Enhanced Oxygen Reduction Activity by Selective Anion Adsorption on Non-Precious-Metal Catalysts

Recent advances in the role of interfacial liquids in electrochemical reactions

The interfacial liquid, situated in proximity to an electrode or catalyst, plays a vital role in determining the activity and selectivity of crucial electrochemical reactions, including hydrogen evolution, oxygen evolution/reduction, and carbon dioxide reduction. Thus, there has been a growing interest in better understanding the behavior and the catalytic effect of its constituents. This minireview examines the impact of interfacial liquids on electrocatalysis, specifically the effects of water molecules and ionic species present at the interface. How the structure of interfacial water, distinct from the bulk, can affect charge transfer kinetics and transport of species is presented. Furthermore, how cations and anions (de)stabilize intermediates and transition states, compete for adsorption with reaction species, and act as local environment modifiers including pH and the surrounding solvent structure are described in detail. These effects can promote or inhibit reactions in various ways. This comprehensive exploration provides valuable insights for tailoring interfacial liquids to optimize electrochemical reactions.

Production of Metallic Alloy Nanowires and Particles Templated Using Tomato Mosaic Virus (ToMV)

We demonstrate a simple, low-energy method whereby tomato mosaic virus (ToMV) particles can be used to template the production of nanowires and particles consisting of alloys of gold (Au), platinum (Pt) and palladium (Pd) in various combinations. Selective nanowire growth within the inner channel of the particles was achieved using the polymeric capping agent polyvinylpyrrolidone (PVPK30) and the reducing agent ascorbic acid. The reaction conditions also resulted in the deposition of alloy nanoparticles on the external surface of the rods in addition to the nanowire structures within the internal cavity. The resulting materials were characterized using a variety of electron microscopic and spectroscopic techniques, which revealed both the structural and chemical composition of the alloys within the nanomaterials.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

The electrostatic effect and its role in promoting electrocatalytic reactions by specifically adsorbed anions

The adsorption of anions and its impact on electrocatalytic reactions are fundamental topics in electrocatalysis. Previous studies revealed that adsorbed anions display an overall poisoning effect in most cases. However, for a few reactions such as the hydrogen evolution reaction (HER), oxidation of small organic molecules (SOMs), and reduction of CO₂ and O₂, some specifically adsorbed anions can promote their reaction kinetics under certain conditions. The promotion effect is frequently attributed to the adsorbate induced modification of the nature of the active sites, the change of the adsorption configuration and free energy of the key reactive intermediate which consequently change the activation energy, the pre-exponential factor of the rate determining step etc. In this paper, we will give a mini review of the indispensable role of the classical double layer effect in enhancing the kinetics of electrocatalytic reactions by anion adsorption. The ubiquitous electrostatic interactions change both the potential distribution and the concentration distribution of ionic species across the electric double layer (EDL), which alters the electrochemical driving force and effective concentration of the reactants. The contribution to the overall kinetics is highlighted by taking HER, oxidation of SOMs, reduction of CO₂ and O₂, as examples.

Elucidating Electrocatalytic Oxygen Reduction Kinetics via Intermediates by Time-Dependent Electrochemiluminescence

Facile evaluation of oxygen reduction reaction (ORR) kinetics for electrocatalysts is critical for sustainable fuel-cell development and industrial H₂ O₂ production. Despite great success in ORR studies using mainstream strategies, such as the membrane electrode assembly, rotation electrodes, and advanced surface-sensitive spectroscopy, the time and spatial distribution of reactive oxygen species (ROS) intermediates in the diffusion layer remain unknown. Using time-dependent electrochemiluminescence (Td-ECL), we report an intermediate-oriented method for ORR kinetics analysis. Owing to multiple ultrasensitive stoichiometric reactions between ROS and the ECL emitter, except for electron transfer numbers and rate constants, the potential-dependent time and spatial distribution of ROS were successfully obtained for the first time. Such exclusively uncovered information would guide the development of electrocatalysts for fuel cells and H₂ O₂ production with maximized activity and durability.

