Tafel Behavior Research Articles

Room Temperature Fluorination/Defluorination of FeFx Positive Electrode for All-Solid-State Fluoride Ion Batteries

IntroductionA fluoride battery is a new concept battery that uses fluoride ions as charge carriers. It is attracting attention as an innovative battery that surpasses lithium-ion batteries due to its high theoretical energy density. Furthermore, by making an all-solid-state battery using a solid electrolyte, it is possible to construct a kinetically advantageous battery. Iron fluoride (FeF3) is a positive electrode material with a high theoretical capacity of 712 mAh g-1. A charge/discharge capacity exceeding 500 mAh g-1 at 160 °C has been reported for a composite electrode using FeF3 powder.1 The main future challenge for FeFx positive electrode is to improve charge/discharge characteristics at low temperatures and to clarify the fluorination/defluorination mechanism. However, the currently available solid electrolytes have low conductivity, causing an increase in ohmic polarization of the composite electrode and making it difficult to evaluate the FeFx positive electrode at low temperatures. In this study, FeFx thin film electrodes were fabricated, in which the effect of ohmic polarization can be suppressed by reducing the current. Using the thin film samples, the charge/discharge characteristics and electrode reaction mechanism of FeFx positive electrodes at room temperature were investigated in detail.ExperimentalOn a LaF3 single crystal substrate (solid electrolyte), a 10 nm thick FeF3 layer and C layer were sequentially deposited as a positive electrode and a current collector, respectively, by sputtering. A PbF2 layer and a Pb foil were laminated on the opposite side of the substrate as a counter electrode. The fabricated FeFx all-solid-state thin film battery was placed in a cell that could be evacuated and heated, and then charged/discharged at the desired temperature and current rate (1C was defined 712 mAh g-1, the theoretical capacity of FeF3). The discharge/charge cutoff-voltage was set to -1.5/1.5 V or -2/3 V (vs. PbF2/Pb). The FeFx electrode reaction was electrochemically analyzed by steady state polarization and AC impedance measurements. The FeFx thin film structure was investigated by SEM and XPS. The electronic state, coordination structure, and crystal structure of the FeFx during charging/discharging were analyzed by operando XAFS/XRD using synchrotron radiation (SPring-8/BL28XU).Result and DiscussionThe experimental results are summarized below.(1) The reversible capacities at 25 °C and 0.1C were 380 and 629 mAh g-1 (53% and 88% of the theoretical capacity) for the voltage ranges of -1.5/1.5 V and -2/3 V, respectively. No capacity degradation was observed up to 30 cycles in the cycle test at 25 °C and 0.1C.(2) The electrode reaction current was proportional to the exponent of the overpotential. This relationship is known as Tafel behavior and indicates that the fluorination/defluorination of the FeFx positive electrode is rate-limited by the charge transfer process.(3) During discharging/charging, reversible changes in Fe valences (Fe3+, Fe2+, Fe0), atomic bonds (Fe–F, Fe–Fe), and crystal structure (FeF3, FeF2, Fe) were observed. The reaction capacity determined from the change in average Fe valence and the reversible capacity measured electrochemically were almost equal.(4) These results indicate that the FeFx positive electrode can be charged/discharged (fluorinated/defluorinated) at room temperature with high capacities close to the theoretical value, at practical current rates, and with excellent cyclability.AcknowledgementThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under RISING3 project (JPNP21006), Japan.

Elucidating the increased ohmic resistances in zero-gap alkaline water electrolysis

This study investigates the increased ohmic resistances observed in zero-gap alkaline water electrolyzers, aiming to provide insights that can help enhance electrolyzer efficiency and enable operation at higher current densities. Electrochemical impedance spectroscopy (EIS) has been employed in combination with chronopotentiometry, utilizing a custom-designed flow cell with nickel perforated electrodes and a Zirfon UTP 500 diaphragm. Observed differences in area-ohmic resistance values obtained through I-V fitting and EIS, are ascribed to a non-linear Tafel slope at higher current densities. Ohmic resistance values measured with EIS are up to 27% higher than the ex-situ determined value, a significantly smaller percentage than expected based on previous studies. The presence of bubbles outside and inside the diaphragm is identified as the key factor contributing to this increased resistance. We recommend the use of an improved fitting approach, accounting for non-linear Tafel behavior, and the use of a 4-terminal configuration when performing EIS measurements to minimize cable and contact resistance.

