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In this study, we report a pair of electrocatalysts for the hydrogen evolution reaction (HER) based on the noninnocent ligand diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-pyridinehydrazone) (H

Developing an efficient catalytic system for electrolysis with reduced platinum (Pt) loading while maintaining performance comparable to bulk platinum metal is important to decrease costs and improve scalability of the hydrogen fuel economy. Here we report the performance of a novel sputter-deposited molybdenum (Mo) thin film with an extremely low co-loading of Pt, where Pt atoms were dispersed on Mo (Ptd-Mo) as an electrocatalyst for the hydrogen evolution reaction (HER) in either alkaline or acidic media. The Ptd-Mo electrocatalyst presents similar catalytic activity to bulk Pt in alkaline media, while the performance is only slightly decreased in acidic media. Differential electrochemical mass spectrometry (DEMS) results confirm that the Ptd-Mo electrocatalyst produced hydrogen at a rate comparable with that of a pristine Pt sample at the same potential. A comparison with Pt-loaded degenerately doped p-type doped silicon (Ptd-Si) suggests that Mo and Pt work synergistically to boost the performance of Ptd-Mo catalysts. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) before and after 1000 cycles of continuous operation confirm the significant durability of the Ptd-Mo performance. Overall, the Ptd-Mo electrocatalyst, with comparable HER activity to bulk Pt despite an ultra-low Pt loading, could be a strong candidate for hydrogen production in either acidic or basic conditions.

The conversion of waste CO

Slurries of semiconductor particles individually capable of unassisted light-driven water-splitting are modeled to have a promising path to low-cost solar hydrogen generation, but they have had poor efficiencies. Tandem microparticle systems are a clear direction to pursue to increase efficiency. However, light absorption must be carefully managed in a tandem to prevent current mismatch in the subcells, which presents a possible challenge for tandem microwire particles suspended in a liquid. In this work, a Ni-catalyzed Si/TiO

In the search for new value-added products from CO2 electroreduction, it is critical that the detected species is derived from conversion of the reactants. Here we characterize contaminants from common ionomer anion- and cation-exchange membranes and demonstrate for one case how this can lead to misinterpretation of the product species by NMR spectroscopy. In particular, the membrane casting solvent N-methyl-2-pyrrolidinone (NMP) is demonstrated to be interpretable by NMR as other possible four-carbon products of the CO2 electroreduction. While the recommended commercial membrane pretreatment procedures greatly reduced the contaminant concentrations, this was not sufficient for all membrane types.

A series of crystalline nickel(II) complexes (1–3) based on inexpensive bis(thiosemicarbazone) ligands diacetylbis(4-methyl-3-thiosemicarbazone) (H2ATSM), diacetylbis(4,4-dimethyl-3-thiosemicarbazone) (H2ATSDM), and diacetylbis[4-(2,2,2-trifluoroethyl)-3-thiosemicarbazone] (H2ATSM-F6) were synthesized and characterized by single-crystal X-ray diffraction and NMR, UV–visible, and Fourier transform infrared spectroscopies. Modified electrodes GC-1–GC-3 were prepared with films of 1–3 deposited on glassy carbon and evaluated as potential hydrogen evolution reaction (HER) catalysts. HER studies in 0.5 M aqueous H2SO4 (10 mA cm–2) revealed dramatic shifts in the overpotential from 0.740 to 0.450 V after extended cycling for 1 and 2. The charge-transfer resistances for GC-1–GC-3 were determined to be 270, 160, and 630 Ω, respectively. Characterization of the modified surfaces for GC-1 and GC-2 by scanning electron microscopy and Raman spectroscopy revealed similar crystalline coatings before HER that changed to...

