William E. Mustain

Anion-exchange membranes in electrochemical energy systems†

This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolysers, redox flow batteries, reverse electrodialysis cells, and bioelectrochemical systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technological and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the references that the authors deemed relevant, and were available on the web by the manuscript submission date (30th April 2014), are included.

Recent progress in the electrochemical conversion and utilization of CO2

Over the past several years, there has been a growing interest in the capture of carbon dioxide emissions and either their permanent immobilization or chemical conversion to industrially relevant products. Several processes have been developed and studied; however, many of these methods are quite expensive since they require either ultra high purity CO2 or are energy intensive. Also, many purely chemical methods show low product selectivity. To address these limitations, several researchers have initiated activities using electrochemical processes to increase reaction pathway selectivity and reduce cost since it allows for direct control of the surface free energy through the electrode potential, which has shown promise. This review article focuses on the advantages and disadvantages of current electrochemical, photoelectrochemical and bioelectrochemical processes for CO2 conversion, and future directions for research in this area are discussed.

High Stability, High Activity Pt/ITO Oxygen Reduction Electrocatalysts

Sn-doped indium oxide (ITO) nanoparticles (NPs) were conceived as a high stability noncarbon support for Pt NPs, and the activity and stability of Pt/ITO for the oxygen reduction reaction (ORR) were probed. Sn was employed as the In(2)O(3) dopant to exploit the strong interaction between Sn and Pt that was previously reported to enhance the activity of Pt on Pt/SnO(2), while concomitantly avoiding the intrinsic stability limitations of SnO(2) and leveraging the high stability of bulk In(2)O(3) at ORR relevant potentials. The mass activity of Pt was extremely high on Pt/ITO, 621 ± 31 mA/mg(Pt), which far exceeded the 2015 DOE goal for Pt mass activity of 440 mA/mg(Pt). The enhanced ORR activity was linked to the faceting of the Pt NPs, which overwhelmingly consisted of Pt (111) facets. The stability of Pt/ITO was also very impressive, with the electrochemically active area unchanged and the Pt half wave potential shifting only 4 mV over 1000 potential cycles to 1.4 V vs RHE, a very harsh condition for ORR electrocatalysts where state-of-the-art Pt/C electrocatalysts typically show very poor stability.

Properties of Nitrogen-Functionalized Ordered Mesoporous Carbon Prepared Using Polypyrrole Precursor

Nitrogen-doped ordered mesoporous carbon was synthesized using SBA-15 as the template and polypyrrole as the nitrogen-containing carbon precursor. Transmission electron microscopy and N 2 Brunauer, Emmett, and Teller adsorption revealed a honeycomb-like ordered mesoporous structure with an average pore diameter of 3.3 nm with a narrow distribution. X-ray photoelectron spectroscopy showed that pyridinic and quaternary nitrogen functionalities were the dominant nitrogen surface functional groups. A high percentage of nitrogen was retained in the carbon surface (C/N = 8.3). The prepared nitrogen-functionalized support had a specific double layer capacitance of 182.5 F/g. Also, its intrinsic oxygen reduction activity was better than that of Vulcan XC-72R. Accelerated degradation test showed that nitrogen-functionalized carbon was highly resistant to electrochemical corrosion.

Electrocatalytic Activity and Stability of Pt clusters on State-of-the-Art Supports: A Review

Instability of supported Pt clusters due to limited bonding with conventional carbon supports and carbon dissolution leads to significant cathode performance losses with time, impeding the development of commercial proton exchange membrane fuel cells. One approach that has recently been gaining momentum is the use of the electrocatalyst support to enhance both the stability and activity of Pt clusters for the oxygen reduction reaction. This review article focuses on four support types: advanced carbons, conductive ceramics, metallic underlayers for Pt monolayer catalysts, and the 3M crystalline organic whiskers. Advantages and disadvantages of each support are summarized and promising future directions for research in this area are discussed.

An optimised synthesis of high performance radiation-grafted anion-exchange membranes

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 μm thick poly(ethylene-co-tetrafluoroethylene) (ETFE) films followed by amination with trimethylamine. Reductions in the 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 state-of-the-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 the 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 a fresh grafting mixture). The lower irradiation doses resulted in mechanically stronger RG-AEMs compared to the reference RG-AEM synthesised using the prior state-of-the-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 state-of-the-art RG-AEM reference). A down-selected RG-AEM with an ion-exchange capacity = 2.0 mmol g−1, that was synthesised using the new greener protocol with a 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 state-of-the-art protocol.

