Photoelectrochemical Water Splitting Cells Research Articles

Improved electrocatalytic activity of electrodeposited Ni3S4 counter electrodes for dye- and quantum dot-sensitized solar cells

Nickel sulfide (Ni3S4) films are deposited onto F-doped SnO2 (FTO) substrates using a facile one-step potentiodynamic electrodeposition without any extra treatment. The potential sweep is between −0.9V and 0.7V (vs. Ag/AgCl) for 4–10 cycles. The series resistance is gradually reduced with increased Ni3S4 film thickness, indicating the metallic conduction of the Ni3S4 phase. The Ni3S4 deposited for 8 cycles (denoted FTO/Ni3S4-8) exhibits full coverage on the FTO substrate, with 110nm thickness and a distinctive structure of porous and nanoscale interconnecting nanoparticular networks, which provide numerous electrochemical active sites to contact with electrolyte. The film not only exhibits prominent electrocatalytic activities but also shows excellent electrochemical stability in both polysulfide and iodide-based electrolytes compared to FTO/Pt. On the contrary, the FTO/Ni3S4-10 shows merged clusters, leading to more compact and less porous morphology, which reduced electrocatalytic activity. The quantum dot-sensitized solar cell (QD-SSC) fabricated using the FTO/Ni3S4-8 counter electrode (CE) exhibits power conversion efficiency (PCE) of 4.57%, short-circuit current (Jsc) of 15.92mAcm−2, open-circuit voltage (Voc) of 0.545V, and fill factor (FF) of 52.63%. In addition, the dye-sensitized solar cell (D-SSC) using FTO/Ni3S4-8 CE achieves a PCE of 8.17%, Jsc of 16.28mAcm−2,Voc of 0.735V, and FF of 68.34%. On the other hand, the PCEs of 2.56% and 7.58% are obtained for QD-SSC and D-SSC with Pt CE, respectively. We suggest that the electrodeposited Ni3S4 should be promising electrocatalytic electrodes for various applications such as QD-SSCs, D-SSCs, supercapacitors, photoelectrochemical water-splitting cells, and batteries.

A Tandem Water Splitting Cell Based on Nanoporous BiVO4 Photoanode Cocatalyzed by Ultrasmall Cobalt Borate Sandwiched with Conformal TiO2 Layers

The BiVO4 photoanode with 2.4 eV band gap and a-Si photocathode with 1.7 eV band gap are ideal to form a self-driven tandem photoelectrochemical (PEC) water-splitting cell. The main obstacles for such tandem cells are the relatively low activity and stability of BiVO4 photoanode. Herein, we demonstrate a highly active undoped BiVO4 cocatalyzed with ultrasmall cobalt borate (CoBi) via photoassisted electrodeposition. The conformal TiO2 layer deposited on BiVO4 surface helps the formation of ultrasmall CoBi cocatalysts with excellent charge-transfer properties. The resulting BiVO4 photoanode exhibited a remarkably stable photocurrent of ∼2.5 mA/cm2 at 1.23 V vs RHE. By combining the highly active and stable BiVO4 photoanode with a-Si photocathode, a solar-driven tandem water-splitting cell is successfully fabricated with up to 3% solar-to-hydrogen (STH) conversion efficiency, a fairly high efficiency yet reported for undoped BiVO4 photoanode.

Sizable Excitonic Effects Undermining the Photocatalytic Efficiency of β-Cu2V2O7.

Copper vanadates have been proposed as promising photoanodes for water-splitting photoelectrochemical cells, but their performance has recently been shown to be severely limited. To understand this behavior, we study the electronic structure and the optical properties of β-Cu2V2O7 both experimentally and computationally. The measured absorption spectrum shows an absorption peak at 1.5 eV followed by the onset of an apparent continuum at 2.26 eV, as generally found for this class of materials. We perform calculations within the framework of the QS GW̃ method and the Bethe-Salpeter equation while including effects of magnetic ordering, nuclear quantum motion, and thermal vibrations. We demonstrate the occurrence of two kinds of excitons with high binding energies upon optical excitation in β-Cu2V2O7, which account for the first absorption peak and the lower edge of the apparent continuum. The results are confirmed by photoluminescence measurements, where sub-band-gap emissions are found for both excitons. These results providean explanation for the low photocatalytic efficiencies of copper vanadates, despite the favorable size of their optical band gaps.

