Non‐Noble‐Metal‐Based Electrocatalysts toward the Oxygen Evolution Reaction

Water electrolysis represents a promising sustainable hydrogen production technology. However, in practical application which requires extremely large current densities (>500 mA cm−2), the oxygen evolution reaction (OER) becomes unstable and kinetically sluggish, which is a major hurdle to large-scale hydrogen production. Herein, we report an exceptionally active and binder-free NiFe nanowire array based OER electrode that allows durable water splitting at current densities up to 1000 mA cm−2 up to 120 hours. Specifically, NiFe oxyhydroxide (shell)–anchored NiFe alloy nanowire (core) arrays are prepared via a magnetic-field-assisted chemical deposition method. The ultrathin (1–5 nm) and amorphous NiFe oxyhydroxide is in situ formed on the NiFe alloy nanowire surface, which is identified as an intrinsically highly active phase for the OER. Additionally, the fine geometry of the hierarchical electrode can substantially improve charge and mass (reactants and oxygen bubbles) transfer. In an alkaline electrolyte, this OER electrode can yield current densities of 500 and 1000 mA cm−2 stably over 120 hours at overpotentials of only 248 mV and 258 mV respectively, which are dramatically lower than any recently reported overpotentials. Notably, the integrated alkaline electrolyzer (with pure Ni nanowires as HER electrode) is demonstrated to reach the current density of 1000 mA cm−2 with super low voltage of 1.76 V, outperforming the state-of-the-art industrial catalysts. Our result may represent a critical step towards an industrial electrolyzer for large-scale hydrogen production by water splitting.

Exceptional performance of hierarchical Ni–Fe oxyhydroxide@NiFe alloy nanowire array electrocatalysts for large current density water splitting

Electrocatalytic oxygen evolution reaction (OER) is a core reaction responsible for converting renewable electricity into storable fuels; yet, it is kinetically challenging, because of the complex ...

Fe-Based Electrocatalysts for Oxygen Evolution Reaction: Progress and Perspectives

Electrocatalysis is at the center of many sustainable energy conversion technologies that are being developed to reduce the dependence on fossil fuels. The past decade has witnessed significant progresses in the exploitation of advanced electrocatalysts for diverse electrochemical reactions involved in electrolyzers and fuel cells, such as the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), the CO2 reduction reaction (CO2 RR), the nitrogen reduction reaction (NRR), and the oxygen evolution reaction (OER). Herein, the recent research advances made in porous electrocatalysts for these five important reactions are reviewed. In the discussions, an attempt is made to highlight the advantages of porous electrocatalysts in multiobjective optimization of surface active sites including not only their density and accessibility but also their intrinsic activity. First, the current knowledge about electrocatalytic active sites is briefly summarized. Then, the electrocatalytic mechanisms of the five above-mentioned reactions (HER, ORR, CO2 RR, NRR, and OER), the current challenges faced by these reactions, and the recent efforts to meet these challenges using porous electrocatalysts are examined. Finally, the future research directions on porous electrocatalysts including synthetic strategies leading to these materials, insights into their active sites, and the standardized tests and the performance requirements involved are discussed.

Active Site Engineering in Porous Electrocatalysts.

The lithiation/delithiation in LiFePO4 is highly anisotropic with lithium-ion diffusion being mainly confined to channels along the b-axis. Controlling the orientation of LiFePO4 crystals therefore plays an important role for efficient mass transport within this material. We report here the preparation of single crystalline LiFePO4 nanosheets with a large percentage of highly oriented {010} facets, which provide the highest pore density for lithium-ion insertion/extraction. The LiFePO4 nanosheets show a high specific capacity at low charge/discharge rates and retain significant capacities at high C-rates, which may benefit the development of lithium batteries with both favorable energy and power density.