Towards the Reduction of Pt Loading in High Temperature Proton Exchange Membrane Fuel Cells - Effect of Fe-N-C in Pt-Alloy Cathodes

Pt poisoning by phosphate in high temperature proton exchange membrane fuel cells (HT-PEMFC) leads to loadings up to 1 mg_Pt  cm^-2 per electrode of costly materials. While cheaper Fe-N-C catalysts are unaffected by phosphate deactivation and contribute to the catalysis of the oxygen reduction reaction, their volumetric activity is substantially lower. In this study, the effect of Pt-loading reduced hybrid cathodes for HT-PEMFC is investigated using commercial Celtec®-P-based assembling. A promising effect of Fe-N-C incorporation in terms of acid attraction and activity retention is found. A longer activation (230 h, 0.3 A cm^-2 ) for the hybrid membrane electrode assembly (MEA) is necessary, due to the slower acid distribution within Fe-N-Cs. This study shows the potential of Pt-content reduction by up to 25 % compared to standard MEA using hybrid electrodes. Moreover, important insights for future strategies of cell activation are revealed for these hybrid MEAs.

Understanding the Activity and Design Principle of Dual-Atom Catalysts Supported on C2N for Oxygen Reduction and Evolution Reactions: From Homonuclear to Heteronuclear

Using large-scale ab initio calculations and taking the two-dimensional C₂N monolayer as a substrate, we sampled a large combinatorial space of C₂N-supported homonuclear and heteronuclear dual-atom catalysts and built a detailed view of catalytic activity and stability toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The results indicate that regulating combinations of metal pairs could widely tune the catalytic performance. Pd₂-, Pt₂-, and PdPt-C₂N could effectively balance the adsorption strength of intermediates and achieve optimal bifunctional activity. The favorable catalytic performance could also be realized on GaPd-C₂N for the ORR and PdRh-C₂N for the OER, surpassing corresponding homonuclear counterparts. The thermodynamic and electrochemical stability simulations reveal that these metal pairs can be stably anchored onto the C₂N matrix. Multiple-level descriptors, including Gibbs free energy, d-band center, and bonding/antibonding orbital population, are established to track the activity trend and reveal the origin of activity, indicating that catalytic activity is intrinsically governed by the d-band center of metal pairs.

Candied Haws-Like Fe-N-C Catalysts with Broadened Carbon Interlayer Spacing for Efficient Zinc-Air Battery

As efficient nonprecious metal catalysts for oxygen reduction reaction (ORR), Fe-N-C materials are one of the most promising alternatives to Pt-based catalysts for fuel cells and metal-air batteries. However, the intrinsically low density of key active sites like FeN₄ moieties hampers their commercial applications. Herein, we provide a smart strategy to construct a candied haws-like Fe-N-C catalyst (CH-FeNC) with broadened carbon interplanar spacing (>4 Å), starting with trehalose as a structure-built brick coupled with a zinc-zeolite imidazole framework (ZIF-8) and polyaniline (PANI) and then followed by copyrolysis carbonization of them. The obtained CH-FeNC exhibits half-wave potentials of 0.92 and 0.90 V (vs RHE) before and after 10,000 cycles in 0.1 M KOH, which are superior to the 0.90 and 0.85 V obtained by commercial Pt/C for ORR. The power density of a homemade zinc-air battery equipped with the catalyst is up to 131 mW cm^-2, greater than that of Pt/C (124 mW cm^-2). The extended X-ray absorption fine structure (EXAFS) results and density functional theory (DFT) theoretical calculations reveal that there exists enriched zigzag or armchair edge-hosted FeN₄ active sites, located at the abundant interface between carbon components in this composite. Furthermore, the unique broadened carbon interlayer spacing plays a key role in deciding the ORR rate in alkaline but not in acidic environments because there exists a fifth ligand of active Fe in the form of FeN₄ centers coupled with SO₄^2- and ClO₄^- from acids.

Identification of Catalytic Active Sites for Durable Proton Exchange Membrane Fuel Cell: Catalytic Degradation and Poisoning Perspectives

Recent progress in synthetic strategies, analysis techniques, and computational modeling assist researchers to develop more active catalysts including metallic clusters to single-atom active sites (SACs). Metal coordinated N-doped carbons (M-N-C) are the most auspicious, with a large number of atomic sites, markedly performing for a series of electrochemical reactions. This perspective sums up the latest innovative and computational comprehension, while giving credit to earlier/pioneering work in carbonaceous assembly materials towards robust electrocatalytic activity for proton exchange membrane fuel cells via inclusive performance assessment of the oxygen reduction reaction (ORR). M-N_x -C_y are exclusively defined active sites for ORR, so there is a unique possibility to intellectually design the relatively new catalysts with much improved activity, selectivity, and durability. Moreover, some SACs structures provide better performance in fuel cells testing with long-term durability. The efforts to understand the connection in SACs based M-N_x -C_y moieties and how these relate to catalytic ORR performance are also conveyed. Owing to comprehensive practical application in the field, this study has covered very encouraging aspects to the current durability status of M-N-C based catalysts for fuel cells followed by degradation mechanisms such as macro-, microdegradation, catalytic poisoning, and future challenges.