Orthorhombic NiSe2 Nanocrystals on Si Nanowires for Efficient Photoelectrochemical Water Splitting.

Photocatalytic water splitting is a vital technology for clean renewable energy. Despite enormous progress, the search for earth-abundant photocatalysts with long-term stability and high catalytic activity is still an important issue. We report three possible polymorphs of nickel selenide (orthorhombic phase NiSe2, cubic phase NiSe2, and hexagonal phase NiSe) as bifunctional catalysts for water-splitting photoelectrochemical (PEC) cells. Photocathodes or photoanodes were fabricated by depositing the nickel selenide nanocrystals (NCs) onto p- or n-type Si nanowire arrays. Detailed structural analysis reveals that compared to the other two types, the orthorhombic NiSe2 NCs are more metallic and form less surface oxides. As a result, the orthorhombic NiSe2 NCs significantly enhanced the performance of water-splitting PEC cells by increasing the photocurrents and shifting the onset potentials. The high photocurrent is ascribed to the excellent catalytic activity toward water splitting, resulting in a low charge-transfer resistance. The onset potential shift can be determined by the shift of the flat-band potential. A large band bending occurs at the electrolyte interface, so that photoelectrons or photoholes are efficiently generated to accelerate the photocatalytic reaction at the active sites of orthorhombic NiSe2. The remarkable bifunctional photocatalytic activity of orthorhombic NiSe2 promises efficient PEC water splitting.

A mini review: Functional nanostructuring with perfectly-ordered anodic aluminum oxide template for energy conversion and storage

Nanostructures have drawn great attentions for functional device applications. Among the various techniques developed for fabricating arrayed nanostructures of functional materials, nanostructuring technique with porous anodic aluminum oxide (AAO) membrane as templates becomes more attractive owing to the superior geometrical characteristics and low-cost preparation process. In this mini review, we summarize our recent progress about functional nanostructuring based on perfectly-ordered AAO membrane to prepare perfectly-ordered nanostructure arrays of functional materials toward constructing high-performance energy conversion and storage devices. By employing the perfectly-ordered AAO membrane as templates, arrayed nanostructures in the form of nanodot, nanorod, nanotube and nanopore have been synthesized over a large area. These as-obtained nanostructure arrays have large specific surface area, high regularity, large-scale implementation, and tunable nanoscale features. All these advanced features enable them to be of great advantage for the performance improvement of energy conversion and storage devices, including photoelectrochemical water splitting cells, supercapacitors, and batteries, etc.

Investigation of p-type Ca2Fe2O5 as a Photocathode for Use in a Water Splitting Photoelectrochemical Cell

Ca2Fe2O5 is a p-type oxide that has not been previously investigated as a photocathode for use in a water splitting photoelectrochemical cell. In this study, Ca2Fe2O5 was synthesized as a polycrystalline electrode, and its properties relevant to photoelectrochemical hydrogen production were investigated. Ca2Fe2O5 electrodes were prepared by thermally annealing electrodeposited FeOOH films with a solution of Ca(NO3)2 drop-casted onto the electrode surface. The bandgap energy, band edge positions, flatband potential, photocurrent generation, and chemical and photoelectrochemical stabilities of Ca2Fe2O5 were examined. The results showed that Ca2Fe2O5 has an ideal bandgap and band positions that can enable the utilization of visible light for overall solar water splitting. Photocurrent–time measurements were also performed and the changes in morphology, crystallinity, and composition of the Ca2Fe2O5 electrodes before and after the photocurrent measurement were investigated. This made it possible to examine th...

Experimental and Computational Investigation of Lanthanide Ion Doping on BiVO4 Photoanodes for Solar Water Splitting

N-type bismuth vanadate (BiVO4) has emerged as one of the most promising photoanodes for use in water-splitting photoelectrochemical cells in recent years. However, its photoelectrochemical propert...

