The notion of metal‐free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal‐free catalysts are prepared using metal‐containing precursors. ...It is highly contested that metal residues in nitrogen‐doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal‐free catalysts is proposed and the differences between NC and M‐Nx/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M‐Nx/C and NC catalysts are discussed to inspire the development of more advanced nonprecious‐metal catalysts for the ORR.
A free agent: Nitrogen‐modified carbon (NC) materials (see scheme) are of emerging importance for electrocatalysis of the oxygen reduction reaction (ORR). However, trace‐metal residues in NC catalysts can significantly influence the ORR, which has raised concerns regarding the validity of the notion of a metal‐free catalyst. A critical discussion of the role of metals in NC catalysts is given to reconcile divergent views.
•High sensitivity and high spatial resolution in localized electrochemical measurements are the key advantages of electroanalysis using nanometer-sized electrodes.•Different protocols for the ...fabrication of needle-type nanoelectrodes are summarized.•Limits of conventional theory to describe electrochemistry at the nanoscale are discussed.•Different applications of nanoelectrodes are highlighted including non-ensemble studies of electrocatalysis at single nanoparticles at high mass transport rates, electrochemical nanosensors for highly localized non-invasive analysis of single living cells and intracellular detection of neurotransmitters and metabolites as well as the use of nanoelectrodes in scanning electrochemical probe techniques.
High sensitivity and high spatial resolution in localized electrochemical measurements are the key advantages of electroanalysis using nanometer-sized electrodes. Due to recent progress in nanoelectrode fabrication and electrochemical instrument development, nanoelectrochemical methods are becoming more widespread. We summarize different protocols for the fabrication of needle-type nanoelectrodes and discuss their properties with regard to various applications. We discuss the limits of conventional theory to describe electrochemistry at the nanoscale and point out technical aspects for characterization and handling of nanometric electrodes. Different applications are highlighted: i) Nanoelectrodes are powerful tools for non-ensemble studies of electrocatalysis at single nanoparticles at high mass transport rates. ii) Electrochemical nanosensors are employed for highly localized non-invasive analysis of single living cells and intracellular detection of neurotransmitters and metabolites. iii) Used in scanning electrochemical probe techniques, nanoprobes afford topographical and truly chemical imaging of samples with high spatial resolution.
The formation of a vast number of different multielement active sites in compositionally complex solid solution materials, often more generally termed high‐entropy alloys, offers new and unique ...concepts in catalyst design, which mitigate existing limitations and change the view on structure–activity relations. We discuss these concepts by summarising the currently existing fundamental knowledge and critically assess the chances and limitations of this material class, also highlighting design strategies. A roadmap is proposed, illustrating which of the characteristic concepts could be exploited using which strategy, and which breakthroughs might be possible to guide future research in this highly promising material class for (electro)catalysis.
High‐entropy alloys offer a huge variety of multielement active sites on a single catalytic surface. This special polyelemental arrangement implies several different fundamental concepts in structure–activity correlations compared to traditional electrocatalysts, which are summarised and their implications for the possibility to adjust the catalytic properties with different limitations are discussed in detail.
Zn metal as anode in rechargeable batteries, such as Zn/air or Zn/Ni, suffers from poor cyclability. The formation of Zn dendrites upon cycling is the key limiting step. We report a systematic study ...of the influence of pulsed electroplating protocols on the formation of Zn dendrites and in turn on strategies to completely prevent Zn dendrite formation. Because of the large number of variables in electroplating protocols, a scanning droplet cell technique was adapted as a high-throughput methodology in which a descriptor of the surface roughness can be in situ derived by means of electrochemical impedance spectroscopy. Upon optimizing the electroplating protocol by controlling nucleation, zincate ion depletion, and zincate ion diffusion, scanning electron microscopy and atomic force microscopy confirmed the growth of uniform and homogenous Zn deposits with a complete prevention of dendrite growth. The implementation of pulsed electroplating as the charging protocol for commercially available Ni–Zn batteries leads to substantially prolonged cyclability demonstrating the benefits of pulsed charging in Zn metal-based batteries.
