An Extreme Energy-Saving Carbohydrazide Oxidization Reaction Directly Driven by Commercial Graphite Paper in Alkali and Near-Neutral Seawater Electrolytes.

The energy-saving anode with low oxidization potential has been an intriguing pursue for earth-abundant seawater electrolysis. In this paper, we first introduced a superior energy-saving carbohydrazide oxidization reaction catalysis system in the anode section, which can be driven by commercial graphite paper with good durability. Combining this catalysis reaction and common graphite paper, the lowest anodic potentials 0.63 V (vs RHE) and 1.09 V (vs RHE) were obtained for driving a 10 mA/cm2 current density in alkali and near-neutral seawater electrolytes, respectively, outperforming all the as-reported alkali or near-neutral seawater catalysts accordingly to the best of our knowledge.

Introduction

Hydrogen as one kind of green energy is gaining more and more attention. Scientists usually employ water electrolysis to generate hydrogen.[1] The water electrolysis process contains hydrogen evolution reaction (HER)[2,3] and oxygen evolution reaction (OER) processes.[4−7] The OER process refers to multielectron transfer and is dynamically sluggish compared with the HER process.[8,9] Initially, noble metal-based catalysts are used to drive the OER process. However, researchers begin to develop earth-abundant catalysts owing to the scarcity of noble metals.[10,11] The as-reported earth-abundant OER catalysts mainly contain compounds that are Ni-based,[12−14] Fe-based,[15] Co-based,[16,17] Cu-based,[18] Mn-based,[19,20] bimetal-based,[21−24] trimetal-based,[25] etc. Although many advanced catalysts have been fabricated, low-cost water electrolysis technology is still highly desired.

To realize more economic water electrolysis, scientists attempt to employ earth-abundant and low-cost seawater electrolysis to replace pure water electrolysis. The seawater electrolysis includes alkali seawater electrolysis and neutral or near-neutral seawater electrolysis. Similarly, the anodic potential consumption dominates the main section during the seawater electrolysis, so researchers pay more attention to the fabrication of highly efficient OER catalysts in the anode section. There exists an oxygen evolution reaction and chloride ion oxidation side reaction for the anode section during seawater electrolysis. The selectivity of the OER process will affect the voltage consumption of the electrolysis process. High-performance OER catalysts always have much higher OER selectivity. Alkali conditions are beneficial for oxygen evolution. Herein, scientists first design some noble catalysts, such as RuO2,[26] Pb2Ru2O7–, and so on. To improve the OER selectivity and catalysis performance, many superior earth-abundant catalysts have been prepared, such as Ni-Fe-LDH,[27] NCFPO/C@CC,[28] NiMoN@NiFeN,[29] S-(Ni,Fe)OOH,[30] and so on. Meanwhile, some researchers also study near-neutral seawater electrolysis and fabricate a lot of OER catalysts, such as Pb2Ru2O7–, Co-Fe LDH,[31] NiFe LDH,[32] Co-P-B/rGO,[33] and so on. Although scientists have made many works, the anodic potentials of catalysts are still more than 1.40 V (vs RHE) and 1.70 V (vs RHE) for driving a 10 mA/cm2 current density in alkali and near-neutral seawater electrolytes till now to the best of our knowledge, respectively. Herein, the high anodic potential of catalysts is still the big obstacle for improving seawater electrolysis performance. As such, to solve this problem, the development of a novel energy-saving oxidization reaction catalysis system in the anode section will be highly desired.

Results and Discussion

Introduction of an Energy-Saving Anodic COR Catalysis System

In this work, we proposed an exceptional energy-saving carbohydrazide oxidization reaction (COR) catalysis system in the anode section for the first time. As shown in Figure a, the carbohydrazide molecule contains abundant N–H bonds with reduction properties, which indicates that the carbohydrazide molecule is more easily oxidized. The literature[34] shows that the theoretical oxidized potential of the carbohydrazide molecule is −1.25 V (vs RHE).[34] The literature[35] demonstrates that a C atom may present a positive charge, a N atom may present a negative charge, and C–N bonds will possibly crack under the catalysis of catalysts for the carbohydrazide molecule similar to the urea molecule.[35] This energy-saving carbohydrazide catalysis system contains carbohydrazide-containing electrolytes and a graphite paper catalytic electrode. As we know, Cl– corrosion is a potential threat to metal-based catalysts for seawater electrolysis. Carbon materials are more resistant to corrosion than metal materials. Graphite paper as a typical carbon material representative is usually widely used as the conductive electrode. Herein, graphite paper is studied as a catalytic anode in this paper. Combining carbohydrazide-containing electrolytes and a common commercial graphite paper electrode, the lowest anodic oxidization potentials 0.63 V (vs RHE) and 1.09 V (vs RHE) for driving a 10 mA/cm2 current density were obtained in alkali or near-neutral seawater electrolytes till now, respectively, surpassing all the as-reported alkali or near-neutral seawater electrolysis systems for the anodic reaction section to the best of our knowledge (Tables S1 and S2).