Impact of Li-ions and CO2 on the electrochemical efficiency of Al and Al-4%Sn in KOH electrolyte for alkaline batteries application

This study investigates the electrochemical behavior of aluminum (Al) and aluminum‑tin (Al-Sn) alloy as anodes in aluminum-air (Al-air) batteries in a potassium hydroxide (KOH) electrolyte with varying concentrations of lithium hydroxide (LiOH), in both the presence and absence of carbon dioxide gas (CO2). Experimental techniques such as potentiodynamic polarization (Tafel behavior), galvanostatic charging-discharging processes, and electrochemical impedance spectroscopy (EIS) were employed. Characterization techniques including X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) were utilized to analyze the surface products of each electrode. The findings indicate a significant decrease in the corrosion current density (icorr.) for both two electrodes with increasing LiOH concentration, however, Al-Sn exhibits lower values than those of commercial Al. Moreover, the presence of Li-ions revealed an interesting phenomenon of Al, where icorr. gradually decreased with temperature rise. Also, in the presence of CO2, icorr. significantly decreased for both two electrodes, particularly Al-Sn, indicating complete repression of the corrosion process on the surface. The corrosion rate of the studied electrodes in basic solutions, with and without CO2, follows the order: KOH > KOH + LiOH > KOH + LiOH + CO2. Galvanostatic charge-discharge measurements demonstrated higher capacity, improved electrochemical performance, and excellent charge-discharge behavior in the presence of LiOH. The electrochemical findings align well with the results from XRD, SEM, and EDX analysis.

Elucidating the Influence of Intercalated Anions in NiFe LDH on the Electrocatalytic Behavior of OER: A Kinetic Study

AbstractThe oxygen evolution reaction (OER) as one half‐cell reaction of electrochemical water splitting has a fundamental impact on water splitting efficiency and thus on the competitiveness of electrochemically generated hydrogen in the energy market. Nickel‐iron layered double hydroxides (NiFe LDH) are among the most promising electrocatalysts for efficient OER under alkaline conditions. Despite intensive research, correlations of the material properties and the resulting kinetically limiting surface processes are poorly investigated. This work focuses on the kinetic behavior of NiFe LDH catalysts containing different anions in the basal spacing in alkaline OER. Steady‐state Tafel plots, impedance measurements as well as reaction order plots were used to elucidate differences in the catalytic performance. All catalysts showed a dual Tafel behavior and fractional reaction orders. For kinetic modelling, the physisorbed hydrogen peroxide mechanism and Temkin adsorption model were adopted to fit experimental data. Our study showed that the intercalated anions affect the kinetics of rate determining steps. The hypophosphite intercalated LDH possessed the highest OER activity and the first step as rate determining. While for both carbonate and borate intercalated NiFe LDH, the second step proved to be rate determining in the low Tafel region, while the first step was found to be rate‐limiting in the high Tafel region.

Comparative study on the corrosion behaviour of CNT-reinforced NiAl alloys in NaCl and Na2SO4 solutions

Various electrochemical techniques were used to study the corrosion behaviour of the CNT-reinforced NiAl alloys under NaCl and Na2SO4 environments. The potentiodynamic polarization curves revealed activation Tafel behaviour when the NiAl alloy specimens were immersed in NaCl solution while passivation followed by slight development of transpassivation resulted under Na2SO4 environment. The reinforcement of pure NiAl alloy with up to 1.0 wt.% CNT increased the corrosion rate from 0.10 to 0.63 mm/yr under NaCl environment, while under Na2SO4 environment, the corrosion rate increased from 0.04 to 0.12 mm/yr. The observed increase in corrosion rate with an increase in CNT reinforcement (under both NaCl and Na2SO4 environments) suggested that the reinforcement of NiAl alloys with CNT reduced the corrosion resistance of NiAl. The X-ray diffraction analysis revealed that the corrosion products consisted of oxides including Al2O3 under NaCl environment, while scanning electron microscope analysis showed a porous passive layer on the surface of the alloy specimens immersed in Na2SO4 solution.Graphical abstract

