Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX pubs.acs.org/CR Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer † Nagappan Ramaswamy and Sanjeev Mukerjee* Northeastern University Center for Renewable Energy Technology, Department of Chemistry and Chemical Biology, Northeastern University, 317 Egan Research Center, 360 Huntington Avenue, Boston, Massachusetts 02115, United States ABSTRACT: Alkaline anion-exchange membrane (AAEM) fuel cells have attracted significant interest in the past decade, thanks to the recent developments in hydroxide- anion conductive membranes. In this article, we compare the performance of current state of the art AAEM fuel cells to proton-exchange membrane (PEM) fuel cells and elucidate the sources of various overpotentials. While the continued development of highly conductive and thermally stable anion-exchange membranes is unambiguously a principal requirement, we attempt to put the focus on the challenges in electrocatalysis and interfacial charge transfer at an alkaline electrode/electrolyte interface. Specifically, a critical analysis presented here details the (i) fundamental causes for higher overpotential in hydrogen oxidation reaction, (ii) mechanistic aspects of oxygen reduction reaction, (iii) carbonate anion poisoning, (iv) unique challenges arising from the specific adsorption of alkaline ionomer cation-exchange head groups on electrocatalysts surfaces, and (v) the potential of alternative small molecule fuel oxidation. This review and analysis encompasses both the precious and nonprecious group metal based electrocatalysts from the perspective of various interfacial charge-transfer phenomena and reaction mechanisms. Finally, a research roadmap for further improvement in AAEM fuel cell performance is delineated here within the purview of electrocatalysis and interfacial charge transfer. CONTENTS 1. INTRODUCTION 1. Introduction A Proton-exchange membrane (PEM) fuel cell performance has 2. Current Status of AAEM Fuel Cell Performance D witnessed enormous progress due to decades of developmental 2.1. PEM vs AEM Electrolyte Bulk Properties F research in electrocatalysts, perfluorinated sulfonic acid (PFSA), 2.2. General Context of Alkaline Electrocatalytic membrane electrolytes and interfacial ionomer solutions, as well Interfaces H as effective integration of these individual components to 3. Hydrogen Oxidation Reaction in Alkaline Media H fabricate a membrane electrode assembly (MEA).1,2 State of the 4. Oxygen Reduction Reaction in Alkaline Electro- art PEM fuel cell performance operating on H2/air gas feeds lyte N using high surface area carbon-supported Pt/Pt-alloy-based 4.1. Nature of the First Electron Transfer Step to catalysts yield power densities ranging roughly 900 to 1000 2 ≥ ° O2 N mW/cm MEA at cell voltages 0.65 V (80 C, 100% RH, 150 4.2. Base Catalysis of the Hydrogen Peroxide 2−4 Downloaded via UNIV OF WINNIPEG on November 9, 2019 at 00:47:05 (UTC). kPaa outlet pressure). This is typically achieved with total 2 Intermediate P MEA precious metal loading of 0.20 to 0.25 mgpgm/cm leading 5. Effect of Carbonate Poisoning of Alkaline Mem- to platinum specific power density values of roughly 0.20 to 0.25 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. branes on Fuel Cell Performance U gPt/kW or alternately 4 to 5 kW/gPt. Recent studies using 6. Effect of Ammonium Cation Poisoning on PtCoMn/PtNi nanostructured thin film (NSTF) cathode Interfacial Charge Transfer W 2 catalysts of total MEA loading 0.15 to 0.25 mgPGM/cm have 7. Alternate Small Molecule Oxidation X 2 ≥ enabled power densities of 800 mW/cm MEA at cell voltages 8. Conclusions Y fi 0.65 V leading to Pt-speci c power densities of roughly 0.3 gPt/ Author Information Z 3,5 kW or alternately 3.3 kW/gPt. While this represents a Corresponding Author Z tremendous progress in performance achieved via both decrease ORCID Z in mass transport induced voltage losses and increase in Pt/Pt- Present Address Z alloy catalyst activity,2 the sustainable use of PEM fuel cell Funding Z technology in large-scale applications still demands further Notes Z improvements in Pt-specific power density in consideration of Biographies Z both the Pt supply and volatile cost. The department of Energy References Z (DOE) has set year 2020 targets for PEMFC electrocatalysts that calls for Pt-specific power density to be improved to 0.125 Received: March 9, 2019 © XXXX American Chemical Society A DOI: 10.1021/acs.chemrev.9b00157 Chem. Rev. XXXX, XXX, XXX−XXX Chemical Reviews Review ∼ 13−15 gPGM/kW or, alternately, 8 kW/gPt ( 8 to 10 g of PGM for 80 alkaline electrolyte based fuel cells. Energy conversion in kWnet automotive fuel cell stack) with total MEA loading of AAEM fuel cell involves