Precatalysts For Oxygen Evolution Reaction Research Articles

Iron Phosphates as Highly Efficient Precatalysts for Oxygen Evolution Reaction and Their in Situ Electrochemical Conversion

Current energy issues have prompted demand for the innovations in constructing a clean and sustainable energy platforms. One of examples for energy carrier to actualize a zero carbon society is hydrogen, which is a promising carbon-free fuel with an excellent weight energy density. Hydrogen production through alkaline water electrolysis using renewable energy resources (light and wind)-derived electricity is an attractive ecofriendly and cost-effective technology because cheap nonprecious electrocatalysts can be employed in alkaline media. However, oxygen evolution reaction (OER) at the anode side in the water electrolysis is basically slow and prevents the widespread applications of water electrolysis. Thus, highly efficient and cheap electrocatalysts for OER are highly desirable.Iron (Fe)-based compounds have recently received much attention as OER electrocatalysts because Fe is a quite earth-abundant metallic element that is a very cheap and non-toxic. Therefore, Fe is a promising candidate as electrocatalyst for OER. Due to the insufficient OER activity on simple Fe oxides, previous studies have attempted to enhance the activity by incorporating Fe and other metallic elements.[1,2] Recently, we established a novel comprehensive structural descriptor for the OER on Fe-based simple and multimetal oxides;[3] i.e., shorter minimum Fe–O bond length in the crystalline structures led to higher OER activity regardless of elemental composition, Fe–O coordination number, and crystal category. Thus, this descriptor enabled us to quickly develop excellent OER catalyst, Ba0.65Ca0.35Fe12O19, comprising extremely short Fe–O bond lengths.[4] Furthermore, we found that post-spinel CaFe2O4 catalyzes OER via "multi-iron-site mechanism" involving a reaction intermediate with O–O direct formation on multiple Fe atoms, and CaFe2O4 consequently displayed outstanding OER activity despite the long Fe–O bond length.[5] However, we speculate that OER activity of Fe-based oxides cannot be further enhanced based on crystal structures, and it necessitates to employ another type of compounds.In this study, we report the OER performances of Fe-based phosphates in alkaline media. We selected three kinds of Fe-based phosphates with distinct crystal structures: trigonal FePO4, trigonal Fe3O3(PO4), and monoclinic Fe4(P2O7)3. The electrochemical measurements and post-characterizations identified the Fe-based phosphates as efficient precatalysts that can be electrochemically converted into highly active Fe-based compounds. Notably, the OER performances of the electrochemically converted Fe-based compounds overcame those of the above-mentioned most active Fe-based multimetal oxides and any previously reported crystalline OER electrocatalysts. Therefore, the Fe-based phosphates are promising candidates as OER precatalysts, which can be in situ converted into excellent OER electrocatalysts, and have further advantages of low price, environmental friendliness, and easy production.[6] The part of this paper is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This paper was funded in part by the JSPS KAKENHI Grant-in-Aid (18H01786), the JST PRESTO (JPMJPR15S3) and CREST (JPMJCR16P3) programs, the Tokuyama Science Foundation, and the “Creation of Life Innovative Materials for Interdisciplinary and International Researcher Development” program of the MEXT. The authors thank Dr. Takeshi Aihara in Tokyo Institute of Technology for fruitful discussions.

Observing the reconstruction of cobalt oxide model catalyst in electrocatalytic water oxidation