Electrocatalytic recycling of waste nitrate (NO
) to valuable ammonia (NH
) at ambient conditions is a green and appealing alternative to the Haber-Bosch process. However, the reaction requires ...multi-step electron and proton transfer, making it a grand challenge to drive high-rate NH
synthesis in an energy-efficient way. Herein, we present a design concept of tandem catalysts, which involves coupling intermediate phases of different transition metals, existing at low applied overpotentials, as cooperative active sites that enable cascade NO
-to-NH
conversion, in turn avoiding the generally encountered scaling relations. We implement the concept by electrochemical transformation of Cu-Co binary sulfides into potential-dependent core-shell Cu/CuO
and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in-situ Raman spectra reveal that the inner Cu/CuO
phases preferentially catalyze NO
reduction to NO
, which is rapidly reduced to NH
at the nearby Co/CoO shell. This unique tandem catalyst system leads to a NO
-to-NH
Faradaic efficiency of 93.3 ± 2.1% in a wide range of NO
concentrations at pH 13, a high NH
yield rate of 1.17 mmol cm
h
in 0.1 M NO
at -0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.
It is demonstrated that amorphous cobalt boride (Co2B) prepared by the chemical reduction of CoCl2 using NaBH4 is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in ...alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm−2 at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen‐doped graphene. Stable performance is maintained at 10 mA cm−2 for at least 60 h. The optimized catalyst, Co2B annealed at 500 °C (Co2B‐500) evolves oxygen more efficiently than RuO2 and IrO2, and its performance matches the best cobalt‐based catalysts reported to date. Co2B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co2B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.
Cobalt boride (Co2B) is presented as a highly efficient water splitting nonprecious catalyst for oxygen and hydrogen evolution in alkaline electrolytes. The catalyst achieves a current density of 10 mA cm−2 at as low as 1.61 VRHE and is among the best reported for cobalt‐based catalysts. Stable performance was maintained for at least 60 h during electrolysis at 10 mA cm−2.
Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as ...regenerative fuel cells and metal–air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core–shell Co@Co3O4 nanoparticles embedded in CNT‐grafted N‐doped carbon‐polyhedra obtained by the pyrolysis of cobalt metal–organic framework (ZIF‐67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2, and RuO2 and thus ranking them among one of the best non‐precious‐metal electrocatalysts for reversible oxygen electrodes.
An electrocatalyst consisting of Co@Co3O4 embedded in carbon nanotube‐grafted N‐doped carbon polyhedra formed in situ has been fabricated from metal–organic frameworks. The electrocatalyst shows high activity towards water oxidation and oxygen reduction and outperforms Pt‐, Ir‐, and Ru‐based electrocatalysts.
A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity ...plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.
Reversible interconversion of water into H2 and O2, and the recombination of H2 and O2 to H2O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy ...conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn3O4 and Co3O4 nanoparticles embedded in nitrogen‐doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N4 macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M–Nx coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1 M) than those for RuO2, IrO2, Pt, NiO, Mn3O4, and Co3O4, thus placing them among the best non‐precious‐metal catalysts for reversible oxygen electrodes reported to date.
A dual finds its right setting: To improve the efficiency of metal–air batteries and unitized regenerative fuel‐cell systems dual‐function catalysts that can accomplish both water oxidation and oxygen reduction are required. Low‐cost bifunctional catalysts based on Co, Mn, and Ni oxides embedded in N‐doped carbon (NC) are synthesized and outperform Pt‐, Ir‐ and, Ru‐based catalysts.
Carbon corrosion at high anodic potentials is a major source of instability, especially in acidic electrolytes and impairs the long‐term functionality of electrodes. In‐depth investigation of carbon ...corrosion in alkaline environment by means of differential electrochemical mass spectrometry (DEMS) is prevented by the conversion of CO2 into CO32−. We report the adaptation of a DEMS system for online CO2 detection as the product of carbon corrosion in alkaline electrolytes. A new cell design allows for in situ acidification of the electrolyte to release initially dissolved CO32− as CO2 in front of the DEMS membrane and its subsequent detection by mass spectrometry. DEMS studies of a carbon‐supported nickel boride (NixB/C) catalyst and Vulcan XC 72 at high anodic potentials suggest protection of carbon in the presence of highly active oxygen evolution electrocatalysts. Most importantly, carbon corrosion is decreased in alkaline solution.
Caught in the act: Direct detection of CO2 as a marker for carbon oxidation in alkaline electrolytes is now possible using a novel DEMS cell (DEMS=differential electrochemical mass spectrometry) and a specially designed experimental procedure. A high oxygen evolution reaction (OER) catalyst loading on carbon suppresses carbon corrosion in alkaline media.
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