Graphdiyne Nanowall for Enhanced Photoelectrochemical Performance of Si Heterojunction Photoanode.

Graphdiyne (GDY), a new member of 2D carbon material family, was introduced into a Si heterojunction (SiHJ)-based photoelectrochemical water splitting cell. With assistance of magnetron-sputtered NiOx, the plateau photocurrent density of SiHJ/GDY/NiO x-10 nm with optimized NiO x film thickness was twice higher than that of SiHJ/NiO x-10 nm, demonstrating the catalytic function of GDY itself as well as the synergistic effect between GDY and NiO x. The results verified that GDY is a promising photoelectrode material candidate to realize highly efficient PEC performance, and pave a novel pathway to further improve Si-based PEC system.

Analysis of the Charge Transfer Dynamics in Oxide Based Photoelectrodes through Small Perturbations Techniques

Photoelectrochemical water splitting using solar energy is an attractive way to produce hydrogen as a clean fuel and energy storage material; use of hydrogen either in fuel cells or a combustion engine only produces water as reaction product. This process requires semiconductor electrodes capable of providing rapid charge transfer at the semiconductor/aqueous solution interface, which exhibit long-term stability and efficiently harvest a large portion of the solar spectrum1. However, the success of efficient water splitting has proved to be an elusive goal, and no material has yet been found that fulfill all criteria for efficient solar water splitting. Some of the main limitations have been the fast recombination of charge carriers, either in the bulk or at the surface, and slow transfer kinetics from the semiconductor to the electrolyte solution2. Unfortunately, the kinetic constants that describe these processes are parameters that are not easily accessible from conventional measurements. Frequency-dependent techniques, such as electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS), are two powerful tools to study the main carrier dynamic mechanisms inside the photoelectrochemical cell. Bertoluzzi and Bisquert have recently proposed an equivalent circuit to describe the main mechanisms in water splitting electrochemical cells3. Ponomarev and Peter proposed a generalized analytical model for IMPS in electrochemical systems4 and showed that EIS and IMPS analysis should provide identical values for rate constants5. In this work, we describe the general employment of small-signal perturbation techniques to determine the rate-determining steps in the charge carrier dynamics in water splitting photoelectrochemical cells, and we apply the theoretical approach to obtain the rate constants of charge transfer and recombination in different systems. The analysis shows how the kinetics of the recombination reaction and charge transfer to the electrolyte solution affect the photocurrent obtained in each case.

(Invited) Metal-Organic Framework-Metal Oxide Composite for Solar Water Splitting

The metal-organic framework (MOF) has been deposited on metal oxide surface layer-by-layer. The synthesis of MOF-inorganic oxide composite has been optimized. The optimized MOF-oxide composite has been used in a photoelectrochemical cell for solar water splitting. The composite shows much higher photocurrent density than the metal oxide alone.

Integrating Semiconducting Catalyst of ReS2 Nanosheets into P-Silicon Photocathode for Enhanced Solar Water Reduction.

Loading electrocatalysts at the semiconductor-electrolyte interface is one of the promising strategies to develop photoelectrochemical water splitting cells. However, the assembly of compatible and synergistic heterojunction between the semiconductor and the selected catalyst remains challenging. Here, we report a hierarchical p-type silicon (p-Si)/ReS2 heterojunction photocathode fabricated through the uniform growth of vertically standing ReS2 nanosheets (NSs) on a planar p-Si substrate for the solar-driven hydrogen evolution reaction (HER). The laden ReS2 NSs not only serve as a high-activity HER catalyst but also render a suitable electronic band coupled with p-Si into a II-type heterojunction, which facilitates the photoinduced charge production, separation, and utilization. As a result, the assembled p-Si/ReS2 photocathode exhibits a 23-fold increased photocurrent density at 0 VRHE and a 35-fold enhanced photoconversion efficiency compared with the pure p-Si counterpart. The bifunctional ReS2 as a catalyst and a semiconductor enables multi-effects in improving light harvesting, charge separation, and catalytic kinetics, highlighting the potential of semiconducting catalysts integrated into solar water splitting devices.

