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Divide and conquer: improving the electrolysis of water



One of the most famous chemical formulas that we have known since the school day is H 2O is hydrogen oxide, i.e. water. Without this seemingly simple substance, life on our planet would be completely different, if at all. In addition to its life-giving functions, water has many other uses, among which it is worth highlighting the production of hydrogen (H). One of the methods for achieving this is the electrolysis of water, when it is divided into components, i.e. for oxygen and hydrogen. This is a fairly complex, costly, but effective method. However, there is no such thing in the world that scientists would not like to improve. A team of researchers from the University of Washington and the Los Alamos National Laboratory found a way to improve the electrolysis of water, significantly reducing the cost of its conduct without reducing the result. What changes had to be introduced into the electrolysis of water, why were these or those substances used,and what results does the updated hydrogen production method show? This will tell us the report of scientists. Go.

Study basis


Hydrogen is in many ways a unique element: it is the lightest among the elements of the periodic table, and its monatomic version is the most common substance in the Universe. In addition, hydrogen is an extremely friendly element that easily forms covalent bonds with most non-metals. In nature, we find hydrogen more often in the composition of a substance, including in water, rather than, so to speak, alone.

Under normal conditions, hydrogen is an odorless and tasteless gas with the chemical formula H 2 . It also has a liquid equivalent - liquid hydrogen, which, although not as popular in mass culture as liquid nitrogen, is no less extreme in terms of temperature: freezing point −259.14 ° C; boiling point −252.87 ° C.

It will take a lot of time to list all the specific applications of hydrogen, since it takes an active part in various fields of production: food industry, metallurgy, electronics, ammonia, etc. Not to mention the use of hydrogen as rocket fuel.

There are also several methods for producing hydrogen: from natural gas, from coal, and through electrolysis of water. According to a rough estimate, about 70 million tons are used annually in the world, of which only 100,000 tons are produced by electrolysis.

This methodological “discrimination” is due to the complexity and cost of electrolysis in conjunction with the resulting volumes of hydrogen compared to other methods. However, there is always an opportunity for improvement, which will be discussed later, but about everything in order.

The driving force of electrolysis of water for its splitting into oxygen and hydrogen is electricity. According to scientists, low-temperature electrolysis of water is of particular interest for the renewable energy sector, since this method can allow storing electricity from renewable sources in chemical bonds in the form of high-purity hydrogen.

In low-temperature electrolysis of water, a concentrated solution of KOH (potassium hydroxide), a proton exchange membrane (PEM fromproton-exchange membrane ) or alkaline anion-exchange membrane (AEM from alkaline anion exchange membrane ).

The main advantage of AEM electrolysis over other options is its cost. That is, for its implementation there is no need to use platinum group metals (PGM from platinum-group metals ) as catalysts. However, there is always a catch, in this case, the instability of the alkaline method, due to its sensitivity to pressure drops and the low rate of hydrogen production.

In traditional alkaline electrolyzers (electrolysis unit), a liquid alkaline electrolyte (30–40 wt.% KOH) circulates through electrodes that are separated by a porous membrane ( 1a ).


Image No. 1: scheme of low-temperature electrolysis of water.

This method works at a current density of 300–400 mA cm –2 at a temperature of 60–90 ° C and a voltage of 1.7–2.4 V. Scientists also note that liquid KOH is very sensitive to CO 2 , which is abundant in air, forming as a result of K 2 CO 3 . This process, in turn, reduces the anodic reaction and ionic conductivity, and the resulting K 2 CO 3 is deposited in the pores of the gas diffusion layer, blocking ion transfer. The conclusion is quite simple - the performance of electrolysis based on KOH is reduced due to its properties, which are extremely difficult to control.

As an alternative, electrolyzers based on ion-exchange membranes can act, which can replace a liquid electrolyte with a polymer electrolyte. PEM electrolyzers usually operate at higher current densities (1-3 A cm -2 at ~ 2.0 V) than alkaline electrolyzers, since the proton exchange membrane has a higher conductivity. ( 1b ).

The use of solid electrolytes in PEM electrolysis of water makes it possible to create a compact system with long-term and stable structural properties at high pressure drops (200–400 psi). But even this method has its drawbacks, in particular, the high cost of the installation for electrolysis, due to expensive acid-resistant equipment and the need for platinum group metals.

There have been several changes in AEM electrolysis over the past few years. One of the most important is the creation of polymer AEM ( 1c ). Alkaline AEM electrolysis combines many of the advantages of other methods: the ability to use catalysts without PGM; the ability to use pure water or a low concentration alkaline solution instead of concentrated alkaline electrolytes; low ohmic losses due to high conductivity and thin AEM. In addition to this, the membrane design of the installation allows it to operate at significant pressure drops, and also reduces its dimensions and weight. Not to mention the reduction in the cost of this device.

