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While photoelectrochemical (PEC) solar-to-hydrogen efficiencies have greatly improved over the past few decades, advances in PEC durability have lagged behind. Corrosion of semiconductor photoabsorbers in the aqueous conditions needed for water splitting is a major challenge that limits device stability. In addition, a precious-metal catalyst is often required to efficiently promote water splitting. Herein, we demonstrate unassisted water splitting using a nonprecious metal molybdenum disulfide nanomaterial catalytic protection layer paired with a GaInAsP/GaAs tandem device. This device was able to achieve stable unassisted water splitting for nearly 12 h, while a sibling sample with a PtRu catalyst was only stable for 2 h, highlighting the advantage of the nonprecious metal catalyst. In situ optical imaging illustrates the progression of macroscopic degradation that causes device failure. In addition, this work compares unassisted water splitting devices across the field in terms of the efficiency and stability, illustrating the need for improved stability.

The recirculation of gases in a sealed reactor system is a broadly useful method in catalytic and electrocatalytic studies. It is especially relevant when a reactant gas reacts slowly with respect to residence time in a catalytic reaction zone and when mass transport control through the reaction zone is necessary. This need is well illustrated in the field of electrocatalytic N2 reduction, where the need for recirculation of 15N2 has recently become more apparent. Herein, we describe the design, fabrication, use, and specifications of a lubricant-free, readily constructed recirculating pump fabricated entirely from glass and inert polymer (poly(ether ether ketone) (PEEK), poly(tetrafluoroethylene) (PTFE)) components. Using these glass and polymer components ensures chemical compatibility between the piston pump and a wide range of chemical environments, including strongly acidic and organic electrolytes often employed in studies of electrocatalytic N2 reduction. The lubricant-free nature of the pump and the presence of components made exclusively of glass and PEEK/PTFE mitigate contamination concerns associated with recirculating gases saturated with corrosive or reactive vapors for extended periods. The gas recirculating glass pump achieved a flow rate of >500 mL min-1 N2 against atmospheric pressure at 15 W peak power input and >100 mL min-1 N2 against a differential pressure of +6 in. H2O (∼15 mbar) at 10 W peak power input.

Nitrates from agricultural runoff and industrial waste streams are a notorious waste product and hazardous pollutant. Traditional electrochemical water remediation approaches aim to solve this prob...

Silicon has shown promise for use as a small band gap (1.1 eV) absorber material in photoelectrochemical (PEC) water splitting. However, the limited stability of silicon in acidic electrolyte requires the use of protection strategies coupled with catalysts. Herein, spin coating is used as a versatile method to directly coat silicon photoanodes with an IrOₓ oxygen evolution reaction (OER) catalyst, reducing the processing complexity compared to conventional fabrication schemes. Biphasic strontium chloride/iridium oxide (SrCl₂:IrOₓ) catalysts are also developed, and both catalysts form photoactive junctions with silicon and demonstrate highphotoanode activity. The iridium oxide photoanode displays a photocurrent onset at 1.06 V vs reversible hydrogen electrode (RHE), while the SrCl₂:IrOₓ photoanode onsets earlier at 0.96 V vs RHE. The differing potentials are consistent with the observed photovoltages of 0.43 and 0.53 V for the IrOₓ and SrCl₂:IrOₓ, respectively. By measuring the oxidation of a reversible redox couple, Fe(CN)₆ ³¯⁄⁴¯, we compare the charge carrier extraction of the devices and show that the addition of SrCl₂ to the IrOx catalyst improves the silicon−electrolyte interface compared to pure IrOₓ. However, the durability of the strontium-containing photoanode remains a challenge, with its photocurrent density decreasing by 90% over 4 h. The IrOₓ photoanode, on the other hand, maintained a stable photocurrent density over this timescale. Characterization of the as-prepared and post-tested material structure via Auger electron spectroscopy identifies catalyst film cracking and delamination as the primary failure modes. We propose that improvements to catalyst adhesion should further the viability of spin coating as a technique for photoanode preparation.

