Hierarchical Electrocatalyst Research Articles

Rational design of ultrahigh porosity Co foam supported flower-like FeNiP-LDH electrocatalysts towards hydrogen evolution reaction

The development of high-efficiency, stable and low-cost electrocatalysts is a matter of cardinal significance for large-scale electrolytic hydrogen production from water. In this study, we report a hierarchical electrocatalyst of flower-like FeNiP-LDH (FeNiP on layered double hydroxide) loaded on ultrahigh porosity Co foam (CF). The structure/component superiorities and hydrogen evolution reaction (HER) performance of this electrode were examined in detail. In alkaline solution, the resulting FeNiP-LDH/CF yields a current density of 10 mA/cm2 at an overpotential of − 39 mV, which is superior than most documented transition metal phosphides electrocatalysts and even Pt catalyst (~ −53 mV). In particular, this electrode with an undamaged microstructure can maintain its HER activity over 16 h at high current density of 500–600 mA/cm2. Such remarkable HER performance originates from the satisfactory porous nature of Co foam as well as the special surface structure and electronic properties of phosphide/hydroxide. This work not only offers a viable modular approach for the synthesis of high-performance HER electrocatalysts, but also allows an in-depth understanding of structure-activity relationships of multistage 3D materials for energy and catalysis application.

Toward High-Performance and Durable Hierarchical “Core-Shell” Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

Proton exchange membrane fuel cells (PEMFCs) are one of the cornerstones of the “hydrogen economy” and are expected to play a pivotal role in today’s worldwide efforts to decarbonize the energy sector. The redox processes taking place at the electrodes of a PEMFC require suitable electrocatalysts (ECs) to minimize the associated overpotentials and achieve the exceptional energy conversion efficiency that characterizes these devices. The largest source of overpotential in direct hydrogen PEMFCs is the oxygen reduction reaction (ORR). State-of-the-art ECs for the ORR still require a significant loading of platinum-group metals, PGMs, to achieve a performance and durability level matching the requirements of practical applications. Hence, the development of advanced, highly active and durable ORR ECs with a minimized loading of PGMs is a major objective of the research.Several approaches have been proposed to obtain ORR ECs beyond the state of the art in terms of improved kinetics and longer durability [1]. However, the corresponding synthetic routes are complex, intrinsically difficult to upscale, and thus liable not to be of high relevance for industry in light of the forthcoming large-scale rollout of PEMFCs in both stationary and automotive applications.A promising avenue to prepare easily large amounts of ORR ECs exhibiting a performance and durability beyond the state of the art has been proposed in our laboratory more than 15 years ago [2]. The procedure is extremely flexible, allowing to tailor crucial EC features such as the chemical composition of the active sites and the morphology. The best ORR ECs exhibit a “core-shell” morphology: a hierarchical carbon-based “core” is covered by a carbon nitride “shell” stabilizing the active sites in C- and N-based “coordination nests”.A critical step in the preparation of ECs by means of the proposed route is the last “activation” yielding the final product. This process reconfigures significantly the most relevant features of the material including the bulk composition, the surface chemistry, the morphology and the structure. This “physicochemical metamorphosis” has a crucial effect on the ORR performance and durability, which are raised dramatically and finally exceed significantly the figures exhibited by state-of-the-art ECs [3].This work elucidates the interplay between the various approaches pursued to achieve the last “activation” (e.g., chemical attack and/or application of a time-dependent electrochemical potential) and the corresponding evolution of the physicochemical properties of the ECs. The general fundamental mechanisms underlying the “physicochemical metamorphosis” are discussed. Lastly, their outcome on the electrochemical properties of the final products are discussed to identify the most promising avenues for the design of ORR ECs able to surpass the performance and durability levels of today’s state of the art. Acknowledgements The research leading to the results reported in this work has received funding from: (a) the European Union’s Horizon 2020 research and innoavation programme under grant agreement 881603; (b) the project ‘Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells’ funded by EIT Raw Materials; and (c) the project ‘Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells e HELPER’ funded by the University of Padova.