Figure 1

Scheme of an exceptional energy-saving carbohydrazide oxidization reaction catalysis system in alkali or near-neutral seawater electrolytes directly driven by graphite paper. (a) Structural model of the carbohydrazide molecule. (b) Design of the energy-saving carbohydrazide oxidization reaction seawater electrolysis cell.

First, the physical property of commercial graphite paper was studied. Figure S1 shows that commercial graphite paper is composed of abundant microscale graphite flakes. As shown in Figure S2, the corresponding matrix carbon element and a small amount of oxygen element were all detected by XPS in the surface of graphite paper. Figure S3 shows that graphite paper presents obvious crystal characterization. PDF #75-2078 shows that the crystal peaks at 26.6 and 54.8° come from the (111) and (222) crystal planes of graphite, respectively.

COR Catalysis Performance of Graphite Paper in Alkali Seawater Electrolytes

Further, the COR performance of graphite paper in alkali seawater electrolytes was first investigated. As shown in Figure a, this graphite paper electrode exhibits good carbohydrazide oxidization reaction catalysis performance in alkali seawater electrolytes, which can drive 10 and 100 mA/cm2 current densities with demands of 0.63 V (vs RHE) and 1.03 V (vs RHE), respectively. The measured double-layer capacitance (Cdl) value of the graphite paper electrode is 24.3 mF/cm2. According to the calculation method in the Supporting Information, the electrochemical active surface area (ECSA) of the graphite paper electrode is 607.5 cm2 per 1 cm2 geometric surface area (GSA) (Figures S4 and S5). The large ECSA value indicates that this electrode has abundant electrochemical active sites.

Figure 2

Carbohydrazide oxidization reaction (COR) performance of graphite paper in alkali seawater electrolytes. (a) Comparison of LSV performance of graphite paper for the OER and COR in alkali water and seawater electrolytes. (b) Comparison of potential at 10, 50, and 100 mA/cm2 current densities of graphite paper for the OER and COR in alkali water and seawater electrolytes. (c) Comparison of the anode oxidization potential between the graphite paper alkali seawater COR catalysis system and other alkali seawater catalysts at a 10 mA/cm2 current density. (d) Comparison of the anode oxidization potential between the graphite paper alkali seawater COR catalysis system and other alkali seawater catalysts at a 100 mA/cm2 current density. Note: the involved catalysts in (c) and (d) come from Table S1 in the Supporting Information.

As shown in Figure a,b, when the graphite paper drives 10, 50, and 100 mA/cm2 current densities, the potentials of the carbohydrazide oxidation reaction for the graphite paper electrode are 1120, 1200, and 1090 mV lower than the potential of the seawater oxidation reaction, respectively. Moreover, linear sweep voltammetry (LSV) performance of graphite paper in alkali water and seawater electrolytes is similar. The result indicates that carbohydrazide oxidization technology shows obvious advantages in the energy-saving hydrogen production area compared with the traditional pure seawater oxidization method.

Notably, this graphite paper electrode can drive 10 and 100 mA/cm2 current densities with the lowest anodic oxidization potentials 0.63 V (vs RHE) and 1.03 V (vs RHE) in alkali seawater electrolytes with the aid of the carbohydrazide oxidization catalysis reaction till now, respectively, outperforming all the as-reported alkali seawater catalysts accordingly to the best of our knowledge (Figure c,d and Table S1). The above obtained results confirm that the graphite paper electrode is superior when driving the carbohydrazide oxidization reaction process.

COR Catalysis Performance of Graphite Paper in Near-Neutral Seawater Electrolytes

Furthermore, the COR performance of graphite paper in near-neutral seawater electrolytes was studied for the first time. As shown in Figure a, this graphite paper electrode also presents excellent carbohydrazide oxidization reaction catalysis performance in near-neutral seawater electrolytes, which can drive 5 and 10 mA/cm2 current densities with demands of 0.96 V (vs RHE) and 1.09 V (vs RHE), respectively. The measured Cdl value of the graphite paper electrode is 3.82 mF/cm2. According to the calculation method in the Supporting Information, the ECSA of the graphite paper electrode is 95.5 cm2 per 1 cm2 GSA (Figures S6 and S7). The large ECSA value demonstrates that this electrode has enriched electrochemical active sites.

Figure 3

Carbohydrazide oxidization reaction (COR) performance of graphite paper in near-neutral seawater electrolytes. (a) Comparison of LSV performance of graphite paper for the OER and COR in near-neutral water and seawater electrolytes. (b) Comparison of potential at 5 and 10 mA/cm2 current densities of graphite paper for the OER and COR in neutral water and seawater electrolytes. (c) Comparison of the anode oxidization potential between the graphite paper near-neutral seawater COR catalysis system and other near-neutral seawater catalysts at a 10 mA/cm2 current density. Note: the involved catalysts in (c) come from Table S2 in the Supporting Information.

