Large-scale storage of renewable energy in the form of hydrogen (H2) fuel via electrolytic water splitting requires the development of water oxidation catalysts that are efficient and abundant. ...Carbon-based nanomaterials such as carbon nanotubes have attracted significant applications for use as substrates for anchoring metal-based nanoparticles. We show that, upon mild surface oxidation, hydrothermal annealing and electrochemical activation, multiwall carbon nanotubes (MWCNTs) themselves are effective water oxidation catalysts, which can initiate the oxygen evolution reaction (OER) at overpotentials of 0.3 V in alkaline media. Oxygen-containing functional groups such as ketonic CO generated on the outer wall of MWCNTs are found to play crucial roles in catalyzing OER by altering the electronic structures of the adjacent carbon atoms and facilitates the adsorption of OER intermediates. The well-preserved microscopic structures and highly conductive inner walls of MWCNTs enable efficient transport of the electrons generated during OER.
Efficient generation of hydrogen from water-splitting is an underpinning chemistry to realize the hydrogen economy. Low cost, transition metals such as nickel and iron-based oxides/hydroxides have ...been regarded as promising catalysts for the oxygen evolution reaction in alkaline media with overpotentials as low as ~200 mV to achieve 10 mA cm
, however, they are generally unsuitable for the hydrogen evolution reaction. Herein, we show a Janus nanoparticle catalyst with a nickel-iron oxide interface and multi-site functionality for a highly efficient hydrogen evolution reaction with a comparable performance to the benchmark platinum on carbon catalyst. Density functional theory calculations reveal that the hydrogen evolution reaction catalytic activity of the nanoparticle is induced by the strong electronic coupling effect between the iron oxide and the nickel at the interface. Remarkably, the catalyst also exhibits extraordinary oxygen evolution reaction activity, enabling an active and stable bi-functional catalyst for whole cell water-splitting with, to the best of our knowledge, the highest energy efficiency (83.7%) reported to date.
Ammonia is of emerging interest as a liquefied, renewable-energy-sourced energy carrier for global use in the future. Electrochemical reduction of N
(NRR) is widely recognised as an alternative to ...the traditional Haber-Bosch production process for ammonia. However, though the challenges of NRR experiments have become better understood, the reported rates are often too low to be convincing that reduction of the highly unreactive N
molecule has actually been achieved. This perspective critically reassesses a wide range of the NRR reports, describes experimental case studies of potential origins of false-positives, and presents an updated, simplified experimental protocol dealing with the recently emerging issues.
Shuttling protons in ammonia synthesis
An electrochemical route to ammonia could substantially lower the greenhouse gas emissions associated with the current thermal Haber-Bosch process. One ...relatively promising option under study involves reductive formation of lithium nitride, which can be protonated to ammonia. However, the ethanol used to date as a local proton source in these studies may degrade under the reaction conditions. Suryanto
et al.
report the use of a tetraalkyl phosphonium salt in place of ethanol (see the Perspective by Westhead
et al.
). This cation can stably undergo deprotonation–reprotonation cycles and, as an added benefit, it enhances the ionic conductivity of the medium.
Science
, abg2371, this issue p.
1187
; see also abi8329, p.
1149
A phosphonium cation acts as a stable proton shuttle during electrochemical ammonia synthesis.
Ammonia (NH
3
) is a globally important commodity for fertilizer production, but its synthesis by the Haber-Bosch process causes substantial emissions of carbon dioxide. Alternative, zero-carbon emission NH
3
synthesis methods being explored include the promising electrochemical lithium-mediated nitrogen reduction reaction, which has nonetheless required sacrificial sources of protons. In this study, a phosphonium salt is introduced as a proton shuttle to help resolve this limitation. The salt also provides additional ionic conductivity, enabling high NH
3
production rates of 53 ± 1 nanomoles per second per square centimeter at 69 ± 1% faradaic efficiency in 20-hour experiments under 0.5-bar hydrogen and 19.5-bar nitrogen. Continuous operation for more than 3 days is demonstrated.