(Invited) Interplay between Surface/Porosimetric, Chemical and Electrochemical Characterization of “Core-Shell” High-Pt ORR Electrocatalysts

A critical bottleneck of PEMFC operation is the performance of the cathode electrode, where the oxygen reduction reaction (ORR) takes place. Two of the most important phenomena that conspire to curtail the cathode performance are: (i) the sluggish kinetics of the oxygen reduction reaction (ORR) [1]; and (ii) the mass transport issues. In particular, the latter become progressively more evident as the current density of the PEMFC is increased upon approaching operating conditions that are relevant for practical applications [2].Both of these shortcomings are addressed by developing new ORR electrocatalysts (ECs). The ECs described here exhibit a “core-shell” morphology: a hierarchical graphene-based support (H-GR) “core” is covered by a carbon nitride “shell” stabilizing the active sites in “coordination nests” [3]. The “core” comprises highly defected graphene nanoplatelets [4] and carbon black nanoparticles; the latter act as spacers and facilitate the charge and mass transport phenomena occurring during the EC operation. The ECs proposed here are characterized by a significant loading of platinum (on the order of ca. 20-40 wt%), that is the “active metal”. Ni and Cu are introduced as the “co-catalysts” to further boost the ORR kinetics of the ECs [3]. The high loading of “active metal” and “co-catalyst” in the ECs is meant to minimize the bulk of the support, facilitating mass transport phenomena.The innovative support of the proposed ECs is very different from that typically adopted in the state of the art in terms of morphology and especially porosity. Thus, this latter feature is to be studied in detail to fully exploit the potential of such support and guide the synthesis of new ECs beyond today’s state of the art. This information is achieved by a comprehensive characterization of the proposed ECs by a variety of porosimetric methods involving the adsorption of suitable probe molecules.Physisorption of N2 is carried out to evaluate the specific surface area of the ECs, together with their micro- and mesoporosity. Water adsorption is executed as well, being a crucial tool to elucidate the hydrophobicity of the ECs. Indeed, the latter feature is critical to prevent electrode flooding during PEMFC operation. Furthermore, through the study of the real ECs’ density a new parameter is proposed, namely the surface area per volume “Σ” (expressed as m2/cc). This latter is of paramount importance to design efficient membrane electrode assemblies (MEAs).As a final step, the results of morphologic/porosimetric studies are correlated with: (i) the outcome of both bulk and surface chemical characterizations carried out by CHNS microanalysis, energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS); and (ii) the “ex-situ” intrinsic ORR performance and selectivity in the 4-electron mechanism determined by cyclic voltammetry with the thin-film rotating ring-disk method (CV-TF-RRDE). Acknowledgements The research leading to the results reported in this work has received funding from: (a) the European Union’s Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; and (c) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells e HELPER” funded by the University of Padova.

(Invited) How to Expand the Scope of Cyclic Voltammetry with the Thin-Film Rotating (Ring) Disk Electrode to Investigate Oxygen Reduction Reaction Electrocatalysts

