Pyrite-Type Nanomaterials for Advanced Electrocatalysis - American

Aug 21, 2017 - Pyrite-Type Nanomaterials for Advanced Electrocatalysis. Min-Rui Gao, Ya-Rong Zheng, Jun Jiang, and Shu-Hong Yu*. Division of Nanomater...
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Pyrite-Type Nanomaterials for Advanced Electrocatalysis Min-Rui Gao, Ya-Rong Zheng, Jun Jiang, and Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, CAS Centre for Excellence in Nanoscience, Hefei Science Centre of CAS, University of Science and Technology of China, Hefei 230026, China CONSPECTUS: Since being proposed by John Bockris in 1970, hydrogen economy has emerged as a very promising alternative to the current hydrocarbon economy. Access to reliable and affordable hydrogen economy, however, requires cost-effective and highly efficient electrocatalytic materials that replace noble metals (e.g., Pt, Ir, Ru) to negotiate electrode processes such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR). Although substantial advances in the development of inexpensive catalysts, successful deployment of these materials in fuel cells and electrolyzers will depend on their improved activity and robustness. Recent research has demonstrated that the nanostructuring of Earth-abundant minerals provides access to newly advanced energy materials, particularly for nanostructured pyrites, which are attracting great interest. Crystalline pyrites commonly contain the characteristic dianion units and have cations occurring in octahedral coordinationwhose generalized formula is MX2, where M can be transition metal of groups 8−12 and X is a chalcogen. The diversity of pyrites that are accessible and their versatile and tunable properties make them attractive for a wide range of applications from photovoltaics to energy storage and electrocatalysis. Pyrite-type structures can be further extended to their ternary analogues, for example, CoAsS (cobaltite), NiAsS (gersdorf f ite), NiSbS (ullmannite), CoPS, and many others. Moreover, improved properties of pyrites can be realized through grafting them with promoter objects (e.g., metal oxides, metal chalcogenides, noble metals, and carbons), which bring favorable interfaces and structural and electronic modulations, thus leading to performance gains. In recent years, research on the synthesis of pyrite nanomaterials and on related structure understanding has dramatically advanced their applications, which offers new perspectives in the search for efficient and robust electrocatalysts, yet a focused review that concentrates the critical developments is still missing. In this Account, we describe our recent progress on the discoveries and applications of nanostructured pyrite-type materials in the area of electrocatalysis. We first briefly highlight some interesting properties of pyrite-type materials and why they are attractive for modern electrocatalysis. Some recent advances on their synthesis that allows access to highly nanostructured pyritetype materials are reviewed, along with the grafting of resultant pyrites with foreign materials (e.g., metal oxides, metal chalcogenides, noble metals, and carbons) to enable improved catalytic performances. We finally spotlight the exciting examples where pyrite nanostructures were used as efficient electrocatalysts to drive the OER, HER, and methanol-tolerant ORR. It is reasonable to assume that, with significant efforts and focus, the next few years will bring new advances on the pyrites and other minerals for electrocatalysis.

1. INTRODUCTION Forty-six years ago, John Bockris coined the term hydrogen economy1 that depicted a clean, safe, and sustainable alternative to the current hydrocarbon economystoring energy from renewable sources (e.g., solar and wind) into the chemical bond of hydrogen (H2) via photo/electrolysis of water, which then can be released through the reverse reaction in fuel cells on demand (Figure 1). Although remarkable achievements have been made toward this target, its realization in reliable and scalable systems still faces formidable challenges in cost and performance of related energy devices. One of the most critical obstacles is the expensive electrocatalyst components that underpin fuel-cell and electrolyzer operations, where noblemetal-based catalysts are adopted to negotiate diverse electrode reactions, including anodic H2 oxidation and cathodic oxygen reduction for fuel cells, as well as anodic water oxidation and © 2017 American Chemical Society

cathodic water reduction for electrolyzers (Figure 1). Innovations in low-cost and high-performance catalytic materials, and their use in engineered systems, are needed for the successful commercial-scale implementation of hydrogen economy. Recent advances in catalyst development for fuel cells/ electrolyzers can be distinguished, depending on whether noble metals are used. Most emerging strategies on noble-metal catalysts are focusing on modulating their surface structure and composition to enable promoted performance with less costly noble metals.2 Despite substantial success, partial replacement of noble metals may not completely address the cost issue, considering the demand/price fluctuations.3 The rapid developReceived: April 14, 2017 Published: August 21, 2017 2194