There are many advantages, you just don’t have time to admire, but there are also disadvantages. A very significant disadvantage of alkaline AEM electrolysis is the very fact of using corrosive concentrated alkaline electrolyte. If you use pure water, then the performance will be ridiculously small (400 mA cm -2 at 1.8 V).

In their study, scientists decided to try to get rid of some of the shortcomings of this method, thereby making it more attractive for mass production of hydrogen. The researchers found that a high concentration of quaternary ammonium compounds is necessary to increase the activity of hydrogen and oxygen evolution reactions in the AEM electrolyzer. It was also found that phenyl groups in the main chain of the ionomer * have a negative effect, forming acid phenols at high anode potentials.
Ionomer * - polymers consisting of electrically neutral and ionized compound units covalently bonded to the polymer backbone in the form of side groups of atoms.
In general, scientists were able to develop polystyrene ionomers with a high degree of quaternization * , which made it possible to create an AEM electrolyzer, whose performance is comparable to modern PEM electrolyzers.
Quaternization * is the conversion of compounds of elements of the 15th group (N, P, As, Sb), the atoms of which have a free electron pair, into quaternary salts when interacting with reagents of the RX type (X is an anionoid group).

Research results


Before you understand what an advanced AEM electrolyzer is capable of, it was necessary to establish what it cannot, that is, find out the factors limiting its performance. To do this, experiments were conducted with a rotating disk electrode (RDE from a rotating disk electrode ). RDE experiments provide information on the various requirements for electrolytes used in fuel cells and electrolysis cells by measuring the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and hydrogen oxidation reaction (HOR).


Image 2: Effect of NaOH (sodium hydroxide) concentration on the activity of electrocatalysts.

The graphs above show the OER polarization curves using IrO 2and HER using a polycrystalline platinum electrode (Pt poly) depending on the concentration of NaOH. The activity of OER and HER for the AEM electrolyzer increased significantly with increasing NaOH concentration from 0.01 M (pH = 12) to 1 M (pH = 14). HOR activity of Pt poly exhibits maximum activity at a NaOH concentration of 0.02 M (insert on 2b ). The loss of HOR activity at a higher NaOH concentration (> 0.1 M) was also accompanied by a lower current density limiting diffusion.

The lower HOR activity of Pt poly with a concentrated NaOH solution is explained by cumulative cation-hydroxide-water co-adsorption, which limits the access of hydrogen to the catalyst surface. However, joint adsorption does not affect the activity of HER and OER, since adsorption occurs from 0 to 0.9 V. The effect of NaOH concentration on the activity of ORR Pt poly showed a trend similar to HOR.

The Pt poly ORR activity increased with increasing NaOH concentration from 0.01 to 0.1 M, then began to decrease with a further increase in NaOH concentration to 1 M.

The experimental results suggest that the concentration of ammonium hydroxide required for AEM electrolyzers and AEM fuel cells can be different. For AEM electrolyzers, an ionomer with a higher ion exchange capacity (IECionexchange capacity ). An ionomer with a medium IEC is better suited for AEM fuel cells, since ions with a higher IEC cause limited gas transfer due to undesired co-adsorption of cation-hydroxide-water.

Then the scientists decided to find out what should be the ionomer for AEM electrolyzers. Experiments with RDE have shown that providing high pH conditions (> 13) in electrodes is important for creating highly efficient AEM electrolyzers.

Currently available anion exchange ionomers have two critical problems that can limit the high pH environment in AEM cells.

The first problem is the presence of phenyl groups in the main chain of the ionomer. A previous study showed that the phenyl group in the main chain of the ionomer can oxidize at OER potentials and form a phenolic compound that is acidic (pK a = 9.6). Unfortunately, most stable alkaline ionomers contain phenyl groups in their structure.

Consequently, AEM electrolyzers using ionomers containing a phenyl group are somehow prone to phenol formation.

In the aspect of the oxidation of the phenyl group, several extremely important features were discovered. The phenol formation rate is related to the adsorption energy of the phenyl group on the surface of the OER catalysts, and unsubstituted phenyl groups in the polymer side chain have a more harmful effect compared to the ammonium-substituted phenyl group.

The structure and size of the backbone fragments in polyaromatic compounds strongly affect the phenyl adsorption, whereas the side-functionalized phenyl group exhibits a much lower adsorption energy due to competing adsorption with ammonium groups. In addition, bimetallic platinum catalysts (e.g., PtRu, PtNi, and PtMo) can effectively reduce phenyl adsorption energy.