ZnO/CuO/M (M = Ag, Au) Hierarchical Nanostructure by Successive Photoreduction Process for Solar Hydrogen Generation.

To date, solar energy generation devices have been widely studied to meet a clean and sustainable energy source. Among them, water splitting photoelectrochemical cell is regarded as a promising energy generation way for splitting water molecules and generating hydrogen by sunlight. While many nanostructured metal oxides are considered as a candidate, most of them have an improper bandgap structure lowering energy transition efficiency. Herein, we introduce a novel wet-based, successive photoreduction process that can improve charge transfer efficiency by surface plasmon effect for a solar-driven water splitting device. The proposed process enables to fabricate ZnO/CuO/Ag or ZnO/CuO/Au hierarchical nanostructure, having an enhanced electrical, optical, photoelectrochemical property. The fabricated hierarchical nanostructures are demonstrated as a photocathode in the photoelectrochemical cell and characterized by using various analytic tools.

(Invited) A New Strategy to Enhance Long-Term Photostability of BiVO4 Photoanodes for Solar Water Splitting

As the performance of photoelectrodes used in in a photoelectrochemical cell (PEC) for water splitting continues to improve, concurrently improving their photostabilities has become an important issue. In this presentation, we report a new strategy to suppress photocorrosion of photoelectrodes using a BiVO4 photoanode as an example. BiVO4 has been recently identified as a promising oxide-based photoanodes for solar water splitting owing to many of its uniquely advantageous features. Anodic photocorrosion of BiVO4 photoanodes involves the loss of V5+ from the BiVO4 lattice by dissolution. Any composition change caused by the anodic photocorrosion of BiVO4 must be coupled with the oxidation of one or more of the elements present in BiVO4. Since V5+ in the BiVO4 lattice cannot be further oxidized, the dissolution of V5+ must be linked with the photooxidation of Bi3+ and/or lattice O2- ions. We show that the use of a V5+-saturated electrolyte, which inhibits the photooxidation-coupled dissolution of BiVO4, can slow down the rate of photocorrosion considerably and serve as a simple yet effective method to suppress anodic photocorrosion of BiVO4. We also discovered that the V5+ species in the solution can incorporate into the oxygen evolution catalyst (OEC) layer present on the BiVO4 surface during water oxidation, further enhancing water oxidation kinetics. The effect of the V5+ species in the electrolyte on both the long-term photostability of BiVO4 and the performance of the OEC layer will be discussed in detail.

Morphological Control Effect of Hierarchical Heterostructure α-Fe2O3/TiO2 Nanotube for Photoelectrochemical Water Splitting

Large-band gap metal oxide TiO2 have suitable band positions for photoelectrochemical cells (PECs) for solar-driven water splitting, but uses only UV light region in the solar spectrum which represent only about 5 % of the energy. On the other hand, α-Fe2O3 with suitable bandgaps for efficient absorption in the solar spectrum require an external bias to drive hydrogen generation at the cathode due to the conduction band of α-Fe2O3 below the H2 evolution potential and have short carrier diffusion lengths. Synthesizing the metal oxide nanomaterials which have both suitable band position to drive reaction and visible light absorbed band gap is one of the major challenge in PECs for water splitting field. Hetero structure of α-Fe2O3 and TiO2 offer a potential solution to improve this problem. However, the inherent low electrical conductivity resulting in the high electron-hole pair recombination rate and short carrier diffusion length of α-Fe2O3 limit its practical use. Here we report a novel hierarchical heterostructure of α-Fe2O3 ultrathin nanoflakes branched on TiO2 nanotube strategy for PECs for water splitting. On the basis of the detailed experimental results and associated theoretical analysis, we demonstrate that suitable morphological control of α-Fe2O3 and TiO2 plays an important role in enhancing the photoelectrochemical water splitting performance.