As shown in Figure a,b, when the graphite paper drives 5 and 10 mA/cm2 current densities, the potentials of the carbohydrazide oxidation reaction for the graphite paper electrode are 800 and 760 mV lower than the potential of the seawater oxidation reaction in near-neutral seawater electrolytes, respectively. Moreover, the LSV performance of graphite paper in near-neutral water and seawater electrolytes is similar. The above result shows that this carbohydrazide oxidization method shows significant advantages in the energy-saving hydrogen production area compared with the traditional pure near-neutral seawater oxidization method. Importantly, this graphite paper electrode can drive a 10 mA/cm2 current density with the lowest anodic oxidization potential 1.09 V (vs RHE) in near-neutral seawater electrolytes by using the carbohydrazide oxidization catalysis reaction till now, outperforming all the as-reported near-neutral seawater catalysts to the best of our knowledge (Figure c and Table S2). The above obtained results also demonstrate that the graphite paper electrode is excellent for driving the carbohydrazide oxidization reaction process in near-neutral seawater electrolytes.

COR Catalysis Durability of Graphite Paper in Alkali or Near-Neutral Seawater Electrolytes

Electrochemical durability is also a key issue for COR catalysts in alkali or near-neutral seawater electrolytes. Figure a shows that the graphite paper can durably drive a 10 mA/cm2 current density carbohydrazide oxidation process for at least 10 h in alkali seawater electrolytes. In addition, as shown in Figure b, the graphite paper can also stably drive the 10 mA/cm2 current density carbohydrazide oxidation process for at least 10 h in near-neutral seawater electrolytes. The above results confirm that the common commercial graphite paper is stable for driving the carbohydrazide oxidization reaction process whether in alkali seawater electrolytes or near-neutral seawater electrolytes.

Figure 4

COR durability of graphite paper in alkali or near-neutral seawater electrolytes. (a) Chronoamperometric measurements of the carbohydrazide oxidation reaction at a 10 mA/cm2 current density for graphite paper in alkali seawater electrolytes. (b) Chronoamperometric measurements of the carbohydrazide oxidation reaction at a 25 mA/cm2 current density for graphite paper in near-neutral seawater electrolytes.

Conclusions

In summary, an exceptional energy-saving carbohydrazide oxidization reaction catalysis system in the anode section was introduced for the first time. Combining common commercial graphite paper and carbohydrazide-containing seawater electrolytes, the lowest anodic oxidization potentials 0.63 V (vs RHE) and 1.09 V (vs RHE) were obtained for driving a 10 mA/cm2 current density in alkali or near-neutral seawater electrolytes till now, respectively, surpassing all the as-reported alkali or near-neutral seawater catalysts accordingly to the best of our knowledge. This carbohydrazide oxidization reaction catalysis system exhibits good commercial application potential for seawater electrolysis in the energy-saving hydrogen production area.

Experimental Section

Materials and Reagents

Highly conductive graphite paper (GP) support was purchased from Latech Scientific Supply Pte. Ltd. Ethyl alcohol, KOH, Na2B4O7·10H2O, and carbohydrazide were bought from Sigma-Aldrich Chemical Reagent Co.

Pretreatment of the Graphite Paper Support

To remove the oil on the surface of GP, a piece of GP was immersed in ethyl alcohol at room temperature for 5 min.

Physical Characterizations

The surface microstructures of different samples were observed using a ZEISS SEM Supra 40 (attached EDS from Oxford Instrument). XRD patterns of varied samples were determined by a Bruker D8 Advanced Diffractometer System. X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD) was employed to investigate the elemental analysis of the surface layer of the samples.

Electrochemical Measurements

We use the typical three-electrode cell connected to a Corrtest CS2350H electrochemical workstation to investigate the COR process. In this cell, the GP, Hg/HgO, and Pt were used as the working electrode (1 × 1 cm2), the reference electrode, and the counter electrode, respectively. According to refs (26) and (29), the alkali or near-neutral seawater electrolytes can be artificially simulated by adding 0.6 M NaCl to 1 M KOH or 0.1 M sodium tetraborate, respectively. Herein, the alkali carbohydrazide-containing seawater electrolyte solution was prepared by adding 0.5 M carbohydrazide into alkali seawater electrolytes and the pH value of the electrolyte is about 14. Similarly, the near-neutral carbohydrazide-containing seawater electrolyte solution was synthesized by adding 0.5 M carbohydrazide into near-neutral seawater electrolytes and the pH value of the electrolyte is about 9. According to the equation E(RHE) = EHg/HgO + 0.059pH + 0.098 V, all the recorded potential values were converted according to the reversible hydrogen electrode (RHE) standard. LSV curves were also recorded with a scan rate of 1 mV/s. Notably, all the electrochemical data in this paper were shown after 100% IR compensation.