Use of Polynomial Approximation in the Electrochemical Frequency Modulation Technique for Determination of Corrosion Rates in Activation-Controlled Systems

The electrochemical frequency modulation (EFM) is a promising nondestructive technique that uses a small biharmonic disturbance signal along with the discrete Fourier transform to calculate the corrosion current and Tafel slopes from a set of equations that rely on harmonic components. Significant results have been published mainly on systems exhibiting Tafel behavior. This work presents the use of polynomial approximation as a data analysis alternative for the EFM technique, which has three main advantages over the procedure proposed by the authors of the EFM technique: (a) reduction of the estimation error caused by the capacitance effect, (b) reduction of the estimation error caused by high harmonic components, and (c) the reduction of computational complexity. This analysis was tested experimentally using a traditional three-electrode cell with a carbon steel working electrode and a sodium chloride solution as the electrolyte. Finally, the results were compared to linear polarization tests and weight loss measurements, where a good agreement was found between the proposed analysis and the other techniques.

Pitting Propagation Behavior on Low Alloy AISI 4130 (UNS G41300) Steel Exposed to Various Alkaline Earth Metal Chlorides Using the 1-D Pit Method

This study examined pit propagation to elucidate whether alkali and alkaline earth metal chloride saltssuch as RbCl affect pitting behavior in distinctly different manner than NaCl. Pit propagation studies were conducted on a low alloy steel using one-dimensional (1-D) pit method over pit depths from 300-1000 µm. LSV and EIS of planar electrodes of 4130 in a range of Cl- solutions were conducted and revealed no differences in impedance, open circuit, corrosion potential (Ecorr), passive current density (ipass), and pitting potential (Epit) as a function of salt type. In the case of one-dimensional pits during fast downward scan rates, the saturation potential (Esat) varied as a function of salt type and pit depth when pits were shallow. Mass transported limited current density also differed with salt type in shallow pits when other alkali metal and alkaline metal cations where present. The pit surface potential (Esurf) of activated pit surfaces reached Ecorrprior to establishing a condition where the pit electrolyte surface concentration (Csurf) was less than the critical concentration for active acidified pitting (Csurf<Ccrit) in this marginally passivating steel. For various Esurf and pit current density (ipit) combinations at constant Csurf where Ccrit< Csurf < Csat, E-log(i) plots were constructed using the method of Li, Tianshu to create IR ohmic voltage corrected Tafel plots at fixed pit solution concentrations. Under these conditions, the influence of salt identity on charge transfer controlled kinetics was reexamined and slight differences in Tafel behavior were found. Differences in metal cations exert no effect on passive planar electrodes and only effect pit propagation stage in shallow pits.

A Perspective on the Repassivation Potential and a Novel Calculation Thereof

Corrosion can occur on metallic surfaces in nearly any humid environment, leading to either early failure or costly repairs. Specifically, localized corrosion, such as pitting, can be difficult to identify at an early stage and can act as crack nucleation sites. Determining under which conditions pitting will take place is therefore important for maintenance and design, yet non-trivial from an electrochemical perspective.The pitting potential (Epit) and repassivation potential (Erp) have long been used in literature to determine a material’s susceptibility to pitting, with the latter acting as the lower threshold at which pitting can occur. A lower threshold of pitting can often be useful for conservative estimates, making Erp a popular parameter. However, a consensus on how to measure and define Erp has yet to be defined. Conventional cyclic potentiodynamic polarization (CPP) scans to measure Erp are shrouded in controversy, as values were found to be dependent on the scan rate and turn-around current density, unless sufficient charge densities had passed. Even once a plateau of Erp is reached at sufficient charge densities, there is debate as to if values are too conservative at that point. Two recent pitting frameworks have proposed alternative measurements of Erp, using both 1-D pit and 3-D bulk electrodes. Breakthroughs and limitations from both of these theories will be considered.In this work, the various methods of measuring and defining Erp will be discussed to provide clarity between different notation and to highlight possible paths going forward. In addition, an alternative calculation of Erp is proposed which is less conservative yet harmonized with the critical pit stability product (ix)crit, as both thresholds should represent the same lower limit of pitting. The proposed equation to calculate Erp includes dependencies on conventional parameters, such as the anodic Tafel behavior within a pit (ba), the transition potential (ET), and the critical percent saturation of metal salts (f). Validation is provided through both results found in literature and the use of Finite Element Method (FEM) modeling in conjunction with the maximum pit model.