Exploiting renewable energy sources to drive CO2 reduction electrochemically into value-added products has gained tremendous attention towards reducing the greenhouse effect. Upstream processes for capturing and purifying the CO2 raise the overall cost of the electrolysis system. Direct reduction of flue gas into value-added C2+ products has attained much consideration in recent years. However, the sensitivity of the cathode catalyst performance in the presence of flue gas contaminants, particularly O2, and strong competition from the hydrogen evolution reaction in aqueous electrolyte are major challenges for such an approach. Herein, the influence of flue gas contaminants on electrochemical reduction of CO2 in acidic nonaqueous methanol to a methyl formate product has been investigated on a Pb-catalyzed electrode in conjunction with aqueous anolyte for the promotion of a sustainable water oxidation half-reaction. The presence of 4% O2, 0.05% SO2, and 0.05% NO is shown to have minimal effect on the total faradaic efficiency of CO2 reduction products. CO2 concentration-dependent measurements reveal declining CO2 reduction faradaic efficiencies corresponding to the decrease in partial pressure, which is attributed to CO2 mass transfer limitations to the electrode surface. XPS analysis displays the relative stability of the Pb working electrode before and after the electrochemical operation during exposure to flue gas components, which further highlights the promising tolerance of this system for direct flue gas conversion. Figure 1

The conversion of waste CO2 to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of the nonaqueous solvent methanol allows for higher CO2 solubility and novel products through the incorporation of methanol as an intermediate. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with a faradaic efficiency for methyl formate as high as 75% measured in pH < 2. Furthermore, strategies to limit the concentration of formic acid and maintain the surface oxide of the catalyst have been shown to be effective at improving the steady-state durability of the CO2-to-methyl formate process. Tests with simulated flue gas as the feedstock have shown a high level of tolerance for the Pb-catalyzed electrolysis to SO2, NO, and dilute O2 contaminants. Finally, a comparative technoeconomic analysis will be discussed to highlight target performance metrics and the viability of alternate pathways for electrochemical conversion of CO2 to methyl formate.

The molecular catalyst diacetyl-bis(N-4-methyl-3-thiosemi-carbazonato)nickel(ii) (NiATSM) was integrated with Si for light-driven hydrogen evolution from water. Compared to an equivalent loading of Ni metal, the NiATSM/p-Si electrode performed better. Durability of the surface-bound catalyst under operation in acid was achieved without covalent attachment by using Nafion binding.

As atmospheric levels of carbon dioxide (CO2) continue to increase, there is an immediate need to balance the carbon cycle. Current approaches require multiple processes to fix CO2 from the atmosphere or flue gas and then reduce it to value-added products. The zinc(II) catalyst Zn(DMTH) (DMTH = diacetyl-2-(4-methyl-3-thiosemicarbazonate)-3-(2-pyridinehydrazonato)) reduces CO2 from air to formate with a faradaic efficiency of 15.1% based on total charge. The catalyst utilizes metal-ligand cooperativity and redox-active ligands to fix, activate, and reduce CO2. This approach provides a new strategy that incorporates sustainable earth-abundant metals that are oxygen and water tolerant.

Efficient electroreduction of carbon dioxide has been a widely pursued goal as a sustainable method to produce value-added chemicals while mitigating greenhouse gas emissions. Processes have been demonstrated for the electroreduction of CO2 to CO at nearly 100 % faradaic efficiency, and as a consequence, there has been growing interest in the further electroreduction of carbon monoxide. Oxide-derived copper catalysts have promising performance for the reduction of CO to hydrocarbons but have still been unable to achieve high selectivity to individual products. A pulsed-bias technique is one strategy for tuning electrochemical selectivity without changing the catalyst. Herein a pulsed-bias electroreduction of CO was investigated on oxide-derived copper catalyst. Increased selectivity for single-carbon products (i.e., formate and methane) was achieved for higher pulse frequencies (

Electrochemical reduction of CO2 to liquid fuels is a promising route to a carbon-neutral, energy-dense storage of intermittent renewable electricity. However, electrocatalysts generally suffer from high overpotential and poor selectivity for multi-carbon products such as ethanol, and efforts to enhance such catalysts are limited by scaling relations which inhibit a simultaneous optimization of each elementary electrochemical step. In this work, the multistep proton-coupled electron-transfer reaction for the conversion of CO2 to C2H5OH was strategically divided into two independently optimized steps in a sequential cascade reaction using heterogeneous electrocatalysts to convert CO2 to CO and CO to C2H5OH within a single integrated electrochemical system. The exclusion of CO2 reactant from the second-stage electrolyzer was observed to be critical for maintaining appreciable ethanol selectivity. The cascade system produced C2H5OH at an overall faradaic efficiency of 11.0% at an average applied potential of −0.52 V vs. RHE, making it highly competitive with known single-step electrocatalysts for ethanol production from CO2. This performance was despite limited conversion of the intermediate CO between cascade steps (∼6.4%), and reactor design improvements to enhance the conversion could lead to significantly enhanced ethanol production performance.