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Besides catalysts described above, nanostructured inorganic minerals, including pyrites,19−24 molybdenites,11,13 bismuthinite,8 pentlandite,25 and others,26,27 were recently observed to advance the non-noble-metal catalyst community. These minerals exhibit versatile chemistry;28 some of them possess surface structures strongly similar to the active metal centers of natural enzymes (e.g., nitrogenase and hydrogenase), thus enabling intriguing catalytic performances.19,23,25 Development opportunities seem to be particularly promising for pyritestructured materials, because they could be made at nanoscale29,30 and because of their diverse properties.28 Although pyrite catalysts are currently experiencing an intensified interest, yet no focused review on this research excitement is available. In this Account, we will provide our recent progress on developing non-noble-metal catalysts from pyrite-structured materials, discuss properties and advances of these minerals, and describe methods that enable their nanostructuring and modifications. We then spotlight the examples where pyrite nanostructures were used to catalyze electrode reactions of fuel cells/electrolyzers, and speculate on future development opportunities.

Figure 1. Schematic illustration of the hydrogen economy based on photo/electricity-driven water splitting and fuel cells.

2. PYRITE-TYPE NANOMATERIALS 2.1. Structure and Properties

ment of non-noble-metal catalysts that are abundant, active, and stable has promised to eliminate noble metals to reduce costs. These impressive low-cost materials include heteroatomdoped nanocarbons,4 transition metal oxides,5,6 chalcogenide7,8 and phosphide nanostructures,9 transition metal complexes,10 and biomimetic catalysts.11−13 In particular, researchers are making progress in revealing the structures of catalytic active sites of different catalysts, for example, perovskite oxide14 and molybdenum disulfide (MoS2),11 thus offering inspiration for designs of new catalytic material systems. Significant advances in non-noble-metal catalyst development has been the topic of many excellent reviews,2,3,10,15 perspectives,16,17 and book chapters.18

Pyrites (e.g., FeS2; fool’s gold) are common minerals that exist in most metamorphosed ores and metamorphic rocks, which crystallize in the cubic system that leads to the formation of well-defined, sometimes very large, cubic crystals (Figure 2a).31 These minerals, whose generalized formula is MX2, where M can be transition metal of groups 8−12 and X is a chalcogen, contain the characteristic dianion units and have cations occurring in octahedral coordination (Figure 2b and c). The electronic structures of pyrites are diverse, which strongly depend on the d-electron count of the transition metals. Figure 2d illustrates that the antibonding d electron in the metal conduction band is progressively increased from FeS2 to ZnS2;

Figure 2. (a) Photograph of the cubic pyrite crystal on a marlstone from Navajún, Rioja, Spain. (b) Crystal structure of pyrite compounds. (c) Sideview of the stable, nonpolar {100} pyrite surface coordination environment. (d) Qualitative schematic illustration showing progressive filling of d orbitals for 3d pyrite-type compounds. EF is Fermi level. Image in (a) courtesy of Eric Greene. Panel d reproduced from ref 28. Copyright 1979 American Physical Society. 2195

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Figure 3. Scanning electron microscopy (SEM) images of FeS2 (a) and CoSe2 (b) nanostructures. (c) High-resolution transmission electron microscopy (HRTEM) image viewed along the lateral thickness direction of the CoSe2 nanobelts. (d) SEM image of NiS2 dodecahedrons. (e) Proposed reaction route that permits kinetic control of pyrite CuSe2 formation through air exposure. Figures reproduced from (a) ref 33, Copyright 2006 Wiley-VCH; (b, c) ref 29, Copyright 2009 American Chemical Society; (d) ref 30, Copyright 2009 Royal Society of Chemistry; (e) ref 35, Copyright 2015 American Chemical Society.

