An initial review (PCET1) on proton-coupled electron transfer (PCET) by Huynh and Meyer appeared in Chemical Reviews in 2007. 1 This is a perennial review, a follow up on the original. It was intended for the special Chemical Reviews edition on Proton Coupled Electron Transfer that appeared in December, 2010 (Volume 110, Issue 12 Pages 6937-710). The reader is referred to it with articles on electrochemical approaches to studying PCET by Costentin and coworkers, 2 theory of electron proton transfer reactions by Hammes-Schiffer and coworkers, 3 proton-coupled electron flow in proteins and enzymes by Gray and coworkers, 4 and the thermochemistry of proton-coupled electron transfer by Mayer and coworkers. 5 Coverage for the current review is intended to be broad, covering all aspects of the topic comprehensively with literature coverage overlapping with the later references in PCET1 until late 2010. There is a growing understanding of the importance of PCET in chemistry and biology and its implications for catalysis and energy conversion. This has led to a series of informative reviews that have appeared since 2007. They include: "The possible role of Proton-coupled electron Transfer (PCET) in Water oxidation by Photosystem II" by Meyer and coworkers in 2007, 6 "Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics" by Hammes-Schiffer and coworkers in 2008, 7 "Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer" by Costentin in 2008, 8 "Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems" by Nocera and Reece in 2009, 9 and "Integrating Proton-Coupled Electron Transfer and Excited States" by Meyer and coworkers in 2010. 10

Photovoltaic cells have considerable potential to satisfy future renewable-energy needs, but efficient and scalable methods of storing the intermittent electricity they produce are required for the large-scale implementation of solar energy. Current solar-to-fuels storage cycles based on water splitting produce hydrogen and oxygen, which are attractive fuels in principle but confront practical limitations from the current energy infrastructure that is based on liquid fuels. In this work, we report the development of a scalable, integrated bioelectrochemical system in which the bacterium Ralstonia eutropha is used to efficiently convert CO2, along with H2 and O2 produced from water splitting, into biomass and fusel alcohols. Water-splitting catalysis was performed using catalysts that are made of earth-abundant metals and enable low overpotential water splitting. In this integrated setup, equivalent solar-to-biomass yields of up to 3.2% of the thermodynamic maximum exceed that of most terrestrial plants. Moreover, engineering of R. eutropha enabled production of the fusel alcohol isopropanol at up to 216 mg/L, the highest bioelectrochemical fuel yield yet reported by >300%. This work demonstrates that catalysts of biotic and abiotic origin can be interfaced to achieve challenging chemical energy-to-fuels transformations.

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Water oxidation is a key half reaction in energy conversion schemes based on solar fuels and targets such as light driven water splitting or carbon dioxide reduction into CO, other oxygenates, or hydrocarbons. Carrying out these reactions at rates that exceed the rate of solar insolation for the extended periods of time required for useful applications presents a major challenge. Water oxidation is the key ''other'' half reaction in these schemes and it is dominated by PCET given its multi-electron, multi-proton character, 2H 2 O / O 2 + 4e À + 4H + . Identification of PCET was an offshoot of experiments designed to investigate energy conversion by electron transfer quenching of molecular excited states. The concepts ''redox potential leveling'' and concerted electron-proton transfer came from measurements on stepwise oxidation of cis-Ru II (bpy) 2 (py)(OH 2 ) 2+ to Ru IV (bpy) 2 (py)(O) 2+ . The Ru ''blue dimer'', cis,cis-, was the first designed catalyst for water oxidation. It undergoes oxidative activation by PCET to give the transient (bpy, O-atom attack on water to give a peroxidic intermediate, and further oxidation and O 2 release. More recently, a class of single site water oxidation catalysts has been identified, e.g., Ru(tpy)(bpm)(OH 2 ) 2+ (tpy is 2,2 0 :6 0 ,2 00terpyridine; bpm is 2,2 0 -bipyrimidine). They undergo stepwise PCET oxidation to Ru IV ¼O 2+ or Ru V (O) 3+ followed by O-atom transfer with formation of peroxidic intermediates which undergo further oxidation and O 2 release. PCET plays a key role in the three zones of water oxidation reactivity: oxidative activation, O/O bond formation, oxidation and O 2 release from peroxidic intermediates. Similar schemes have been identified for electrocatalytic water oxidation on oxide electrode surfaces based on phosphonated derivatives such as [Ru(Mebimpy)(4,4 0 -(PO 3 H 2 CH 2 ) 2 bpy)(OH 2 )] 2+ . A PCET barrier to Ru III -OH 2+ / Ru IV ¼O 2+ oxidation arises from the large difference in pK a values between Ru III -OH 2+ and Ru IV (OH) 3+ . On oxide surfaces this oxidation occurs by multiple pathways. Kinetic, mechanistic, and DFT results on single site catalysts reveal a new pathway for the O/O bond forming step (Atom-Proton Transfer, APT), significant rate enhancements by added proton acceptor bases, and accelerated water oxidation in propylene carbonate as solvent with water added as a stoichiometric reagent. Lessons learned about water oxidation and the role of PCET and concerted pathways appear to have direct relevance for water oxidation in Photosystem II (PSII) with PSII a spectacular example of PCET in action. This includes a key role for Multiple Site-Electron Proton Transfer in oxidative activation of the Oxygen Evolving Complex (OEC) in the S 0 / S 1 transition in the Kok cycle.

Benzoquinone/hydroquinone redox interconversion by the reversible Os(dmb)(3)(3+/2+) couple over an extended pH range with added acids and bases has revealed the existence of seven discrete pathways. Application of spectrophotometric monitoring with stopped-flow mixing has been used to explore the role of PCET. The results have revealed a role for phosphoric acid and acetate as proton donor and acceptor in the concerted electron-proton transfer reduction of benzoquinone and oxidation of hydroquinone, respectively.