocyclic-QA head groups were down-selected with higher relative stabilities in aqueous KOH
(1 mol dm-3) at 80°C (compared to the benchmark): these 100 ¼m thick (fully hydrated) ETFE-g-poly(vinylbenzyl-Nmethylpiperidinium)-
and ETFE-g-poly(vinylbenzyl-N-methylpyrrolidinium)-based AEMs had as-synthesised ion-exchange
capacities (IEC) of 1.64 and 1.66 mmol g-1, respectively, which reduced to 1.36 mmol dm-3 (ca. 17 ? 18% loss of IEC) after
alkali ageing (the benchmark AEM showed 30% loss of IEC under the same conditions). These down-selected AEMs exhibited
as-synthesised Cl- ion conductivities of 49 and 52 mS cm-1, respectively, at 90°C in a 95% relative humidity atmosphere, while
the OH- forms exhibited conductivities of 138 and 159 mS cm-1, respectively, at 80°C in a 95% relative humidity atmosphere.
The ETFE-g-poly(vinylbenzyl-N-methylpyrrolidinium)-based AEM produced the highest performances when tested as
catalyst coated membranes in H2/O2 alkaline polymer electrolyte fuel cells at 60°C with PtRu/C anodes, Pt/C cathodes, and
a polysulfone ionomer: the 100 ¼m thick variant (synthesised from 50 ¼m thick ETFE) yielded peak power densities of 800
and 630 mW cm-2 (with and without 0.1 MPa back pressurisation, respectively), while a 52 ¼m thick variant (synthesised
from 25 ¼m thick ETFE) yielded 980 and 800 mW cm-2 under the same conditions. From these results, we make the
recommendation that developers of AEMs, especially pendent benzyl-QA types, should consider the benzyl-Nmethylpyrrolidinium
head-group as an improvement to the current de facto benchmark benzyltrimethylammonium headgroup.
Anion exchange membrane fuel cells (AEMFCs) offer several potential advantages over proton exchange membrane fuel cells (PEMFCs), most notably to overcome the cost barrier that has slowed the growth and large scale implementation of fuel cells for transportation. However, limitations in performance have held back AEMFCs, specifically in the areas of stability, carbonation, and maximum achievable current and power densities. In order for AEMFCs to contend with PEMFCs for market viability, it is necessary to realize a competitive cell performance. This work demonstrates a new benchmark for a H2/O2 AEMFC with a peak power density of 1.4 W cm?2 at 60 °C. This was accomplished by taking a more precise look at balancing necessary membrane hydration while preventing electrode flooding, which somewhat surprisingly can occur both at the anode and the cathode. Specifically, radiation-grafted ETFE-based anion exchange membranes and anion exchange ionomer powder, functionalized with benchmark benzyltrimethylammonium groups, were utilized to examine the effects of the following parameters on AEMFC performance: feed gas flow rate, the use of hydrophobic vs. hydrophilic gas diffusion layers, and gas feed dew points.
Radiation-grafted anion-exchange membrane (RG-AEM) research has predominantly focused on the chemical stability of
the polymer-bound positively-charged head-groups that enable anion conduction. The effect of the backbone polymer
chemistry, of the precursor film, on RG-AEM stability has been studied to a lesser extent and not for RG-AEMs made from
pre-irradiation grafting of polymer films in air (peroxidation). The mechanical strength of polymer films is generally
weakened by exposure to high radiation doses (e.g. from a high-energy e?-beam) and this is mediated by chemical
degradation of the main chains: fluorinated films mechanically weaken at lower absorbed doses compared to nonfluorinated
films. This study systematically compares the performance difference between RG-AEMs synthesised from a
non-fluorinated polymer film (low-density polyethylene ? LDPE) and a partially-fluorinated polymer film (poly(ethylene-cotetrafluoroethylene)
? ETFE) using the peroxidation method (pre-irradiation in air using an e?-beam). Both the LDPE and
ETFE precursor films used were 25 ¼m in thickness, which led to RG-AEMs of hydrated thicknesses in the range 52 ? 60 ¼m.