Oxygen evolution reaction (OER) is the rate-determining step in water electrolysis. Surface reconstruction induced by external environments and potentials has been proven to enable inactive OER pre-catalysts to become superior oxides/oxyhydroxides that are comparable to the benchmark IrO2/RuO2 catalysts. However, it is still highly challenging to operando probe the reconstruction and precisely identify the true active sites due to the complex composition and structural diversity of practical catalysts. To address the issue, three cobalt-based OER model catalysts are prepared by different treatments of commercial cobalt foil via ultra-high vacuum approaches. The derived Co3O4-film model catalyst exhibits a substantially improved OER activity compared with the other two counterparts, Co-metal and acid etching cobalt sample. Multiple vacuum-connected technologies, including quasi-in situ XPS and time-of-flight secondary ion mass spectroscopy, are conducted and reveal the quantitative compositions and structures of the three-dimensional (3D) active oxides/oxyhydroxides layer reconstructed near the surface. The results suggest that the hydroxyl species and oxygen vacancies coordinated to reversible Co2+ sites within superficial layer are responsible for the OER activity. This study provides valuable insights for the near-surface investigation of model catalysts in OER reconstruction.

Understanding the Anion Effect of Basic Cobalt Salts for the Electrocatalytic Oxygen Evolution Reaction

Basic cobalt salts with distinct acidic anion Co(OH)x(A)y are efficient electrocatalysts toward the oxygen evolution reaction (OER). However, plenty of present studies are still in the try-and-wrong stage, while the underlying anion effect remains unclear. Herein, a series of Co(OH)x(A)y (A = F–, Cl–, and CO32–) are synthesized via a hydrothermal strategy, and the order of the OER activity is determined to be Co(OH)(CO3)0.5 > Co2(OH)3Cl > Co(OH)F > Co(OH)2. X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy studies reveal that these Co(OH)x(A)y materials undergo surface oxidation during the OER process, and the highly active Co(IV) content is the dominant factor in deciding the OER performance. Furthermore, quantitative analysis of anion concentrations in the electrolyte solution reveals that the anion leaching ability in Co(OH)x(A)y directly relates to the Co(IV) content and finally the OER catalytic activity, which is the essence of the anion effect and can be summarized as an “anion leaching─metal oxidation” model. Our work not only provides deep understanding of the anion effect of metal basic salt-based OER catalysts but also profound insights for the activation process of the OER pre-catalysts.

Molybdenum-leaching induced rapid surface reconstruction of amorphous/crystalline heterostructured trimetal oxides pre-catalyst for efficient water splitting and Zn-air batteries

Crystalline and amorphous structure can entitle a catalyst with high stability and activity, respectively. Oxygen evolution reaction (OER) catalysts, which widely used in water electrolysis and rechargeable Zn-air batteries, often undergo a surface phase reconstruction process and generate amorphous active phases under applied anodic potential. Although widely known, few studies and strategies have been reported to rationally tune OER pre-catalysts for enhanced reaction kinetics. Herein, we report a trimetallic oxides (a/c-NiFeMoOx) OER per-catalyst with rationally tunable amorphous/crystalline heterostructure degrees by a precise-tuning component strategy. The best a/c-NiFeMoOx electrode exhibits an OER overpotential merely of 256 mV and a small cell-voltage of 1.52 V to reach 10 mA cm–2 for water electrolysis, respectively. It is find that Mo leaching with tailored amorphous/crystalline heterostructure via the rational tuned degree of amorphousness promotes a rapid surface reconstruction of the a/c-NiFeMoOx pre-catalyst to form (oxy)hydroxide active species, whilst operando Raman, ex-situ X-ray photoelectron spectroscopy and density functional theory (DFT) analysis show the ample oxygen vacancies generated by phase transition significantly accelerates the deprotonation of OH* and lower the O* ➝ OOH* free energy for a fast oxygen evolution kinetics. Additionally, the practical application of a/c-NiFeMoOx cathode in rechargeable Zn-air battery delivers a robust long-term cycling (over 840 cycles).

Electrocatalytic activity and surface oxide reconstruction of bimetallic iron-cobalt nanocarbide electrocatalysts for the oxygen evolution reaction.

