Hydrogen Details

Hydrogen: Facts

Periodic Symbol: H

Periodic Number: 1

Standard atomic weight: 1.008 atomic mass units (amu) (lightest element, 14x lighter than air)

Composition: 1 election, 1 proton

Prevalence: Roughly 75% of all baryonic mass (most abundant element in the universe)

Appearance: Colorless (also odorless, tasteless and non-toxic)

Melting Point: -259.14° C

Boiling Point: -252.87° C

Specific Energy: 142 MJ/kg (compared to gasoline: 46.4 MJ/kg, Lithium-ion batteries: .6 MJ/kg)

Discovered: Henry Cavendish in 1766

Name Origin: Hydro (Greek for “water”) Gene (Greek for “forming”)

Hydrogen: Basics

Hydrogen is the lightest, most abundant element in the universe, and the simplest. Its atomic number is 1 (of 118), with an atom composed of a single proton and electron.

Hydrogen is non-toxic, colorless and odorless. It's stored in water, hydrocarbons (such as methane), and other organic matter. A key challenge for the use of hydrogen as a fuel is the ability to extract it cleanly and efficiently. Skai uses a method called Electrolysis, which uses water in a non-polluting process that’s powered by renewable energy sources like solar, wind and hydroelectric.

Under the US Energy Policy Act of 1992, hydrogen is considered an alternative fuel. There is increasing global interest in hydrogen as a transportation fuel for its ability to power non-polluting electric vehicles, its promise of energy independence, its rapid refueling, and the high efficiency that fuel cells offer.

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Energy Density

The specific energy of hydrogen – that is, its stored energy by weight – is 142 MJ/kg, the highest of any practical fuel. That’s more than 3x that of gasoline (46 MJ/kg) and more than 200x that of lithium-ion batteries (approximately .6 MJ/kg). In fact, only radioactive fuels like plutonium and uranium offer more energy per kilogram.

Hydrogen is superior to gasoline not only in terms of energy, but in terms of clean performance – gas is a toxic fossil fuel with harmful emissions.

While batteries have zero emissions during use, their overall environmental footprint is significant. In terms of energy, it’s not even close. Battery-powered air mobility vehicles are projected to have flight durations of less than an half an hour before needing to recharge – Skai's hydrogen fuel cells give it the ability to fly continuously for up to 4 hours or more with higher capacity auxiliary tanks.

Eco-Sensitivity and Life-Cycle Assessment

In order to truly reduce greenhouse gases and carbon in the environment, the entire life-cycle costs associated with production, manufacturing, operation, reuse and recycling must be considered. The Skai hydrogen fuel cell vehicle was designed for the lowest possible impact from "cradle-to-grave."

As a company, we are keenly focused on these metrics, and on continuing to reduce our environmental footprint. Our engineering facility draws power from solar panels on the roof, and will soon receive additional power from hydrogen fuel cells repurposed from Skai vehicles.

Our vehicles are designed to emit only heat and water as byproducts. Our fuel cells provide thousands of hours of useful life per vehicle, and then can be reused by replacing the fuel-cell stack and returning to service. The stacks themselves are then recycled to capture the raw materials for later recycling and reuse.

The hydrogen we use comes from hydroelectric, solar, or wind-generation sources. Our fuel cells are reusable and recyclable, to minimize our environmental impact. All told, our Life-Cycle Assessment compares very favorably against the systems and products built around lithium-ion or hybrid solutions.

Safety Standards

Hydrogen has been used extensively in industry and aerospace for 80+ years. Over that time, rigorous safety standards have been established, in the same way that safe practices for gas or batteries have been developed. In fact, many experts consider hydrogen to be safer than gasoline.

Key safety concerns center around hydrogen’s flammability. However, the hazards associated with it are significantly reduced by its light weight. Hydrogen is 14x lighter than air, rising rapidly and dissipating quickly, thereby preventing conflagrations. For example, hydrogen fuel cell-equipped car fires typically burn out very quickly – causing little damage to the vehicle, while a similar fire in a gas- or battery-powered vehicle can cause massive, uncontainable damage.