A nitrogen plasma was incorporated into the cathode side of an electrolyzer to provide energetically activated N2 species to the electrocatalyst surface. At an applied bias of ∼3.5 V across the electrolyzer, plasma-assisted operation was observed to produce 47% more ammonia than the combination of plasma-without-bias and bias-without-plasma conditions.

Electrochemical reduction of CO2 to fuels and chemicals is currently a focus of significant research effort as a technology that can simultaneously mitigate greenhouse gas emissions while storing renewable electricity for use on demand. Liquid products are particularly desirable as an easily storable and portable energy-dense form. To be widely implemented, CO2 electroreduction technologies must be able to produce chemicals at costs that are economically competitive with existing commercial prices. In this work, four possible routes to the electrochemical synthesis of liquid products from CO2 derived from post-combustion flue gas were compared with one consistent approach to technoeconomic analysis. In the first case, diesel fuel was produced from electrosynthesized CO plus H2 to make syngas which was subsequently converted through the Fischer–Tropsch process. Liquid ethanol was modeled through two comparable approaches, a one-step electrolysis and a two-step cascade electrolysis. Lastly, the direct electrosynthesis of formic acid from CO2 was considered. In the base case scenarios established on current state-of-the-art CO2 reduction research, none of the processes were modeled to be competitive with present fuel prices. High capital expense for the electrolyzer units was the primary limiting factor. With conceivable improvements in an optimistic scenario, the diesel process was projected to have the best pathway to making cost-effective fuels, while ethanol would be prohibitively expensive without major improvements to the present electrosynthesis performance. Formic acid, though projected to be expensive relative to its stored energy content, was projected to have perhaps the simplest pathway to production at costs competitive with its commercial bulk price. In each case, the levelized cost of the liquid product was most strongly influenced by parameters that affect the electrolyzer capital cost (i.e., current density, faradaic efficiency, and cost per electrode area).

Electrochemical reduction of CO2 to liquid products offers a route for the energy-dense storage of intermittent renewable electricity while simultaneously helping to mitigate greenhouse gas emissions. In this work, high-quality Si photocathodes decorated with an earth-abundant Sn porous nanowire catalyst utilized the energy from visible light absorption to provide a photovoltage-assisted conversion of CO2 to liquid HCOOH. The Sn porous nanowire catalysts were selected for their high density of grain boundaries which was previously shown to enhance activity for formic acid formation. A faradaic efficiency of ∼60% with a partial current density of 10 mA cm−2 for HCOOH was achieved at −0.4 V vs. RHE under illumination, which reflected a positive potential shift of ∼400 mV compared to the dark electrocatalytic behavior. The photo-assisted electrolysis efficiency for formic acid was calculated to be 11.0%. The results represent a promising photocathode for a narrow bandgap subcell for a tandem photoelectrode system for unbiased light-driven CO2 electroreduction.

Electrochemical conversion of CO2 into energy-dense liquids, such as formic acid, is desirable as a hydrogen carrier and a chemical feedstock. SnOx is one of the few catalysts that reduce CO2 into formic acid with high selectivity but at high overpotential and low current density. We show that an electrochemically reduced SnO2 porous nanowire catalyst (Sn-pNWs) with a high density of grain boundaries (GBs) exhibits an energy conversion efficiency of CO2-into-HCOOH higher than analogous catalysts. HCOOH formation begins at lower overpotential (350 mV) and reaches a steady Faradaic efficiency of ca. 80 % at only −0.8 V vs. RHE. A comparison with commercial SnO2 nanoparticles confirms that the improved CO2 reduction performance of Sn-pNWs is due to the density of GBs within the porous structure, which introduce new catalytically active sites. Produced with a scalable plasma synthesis technology, the catalysts have potential for application in the CO2 conversion industry.