Acid anion electrolyte effects on platinum for oxygen and hydrogen electrocatalysis

Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO₄ > HNO₃ > H₂SO₄, and the OER activity trend of HClO₄ [Formula: see text] HNO₃ ∼ H₂SO₄. HER/HOR performance is similar across all three electrolytes. Notably, we demonstrate that ORR performance can be improved 4-fold in nitric acid compared to in sulfuric acid. Assessing the potential-dependent role of relative anion competitive adsorption with density functional theory, we calculate unfavorable adsorption on Pt(111) for all the anions at HER/HOR conditions while under ORR/OER conditions [Formula: see text] binds the weakest followed by [Formula: see text] and [Formula: see text]. Our combined experimental-theoretical work highlights the importance of understanding the role of anions across a large potential range and reveals nitrate-like electrolyte microenvironments as interesting possible sulfonate alternatives to mitigate the catalyst poisoning effects of polymer membranes/ionomers in electrochemical systems. These findings help inform rational design approaches to further enhance catalyst activity via microenvironment engineering.

Desalination Fuel Cells with High Thermodynamic Energy Efficiency

Sustainably-produced hydrogen is currently intensively investigated as an energy carrier to replace fossil fuels. We here characterize an emerging electrochemical cell termed a desalination fuel cell (DFC) that can continuously generate electricity and desalinate water while using hydrogen and oxygen gases as inputs. We investigated two operational modes, a near-neutral pH operation with H₂, O₂, and feedwater inputs (H₂|O₂), and a pH-gradient mode with H₂, O₂, feedwater, acid, and base inputs (H₂ + B|O₂ + A). We show that our cell can desalinate water with 30 g/L of salt content to near-zero salt concentration, while generating an enormous amount of electricity of up to 8.6 kW h per m³ of treated water when operated in the pH-gradient mode and up to about 1 kW h per m³ for the near-neutral mode. We quantify the thermodynamic energy efficiency of our device in both operational modes, showing that significantly higher efficiency is achievable in the pH-gradient mode, with up to 95.6%. Further, we present results elucidating the key bottlenecks in the DFC process, showing that the cell current and voltage are limited in the near-neutral pH operation due to a lack of H⁺ to serve as a reactant, and further reinforce the deleterious effect of halide poisoning on the cathode Pt catalyst and cell open circuit voltage. Such findings demonstrate that new fuel cell catalyst materials, tailored for environments associated with water treatment, can unlock yet-improved performance.

Proton Exchange Membrane Fuel Cells (PEMFCs): Advances and Challenges

The study of the electrochemical catalyst conversion of renewable electricity and carbon oxides into chemical fuels attracts a great deal of attention by different researchers. The main role of this process is in mitigating the worldwide energy crisis through a closed technological carbon cycle, where chemical fuels, such as hydrogen, are stored and reconverted to electricity via electrochemical reaction processes in fuel cells. The scientific community focuses its efforts on the development of high-performance polymeric membranes together with nanomaterials with high catalytic activity and stability in order to reduce the platinum group metal applied as a cathode to build stacks of proton exchange membrane fuel cells (PEMFCs) to work at low and moderate temperatures. The design of new conductive membranes and nanoparticles (NPs) whose morphology directly affects their catalytic properties is of utmost importance. Nanoparticle morphologies, like cubes, octahedrons, icosahedrons, bipyramids, plates, and polyhedrons, among others, are widely studied for catalysis applications. The recent progress around the high catalytic activity has focused on the stabilizing agents and their potential impact on nanomaterial synthesis to induce changes in the morphology of NPs.

Catalytic Role of Adsorption of Electrolyte/Molecules as Functional Ligands on Two-Dimensional TM-N4 Monolayer Catalysts for the Electrocatalytic Nitrogen Reduction Reaction

