1. Setting the Scope of the Challenge 6474 1.1. The Need for Solar Energy Supply and Storage 6474 1.2. An Imperative for Discovery Research 6477 1.3. Scope of Review 6478 2. Large-Scale Centralized Energy Storage 6478 2.1. Pumped Hydroelectric Energy Storage (PHES) 6479 2.2. Compressed Air Energy Storage (CAES) 6480 3. Smaller Scale Grid and Distributed Energy Storage 6481 3.1. Flywheel Energy Storage (FES) 6481 3.2. Superconducting Magnetic Energy Storage 6482 4. Chemical Energy Storage: Electrochemical 6482 4.1. Batteries 6482 4.1.1. Lead-Acid Batteries 6483 4.1.2. Alkaline Batteries 6484 4.1.3. Lithium-Ion Batteries 6484 4.1.4. High-Temperature Sodium Batteries 6484 4.1.5. Liquid Flow Batteries 6485 4.1.6. Metal-Air Batteries 6485 4.2. Capacitors 6485 5. Chemical Energy Storage: Solar Fuels 6486 5.1. Solar Fuels in Nature 6486 5.2. Artificial Photosynthesis and General Considerations of Water Splitting 6486

Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds

With the approaching commercialization of PEM fuel cell technology, developing active, inexpensive non-precious metal ORR catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost. This review paper highlights the progress made over the past 40 years with a detailed discussion of recent works in the area of non-precious metal electrocatalysts for oxygen reduction reaction, a necessary reaction at the PEM fuel cell cathode. Several important kinds of unsupported or carbon supported non-precious metal electrocatalysts for ORR are reviewed, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, conductive polymer-based catalysts, transition metal chalcogenides, metal oxides/carbides/nitrides/oxynitrides/carbonitrides, and enzymatic compounds. Among these candidates, pyrolyzed transition metal nitrogen-containing complexes supported on carbon materials (M–Nx/C) are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalysts. Although great progress has been achieved in this area of research and development, there are still some challenges in both their ORR activity and stability. Regarding the ORR activity, the actual volumetric activity of the most active non-precious metal catalyst is still well below the DOE 2015 target. Regarding the ORR stability, stability tests are generally run at low current densities or low power levels, and the lifetime is far shorter than targets set by DOE. Therefore, improving both the ORR activity and stability are the major short and long term focuses of non-precious metal catalyst research and development. Based on the results achieved in this area, several future research directions are also proposed and discussed in this paper.

A review on non-precious metal electrocatalysts for PEM fuel cells

Advances in electrocatalysis at solid-liquid interfaces are vital for driving the technological innovations that are needed to deliver reliable, affordable and environmentally friendly energy. Here, we highlight the key achievements in the development of new materials for efficient hydrogen and oxygen production in electrolysers and, in reverse, their use in fuel cells. A key issue addressed here is the degree to which the fundamental understanding of the synergy between covalent and non-covalent interactions can form the basis for any predictive ability in tailor-making real-world catalysts. Common descriptors such as the substrate-hydroxide binding energy and the interactions in the double layer between hydroxide-oxides and H---OH are found to control individual parts of the hydrogen and oxygen electrochemistry that govern the efficiency of water-based energy conversion and storage systems. Links between aqueous- and organic-based environments are also established, encouraging the 'fuel cell' and 'battery' communities to move forward together.