The RG-AEMs (designated LDPE-AEM and ETFE-AEM, respectively) all contained identical covalently-bound
benzyltrimethylammonium (BTMA) cationic head-groups. An LDPE-AEM achieved a OH? anion conductivity of 145 mS cm-1
at 80 °C in a 95% relative humidity environment and a chloride Cl? anion conductivity of 76 mS cm-1 at 80 °C when fully
hydrated. Alkali stability testing showed that the LDPE-AEM mechanically weakened to a much lower extent when treated
in aqueous alkaline solution compared to the ETFE-AEM. This LDPE-AEM outperformed the ETFE-AEM in H2/O2 anionexchange
membrane fuel cell (AEMFC) tests due to high anion conductivity and enhanced in situ water transport (due to the
lower density of the LDPE precursor): a maximum power density of 1.45 W cm-2 at 80 °C was achieved with an LDPE-AEM
alongside a Pt-based anode and cathode (cf. 1.21 mW cm-2 for the benchmark ETFE-AEM). The development of more
mechanically robust RG-AEMs has, for the first time, led to the ability to routinely test them in fuel cells at 80 °C (cf. 60 °C
was the prior maximum temperature that could be routinely used with ETFE-based RG-AEMs). This development facilitates
the application of non-Pt catalysts: 931 mW cm-2 was obtained with the use of a Ag/C cathode at 80 °C and a Ag loading of
0.8 mg cm-2 (only 711 mW cm-2 was obtained at 60 °C). This first report on the synthesis of large batch size LDPE-based RGAEMs,
using the commercially amenable peroxidation-type radiation-grafting process, concludes that the resulting LDPEAEMs
are superior to ETFE-AEMs (for the intended applications).
Developing the low-cost, highly active carbonaceous materials for oxygen reduction reaction (ORR) catalysts has been a high-priority research direction for durable fuel cells. In this paper, two novel N-doped carbonaceous materials with flaky and rod-like morphology using the natural halloysite as template are obtained from urea nitrogen source as well as glucose (denoted as GU) and furfural (denoted as FU) carbon precursors, respectively, which can be directly applied as metal-free electrocatalysts for ORR in alkaline electrolyte. Importantly, compared with a benchmark Pt/C (20wt%) catalyst, the as-prepared carbon catalysts demonstrate higher retention in diffusion limiting current density (after 3000 cycles) and enhanced methanol tolerances with only 50-60mV negative shift in half-wave potentials. In addition, electrocatalytic activity, durability and methanol tolerant capability of the two N-doped carbon catalysts are systematically evaluated, and the underneath reasons of the outperformance of rod-like catalysts over the flaky are revealed. At last, the produced carbonaceous catalysts are also used as cathodes in the single cell H2/O2 anion exchange membrane fuel cell (AEMFC), in which the rod-like FU delivers a peak power density as high as 703 mW cm?2 (vs. 1106 mW cm?2 with a Pt/C benchmark cathode catalyst).
A majority of anion exchange membrane fuel cells (AEMFCs) reported in the literature have been unable to achieve high current or power. A recently proposed theory is that the achievable current is largely limited by poorly balanced water during cell operation. In this work, we present convincing experimental results ? coupling operando electrochemical measurements and neutron imaging ? supporting this theory and allowing the amount and distribution of water, and its impact on AEMFC performance, to be quantified for the first time. We also create new electrode compositions by systematically manipulating the ionomer and carbon content in the anode catalyst layer, which allowed us to alleviate the mass transport behavior limitations of H2/O2 AEMFCs and achieve a new record-setting peak power density of 1.9 W cm?2 ? a step-change to existing literature. Our efforts cast a new light on the design and optimization of AEMFCs ? potentially changing the way that AEMFCs are constructed and operated.
This article describes the development of a sub-30 ¼m thick LDPE-based radiation-grafted anion-exchange membrane (RGAEM)
with high performance characteristics when fully hydrated. This RG-AEM had a OH? anion conductivity of 200 mS cm-
1 (80°C in 100% relative humidity (RH) environments), which led to a H2/O2 anion-exchange membrane fuel cell (AEMFC)
performance of 2.0 W cm-2 (80°C, RH = 92% environments, PtRu/C anode, and a Pt/C cathode) and a H2/air(CO2-free)
AEMFC peak power density of 850 mW cm-2 with a (non-platinum-group) Ag/C cathode electrocatalyst. When hydrated in
a RH = 100% N2 (CO2-free) atmosphere, the OH? form of this RG-AEM shows Â 7% degradation after 500 h at 80°C, with
the extent of degradation being highly similar when measured using three different techniques (decrease in conductivity,
decrease in ammonium content as measured using Raman spectroscopy, and decrease in ion-exchange capacity).