solids commonly require high temperature (>1000 °C), high pressure (>60 kbars), and/or long time.32 Sustainable synthesis of pyrite-structured materials was informed by prior advances in hydro- and solvothermal reactions.7 We experienced that a series of unique pyrite structures, for example, FeS2, CoSe2, and NiS2, can be successfully prepared via this low-temperature synthesis technique (Figure 3a−d).29,30,33 Autogeneous pressure developed in the closed reaction system and the high-level thermal management enabled desired minerals to be formed at temperatures less than 200 °C, with significant size and dimensional and morphological diversities (Figure 3a−d). An example is the elegant synthesis of CoSe2 that crystallizes in a graphite-like layered structure in which each layer is atomically thin (close to the lattice parameter of a = 0.586 nm; Figure 3c).29 Here a small organic solvent molecule, i.e., linear diethylenetriamine, leads to the formation of the unique beltlike stacking structure, which can be exfoliated into mono- or fewlayers,34 thus exhibiting additional characteristics. Figure 3d shows that pyrite NiS2 dodecahedrons could be produced in an ethylenediamine-glycol solvothermal system; these well-defined polyhedral crystals enable dense packing and exhibit antiferromagnetic property.30 Nevertheless, pyrite-structured materials with limited phase stabilities, for example CuSe2, are challenging to prepare, as they need very high pressure to stabilize the resultant compounds. Recently, Neilson’s group reported a solid-state metathesis method that permitted the kinetic control formation of pyrite CuSe2 under air exposure (Figure 3e).35 Although it is an attractive protocol, the poor nanostructuring of achieved product suggests the need of modified means that allows the preparation of nanostructured metastable pyrites.

Figure 4. SEM image of ternary pyrite-type CoPS nanoplates (a) and corresponding crystal structure (b). Reproduced from ref 23. Copyright 2015 Nature Publishing Group. (c) Elemental mapping images of pyrite-type CoSeP nanobelts.

this leads to remarkably different properties of pyrites, ranging from insulators such as NiS2, semiconductors such as FeS2, metals such as CoS2 and CoSe2, to even superconductivity such as CuS2 and CuSe2.28 Moreover, pyrite-type structures can be extended to transition metal chalcogenides consisting of ternary elements, for example, CoAsS (cobaltite), NiAsS (gersdorf fite), NiSbS (ullmannite), and CoPS as well as transition metal nitrides, phosphides, and arsenides such as PtN2, NiP2, and NiAs2. In recent years, research on pyrites has yielded dramatic advances, including their synthesis and applications for electrocatalysis.

2.3. Synthesis of Ternary Pyrite-Type Nanomaterials

Besides binary pyrites, synthetic advances that permit the formation of ternary pyrites are also developing. In contrast to their binary analogues, ternary pyrites possess dianions that composed of two different elements, such as natural minerals CoAsS, NiAsS, and NiSbS, which might lead to modulated electronic structure and thus modified properties due to the perturbed dianions.23,36 Nanostructuring ternary pyrites can

2.2. Synthesis of Binary Pyrite-Type Nanomaterials

Perhaps a main obstacle needs to be overcome for the use of pyrites as electrocatalysts is to make high-quality materials at nanoscale. Conventional pathways to access pyrite-structured 2196

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Figure 5. (a) Schematic illustration showing the concepts from plant grafting to materials grafting. (b−d) Structure models and the corresponding characterizations of various CoSe2-based composite catalysts: metal oxides-CoSe2 (b), carbons-CoSe2 (c), and Pt/CoSe2 as well as MoS2/CoSe2 (d). Reproduced from ref 19, Copyright 2015 Nature Publishing Group; ref 20, Copyright 2012 American Chemical Society; ref 21, Copyright 2011 Wiley-VCH; ref 22, Copyright 2013 Wiley-VCH; ref 39, Copyright 2010 Royal Society of Chemistry; ref 40, Copyright 2015 Wiley-VCH; ref 41, Copyright 2014 American Chemical Society; ref 42, Copyright 2015 Royal Society of Chemist

beltlike shape of the CoSe2 template.38 Making ternary pyrite nanostructures with high phase purity has yet to emerge and remains challenging, and further advancements in related synthesis are awaiting.

help to increase the active surface areas. An elegant synthesis of phosphosulfide (CoPS) nanostructures was recently described by Cabán-Acevedo and co-workers, through converting Cobased nanostructured precursors at 500 °C in a thiophosphate (PxSy) atmosphere under Ar protection (Figure 4a).23 Analogous to the natural mineral CoAsS, the pyrite CoPS has Co3+ octahedra and dumbbells with a homogeneous distribution of P2− and S− atoms (Figure 4b). Although CoPS performs like a semiconductor, the P2− ligands in the structure offer high electron-donating character, enabling it to exhibit superior HER activity to that of metallic CoS2. Later, Liu and co-workers reported the growth of CoPS nanoparticles onto carbon nanotubes in order to gain good electrical conductivity and therefore improved catalytic performance.37 In the search for the possibility of extending ternary pyrites to other systems, we found very recently that CoSeP with uniform element distribution could be gained by heating as-synthesized binary CoSe2 with NaH2PO2 at 400 °C under Ar flow (Figure 4c), leading to highly nanostructured product that retains the