GaInP

Using renewable electricity to convert CO/CO2 into liquid products is touted as a sustainable process to produce fuels and chemicals, yet requires further advances in electrocatalyst understanding, development and device integration. The roughness factor of an electrode has generally been used to increase total rates of production, although rarely as a means to improve selectivity. Here we demonstrate that increasing the roughness factor of Cu electrodes is an effective design principle to direct the selectivity of CO reduction towards multicarbon oxygenates at low overpotentials and concurrently suppressing hydrocarbon and hydrogen production. The nanostructured Cu electrodes are capable of achieving almost full selectivity towards multicarbon oxygenates at an electrode potential of only –0.23 V versus the reversible hydrogen electrode. The successful implementation of this catalytic system has enabled an excellent CO reduction performance and elucidated viable pathways to improve the energy efficiency towards liquid fuels in high-power conversion electrolysers. The roughness factor of an electrode has been generally used to increase total rates of production, though rarely as a means to improve selectivity. Now, Jaramillo, Hahn and co-workers direct the selectivity of CO reduction to multicarbon oxygenates at low overpotentials by increasing the roughness factor of nanostructured Cu electrodes.

Photoelectrochemical water splitting is a promising route for sustainable hydrogen production. Herein, we demonstrate a photoelectrode motif that enables a nanostructured large-surface area electrocatalyst without requiring a nanostructured semiconductor surface with the goal of promoting electrocatalysis while minimizing surface recombination. We compare the photoelectrochemical H2 evolution activity of two silicon photocathode nanostructuring strategies: (1) direct nanostructuring of the silicon surface and (2) incorporation of nanostructured zinc oxide to increase the electrocatalyst surface area on planar silicon. We observed that silicon photocathodes that utilized nanostructured ZnO supports outperformed nanostructured silicon electrodes by ∼50 mV at open circuit under 1 sun illumination and demonstrated comparable electrocatalytic activity.

Electrochemical transformation of potent greenhouse gases such as CO2 and CH4 to produce useful carbon-based products is a highly desirable sustainability goal. However, selectivity challenges remain in aqueous electrochemical processes as selective CO2 reduction to desired products is difficult and electrochemical CH4 oxidation often proceeds at very low rates. The formation of C-C coupled products in these fields is particularly desirable as this provides a path for the production of high-value fuels and chemicals. We have developed a cyclic electrochemical strategy which can produce acetylene, a C-C coupled product, from such carbon sources and water, with favorable current density and selectivity. This strategy is exemplified with a lithium-mediated cycle: an active Li0 surface is electrochemically generated from LiOH, the newly formed Li0 reacts with a carbon source to form Li2C2, and Li2C2 is hydrolyzed to form acetylene and regenerate LiOH. We demonstrate this process primarily using CO2 gas, achieving a current efficiency of 15% to acetylene (which represents 82% of the maximum based on stoichiometric production of oxygenated byproducts, e.g.LiCO3 and/or Li2O), as verified by gas chromatography and Fourier transform infrared radiation studies. We also explore CH4, CO, and C as alternative precursors in the acetylene synthesis. Notably, the use of graphitic carbon at higher temperatures resulted in over 55% current efficiency to acetylene, with opportunity for further optimization. Importantly, this cycling method avoids the formation of common side products observed during aqueous electrochemical CO2 and CH4 redox reactions, such as H2, CO, HCO2−, or CO2. Theoretical considerations elucidate the feasibility and general applicability of this cycle and the process steps have been characterized with specific electrochemical and materials chemistry techniques. The continued development of this strategy may lead to a viable route for the sustainable production of C-C coupled carbon fuels and chemicals.

Electrochemical processes for ammonia synthesis could potentially replace the high temperature and pressure conditions of the Haber‐Bosch process, with voltage offering a pathway to distributed fertilizer production that leverages the rapidly decreasing cost of renewable electricity. However, nitrogen is an unreactive molecule and the hydrogen evolution reaction presents a major selectivity challenge. An electrode of electrodeposited lithium in tetrahydrofuran solvent overcomes both problems by providing a surface that easily reacts with nitrogen and by limiting the access of protons with a nonaqueous electrolyte. Under these conditions, we measure relatively high faradaic efficiencies (ca. 10 %) and rates (0.1 mA cm−2) toward NH3. We observe the development of a solid electrolyte interface layer as well as the accumulation of lithium and lithium‐containing species. Detailed DFT studies suggest lithium nitride and hydride to be catalytically active phases given their thermodynamic and kinetic stability relative to metallic lithium under reaction conditions and the fast diffusion of nitrogen in lithium.