(Invited) Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) recombination. In order to circumvent these limitations considerable research efforts have been directed at creating more complex architectures involving e.g. tandem cells, nanostructures, and buried junctions to correctly position the bands and provide enough driving force to surmount overpotentials and the 1.23 eV energetic barrier of the water splitting reactions. Therefore, developing novel III-V photoabsorbers with adequate band energetics is key to conceiving simpler, yet efficient single semiconductor PEC cells for production of solar fuels at a competitive cost. First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb narrows the band gap in Ga(Sbx)N1-x and changes the electronic band gap from indirect to direct in GaSbP. Theoretical computations predict that, with band gaps in the order of 2 eV, these materials straddle the potential window for water oxidation and proton reduction in acidic solution. Single crystalline films of these two materials have been deposited by halide/hydride vapor phase epitaxy and metalorganic chemical vapor deposition systems, in a wide range of Sb incorporation without phase segregation. Experimental results corroborate the significant band gap reduction in GaSbN from 3.4 to 1.5 eV(Fig 1a) and suggest the conversion of excitonic transitions in the electronic band structure of GaSbP (Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data and low temperature photoluminescence spectroscopy. Electrodes comprising Ga(Sbx)N1-x and Ga(Sbx)P1-x have been benchmarked employing 2- and 3-electrode standard methods for assessment of their characteristic attributes, i.e. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situ fluorescence probing. This presentation will highlight recent advances in the understanding of the inter-relationship of processing/synthesis, material structure and photoelectrochemical properties of this new class of materials. Also, preliminary data on un-assisted photoactivity involving p-Si cathode and n-type GaSbP anode will be presented. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909). Collaborations with Dr. Todd Deutsch & Dr. James Young of NREL, and Dr. Madhu Menon of Univ. of Kentucky are acknowledged. Also acknowledge contributions of prior PhD students (Alejandro Garcia and Swathi Sunkara).

WO3 nanoflakes decorated with CuO clusters for enhanced photoelectrochemical water splitting

The low quantum efficiency arising from poor charges transfer and insufficient light absorption is one of the critical challenges toward achieving highly efficient water splitting in photoelectrochemical cells. Three dimensions (3D) structures and heterojunctions have received intensive research interests recent years due to their excellent ability to separate photo-generated charges as well as the enhanced light harvesting property. Herein, 3D CuO/WO3 structure was fabricated through a facile solvothermal method followed by chemical bath deposition. The loading of CuO clusters on WO3 nanoflake arrays results in a much improved photocurrent density compared with that of pristine WO3 nanoflake arrays, which reaches 1.8 mA/cm2 at 1.23 V vs. the reversible hydrogen electrode. The electrochemical impedance spectroscopy measurement demonstrates that the improved performance of CuO/WO3 electrode is attributed to the accelerated charge transfer kinetics as a result of the desirable band alignment in CuO/WO3 heterojunction. This work demonstrates a facile strategy to construct superior WO3 electrode, which will ultimately allow for efficient storage of solar energy into hydrogen.

Comparative investigation of the molybdenum sulphide doped with cobalt and selenium towards hydrogen evolution reaction

Hydrogen evolution reaction (HER) using earth abundant catalyst is one of the most promising approaches to replace precious platinum (Pt) and Pt-based catalysts for a green-energy environment. In this work, the HER performance of pristine MoS2 is enhanced by doping of Co and Se atoms, which serve as additional active sites and oxygen defects on HER performance respectively, thereby tuning the electronic structure of pristine MoS2 and further optimizing its hydrogen adsorption energy. The structural and morphological characterizations reveal the presence of the cuboid and raspberry nanostructures with a high order of crystallinity and extended interlayer spacing. The Co- and Se-doped MoS2, display enhanced HER activity than pristine MoS2 with a low onset potential of 181 mV and 166 mV, and a low overpotential of 218 mV and 207 mV at a cathodic current density of 10 mA cm−2. More importantly, the Se-doped MoS2 shows good catalytic stability with a low Tafel slope of 44 mV dec−1 as compared with that of Co-doped MoS2 (50 mV dec−1) and other reported values in the literature. Furthermore, the density functional theory calculations were carried out to get a fundamental understanding of electrocatalytic activity of Co and Se doping onto MoS2. This work can serve as a general methodology to enhance HER activity for the upcoming potential catalysts in the future and may uncover several applications in the field of solar photo-electrochemical (PEC) water-splitting cells and proton exchange membrane (PEM) electrolyzers.