The performance of electrochemical energy conversion and storage (EECS) devices is modulated by a cascade of phenomena that include: (i) the redox processes taking place at the electrodes; (ii) the transport of charge, with a particular reference to the migration of ions through the electrolyte; and (iii) the transport of mass involved in the provision of reactants/removal of products to/from the electrodes. These phenomena are intimately correlated with one another and with all the different features of the functional materials adopted in the fabrication of the specific EECS device. Thus, it is often very complex to unravel the different contributions to overall system performance. In addition, it is not practical to pursue a purely “trial-and-error” approach where all the possible combinations of functional materials are used to fabricate and test full-fledged EECS prototypes, as this would entail too massive an investment in terms of both resources and time.For this reason, there is a great need to implement experimental approaches able to elucidate the details of specific electrochemical features (e.g., reaction kinetics) of the various functional materials used in EECS devices, at the same time minimizing the cloaking effects due to: (i) the formation of complex interfaces with other components of the device; and (ii) spatial and temporal inhomogeneity in the motion of both charged and neutral species through the system. The resulting information is critical in the development of advanced functional materials bestowing to the EECS device a performance and durability beyond the state of the art.Perhaps the most successful and widely adopted of such approaches is cyclic voltammetry conducted with a thin-film rotating disk electrode or ring-disk electrode (CV-TF-R(R)DE) [1]. CV-TF-R(R)DE is typically used to screen quickly the performance and the details of the reaction mechanism of electrocatalysts (ECs) for the oxygen reduction reaction (ORR). The latter process is widely exploited in a broad variety of EECS devices including fuel cells (FCs) and metal-air batteries. The ORR is particularly critical since it often bottlenecks the operation of the entire system. Even though the CV-TF-R(R)DE method is very popular and broadly used in the scientific community, most researchers do not use all its remarkable capabilities.Here it is shown how to implement both innovative and more traditional (but no longer widespread) approaches to exploit CV-TF-R(R)DE to its full potential, with the purpose to study the most relevant electrochemical features of ORR ECs and quickly identify the most promising candidates for application in FCs or other electrochemical devices [2]. Particular emphasis is devoted to analysis techniques meant to study in detail the ORR kinetic features of vastly different ECs, allowing for quantitative performance comparisons at the same high level of accuracy without risking serious distortions due to an excessive impact of corrections during data analysis. It is also shown how to adopt the CV-TF-R(R)DE approach to understand quantitatively whether an ORR EC exhibits improved morphology/mass transport features in comparison with a Pt/C benchmark. In summary, it is elucidated how to expand significantly the scope of CV-TF-R(R)DE studies with respect to the level typically achieved in the state of the art, taking into consideration both kinetic and morphology/mass transport features by means of a simple morphokinetic (MK) correlation map. Acknowledgements The research leading to the results reported in this work has received funding from: (a) the European Union’s Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; and (c) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells e HELPER” funded by the University of Padova.

(Keynote) Hybrid Inorganic-Organic Ion-Exchange Membranes for Electrochemical Applications: Electrical Response and Conductivity Mechanism

Electrochemical energy conversion and storage (EECS) systems are of major interest for both industry and scientific community due to their high energy conversion efficiency and easy scalability [1]. Two families of EECS systems are attracting particular attention: ion-exchange membrane fuel cells and redox flow batteries (RFBs). On one hand, ion-exchange membrane fuel cells are particularly suited for light-duty vehicles and small-scale stationary systems (e.g., auxiliary power units) owing to their high energy and power densities. On the other hand, RFBs (with a particular reference to all-vanadium redox flow batteries, VRFBs) are ideally suited to the large-scale storage of energy for the power grid as they are characterized by a very high turnover efficiency and an outstanding cyclability.Electrolyte membranes (EMs) are found at the core of both ion-exchange membrane fuel cells and VRFBs. The role of such EMs is to: (i) keep separated the electroactive species involved in the redox reactions taking place at the electrodes and, at the same time, (ii) ensure a facile and selective migration of ions to prevent the polarization of the device and allow for the establishment of large current densities. As of today, the most widely adopted EMs for both ion-exchange membrane fuel cells and VRFBs are based on perfluorinated ionomers such as Nafion™ and other similar macromolecules [2]. These systems exhibit an outstanding proton conductivity and a very high chemical and electrochemical stability; however, they also suffer from important drawbacks such as: (i) a dramatic drop in conductivity at T > 80°C and in dry conditions; (ii) poor mechanical properties; and (iii) a large permeability to vanadium species [3, 4]. Consequently, it is often necessary to: (i) add humidification modules to ion-exchange membrane fuel cells, thus raising the bulk and cost of the device; or (ii) especially in the case of VRFBs, use thick EMs that curtail the current density.These drawbacks can be addressed by implementing several different strategies, that include: (i) the introduction of a filler in the EM, giving so rise to a hybrid inorganic-organic membrane [5]; (ii) the fabrication of the EM with a different macromolecule (e.g., a sulfonated polyaromatic system such as sulfonated polyether ether ketone, SPEEK [6]) and (iii) the doping of the EM with an ion-conducting medium (e.g., phosphoric acid [7], or a proton-conducting ionic liquid [8]). All of these strategies operate by modulating the physicochemical properties of the EMs, with a particular reference to the details of the phase segregation at the nano/mesoscale.This work overviews the study of the interplay between the physicochemical properties of hybrid ion-exchange membranes (with a particular reference to the chemical composition, thermoanalytical properties, morphology and structure), and the electrical response as determined by means of advanced investigation techniques such as Broadband Electrical Spectroscopy (BES). Results allow to: (i) elucidate the interactions between the different phases and components within each EM; and (ii) clarify the conductivity mechanism. On these bases, it is possible to identify the most promising avenues of research to devise EMs for application in ion-exchange membrane fuel cells and RFBs exhibiting a performance and a cyclability beyond the state of the art. Acknowledgement We acknowledge the support from: (a) the European Union’s Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; and (c) the project “HELPER” funded by the University of Padova; and (d) the BIRD 2020 program of UNIPD.

