The energy-conversion efficiency is a key metric that facilitates comparison of the performance of various approaches to solar-energy conversion. However, a suite of disparate methodologies has been proposed and used historically to evaluate the efficiency of systems that produce fuels, either directly or indirectly, with sunlight and/or electrical power as the system inputs. A general expression for the system efficiency is given as the ratio of the total output power (electrical plus chemical) divided by the total input power (electrical plus solar). The solar-to-hydrogen (STH) efficiency follows from this globally applicable system efficiency but only is applicable in the special case for systems in which the only input power is sunlight and the only output power is in the form of hydrogen fuel derived from solar-driven water splitting. Herein, system-level efficiencies, beyond the STH efficiency, as well as component-level figures-of-merit, are defined and discussed to describe the relative energy-conversion performance of key photoactive components of complete systems. These figures-of-merit facilitate the comparison of electrode materials and interfaces without conflating their fundamental properties with the engineering of the cell setup. The resulting information about the components can then be used in conjunction with a graphical circuit analysis formalism to obtain "optimal" system efficiencies that can be compared between various approaches, when the component of concern is used in a reference fuel-producing energy-conversion system. The approach provides a consistent method for comparison of the performance at the system and component levels of various technologies that produce fuels and/or electricity from sunlight.As the fields of photoelectrochemical (PEC) energy conversion and solar fuels have grown, a number of metrics have been adopted for evaluating the performance of electrodes and systems. These metrics are often contradictory, irreproducible, or not properly standardized, which prevents researchers from accurately comparing the performance of materials, even within the PEC community itself. We explore herein these different metrics to evaluate their strengths and applicability, as well as to demonstrate the knowledge derived from each approach. We also present a framework for reporting these metrics in an unambiguous and reproducible manner. Additionally, we outline a method to estimate two-electrode system efficiencies from three-electrode potentiostatic measurements, to accelerate the identification of promising system components without requiring the actual construction of a full system. Clarifying these issues will benefit the PEC community by facilitating the consistent reporting of electrode performance metrics, and will allow photoelectrodes and solar fuels systems to be appropriately compared in performance to other solar energy-conversion technologies. Table of contents graphic textWe outline the significance and advantages of different metrics used to characterize photoelectrodes ...

Tandem junction (n-p + -Si/ITO/WO 3 /liquid) core-shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H 2 , respectively, due to the non-optimal photovoltage and band-gap of the WO 3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes. Broader contextDirect photoelectrochemical conversion of sunlight into a storable, energy-dense fuel has the opportunity to provide a predictable, carbon-neutral energy source to displace current carbon-based technologies. Solar hydrogen generation via water splitting is an important goal because the voltage requirements for this process are well matched to the maximum power point of high-efficiency tandem photovoltaics. In addition to including light-absorbing materials that provide sufficient voltage for water splitting, an integrated solar fuels device requires catalysts connected to the light absorbers and an ionic transport pathway between the anode and cathode to complete the circuit while maintaining product separation below the lower explosive limits, for safety purposes. Integration of these different active materials is important to further development of this technology. Single-crystalline Si microwire arrays represent an architecture that can allow the system operation and integration requirements to be met, because photoactive Si microwires have been previously embedded into ionically conductive, gasblocking membranes. However, Si microwires do not produce enough photovoltage for unassisted water splitting even in a tandem Si-based structure. We describe a Si microwire based tandem junction device that produces sufficient photovoltage for unassisted water splitting, by use of WO 3 in a core-shell tandem structure. This system provides a proof-of-principle for this design, which can be improved signicantly through the incorporation of higher efficiency wide band-gap semiconductors as they become available and are stable under the same conditions as the rest of the components of the device.

The TAT protein transduction domain (PTD) of the human immunodeficiency virus (HIV-1) can cross cell membranes with unusual efficiency [1] and has many potential biotechnological applications. [2][3][4] Extant work has provided important clues to the molecular mechanism underlying the activity of this peptide, which consists of 11 amino acids, 8 of which are cationic and 6 of these are arginines. TAT PTD synthesized with d-amino acids enters cells as efficiently as the native form, [5] thereby indicating that the mechanism of transduction is receptor independent; this conclusion is consistent with recent results that suggest that the TAT PTD may enter cells through receptor-independent macropinocytosis.[6] Substitution of any of the PTDs cationic residues with neutral alanine decreases activity, while substitution of neutral residues has no effect. [5] This indicates the importance of electrostatic interactions between cationic TAT PTD and anionic phospholipid membranes. Recent work has shown that the physics of electrostatic interactions can drive a rich polymorphism of self-assembled structures that depend on parameters such as charge density [7,8] and intrinsic membrane curvature. [9,10] However, although arginine-rich polycations can enter cells, cationic polylysine cannot. [11] This shows that electrostatic interactions alone are insufficient for PTD activity and that the arginine residues play a specific, essential role.We use confocal microscopy and synchrotron X-ray scattering (SAXS) to study the interaction of the TAT PTD with model membranes at room temperature. We find that the transduction activity correlates with induction of negative Gaussian ("saddle-splay") membrane curvature, which is topologically required for pore formation. Moreover, we show that the TAT PTD can drastically remodel vesicles into a porous bicontinuous phase with analogues in block-copolymer systems, [12][13][14] and we propose a geometric mechanism facilitated by both electrostatics and bidentate hydrogen bonding. The latter is possible for the TAT PTD but not for similarly cationic, nonarginated polypeptides.Cell membranes are composed of lipids that have fundamentally different interactions with cationic macroions such as TAT PTD. We examine representative model membranes composed of lipids with different charges and intrinsic curvatures: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) have zwitterionic headgroups, while 1,2-dioleoyl-sn-glycero-3-[phospho-l-serine] (sodium salt) (DOPS) and 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (sodium salt) (DOPG) have anionic headgroups; all have zero intrinsic curvature [15] (C 0 = 0, "cylinder-shaped") except for DOPE, which has negative intrinsic curvature (C 0 < 0, "cone-shaped"). When rhodamine-tagged TAT PTD (Rh-PTD) is applied to the exterior of giant unilamellar vesicles (GUVs, diameters of 5-30 mm) with low DOPE content (0 and 20 %), rhodamine fluorescence is seen only outside the GUVs (Figure 1 a), thereby indi...