2.4. Pyrite-Type Composite Catalysts

In the past, many pyrite-type nanomaterials were overlooked in that their structures and properties may not be very appropriate for specific energy applications. Notably, materials grafting has recently demonstrated great promise to tune the properties of pyrites and enable promotional effects, like plant grafting in botany (Figure 5a, left). The diverse chemistry of pyrite-type compounds reasonably raises opportunities for engineering their structural features to achieve modulated properties (Figure 5a, right). This concept was elaborated by our group to synthesize numberous pyrite-type composite electrocatalysts through grafting CoSe2 nanobelts with different foreign materials, giving rise to metal oxides-CoSe2,20,22,39,40 carbonsCoSe2,41,42 Pt/CoSe2,21 and MoS2/CoSe2 (Figure 5b−d).19 A small amount of amino groups on the surface of CoSe2 can act 2197

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improved properties, we observed that chemically grafting an appropriate material onto CoSe2 can substantially promote its original catalytic performances. These works have stimulated researchers to develop low-cost and efficient OER/HER electrocatalysts based on our developed CoSe2 nanobelts and other pyrite materials, and quickly yielded dramatic advances, which are spotlighted in the representative examples that follow.

as nucleation sites to couple reactant species and result in desired material grafting,29 with the ability to control their sizes, dimensions, and shapes owing to the confined growth effect; otherwise, the identical material might take entirely different growth behaviors. Although the catalytic performances of single CoSe2 are ordinary, we witnessed that materials grafting indeed offers a means of promoting its intrinsic catalytic properties. It notes that the gain is often remarkable, as materials grafting would induce modified electrocatalytic interface, engineered electronic structure, and combined catalytic proficiencies to pyrite nanomaterials, which will be discussed later. Doubtless, as this strategy develops, low-cost pyrite-type electrocatalysts show promise to be far more efficient in future.

3.1. OER Electrocatalysts

OER is the central reaction that determines the efficiencies of sunlight/electricity-driven water splitting.9,14 The formidable complexity of this oxidative half reaction, including fourelectron/four-proton involved O−H bond breaking and O−O bond formation, causes slow kinetics and thus considerable overpotential (η) requirement for device applications. The need for cheap and efficient electrocatalysts is perhaps the biggest obstacle limiting the large-scale electrolysis of water for H2 production. Pyrites are the materials of choice for the OER because of their low cost and their versatile and tunable properties. Five years ago we studied the OER properties of pyrite materials,20 revealing that pyrite CoSe2 can give the current density of 10 mA cm−2 at η of 0.36 V in 0.1 M KOH, and performs robustly. Slightly more active pyrite OER catalyst followed, such as hierarchical CoTe2 nanofleeces developed lately in our group, which require η of 0.35 V to reach this current density.46 Although promising, they did not show the optimized catalytic efficiency, for example, metallic CoSe2 with electronic configuration of t2g6eg1 might not possess optimal catalytic surfaces. Grafting foreign materials onto CoSe2 is considered possible to modulate the electronic structure of CoSe2, tuning the bond strength between its surface and the OER intermediates and thus the OER activity. Our recent focused works set the stage for better pyrite-based OER catalysts through materials grafting.20,40,41 Figure 7a compares the OER polarization curves of pyrite CoSe2 and various CoSe2-based hybrids, with Pt/C and RuO2 as references. It is clear that grafting foreign materials onto CoSe2 surface has drastically enhanced its OER activity and some hybrids even outperform the RuO2, which is state of the art. Markedly, grafting oxygen-vacancies-riched CeO2 with CoSe2 results in pyrite-based catalyst exhibiting an onset potential of 1.39 V vs RHE, a Tafel slope of 44 mV dec−1, and a small η of 0.29 V at 10 mA cm−2 (Figure 7b), which represents the most impressive advancement thus far.40 The smallest Tafel slope of 40 mV dec−1 was observed for NG-CoSe2, enabling it to be a superior catalyst at large operating currents.41 Another notability here is that these CoSe2-based catalysts are very stable under OER cycling conditions (Figure 7c), which lies with the high oxidation potential of CoSe2, making it intact during the electrochemical window of water oxidation, analogous to the OER robustness observed recently for CoTe2.46 But this is not the case for pyrites where oxidation waves appeared before the OER, which will tend to yield corresponding oxides/hydroxides in situ that are OER active. Characterizing the hybrid materials down to atomic scale can provide fundamental understanding of the enhanced mechanism. Figure 7c compares Co 2p3/2 X-ray photoelectron spectroscopy (XPS) of CoSe2 and various CoSe2-based hybrids, which reveal the binding energy of 778.5 eV for CoSe2. With foreign materials like CeO2, Mn3O4, and NG grafted with CoSe2, they lower the Co2p3/2 binding energy in the order of CeO2/CoSe2 > Mn3O4/CoSe2 > NG-CoSe2, suggesting