Electrocatalysis and photoelectrochemistry are critical to technologies like fuel cells, electrolysis, and solar fuels. Material stability and interfacial phenomena are central to the performance and long-term viability of these technologies. Researchers need tools to uncover the fundamental processes occurring at the electrode/electrolyte interface. Numerous analytical instruments are well-developed for material characterization, but many are ex situ techniques often performed under vacuum and without applied bias. Such measurements miss dynamic phenomena in the electrolyte under operational conditions. However, innovative advancements have allowed modification of these techniques for in situ characterization in liquid environments at electrochemically relevant conditions. This review explains some of the main in situ electrochemical characterization techniques, briefly explaining the principle of operation and highlighting key work in applying the method to investigate material stability and interfacial properties for electrocatalysts and photoelectrodes. Covered methods include spectroscopy (in situ UV-vis, ambient pressure X-ray photoelectron spectroscopy (APXPS), and in situ Raman), mass spectrometry (on-line inductively coupled plasma mass spectrometry (ICP-MS) and differential electrochemical mass spectrometry (DEMS)), and microscopy (in situ transmission electron microscopy (TEM), electrochemical atomic force microscopy (EC-AFM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical microscopy (SECM)). Each technique's capabilities and advantages/disadvantages are discussed and summarized for comparison.

Plasmas have a number of unique properties which researchers may be able to leverage to enhance desired electrocatalyst material properties and steer electrochemical reaction performance. Two examples of the recent application of plasmas for advancements in electrochemical fuel production are presented. In the first, the nonequilibrium reaction environment of an air plasma enabled the synthesis of homogeneous phases of metastable compositions of a mixed W1-xIrxO3-δ alloy. For a low-platinum-group-metal (PGM) composition of only 1% Ir, plasma-synthesized W0.99Ir0.01O3-δ was observed to be an acid-stable oxygen evolution reaction (OER) catalyst of competitive overpotential. Moreover, the plasma-synthesized alloy was observed to have an OER overpotential ~ 570 mV less than that of a thermally oxidized alloy of the same composition. In the second example of a plasma enhancement, a nitrogen plasma was coupled into the cathode of a proton exchange membrane electrolyzer to provide energetically activated N2 species to the electrocatalyst surface and demonstrate a proof-of-concept synergistic enhancement in the rate of ammonia production. At an applied bias of ~3.5 V across the electrolyzer, plasma-assisted operation was observed to produce 47% more ammonia than the combination of plasma-without-bias and bias-without-plasma conditions.

The efficient conversion of solar energy to fuels through electrochemical processes requires optimizing the photovoltage and current for an ideal coupling with the electrolysis reaction. A modular architecture for tandem photovoltaics is explored and modeled as a strategy to drive an arbitrary electrolysis reaction from sunlight to produce the maximum fuel product in a day. Non-monolithic tandem solar cells based on Si and organometal halide perovskites are simulated in two-terminal and four-terminal arrangements and coupled with experimental data on water-splitting and CO2 reduction to predict the performance of an integrated solar fuels system. An appropriately designed four-terminal system is modeled to match or exceed the output of a two-terminal system. The four-terminal configuration leads to a 15.8% increase in daily H2 production with a 1.5 eV/1.12 eV system, and a 5.3% increase with a more ideal 1.74 eV/1.12 eV combination. The four-terminal system is also simulated to match the production of formic acid and increase the production of ethylene by 20.4% in a Cu-catalyzed CO2 reduction process compared to a two-terminal tandem arrangement. The effects of series resistance in non-monolithic tandem devices are modeled as well, showing a much greater tolerance to cell width in the four-terminal systems.

Acid-based electrolysis has many advantages, but to achieve simultaneous activity and stability, commercial water oxidation catalysts rely on noble metal oxides that are expensive and too rare for the global scale. Here, earth-abundant tungsten was used as a structural metal to dilute the noble metal iridium content while maintaining high activity and stability in acid. Mixed-metal oxide catalysts were synthesized using rapid plasma oxidation in which the non-equilibrium reaction environment permitted better formation of a homogenous W1−xIrxO3−δ phase. With an Ir metal content as low as 1%, a competitive and durable overpotential for oxygen evolution was achieved. Relative to high Ir content, low Ir compositions consisted of a more highly crystalline, phase-pure iridium polytungstate which was more catalytically active per Ir content. Moreover, the plasma-synthesized material had a sharp electrocatalytic improvement over an equivalent composition synthesized via standard thermal oxidation, demonstrating the value of non-equilibrium synthesis to find new catalysts.