Pitting Propagation Behavior on Low Alloy AISI 4130 (UNS G41300) Steel Exposed to Various Alkali and Alkaline Earth Metal Chlorides

This study examined pit propagation to elucidate whether alkali and alkaline earth metal chloride salts such as RbCl affect pitting in some manner previously not expected compared to NaCl. Pit propagation studies were conducted on low alloy steel using one-dimensional (1D) pit method over pit depths from 300 µm to 1,000 µm. Linear sweep voltammetry and electrochemical impedance spectroscopy on planar 4130 electrodes over a range of Cl− concentrations revealed no differences in impedance, open circuit, corrosion potential (Ecorr), passive current density (ipass), and pitting potential (Epit) as a function of salt type. In the case of 1D pits evaluated during fast downward scan rates, the saturation potential (Esat) varied as a function of salt type and at shallow pit depths. Mass transported limited current density also varied with salt type in shallow pits when other alkali metal and alkaline metal cations where present. The potential (Esurf) of activated pit surfaces reached Ecorr prior to establishing a condition where the pit electrolyte surface concentration (Csurf) was less than the critical concentration for active acidified pitting (Csurf &amp;lt; Ccrit) in this marginally passivating steel. For various Esurf and pit current density (ipit) combinations at constant Csurf where Ccrit &amp;lt; Csurf &amp;lt; Csat, E-log(i) plots were constructed using the method of Li Tianshu to unmask IR ohmic voltage corrected Tafel plots at fixed pit solution concentrations. Under these conditions, the influence of salt identity on charge-transfer-controlled kinetics was re-examined and slight differences in Tafel behavior were found. Differences in metal cations have little effect on passive planar electrodes and only affect pit propagation stage in shallow pits.

Correlative operando microscopy of oxygen evolution electrocatalysts

Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1-3. The properties of these materials evolve dynamically and heterogeneously4 with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation5. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline β-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an α-CoO2H1.5·0.5H2O-like structure-produced through hydroxide intercalation-in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted β-CoOOH particles that contain Co3+ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co3+ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity.

Effect of pressure and temperature on carbon dioxide reduction at a plasmonically active silver cathode

Carbon dioxide (CO2) reduction at a plasmonically active silver cathode was investigated by varying the pressure and temperature at multiple applied potentials under both dark and illuminated conditions to understand the mechanism of selectivity changes driven by plasmon-enhanced electrochemical conversion. CO2 partial pressures (PCO2) from 0.2 to 1 atm were studied during linear sweep voltammetry and chronoamperometry at −0.7, −0.9, and −1.1 VRHE. At a given applied overpotential the total current density increased with increasing PCO2 in both the dark and the light, but there were significant differences in the Tafel behavior between dark and illuminated conditions. The reduction of CO2 to carbon monoxide (CO) was found to have first-order behavior with respect to PCO2 at all applied potentials in both the dark and the light, likely indicating no change in the rate-determining step upon illumination. Activity for the hydrogen (H2) evolution reaction decreased with increasing PCO2 at slightly different rates in the dark and the light at each applied potential, making it unclear if light is influencing CO or H2 intermediate adsorbate coverage. Both formate and methanol production showed no dependence on PCO2 under any conditions, but the true reaction orders may be masked by the much higher activity for CO and H2 at the silver cathode. The investigation of product distribution with temperature at 14, 22, and 32∘C at −0.7, −0.9, and −1.1 VRHE in both the dark and the light demonstrated that the selectivity changes observed upon illumination are not caused by local heating of the cathode surface.