3. ELECTROCATALYSIS APPLICATIONS Beginning in the 1970s, transition metal chalcogenides found potenial use for catalyzing the cathode oxygen reduction

Figure 6. Polarization curves for OER (red), HER (blue), and ORR (green) on single CoSe2 modified glass carbon (GC) electrode. OER measurement was performed in O2-saturated 0.1 M KOH (pH ∼ 13). Scan rate: 5 mV s−1. Catalyst loading: ∼0.2 mg cm−2. HER measurement was performed in H2-saturated 0.5 M H2SO4 (pH ∼ 0). Scan rate: 2 mV s−1. Catalyst loading: ∼0.28 mg cm−2. ORR measurements were performed in O2-saturated 0.5 M H2SO4 (pH ∼ 0). Scan rate: 10 mV s−1. Catalyst loading: ∼0.13 mg cm−2.

reaction (ORR) of fuel cells,43 and a breakthrough came in 1986 by using Chevrel-phase Mo4Ru2Se8 that enabled the fourelectron ORR properties,44 but with expensive Ru. It was subsequently observed that 3d metal chalcogenides, particularly those with pyrite structure, show promising ORR activities, due to the optimized energy difference between the O 2p orbital and the highest occupied d orbital of chalcogen.43 For example, we have reported that pyrite CoSe2 nanobelts have a decent ORR onset potential of ∼0.71 V versus reversible hydrogen electrode (RHE; Figure 6, green curves), and further performance gains can be accessible when coupling CoSe2 with Fe3O4 nanoparticles.39 Several recent reviews and chapters describing this application are available,43−45 so we do not provide a detailed discussion here. Nanocrystalline pyrites have recently demonstrated great potential as electrocatalysts for the anode oxygen evolution reaction (OER) and cathode hydrogen evolution reaction (HER) of the water electrolyzers. In 2012, we disclosed the catalysis of the OER by using pyrite CoSe2 nanobelts (Figure 6, red curve),20 which were later found to be also active for catalyzing the HER (Figure 6, blue curve).19,22 In the search for 2198

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Figure 7. (a) Polarization curves for OER on RuO2, single CoSe2, and various CoSe2-based composite catalysts. (b) Comparison of catalytic parameters of various OER catalysts. (c) Comparison of OER stability of various catalysts. All the measurements were performed in O2-saturated 0.1 M KOH (pH ∼ 13). Scan rate: 5 mV s−1. Catalyst loading: ∼0.2 mg cm−2. (d−f) Co 2p3/2 XPS spectra (d), Co K-edge XANES spectra (e), and Raman spectra (f) for single CoSe2 and different CoSe2-based composites. Insets in (e) and (f) are enlarged spectra region of interest. Reproduced from ref 20, Copyright 2012 American Chemical Society; ref 29, Copyright 2009 American Chemical Society; ref 40, Copyright 2015 Wiley-VCH; ref 41, Copyright 2014 American Chemical Society.

layers,34 yielding improved OER performances. All these advancements were made based on the seminal syntheses of pyrite CoSe2 nanobelts reported in 2009.29

electron donation from these foreigners to the CoSe2 host (Figure 7c). As to d7 CoSe2 with low-spin state of t2g6eg1, the donated electron will increase eg-orbital occupancy of CoSe2 and thereby modify its surface electronic structure. Recently, Suntivich et al.14 reported that transition metal oxide with eg filling close to 1.2 possesses the highest OER activity due to the optimized binding of OER intermediates to the metal surface. Likewise, electrons from grafted materials enable a higher eg filling of CoSe2 that approaches the optimal eg1.2, giving rise to higher OER activities. Indeed, such electron donation behavior can be further evidenced by X-ray absorption near edge spectra and Raman spectra (Figure 7e and f). After these initial reports, there are many new cases where materials grafting, for example, with Au nanoclusters,47 Ag,48 and CoO,49 allows access to pyrite-based OER catalysts with high activities. Additional advances were realized through exfoliation of layered CoSe2 nanobelts into few or single