The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1–4 to the energy-intensive Haber–Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation7–9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia. A protocol for the electrochemical reduction of nitrogen to ammonia enables isotope-sensitive quantification of the ammonia produced and the identification and removal of contaminants.

Electrocatalytic N-2 reduction to ammonia has recently attracted a great deal of interest as a possible renewable energy-driven alternative to the Haber-Bosch process. However, the detection of NH3 after attempting electrocatalytic reduction of N-2 can be hampered by low NH3 yields, ambient NH3 contamination, and the need for multistep chemical separation of NH3 from the electrolyte. Herein, we report a frequency-selective pulse nuclear magnetic resonance (NMR) method and quantify the efficacy of this method to measure the concentration of NH3 (present in the assay as NH4+) in an electrolyte after electrocatalysis. This NMR method was demonstrated to be effective in a variety of nondeuterated, nonaqueous and aqueous electrolytes, and did not require the separation of NH3 from the electrolyte. NH3 sensitivity down to 1 mu M was readily achieved with isotopic and chemical specificity. Compatible electrolytes and solvents included ethanol, tetrahydrofuran, dimethyl sulfoxide, acetonitrile, propylene carbonate, diethyl either, hexanes, and water. The efficacy of the commonly employed Berthelot method was also quantified and compared to the NMR method in a range of nonaqueous and aqueous electrolytes, including ethanol, THF, propylene carbonate, and water.

The electrocatalytic reduction of nitrogen gas to ammonia (N2RR) is touted as a sustainable, low-emissions alternative to the energy-intensive Haber-Bosch process, which occurs at high temperature and pressure and accounts for 1% of global CO2 emissions. However, creating device architectures that produce both high Faradaic efficiencies and high currents is challenging due to low demonstrated selectivity toward ammonia, which results in low product yields commensurate with the quantities of ammonia that can be introduced by contamination. Thus, the ability to accurately distinguish and quantify ammonia formed electrocatalytically and contaminant NH3 is of the utmost importance. Herein, we discuss work by our group and others to establish a protocol for rigorous N2RR testing to enable the progression of the field while minimizing reports of false positives. Using purified, isotope-labeled 15N2 was found to be critical for the reliable quantification of synthesized ammonia, and we review our development of ammonia-detection strategies including colorimetric methods, quantitative NMR, and LC-MS. We recently reported our construction of a custom-built glass piston pump with carefully controlled pressure and headspace to be used in a variety of electrochemical conditions, and we discuss the advantages and disadvantages of closed electrochemical systems for the N2RR. Using these methods, we continue to investigate catalyst and electrolyte conditions to promote the N2RR, and we present our latest efforts in developing an improved theoretical and mechanistic understanding of this reaction.