The electrochemical conversion of CO 2 into energy-rich fuels and chemicals has gained significant interest as a potential strategy for simultaneously mitigating increasing global CO 2 concentration and effectively storing intermittent renewable energy from sources such as solar and wind. This process recycles CO 2 , permitting a carbon-neutral, closed-loop of fuel combustion and waste CO 2 reduction to help prevent a rising concentration of this greenhouse gas in the atmosphere. At the same time, intermittent electricity generation can be stored in an energy-dense, portable form in chemical bonds. However, the stability of CO 2 makes its conversion kinetically challenging, generally requiring a large overpotential, and thus the efficiency of electrochemical CO 2 reduction is strongly dependent on the activity and selectivity of the cathodic electrocatalyst. In this review, we discuss the current state-of-the-art of unconventional heterogeneous catalysts with a focus on activity and product selectivity, even if the CO 2 reduction reaction mechanism remains uncertain. Various emerging approaches to enhance the yield of specific products and the overall rate of reaction will also be addressed. Finally, prospects for the development of next-generation catalysts will be discussed.

The electrochemical reduction of CO2 is a promising method for sustainable, carbon-neutral chemical synthesis as well as the storage of intermittent renewable energy in the form of energy-dense fuels compatible with existing infrastructure. In this work, we investigated a pulsed-bias technique for CO2 reduction on Cu, which led to a major shift in the product selectivity relative to potentiostatic electrolysis conditions. With applied voltage pulses in the millisecond time regime, syngas (CO + H2) became the only product and had a pulse-time-dependent H2:CO molar ratio, ranging from ∼32:1 to 9:16 for pulse times between 10 and 80 ms, respectively, at the same applied working potential. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) data suggested that in situ oxidation and reduction of the Cu partially caused the preference for CO formation over other carbon products on polycrystalline Cu. Significant nonfaradaic current arising from electrical double layer charging and discharging was...

The synthesis technique that can be used to accelerate the discovery of materials for various energy conversion and storage applications is presented. Specifically, this technique allows a rapid and controlled synthesis of mixed metal oxide particles using plasma oxidation of liquid droplets containing mixed metal precursors. The conventional wet chemical methods for synthesis of multimetal oxide solid solutions often require time-consuming high pressure and temperature processes, and so the challenge is to develop rapid and scalable techniques with precise compositional control. The concept is demonstrated by synthesizing binary and ternary transition metal oxide solid solutions with control over entire composition range using metal precursor solution droplets oxidized using atmospheric oxygen plasma. The results show the selective formation of metastable spinel and the rocksalt solid solution phases with compositions over the entire range by tuning the metal precursor composition. The synthesized manganese doped nickel ferrite nanoparticles, NiMnzFe2−zO4 (0 ≤ z ≤ 1), exhibits considerable electrocatalytic activity toward oxygen evolution reaction, achieving an overpotential of 0.39 V at a benchmarking current density of 10 mA/cm2 for a low manganese content of z = 0.20.

In this study, boron doped diamond (BDD) is utilized as a redox flow battery (RFB) electrode, demonstrating its capability with several low-cost redox couples for the first time. Active species cost, electrode corrosion, energy density, and power density are the current issues hindering the widespread adoption of RFBs. Several high-potential and low-cost redox couples, including Ce3+/Ce4+ and Mn2+/Mn3+, are shown to exhibit low overpotentials, high efficiency, and good cyclability on BDD electrodes. Using the Ce3+/Ce4+ redox couple, a formal potential of 1.67 V vs. SHE and a peak separation of 288 mV during cyclic voltammetry was obtained. Low-cost redox species significantly decrease RFB system cost while high potentials increase energy and power density. Demonstrated here, high potentials cannot be supported by traditional carbon-based RFB electrodes due to significant corrosion and gas evolution; however, the inherent properties of BDD negate this effect. This study exhibits the potential of BDD, as an alternative to traditional carbon-based electrodes, to enable a long cycle life, at high coulombic efficiencies, and with high-potential and low-cost redox couples.