Electrochemical and Raman analysis of the corrosion products formed over hot dip galvanised steel wires exposed in different environmental sites

ABSTRACT In this work, hot-dip galvanised steel wires were exposed in Belgium and Indonesia for 4.5 years and correlation of corrosion products formed on the exposed wires with their corrosion properties was studied. Corrosion products were identified with local Raman analysis and correlated with corrosion activity based on odd random phase electrochemical impedance spectroscopy and Tafel behaviour. Raman spectroscopy results confirmed that corrosion products formed in both atmospheric conditions differed either in quantity and/or in chemical nature, which indicated that ‘aged in Belgium’ and ‘aged in Indonesia’ wires did not have similar surface compositions. The comparison of the instantaneous corrosion behaviour of pristine and aged wires based on corrosion currents (polarisation curve) and impedance values in 0.1 M NaCl and 0.1 M Na2SO4 declared that aged in Belgium (AIB) and aged in Indonesia (AII) wires exhibited different corrosion behaviour, which confirmed that corrosion products formed in the environments of the exposed wires were dominating their corrosion behaviour.

Interpreting Tafel behavior of consecutive electrochemical reactions through combined thermodynamic and steady state microkinetic approaches

A combined DFT and microkinetic model is developed to interpret the experimental oxygen evolution reaction Tafel behavior on CoOx(OH)2−x electrocatalysts.

Reversible Decay of Oxygen Evolution Activity of Iridium Catalysts

Iridium has long been recognized as one of the best oxygen evolution catalysts in terms of activity and stability over a wide range of pH. Despite exhibiting initially high activity for the oxygen evolution reaction (OER), a rapid and reversible activity decay is observed during continuous operation. The potential dependence of recovery of the activity is explored, and 0.0 V vs NHE is found to be an effective potential to recover the initial water oxidation performance. Iridium thin films on rotating disk electrodes are used to show that this OER activity decay is neither explained by progressive oxidation of the iridium nor by reduced mass transport to/from the electrode surface. Careful examination of the time dependence of the activity decay reveals that it is well described by a t−1/4 functional dependence across multiple electrochemical cell geometries. Tafel behavior is analyzed by normal pulse voltammetry, suggesting that, after 10 minutes of activity decay, the catalyst exhibits a five-fold decrease in active site density while the mechanism of water oxidation is not altered. We hypothesize that this decay may result from a loss of active sites capable of forming the Ir(V) = O species, possibly via progressive cross-linking of iridium sites by bridging mu-oxides.

Electrochemical Kinetic Modeling for the Conversion of Carbon Dioxide to Carbon Monoxide on Gold Surfaces

Recent advances in catalyst materials1 and reactor configurations2 for the electrochemical reduction of carbon dioxide (CO2) have led to significant increases in current efficiency and product throughput, but further improvements are necessary for process viability. In particular, understanding and controlling reaction mechanisms and kinetic rates are critical for balancing voltage efficiency and current density. To this end, the ubiquitous Butler-Volmer (BV) kinetic model, from which Tafel analysis is derived, has been used extensively to evaluate current-potential relationships for CO2 reduction reactions to various products on a range of catalytic surfaces. Though relatively easy to implement, the BV model has a phenomenological basis that is not as strongly rooted in physics as alternative electrokinetic models, which, in turn, can limit its ability to describe complex reactions that exhibit nonlinear Tafel behavior. Here we investigate the applicability of several different electrokinetic models for describing the electroreduction of CO2 to carbon monoxide on gold (Au) surfaces. Specifically, we compare the BV model to the Marcus-Hush-Chidsey (MHC) model, which has been successfully demonstrated in LiFePO4-based Li-ion batteries3, and a BV model that incorporates a series resistance term (BV+R). These models are applied to kinetic data collected from gas diffusion electrodes coated with Au nanoparticles4 and integrated into an instrumented flow electrolyzer that enables in-situ single electrode analysis coupled with product quantification. The resulting current-potential response, shown in Figure 1a, displays sublinear behavior at high overpotentials and, based on a literature survey, appears characteristic of Au surfaces irrespective of catalyst morphology, reactor configuration, or reactant delivery mode. In this presentation, we seek to describe the underlying causes of this general behavior and to apply appropriate kinetic modeling that accurately represent this electrochemical response (Figure 1b).

