To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.

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Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte

Carbon dioxide (CO 2 ) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO 2 to ethylene with 70% faradaic efficiency at a potential of −0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO 2 reduction and carbon monoxide (CO)–CO coupling activation energy barriers; as a result, onset of ethylene evolution at −0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.

http://science.sciencemag.org/content/sci/360/6390/783.full.pdf

CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface

Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

/pdf/a-review-on-fundamentals-for-designing-oxygen-evolution-3mw3rcvtbe.pdf

A review on fundamentals for designing oxygen evolution electrocatalysts

Non‐Noble‐Metal‐Based Electrocatalysts toward the Oxygen Evolution Reaction

Zn-air batteries are becoming the promising power sources for portable and wearable electronic devices and hybrid/electric vehicles because of their high specific energy density and the low cost for next-generation green and sustainable energy technologies. An air electrode integrated with an oxygen electrocatalyst is the most important component and inevitably determines the performance and cost of a Zn-air battery. This article presents exciting advances and challenges related to air electrodes and their relatives. After a brief introduction of the Zn-air battery, the architectures and oxygen electrocatalysts of air electrodes and relevant electrolytes are highlighted in primary and rechargeable types with different configurations, respectively. Moreover, the individual components and major issues of flexible Zn-air batteries are also highlighted, along with the strategies to enhance the battery performance. Finally, a perspective for design, preparation, and assembly of air electrodes is proposed for the future innovations of Zn-air batteries with high performance.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700691

Advanced Architectures and Relatives of Air Electrodes in Zn-Air Batteries.

The oxygen evolution reaction (OER) is a key process in electrochemical energy conversion devices. Understanding the origins of the lattice oxygen oxidation mechanism is crucial because OER catalysts operating via this mechanism could bypass certain limitations associated with those operating by the conventional adsorbate evolution mechanism. Transition metal oxyhydroxides are often considered to be the real catalytic species in a variety of OER catalysts and their low-dimensional layered structures readily allow direct formation of the O–O bond. Here, we incorporate catalytically inactive Zn2+ into CoOOH and suggest that the OER mechanism is dependent on the amount of Zn2+ in the catalyst. The inclusion of the Zn2+ ions gives rise to oxygen non-bonding states with different local configurations that depend on the quantity of Zn2+. We propose that the OER proceeds via the lattice oxygen oxidation mechanism pathway on the metal oxyhydroxides only if two neighbouring oxidized oxygens can hybridize their oxygen holes without sacrificing metal–oxygen hybridization significantly, finding that Zn0.2Co0.8OOH has the optimum activity. Oxygen evolution is one half of the overall water splitting reaction to produce hydrogen. Although this reaction is well studied, there remains debate over the particulars of the catalytic mechanism. Here, the authors investigate Co–Zn oxyhydroxide electrocatalysts, and suggest that the mechanism depends on the amount of Zn2+ they contain.

Chemical and structural origin of lattice oxygen oxidation in Co–Zn oxyhydroxide oxygen evolution electrocatalysts

Cost-effective and efficient oxygen-evolving electrocatalysts are urgently required for energy storage and conversion technologies. In this work, an amorphous trimetallic boride nanocatalyst (Fe–Co–2.3Ni–B) prepared by a simple approach is reported as a highly efficient oxygen evolution reaction electrocatalyst. It exhibits an overpotential (η) of 274 mV to deliver a geometric current density (jgeo) of 10 mA cm−2, a small Tafel slope of 38 mV dec−1, and excellent long-term durability at a mass loading of 0.3 mg cm−2. The impressive electrocatalytic performance originates from the unique amorphous multimetal–metalloid complex nanostructure. From application's point of view, this work holds great promise as this process is simple and allows for large-scale production of cheap, yet efficient, material.

/pdf/an-efficient-and-earth-abundant-oxygen-evolving-gfx8jypfya.pdf

An Efficient and Earth-Abundant Oxygen-Evolving Electrocatalyst Based on Amorphous Metal Borides

Highly Efficient Oxygen Reduction Reaction Activity of N‐Doped Carbon–Cobalt Boride Heterointerfaces

V. Jose Energy Research Institute @ NTU ERI@N Interdisciplinary Graduate School Nanyang Technological University 50 Nanyang Drive, Singapore 637553, Singapore V. Jose, Dr. J. M. V. Nsanzimana, Prof. J.-M. Lee School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459, Singapore E-mail: jmlee@ntu.edu.sg H. Hu, Prof. J. Choi College of Energy, Soochow Institute for Energy and Materials Innovations, and Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province Soochow University Suzhou 215006, China E-mail: jhchoi@suda.edu.cn Dr. E. Edison, Dr. W. Manalastas Jr., Dr. H. Ren, Prof. M. Srinivasan School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Ave, Singapore 639798, Singapore Dr. P. Kidkhunthod Synchrotron Light Research Institute Nakhon Ratchasima 30000, Thailand Dr. S. Sreejith National University of Singapore 14 Medical Drive, Singapore 117599, Singapore Dr. A. Jayakumar School of GeoSciences Biochar Research Centre University of Edinburgh Edinburgh EH9 3FF, UK

Jean Marie Vianney Nsanzimana

Papers

Chemical and structural origin of lattice oxygen oxidation in Co–Zn oxyhydroxide oxygen evolution electrocatalysts

An Efficient and Earth-Abundant Oxygen-Evolving Electrocatalyst Based on Amorphous Metal Borides

Highly Efficient Oxygen Reduction Reaction Activity of N‐Doped Carbon–Cobalt Boride Heterointerfaces

Modulation of Single Atomic Co and Fe Sites on Hollow Carbon Nanospheres as Oxygen Electrodes for Rechargeable Zn–Air Batteries

Tailoring of Metal Boride Morphology via Anion for Efficient Water Oxidation