3.2. HER Electrocatalysts

HER is fundamentally important for sunlight/electricity-driven H2 production and H2−O2 fuel cells (as the reverse reaction).11 Compared with O2-electrode reactions, this two-electron H2O/ H2 reaction is relatively easier. Expensive Pt is the best catalyst to overcome the kinetic barriers of HER. The challenge regarding to this reaction is the design of Pt-alternative catalysts that made from Earth-abundant elements to convert H2O to H2 rapidly without a large excess thermodynamic cost. Nature evolved hydrogenases and nitrogenase as effective catalysts for HER using only Fe, Ni, and Mo as metal centers.13 The Gibbs free energies of adsorbed hydrogen (ΔGH) is a good descriptor that predict the HER activity.13 Computational studies show that hydrogenases, nitrogenases, and Pt all show a 2199

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Figure 8. (a) Polarization curves for HER on bare GC electrode and modified GC electrodes comprising different catalysts. Scan rate: 2 mV s−1. Catalyst loading: 0.28 mg cm−2. (b) Tafel plots for different catalysts derived from (a). (c) Chronoamperometric responses (j ∼ t) recorded on MoS2/CoSe2 and single MoS2 at a constant applied potential of −0.7 V vs SCE. Catalyst loading: 1 mg cm−2. Inset digital photo shows the H2 bubbles formed on MoS2/CoSe2-modified electrode at the time point of 20 h. All the measurements were performed in H2-saturated 0.5 M H2SO4. (d) S 2p XPS spectra for single MoS2, MoS2/CoSe2, and MoS2/CoSe2 after stability test. (e) Reaction pathway of HER on MoS2/CoSe2 according to the Volmer−Tafel route. The calculated distance of two hydrogen atoms and energies are displayed in Å and eV. Blue, orange, azure, yellow, and pink indicate Co, Se, Mo, S, and H atoms, respectively. Reproduced from ref 19. Copyright 2015 Nature Publishing Group.

ΔGH ≈ 0, indicating the optimal binding energies between their surfaces and adsorbed hydrogen.13 In 2005, Nørskov and coworkers13 calculated that Mo-edge ΔGH of MoS2 (molybdenite) is slightly positive at 0.08 eV, suggesting the promise of MoS2 as effective HER catalyst, which was later verified experimentally by Jaramillo et al.11 in 2007. It was further found that incorporating Co into the edge structures of MoS2 can reduce S-edge ΔGH from 0.18 to 0.10 eV and thus offer HER promotions.50 Motivated by the great success of nanostructured molybdenites for catalyzing HER, we extended the material scope to pyrite minerals for searching newly advanced HER catalysts. Our exploration of HER catalysts actually begins from a new nanostructured NiSe,51 which showed decent HER activity but poor stability in 0.5 M H2SO4. Afterward, we disclosed that pyrite CoSe2 nanobelts can exhibit better HER activity with an onset potential of −50 mV vs RHE and are very stable in the same electrolyte (Figure 6).22 We then developed superior HER catalysts from CoSe2 by grafting them with hydrogeses favoring Ni and subsequently annealed the obtained Ni/CoSe2 in air to create novel Ni/NiO/CoSe2 hybrid (Figure 5b).22 The onset potential of −30 mV vs RHE, Tafel slope of 39 mV dec−1, and exchange current density (j0) of 1.4 × 10−2 mA cm−2 demonstrate substantially enhanced HER activity of Ni/NiO/ CoSe2 hybrid as compared to pure CoSe2. The compact and balanced configuration of the Ni/NiO core/shell structure on