the hydrogen oxidation reaction 2 3 − → 0.125 mgPGM/cm . Researchers in the automotive industry have (HOR) at the anode (H2 + 2OH 2H2O+2e)̅ with stretched this target even higher up to 16 kW/gPt to reduce the hydroxide anions that are produced as a result of oxygen ∼ 4 − → total stack PGM content of 6gPGM/vehicle. This has to be reduction reaction (ORR) at the cathode (O2 +2H2O+4e accomplished without compromising on the performance (i.e., 4OH−) leading to the formation of final product water on the 2 ≥ power density of at least 1.0 W/cm MEA at operating potentials anodeside.Theprimaryfunction of AAEM polymeric 0.65 V) and durability at low PGM loadings (5000 h of electrolyte is to spatially separate the electrodes and enable operation for automotive applications equivalent to 150000 the selective transport of hydroxide anions from cathode to miles, and 40000 h of reliable operation for stationary anode and transport of water from anode to cathode without − applications).3 6 Durability issues lead to constant decrease in reactant/oxidant gas crossover. Water produced as a byproduct PEM fuel cell performance (i.e., power/current density loss with on the anode is consumed as a stoichiometric reactant in the lifetime) due to the manifestation of a wide range of degrading ORR process at the cathode presumably leading to better water factors related to the materials (membrane, electrocatalysts, management and decreased cathode flooding at high current support, ionomer, and gas diffusion media) and operating densities. To date, a majority of the AAEM polymer chemistry conditions (temperature, pressure, humidity, impurities, and investigated typically involves a hydrocarbon membrane back- potential excursions).7 The projected cost target of PEM fuel bone (polysulfone,16,17 polyphenylene,18 and polyethylene19) cell system (stack plus balance of plant for an 80 kWnet system) is covalently functionalized with positively charged benzyltrimeth- $30/kW by 2020 out of which ∼25% of the cost arises from the yl quaternary ammonium (QA) cation headgroup side-chains − use of precious metal cathode catalysts.6 The major challenges in for hydroxide-anion transport.14,20 27 Use of solid state AAEM PEM fuel cells to be overcome prior to widespread deployment electrolytes free of mobile alkali metal cations implies that the can be broadly classified as related to cost minimization without carbonate precipitation found in liquid electrolyte AFC is also affecting performance and durability. There is significant effort largely mitigated (vide infra). Typical AAEM solid electrolytes related to the development of non-PGM catalysts for PEM fuel exhibit ∼20−40 mS cm−1 hydroxide anion conductivity with cells to mitigate the cost arising from the use of Pt in the cathode. ion-exchange capacities of ∼1.5 to 2.0 mmol g−1 at room So far, some of the leading non-PGM cathode catalyst temperature and well-hydrated conditions to enable moderate candidates based on the metal−nitrogen coordination centers current densities during fuel cell operation.14 For applications in − ∼ 2 (Fe Nx) have shown H2/air MEA performance of 0.8 A/cm the watts to a few kilowatts scale, AMFC technology has the ∼ 2 ° 8 at 0.65 V leading to 500 mW/cm MEA at 80 C and 100% RH. potential to capture the early market in applications such as While this represents tremendous progress, the non-PGM auxiliary power sources in automotive applications, critical catalysts suffer from poor stability and still unacceptably low backup power, material handling equipment, and micro-CHP performance at high power.4 Despite worldwide research effort applications.28 in nonprecious catalyst developments9 and durability improve- In the first part of this treatise, we will establish the current ments,7 the chronic dependence of acidic PEM fuel cells on state of the art AAEM fuel cell performance operating on PGM PGM catalysts causes significant liabilities for both cost and and non-PGM catalysts in comparison to PEM fuel cells. For this durability targets. This strongly predicates widespread commer- purpose, we utilize Tokuyama A201 anion-exchange membrane cialization on retrenching component costs and striking an along with commercially available Pt/C and in-house developed optimum balance between performance and durability. non-PGM electrocatalysts. It should be noted that arguments in Alkaline electrolyte provides a paradigm shift with the the PEM fuel cell literature has shifted to Pt-specific power possibility of complete elimination of precious metal catalysts densities due to the pertinent requirement of reduced Pt- and potentially enhanced material stability. The advantages of loading, whereas AAEM fuel cell is in an early stage of traditional alkaline