Humans have drastically altered the global nitrogen cycle, asymmetrically doubling its throughput due to industrial Haber-Bosch fertilizer production.1,2 Fertilizer runoff and inadequate wastewater removal cause waterborne emissions of reactive nitrogen (e.g., NH4 +, NO3 -) that induce harmful algal blooms.3,4 Meanwhile, both water purification and ammonia synthesis are conducted separately in centralized facilities with severe limitations: only 20% of wastewater is collected at treatment plants,5 and ammonia synthesis requires high temperature (400 °C), pressure (200 atm), energy input (1.2% of global energy), and greenhouse gas emissions (5% of industrial CO2).6,7 To couple water purification and ammonia production more efficiently, the following two reactions can be leveraged to synthesize and recover ammonia from nitrogen-polluted wastewater. Ammonia synthesis via electrochemical nitrate reduction: NO3 - + 9H++ 8e- → NH3 + 3H2O Ammonia recovery via electrodialysis and electrochemical pH control: NH4 + + OH- ↔ NH3 + H2O In our previous work, we have successfully demonstrated electrochemical stripping (ECS), a novel process combining electrodialysis and membrane stripping that selectively extracts (NH4)2SO4 from wastewater with > 93% recovery efficiency.8 We have also constructed a nitrogen mass transfer model for this process to inform rational decision of operating parameters.9 Building on these results, we reimagine nitrogen pollutants as products and propose electrodialysis and nitrate Reduction (EDNR). This novel unit process consists of three stages: (1) electrodialysis to separate influent NO3 - and NH4 +; (2) electrocatalytic reduction of NO3 - to NH3 (NO3RR), along with migration of undesired anions; and (3) product purification via migration of undesired cations (see Figure 1). We have built a proof-of-concept EDNR reactor using Ti/IrO2-Ta2O5 mesh (left and middle chambers) and stainless steel electrodes (right chamber). With recirculating batches of synthetic ion exchange brine (NaNO3/NH4Cl solution) and LiCl supporting electrolyte, we demonstrated that (1) over 99% of influent NH4 + is recovered to the right chamber; (2) 12% of initial NO3 - reacts, reaching 32% peak Faradaic efficiency towards NH3; (3) Cl- migrates out of the left chamber during NO3RR as desired and Cl2 gas evolution is minimal; and (4) ideal pH values are sustained in all three chambers to maximize NH3 synthesis and recovery. Aiming at selective NO3RR to NH3, we have also explored the effect of varying electrochemical conditions (e.g., pH, NO3 - concentration, applied potential) at a titanium foil cathode, which has shown high NH3 selectivity and corrosion resistance. Moving forward, we plan to use Ti foil electrode in the left chamber of EDNR, in which it will perform the oxygen evolution reaction (OER) in stage 1 and NO3RR in stage 2. To better understand the evolution of the catalyst during reactions and rationally identify EDNR operating potentials and cycle durations that maximize active catalyst stability, we perform characterizations of the Ti surface. By enabling automated, distributed ammonia manufacturing with minimal environmental impacts from prevalent wastewater contaminants, EDNR furthers our endeavor to re-engineer the nitrogen cycle. References (1) Galloway, J. N.; Townsend, A. R.; Erisman, J. W.; Bekunda, M.; Cai, Z.; Freney, J. R.; Martinelli, L. A.; Seitzinger, S. P.; Sutton, M. A. Transformation of the Nitrogen Cycle: Recent Trends, Questions, and Potential Solutions. Science 2008, 320 (5878), 889–892. (2) Fields, S. Global Nitrogen: Cycling out of Control. Environ Health Perspect 2004, 112 (10), A556–A563. (3) Dodds, W. K.; Bouska, W. W.; Eitzmann, J. L.; Pilger, T. J.; Pitts, K. L.; Riley, A. J.; Schloesser, J. T.; Thornbrugh, D. J. Eutrophication of U.S. Freshwaters: Analysis of Potential Economic Damages. Environ. Sci. Technol. 2009, 43 (1), 12–19. (4) Nyenje, P. M.; Foppen, J. W.; Uhlenbrook, S.; Kulabako, R.; Muwanga, A. Eutrophication and Nutrient Release in Urban Areas of Sub-Saharan Africa — A Review. Science of The Total Environment 2010, 408 (3), 447–455. (5) Larsen, T. A.; Hoffmann, S.; Lüthi, C.; Truffer, B.; Maurer, M. Emerging Solutions to the Water Challenges of an Urbanizing World. Science 2016, 352 (6288), 928–933. (6) Singh, A. R.; Rohr, B. A.; Schwalbe, J. A.; Cargnello, M.; Chan, K.; Jaramillo, T. F.; Chorkendorff, I.; Nørskov, J. K. Electrochemical Ammonia Synthesis—The Selectivity Challenge. ACS Catal. 2017, 7 (1), 706–709. (7) Norskov, J.; Chen, J. Sustainable Ammonia Synthesis; Department of Energy, 2016. (8) Tarpeh, W. A.; Barazesh, J. M.; Cath, T. Y.; Nelson, K. L. Electrochemical Stripping to Recover Nitrogen from Source-Separated Urine. Environ. Sci. Technol. 2018, 52 (3), 1453–1460. (9) Liu, M. J.; Neo, B. S.; Tarpeh, W. A. Building an Operational Framework for Selective Nitrogen Recovery via Electrochemical Stripping. Water Research 2020, 169, 115226. Figure 1