Semiconductors of the III-V class are among the most promising materials for high efficiency solar fuels applications, particularly in tandem devices. Unfortunately, these materials are known to undergo oxidative corrosion as photoanodes in strongly acidic or alkaline electrolyte. In striving to overcome this challenge, researchers need to understand the fundamental degradation behavior during photoelectrochemical operation. Studying the in-situ photocorrosion process has largely been limited thus far to expensive and labor-intensive techniques that are not widely available in most labs. Herein, the corrosion of n-GaP, a promising III-V material for tandem top subcells, was investigated in strongly acidic electrolyte using an in-situ UV-Vis spectroscopy technique to monitor dissolved Ga and P species as a function of applied bias and time. The changing faradaic efficiency of the electrochemical GaP oxidation reaction was calculated from this data and used to interpret the corrosion process in conjunction with SEM and XPS characterization. Most notably, p+-GaP and n-GaP displayed strikingly different corrosion behavior, with p+-GaP dissolving uniformly across the active area and the corrosion faradaic efficiency increasing to a steady value. Illuminated n-GaP, in contrast, showed initially high corrosion faradaic efficiency which decreased during operation and was attributed to the phenomena of anisotropic surface etching and micropore formation during the beginning stages of photoanodic operation. In addition, corrosion measurements were made with thin conformal coatings of TiO2 as a protective barrier layer on the GaP surface. Although the protective coating slowed the rate of GaP dissolution, the TiO2 layers produced herein contributed significant charge-transfer resistance and still showed similar trends in the corrosion faradaic efficiency vs. time as the bare n-GaP.

Plasmas have a number of unique properties which researchers may be able to leverage to enhance desired electrocatalyst material properties and steer electrochemical reaction performance. Two examples of the recent application of plasmas for advancements in electrochemical fuel production are presented. In the first, the nonequilibrium reaction environment of an air plasma enabled the synthesis of homogeneous phases of metastable compositions of a mixed W1-xIrxO3-δ alloy. For a low-platinum-group-metal (PGM) composition of only 1% Ir, plasma-synthesized W0.99Ir0.01O3-δ was observed to be an acid-stable oxygen evolution reaction (OER) catalyst of competitive overpotential. Moreover, the plasma-synthesized alloy was observed to have an OER overpotential ~ 570 mV less than that of a thermally oxidized alloy of the same composition. In the second example of a plasma enhancement, a nitrogen plasma was coupled into the cathode of a proton exchange membrane electrolyzer to provide energetically activated N2 species to the electrocatalyst surface and demonstrate a proof-of-concept synergistic enhancement in the rate of ammonia production. At an applied bias of ~3.5 V across the electrolyzer, plasma-assisted operation was observed to produce 47% more ammonia than the combination of plasma-without-bias and bias-without-plasma conditions.

© 2018 The Royal Society of Chemistry. Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.

At the Renewable Energy Research & Education 2018 conference in Rajahmundry, India, scientists from the Conn Center for Renewable Energy Research presented educational lectures and research results from several key areas of renewable energy. Among the possible paths forward for a global sustainable energy future, the production of clean fuels through solar energy is a promising technology to store the intermittent energy of sunlight in a dense and portable form. Solar fuels, or artificial photosynthesis, is an approach that integrates light-absorbing semiconductors with electrocatalysts in an electrolyte to produce photogenerated charge-carriers capable of directly driving an electrochemical reaction to produce a chemical fuel. Key criteria for the development of an efficient solar fuels system include optimizing the semiconductor bandgaps and charge-collection properties for the electrochemical reaction, minimizing the anodic and cathodic activation overpotentials through highly active electrocatalysts, and ensuring a stable system that does not rapidly corrode under operation. Solar fuels research at the Conn Center strives to advance this technology and improve its practicality by working on key challenges in the areas of electrocatalysis, carbon-neutral liquid fuel production, photoelectrochemistry, and novel device design. A review of Conn Center progress in these areas is provided including acid-stable water oxidation catalysis, enhancing the selectivity for CO2 reduction, novel light-driven systems, and a demonstration of solar hydrogen production from ambient humidity.

