Electrocatalysis: Holding the Keys to Advanced Energy Materials and Systems (2016)

This Special Issue comprises 23 invited contributions on the topic of electrocatalysis. Research in the area of electrocatalysis is driven by the escalating need for advanced catalyst materials in electrochemical energy technologies. Progress in this area continues to be of primary relevance for the on-going transition to renewable energy infrastructures, which demands large-scale and long-term energy storage as well as highly efficient and environmental benign energy conversion from electrical to chemical and back. So far, the ability to meet performance and stability targets of water-based electrochemical devices, such as polymer electrolyte fuel cells and PEM-based electrolyzers, hinges on the use of Pt-group metals (PGM). However, the limited abundance of PGM, in particular of Pt, is a massive obstacle to the widespread deployment of electrochemical energy technologies. Research strategies aim at reducing the use of PGM by specifically designed and stabilized interfacial architectures that boost catalyst effectiveness and longevity, by use of alloying strategies to improve the intrinsic activity and surface area utilization of the catalyst, and eventually by replacing noble metal catalysts by nature-inspired catalysts that can be fabricated from earth-abundant elements. Promising approaches in catalyst design explore materials with nitrogen-coordinated metal ions, porous transition metal oxides, metal nitrides, and (oxy)nitrides [Vukmirovic et al., Wassner et al., Chong et al., Gómez-Marín et al., Lopes et al., Ryabova et al., Li et al., Zhang et al., Gusmão et al., An et al.]. Furthermore, introducing new catalyst materials entails a critical need to re-design porous composite host media that accommodate the catalyst; the objective of this materials design exercise are twofold: 1) to guarantee rapid supply and removal of reactant and product species to and from active catalyst sites and 2) to ensure optimal adaptation of operating conditions to electronic, electrostatic, catalytic, and transport properties of novel catalyst and support materials. The formidable challenges inherent in devising optimal host media and operating conditions around new classes of electrocatalyst materials are known from efforts in developing PGM-free catalysts for polymer electrolyte fuel cells. Another core issue of electrocatalysis research for electrochemical energy systems is concerned with the stability of complex electrode assemblies that consist of nanoscale catalyst particles [Timperman et al.], high-surface-area porous support materials, and an embedded phase of self-organized ionomer as a proton reservoir and binder. Nanoparticle dissolution, detachment, and coagulation as well as the corrosion of high-surface-area carbon materials have been singled out as the main degradation mechanisms in low-temperature polymer electrolyte fuel cells. Understanding the different degradation mechanisms under relevant operating conditions and potential cycling protocols provides rational criteria for material selection and material preparation. Pertinent materials modification strategies are addressed in various articles to this Special Issue, taking a fresh look at alternative support materials such as thin films and nanosheets of transition metal oxides and carbides that may come with enhanced stability but may at the same time affect the electrocatalytic activity through modification of the catalyst–support interaction [Gebauer et al., Kabir et al., Celebanska and Opallo]. In the challenge to unravel the complex interplay of desired and undesired electrocatalytic processes, seamless advances in first-principles simulations [McCrum et al.] and electrochemical characterization [Martínez-Rodríguez et al.] become increasingly important. Ideally, the complete characterization of novel electrocatalyst and support materials as well as the electrode structures they form would allow reliable predictions for performance and stability at the fuel cell level to be made. In return, diagnostic measurements on whole cells and model-based analyses are needed to deconvolute and quantify different degradation contributions and thereby single out and eventually eliminate the weakest materials and components. Several model-based approaches in this direction have been developed [Asset et al., Mezzavilla et al.]. A critical condition for the success of this strategy would be for researchers in academia and industry to agree on harmonized and standardized characterization methods and tools as well as test conditions and protocols. Advances in this direction are underway, for example, in the Fuel Cell Consortium for Performance and Durability (FC-PAD), recently formed in the US, and similar consortium-type initiatives in the EU [FCH-JU Automotive projects: Cathcat, Impact, Catapult, Autostack core, and so on]. A review paper [Mezzavilla et al.] in this Special Issue focuses on the development and application of new cross-correlative techniques to rationalize relations among structure, electrochemical properties, performance, and stability of electrocatalyst materials. Several other articles pick up the topic of fuel cell diagnostics and modelling to unveil these dependences. It has been a particular intention of the Guest Editors to demonstrate the importance of electrocatalysis beyond the oxygen and hydrogen electrode in fuel cell technology. In the realm of fuel cell technology, the reduction or catalytic dismutation of hydrogen peroxide intermediates receives some attention, as reflected in this issue [Ryabova et al.]. This is both for its role as intermediate but also for laboratory experiments with hydrogen peroxide to establish a mechanistic understanding for metal oxide catalysts. The oxygen reduction reaction lies also at the heart of the many problems in bringing the lithium–air battery to fruition [del Pozo et al.]. At a first glance, challenges seem to reflect those related to the oxygen reduction reaction in fuel cells. However, because of the organic solvents, different reaction pathways become important in Li–air batteries. Oxidation reactions of alternative fuels to hydrogen are another unsolved problem. In particular, ethanol oxidation with scission of the carbon–carbon bond would open the door for direct conversion of bioethanol to electricity. Dimethyl ether is another small organic compound suitable as fuel. It is particularly interesting because it may be formed as a product of the electrocatalytic reduction of carbon dioxide [Dumont et al.]. Realizing a complete carbon cycle by electrochemical reactions is another future vision, with some of the steps towards it being documented in this issue [Li et al.]. Often this is connected with the production of solar fuels, a high-impact topic that receives attention in the research community and beyond [Bae et al.]. Electrocatalysis is of key importance in this field because after initial charge separation in a solid-state material, charge carriers must be efficiently converted into liquid fuels to allow for high chemical storage with high energy density. The storage molecules should be formed fast in the device but should not react back spontaneously—a situation that is familiar to electrochemists from kinetically inert high-energy molecules such as oxygen or hydrocarbons. The realization of the concept of solar fuel will require electrocatalysts specifically adapted to the requirements of the solid-state semiconductor onto which they are deposited [Bae et al.]. Electrolytic production of fuels and value-added chemicals is another focus for the community of researchers in electrocatalysis [Gonzáles-Cobos et al.]. The goal is to make use of the temporary surplus of energy produced from renewable energy technologies. Key requirements for fuel production are electrolyzers that tolerate rather rapid load changes, that use abundant materials to be viable for large-scale deployment, and that drastically bring down investment costs. The Special Issue provides examples for research in this direction based on carbon materials, modified noble metal alloy catalysts, and modified metal carbides [An et al., Gusmão et al.]. Other application areas for electrocatalysis are also represented in this Special Issue, featuring research directed towards management of water resources in its widest sense. Important applications comprise removal of problematic ions from drinking water (nitrite, nitrate) [Celebanska and Opallo] as well as new approaches for disinfection based on electrocatalytic formation of hydrogen peroxide [Li et al.]. The Guest Editors hope that the readers will not only appreciate the individual contributions closest to their own field of research, but benefit from the variety of research topics that employ electrocatalysis in its many facets.