Decorating ZIF-67-derived cobalt–nitrogen doped carbon nanocapsules on 3D carbon frameworks for efficient oxygen reduction and oxygen evolution

The rational construct of bifunctional electrocatalysts with high active and excellent cycling durability is the top priority for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Recently, metal-nitrogen-doped carbon (M-N-C) components formed by transition metal (TM) combined with nitrogen-doped carbon materials have drawn extensive interests due to their inexpensive cost and great activity. Herein, a straightforward evaporation-pyrolysis strategy is designed for the preparation of 3D hierarchical electrocatalyst (abbreviated as Co@N–HCCs@NG, which represented carbonization products at 900 °C) made of nitrogen-doped hollow carbon nanocapsules combined with Co nanoparticles bridged N-doped graphene by carbonization of a pristine ZIF-67/GO, enables hierarchical porous structure, high surface area (618.12 m2 g-1) and a hollow capsule-like nanostructure. Of these, the Co@N–HCCs@NG composite displays outstanding electrocatalytic performances for OER (overpotential of 298 mV @ 10 mA cm-2) and ORR (0.86 V @ E1/2) in alkaline electrolyte, which could rival the most of reported materials. The tactics presented in this research may provide a powerful platform to engineer non-precious highly electroactive catalysts with a well-excogitated structure and preferable activity for clean energy conversion technologies.

(Invited) Interplay between the Physicochemical Properties and the Electrochemical Performance of Hierarchial Electrocatalysts for the ORR Comprising a Graphene-based ‘Core’ and a Carbon Nitride ‘Shell’

The sluggishness of the oxygen reduction reaction (ORR) is a major bottleneck in the operation of low-temperature fuel cells (FCs), with a particular reference to proton-exchange membrane fuel cells (PEMFCs) [1]. Hence, it is necessary to devise efficient and affordable electrocatalysts (ECs) able to minimize the overpotentials associated with the ORR. This issue is further exacerbated considering that the single chemical element affording the lowest ORR overpotentials in the highly acidic environment found at the PEMFC cathode is Pt, whose abundance in Earth’s crust is very limited [2]. This raises serious concerns towards the incipient large-scale rollout of PEMFCs, that may face serious supply bottlenecks.On these bases, massive research efforts are currently devoted to develop new ORR ECs. A serious issue faced by the researchers is that it is often very difficult to transfer the intrinsic ORR performance of an EC into a single FC. The performance of a single FC is limited by transport phenomena, that are not easy to gauge by means of the well-assessed “ex-situ” techniques routinely adopted to determine the physicochemical properties and the “pure” ORR kinetics of an EC taken on its own. Thus, the optimization of the cathodic electrocatalytic layer of the FC (that hosts the ORR EC) is a crucial step to transfer effectively the ORR performance and ultimately to maximize the current and power outputs of the FC as a whole.In order to address this issue, this work describes a characterization approach that, by combining the information derived from single fuel cell measurements collected in different current density regimes, and upon modulating the partial pressure of oxygen at the cathode, allows for the identification and quantification a few simple figures of merit. The latter: (i) picture the extent with which the intrinsic ORR performance of the cathodic EC is transferred to the single FC; and (ii) gauge the contribution of different transport phenomena in the determination of single FC performance.The results are studied with an innovative analysis framework, yielding information that is then correlated to the physicochemical properties of the cathodic EC and to its electrochemical performance as determined by “ex-situ” measurements. Important insights are thus achieved, that play a crucial role in the optimization of the cathodic electrocatalytic layer with the final purpose to maximize the performance of the FC.The proposed characterization approach and subsequent analysis framework are straightforward, and suitable for a broad variety of FCs. The latter may also include cathodic ECs (e.g., low-Pt materials exhibiting a “core-shell” morphology [3], or completely “Pt-free” systems) that are very different from the conventional Pt/C of the state of the art. In conclusion this work describes a tool, simple and of broad applicability, able to provide important contributions to facilitate the implementation of developmental cathodic ECs in single FCs in the ongoing quest to realize efficient and low-carbon energy conversion systems. Acknowledgements This work has received funding from: (a) the European Union's Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; (c) the program “Budget Integrato per la Ricerca Interdipartimentale - BIRD 2018” of the University of Padova (protocol BIRD187913); and (d) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells – HELPER” funded by the University of Padova.