The electrochemistry of pyrite in chloride solutions

A detailed study of the anodic and cathodic behaviour of natural pyrite in acidic chloride solutions containing various oxidants has been conducted as part of an overall program on the fundamental aspects of the heap leaching of copper sulphide minerals.The stoichiometry of the anodic dissolution reaction depends on the potential in that it varies from less than 4F/mol Fe dissolved at potentials below 0.8 V(SHE) to 15F/mol Fe at potentials above about 1.0 V(SHE).The mixed potentials of pyrite in chloride solutions containing iron(III) are greater than those in the presence of copper(II) and both increase with agitation as a result of enhanced transport of iron(II) and copper(I) from the surface of the dissolving mineral. The mixed potentials are unaffected by the presence of dissolved oxygen confirming the low reactivity for the cathodic reduction of oxygen.The rate of anodic dissolution of pyrite in chloride solutions is independent of the acid and chloride concentration except at high chloride concentrations when the rate decreases slightly. The potential dependence of the anodic reaction roughly follows Tafel behaviour up to about 1.0 V but mechanistic conclusions are excluded due to the variable stoichiometry.Cathodic reduction of oxygen is some 30 times slower than that of 1 g/L iron(III). The reduction of copper(II) is also less significant in the leaching of pyrite due to the lower formal potential of the copper(II)/copper(I) couple than that of the iron(III)/iron(II) couple in concentrated chloride solutions.Comparative measurements have shown that pyrite will dissolve more slowly than chalcopyrite in chloride solutions and, this coupled to the low sulfate yield at low potentials, suggests that oxidation of pyrite as a source of heat in abiotic heap leaching is unlikely.

The oxygen evolution reaction mechanism at IrxRu1−xO2 powders produced by hydrolysis synthesis

A mechanistic study of the oxygen evolution reaction (OER) has been performed for IrxRu1−xO2, x = 1, 0.6, 0.3 and 0, prepared by the hydrolysis synthesis. The oxides were characterized by X-ray diffraction, cyclic voltammetry and steady state polarization measurements. The electrolyte pH was varied in order to study the reaction order with respect to protons. The polarization curves recorded could be well fitted to a model consisting of a series of concerted electron-proton transfer reactions (mononuclear mechanism) with either of the second, third, or fourth step being rate determining. The expected trends for this mechanism with respect to potential and pH were observed in the experimental data and are consistent with DFT results for the adsorption energies of the adsorbates [Rossmeisl et al., J. Electroanal. Chem.607 (2007) 83–89] if the third or fourth step is rate-determining for RuO2 and IrO2, respectively.The fitting procedures also demonstrate the advantages of working with the full current-voltage expression when analyzing polarization curves, since Tafel behavior may only prevail in a limited potential region.

Kinetics of iron electrochemical reduction into liquid metal at 1823 K in a molten oxide electrolyte

The goal of this study was the measurement of the kinetic parameters of the electrochemical reduction of iron oxide into liquid iron metal in a molten oxide electrolyte, Al2O3-FeOx-MgO-SiO2, at 1823 K. A large surface anode was used to circumvent the need of a reference electrode and a stepped linear scan voltammetry was applied to an iron-bearing silicate electrolyte over a cell-voltage range of 0.8 V to 1.9 V. In order to extract the kinetic parameters the current response is treated concerning the electronic charge transfer and the ohmic drop due to the electrolyte in the electrochemical cell. The cathode half-reaction identified by this method is the reduction of ferrous iron into its metal state. Derived parameters imply that electrode kinetics follow Tafel behavior. An order of reaction close to unity with transfer coefficients between 0.54 and 0.66 is identified.