The second problem is the low concentration of the functional groups of ammonium hydroxide in the anion-exchange ionomers. The IEC value of the classic anion-exchange ionomers developed for AEM fuel cells is usually about 1.5 mEq g -1 (mEq - milliequivalent). For the ionomer, the estimated ammonium concentration in the electrode filled with water is relatively low (~ 0.1 M). The inhomogeneous distribution of the ionomer in the electrode further reduces the reaction efficiency and the conductivity of the hydroxide. Therefore, higher IEC ionomers should be useful to increase the performance of the AEM electrolyzer.

However, one pulls the other, since several criteria must be taken into account for the synthesis of an ionomer with high IEC.

Firstly, there is a limitation on the maximum number of ammonium groups per polymer compound (the group of atoms that make up the polymer).

Secondly, high IEC anion exchange ionomers often undergo crosslinking reactions during the functionalization process, which complicates further processing.

And thirdly, when anion-exchange ionomers are synthesized with high IEC, they often become soluble in water, which is unsuitable for use in electrodes.

Like it or not, it will not be so simple to get around all these restrictions. Nevertheless, based on the above data, scientists prepared several polystyrene ionomers functionalized with trimethylammonium ( 3a ).


Image No. 3: chemical structure of polymeric materials used in the study.

The created ionomers possessed rather unique characteristics compared to conventional ionomer binders developed for AEM fuel cells.

Firstly, the main aliphatic polymer chain does not contain a phenyl group. The absence of a phenyl group in the polymer backbone excludes the possibility of phenyl adsorption and the formation of acid phenol.

Secondly, the polymer backbone does not contain long nonionic alkyl chains that can reduce the solubility of the polymer.

Thirdly, all phenyl groups in the side chains have substituted ammonium or amine groups, which minimize the adsorption of phenyl groups and help maintain a high pH.

Upon completion of the synthesis of ionomers, it was found that their IEC varies in the range from 2.2 to 3.3. For AEM, HTMA-DAPP was prepared, i.e. Diels-Alder polyphenylene functionalized with hexamethyltrimethylammonium ( 3b ). The hydroxide conductivity of HTMA-DAPP was 120 mS / cm at 80 ° C (mS - millisiemens; Siemens - unit of electrical conductivity).

The polyphenylene backbone in the HTMA-DAPP high molecular weight polymer provides excellent mechanical strength (tensile stress> 20 MPa at 90% relative humidity at 50 ° C). But quaternized polystyrene is too fragile to create membranes and therefore is not suitable for applications with an aqueous AEM electrolyzer that requires mechanically stable AEM.

The alkaline stability of HTMA-DAPP is also quite high: minimal decomposition for> 3000 hours in 4 M NaOH at 80 ° C. This indicator guarantees AEM electrolysis tests at an operating temperature of 85 ° C.

In conditions of using pure water, the membrane electrode shows a current density of 107 mA cm -2 at 1.8 V and 60 ° C. If you add 0.1 M NaOH to the water, then the indicators increase 3.5 times to 376 mA cm -2 at 1.8 V and 60 ° C.

If, when using pure water, the operating temperature is increased to 85 degrees, the current density in the electrolyzer will increase to 224 mA cm -2 .


Image 4: The effect of ionomers on AEM performance.

On chart 4ademonstrated progress in improving the performance of the electrolyzer due to the studied ionomer.

To obtain this improvement, it was first necessary to accurately determine the required content of ionomer. As a result, the current density of the membrane electrode with 9 wt% of the ionomer (two times higher than the baseline value) was 405 mA cm −2 at 1.8 V (red curve), which is 1.8 times higher than the baseline value of the membrane electrode (MEA).

Next, the membrane electrode and trimethylamine (CH3) 3N ionomers were integrated, which showed higher IEC values ​​at a higher ionomer content. MEA performance using TMA-53 (IEC = 2.6) increased significantly (blue curve). At 1.8 V, the current density was 791 mA cm -2, which is 2.0 times more than MEA with TMA-45. The current density at 1.8 V MES with TMA-62 (purple curve) and TMA-70 (green curve) additionally increased to 860 and 1360 mA cm -2, respectively. The current density of MEA with TMA-70 was 1.7 times higher than that of MEA with TMA-53, and 6 times higher than that of the base MEA at 1.8 V.

In addition to the effect of ionomers on the electrolyzer performance, scientists also investigated the effect of the phenyl group in ionomer ( 4b ). For this, experiments were conducted in which two MEA were compared, which were the same, with the exception of the bonding electrode. The first electrode is MEA with HTMA-DAP, and the second is with TMA-53. The ionomer content (9 wt.%) And the IEC value (2.6) for both electrodes were also the same.