Two-dimensional single-atom catalysts (2D SACs) have been widely studied on the nitrogen reduction reaction (NRR). The characteristics of 2D catalysts imply that both sides of the monolayer can be catalytic sites and adsorb electrolyte ions or molecules from solutions. Overstrong adsorption of electrolyte ions or molecules on both sides of the catalyst site will poison the catalyst, while the adsorbate on one side of the catalytic site will modify the activity and selectivity of the other side for NRR. Discovering the influence of adsorption of electrolyte ions or molecules as a functional ligand on catalyst performance on the NRR is crucial to improve NRR efficiency. Here, we report this work using the density functional theory (DFT) method to investigate adsorption of electrolyte ions or molecules as a functional ligand. Among all of the studied 18 functional ligands and 3 transition metals (TMs), the results showed that Ru&F, Ru&COOH, and Mo&H₂O combinations were screened as electrocatalysis systems with high activity and selectivity. Particularly, the Mo&H₂O combination possesses the highest activity with a low ΔG_MAX of 0.44 eV through the distal pathway. The superior catalytic performance of the Mo&H₂O combination is mainly attributed to the electron donation from the metal d orbital. Furthermore, the functional ligands can occupy the active sites and block the competing vigorous hydrogen evolution reaction. Our findings offer an effective and practical strategy to design the combination of the catalyst and electrolyte to improve electrocatalytic NRR efficiency.

Selectivity Map for the Late Stages of CO and CO2 Reduction to C2 Species on Copper Electrodes

The electrochemical CO and CO₂ reduction reactions (CORR and CO₂ RR) using copper catalysts and renewable electricity hold promise as a carbon-neutral route to produce commodity chemicals and fuels. However, the exact mechanisms and structure sensitivity of Cu electrodes toward C₂ products are still under debate. Herein, we investigate ethylene oxide reduction (EOR) as a proxy to the late stages of CORR to ethylene, and the results are compared to those of acetaldehyde reduction to ethanol. Density functional theory (DFT) calculations show that ethylene oxide undergoes ring opening before exclusively reducing to ethylene via *OH formation. Based on generalized coordination numbers (CN), a selectivity map for the late stages of CORR and CO₂ RR shows that sites with moderate coordination (5.9 < CN < 7.5) are efficient for ethylene production, with pristine Cu(100) being more active than defective surfaces such as Cu(311). In contrast, kinks and edges are more active for ethanol production, while (111) terraces are relatively inert.

One-step pyrolysis synthesis of nitrogen, manganese-codoped porous carbon encapsulated cobalt-iron nanoparticles with superior catalytic activity for oxygen reduction reaction

Replacing precious metal catalysts with low-price and abundant catalysts is one of urgent goals for green and sustainable energy development. It is imperative yet challenging to search low-cost, high-efficiency, and long-durability electrocatalysts for oxygen reduction reaction (ORR) in energy conversion devices. Herein, three-dimensional low-cost Co₃Fe₇ nanoparticles/nitrogen, manganese-codoped porous carbon (Co₃Fe₇/N, Mn-PC) was synthesized with the mixture of dicyandiamide, cobalt (II) tetramethoxyphenylporphyrin (Co(II)TMOPP), hemin, and manganese acetate by one-step pyrolysis and then acid etching. The resultant Co₃Fe₇/N, Mn-PC exhibited excellent durability and prominent ORR activity with more positive onset potential (E_onset, 0.98 V) and half-wave potential (E_1/2, 0.87 V) in 0.1 M KOH electrolyte, coupled with strong methanol resistance. The pyrolysis temperature and optimal balance of graphite with pyridine-nitrogen are of significance for the ORR performance. The prepared Co₃Fe₇/N, Mn-PC displayed excellent ORR performance over commercial Pt/C in the identical environment. It was ascribed to the uniform 3D architecture, Mn- and N-doping effects by finely adjusting the electronic structures, coupled with the synergistic catalytic effects of multi-compositions and multi-active sites. This work provides some constructive guidelines for preparation of low-cost and high-efficiency ORR electrocatalysts.

Improving the Activity of M-N4 Catalysts for the Oxygen Reduction Reaction by Electrolyte Adsorption

Metal and nitrogen codoped carbons (M-N/Cs) have emerged as promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). DFT calculations are used to investigate the adsorption of anions and impurities from the electrolyte on the active site, modeled as an M-N₄ motif embedded in a planar carbon sheet (M=Cr, Mn, Fe, Co). The two-dimensional catalyst structure implies that each metal atom has two potential active sites, one on each side of the sheet. Adsorption of anions or impurities on both sites results in poisoning, but adsorption on one of the sites leads to a modified ORR activity on the remaining site. The calculated adsorption energies show that a number of species adsorb only on one of the two sites under realistic experimental conditions. Furthermore, a few of these adsorbates modify the adsorption energies of the ORR intermediates on the remaining site, in such a way that the limiting potential is improved.

Co(ii)Tetraphenyltetraphenanthroporphyrin@MWCNTs: enhanced π–π interaction for robust electrochemical catalysis

A cobalt(ii)tetraphenyltetraphenanthroporphyrin with phenanthrene-fused pyrrole rings was applied for robust HER and ORR.