Hydrogen, a versatile element with a myriad of applications, serves as a crucial feedstock for various chemical processes. Beyond its role in chemical synthesis, hydrogen also holds promise as a cleaner energy alternative to traditional fossil fuels. Its clean-burning properties make it an attractive option for power stations, offering a pathway to reduce greenhouse gas emissions and combat climate change. In transportation, it also offers a cleaner fuel alternative for heavy vehicles such as trucks, buses, and construction machinery. There is even the potential for hydrogen-powered ships navigating our seas. The current method of creating hydrogen, however, steam methane reforming (SMR), raises environmental concerns due to its carbon emissions. This prompts a closer examination of alternative hydrogen production methods, particularly water electrolysis. In this article, we compare the established method of hydrogen production, SMR, with emerging water electrolysis techniques. By weighing the advantages and disadvantages of each approach, we aim to shed light on the quest for a more sustainable hydrogen production process that aligns with our collective goal of a greener future.

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Creating Hydrogen from Steam Methane Reforming (SMR)

Today, the predominant method of creating hydrogen is steam methane reforming or SMR (also known as natural gas reforming). In SMR, methane (CH4) and steam (H2O) are combined in the presence of a catalyst to produce hydrogen (H2) and carbon monoxide (CO). This hydrogen-carbon monoxide mixture, known as 'Syngas', is the principle feedstock for the synthesis of other compounds, such as ammonia.

CH4 + H2O -> CO + 3H2

In addition to this main reaction, a further chemical transformation also occurs to a limited degree, known as the water-gas shift reaction (WGSR). This WGSR occurs when the carbon monoxide, produced in the initial reaction, combines further with the steam creating more hydrogen but, this time, with carbon dioxide (CO2) as the by-product:

CO + H2O -> CO2 + H2

If we are to use hydrogen as a fuel of the future, we need to remove the by-products of its synthesis in an environmentally-friendly way. In this era of global warming, we cannot release the carbon monoxide, which is toxic and negatively affects the atmospheric greenhouse gas levels. Nor can we release the carbon dioxide, the prototypical greenhouse gas, into the atmosphere. Furthermore, the use of methane as one of the key starting reactants in the production process invariably leads to leaks and losses during its handling. And methane is an even more potent greenhouse gas than carbon dioxide! Ultimately, SMR is an environmentally-unfriendly process and we urgently need a better method of creating hydrogen if we are to make it to a fossil fuel-absent future.

Creating Hydrogen from Water Electrolysis

Another way of creating hydrogen is via water electrolysis. The concept of electrolysis has been known about for over a hundred years. Attaching a power source to electrodes in a solution of ions causes the positive ions to move to the negative electrode or cathode, and the negative ions to move to the positive electrode or anode. The electrodes themselves act as catalysts, driving forward chemical reactions on their surface involving these ions. In the case of water electrolysis, water splits into its hydrogen and oxygen components. However, water itself is only weakly charged or polar so electrolysis happens at a glacial pace, if at all. To speed up the electrolysis of water, the addition of other compounds (salts, acids or bases) that more readily form ions, create a more ionic electrolyte. 

Alkaline Electrolyser (AEL / AWE)

The alkaline electrolyser (abbreviated as AEL or AWE) is one type of water electrolyser under development today that most resembles the original forms of electrolysis. In AEL, the electrodes (usually made of nickel) sit in an alkaline solution, usually a potassium hydroxide (KOH) or sodium hydroxide (NaOH) solution. A diaphragm or separator keeps the two sides of the electrochemical cell apart, selectively allowing the passage of hydroxide ions (OH-).

Applying an electrical potential across the electrodes causes electrolysis to begin. At the cathode or negative electrode, water splits into hydrogen gas and hydroxide ions. While at the anode, hydroxide ions react to form water, oxygen, and some electrons. Importantly, the reaction at the anode uses up the hydroxide ions in the electrolyte, while the reaction at the cathode regenerates them. Therefore, these ions pass through the diaphragm to the anode keeping electrolysis going. As for the electrons produced at the anode, these travel back into the negative side of the electrical cell through the powered connection between the electrodes. This provides a steady stream of electrons at the cathode which replenishes those used up in its reaction.