For renewable energy technology to become ubiquitous, it is imperative to develop efficient oxygen evolution reaction (OER) electrocatalysts, which is challenging due to the kinetically and thermodynamically unfavorable OER mechanism. Transition metal carbides (TMCs) have recently been investigated as desirable OER pre-catalysts, but the ability to tune electrocatalytic performance of bimetallic catalysts and understand their transformation under electrochemical oxidation requires further study. In an effort to understand the tunable TMC material properties for enhancing electrocatalytic activity, we synthesized bimetallic FeCo nanocarbides with a complex mixture of FeCo carbide crystal phases. The synthesized FeCo nanocarbides were tuned by percent proportion Fe (i.e. % Fe), and analysis revealed a non-linear dependence of OER electrocatalytic activity on % Fe, with a minimum overpotential of 0.42 V (15-20% Fe) in alkaline conditions. In an effort to understand the effects of Fe composition on electrocatalytic performance of FeCo nanocarbides, we assessed the structural phase and electronic state of the carbides. Although we did not identify a single activity descriptor for tuning activity for FeCo nanocarbides, we found that surface reconstruction of the carbide surface to oxide during water oxidation plays a pivotal role in defining electrocatalytic activity over time. We observed that a rapid increase of the (FexCo1-x)2O4 phase on the carbide surface correlated with lower electrocatalytic activity (i.e. higher overpotential). We have demonstrated that the electrochemical performance of carbides under harsh alkaline conditions has the potential to be fine-tuned via Fe incorporation and with control, or suppression, of the growth of the oxide phase.

Amorphous Ni-P-S@FeOOH/CC catalyst for high oxygen evolution Activity: Preparation, characterization and modeling

The oxygen evolution reaction (OER) is the key to prepare electrocatalysts for water splitting. Heteroatom doping is an effective way to improve the catalytic performance of OER by modulating the local electronic environment and stimulating the synergy. However, the complicated manufacturing methods have hindered their practical application. Herein, we demonstrate a method based on FeOOH/CC precursor to realize the preparation of Ni-P-S@FeOOH/CC catalyst through fast electrodeposition method. The overpotential of Ni-P-S@FeOOH/CC catalyst is 210 mV (10 mA cm−2), and still has an initial voltage of 97.35 % for 48 h. The surface area analysis shows a higher electrochemically active surface area for the amorphous structure prepared by electrodeposition method. XPS and DFT results show that the co-doping of P and S can stimulate the synergistic effect and the minimum energy of adsorption/desorption in OER process is only 0.83 eV. The strategy shows good prospects for preparing efficient OER pre-catalysts.

Dual-anion etching induced in situ interfacial engineering for high-efficiency oxygen evolution

Designing novel catalysts for oxygen evolution reaction (OER) with high cost-effectiveness plays a central role in sustainably driving renewable energy conversion and storage. Here we demonstrate the in situ interfacial engineering for constructing efficient OER catalysts based on the electrochemical dual-anion etching of natural arsenopyrite. The OER catalyst (FeAsS) prepared from natural arsenopyrite via an environment-friendly ball milling approach achieves a current density of 10 mA cm−2 at an overpotential of 200 mV, outperforming many state-of-the-art catalysts. The in-depth study indicates that the co-etching of lattice As and S under the OER conditions triggers the in situ surface self-reconstruction, and a self-optimized catalytic active and stable FeAsS/α-FeOOH interface has been developed. Computational studies further confirm that the strong electronic coupling effect between α-FeOOH and FeAsS significantly tunes the binding energy between reaction intermediates and active sites, finally leading to an enhanced OER activity. The dual-anion etching of precatalysts induced in situ interfacial engineering demonstrated here expands the way of exploring other multiple nonmetallic elements involved nanomaterials as efficient OER precatalysts. This study also stimulates further study on the eco-design of electroactive materials for advanced energy conversion/storage applications from earth-abundance natural resources.