CuO@CoFe Layered Double Hydroxide Core-Shell Heterostructure as an Efficient Water Oxidation Electrocatalyst under Mild Alkaline Conditions.

Designing hierarchical electrocatalysts with superior water oxidation performance is highly desirable for the production of renewable chemical fuels. Here, we report the development of a CuO@CoFe layered double hydroxide core-shell heterostructure supported on Cu foil (CuO@CoFe-LDH/CF) as a highly active catalyst electrode for water oxidation under mild alkaline conditions. In a 0.2 M carbonate buffer solution (pH 11), it only needs a small overpotential of 213 mV to achieve 10 mA cm-2, outperforming all reported electrocatalysts at comparable testing conditions. It also shows outstanding long-term durability.

(Invited) A General Framework to Gauge Transport Phenomena in the Cathodic Electrocatalytic Layer of a Fuel Cell

The sluggishness of the oxygen reduction reaction (ORR) is a major bottleneck in the operation of low-temperature fuel cells (FCs), with a particular reference to proton-exchange membrane fuel cells (PEMFCs) [1]. Hence, it is necessary to devise efficient and affordable electrocatalysts (ECs) able to minimize the overpotentials associated with the ORR. This issue is further exacerbated considering that the single chemical element affording the lowest ORR overpotentials in the highly acidic environment found at the PEMFC cathode is Pt, whose abundance in Earth’s crust is very limited [2]. This raises serious concerns towards the incipient large-scale rollout of PEMFCs, that may face serious supply bottlenecks.On these bases, massive research efforts are currently devoted to develop new ORR ECs. A serious issue faced by the researchers is that it is often very difficult to transfer the intrinsic ORR performance of an EC into a single FC. The performance of a single FC is limited by transport phenomena, that are not easy to gauge by means of the well-assessed “ex-situ” techniques routinely adopted to determine the physicochemical properties and the “pure” ORR kinetics of an EC taken on its own. Thus, the optimization of the cathodic electrocatalytic layer of the FC (that hosts the ORR EC) is a crucial step to transfer effectively the ORR performance and ultimately to maximize the current and power outputs of the FC as a whole.In order to address this issue, this work describes a characterization approach that, by combining the information derived from single fuel cell measurements collected in different current density regimes, and upon modulating the partial pressure of oxygen at the cathode, allows for the identification and quantification a few simple figures of merit. The latter: (i) picture the extent with which the intrinsic ORR performance of the cathodic EC is transferred to the single FC; and (ii) gauge the contribution of different transport phenomena in the determination of single FC performance.The results are studied with an innovative analysis framework, yielding information that is then correlated to the physicochemical properties of the cathodic EC and to its electrochemical performance as determined by “ex-situ” measurements. Important insights are thus achieved, that play a crucial role in the optimization of the cathodic electrocatalytic layer with the final purpose to maximize the performance of the FC.The proposed characterization approach and subsequent analysis framework are straightforward, and suitable for a broad variety of FCs. The latter may also include cathodic ECs (e.g., low-Pt materials exhibiting a “core-shell” morphology [3], or completely “Pt-free” systems) that are very different from the conventional Pt/C of the state of the art. In conclusion this work describes a tool, simple and of broad applicability, able to provide important contributions to facilitate the implementation of developmental cathodic ECs in single FCs in the ongoing quest to realize efficient and low-carbon energy conversion systems. Acknowledgements This work has received funding from: (a) the European Union’s Horizon 2020 research and innovation program under grant agreement Graphene Core 2 785219; and (b) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells – HELPER” funded by the University of Padova.