Platinum–copper nanotube electrocatalyst with enhanced activity and durability for oxygen reduction reactions

The activity and the durability of Pt based electrocatalysts constitute the major challenges in the current fuel cell technology. Motivated by the improved activity and ameliorated durability for Pt based cathode catalysts achieved from the compositional and morphological control, respectively, in this paper, a rationally designed, PtCu based, 1-dimensional electrocatalyst was prepared via a facile and scalable procedure. The detailed materials characterization revealed that the as-prepared nanotubes (NTs) were composed of a PtCu alloy bulk and a Pt-enriched surface with downshifted d-band center position. Towards the electrocatalysis of oxygen reduction reaction, PtCu NTs have displayed a distinguished specific activity with more than a 10-fold improvement relative to the commercial catalysts and more than a 3-fold improvement relative to the 2015 DOE technical target (0.72 mA cmPt−2 @ 0.9 V). Meanwhile, PtCu NTs have shown greatly ameliorated durability in the aspects of both ECSA and mass activity compared to Pt/C (40%) and Pt black after a 6000-cycle accelerated durability test.

Preparation of radiation-grafted powders for use as anion exchange ionomers in alkaline polymer electrolyte fuel cells

A novel alkaline exchange ionomer (AEI) was prepared from the radiation-grafting of vinylbenzyl chloride (VBC) onto poly(ethylene-co-tetrafluoroethylene) [ETFE] powders with powder particle sizes of less than 100 μm diameter. Quaternisation of the VBC grafted ETFE powders with trimethylamine resulted in AEIs that were chemically the same as the ETFE-based radiation-grafted alkaline anion exchange membranes (AAEM) that had been previously developed for use in low temperature alkaline polymer electrolyte fuel cells (APEFC). The integration of the AEI powders into the catalyst layers (CL) of both electrodes resulted in a H2/O2 fuel cell peak power density of 240 mW cm−2 at 50 °C (compared to 180 mW cm−2 with a benchmark membrane electrode assembly containing identical components apart from the use of a previous generation AEI). This result is promising considering the wholly un-optimised nature of the AEI inclusion into the catalyst layers.

Investigation of metal oxide anode degradation in lithium-ion batteries via identical-location TEM

Metal oxides are one of the most promising classes of materials to replace graphite anodes in future high energy density, high power density applications for lithium-ion batteries (LIBs), including (hybrid) electric vehicles and grid-scale energy storage. Unlike graphite, which undergoes lithium ion intercalation, nearly all metal oxides (MOs) experience structural decomposition and phase separation during charge–discharge. In this work, ordered mesoporous nickel oxide (OMNiO) was synthesized and used as a representative MO to study the nanoscale structural changes that occur during charge–discharge via identical-location transmission electron microscopy (IL-TEM). We connect IL-TEM with coin cell electrochemical studies to explain the root cause for the rapid capacity fade that is common to MOs generally, and NiO specifically. We show that the electronic conductivity, in addition to MO structure, plays a crucial role in maintaining high capacity over repeated charge–discharge cycling.

Hydrogen and Methanol Oxidation Reaction in Hydroxide and Carbonate Alkaline Media

The oxidation of hydrogen and methanol by hydroxide and carbonate anions in low temperature alkaline electrolytes was investigated on a polycrystalline platinum rotating disk electrode. The electron equivalence was experimentally determined as 2.0 and 1.9 for oxidation of hydrogen with hydroxide and carbonate anions, respectively. The exchange current density for hydrogen oxidation was measured as 0.14 mA cm−2 by hydroxide (1 M KOH), while in carbonate electrolytes, the exchange current density was 0.24 mA cm−2 (0.3 M CO3 −2) and 0.32 mA cm−2 (0.5 M CO3 −2). The increased exchange current density through the carbonate pathway was attributed to the ease of bond reorganization between hydrogen and carbonate compared to hydrogen and hydroxide, which results in a more thermodynamically favored process. Also, a more complete methanol oxidation was observed in the presence of hydroxide, though the difference compared to carbonate was not significant.