Transition-Metal-Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review.

Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is one of the most promising approaches for a sustainable energy supply. Highly active, cost-effective, and robust photoelectrodes are undoubtedly crucial for the PEC technology. To achieve this goal, transition-metal-based electrocatalysts have been widely used as cocatalysts to improve the performance of PEC cells for water splitting. Herein, this Review summarizes the recent progresses of the design, synthesis, and application of transition-metal-based electrocatalysts as cocatalysts for PEC water splitting. Mo, Ni, Co-based electrocatalysts for the hydrogen evolution reaction (HER) and Co, Ni, Fe-based electrocatalysts for the oxygen evolution reaction (OER) are emphasized as cocatalysts for efficient PEC HER and OER, respectively. Particularly, some most efficient and robust photoelectrode systems with record photocurrent density or durability for the half reactions of HER and OER are highlighted and discussed. In addition, the self-biased PEC devices with high solar-to-hydrogen efficiency based on earth-abundant materials are also addressed. Finally, this Review is concluded with a summary and remarks on some challenges and opportunities for the further development of transition-metal-based electrocatalysts as cocatalysts for PEC water splitting.

Electrochemically active and robust cobalt doped copper phosphosulfide electro-catalysts for hydrogen evolution reaction in electrolytic and photoelectrochemical water splitting

The area of non-noble metals based electro-catalysts with electrochemical activity and stability similar or superior to that of noble metal electro-catalyst for efficient hydrogen production from electrolytic and photoelectrochemical (PEC) water splitting is a subject of intense research. In the current study, exploiting theoretical first principles study involving determination of hydrogen binding energy to the surface of the electro-catalyst, we have identified the (Cu0.83Co0.17)3P: x at. % S system displaying excellent electrochemical activity for hydrogen evolution reaction (HER). Accordingly, we have experimentally synthesized (Cu0.83Co0.17)3P: x at. % S (x = 10, 20, 30) demonstrating excellent electrochemical activity with an onset overpotential for HER similar to Pt/C in acidic, neutral as well as basic media. The highest electrochemical activity is exhibited by (Cu0.83Co0.17)3P:30 at. % S nanoparticles (NPs) displaying overpotential to reach 100 mA cm−2 in acidic, neutral and basic media similar to Pt/C. The (Cu0.83Co0.17)3P:30 at. % S NPs also display excellent electrochemical stability in acidic media for long term electrolytic and PEC water splitting process [using our previously reported (Sn0.95Nb0.05) O2: N-600 nanotubes (NTs) as the photoanode]. The applied bias photon-to-current efficiency obtained using (Cu0.83Co0.17)3P:30 at. % S NPs as the cathode electro-catalyst for HER in an H-type PEC water splitting cell (∼4%) is similar to that obtained using Pt/C (∼4.1%) attesting to the promise of this exciting non-noble metal containing system.

(In, Cu) Co-doped ZnS nanoparticles for photoelectrochemical hydrogen production

Abstract In and Cu co-doped ZnS nanoparticles were successfully synthesized in DI water and ethanol solvent by a sonochemical approach using citric acid as surfactants in aqueous medium. FESEM micrographs show that In and Cu co-doped ZnS crystallites have a rough surface nanostructure and the as-synthesized photocatalysts were tested for the photocatalytic hydrogen evolution from water splitting via the irradiation of simulated sunlight. Among In and Cu co-doped ZnS products, 4In4CuZnS photocatalyst can achieve the maximum hydrogen production rate (752.7 μmol h−1 g−1) in 360 min under simulated sunlight illumination. Meanwhile, we separated the hydrogen and oxygen cells using an ion exchange membrane. Both electrodes (working electrode and Pt electrode) are dipped into each cell containing an aqueous solution containing 0.1 M Na2S at pH 3 to convert water into hydrogen and oxygen under solar irradiation. As expected, the photoelectrochemical water splitting cells could significantly improve the photocatalytic activity, where the 4In4CuZnS nanoparticles shows the photoelectrochemical performance with photocurrent density of 12.2 mA cm−2 at 1.1 V and hydrogen evolution rate of 1189.4 μmol h−1 g−1.