Intriguing Catalytic Activity of Surface Active Gold‐Platinum Islands on Nano‐Porous Au in Determining Efficient Direct Formic Acid Oxidation Pathway

AbstractGold‐platinum nano‐porous structures (Au100‐xPtx NPoS) with unique surface enriched Au−Pt islands are synthesised through a galvanic replacement strategy from Au100‐xAg2x nano‐alloys with ultra‐low platinum loading (x=1.25, 2.5 and 5). Formic acid oxidation reaction (FAOR) on Au100‐xPtx/C NPoS shows a distinct peak at around 0.5 V related to CO2 formation. A characteristic peak at around 1.5 V increases with increasing FA concentration owing to the direct FAOR by nano‐porous Au centers. FAOR peaks and If/Ib peak current ratios indicates that, Au100‐xPtx/C NPoS facilitates direct FAOR pathway unlike HiSPEC Pt/C (dehydration pathway). Based on the enhanced chrono‐amperometric response, Tafel behaviour and mass activity values, Au97.5Pt2.5/C NPoS (5.5 A/mgPt) reflects as the optimal catalyst for efficient FAOR. The intriguing catalytic role of surface enriched Au−Pt islands present on nano‐porous Au greatly helps in determining the superior FAOR activity in Au97.5Pt2.5/C NPoS towards direct dehydrogenation pathway at ultra‐low Pt content compared to other NPoS and HiSPEC Pt/C catalysts.

Investigating the Role of Fe in Transition-Metal (Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

The efficiency of water electrolysis for hydrogen production is limited in part by the slow kinetics of the oxygen evolution reaction (OER).1,2 Nickel-iron and cobalt-iron (oxy)hydroxides have been shown to be the most active, low-cost electrocatalysts for the OER in alkaline conditions.3–5 Although it is evident that Fe is essential for high activity,6,7 its role in the catalytic active site is still largely debated.8,9 We investigate the role of Fe in NiOOH by comparing the effects of Ti, Mn, La, and Ce incorporation on the OER activity and electrochemical behavior of NiOOH in Fe-free 1 M KOH.10 We show that other metal cations in NiOOH can increase its reduction potential in a similar way to Fe, but without the same orders of magnitude increase in OER activity. This suggests that Ni is not likely the active site as its electronic properties are not correlated to activity. We also evaluate the OER activity and Tafel behavior of Fe3+ impurities on different noble metal substrates. We find that the activity varies by substrate suggesting that the local atomic and electronic structure of [FeO6] units play an important role in catalysis of the OER as it can be tuned by substrate interactions. Finally, through in situ and in operando X-ray absorption spectroscopy experiments, we find that the local structure around Fe atoms in Co(Fe)OOH changes during OER while that of Co stays the same. Given that the OER activity on a per Fe basis is independent of [Fe] in Co(Fe)OOH, the potential dependent structural changes further indicate that Fe is essential to the catalytic active site for the OER on transition-metal (oxy)hydroxides. References (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110(11), 6446. (2) Ursua, A.; Gandia, L. M.; Sanchis, P. Proc. IEEE 2012, 100(2), 410. (3) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. J. Am. Chem. Soc. 2012, 134(41), 17253. (4) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W. J. Phys. Chem. Lett. 2015, 6(18), 3737. (5) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Chem. Mater. 2015, 27(22), 7549. (6) Corrigan, D. A. J. Electrochem. Soc. 1987, 134(2), 377. (7) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. J. Am. Chem. Soc. 2014, 136(18), 6744. (8) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Nørskov, J. K.; Nilsson, A.; Bell, A. T. J. Am. Chem. Soc. 2015, 137(3), 1305. (9) Chen, J. Y. C.; Dang, L.; Liang, H.; Bi, W.; Gerken, J. B.; Jin, S.; Alp, E. E.; Stahl, S. S. J. Am. Chem. Soc. 2015, 137(48), 15090. (10) Enman, L. J.; Burke, M. S.; Batchellor, A. S.; Boettcher, S. W. ACS Catal. 2016, 6 (4), 2416.