Advancing Materials Electrochemistry for Chemical Transformation

Chemical transformations provide energy, fuel, and chemicals for today's society, which shape human beings' daily life, prosperity, sustainability. Electrochemistry and photoelectrochemistry offer the possibility of driving chemical transformations under ambient temperature and ambient pressure in a more ecofriendly way. More importantly, the interconversion of chemical and electrical energy by electrochemical and photoelectrochemical means (e.g., electrocatalysis or photoelectrocatalysis), to use the electron potential for controlling the directions and rates of chemical processes, provides a flexible and scalable solution to store energy in chemical bonds and retrieve this energy wherever and whenever needed. With the increasing penetration of renewable energy, electrification of transportation, and environmental regulations into human society, electrochemistry- and photoelectrochemistry-enabled chemical transformation are becoming more important than ever for clean power and energy storage, requiring not only new concepts, materials, devices, and technologies, but also highly trained scientists and engineers to realize them. In the past few years, there has been significantly increased effort toward the research and development of electrochemistry- and photoelectrochemistry-enabled chemical transformation for energy-related applications. However, there are still several grand challenges from the materials perspective to achieve the technological development goals and, more broadly, to meet the requirements of human society. First, for the technologies to meet the needs of human society at the global scale, the materials used in the technologies have to be either earth-abundant or be made extremely efficient and durable if rare materials are used; this asks for new theory and new experimental tools for materials synthesis and characterization. Second, in-depth understanding of materials degradation, materials compatibility, and materials interface/interphase is necessary to provide a solid scientific foundation beyond today's knowledge for new technologies to develop. Third, material production and materials processing have to be scalable and sustainable. This special issue is organized to critically review recent progress and remaining challenges in this field. We include (photo)electrochemical molecular transformations such as the so-called three cycles, i.e., the water cycle (2H2 + O2 ↔ 2H2O), the carbon cycle (CxHyOz + O2 ↔ CO2 + H2O), and the nitrogen cycle (N2 + 3H2 ↔ 2NH3), and their application in relevant technologies like fuel cells, water splitting, solar fuels, CO2 utilization, and ammonia synthesis; we also include metal–air batteries, which could provide a transformational energy-storage solution for future mobile applications. We focus on emerging materials, concepts, and devices in electrochemistry- and photoelectrochemistry-enabling chemical transformations. The (photo)electrochemical transformation between 2H2 + O2 ↔ 2H2O is the most widely studied. We understand that reliable and accurate measurements of materials' structures and properties are the foundation for material research and development. Yang Shao-Horn, Zhichuan Xu, and co-workers review recommended practices and benchmark activity for hydrogen and oxygen electrocatalysis in water splitting and fuel cells (article number 1806296). Following this, we present papers on oxygen reduction reaction (ORR) electrocatalysts based on platinum group metals (PGM)-free materials, including Fe-based catalysts (Piotr Zelenay and co-workers (article number 1806545)), Co-based catalysts (Gang Wu, Yuyan Shao, and co-workers (article number 1805126)), and metal-free catalysts (Liming Dai, Lin Zhu, Hao Chen, and co-workers (article number 1806403)). We include key experimental tools (e.g., X-ray absorption spectroscopy, 57Fe Mössbauer spectroscopy) for the study of PGM-free catalysts (Sanjeev Mukerjee and co-workers (article number 1805157) and Ulrike Kramm and co-workers (article number 1805623)). The stability challenges and the integration of PGM-free catalysts in fuel cells are reviewed by Yuyan Shao and co-workers (article number 1807615) and Dustin Banham, Siyu Ye, and co-workers (article number 1804846). Recent progress on precious-metal ORR catalysts is also reviewed by Peter Strasser and co-workers (article number 1805617), Alexander Eychmüller and Bin Cai (article number 1804881), and Yu Huang, Yujing Li, and co-workers (article number 1808115)). Regarding the topic of water splitting, we present papers related to electrochemical water splitting (Xinliang Feng and co-workers (article number 1808167)) and photoelectrochemical water splitting (Alexandra Boltasseva, Radek Zbořil, Alberto Naldoni, and co-workers (article number 1805513)), and include hydrogen electrocatalysis (Wenchao Sheng and co-workers (article number 1808066), and Dai, Zhu, Chen, and co-workers, (article number 1806403)), and hydroxide exchange membrane electrolyzers (Yushan Yan and co-workers (article number 1805876)). Topics related to carbon cycles are presented, such as electrochemical CO2 reduction (Edward Sargent and co-workers (article number 1807166); Strasser and co-workers (article number 1805617); and Dai, Zhu, Chen, and co-workers (article number 1806403)), photoelectrochemical CO2 reduction (Jinlong Gong and co-workers (article number 1804710)), and alcohol oxidation (Eychmüller and Cai (article number 1804881)). Feng Jiao and Bingjun Xu review the nitrogen cycle, including electrochemical ammonia synthesis and ammonia fuel cells (article number 1805173). Recent progress on metal–air batteries, i.e., Zn–air batteries (Zhongwei Chen, Zhenyu Bai, and co-workers (article number 1805230)) and Li–air batteries (Jun Lu, Khalil Amine, and co-workers (article number 1805602)), is also reviewed. Through closing this special issue with two papers on energy storage, we expect that (photo)electrochemical chemical transformation, including chemical-transformation-enabled batteries, will play a more and more critical role in future sustainable transportation, renewable energy deployment, and reliable power grid. Readers will notice that a few papers cover more than one topic; we hope they can be used for cross-referencing and to well integrate this special issue. We understand that one special issue could not cover all of the important research directions in this rapidly growing and expanding field of materials electrochemistry for chemical transformation. We apologize for some of our invited papers did not arrive before the publication of this special issue because of limited time. We want to thank the editorial team at Advanced Materials for their tremendous support, and all the authors, including those who cannot submit in their papers on time, for their significant efforts to make this special issue successful. We do hope this special issue will bring knowledge, insight, and inspiration to readers.