the CoSe2 surface plays a key role in promoting the HER activity. The conductive Ni cores can lower the internal resistance, and the thin NiO shells can synergistically facilitate the dissociation of H 2 O to H ads , which subsequently recombined on the CoSe2 surface to generate H2.22 An unexpected HER activity was recently realized through grafting pyrite CoSe2 with molybdenite MoS2 (i.e., MoS2/ CoSe2).19 In acidic water, this new catalyst possesses an onset potential of −11 mV vs RHE, Tafel slope of 36 mV dec−1, and j0 of 7.3 × 10−2 mA cm−2, approaching the performance of the Pt/C catalyst (Figure 8a and b). Quasi-electrolysis experiment showed that MoS2/CoSe2 catalyst allows sustained production of H2 for 24 h with continuously increased current density (Figure 8c and inset). After materials grafting, Co from CoSe2 substrate chemically interacted with MoS2 by forming S−Co bonds (shaded area in Figure 8d), which promoted the HER kinetics via lowering the S-edge ΔGH of grafted MoS2.50 The degradation of external MoS2 during the electrolysis process enables more reactants to access the Co-promoted MoS2− CoSe2 interfaces, yielding the increased HER current density. Moreover, S 2p XPS studies exhibit a dramatically decreased electron energy (by ∼1.3 eV) after growing MoS2 onto CoSe2 surfaces, suggesting the formation of more terminal S22− and S2− ions, which are HER active (Figure 8d). XPS analyses performed after 24 h of operation demonstrated no obvious chemical state change of HER-active S (Figure 8d), indicating 2200

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Accounts of Chemical Research 3.3. Methanol-Tolerant ORR Electrocatalysts

Direct methanol fuel cells (DMFCs) have received persistent attention because methanol is a liquid fuel with reasonably high energy density, is readily available from industrial process, and allows use of current infrastructure for transport and storage. A standard DMFC device involves the methanol oxidation at the anode and oxygen reduction at the cathode, which are separated by a proton exchange membrane such as Nafion. However, diffusion of methanol from the anode, across the Nafion membrane, to the cathode will reduce the open-circuit potential by about 0.15−0.2 V and poison the Pt electrocatalysts at the cathode.55 Although progress made in the modification of current PEM membrane can suppress about 50% methanol crossover, it simultaneously reduces the proton conductivity and the mechanical properties due to the altered membrane microstructure. Another effective method to overcome this issue is the development of methanol-tolerant electrocatalysts with high ORR activity. Recent advances show that Ru-based chalcogenides, transition metal macrocycles, and some Pt-based alloys bear methanol tolerance while maintaining ORR activity.43 However, Pt-free materials show ordinary activity and stability under fuel-cell conditions, while Pt-based alloys take effect commonly at low metal loading, which are not suitable for DMFC cathodes with typical loading of about 1−2 mgPt cm−2.55 Numerous transition metal chalcogenides, including pyrite materials, have recently demonstrated promise as “poisontolerant” DMFC cathode electrocatalysts because of their excellent tolerance to methanol.44,56 Computational studies suggested that a high energy barrier for the initial dehydrogenation of methanol on a chalcogen-containing surface leads to such methanol-tolerant feature.56 One dilemma of using pyrite materials for DMFC cathode is their relative low ORR activity. To circumvent this issue, we developed a new catalyst design that attaches Pt nanoparticles to the surface of pyrite CoSe2 by taking advantage of high ORR activity of Pt and high methanoltolerant property of CoSe2.21 In 0.5 M H2SO4, the new Pt/ CoSe2 (22.1 wt % Pt) catalyst shows almost the same onset potential with 20 wt % Pt/C reference. The transferred electron number increased from 2.1 for CoSe2 to 3.8 for Pt/CoSe2, suggesting nearly complete reduction of O2 to H2O on the Pt/ CoSe2 catalyst. Importantly, this pyrite-based catalyst exhibits excellent methanol-tolerant property. The ORR onset potential and current density reduced significantly for Pt/C in the presence of methanol even at a very low concentration of 0.5 M (Figure 9a). In contrast, the Pt/CoSe2 can keep the ORR property unchanged even up to a high methanol concentration of 5 M (Figure 9b). The methanol-tolerant performance of this pyrite-based catalyst is important because more fuel can be fed at the anode without considering the poisoning effect at the cathode, thus endowing DMFCs with higher power capability.

Figure 9. Polarization curves for ORR on 20 wt % Pt/C catalyst (a) and 22.1 wt % Pt/CoSe2 composite catalyst (b) in O2-saturated 0.5 M H2SO4 solution containing 0, 0.5, 1, and 5 M methanol at a rotation rate of 2500 rpm. Scan rate: 50 mV s−1. Catalyst loading: 20 μgPt cm−2. Reproduced from ref 21. Copyright 2011 Wiley-VCH.