This work reports a high power, stable, completely Pt-free anion exchange membrane fuel cell (AEMFC) comprised of highly active catalysts ? Pd-CeO2/C at the anode and PdCu/C alloy at the cathode for the hydrogen oxidation and oxygen reduction reactions, respectively. The resulting AEMFC shows outstanding performance, reaching a peak power density of 1 W cm?2, twice the value of the best performance for Pt-free cells reported in the literature to date. The AEMFC also shows a low voltage degradation rate when operated continuously for more than 100 h at a constant 0.5 A cm?2, with a voltage degradation rate of only 2.5 mV h-1, which is excellent when compared to nearly all of the AEMFCs reported in the literature to date. This combination of high performance and high stability in the absence of Pt-based catalysts represents a significant landmark in the progress of the AEMFC technology.
Gonçalves Biancolli Ana Laura, Herranz Daniel, Wang Lianqin, Stehlíkov Gabriela, Bance-Soualhi Rachida, Ponce-González Julia, Ocón Pilar, Ticianelli Edson A., Whelligan Daniel K., Varcoe John R., Santiago Elisabete I. (2018) ETFE-based anion-exchange membrane ionomer powders for alkaline membrane fuel cells: a first performance comparison of head-group chemistry, Journal of Materials Chemistry A
Royal Society of Chemistry
In the last few years, the development of radiation-grafted powder-form anion-exchange ionomers (AEI), used in
combination with anion-exchange membranes (AEM), have led to the assembly of AEM-based fuel cells (AEMFC) that
routinely yield power densities ranging between 1 ? 2 W cm-2 (with a variety of catalysts). However, to date, only
benzyltrimethyammonium-type powder AEIs have been evaluated in AEMFCs. This study presents an initial evaluation of
the relative AEMFC power outputs when using a combination of ETFE-based radiation-grafted AEMs and AEIs containing
three different head-group chemistries: benzyltrimethylammonium (TMA), benzyl-N-methylpyrrolidinium (MPY), and
benzyl-N-methylpiperidinum (MPRD). The results from this study strongly suggest that future research should focus on the
development and operando long-term durability testing of AEMs and AEIs containing the MPRD head-group chemistry.
Zhu Yuan, Ding Liang, Liang Xian, Shehzad Muhammad A., Wang Lianqin, Ge Xiaolin, He Yubin, Wu Liang, Varcoe John R, Xu Tongwen (2018) Beneficial use of rotatable-spacer side-chains in alkaline anion exchange membranes for fuel cells, Energy & Environmental Science 11 (12) pp. 3472-3479
Royal Society of Chemistry
Side-chain-type polymer architectures have been extensively studied for development of highly conductive fuel cell membranes. However, the commonly used rigid, hydrophobic spacers (between the ionic end-group and polymer backbone) limit self-assembly of ionic side-chains and, therefore, ion transport. Herein, we report a flexible, hydrophilic side-chain-type anion exchange membrane (AEM), where ethylene oxide spacers are incorporated into imidazolium-containing cationic side-chains. AFM and SAXS analysis confirm that the flexible spacers facilitate self-assembly of the ionic side-chains to form continuous conducting channels. Most importantly, both in situ FTIR spectroscopy and molecular dynamic theory simulations indicate that the ethylene oxide spacers are capable of hydrogen bonding to both H2O molecules and hydrated OH? ions. This unique auxiliary function facilitates both ion and H2O transport during fuel cell operation. The resultant AEM exhibits a peak power density of 437 mW cm?2 at 65 °C when tested in a H2/O2 single-cell anion-exchange membrane fuel cell, which is among the highest reported for comparable side-chain-type AEMs.
Herein we detail the development of a new high-density polyethylene-(HDPE)-based radiation-grafted anion-exchange membrane (RG-AEM) that achieves a surprisingly high peak power density and a low in situ degradation rate (with configurations tailored to each). We also show that this new AEM can be successfully paired with an exemplar non-Pt-group cathode.
Broader context: A primary motivation for the development of anion-exchange membrane (AEM) fuel cells (AEMFCs) is the broader range of sustainable, non-precious-metal catalysts that are feasible; if costs are lowered enough, AEMFCs would be deployable in a range of stationary power sectors (e.g. back-up and off-grid). However, as the performance of AEMFCs typically drop when Pt-based electrodes are replaced with non-Pt types, it is essential that the highest performing polyelectrolytes are developed, both membranes and ionomers (the latter incorporated to impart ionic conductivity in the electrodes). The findings with the high conductivity AEM reported herein will also be of interest to developers of AEMs for metal?air and redox-flow batteries, electrolysers (both H2O H2 and CO2 high-value chemicals and fuels), and salinity gradient power.