When using 0.1 M NaOH electrolyte, the electrolyzer performance was very similar for both electrodes: 954 mA cm -2 for HTMA-DAPP MEA and 1.052 mA cm -2 for TMA-53 MEA. However, if pure water was used, MEA with TMA-53 (630 mA cm -2 ) showed significantly higher performance compared to MEA with DAPP-HTMA (484 mA cm -2 ).

Similar observations indicate that the operation of the cell is less sensitive to 0.1 M NaOH. This can be explained by the fact that acid phenols from the oxidation of the phenyl group were neutralized by an alkaline solution.

Further, a more detailed study of the characteristics of MEA with TMA-70 was carried out using catalysts not containing platinum group metals. A catalyst based on NiFe nanofoam was used as an anode.

Verification of the characteristics of MEA with different content of ionomers in the anode NiFe catalyst made it possible to determine that 20 wt.% The content of ionomer is the most effective.


Image No. 5: AEM capacity of an electrolytic cell with an anode catalyst without the use of platinum group metals.

The graph above shows the performance indicators of an AEM electrolyzer with an anode NiFe catalyst under the conditions of supply of 1 M and 0.1 M solutions of NaOH (60 ° ) and pure water (85 ° ) at 1.8 V: 5.3 A cm -2 (1 M NaOH); 3.2 A cm -2(0.1 M NaOH); 2.7 A cm -2 (clear water).

Further, for greater clarity, the performance of MEA with an anode NiFe catalyst was compared with the characteristics of a proton exchange membrane (PEM) electrolytic cell using platinum group metal catalysts.

In the kinetic region at voltages less than 1.58 V, MEA with a NiFe catalyst surpassed the PEM electrolyzer (insert in image No. 5). If in more detail, then at 1.5 V the current density MEA was 300 mA cm -2 , and this is two times higher than that of the PEM electrolyzer, where IrO 2 (iridium oxide) and nanostructured thin films are used.


Image 6: Strength indicators of AEM electrolytic cells with NiFe catalyst.

Among other things, scientists also decided to test the strength of the AEM electrolyzer in clean water. 6a shows a short-term strength test of an AEM of an electrolyzer with a NiFe catalyst at a constant current density of 200 mA cm -2 . Both at 60 and at 85 ° C, the voltage rapidly increased over ~ 10 hours.

It was found that the catalyst particles were washed from both the anodic and cathodic outlet streams. This may indicate that a high IEC (TMA-70) ionomer did not retain catalyst particles during continuous operation.

An increase in ionomer binding strength could be achieved by using the same ionomer with a lower IEC at 60 ° C.

On 6bA short-term strength test of an AEM electrolyzer using a TMA-53 ionomer is shown. The results clearly show that the system works stably for more than 100 hours after the initial increase in voltage from 1.75 to 2.1 V. The initial increase in voltage during the first 40 hours is probably due to the oxidation of phenyl.

The conclusion is quite sad - although the system shows excellent results in terms of performance, it cannot boast the same in terms of durability.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists .

Epilogue


In this work, researchers demonstrated a model of a binder system for electrodes that can increase the performance of an AEM electrolyzer. This option in its characteristics is not inferior to modern PEM electrolyzers, while it does not need platinum group metals, which significantly reduces the cost of the entire system.

A binder for the electrodes was synthesized based on the results of experiments with a rotating disk electrode, which showed the importance of a high local pH for an effective hydrogen evolution reaction and oxygen evolution reaction.

Removing phenyl groups from the polymer backbone prevents the formation of acid phenols, which can neutralize quaternary ammonium hydroxide and lower the pH of the electrolyte. In addition, increasing the pH of the electrodes was achieved by increasing the content of ionomer and IEC.

An AEM electrolyzer using a quaternized ammonium polystyrene ionomer has shown excellent performance even without a circulating alkaline solution.

Of course, there were some flaws. In the future, scientists intend to conduct a number of additional studies to improve the performance of the developed system and to increase its durability.

In total, all the considered observations are additional information in the field of developing highly efficient systems of electrolyzers, and also allow us to understand how to store renewable energy more efficiently.

As the authors of the study themselves say, the bottom line is that renewable energy sources are very unstable. For the same period of time, you can get a different amount of energy, because there can be different conditions (for example, a wind generator does not work very effectively in calm weather). However, sometimes there is an excess of energy that must be disposed of efficiently. The authors of this work believe that the use of this renewable energy is necessary for the production of hydrogen, the need for which is only growing from year to year.

Thank you for your attention, remain curious and have a good working week, guys. :)

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