Another issue addressed by safety practices is the unique quality of hydrogen flames. They are hard to see with the naked eye, and ultraviolet flame detectors are used to mitigate this risk in fueling stations and storage depots.

As hydrogen is odorless, colorless and non-toxic, detecting a leak requires hydrogen detectors. These detectors have been used for decades with a high safety record. There are multiple hydrogen detectors onboard Skai, tied into an alert system for the pilot, and available as telemetry to ground control. The vehicle has positive airflow through all compartments, and a vent stack to allow any gas to escape.

Hydrogen offers clean, efficient energy, and is gaining momentum worldwide. But as with any fuel, proper steps must be taken to reduce risks to their lowest possible levels.

Fuel Cells: Basics

A fuel cell uses hydrogen or another fuel to produce electricity through an electrochemical reaction. If hydrogen is the fuel, then electricity, water, and heat are the only products of the system. Fuel cells have the unique ability to provide large-scale power for a building or even a city, and small systems such as a smartphone charger.

Fuel cells are similar to batteries in that they deliver electricity. The key difference is that batteries store electricity but can run down and require recharging. Fuel cells will continue to operate as long as fuel is provided.

Skai uses a specific type of fuel cell, called a Polymer Electrolyte Membrane (PEM) fuel cell (also known as a Proton Exchange Membrane fuel cell). The advantage of this system is its high power density and lower weight and size compared with other types – all critical when it comes to an air mobility solution.

PEM hydrogen fuel cells also operate at lower temperatures than other fuel cells, which speeds start-up time and reduces wear on the system. It uses a platinum “catalyst” to separate protons and electrons from the hydrogen molecules and produce electricity.

US Department of Energy: 


National Geographic:


Fuel Cells:
How they work

Fuel cells produce electricity through an electrochemical reaction. The system has two electrodes, an Anode and a Cathode, where the electrical generation takes place. Another key component is a catalyst –  a thin membrane made of platinum in the case of Skai’s Polymer Electrolyte Membrane (PEM) fuel cell. This membrane is crucial to the electrochemical reaction.

Here’s how the system works:
Hydrogen molecules (H2) enter the fuel cell at the Anode. The platinum Catalyst separates electrons from the hydrogen molecules, while allowing the rest of the hydrogen molecule – now ionized – to pass through. The stripped electrons travel along the Anode, creating an electrical current. Meanwhile, oxygen molecules (O2) enter at the Cathode. Here, the hydrogen ions – and the electrons returning from the electrical circuit started at the Anode – bind with the oxygen molecules to produce pure water (H2O) and heat.

Interestingly, Skai repurposes this heat byproduct to warm the cabin when it’s cold out – a perfect example of the elegance and efficiency of its fuel cell system.

Fuel Cell & Hydrogen Energy Association (FCHEA):


Fuel Cells: Background

Fuel cells offer clean, efficient performance, and while they’ve been optimized over the years, they weren’t invented yesterday.

Credit for the invention of the first fuel cells is typically given to Sir William Robert Grove, a Welsh judge and early physicist who published his findings in 1838. Interestingly, German-Swiss chemist Christian Friedrich Schönbein discovered the electrochemical process at the same time, but did not publish until the following year.

Fuel cells were first used commercially by NASA, to provide electricity for the Gemini and Apollo space programs. In addition to providing enough electrical power for the command modules’ operation, the fuel cells provided pure drinking water for the crew. NASA continued perfecting the technology, and later used fuel cells to power the Space Shuttle’s systems.

Today, fuel cells are gaining momentum as a clean, capable energy source. They are used in multiple applications, from transportation (cars, buses, boats, forklifts), to backup power generators, to small charging units for consumer electronics.