The photoelectrochemical behavior of n-type GaAs(100) electrodes coated with a single layer of graphene was compared with the behavior of bare, freshly etched n-type GaAs(100) electrodes, both for electrodes in contact with an aqueous solution containing K3[Fe(CN)6]/K4[Fe(CN)6] and for electrodes in contact with nonaqueous solutions containing a series of one-electron redox couples selected such that the Nernstian solution potentials spanned a range greater than 1 V. Under simulated 1 Sun illumination, the graphene-coated electrodes produced a short-circuit photocurrent density of 20 mA cm–2 for up to 8 h of continuous operation in nonaqueous electrolytes (H2O concentration 0.1%, v/v), while, under the same conditions, the unprotected n-GaAs electrodes showed a rapid decay of the photocurrent density within ∼400 s. Although the graphene monolayers enhanced the stability of n-GaAs photoanodes in nonaqueous electrolytes, the graphene did not fully protect photoanodes operated in contact with Fe(CN)63–/4–(aq...

Introduction of an ultrathin (2 nm) film of cobalt oxide (CoO_x) onto n-Si photoanodes prior to sputter-deposition of a thick multifunctional NiO_x coating yields stable photoelectrodes with photocurrent-onset potentials of ~−240 mV relative to the equilibrium potential for O2(g) evolution and current densities of ~28 mA cm^(−2) at the equilibrium potential for water oxidation when in contact with 1.0 M KOH(aq) under 1 sun of simulated solar illumination. The photoelectrochemical performance of these electrodes was very close to the Shockley diode limit for moderately doped n-Si(100) photoelectrodes, and was comparable to that of typical protected Si photoanodes that contained np+ buried homojunctions.

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. 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.

Nitrate-containing fertilizers are prolific in the agricultural industry due to the ease with which nitrates can be used as a nutrient source by plants.1 Concentrated forms of nitrates can be found in industrial waste streams such as those originating from uranium purification and processing,2 saltpeter mines,3 and agricultural runoff water.4 These nitrates often appear in the form of cation-containing salts, which can themselves be used as fertilizers. However, nitrate salts are generally avoided due to possible buildup of cations in the soil. Accumulation of ions such as sodium and calcium can break down soil structure, affect water intake by plants, and decrease plant metabolism, all of which lower crop yield.1 Consequently, a technology with the ability to convert various nitrate sources to a form of fertilizer that is less detrimental to crop growth is desirable. Direct electrochemical conversion of nitrates to ammonia is attractive due to its feasibility under a wide variety of electrolyte conditions, as well as its potential to be coupled to renewable electricity sources. While many studies have focused on electrochemical reduction of nitrates to environmentally-benign N2,5-7 recycling these nitrates into a chemical commodity such as ammonia could provide a means of producing a valuable fertilizer, ammonium nitrate, that is less potentially harmful to crop health than mineral- and metal-containing nitrate salts. To this end, a titanium-based cathode was used to electrochemically reduce NO3 - to ammonia. Applied potential, electrolyte pH, and nitrate concentration were varied to map selectivity toward ammonia under a variety of electrode/electrolyte conditions. Selectivity varied depending on proton and nitrate anion availability, with a maximum Faradaic efficiency (FE) toward NH3 of 78% occurring at a pH of 1 and 0.4 M NO3 - under an applied potential of -1 V vs RHE and current density of 23 mA/cm2. We observed titanium hydride formation on the surface of the electrode during electrolysis experiments and found that pre-treating the cathode may improve FE toward NH3. Additionally, the cathode was stable over 8 hours and demonstrated 50% FE toward ammonia, suggesting titanium is reasonably stable under peak selectivity conditions. Our preliminary technoeconomic analysis suggests that, at our total cell voltage of 1 V and the above selectivity toward ammonia, there may be an opportunity to produce ammonium nitrate, a typical fertilizer product, at or below the USDA cost range of NH4NO3. References Day, A. D.; Ludeke, K. L., Soil Alkalinity. In Plant Nutrients in Desert Environments, Springer Berlin Heidelberg: Berlin, Heidelberg, 1993; pp 35-37. Holleman, A. F.; Wiberg, E.; Wiberg, N.; Eagleson, M.; Brewer, W., Inorganic Chemistry. Academic Press: 2001. Gustafson, A. F., Handbook of Fertilizers - Their Sources, Make-Up, Effects, And Use. Read Books Limited: 2013. Davidson, E. A.; David, M. B.; Galloway, J. N.; Goodale, C. L.; Haeuber, R.; Harrison, J. A.; Howarth, R. W.; Jaynes, D. B.; Lowrance, R. R.; Nolan, B. T.; Peel, J. L.; Pinder, R. W.; Porter, E.; Snyder, C. S.; Townsend, A. R.; Ward, M. H., Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions. Issues in Ecology 2012, 12, 1-1. Katsounaros, I.; Kyriacou, G., Influence of the concentration and the nature of the supporting electrolyte on the electrochemical reduction of nitrate on tin cathode. Electrochimica Acta 2007, 52 (23), 6412-6420. Dortsiou, M.; Katsounaros, I.; Polatides, C.; Kyriacou, G., Influence of the electrode and the pH on the rate and the product distribution of the electrochemical removal of nitrate. Environmental Technology 2013, 34 (3), 373-38. Dima, G. E.; de Vooys, A. C. A.; Koper, M. T. M., Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions. J Electroanal Chem 2003, 554-555, 15-23. Figure 1

