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

Turbulent combustion modeling

Hydrogen from renewable electricity: An international review of power-to-gas pilot plants for stationary applications

The number of research reports published in recent years on electrochemical water splitting for hydrogen generation is higher than for many other fields of energy research. In fact, electrochemical water splitting, which is conventionally known as water electrolysis, has the potential to meet primary energy requirements in the near future when coal and hydrocarbons are completely consumed. Due to the sudden and exponentially increasing attention on this field, many researchers across the world, including our group, have been exerting immense efforts to improve the electrocatalytic properties of the materials that catalyze the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode, aided by the recent revolutionary discovery of nanomaterials. However, the pressure on the researchers to publish their findings rapidly has caused them to make many unnoticed and unintentional errors, which is mainly due to lack of clear insight on the activity parameters. In this perspective, we have discussed the use and validity of ten important parameters, namely overpotential at a defined current density, iR-corrected overpotential at a defined current density, Tafel slope, exchange current density (j0), mass activity, specific activity, faradaic efficiency (FE), turnover frequency (TOF), electrochemically active surface area (ECSA) and measurement of double layer capacitance (Cdl) for different electrocatalytic materials that are frequently employed in both OER and HER. Experimental results have also been provided in support of our discussions wherever required. Using our critical assessments of the activity parameters of water splitting electrocatalysis, researchers can ensure precision and correctness when presenting their data regarding the activity of an electrocatalyst.

Precision and correctness in the evaluation of electrocatalytic water splitting: revisiting activity parameters with a critical assessment

The concept of a fuel cell dates back to 1839, from independent studies by Grove and Schoenbein. Like a battery, a fuel cell is a device for obtaining electrical energy directly from a chemical reaction, but unlike a battery, electrical power is sustained as long as the reacting chemicals are supplied to each electrode with the cathode receiving oxidant and the anode receiving reductant or “fuel”, hence “fuel cell”. There are environmental advantages over combustion because fuel cells avoid the high temperatures that cause NOx production, and they are usually reported as operating at a higher efficiency (typically 50-60%) than internal combustion engines (20-25%). Applications of fuel cells were largely neglected until the “space age” (1960s) when there became a need for reliable electrical power in challenging, niche situations. In addition to environmental driving forces, energy demands for niche applications continue to drive fuel cell development today. Fuel cells vary greatly in their power output, ranging from large-scale (kW) building-integrated systems, known as “combined heat and power” systems, to those that provide just enough power to operate electronics in special circumstances, such as an implantable device for sensing and controlling glucose levels in the body. As we outline below, the power output of a fuel cell can be limited by the electrochemical reactions occurring at either of the two electrodes, the anode for oxidizing fuel and the cathode for reducing oxidant, and so the electrodes are usually coated with electrocatalysts. An enzyme fuel cell uses an enzyme as the electrocatalyst, either at both cathode and anode, or at just one of the electrodes. The catalytic properties of redox enzymes offer some interesting advantages in fuel cell applications, although examples of devices exploiting enzyme electrocatalysis are almost exclusively at a “proof of concept” stage. Conventional low-temperature fuel cells are generally limited to H2 or primary alcohols as fuels; however, the use of an enzyme as the anode electrocatalyst means that any substance that is oxidized by an organism can become a useful fuel because it is obvious that enzymes for carrying out that specific task must exist. Not only are enzymes capable of very high activity (on a per mole basis), but they are usually highly selective for their substrates. This simplifies the design of a fuel cell because fuel and oxidant need not be separated by an ionically conducting membrane, and they can be introduced as a mixture, that is, mixed reactant fuel cells are possible. This also makes it possible to miniaturize the fuel cell to an extremely small scale. Enzymes are also renewable, meaning that their components are fully recycled using sunlight as an energy source. The main disadvantages of enzymes as electrocatalysts are as follows. First, they are usually very large molecules, so that although the active sites may be extremely active in comparison to the catalytic site of a conventional metal electrode, the catalytic (volume) density is low, and hence multilayers of enzyme are likely to be needed to provide sufficient current. Second, the catalytically active sites are usually buried, so that fast electron transfer to or from the electrode requires either use of an intrinsic electron relay system in the protein (such as a series of FeS clusters) or an extrinsic mediator that can penetrate sufficiently close to the active site. Third, enzymes are often unstable outside ambient / To whom correspondence should be addressed. E-mail: fraser.armstrong@ chem.ox.uk. Chem. Rev. 2008, 108, 2439–2461 2439

Andrew E. Lutz

Papers

The hydrogen-fueled internal combustion engine : a technical review.

Exergy analysis of hydrogen production via steam methane reforming

Thermodynamic analysis of hydrogen production by steam reforming

Thermodynamic analysis of hydrogen production by partial oxidation reforming

Thermodynamic comparison of fuel cells to the Carnot cycle