Single-Crystalline LiFePO4 Nanosheets for High-Rate Lithium Batteries

The oxygen evolution reaction (OER) is a key half-reaction involved in many important electrochemical reactions, but this process is quite sluggish and the materials needed usually show unsatisfactory activity, stability or corrosion resistance in the harsh electrocatalysis environment. Here we report an effective boronization strategy for the value-added transformation of inexpensive, commercially available metal sheets (Ni, Co, Fe, NiFe alloys and steel sheets) into highly active and stable, corrosion resistant oxygen evolution electrodes. The boronized metal sheets exhibit OER activities that are an order of magnitude higher than those of the corresponding metal sheets, and show significantly improved catalytic stability and corrosion resistance in the operating environment. The in situ formed, ultrathin (2–5 nm), metaborate-containing oxyhydroxide thin films on metal boride surfaces are identified as a self-functionalized, highly active catalytic phase for the OER. In particular, a boronized NiFe alloy sheet is demonstrated to exhibit intrinsic catalytic activity higher than those of the state-of-the-art materials in 1 M KOH, while retaining such catalytic activity for over 3000 hours. Additionally, the boronized NiFe alloy and steel sheets are also demonstrated to have good catalytic activity as well as excellent catalytic stability and corrosion resistance in 30% KOH solution, a widely-adopted electrolyte in commercial water–alkali electrolyzers.

High-performance oxygen evolution electrocatalysis by boronized metal sheets with self-functionalized surfaces

A class of metal diboride electrocatalysts synthesized by a molten salt-assisted reaction for the hydrogen evolution reaction

Hollow micro-/nanostructures have a wide range of applications in catalysis, rechargeable batteries, drug delivery, and gas sensors, as well as energy storage and conversion. Herein, we report a facile, template-free, precursor-mediated method to synthesize V2O5 nanomaterials with three different architectures: double-shelled hollow nanospheres, single-shelled hollow nanospheres and nanoparticles. These V2O5 nanostructures are obtained via a simple thermal treatment in air of a “pre-synthesized” vanadyl glycerolate precursor, and their morphologies can be easily tuned by varying the thermal treatment temperatures. Electrochemical studies show that the double-shelled V2O5 hollow nanospheres as a cathode material for lithium-ion batteries deliver an initial capacity of 256.7 mA h g−1 with a Coulombic efficiency of nearly 100%, and their capacity is superior to the two other V2O5 nanostructures (i.e., single-shelled hollow nanospheres and nanoparticles), mainly due to their unique double-shelled hollow structure.

Precursor-mediated synthesis of double-shelled V2O5 hollow nanospheres as cathode material for lithium-ion batteries

We report on uniform Li3V2(PO4)3 microspheres with a size distribution of 1–3 μm assembled with nanoparticles embedded in a carbon matrix. LVP particles have a size of about 50–100 nm and carbon accounts for about 6% in total mass for the one with the best electrochemical performance. An initial capacity of 121 mA h g−1 or 101 mA h g−1 was achieved when cycled at 1 C or 10 C at room temperature with an admissible capacity fading of 8% after 100 cycles. The high rate capability and cycling stability may attribute to the unique microsize, electron conductive continuous carbon matrix and stable 3D skeleton of Li3V2(PO4)3.

Enhanced electrochemical performance of Li3V2(PO4)3 microspheres assembled with nanoparticles embedded in a carbon matrix

Porous N-doped carbon stabilized Li3V2(PO4)3 particles are prepared by a modified sol–gel method. Both the in situ doping of nitrogen and the formation of porous structure can be attributed to the addition of dicyandiamide in the sol–gel process. The composite material exhibits a high discharge capacity of 114.7 mA h g−1 at 1 C in the voltage range of 3–4.3 V after 600 cycles. Even at a high rate of 5 C, the discharge capacity of 90.5 mA h g−1 is achieved after 600 cycles. The superior electrochemical performance mainly benefits from the porous and nitrogen doped carbon.

Feifan Guo

Papers

High-performance oxygen evolution electrocatalysis by boronized metal sheets with self-functionalized surfaces

A class of metal diboride electrocatalysts synthesized by a molten salt-assisted reaction for the hydrogen evolution reaction

Precursor-mediated synthesis of double-shelled V2O5 hollow nanospheres as cathode material for lithium-ion batteries

Enhanced electrochemical performance of Li3V2(PO4)3 microspheres assembled with nanoparticles embedded in a carbon matrix

Li3V2(PO4)3 particles embedded in porous N-doped carbon as high-rate and long-life cathode material for Li-ion batteries