Halides-assisted electrochemical synthesis of Cu/Cu2O/CuO core-shell electrocatalyst for oxygen evolution reaction

Constructing transition metal/metal oxides core-shell structures has been demonstrated as an efficient way to improve the poor electrical conductivity of bare metal oxides for application in oxygen evolution reaction (OER). However, the rational design and facile fabrication of metal oxides shell with high activity and high surface area remain challenging. Here, we report a facile halides-assisted electrooxidation method for the synthesis of three dimensional (3D) Cu/Cu2O/CuO hybrid foams as free-standing OER electrodes. The hybrid foams are composed of Cu core with high conductivity and Cu2O/CuO hybrid shell with nanoarray architectures which can increase the surface area and enhance the diffusion of liquid reactants and gas products. As expected, the as-prepared Cu/Cu2O/CuO electrocatalyst exhibits an overpotential of 0.289 V at 10 mA cm−2 as well as a long-term durability. The results of this study may be beneficial for the fabrication of hybrid hierarchical electrocatalysts and the designing of core-shell structures for gas-involving electrocatalytic reactions.

The self-supported Zn-doped CoNiP microsphere/thorn hierarchical structures as efficient bifunctional catalysts for water splitting

The hierarchical electrocatalysts with the superaerophobic property are in favor of removal of gas bubbles and supply of numerous active sites to improve the electrochemical catalytic activity for water splitting. In this work, the self-supported Zn-doped CoNiP (Zn–CoNiP/NF) hierarchical structure that consisted of nanothorns and radial microspheres were prepared with via a facile precursor phosphorization method. Zn-doping could tailor the initial electronic state of CoNiP to improve the hydrogen or oxygen atom binding, which effectively promoted the catalytic kinetics of water splitting. The hierarchical structure not only offered more catalytic active sites for water splitting, but also facilitated gas releasing due to the superaerophobic interface. The hierarchical Zn–CoNiP structure showed a high hydrogen evolution reaction (HER) efficiency in electrolytes with pH 0–14, which yielded low overpotentials of 73 mV in acidic solution and 34 mV in alkaline media at current density of 10 mA cm−2. Furthermore, it also had evidenced by an overpotential of 310 mV for oxygen evolution reaction (OER) at the current density of 50 mA cm−2. Note that the required cell initial voltage was 1.51 V for overall water splitting. These results were comparable or even better than many non-noble catalysts.

Ni loaded on N-doped carbon encapsulated tungsten oxide nanowires as an alkaline-stable electrocatalyst for water reduction

A hierarchical electrocatalyst, Ni on N-doped carbon shell coated oxygen-vacancy-rich WOx nanowires, exhibits excellent stability and superb alkaline HER activity.

Hierarchical three-dimensional framework interface assembled from oxygen-doped cobalt phosphide layer-shelled metal nanowires for efficient electrocatalytic water splitting

A novel hierarchical electrocatalyst of oxygen-doped cobalt phosphide layer-shelled copper nanowires vertically attached on surface of three-dimensional framework was fabricated through a simple and cost-effective approach. Such unique architecture was demonstrated to produce significant defects and modulated surface chemistry with rich-electroactive sites, adjusted adsorption energy, and enhanced surface area owning abundant channels for diffusion process. The catalyst achieved low overpotentials of 101 and 270 mV to acquire current response of 10 mA cm−2 towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline, respectively. Rapid kinetics were also obtained with low Tafel slopes of 69.4 mV dec-1 for HER and 74.4 mV dec-1 for OER. A practical electrolyzer of the electrocatalyst for water splitting exhibited small cell voltages of 1.54 and 1.71 V at 10 and 50 mA cm-2, respectively, along with excellent stability, which are superior to those of a similar device based on RuO2-C + Pt-C.