In situ unraveling surface reconstruction of Ni5P4@FeP nanosheet array for superior alkaline oxygen evolution reaction

Oxygen evolution reaction (OER) is a key step for electrochemical water splitting and understanding the surface reconstruction of OER pre-catalysts is of vital importance. Herein, hybrid Ni5P4 @FeP nanosheet arrays were evaluated as promising OER pre-catalysts. The dynamic surface evolution was probed by in situ Raman spectroscopy, which revealed that Ni5P4 @FeP was rapidly reconstructed to NiFe2O4 during the anodic scan. The structural instability of amorphous NiFe2O4 led to partial reconstitution to Ni/FeOOH at high oxidation potentials. As-formed Ni/FeOOH@NiFe2O4 hybrid with high structural reversibility was established as a truly active species, which exhibited excellent alkaline OER performance with a low overpotential of 205 and 242 mV under current densities of 10 and 100 mA cm−2, respectively. This work provides a facile strategy to in situ construct an amorphous spinel/oxyhydroxide hybrid structure using electrochemical activation that holds strong promise for potential application in electrochemical water splitting and related energy devices.

Efficient NiFe-based oxygen evolution electrocatalysts and origin of their distinct activity

Efficient oxygen evolution reaction (OER) electrocatalyst is essential for water electrolysis. Herein, high-performance NiFe-based layered double hydroxides (LDH), phosphide and sulfide OER pre-catalysts were fabricated and their distinct activity was unveiled. The as-prepared NixFe1−xS exhibits ultralow OER overpotential of 122 mV at 10 mA cm−2 in 1 M KOH. The alkali-electrolyzer using NixFe1−xS electrodes achieve superior performance exhibiting a voltage of 1.46 V at 10 mA cm−2. Experimental analysis reveals that, during OER, Fe dissolution into electrolyte occurs for NiFe LDH and NixFe1−xP, which were both converted into NiOOH, well explaining their similar activity. Interestingly, Fe dissolution is significantly mitigated in NixFe1−xS, forming partially oxidized Fe2O3/FeOOH species. Theoretical calculations confirmed that Fe2O3/FeOOH is responsible for the enhanced OER energetics of NixFe1−xS. These observations provide new insights on the distinct activity of NiFe-based electrocatalysts, guiding their rational design as well.

In-situ probing the rapid reconstruction of FeOOH-decorated NiMoO4 nanowires with boosted oxygen evolution activity

Understanding the dynamic surface reconstruction is of vital importance for the development of oxygen evolution reaction (OER) electrocatalysts. Herein, we report FeOOH-decorated NiMoO4 nanowires as OER pre-catalysts. In-situ Raman spectroscopy elucidates that modified NiMoO4 undergoes fast reconstruction, including significant Mo leaching and simultaneous formation of Ni(Fe)OOH intermediates, which are determined as the real active species toward the OER. Rapid and deep reconstruction results in amorphous nanowires with uniform Fe distribution and great size expansion. Impressively, the reconstructed pre-catalyst exhibits outstanding catalytic activity, ultra-low overpotential, and extraordinary long-term stability under high current densities. Our work provides a facile and effective surface engineering approach of pre-catalysts and highlights the evolution of self-reconstruction chemistry for accelerating the kinetics of the OER.

Tuning electronic property and surface reconstruction of amorphous iron borides via W-P co-doping for highly efficient oxygen evolution

Designing cost-effective oxygen evolution reaction (OER) electrocatalysts is essential for sustainable water splitting. Recently, amorphous transition metal borides (TMBs) as OER pre-catalysts have acquired growing attention due to their favorable characteristics such as high conductivity, compositional and structural flexibility. Nevertheless, rational design of boride-based OER pre-catalysts remains an ongoing challenge. Herein, an efficient pre-catalyst derived from FeB with accelerated surface reconstruction and regulated intrinsic activity of evolved FeOOH is obtained by W and P co-doping. The obtained catalyst demonstrates an excellent OER activity with a low overpotential of 209 mV at a current density of 10 mA cm−2, and good stability in alkaline electrolyte, which surpasses most of boride-based OER catalysts. Specifically, the anion etching facilitates the surface reconstruction and accelerates the mass/charge transfer. Density functional theory calculations suggest W doping can enhance intrinsic catalytic activity via optimizing the adsorption free energy of reaction intermediates and improving the conductivity. Additionally, the hierarchical structure and amorphous feature also benefit the OER process. This study provides a fundamental insight into the correlation between surface structure and catalytic activity, and a powerful strategy to construct efficient OER pre-catalysts.

