We report the electrocatalytic reduction of CO 2 to the highly reduced C 2 products, ethylene and ethane, as well as to the fully reduced C 1 product, methane, on three different phases of nickel−gallium (NiGa, Ni 3 Ga, and Ni 5 Ga 3 ) films prepared by drop-casting. In aqueous bicarbonate electrolytes at neutral pH, the onset potential for methane, ethylene, and ethane production on all three phases was found to be −0.48 V versus the reversible hydrogen electrode (RHE), among the lowest onset potentials reported to date for the production of C 2 products from CO 2 . Similar product distributions and onset potentials were observed for all three nickel− gallium stoichiometries tested. The onset potential for the reduction of CO 2 to C 2 products at low current densities catalyzed by nickel−gallium was >250 mV more positive than that of polycrystalline copper, and approximately equal to that of single crystals of copper, which have some of the lowest overpotentials to date for the reduction of CO 2 to C 2 products and methane. The nickel−gallium films also reduced CO to ethylene, ethane, and methane, consistent with a CO 2 reduction mechanism that first involves the reduction of CO 2 to CO. Isotopic labeling experiments with 13 CO 2 confirmed that the detected products were produced exclusively by the reduction of CO 2 .

A number of approaches to solar fuels generation are being developed, each of which has associated advantages and challenges. Many of these solar fuels generators are identified as "photoelectrochemical cells" even though these systems collectively operate based on a suite of fundamentally different physical principles. To facilitate appropriate comparisons between solar fuels generators, as well as to enable concise and consistent identification of the state-of-the-art for designs based on comparable operating principles, we have developed a taxonomy and nomenclature for solar fuels generators based on the source of the asymmetry that separates photogenerated electrons and holes. Three basic device types have been identified: photovoltaic cells, photoelectrochemical cells, and particulate/molecular photocatalysts. We outline the advantages and technological challenges associated with each type, and provide illustrative examples for each approach as well as for hybrid approaches. Broader contextSolar fuels generators are devices that harness energy from sunlight to drive the synthesis of chemical fuels. A number of approaches to solar fuels generation are being pursued, many of which can be differentiated by the physical principles on which they are based. Herein, we propose a nomenclature and taxonomy based on three basic device types: photovoltaic cells, photoelectrochemical cells, and photoelectrosynthetic particulate/molecular photocatalysts. An understanding of the inherent operating principles and the advantages and challenges associated with each of these device types will facilitate clear comparisons between devices as well as help guide research efforts toward improving these devices and achieving the ultimate goal of sustainable fuel production.

A solar-driven CO2 reduction (CO2R) cell was constructed, consisting of a tandem GaAs/InGaP/TiO2/Ni photoanode in 1.0 M KOH­(aq) (pH = 13.7) to facilitate the oxygen-evolution reaction (OER), a Pd/C nanoparticle-coated Ti mesh cathode in 2.8 M KHCO3(aq) (pH = 8.0) to perform the CO2R reaction, and a bipolar membrane to allow for steady-state operation of the catholyte and anolyte at different bulk pH values. At the operational current density of 8.5 mA cm–2, in 2.8 M KHCO3(aq), the cathode exhibited <100 mV overpotential and >94% Faradaic efficiency for the reduction of 1 atm of CO2(g) to formate. The anode exhibited a 320 ± 7 mV overpotential for the OER in 1.0 M KOH­(aq), and the bipolar membrane exhibited ∼480 mV voltage loss with minimal product crossovers and >90 and >95% selectivity for protons and hydroxide ions, respectively. The bipolar membrane facilitated coupling between two electrodes and electrolytes, one for the CO2R reaction and one for the OER, that typically operate at mutually different pH values and produced a lower total cell overvoltage than known single-electrolyte CO2R systems while exhibiting ∼10% solar-to-fuels energy-conversion efficiency.

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 ...