The electrocatalytic behavior of Ni and Pt nanoparticles for the hydrogen-evolution reaction (HER) on p-type Si photocathodes was measured experimentally and the current density vs. potential (J–E) characteristics of a general metal catalyst on p-Si was modeled as a combination of a Si photodiode in series electrically with metal electrocatalysts. Relative to the rest potential, the J–E characteristics produced by the model showed an increase in total overpotential required to reach a specified current density for the metallized photoelectrodes relative to that of a metal electrode. This prediction was in accord with the experimentally observed behavior of Pt on p-Si, but was in contrast to the behavior observed for Ni on p-Si. Properly accounting for junction energetics and kinetics of the HER is critical to accurate predictions of the solar-to-hydrogen (STH) energy-conversion efficiency of metallized integrated photoelectrochemical systems. Further, models that accurately predict the performance of metal catalysts on semiconductor light absorbers are required to optimize the catalytic performance of metallized photoelectrodes.

Ultrathin dual layers of TiO_2 and Ni have been used to stabilize polycrystalline BiVO_4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte. Conformal, amorphous TiO_2 layers were deposited on BiVO_4 thin films by atomic-layer deposition, with Ni deposited onto the TiO_2 by sputtering. Under simulated air mass 1.5 illumination, the dual-layer coating extended the lifetime of the BiVO4 photoanodes during photoelectrochemical water oxidation from minutes, for bare BiVO4, to hours, for the modified electrodes. X-ray photoelectron spectroscopy showed that these layers imparted chemical stability to the semiconductor/electrolyte interface. Transmission electron microscopy revealed the structure and morphology of the polycrystalline BiVO_4 film as well as of the thin coating layers. This work demonstrates that protection schemes based on ultrathin corrosion-resistant overlayers can be applied beneficially to polycrystalline photoanode materials under conditions relevant to efficient solar-driven water-splitting systems.

The electrolytic conversion of CO2 to useful fuels and chemicals is a promising technology for greenhouse gas reduction and waste utilization, but converting CO2 to larger multicarbon products with any selectivity is a major challenge. However, commercial electrolyzers for the conversion of CO2 to CO at high current density and high yield are in development. Subsequent electrochemical reduction of carbon monoxide affords the possibility of promoting a different reaction pathway for higher multicarbon product selectivity. The reaction conditions for the aqueous conversion of CO on Cu/CuOx catalyst have been explored including applied bias, gas flow rate, and oxide thickness. Significant faradaic efficiencies for C4 products (> 5% FE), including gamma-hydroxybutryic acid and succinic acid, were observed and quantified via NMR. The gas flow rate and resulting hydrodynamics of CO near the catalyst surface play a crucial role, with little C4 species observed at lower flow rates. A proposed reaction pathway for the formation of these C4 products will be presented as well.

An important approach for solving the world's sustainable energy challenges is the conversion of solar energy to chemical fuels. Semiconductors can be used to convert/store solar energy to chemical bonds in an energy-dense fuel. Photoelectrochemical (PEC) water-splitting cells, with semiconductor electrodes, use sunlight and water to generate hydrogen. Herein, recent studies on improving the efficiency of semiconductor-based solar water-splitting devices by the introduction of surface passivation layers are reviewed. We show that passivation layers have been used as an effective strategy to improve the charge-separation and transfer processes across semiconductor–liquid interfaces, and thereby increase overall solar energy conversion efficiencies. We also summarize the demonstrated passivation effects brought by these thin layers, which include reducing charge recombination at surface states, increasing the reaction kinetics, and protecting the semiconductor from chemical corrosion. These benefits of passivation layers play a crucial role in achieving highly efficient water-splitting devices in the near future.