The AEL water electrolysis mechanism

Alkaline Water Electrolysis (AEL / AWE)

Anode : 4OH-> 2H2O + O2  4e-

Cathode : 4H2O4e- -> 2H2 + 4OH-

Advantages of Alkaline Electrolysis

  • Uses cheaper nickel-based metal electrodes
  • Operated at relatively low temperatures (< 100 ºC)

Disadvantages of Alkaline Electrolysis

  • Slow start-up times - inappropriate for renewable power sources
  • Electrolyser stack is usually bulky and heavy
  • Corrosive alkaline electrolyte needs to be handled carefully

Proton Exchange Membrane Electrolyser (PEM)

Another type of water electrolysis of a more recent pedigree occurs in a Proton Exchange Membrane (PEM) electrolyser. PEM electrolysis also exists under the moniker Polymer Electrolyte Membrane electrolysis which, conveniently, carries the same acronym. In a PEM electrolyser, the electrodes (usually made of platinum or platinum-type metals) press against a semi-permeable membrane. This barrier allows protons to pass through it while blocking electrons. The application of an electrical voltage across the electrodes, splits water into oxygen, protons (H+), and electrons at the positive anode. The free protons travel across the membrane to the negative side of the cell. At the same time, the electrical connection between the electrodes forces the electrons to the negative cathode. At the cathode the cycle completes with the protons and electrons combining to form hydrogen gas.

Anode : 2H2O -> O2 + 4H+ +  4e-

Cathode : 4H+ +  4e- -> 2H2

The PEM water electrolysis mechanism

Proton Exchange Membrane (PEM) Electrolysis

Advantages of Proton Exchange Membrane Electrolysis

  • Fast start-up times - useful when used with intermittent renewable power sources 
  • Operated at relatively low temperatures (70 - 90 ºC)
  • Easier to maintain than alkaline electrolysers

Disadvantages of Proton Exchange Membrane Electrolysis

  • Uses expensive platinum-group metal electrodes

Anion Exchange Membrane Electrolyser (AEM)

In Anion Exchange Membrane (AEM) electrolysis, the electrodes (usually made of steel or nickel alloy metals) are pressed against a membrane similar to a PEM electrolyser. However, the membrane in AEM selectively allows the passage of hydroxide ions rather than protons. Once again, the application of an electrical potential across the cell splits water into hydrogen gas and hydroxide ions at the negative electrode or cathode. The negative hydroxide ions, attracted by the positive electrode, cross the semi-permeable membrane to the anode. Once at the anode, the hydroxide ions react to form water, oxygen gas and some free electrons. As in other electrolyser types, the electrons are pumped back into the cathode via the wired connection. There they replenish the ones used up in the cathode reaction completing the cycle.

Anode : 4OH- -> 2H2O + O2 + 4e-

Cathode : 4H2O + 4e- -> 2H2 +  4OH-

The AEM water electrolysis mechanism

Anion Exchange Membrane (AEM) Electrolysis

Advantages of Anion Exchange Membrane Electrolysis

  • Uses cheaper steel or nickel-based metal alloy electrodes
  • Can tolerate lower water purity levels eg. rain or tap water
  • Can operate using water alone or slightly alkaline electrolyte unlike the corrosive electrolytes needed for AEL

Disadvantages of Antion Exchange Membrane Electrolysis

  • Lower conductivity, therefore lower performance
  • Shorter lifetimes of the electrolyser stack
  • Degrades at high temperatures, emitting carbon dioxide

Solid Oxide Electrolyser (SOE)

The final main type of water electrolyser in development today is the solid oxide electrolyser (SOE). In solid oxide electrolysis, the splitting of water, in the form of steam, produces hydrogen gas and oxide ions at the negative cathode. The hydrogen gas is collected, while the oxide ions travel through the solid oxide electrolyte (usually consisting of a ceramic material) that prevents the other molecules in the reaction chamber from passing. At the anode or positive electrode, the oxide ions lose their excess electrons to form oxygen gas. Just as in other types of water electrolysis, these spare electrons are pumped back into the cathode via the electrical connection. There they replace the electrons used to reduce the hydrogen ions to hydrogen gas, keeping the electrolytic cycle going.

Anode : 2O2- -> O2 + 4e-

Cathode : H2O + 2e- -> H2 +  O2-

The SOE water electrolysis mechanism

Solid Oxide Electrolysis (SOE)

Advantages of Solid Oxide Electrolysis

  • Operates at close to 100% efficiency (i.e. no input energy loss)
  • Good for industries that generate a lot of waste heat which is useful for pre-heating the electrolytic stack

Disadvantages of Solid Oxide Electrolysis

  • Operates at high temperatures (500 - 800 ºC), so SOE requires a lot of energy to start and maintain the process.
  • Very slow on-off times - inappropriate for renewable power sources

Summary

In this exploration of hydrogen production methods, we've delved into the current predominant technique of steam methane reforming (SMR) alongside emerging electrolysis technologies. Hydrogen already serves as a vital feedstock for chemical processes and it holds immense potential as a cleaner fuel alternative. However, concerns regarding its environmental impact necessitate a closer look at sustainable production methods. Through the our examination of Alkaline Electrolysers (AEL), Proton Exchange Membrane Electrolysers (PEM), Anion Exchange Membrane Electrolysers (AEM), and Solid Oxide Electrolysers (SOE), we can discover the advantages and disadvantages of each approach. By fostering a deeper understanding of hydrogen production, we can make better informed decisions for a greener, more sustainable future.