In this talk, we present our experimental efforts to increase selectivity in electrochemical ammonia synthesis with electrolyte engineering. Ammonia synthesis is a vital chemical reaction for the modern world. The Haber-Bosh process is routinely credited with supporting somewhere around half of our current population, at a cost of about 1% of the worlds energy consumption. It also requires hydrogen, typically provided through hydrocarbon reforming and is concomitant with centralized infrastructure and distribution. Clearly, this presents an opportunity to develop a process that is not reliant on large capital infrastructure or hydrocarbon derived hydrogen. One sought after option is a direct electrochemical process. This would enable ammonia to be produced directly using water and electricity from renewable resources. Unfortunately, faradaic efficiencies for ammonia synthesis in aqueous electrolytes are typically observed to be prohibitively low due to the competing hydrogen evolution reaction. Density functional calculations predict that ammonia synthesis will only proceed at potentials more negative than 1 volt, a potential at which most metal surfaces evolve hydrogen rapidly. While, in principle, it may be possible to find a material that activates nitrogen and not protons, we instead focus on changing the electrolyte to reduce HER. According to our simple kinetic model, reducing the availability of protons at the surface will allow nitrogen binding, the presumed rate limiting step, to compete with proton reduction.[1] While there are a number of possible electrolyte systems that may achieve this, we use aprotic, organic electrolytes with alcohols added as proton donors. By using alcohols as a proton donor, we hope to access an additional degree of freedom when engineering the reactivity of the interface. In preliminary measurements, we see faradaic efficiencies approaching 20%. We use both argon and nitrogen-15 labelled control experiments. We see no ammonia produced under argon purge and only labelled ammonia produced when using nitrogen-15 labelled N2. [1] Singh, A. R., Rohr, B. A., Schwalbe, J. A., Cargnello, M., Chan, K., Jaramillo, T. F., Nørskov, J. K. (2016). ACS Catalysis, 706–709. doi: 10.1021/acscatal.6b03035

The activation of molecular nitrogen toward reduction is key to the efficient synthesis of valuable ammonia-based agricultural fertilizers. With this in mind, we have explored the electrocatalytic reduction and oxidation of N2 with a range of materials, including Re, Ni, and Fe metals predicted to have high nitrogen reduction activity via DFT calculations. The activity of these electrocatalytic materials toward nitrogen reduction and oxidation was evaluated under a range pressures (1-50 bar) and under a range of electrolyte conditions, including various pH levels, solvents (H2O, ethanol, THF), and supporting electrolyte salts (Li+, Na+, K+) to understand and minimize the loss of Faradaic efficiency for nitrogen due to concurrent hydrogen evolution. Our experimental results were compared to computationally predicted nitrogen reduction electrocatalytic activities in order to understand the mechanism of N2 activation as well as improve computational methods for further theory-based hypothesis generation and catalyst selection.

