# Sensing Hydrogen Seeps in the Subsurface for Natural Hydrogen Exploration
## 1 Introduction  
The transition to low emissions is gaining widespread acceptance, with hydrogen expected to play a major role in achieving a net zero emissions energy future. Currently, hydrogen is primarily produced via two pathways, either through thermochemical means, which involve the use of fossil fuel feedstocks, or through electrochemical means, which use an electrical current to split water into hydrogen and oxygen [1]. While the hydrogen industry and the technologies developed are scaling up [2], currently, there is no method that can be used to deliver the required quantities of hydrogen. The exploration and production of natural hydrogen, also called \"white or gold\" hydrogen, is one promising way to obtain potentially large quantities of green hydrogen compared with the \"blue\" one produced from methane [3, 4, 5]. There is significant interest in understanding natural hydrogen geological systems with a focus on seeps, as the modern history of oil and gas exploration began in the 19 th century with the digging and drilling of oil seeps [6, 7]. Surface emanations called \"fairy circles\" are often associated with high hydrogen soil gas measurement and have been described in numerous countries such as Russia, Brazil, Mali, Australia, and the United States [8, 9, 10, 11, 12]. Determining the hydrogen concentration and the assessment of hydrogen gas seeps in the environment is an important part of not only hydrogen exploration, but also for ensuring that any hazardous leaks are minimal. The hydrogen content of soil-gas is typically tested at the surface of natural hydrogen seeps with various analytical techniques. Commonly, airand gas samples are collected and sent to a laboratory for analysis using techniques such as gas chromatography. Although this method can provide relatively accurate quantitative information regarding the hydrogen concentration in air samples, it is unable to deliver real-time data and is not suitable for long-term field deployment. To address the limitations associated with existing laboratory techniques, there has been increasing interest in the development of sensors and sensing materials for hydrogen detection. More importantly, a few analytical techniques and sensors have been used to monitor hydrogen leaks from geological formations [5; 13; 14]. Recently, it has been shown that certain portable analysers may provide more reliable information regarding the presence and absence of hydrogen near surface settings in Western Australia [12]. There are several types of sensors that are currently useful for hydrogen detection in atmospheric environments [15; 16; 17]. Electrical resistance metal oxide sensors are a very common type of gas sensor and are widely available. In these sensors, the interaction of a gas species changes the electrical resistance of a metal oxide (often SnO\({}_{2}\)), which forms the sensor signal. Metal oxide sensors are highly sensitive and can often achieve lower detection limits of 5-10 ppm. Unfortunately, they are sensitive to humidity changes and have multiple cross sensitivities. In principle, they respond to almost any gas, including water vapour; however, when doping the metal oxide with various materials it can make the sensor more responsive to one gas molecule over another [16]. These types of sensors are often very low cost and by using a combination of sensors and appropriate data processing, it may be possible to deconvolute the response from a mixture of gases. Some types use a proprietary molecular sieve coating that prevents cross sensitivity to many other gases including methane. Electrochemical sensors that employ amperometric methods are also commonly used for hydrogen detection. These sensors comprise a miniature electrochemical cell including a working electrode (where hydrogen is oxidised) and a counter electrode allowing a reduction process to take place (typically oxygen from air) [16; 18]. Most sensors use a liquid electrolyte such as sulfuric acid and a reference electrode.