Electrochemical conversion of CO

Artificial photosynthesis relies on the availability of semiconductors that are chemically stable and can efficiently capture solar energy. Although metal oxide semiconductors have been investigated for their promise to resist oxidative attack, materials in this class can suffer from chemical and photochemical instability. Here we present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. Computational modelling indicates that photoexcited charge carriers accumulated at the surface destabilize the lattice, and that self-passivation by formation of a chemically stable surface phase is kinetically hindered. Although chemical stability of metal oxides cannot be assumed, insight into corrosion mechanisms aids development of protection strategies and discovery of semiconductors with improved stability., Metal oxide semiconductors are promising materials for solar energy capture but can suffer from stability problems. Here, the authors present a methodology for evaluating corrosion mechanisms and apply it to BiVO4, revealing chemical instabilities that are not predicted from thermodynamic considerations alone.

Degradation of metal-organic halide perovskites when exposed to ambient conditions is a crucial issue that needs to be addressed for commercial viability of perovskite solar cells (PSCs). Here, a concept of encapsulating CH3NH3PbI3 perovskite crystals with a multi-functional graphene-polyaniline (PANI) composite coating to protect the perovskite against degradation from moisture, oxygen and UV light is presented. Hole-conducting polymers containing 2D layered sheet materials are presented here as multi-functional materials with oxygen and moisture impermeability. Specific studies involving PANI and graphene composites as coatings for perovskite crystals exhibited resistance to moisture and oxygen under continued exposure to UV and visible light. Most importantly, no perovskite degradation was observed even after 96 h of exposure of the PSCs to extremely high humidity (99% relative humidity). Our observations and results on perovskite protection with graphene/conducting polymer composites open up opportunities for glove-box-free and atmospheric processing of PSCs.

Perovskite solar cells utilizing a two-step deposited CH3NH3PbI3 thin film were rapidly sintered using an intense pulsed light source. For the first time, a heat treatment has shown the capability of sintering methylammonium lead iodide perovskite and creating large crystal sizes approaching 1 μm without sacrificing surface coverage. Solar cells with an average efficiency of 11.5% and a champion device of 12.3% are reported. The methylammonium lead iodide perovskite was subjected to 2000 J of energy in a 2 ms pulse of light generated by a xenon lamp, resulting in temperatures significantly exceeding the degradation temperature of 150 °C. The process opens up new opportunities in the manufacturability of perovskite solar cells by eliminating the rate-limiting annealing step, and makes it possible to envision a continuous roll-to-roll process similar to the printing press used in the newspaper industry.

Arrays of oriented, crystalline Si wires are transferred into flexible, transparent polymer films. The polymer-supported Si wire arrays in liquid-junction photoelectrochemical cells yield current-potential behavior similar to the Si wires attached to the brittle growth substrate. These systems offer the potential for attaining high solar energy-conversion efficiencies using modest diffusion length, readily grown, crystalline Si in a flexible, processable form.

Analysis of the full device generation, transport, and recombination equations has shown that nanorod junction arrays can potentially offer improved photovoltaic performance relative to planar junctions for a carrier-collection limited absorber material not characterized by an excessively high rate of depletion-region recombination. To test this hypothesis, we have characterized planar and nanorod array photoelectrodes prepared by electrodeposition of Cd(Se, Te). The photoelectrochemical behavior of each type of photoelectrode was measured in contact with a liquid electrolyte consisting of aqueous 1 M S_2^(2-)/S^(2-), 1 M NaOH. The open-circuit photovoltage, V_(oc), short-circuit current density, J_(sc), fill factor, and overall energy conversion efficiency for both types of electrodes was measured under simulated 100 mW cm^(-2), Air Mass 1.5 conditions. V_(oc), J_(sc), and overall efficiencies were lower, on average, for nanorod array Cd(Se,Te) photoelectrodes, while the fill factors of the nanorod array photoelectrodes were generally superior to those of the planar junction devices. Importantly, the spectral response of the nanorod array photoelectrodes exhibited better quantum yields for collection of near-IR photons relative to collection of high-energy photons than did the planar photoelectrodes, in agreement with predictions of the theoretical model. The effects of surface recombination and junction area for both electrode designs have also been evaluated relative to planar photoelectrode junctions, using the Cd(Se,Te) electrode as a model system for the properties of nanorod array photoelectrodes.