Autogenous Growth of Hierarchical NiFe(OH)x/FeS Nanosheet‐On‐Microsheet Arrays for Synergistically Enhanced High‐Output Water Oxidation

AbstractPractical electrochemical water splitting requires cost‐effective electrodes capable of steadily working at high output, leading to the challenges for efficient and stable electrodes for the oxygen evolution reaction (OER). Herein, by simply using conductive FeS microsheet arrays vertically pre‐grown on iron foam (FeS/IF) as both substrate and source to in situ form vertically aligned NiFe(OH)x nanosheets arrays, a hierarchical electrode with a nano/micro sheet‐on‐sheet structure (NiFe(OH)x/FeS/IF) can be readily achieved to meet the requirements. Such hierarchical electrode architecture with a superhydrophilic surface also allows for prompt gas release even at high output. As a result, NiFe(OH)x/FeS/IF exhibits superior OER activity with an overpotential of 245 mV at 50 mA cm−2 and can steadily output 1000 mA cm−2 at a low overpotential of 332 mV. The water‐alkali electrolyzer using NiFe(OH)x/FeS/IF as the anode can deliver 10 mA cm−2 at 1.50 V and steadily operate at 300 mA cm−2 with a small cell voltage for 70 h. Furthermore, a solar‐driven electrolyzer using the developed electrode demonstrates an exceptionally high solar‐to‐hydrogen efficiency of 18.6%. Such performance together with low‐cost Fe‐based materials and facile mass production suggest the present strategy may open up opportunities for rationally designing hierarchical electrocatalysts for practical water splitting or diverse applications.

Construction of NiFeP/CoP nanosheets/nanowires hierarchical array as advanced electrocatalysts for water oxidation

Cost-effective hierarchical electrocatalysts with excellent performance and high stability for water splitting play an important role in promoting social sustainable development. Herein, we report a three-dimensional (3D) trimetallic nickel-iron-cobalt phosphide with a hierarchical nanoarray structure grown in situ on carbon cloth via a combined method of two-step hydrothermal reaction and following low-temperature phosphating using a carbon cloth as a conductive substrate. The synthesized NiFeP/CoP/CC exhibits excellent oxygen evolution reaction (OER) catalytic performance, giving small overpotentials of 250 mV, 270 mV and 320 mV at current densities of 20 mA cm−2, 50 mA cm−2 and 200 mA cm−2 in alkaline electrolyte, respectively, with a small Tafel slope of 51 mV dec−1. In addition, the catalytic activity of the material remains stable for at least 50 h, and the turnover frequency (TOF) is 0.466 mol O2 s−1 at an over potential of 300 mV. These properties of the material are comparable to those of the noble metal catalyst RuO2/CC, which may be ascribed to the hierarchical microstructure, good conductivity and desirable synergistic effect among metal ions.

Boosting electrocatalysis by heteroatom doping and oxygen vacancies in hierarchical Ni-Co based nitride phosphide hybrid

Designing highly active and low-cost electrocatalysts towards oxygen evolution reaction has ignited much interest in recent years. Thereby, rational design of those catalysts is an urgent topic that must be addressed. Herein, an effective strategy is demonstrated to be beneficial to the formation of hierarchical nitride phosphide hybrid with abundant oxygen vacancies. This hybrid catalyst exhibits an excellent OER catalytic performance, as demonstrated by an ultralow Tafel slope of 41 mV·dec−1 and an overpotential of 290 mV at 10 mA cm−2 in 1.0 M KOH solution, which is superior to those of the pure nitride, phosphide, and the commercial IrO2 catalysts. The outstanding catalytic performance can be attributed to the specific heterojunction structure, accessible active sites, and abundant oxygen vacancies, which can facilitate the charge transport and the release of the O2 bubbles. This study not only provides a facile strategy for constructing surface oxygen vacancies onto phosphide/nitride hybrid, but also exhibits an insight into the nature of hierarchical electrocatalyst with multiple active sites.