Fuel Cells: Safety

Fuel cells are similar to batteries in that they deliver electricity. Batteries store their “fuel," while Skai’s fuel cell uses hydrogen from an attached tank. Like gasoline and lithium-ion batteries, hydrogen comes with a degree of risk, so industry safety standards are followed. Skai has gone a step further, with fuel tanks that can stop a bullet and an array of sensors and components to mitigate any potential risks.

Fuel Tank: The liquid hydrogen fuel tank is made from double-walled stainless steel, and is strong enough to withstand a .45 caliber bullet without penetration.

Fuel Cell: Skai’s hydrogen fuel cell is a highly fault-tolerant component. In addition, each Skai vehicle has a redundant system of three fuel cells. In the unlikely event that one fuel cell fails, the vehicle can continue safely to its destination.

Sustainable Hydrogen Production

Hydrogen has been produced at scale for more than 80 years.

Skai uses a technique called Electrolysis. This process uses electricity to separate water molecules into hydrogen and oxygen. In the past, this system was not economically viable, but with increased volume it’s becoming more cost-competitive. In addition, the power required can be generated using solar, wind or other renewable energy sources, which is Skai’s approach.

The most common method of hydrogen production is called Steam Reformation. This method is not sustainable – it uses fossil fuels and typically releases carbon dioxide into the atmosphere. In addition, the process causes impurities in the hydrogen molecules, which can negatively affect fuel cells.

Here’s how the Electrolysis process works: two electrodes (Anode and Cathode) are placed in water. DC current is passed through the electrodes. Hydrogen is produced at the Cathode, and oxygen is produced at the Anode. Electrolyzers are used to increase efficiency by helping the electricity to flow more easily. These electrolyzers can be recycled for a more sustainable and efficient end-to-end system. Current research efforts will soon optimize the Electrolysis process for an even more cost-effective solution.

US Department of Energy:


Increasing Global Utilization

Hydrogen and fuel cell usage is increasing around the world. Higher utilization and the accompanying increase in volume will lower the costs of hydrogen production and fuel cell components. This in turn will help Skai realize its vision to deliver affordable, accessible air mobility for all.

Global fuel cell power shipped in 2018 is forecasted to be 800 Megawatts (MW), up from 650 MW in 2017, with North America and Asia as the largest users. The increase is primarily from transportation applications.

North America’s unit shipments saw a 4% increase to 9,800 units. Almost 75% of these were for transportation applications, with 2,800 fuel cell electric vehicles (FCEV), and roughly 5,000 material handling vehicles such as forklifts – with buses, trucks and UAV applications making up the remainder. In 2019, there are plans for approximately 1,000 backup power and off-grid systems to be shipped this year in North America, including both stationary and portable units.

California has been a US leader in hydrogen fuel cell adoption – currently the state has over 30 hydrogen fuel stations. There is funding for 200 stations by 2025, and a goal of 1,000 by 2030. This 2030 goal will mean a reduction of roughly 2.7 million metric tons per year of greenhouse gases (GHG) and 3,900 metric tons per year nitrogen oxide pollution (NOx).  

California Fuel Cell Partnership:


E4Tech Industry Review: (requires registration): http://www.fuelcellindustryreview.com/

Energy Independence

One of the key advantages of using hydrogen as an energy source is increased energy independence. In a general sense, this refers to nations or regions being free from a dependency on energy resources, often from other countries. A higher dependence on foreign energy can negatively affect a nation’s security due to trade disputes or military conflicts.

Hydrogen’s abundance and efficiency as a fuel means that we’ll never run out. It can replace fossil fuels to power transportation and other applications. Hydrogen also reduces the need for the raw materials required for battery production, including lithium and cobalt – finite resources that cannot be sourced domestically in the US.

In addition, there is the promise of a new hydrogen economy. All of the overseas jobs and investments into imported fossil fuels and battery raw materials would be returned to domestic economies. In the US, hydrogen could be sustainably produced in every state, creating local jobs across the entire nation.

US Dept. of Energy H2 at Scale Program:


Hydrogen Energy Center, Hydrogen Economy: