Oxygen-oxygen bonds : catalytic redox pathways in energy storage

Abstract

Introduction: The present understanding of energy - its many forms, and its governing role in the time evolution of physical systems - underlies many of the most fundamental and unifying principles furnished by scientific theories. We are now deeply aware that energy is inherently quantized and is associated with stationary states (from quantum theory), that it is conserved (from the first law of thermodynamics), but that its conversion is asymmetric and not invariant to time reversal (from the second law of thermodynamics). The transaction of energy from one system to another system; from one form to another form, is deeply embedded in our interpretation of Nature, and refining what precisely energy is tells a large portion of the story that is Science. In contrast, energy technologies allow mankind to harvest (by converting the form of) natural "banks" of energy - primarily the chemical bonding energy of organic molecules in reduced states - in order to perform tasks to our liking: there are many such tasks, but a significant portion of them involves the transformation into electricity. It is certain that one of the most significant challenges facing developed society in the 21st century will be devising how to derive energy in the form of electricity in large quantities from sources other than fossil fuels. Because fossil fuels have been used so monolithically, new methods of providing useful energy without them have gone undeveloped until only the last few decades. A large number of efforts in scientific research from a panoply of disciplines today are motivated by this challenge. There are many ways of addressing such a far-reaching problem, but one attractive and thoughtful strategy to probe it is to consider by what means all the chemical energy that has been harvested from fuels since the Industrial Revolution was "made" in the first place. Essentially all terrestrial banks of energy are derived originally from solar radiation, and the development of the photosynthetic process by cyanobacteria some 3.5 billion years ago,1 which (in general terms) couples light energy to drive endothermic electron transfer reactions, is perhaps the most significant development in geological history. However, how this solar energy is actually used and stored in nature is intimately tied to the chemical bond between two oxygen atoms. The common theme that ties my research projects in the Nocera group in the past four years is an interest in systems that transfer energy through the making and breaking of oxygen-oxygen bonds. 0-0 bonds are some of the weakest covalent bonds in nature, with bond dissociation enthalpy (BDE) values near 33 kcal mol-1 in H202 . By simple thermodynamic arguments, the formation of these bonds from energy-rich O-H and C=O bonds is uphill, and the consumption of these bonds in reactions that oxidize other reagents are exothermic, and if not carefully controlled, explosive. Indeed Nature decided to employ the free energy liberated by the reduction of molecular 02 to obtain the vast majority of energy available to aerobic heterotrophic organisms via respiration. Presently, the highly active area of fuel cell technology is tied intrinsically to a question that cells had to answer long ago in evolutionary history: how does one maximize an output potential (i.e., minimize the overpotential) generated by the reduction of 02 with H2 as reductant 2 Nature's own fuel cell, cytochrome c oxidase (CcO), evolved to drive the complete reduction of 02 by an "H2 equivalent" in the form of NADH (the reduced form of nicotinamide adenine dinucleotide), which is exothermic by 1.229 electron volts (eV) per elementary charge transferred as shown on the left side of Scheme 1. An incomplete reduction of 02 leads to the formation of hydrogen peroxide (H2O2) ...