Semiconductors with small band gaps (2 eV) must be stabilized against corrosion or passivation in aqueous electrolytes before such materials can be used as photoelectrodes to directly produce fuels from sunlight. In addition, incorporation of electrocatalysts on the surface of photoelectrodes is required for efficient oxidation of H2O to O2(g) and reduction of H2O or H2O and CO2 to fuels. We report herein the stabilization of np(+)-Si(100) and n-Si(111) photoanodes for over 1200 h of continuous light-driven evolution of O2(g) in 1.0 M KOH(aq) by an earth-abundant, optically transparent, electrocatalytic, stable, conducting nickel oxide layer. Under simulated solar illumination and with optimized index-matching for proper antireflection, NiOx-coated np(+)-Si(100) photoanodes produced photocurrent-onset potentials of -180 ± 20 mV referenced to the equilibrium potential for evolution of O2(g), photocurrent densities of 29 ± 1.8 mA cm(-2) at the equilibrium potential for evolution of O2(g), and a solar-to-O2(g) conversion figure-of-merit of 2.1%.

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.

Development of efficient and robust photoelectrochemical systems compromised of semiconductor electrodes which harness light to drive useful chemical transformations is hindered by a paucity of appropriate protective coatings to enable operation in aqueous media. Optically transparent, electrically conductive and physically stable coatings must be generated in order to avoid deleterious surface reactions associated with the corrosion and passivation of the semiconductor in solution. Graphene is the thinnest protective coating (0.3 nm) shown to extend stability of a semiconductor in aqueous solution, due in part to its relative impermeability to most gases and liquids. However, large-scale graphene growth via chemical vapor deposition – vital to integration of graphene into commercial devices – yields polycrystalline graphene, which contains a number of defects, such as dangling bonds found at grain boundaries and the edges of a sheet or point dislocations found within the sheet. This work demonstrated that capping these defects utilizing fluorine moieties can lead to significantly enhanced stability of silicon photoanodes under anodic conditions in aqueous media, which are typically too harsh for carbon-based layers to withstand for extended periods of time. N-type silicon (n-Si) (111) photoelectrodes with a hydride surface functionalization were biased anodically at short-circuit in aqueous Fe(CN)6 3-/4- under illumination (∼33 mW cm–2 ENH-type W-halogen lamp), and the generated photocurrent was observed to decay almost to zero within 100s, consistent with rapid corrosion of the semiconductor. In comparison, identical photoelectrodes protected with a single sheet of fluorinated graphene (F–Gr) demonstrated photocurrent densities of ~ 10 mA cm-2 for > 100,000s (> 24h). X-ray photoelectron spectra collected before and after exposure to aqueous anodic conditions showed that oxide formation was significantly inhibited for the n-Si electrodes covered with F–Gr in comparison with bare n-Si–H electrodes tested under the same conditions. Variations in the open-circuit voltage (VOC) in contact with several different redox couples with formal potentials that span the bandgap of silicon indicated that the photoanodes coated with F–Gr did not form a buried junction with respect to solution, which shows promise for the use of F–Gr as a protective layer for semiconductors beyond silicon. In addition, n-Si photoanodes coated with F–Gr exposed to a HBr/Br2 electrolyte solution under anodic conditions showed enhanced stability and formed a halide-splitting cell with up to 5% efficiency with the addition of a platinum catalyst, demonstrating that these protective layers can successfully be integrated into devices for the production of chemical fuels.

The behavior of graphene-coated n-type Si(111) photoanodes was compared to the behavior of H-terminated n-type Si(111) photoanodes in contact with aqueous K_3[Fe(CN)_6]/K_4[Fe(CN)_6] as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. The n-Si/Graphene electrodes exhibited stable short-circuit photocurrent densities of over 10 mA cm^(–2) for >1000 s of continuous operation in aqueous electrolytes, whereas n-Si–H electrodes yielded a nearly complete decay of the current density within 100 s. The values of the open-circuit photovoltages and the flat-band potentials of the Si were a function of both the Fermi level of the graphene and the electrochemical potential of the electrolyte solution, indicating that the n-Si/Graphene did not form a buried junction with respect to the solution contact.