the robustness of this mineral catalyst. Further, our computational study revealed that the activation barrier for the ratedetermining Tafel-step is mere 1.13 eV (30.7 kcal mol−1), which can be readily overcome by providing a slight η (Figure 8e), agreeing with the experimentally observed fast HER kinetics. The optimized electrocatalytic interfaces and synergies between the two nanostructured minerals together lead to this advancement. Subsequent studies, notably by Cui et al.52 and us,42 showed that growing pyrite CoSe2 catalysts onto commercial carbon substrates can give efficient HER block electrodes with significant cost and throughput advantages. Other advances in this area of research emerge rapidly. The Jin group53 demonstrated that micro/nanostructuring pyrite CoS2 can enable striking HER enhancement as compared to flat CoS2 film, owing largely to the easy release of evolved H2 bubbles from the electrode surface. Zhang and co-workers described the creation of a promising photocathode based on p-Si and NiCoSex for photoelectrochemical HER.54 Very recently, new advances were made to allow for the use of ternary pyrites as HER catalysts, such as nanostructured CoPS23 and its composites.37

4. CONCLUSIONS AND PROSPECTS Pyrite minerals are unique materials in the sense that they exhibit versatile properties, from semiconductors, metals, to even superconductors. Nanostructured pyrites are currently experiencing a considerable research interest for electrocatalysis. This Account has focused on the recent advancements in the synthesis, properties, and applications of pyrite materials those enable efficient catalysis of different reactions such as OER, HER, and methanol-tolerant ORR. Various aspects of the development of pyrites with controlled sizes, components, 2201

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Accounts of Chemical Research nanostructuring, and surface modifications have been discussed, with special emphasis placed on the benefits of materials grafting, which permits much improved catalytic performances because of the optimized electronic structures and combined catalytic proficiencies. Although there is clear progress, the research of exploring pyrite nanostructures for electrocatalysis has only recently begun and needs to continue unabated. Better synthetic routes that allow for a remarkable level of control over size, dimensionality, morphology, and component of pyrites are still highly needed. Insights into the fundamental behaviors of pyrite-based catalysts, including materials grafting, electronic structures, active sites, defects, doping, and how these influence catalytic properties, are important aspects that remain not well understood. In addition to experimental works, future computational efforts might be capable of shedding light upon these matters. Furthermore, it would be interesting to see the advancements on the integration of pyrite catalysts into solar-driven photoelectrochemical cells.54 All in all, we expect great strides in the development of pyrite-type materials for electrocatalysis for years to come, when they can possess striking inherent properties that provide an alternative to costly noble-metal electrocatalysts.

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research in the same group. Her interest focuses on bioinspired materials and their applications. Shu-Hong Yu received Ph.D. in inorganic chemistry in 1998 from USTC. After he finished Postdoctoral research in the Tokyo Institute of Technology and the Max Planck Institute of Colloids and Interfaces, he was appointed as a full professor in 2002 at USTC, and was awarded the Cheung Kong Professorship in 2006. He serves as a senior editor for Langmuir (2017−), and is an editorial or advisory board member of journals Account of Chemical Research, Chemistry of Materials, Chemical Science, Materials Horizons, Nano Research, ChemNanoMater, ChemPlusChem, and Crystals. His research interests include bioinspired synthesis and self-assembly of nanoscale building blocks and nanocomposites, and their related applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Min-Rui Gao: 0000-0002-7805-803X Shu-Hong Yu: 0000-0003-3732-1011 Funding

This work was supported by the National Natural Science Foundation of China (21431006, 21761132008), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21521001), Key Research Program of Frontier Sciences, CAS (QYZDJ-SSW-SLH036), the National Basic Research Program of China (2014CB931800), the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS (2015HSCUE007), the Fundamental Research Funds for the Central Universities (WK2340000076), and the Recruitment Program of Global Youth Experts. Notes

The authors declare no competing financial interest. Biographies Min-Rui Gao received his Ph.D. in Nano Chemistry in 2012 from University of Science and Technology of China (USTC) under Prof. Shu-Hong Yu. After he finished postdoctoral research at the University of Delaware, Argonne National Laboratory, and the Max Planck Institute of Colloids and Interfaces, he was appointed as a professor in 2016 at the USTC. His research interest involves the design and synthesis of functional nanostructured materials and their applications in energy areas. Ya-Rong Zheng received his Ph.D. in inorganic chemistry from USTC in 2015 under Prof. Shu-Hong Yu. He is currently carrying out postdoctoral research in the same group, working on new catalyst development. Jun Jiang received her Ph.D. in inorganic chemistry from USTC in 2012 under Prof. Shu-Hong Yu. She is currently doing postdoctoral 2202

DOI: 10.1021/acs.accounts.7b00187 Acc. Chem. Res. 2017, 50, 2194−2204

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