Iron-substituted Co-Ni phosphides immobilized on Ni foam as efficient self-supported 3D hierarchical electrocatalysts for oxygen evolution reaction

Electrocatalytic water splitting for hydrogen evolution is significantly impeded by the kinetically sluggish oxygen evolution reaction (OER). Thus, the development of highly efficient and durably stable non-noble-metal OER electrocatalyst is necessary and challenging for the large-scale electrocatalytic water splitting. Herein, a series of iron-substituted cobalt-nickel phosphides grown on Ni foam (FeCoNi-P/NFs) were easily prepared though successive hydrothermal and phosphorization treatments. The chemical compositions, crystalline and electronic structures as well as surface morphologies of these resulting electrocatalysts are strongly related with the iron substitution ratio. More interestingly, the FeCoNi-P/NF-2 nanosheet arrays prepared from equivalent molar ratio of iron and cobalt precursors exhibit the best OER performance with a low overpotential of 266 mV to produce a current density of 50 mA cm−2 and a low Tafel slope of 61.2 mV dec−1 in 1.0 M KOH condition, which is comparable to the reported state-of-the-art OER electrocatalysts. Additionally, the FeCoNi-P/NF-2 nanosheet arrays also show satisfactory long-term durability over 60 h. The superior OER activity of the electrocatalyst is essentially attributed to the heteroatomic substitution and the unique three-dimensional hierarchical morphology, which greatly increase the electrical conductivity, afford more active sites and facilitate the efficient charge transfer ability.

Ru2P Nanoparticle Decorated P/N-Doped Carbon Nanofibers on Carbon Cloth as a Robust Hierarchical Electrocatalyst with Platinum-Comparable Activity toward Hydrogen Evolution

It is desirable yet challenging to develop highly active and durable hydrogen evolution reaction (HER) electrocatalysts with Pt-comparable activity for future energy devices. In this work, we report Ru2P nanoparticle decorated P/N dual-doped carbon nanofibers on carbon cloth (Ru2P@PNC/CC-900) as a highly efficient and durable hierarchical HER electrocatalyst in both acidic and alkaline media. Electrochemical tests show that this Ru2P@PNC/CC-900 possesses Pt-comparable HER activity to support 10 mA cm–2 HER current density at low overpotential of 15 and 50 mV in acidic and alkaline condition, respectively. Density functional theory calculations reveal that coupling Ru2P nanoparticles with heteroatom-doped carbon fibers leads to enhanced intrinsic HER activity. The integrative hierarchical architecture further endows high surface areas with good mechanical robustness to support abundant catalytically active sites and possesses excellent electrical conductivity and efficient access for mass transportation to...

Metallic Intermediate Phase Inducing Morphological Transformation in Thermal Nitridation: Ni3FeN-Based Three-Dimensional Hierarchical Electrocatalyst for Water Splitting.

Transition-metal nitrides have attracted a great deal of interest as electrocatalysts for water splitting due to their super metallic performance, high efficiency, and good stability. Herein, we report a novel design of hierarchical electrocatalyst based on Ni3FeN, where the presence of carbon fiber cloth as a scaffold can effectively alleviate the aggregation of Ni3FeN nanostructure and form three-dimensional conducting networks to enlarge the surface area and simultaneously enhance the charge transfer. The composition and morphological variations of NiFe precursors during annealing in different atmospheres were investigated. Such Ni3FeN/CC hierarchical electrocatalyst shows much improved electrochemical properties for water splitting in terms of overpotentials (105 and 190 mV at 10 mA/cm2 for hydrogen evolution reaction and oxygen evolution reaction, respectively) and stability.

Co3O4 based non-enzymatic glucose sensor with high sensitivity and reliable stability derived from hollow hierarchical architecture

Inspired by kinetics, the design of hollow hierarchical electrocatalysts through large-scale integration of building blocks is recognized as an effective approach to the achievement of superior electrocatalytic performance. In this work, a hollow, hierarchical Co3O4 architecture (Co3O4 HHA) was constructed using a coordinated etching and precipitation (CEP) method followed by calcination. The resulting Co3O4 HHA electrode exhibited excellent electrocatalytic activity in terms of high sensitivity (839.3 μA mM−1 cm−2) and reliable stability in glucose detection. The high sensitivity could be attributed to the large specific surface area (SSA), ample unimpeded penetration diffusion paths and high electron transfer rate originating from the unique two-dimensional (2D) sheet-like character and hollow porous architecture. The hollow hierarchical structure also affords sufficient interspace for accommodation of volume change and structural strain, resulting in enhanced stability. The results indicate that Co3O4 HHA could have potential for application in the design of non-enzymatic glucose sensors, and that the construction of hollow hierarchical architecture provides an efficient way to design highly active, stable electrocatalysts.