Energy and fuels from electrochemical interfaces

With the development of renewable energy and electrified transportation, electrochemical energy storage will be more important in the future than it has ever been in the past. Although lithium-ion batteries (LIBs) are traditionally considered to be the most likeliest candidate thanks to their relatively long cycle life and high energy efficiency, their limited energy density as well as cost are still causing a bottleneck for their long-term application. Alternatively, metal–air batteries have been proposed as a very promising large-scale electricity storage technology with the replacement of the intercalation reaction mechanism by the catalytic redox reaction of a light weight metal–oxygen couple. Generally, based on the electrolyte, these metal–air batteries can be divided into aqueous and nonaqueous systems, corresponding to two typical batteries of Zn–air and Li–air, respectively. The prominent feature of both batteries are their extremely high theoretical energy density, especially for nonaqueous Li–air batteries, which far exceeds the best that can be achieved with LIBs. In this review, we focus on the major obstacle of sluggish kinetics of the cathode in both batteries, and summarize the fundamentals and recent advances related to the oxygen catalyst materials. According to the electrolyte, the aqueous and nonaqueous electrocatalytic mechanisms of the oxygen reduction and evolution reactions are discussed. Subsequently, seven groups of oxygen catalysts, which have played catalytic roles in both systems, are selectively reviewed, including transition metal oxides (single-metal oxides and mixed-metal oxides), functional carbon materials (nanostructured carbons and doped carbons), metal oxide–nanocarbon hybrid materials, metal–nitrogen complexes (non-pyrolyzed and pyrolyzed), transition metal nitrides, conductive polymers, and precious metals (alloys). Nonaqueous systems have the advantages of energy density and rechargeability over aqueous systems and have gradually become the research focus of metal–air batteries. However, there are considerable challenges beyond catalysts from aqueous to nonaqueous electrolytes, which are also discussed in this review. Finally, several future research directions are proposed based on the results achieved in this field, with emphasis on nonaqueous Li–air batteries.

Oxygen electrocatalysts in metal–air batteries: from aqueous to nonaqueous electrolytes

This article reviews recent significant advances in the field of water splitting. Catalysts play very important roles in two half reactions of water splitting - water reduction and water oxidation. Considering potential future applications, catalysts made of cheap and earth abundant element(s) are especially important for economically viable energy conversion. This article focuses only on catalysts made of cobalt (Co), nickel (Ni) and iron (Fe) elements for water reduction and water oxidation. Different series of catalysts that can be applied in electrocatalytic and photocatalytic water spitting are discussed in detail and their catalytic mechanisms are introduced. Finally, the future outlook and perspective of catalysts made of earth abundant elements will be discussed.

Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: Recent progress and future challenges

Sol-gel derived spinel MxCo3−xO4 (M=Ni, Cu; 0≤x≤1) films and oxygen evolution ☆

Oxygen reduction on oxide/polypyrrole composite electrodes: effect of doping anions

Studies on semiconducting thin films prepared by the spray pyrolysis technique for photoelectrochemical solar cell applications: Preparation and properties of ZnO

Oxygen reduction on NixCo3−xO4 spinel particles/polypyrrole composite electrodes: hydrogen peroxide formation

Composite electrodes based on mixed valence oxide nanoparticles (Ox) embedded in an electrically conductive polymer (ECP) were investigated toward the oxygen reduction reaction (ORR). ECP was polypyrrole (PPy), and Ox was the spinel mixed valence oxide Cu 1.4 Mn 1.6 O 4 , which is known to be an electrocatalyst of the ORR in neutral and alkaline media. The main findings were that in such a composite electrode (i) the spinel lattice and the electrocatalytic activity of the oxide remained remarkably stable in acidic media where normally this oxide would be electrochemically reduced, and (ii) PPy retained its electrical conductivity at cathodic potentials where it would be normally in its insulating reduced state. Those composite electrodes sustained the ORR for hours without signs of deterioration.

P. Chartier

Papers

Sol-gel derived spinel MxCo3−xO4 (M=Ni, Cu; 0≤x≤1) films and oxygen evolution ☆

Oxygen reduction on oxide/polypyrrole composite electrodes: effect of doping anions

Studies on semiconducting thin films prepared by the spray pyrolysis technique for photoelectrochemical solar cell applications: Preparation and properties of ZnO

Oxygen reduction on NixCo3−xO4 spinel particles/polypyrrole composite electrodes: hydrogen peroxide formation

Electrocatalysis of Oxygen Reduction on Polypyrrole/Mixed Valence Spinel Oxide Nanoparticles