Non‐Precious‐Metal Oxygen Reduction Reaction Electrocatalysis

Abstract

The story of non-precious-metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) started more than 50 years ago, when Jasinski employed cobalt phthalocyanine in ORR electrocatalysis.1 For several decades, it was of substantial interest to study the effect of the central metal ions and ligands of metal macrocyclic compounds on the ORR electrocatalytic activity as alternative system to costly and rare Pt catalysts. In this regard, MN4 macrocyclic complexes, in particular metal phthalocyanines and metal porphyrins, have been explored as non-precious model systems, and large sets of ORR data have been collected. Zagal and Koper proposed activity descriptors, which relate the ORR activity to the oxygen binding energy, yielding volcano-type correlations.2 However, it was soon found that non-pyrolyzed MN4 macrocyclic compounds were not stable for long-term operation, rendering them unsuitable for fuel cells. At the end of 1970s, it was reported by Bagotzky et al. that annealing at high temperatures improves the ORR activity as well as the durability of MN4-macrocycle-derived catalysts.3 There has long been a debate, which still continues, about the chemical nature of active centers formed upon annealing. A strong impetus to the study of NPMC materials was made by Yeager and co-workers, who replaced MN4 macrocyclic complexes with simple nitrogen-containing compounds and transition-metal salts, producing rather active ORR catalysts through pyrolysis.4 Throughout the last decade, an enormous amount of research has been dedicated toward the development of NPMC materials for ORR, which can be used in low-temperature fuel cells and metal–air batteries. Special attention has been devoted to transition metal–nitrogen–carbon (M-N-C) catalysts, which are the flagship of all NPMC materials. Around 50 nitrogen-containing compounds (both low molecular weight and polymeric) and different heat-treatment protocols have been exploited to synthesize the best high-performance ORR catalysts. Large discussions about the ORR active sites of M-N-C type catalysts are still ongoing. Thorough physicochemical characterization of these materials is possible by using various spectroscopic and microscopic techniques in combination with electrochemical methods to elucidate the nature of the active centers. Unfortunately, most of the surface analysis results have been obtained in ultrahigh vacuum, and in situ characterization in electrochemical conditions has been less frequently employed. However, it is of utmost importance to further elucidate the origin of electrocatalysis with this type of material. Metal-free heteroatom-doped (in particular, nitrogen-doped) carbon nanomaterials offer a variation on typical NPMCs for the ORR. A remarkable electrocatalytic activity of heteroatom-doped nanocarbons has been reported in alkaline media, whereas these materials show considerably lower activity in acidic conditions. Again, a better understanding of ORR electrocatalysis with this type of material would enable us to design more active catalysts. An important issue is the influence of trace transition metals on the electrocatalytic properties of heteroatom-doped carbons. It appears that many researchers have claimed the electrocatalytic activity is caused by residual transition metals in the catalyst materials. This Special Issue in ChemElectroChem is devoted to the latest findings in NPMCs for ORR electrocatalysis. There are more than 30 original research pieces and four review-type articles presented in this Special Issue, and we are very thankful to all contributors. All of the main types of NPMC materials are dealt with in these articles and reviews, thereby giving a good overview in this area of research. The application of NPMC materials as cathode catalysts in proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs) has been reported and, in several studies, these catalysts have been used in Zn–air batteries, showing a great promise of such non-Pt ORR catalysts in electrochemical energy applications. There are still many challenges remaining in the development of novel NPMC materials for the ORR in low- and high-temperature fuel cells, including automotive applications. In a futuristic view of energy production, carbon-based energy systems should decrease and hydrogen-based energy should play a greater role in our society. This is mostly related to environmental concerns, and the production of energy from renewable resources is gradually increasing. It is well known that wind and solar energy are produced in an irregular manner and, at peak times, this energy can be used for the generation of hydrogen through water electrolysis. H2 can then be used to produce electrical energy in a fuel cell as and when it is needed. This means that the whole energy cycle could be free of pollutants and, therefore, fuel cells will be significant in contributing to future energy and environment targets. In this regard, this Special Issue of ChemElectroChem is timely and proper as an important research field in non-Pt fuel cell development. Slow ORR is the bottleneck for fuel cell technologies and highly active and stable NPMCs are urgently needed. Kaido Tammeveski was born in Tartu (Estonia) in 1964. In 1989, he graduated from the University of Tartu (Estonia) as a Chemist. He was awarded a PhD degree in 1998 at the same university. In 1999/2000, he was a Postdoctoral Fellow at the University of Liverpool (UK). He has been Associate Professor at the Institute of Chemistry of the University of Tartu since 2001. His main research interest is oxygen reduction electrocatalysis and he has also been involved in fuel cell research. In 2014, he received the National Science Award. Jong-Sung Yu earned a B.Sc. in Chemistry from Sogang University in Seoul (Republic of Korea) and a Ph.D. from the University of Houston (USA) in 1990 before postdoctoral work at Ohio State University (USA). He also worked as a visiting scientist in Pennsylvania State University (USA) in 1998–1999 and in Northwestern University (USA) in 2004–2005. He was a Professor in Korea University during 2008–2015 before he moved to Daegu Gyeongbuk Institute of Science and Technology (DGIST, Republic of Korea). Currently, he is a supervisor for graduate students and postdocs of the Light, Salts and Water Research Lab in the Energy Science and Engineering Department of DGIST, where his research focuses on nanostructured materials, including nanoscale 0–3D materials and their composites, and their energy applications to fuel cells, batteries, supercapacitors, sensors, and photo-/electrocatalytic systems. Zhongwei Chen is Professor and Canada Research Chair in Advanced Materials for Clean Energy at the University of Waterloo (Canada), Fellow of the Canadian Academy of Engineering, and Vice President of International Academy of Electrochemical Energy Science (IAOEES). His research interests are in the development of advanced energy materials and electrodes for fuel cells, metal–air batteries, and lithium-ion batteries. He was the recipient of the 2016 E.W.R Steacie Memorial Fellowship and elected as the member of the Royal Society of Canada's College of New Scholars, Artists and Scientists in 2016, and was also elected as the fellow of the Canadian Academy of Engineering in 2017, the Rutherford memorial medal from The Royal Society of Canada in 2017, which followed shortly upon several other prestigious honors, including the Ontario Early Researcher Award, an NSERC Discovery Supplement Award, the Distinguished Performance and Research.