Anion Etching for Accessing Rapid and Deep Self-Reconstruction of Precatalysts for Water Oxidation

<h2>Summary</h2> Transition metal-based nanomaterials represent an emerging class of highly active and low-cost precatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, most OER precatalysts undergo slow or incomplete self-reconstructions to generate real active sites, which is a time-consuming process for achieving high OER performance. Thus, we report a new class of OER precatalysts that can achieve highly active OER species by a rapid and deep self-reconstruction (denoted by SELF-RECON). The precatalysts with a core-shell structure comprising NiMoO₄ (core) and NiFe/NiFeO_x nanoparticles in N-doped amorphous carbons (shell) (denoted by NiMoFeO@NC), can realize rapid MoO₄²⁻ dissolution and fast formation of NiOOH with Fe incorporation simultaneously. <i>In situ</i> Raman spectroscopy together with electron microscopy, X-ray photoelectron spectroscopy, and electrochemical tests indicate that the obtained NiFeOOH/NiFe-LDH after SELF-RECON behave as the real active species that outperform NiMoFeO@NC, with ultralow overpotentials and extraordinary long-term stability.

Heterostructural Ni3S2–Fe5Ni4S8 hybrids for efficient electrocatalytic oxygen evolution

Developing efficient and durable oxygen evolution reaction (OER) electrocatalysts with non-noble elements is particularly urgent for electrochemical water splitting. Herein, we report heterostructural Ni3S2–Fe5Ni4S8 hybrids directly grown on redox-NiFe foams (denoted as NFS/rNF) as excellent pre-catalysts for OER in 1 M KOH media. The electrochemical performance can be tuned by amount of sulfur source, and the optimized electrode only needs low overpotentials of 199 mV and 240 mV to attain 10 mA cm−2 and 100 mA cm−2, respectively. Moreover, the overpotential is further reduced to 231 mV to deliver 100 mA cm−2 after multi-cyclic voltammogram scans, due to the exposure of abundant in situ generated Ni(Fe)OOH species. Excellent stability with continuous 40 h operation is also demonstrated. Our study provides a new strategy for direct synthesis of self-standing integrated hybrid electrodes for efficient and durable water oxidation.

Amorphous CoFeBO nanoparticles as highly active electrocatalysts for efficient water oxidation reaction

The precise control of designing and synthesizing highly efficient water oxidation catalysts is a key role in enhancing the electrochemical processes such as water electrolysis. Herein, we fabricate the ultrafine Fe (variations on Fe amounts, χFe = MFe/M(Co+Fe)) and B-modulated CoFeBO nanoparticles as efficient water oxidation precatalysts. In view of intrinsic amorphous nanostructure, optimal Fe-doping amounts and borate-rich CoFeOOH active nanosheets formed on the surface of CoFeBO nanoparticles during oxygen evolution reaction (OER) process, the CoFeBO (χFe = 0.3) precatalyst has demonstrated active electrochemical water oxidation properties under widen pH value from neutral to alkaline conditions. It exhibits only 263 mV in overpotential to reach a current density of 10 mA/cm2 on glassy carbon (GC) electrode with the lowest Tafel slope of 39 mV/dec in 1 M KOH solution, which outperforms those of individual CoBO and FeBO as well as commercial RuO2 electrocatalyst in the same electrolyte. Moreover, continuous stability tests in 1 M KOH solution by recording potential response at 10 mA/cm2 for 28 h and current response at overpotential of 270 mV for another 28 h show no obvious fluctuation. Combining the merits of cost-efficient, scalable synthesis and active water oxidation performances, amorphous CoFeBO nanoparticles could be employed as highly active OER precatalysts for OER.