The electrochemical nitrogen reduction reaction (NRR) under ambient conditions is a promising alternative to the traditional energy‐intensive Haber–Bosch process to produce NH3. The challenge is to ...achieve a sufficient energy efficiency, yield rate, and selectivity to make the process practical. Here, we demonstrate that Ru nanoparticles (NPs) enable NRR in 0.01 m HCl aqueous solution at very high energy efficiency, that is, very low overpotentials. Remarkably, the NRR occurs at a potential close to or even above the H+/H2 reversible potential, significantly enhancing the NRR selectivity versus the production of H2. NH3 yield rates as high as ≈5.5 mg h−1 m−2 at 20 °C and 21.4 mg h−1 m−2 at 60 °C were achieved at a redox potential (E) of −100 mV versus the reversible hydrogen electrode (RHE), whereas a highest Faradaic efficiency (FE) of ≈5.4 % is achievable at E=+10 mV vs. RHE. This work demonstrates the potential use of Ru NPs as an efficient catalyst for NRR at ambient conditions. This ability to catalyze NRR at potentials near or above RHE is imperative in improving the NRR selectivity towards a practical process as well as rendering the H2 viable as byproduct. Density functional theory calculations of the mechanism suggest that the efficient NRR process occurring on these predominantly Ru (0 0 1) surfaces is catalyzed by a dissociative mechanism.
Ru is not under pressure: Electrochemical nitrogen reduction reaction on Ru nanoparticles was studied experimentally, together with theoretical density functional theory calculations, demonstrating the possibility of using Ru as catalyst for ammonia synthesis under ambient conditions.
It has become apparent that renewable energy sources are plentiful in many, often remote, parts of the world, such that storing and transporting that energy has become the key challenge. For ...long‐distance transportation by pipeline and bulk tanker, a liquid form of energy carrier is ideal, focusing attention on liquid hydrogen and ammonia. Development of high‐activity and selectivity electrocatalyst materials to produce these energy carriers by reductive electrochemistry has therefore become an important area of research. Here, recent developments and challenges in the field of electrocatalytic materials for these processes are discussed, including the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the nitrogen reduction reaction (NRR). Some of the mis‐steps currently plaguing the nitrogen reduction to ammonia field are highlighted. The rapidly growing roles that in situ/operando and quantum chemical studies can play in new electromaterials discovery are also surveyed.
Australia has an enormous potential to generate renewable energy from wind and solar, vastly more than is needed for domestic use. Most of this generation capacity is in remote regions such that means of transporting and storing this energy is required. Advances in the electrochemical production of chemical energy carriers including hydrogen, reduced carbon compounds, and ammonia are discussed.
Carbon black (CB) is popularly used as a catalyst support for metal/metal oxide nanoparticles due to its large surface area, excellent conductivity and stability. Herein, we show that surface ...oxidized CB itself, after acidic treatment and electrochemical oxidation, exhibits significant catalytic activity for the electrochemical oxidation of water and alcohols.
The role of carbon nanotubes in the advancement of energy conversion and storage technologies is undeniable. In particular, carbon nanotubes have attracted significant applications for ...electrocatalysis. However, one central issue related to the use of carbon nanotubes is the required oxidative pretreatment that often leads to significant damage of graphitic structures which deteriorates their electrochemical properties. Traditionally, the oxidized carbon nanomaterials are treated at high temperature under an inert atmosphere to repair the oxidation-induced defect sites, which simultaneously removes a significant number of oxygen functional groups. Nevertheless, recent studies have shown that oxygen functional groups on the surface of MWCNT are the essential active centers for a number of important electrocatalytic reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein we first show that hydrothermal treatment as a mild method to improve the electrochemical properties and activities of surface-oxidized MWCNT for OER, HER, and ORR without significantly altering the oxygen content. The results indicate that hydrothermal treatment could potentially repair the defects without significantly reducing the pre-existing oxygen content, which has never been achieved before with conventional high-temperature annealing treatment.
With the recent advances in sustainable ammonia production, the ammonia oxidation reaction (AOR) has gained interest. In this work, we present an evaluation of the performance of electrochemically ...roughened platinum electrodes (PtR) for aqueous AOR. Platinum electrodes were modified using square wave voltammetry at various frequencies in 0.5 M H2SO4. Platinum electrodes modified at frequencies in excess of 1 kHz tended to develop nano-structured surfaces with corresponding increases in the proportion of the active {311} phase of platinum resulting in improved AOR activity. Of the different surface structures formed by the roughening process, trigonal nano-pyramids (TNPs) with small average cross sections of 20 ± 5 nm showed the greatest degree of enhancement (550% increase in peak current density relative to polycrystalline platinum. With further optimization of these roughening techniques, PtR electrodes may prove useful in as anodes in ammonia sensing applications and in fuel cells.
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