Reduction of carbon dioxide in aqueous electrolytes at single-crystal MoS 2 or thin-film MoS 2 electrodes yields 1-propanol as the major CO 2 reduction product, along with hydrogen from water reduction as the predominant reduction process. Lower levels of formate, ethylene glycol, and t-butanol were also produced. At an applied potential of -0.59 V versus a reversible hydrogen electrode, the Faradaic efficiencies for reduction of CO 2 to 1-propanol were ~3.5% for MoS 2 single crystals and ~1% for thin films with low edge-site densities. Reduction of CO 2 to 1-propanol is a kinetically challenging reaction that requires the overall transfer of 18 e -and 18 H + in a process that involves the formation of 2 C-C bonds. NMR analyses using 13 CO 2 showed the production of 13 C-labelled 1-propanol. In all cases, the vast majority of the Faradaic current resulted in hydrogen evolution via water reduction. H 2 S was detected qualitatively when single-crystal MoS 2 electrodes were used, indicating that some desulfidization of single crystals occurred under these conditions.

A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO 2 reduction (CO 2 R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO 2 R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO 2 R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO 2 R. On Ni-MoS 2 , a limitation is the kinetic barrier for the proton−electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO 2 R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers.

A solar-driven CO2-reduction (CO2R) cell, consisting of a tandem GaAs/InGaP/TiO2/Ni photoanode in 1.0 M KOH(aq) (pH=13.7) to facilitate the oxygen-evolution reaction (OER), a Pd/C nanoparticle-coated Ti mesh cathode in 2.8 M KHCO3(aq) (pH=8.0) to perform the CO2R reaction, and a bipolar membrane to allow for steady-state operation of the catholyte and anolyte at different bulk pH values, was constructed. At the operational current density of 8.5 mA cm -2 , in 2.8 M KHCO3(aq), the cathode exhibited <100 mV overpotential and >94% Faradaic efficiency for the reduction of 1 atm of CO2(g) to formate. The anode exhibited 320 ± 7 mV overpotential for the OER in 1.0 M KOH(aq), and the bipolar membrane exhibited ~480 mV voltage loss with minimal product crossover as well as >90% and >95% selectivity for protons and hydroxide ions, respectively. The solardriven CO2R cell converted sunlight to fuels at an energyconversion efficiency of ~10%.

Certain alloys of nickel have recently been shown to reduce CO 2 to multi-carbon products electrochemically without the need for copper. Here we show that Ni 3 Ga thin film electrocatalysts on carbon electrodes discriminate between CO 2 reduction pathways and products based on their surface morphologies, which are controlled by catalyst-carbon support interactions. It is also observed that unsupported, bulk Ni 3 Ga reduces CO but not CO 2 . With this understanding, a tandem electrocatalyst utilizing two variants of the Ni 3 Ga material-one supported and one unsupported-was developed. In this two-electrode system, CO is generated from CO 2 on an electrode optimized for this process, and the CO is then further reduced to methanol in the same reactor. It appears that choice of carbon support impacts the morphology of Ni 3 Ga during the synthesis of the catalyst, thereby influencing the electrolysis product distribution. The discovery or design of CO 2 reduction catalysts has been a focus of current electrochemical studies due to the rising concentration of CO 2 in the atmosphere coupled with global challenges accompanying such a phenomenon.1,2 Materials that facilitate the transformation of CO 2 are wide-ranging, but a deficiency in our understanding of how they function is exemplified by several recent studies reporting on what is nominally the same electrode material after modifying the structure, morphology, or composition and eliciting a new or improved response toward CO 2 reduction. 3-10When a known CO 2 -reducing material is altered to include novel structures or morphologies, it is anticipated that the newly introduced characteristic may change the base material's activity by increasing the concentration of surface active sites or improving intermediate stability, thereby impacting catalysis. As an example, the Kanan group has reported that copper catalyst morphology influences the distribution and faradaic efficiencies of CO 2 reduction products. 11 The importance of morphology is one reason why nanoparticles and thin films have garnered attention. Selection of heterogeneous catalysts in non-bulk form involves immobilizing nanoparticles, thin films, and other specialty structures on electrodes, and thus a second material, the solid support, is added to the electrochemical system.It is well-established that solid support identity can directly impact material or catalytic properties. For example, Rakhi et al. grew Co 3 O 4 nanowires on carbon fiber paper and planar graphitized carbon paper and achieved startlingly different morphologies, 12 while other authors witnessed similar morphological impacts for different systems. 13,14 Superconductivity, 15,16 material hardness, 17 and especially catalytic efficacy in the oxygen reduction reaction [18][19][20][21] have been attributed to solid supports interacting with or changing properties of surface materials that are typically the focus of the study. These works suggest that properties such as catalyst morphology, which is important for CO 2 electrocatalysis, ma...